France



Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have advantages of high packing density due to their high thermal stability, efficient current-induced magnetic switching, and unrestricted cell aspect ratio.[1] Therefore, PMA-based MTJs (p-MTJs) have broad application prospects on high-density and non-volatile spin-transfer torque magnetic random access memories (STT-MRAMs) and other spintronics devices.[2-4] In particular, p-MTJs with CoFeB/MgO structures show high tunnel magnetoresistance ratio.[4] In the W/CoFeB/MgO stacks, the interfacial magnetic anisotropy (&lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt;) of 1.98 mJ/m&lt;sup&gt;2&lt;/sup&gt; is acquired after annealing at 350 °C.[5] To further improve &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt;, it is an effective way to use MgO/CoFeB/X/CoFeB/MgO double-interface free layer structures, where the X represents the spacer material. [6] However, higher &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; is needed to scale down the MTJs devices and maintain high thermal stability.[7]&lt;br/&gt;In this work [8], we study the magnetic anisotropy of the double-interface free layer stacks with different annealing temperatures and different spacer materials. The samples of Ta(5)/MgO(2)/Co&lt;sub&gt;20&lt;/sub&gt;Fe&lt;sub&gt;60&lt;/sub&gt;B&lt;sub&gt;20&lt;/sub&gt;(t)/X(0.4) /Co&lt;sub&gt;20&lt;/sub&gt;Fe&lt;sub&gt;60&lt;/sub&gt;B&lt;sub&gt;20&lt;/sub&gt;(1.2)/MgO(2)/Ta(3) are deposited by sputtering system, where the numbers in parentheses indicate the thickness in nanometers, X including Ta, W, and Mo. The thickness of the bottom CoFeB layer &lt;i&gt;t&lt;/i&gt; ranges from 0.4~3.2 nm. The films with Mo spacer layer exhibit the highest &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; of 4.06 mJ/m&lt;sup&gt;2&lt;/sup&gt; compared with the Ta- and W-based films. Furthermore, the &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; of Mo-based films remains at 2.93 mJ/m&lt;sup&gt;2&lt;/sup&gt; even after annealing at 500 °C for 1 h.&lt;br/&gt;Fig.1a and 1b show the structures with bottom CoFeB thickness of 0.8 nm and with spacer materials of Mo and W. As shown in the &lt;i&gt;M&lt;/i&gt;-&lt;i&gt;H&lt;/i&gt; (Magnetization &lt;i&gt;M&lt;/i&gt; versus magnetic field &lt;i&gt;H&lt;/i&gt;) curves, all the films show good PMA. Fig.1c and 1d show the corresponding high-resolution transmission electron microscopy (HR-TEM) and energy-dispersive X-ray spectroscopy (EDX) images.&lt;br/&gt;Saturation magnetization &lt;i&gt;M&lt;/i&gt;&lt;sub&gt;s&lt;/sub&gt; and the total thickness of magnetic dead layer &lt;i&gt;t&lt;/i&gt;&lt;sub&gt;DL&lt;/sub&gt; can be extracted from the curves ( &lt;i&gt;M&lt;/i&gt;/&lt;i&gt;A&lt;/i&gt; versus &lt;i&gt;t&lt;/i&gt;&lt;sub&gt;CoFeB&lt;/sub&gt; curves), where &lt;i&gt;A&lt;/i&gt; is the area of the sample and &lt;i&gt;t&lt;/i&gt;&lt;sub&gt;CoFeB &lt;/sub&gt;is the total thickness of CoFeB layers. The &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; can be extracted from the intercept of the &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;eff&lt;/sub&gt;*&lt;i&gt;t&lt;/i&gt;&lt;sub&gt;eff &lt;/sub&gt;versus &lt;i&gt;t&lt;/i&gt;&lt;sub&gt;eff &lt;/sub&gt;curves, where &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;eff &lt;/sub&gt;is effective magnetic anisotropy energy density and &lt;i&gt;t&lt;/i&gt;&lt;sub&gt;eff &lt;/sub&gt;= &lt;i&gt;t&lt;/i&gt;&lt;sub&gt;CoFeB&lt;/sub&gt;- &lt;i&gt;t&lt;/i&gt;&lt;sub&gt;DL&lt;/sub&gt;. As shown in Fig.2a and 2b, &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; of 2.15±0.11 mJ/m&lt;sup&gt;2&lt;/sup&gt; and 2.90±0.34 mJ/m&lt;sup&gt;2&lt;/sup&gt; can be obtained for Ta- and W-based stacks, respectively. More importantly, the maximum of 4.06±0.14 mJ/m&lt;sup&gt;2&lt;/sup&gt; is acquired for the Mo-based structure after annealing at 350 °C as shown in Fig.2c. According to previous study, when the required thermal stability factor is 70, if the &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; reached 4 mJ/m&lt;sup&gt;2&lt;/sup&gt; , the device size can be reduced to 10 nm.[8] It can be shown in Fig.2d- 2f that the &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; decreases with the increase of annealing temperature. Note that the &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; reaches 2.93 mJ/m&lt;sup&gt;2&lt;/sup&gt; even after annealing at 500 °C for 1 h, which demonstrates the high thermal stability of this structure.&lt;br/&gt;Revealed by the HR-TEM and EDX images, the sharper interface, weaker interdiffusion, better interfacial uniformity and higher crystallinity can be seen in the Mo-based stack than the W-based stack. These results show that Mo plays an important role in improving the film quality. When the annealing temperature increases, the interfacial mixing intensifies, which leads to the decrease of &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i &lt;/sub&gt;in Mo-based films.&lt;br/&gt;In summary, we study the dependence of magnetic anisotropy and film quality of the MgO/CoFeB/X/CoFeB/MgO double-interface free layer structures on the spacer material and the annealing temperature. We find that, the Mo-based structure exhibits the strongest PMA (4.06 mJ/m&lt;sup&gt;2&lt;/sup&gt;). Furthermore, &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i &lt;/sub&gt;in the Mo-based stack is still as strong as 2.93 mJ/m&lt;sup&gt;2&lt;/sup&gt; even after annealing at 500 °C. The HR-TEM and EDX images of W- and Mo-based stacks show that the high thermal stability can be attributed to the high film quality in the Mo-based films. Our work provides a practical solution to scale down MRAM device while maintaining a high thermal stability.

**References:**

[1] Ikeda, S., Miura, K., Ohno, H. Nature materials, 9(9), 721-724 (2010).&lt;br/&gt;[2] Wang, M., Cai, W., Zhao, W. Nature electronics, 1(11), 582-588 (2018).&lt;br/&gt;[3] Peng, S., Zhu, D., Zhao, W. Nature Electronics, 1-8 (2020).&lt;br/&gt;[4] Wang, M., Cai, W., Zhao, W. Nature communications, 9(1), 1-7 (2018).&lt;br/&gt;[5] An, G. G., Lee, J. B., Hong, J. P. Acta Materialia, 87, 259-265 (2015).&lt;br/&gt;[6] Hwang, C. S. Advanced Electronic Materials, 1(6), 1400056 (2015).&lt;br/&gt;[7] Peng, S., Kang, W., Zhao, W. IEEE Magnetics Letters, 8, 1-5 (2017).&lt;br/&gt;[8] Cheng, H., Chen, J., Zhao, W. Advanced Electronic Materials, 6(8), 2000271 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c74384cddd3ce3d115a89232da765fba.jpg)
Fig.1. (a,b) The schematic of Ta(5)/MgO(2)/Co&lt;sub&gt;20&lt;/sub&gt;Fe&lt;sub&gt;60&lt;/sub&gt;B&lt;sub&gt;20&lt;/sub&gt;(0.8)/X(0.4) /Co&lt;sub&gt;20&lt;/sub&gt;Fe&lt;sub&gt;60&lt;/sub&gt;B&lt;sub&gt;20&lt;/sub&gt;(1.2)/MgO(2)/Ta(3) with spacer material of (a) X=Mo and (b) X=W. (c,d) The corresponding HR-TEM images and EDX line profiles with spacer material of (c) X=Mo and (d) X=W. (e,f) The corresponding hysteresis loops with the in-plane (IP) field (red line) and out-of-plane (OP) field (black line) for different spacer materials of (e) X=Mo and (f) X=W. In the inset of (e) and (f), a &lt;i&gt;M&lt;/i&gt;-&lt;i&gt;H&lt;/i&gt; image under the small OP field with spacer material of (e) X=Mo and (f) X=W.

![](https://assets.underline.io/uploads/markdown_image/1/image/75ebcd2b300cee56aef34e4a13c0417a.jpg)
Fig.2. (a-c) The &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; fittings for MgO (2)/Co&lt;sub&gt;20&lt;/sub&gt;Fe&lt;sub&gt;60&lt;/sub&gt;B&lt;sub&gt;20&lt;/sub&gt;(&lt;i&gt;t &lt;/i&gt;)/X(0.4)/ Co&lt;sub&gt;20&lt;/sub&gt;Fe&lt;sub&gt;60&lt;/sub&gt;B&lt;sub&gt;20&lt;/sub&gt;(1.2)/MgO(2) stacks under annealing at 350 °C with spacer layer X of (a) Ta, (b) W, and (c) Mo. (d-f) The &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;i&lt;/sub&gt; fittings for MgO(2)/Co&lt;sub&gt;20&lt;/sub&gt;Fe&lt;sub&gt;60&lt;/sub&gt;B&lt;sub&gt;20&lt;/sub&gt;(&lt;i&gt;t &lt;/i&gt;) /Mo(0.4)/Co&lt;sub&gt;20&lt;/sub&gt;Fe&lt;sub&gt;60&lt;/sub&gt;B&lt;sub&gt;20&lt;/sub&gt;(1.2)/MgO(2) stacks with the annealing temperature of (d) 400 °C, (e) 450 °C, and (f) 500 °C.

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 Giant perpendicular magnetic anisotropy in Mo-based double-interface free layer structure for advanced magnetic tunnel junctions

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Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have advantages of high packing density due to their high thermal stability, efficient current-induced magnetic switching, and unrestricted cell aspect ratio.[1] Therefore, PMA-based MTJs (p-MTJs) have broad application prospects on high-density and non-volatile spin-transfer torque magnetic random access memories (STT-MRAMs) and other spintronics devices.[2-4] In particular, p-MTJs with CoFeB/MgO structures show high tunnel magnetoresistance ratio.[4] In the W/CoFeB/MgO stacks, the interfacial magnetic anisotropy (Ki) of 1.98 mJ/m2 is acquired after annealing at 350 °C.[5] To further improve Ki, it is an effective way to use MgO/CoFeB/X/CoFeB/MgO double-interface free layer structures, where the X represents the spacer material. [6] However, higher Ki is needed to scale down the MTJs devices and maintain high thermal stability.[7] In this work [8], we study the magnetic anisotropy of the double-interface free layer stacks with different annealing temperatures and different spacer materials. The samples of Ta(5)/MgO(2)/Co20Fe60B20(t)/X(0.4) /Co20Fe60B20(1.2)/MgO(2)/Ta(3) are deposited by sputtering system, where the numbers in parentheses indicate the thickness in nanometers, X including Ta, W, and Mo. The thickness of the bottom CoFeB layer t ranges from 0.4~3.2 nm. The films with Mo spacer layer exhibit the highest Ki of 4.06 mJ/m2 compared with the Ta- and W-based films. Furthermore, the Ki of Mo-based films remains at 2.93 mJ/m2 even after annealing at 500 °C for 1 h. Fig.1a and 1b show the structures with bottom CoFeB thickness of 0.8 nm and with spacer materials of Mo and W. As shown in the M-H (Magnetization M versus magnetic field H) curves, all the films show good PMA. Fig.1c and 1d show the corresponding high-resolution transmission electron microscopy (HR-TEM) and energy-dispersive X-ray spectroscopy (EDX) images. Saturation magnetization Ms and the total thickness of magnetic dead layer tDL can be extracted from the curves ( M/A versus tCoFeB curves), where A is the area of the sample and tCoFeB is the total thickness of CoFeB layers. The Ki can be extracted from the intercept of the Keff*teff versus teff curves, where Keff is effective magnetic anisotropy energy density and teff = tCoFeB- tDL. As shown in Fig.2a and 2b, Ki of 2.15±0.11 mJ/m2 and 2.90±0.34 mJ/m2 can be obtained for Ta- and W-based stacks, respectively. More importantly, the maximum of 4.06±0.14 mJ/m2 is acquired for the Mo-based structure after annealing at 350 °C as shown in Fig.2c. According to previous study, when the required thermal stability factor is 70, if the Ki reached 4 mJ/m2 , the device size can be reduced to 10 nm.[8] It can be shown in Fig.2d- 2f that the Ki decreases with the increase of annealing temperature. Note that the Ki reaches 2.93 mJ/m2 even after annealing at 500 °C for 1 h, which demonstrates the high thermal stability of this structure. Revealed by the HR-TEM and EDX images, the sharper interface, weaker interdiffusion, better interfacial uniformity and higher crystallinity can be seen in the Mo-based stack than the W-based stack. These results show that Mo plays an important role in improving the film quality. When the annealing temperature increases, the interfacial mixing intensifies, which leads to the decrease of Ki in Mo-based films. In summary, we study the dependence of magnetic anisotropy and film quality of the MgO/CoFeB/X/CoFeB/MgO double-interface free layer structures on the spacer material and the annealing temperature. We find that, the Mo-based structure exhibits the strongest PMA (4.06 mJ/m2). Furthermore, Ki in the Mo-based stack is still as strong as 2.93 mJ/m2 even after annealing at 500 °C. The HR-TEM and EDX images of W- and Mo-based stacks show that the high thermal stability can be attributed to the high film quality in the Mo-based films. Our work provides a practical solution to scale down MRAM device while maintaining a high thermal stability.

**References:**

[1] Ikeda, S., Miura, K., Ohno, H. Nature materials, 9(9), 721-724 (2010). [2] Wang, M., Cai, W., Zhao, W. Nature electronics, 1(11), 582-588 (2018). [3] Peng, S., Zhu, D., Zhao, W. Nature Electronics, 1-8 (2020). [4] Wang, M., Cai, W., Zhao, W. Nature communications, 9(1), 1-7 (2018). [5] An, G. G., Lee, J. B., Hong, J. P. Acta Materialia, 87, 259-265 (2015). [6] Hwang, C. S. Advanced Electronic Materials, 1(6), 1400056 (2015). [7] Peng, S., Kang, W., Zhao, W. IEEE Magnetics Letters, 8, 1-5 (2017). [8] Cheng, H., Chen, J., Zhao, W. Advanced Electronic Materials, 6(8), 2000271 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c74384cddd3ce3d115a89232da765fba.jpg)
Fig.1. (a,b) The schematic of Ta(5)/MgO(2)/Co20Fe60B20(0.8)/X(0.4) /Co20Fe60B20(1.2)/MgO(2)/Ta(3) with spacer material of (a) X=Mo and (b) X=W. (c,d) The corresponding HR-TEM images and EDX line profiles with spacer material of (c) X=Mo and (d) X=W. (e,f) The corresponding hysteresis loops with the in-plane (IP) field (red line) and out-of-plane (OP) field (black line) for different spacer materials of (e) X=Mo and (f) X=W. In the inset of (e) and (f), a M-H image under the small OP field with spacer material of (e) X=Mo and (f) X=W.

![](https://assets.underline.io/uploads/markdown_image/1/image/75ebcd2b300cee56aef34e4a13c0417a.jpg)
Fig.2. (a-c) The Ki fittings for MgO (2)/Co20Fe60B20(t )/X(0.4)/ Co20Fe60B20(1.2)/MgO(2) stacks under annealing at 350 °C with spacer layer X of (a) Ta, (b) W, and (c) Mo. (d-f) The Ki fittings for MgO(2)/Co20Fe60B20(t ) /Mo(0.4)/Co20Fe60B20(1.2)/MgO(2) stacks with the annealing temperature of (d) 400 °C, (e) 450 °C, and (f) 500 °C.

technical paper

Systematic studies of magnetic properties observed in epitaxial W/Co/Pt layered structures grown by molecular beam epitaxy are discussed. In this system contributions to interfacial Dzyaloshinskii-Moriya interaction (DMI) from the asymmetric interfaces between ferromagnet (Co) and non-magnetic heavy metals with high spin-orbit coupling (W or Pt) are of additive nature. DMI can stabilize the chiral magnetic texture, vortex and skyrmions, being promising in future data storage technology. Presented progressive investigations encompass layered structures containing: a single Co layer (magnetic anisotropy), two Co layers (interlayer coupling) and multilayers (dipolar interactions, complex domain structure). Layered structures containing a single Co layer exhibit spin reorientation transition (SRT) with increasing thickness, dCo,from perpendicular (perpendicular magnetic anisotropy, PMA) to in-plane aligned magnetisation. The range of dCo with PMA and the critical thickness dSRT depend on the type and sequence of adjacent covers. In the Pt/W/Co/Pt system dCo range with PMA shifts towards higher values (from 0.6-0.9 nm to 2.1-2.4 nm) with increasing bottom W layer thickness, dW, from 2.4 nm to 8.0 nm. This behaviour differs from the similar system with Mo component leyer, studied earlier [1]. Hysteresis loops measured by polar magnetooptical Kerr effect (PMOKE) magnetometry exhibit shapes typical of fine domain structure with perpendicular magnetisation. In the W/Co(1.0 nm)/Pt(dPt)/W(dW)/Co(1.0 nm)/Pt interlayer antiferromagnetic coupling of perpendicular magnetization with the strength as high as 0.6 kOe is very distinct for the component spacers of equal thickness, dPt = dW, in the range between 0.7 and 1.1 nm. For dW ≠ dPt (keeping the same total thickness dW + dPt) the coupling strength decreases. Particularly, the single spacer systems (Pt or W; dPt or dW kept in the corresponding range from 1.4 nm to 2.0 nm) do not exhibit such feature, indicating a crucial role of the complex spacer structure affecting both coupling and PMA (Figure 1). The multilayer structures W/Co(0.6 &lt; dCo &lt; 1.0 nm)/Pt (10-20 repetitions) are magnetised in perpendicular direction to the film plane and display at the remanent state typical labyrinth domain structure with tendency towards bubble domains formation with diameter of around 75 nm (Figure 2). DMI strength determined form (aligned) domain structure reaches the value of D = 2.49 mJ/m2 [2]. Such high value of D is a result of high quality crystalline structure characteristic for epitaxial systems and additive contribution to DMI from asymmetric well-defined interfaces. The above discussed structures are compared to those with different component layer thickness and opposite sequence (i.e. Pt/Co/W). This work is supported by IEEE educational seed funding 2020 (beneficiary:-Sukanta Kumar Jena), Foundation for Polish Science (FNP) under the European Regional Development Fund – Program REINTEGRATION 2017 OPIE 14-20 and by the National Science Centre in Poland, projects no: 2016/23/G/ST3/04196 and 2020/37/B/ST5/02299.

**References:**

[1] A Wawro et al. J. Phys. D: Appl. Phys. 50 215004 (2017). [2] S.K. Jena et al. to be submitted (2021).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/f62fbee3caf0455305aedeea9ea695c0.jpg)
“Fig.1” Hysteresis PMOKE loop from W/Co/Pt/W/Co/Pt layered structures composed of two Co (1nm) layers separated by the Pt/W spacer with total thickness of 1.4 nm showing distinct antiferromagnetic coupling (red). Corresponding structures with a single spacer: W (black) or Pt (green) 1.4 nm thick do not exhibit such coupling.

![](https://assets.underline.io/uploads/markdown_image/1/image/5285c84da6791eb5744546eceba7f522.jpg)
"Fig.2" Magnetic force microscopy image from as grown W/Co/Pt multilayer with basic trilayer repetition number equal to 20.

 Magnetic anisotropy, interlayer coupling and Dzyaloshinskii-Moriya interaction in epitaxial W/Co/Pt multilayers

The interfacial Dzyaloshinskii-Moriya Interaction has had profound consequences on the possible magnetic configurations that can be stabilized in multi-layer thin films, most notably chiral Neel skyrmions and domain walls.[1] More recently, there have been theoretical calculations and experimental work confirming the existence of an appreciable anisotropic DMI in epitaxial thin films with further reduced symmetry – namely the C2v (or mm2) point group.[2] However, so far the systems under consideration have been based on cubic crystals where the magnetocrystalline anisotropy is nearly isotropic. Here, we identify a new epitaxial relationship with C2v symmetry based on HCP Co(10.0)/FCC Pt(110). Although it is quite common to grow FCC Co(111)/Pt(111), for the case of a Pt(110) underlayer, the lattice mismatch is actually smaller for the HCP Co(10.0) orientation [as opposed to FCC Co(110)] where the c-axis lies along a single direction in the plane of the film (figure 1). Such an orientation is expected to offer some degree of anisotropic DMI based on the heavy metal Pt and reduced symmetry as well as the added feature of significant uniaxial in-plane magnetocrystalline anisotropy. Samples were prepared by DC magnetron sputtering on HF-etched Si(110) single crystal substrates. Following the work of Yang et al., we use a Ag (10nm) buffer layer, which is known to grow with (110) orientation on Si(110) at room temperature, thereby avoiding further unwanted interface diffusion.[3] This is followed by deposition of the Pt (10nm) /Co (10-50nm) bilayers, which are the focus of this work. All depositions were done in an Ar atmosphere with the working pressure fixed at 2.5 mTorr. The films were characterized structurally by standard θ/2θ x-ray diffraction (XRD) measurements and phi-scans to characterize the epitaxial relationship. The XRD scan shows primarily Ag(220), Pt(220), and Co(10.0) peaks with no identifiable Co(220) peak suggesting the film is predominantly HCP. Phi scans (figure 2) on Pt(111), Ag(111), and Co(10.1) confirm the epitaxial relationship shown in figure 1. Moreover, reciprocal space q-scans on the HCP Co spots were leveraged to characterize directional peak broadening due to stacking faults. As it is known that stacking faults cause broadening only of certain peaks (h-k ≠ 3m) along the c* direction (with breadth also determined by the parity of l), it is possible to extract the density of growth and deformation faults in the Co layer, which are found to be approximately 10% and 3% respectively in 50nm Co films. Details on how these stacking fault densities are calculated will presented in detail. Alternating gradient field magnetometry was used to measure M-H loops along different crystallographic directions in the plane of the film. A clear in-plane easy axis is identified along Si[001] (parallel to Co[0001]). The hard axis saturation field of 1.5 kOe suggests a notably smaller magnetocrystalline anisotropy than would be expected for bulk cobalt, likely due to the aforementioned stacking faults. Although the relatively thick Co layer in these films makes any interface induced DMI unlikely, the epitaxial relationship is expected to be preserved in thinner samples along with the reported uniaxial anisotropy. Together with the expected interfacial anisotropic DMI, this in-plane anisotropy makes the proposed material system an interesting playground for exploring new chiral magnetic ground states.

**References:**

[1] Thiaville, A., Rohart, S., Jué, É., Cros, V. and Fert, A., 2012. Dynamics of Dzyaloshinskii domain walls in ultrathin magnetic films. EPL (Europhysics Letters), 100(5), p.57002. [2] Hoffmann, M., Zimmermann, B., Müller, G.P., Schürhoff, D., Kiselev, N.S., Melcher, C. and Blügel, S., 2017. Antiskyrmions stabilized at interfaces by anisotropic Dzyaloshinskii-Moriya interactions. Nature communications, 8(1), pp.1-9. [3] Yang, W., Lambeth, D.N. and Laughlin, D.E., 1999. Unicrystal Co–alloy media on Si (110). Journal of applied physics, 85(8), pp.4723-4725.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/cf10786ba9d1e164cc2eadffc57f6d46.jpg)
Fig 1. Schematic of the lattice matching for Ag(110)/Pt(110) with bottom left) FCC Co(110) or bottom right) HCP Co(10.0). The HCP (10.0) orientation has a better match along the Pt [001] direction.

![](https://assets.underline.io/uploads/markdown_image/1/image/78b7907db3cc501cd82fd177111f404e.jpg)
Fig 2. Phi scans for Pt (111), Ag (111), and Co (10.1) confirming the epitaxial relationship shown in figure 1.

 Structural and Magnetic Characterization of Epitaxial Co(10.0)/Pt(110) Multi-layers for Future Anisotropic DMI Systems Based on C2v Symmetry

Skyrmions typically appear in systems with perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii-Moriya interactions (DMI). Bloch skyrmion states can be stabilized in magnetic dots with no need of DMI [1]. However, PMA is typically considered to be necessary. Nanoparticle geometry is an additional important factor for the skyrmion stabilization due to the influence of magnetostatic energy. Therefore, the sample shape (e.g. spherical) can result in stabilization of the skyrmions [2]. Recently, stabilization of the half-hedgehog skyrmions in soft magnetic (Permalloy) hemispherical dots by the MFM tip was demonstrated [3]. Here we conduct micromagnetic simulations in 3D soft magnetic dots with planar (cylindrical) or curved (spherical cap) geometry with neither PMA nor DMI. The geometries of both magnetic elements correspond to the base radius 60-120 nm and the aspect ratio thickness/ radius ranged from 0.4 to 2 and magnetic parameters of permalloy. In a wide range of the dot sizes, we observed the co-existence of multiple magnetic states. Particularly, in the spherical caps we stabilized for the same geometry, the in-plane, the out-of-plane (flower-like), the 3D vortex-like (Bloch skyrmion), the 3D Néel skyrmion –like (see Fig.1) and the Bloch point-like states. The Bloch skyrmion is characterized by a vortex configuration in the basal plane while the Neel-like skyrmon – by the radial vortex configuration and is similar to the half hedgehog structure. The existence of magnetic skyrmions without DMI or PMA in cylindrical nanodots was demonstrated by analytical calculations using the Belavin-Polyakov ansatz [4]. Both Néel and Bloch (vortex-like) skyrmions structures have been proven to be metastable or stable states in some range of dot sizes. The Néel skyrmion always has larger energy than the Bloch skyrmion. Direct micromagnetic simulations confirm the co-existence of both types of skyrmions, although in a smaller range of geometrical parameters than predicted by analytical calculations. In spherical caps, the curved geometry provides an additional factor for the Néel skyrmion stabilization which was obtained for a wide range of geometrical parameters. Typically, the Néel skyrmion has the largest energy of all structures, however its energy decreases as a function of the cap basal diameter and aspect ratio. Consequently, for some geometrical parameters we found a Néel skyrmion having smaller energy than that of the Bloch skyrmion. The spherical geometry also leads to a coupling between the skyrmion polarization and radial magnetization components, so that only the skyrmions with positive Polarity (P) and outward chirality (Q) or negative polarity and inward chirality are possible, see Fig. 1 Finally, we present the state diagram for skyrmions in soft magnetic caps and nanopillars. We have also calculated 2D topological charges and the gyrovector values, both depending on the geometrical parameters.

**References:**

[1] K. Guslienko, K. Y. IEEE Magn. Lett. Vol. 6, p,4000104 (2015) [2] V.P. Kravchuk et al Phys. Rev. B, Vol. 85, p.144433 (2012). [3] E.Berganza et al, Nanoscale, Vol.12, p.18646 (2020) [4] A. A. Belavin and A. M. Polyakov, Pis’ma Zh. Eksp. Teor. Fiz. Vol. 22, p. 503 (1975) .

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c0723cd91b5db4d72e227b49f23c8980.jpg)
Figure.1 (a-b) Chiral (Neel-like) skyrmion magnetic configurations in permalloy caps with diameter 60 nm and height 30 nm colored by the axial-component mz. Left and right figures are cross sections in the XZ plane and in the XY basis plane respectively. P and Q stand for the polarity and the chirality of the skyrmion respectively.

 Skyrmions in 3D soft magnetic nanodots with no Dzyaloshinskii-Moriya interactions

Exchange-spring nanomagnets [1–3] are composed of the interaction of the magnetic exchange coupling between magnetically hard and soft phases. Such frameworks are promising for cutting edge perpetual magnetic applications, as they have a high energy product - the mix of permanent magnetic field and magnetization - contrasted with the conventional, single-phase magnetic materials. Hence, for the last three decades, these exchange spring nanomagnets have taken the keen attention of the scientific community. However, the necessary condition that both the hard and soft phases are controlled at the nanometer scale, to ensure effective exchange coupling, has posed significant preparation challenges. In pursuit of preparing multi-nanolayered-structured nanomagnetic material, sputtering, electron beam lithography/ evaporation (EBL/EBE) have inspired a wealth of experimental results. However, to date, there is a very limited experimental understanding of preparing in situ multilayered nanomagnetic material using electrodeposition technique and the elucidation of their magnetic behavior. Towards this end, using electrodeposition technique with a modified and stable acidic bath chemistry at room temperature, we have prepared in situ multilayered nanomagnetic cobalt phosphorus (CoP) thin film and investigated its characteristics structurally and magnetically combined with micromagnetic simulation utilizing MuMax3. Despite the availability of various techniques, the electrodeposition technique [4–7] has been chosen for the preparation of the nanomagnetic cobalt phosphorous thin films at room temperature using acidic bath chemistry due to its relatively higher deposition rate and low cost. Here, the crossover from the exchange spring structure to exchange coupled structures by the manipulation of thickness in nanometer level in a single amorphous system of cobalt phosphorus nanomagnetic thin film has been demonstrated. We have characterized the developed smooth, stress-free, and shiny electroplated samples structurally by XRD (shown in FIG. 1), SEM, and magnetically by the hysteresis loop tracer (ShB Instruments, USA). The SEM micrographs confirm the fine grain surface morphology of the cobalt phosphorous nanomagnetic thin films. The XRD analysis further ensures the amorphousness of the thin film, as in the XRD pattern, the three dominating peaks only appears from the substrate, i.e., the largest peak near 2θ = 69.22° is due to the Si-wafer and the rest of the two peaks near 2θ = 38° and 83° are from the top conducting seed layer of gold (Au). The elemental analysis by energy dispersive X-Ray (EDX) yields the stoichiometry of the electrodeposited cobalt phosphorous. An optimum exchange coupling can be obtained by tuning the spin configuration with respect to the multi-nano layer thicknesses. The static in-plane magnetic measurement by the hysteresis loop tracer shown in FIG. 2 reveals different hysteresis loops of electrodeposited CoP thin film. FIG. 2(a) confirms the exchange spring amorphous cobalt phosphorus films with a relatively higher coercivity of ~197 Oe for 0.68 µm CoP thin film while the FIG. 2(b) affirms the flat hysteresis loop with a coercivity of 162 Oe for the 11.54 µm thick CoP thin film which signifies the exchange coupled structure. Utilizing the finite difference method-based freely available GPU-accelerated micromagnetic simulator, MuMax3 [8], we have also verified the experimentally obtained results further by doing a micromagnetic simulation. Considering the typical magnetic material parameters (i.e., Saturation Magnetization (MS) as obtained from the static measurement, Exchange Stiffness Constant (AEx), Anisotropy Constant (Ku1)), MuMax3 solves the well-known Landau-Lifshitz-Gilbert (LLG) equations, and so forth gives the static micromagnetic evolution of the nanomagnetic thin films. To proceed with the simulation, we have discretized the overall amorphous cobalt phosphorous thin film into the cells of dimension 5 x 5 x 5 nm3. To include all the exchange interactions in the static magnetization of nanoscale thin film, this cell size in each direction is considered lower than the exchange length (lex = 5.1 nm) of the system, as well as to precisely reproduce the nanomagnetic impression of the thin film under study. The dimensions of the whole atlas we used for the simulation are (l x w x t = 8000 x 6000 x 5400 nm3), where l, w, and t are the length, width, and thickness of the atlas, respectively. Finally, we report the micromagnetic spin configuration at every coordinate point within the nanomagnetic thin film at every instant of field determined by the simulator, which confirms the formation of a multidomain state from a single domain state (shown in FIG. 2(e))

**References:**

[1] E. F. Kneller and R. Hawig, IEEE Trans. Magn. 27, 3560 (1991). [2] E. E. Fullerton, J. S. Jiang, M. Grimsditch, Phys. Rev. B 58, 12193 (1998). [3] H. Zeng, J. Li, J. P. Liu, Z. L. Wang, Nature 420, 395 (2002). [4] P. McCloskey, B. Jamieson, T. O’Donnell, J. Magn. Magn. Mater. 320, 2509 (2008). [5] P. McCloskey, B. Jamieson, T. O’Donnell, J. Magn. Magn. Mater. 322, 1536 (2010). [6] P. McCloskey, T. O’Donnell, B. Jamieson, in Handb. Nanoelectrochem. Electrochem. Synth. Methods, Prop. Charact. Tech.,2016, pp. 1–27. [7] S. Roy, A. Connell, M. Ludwig, J. Magn. Magn. Mater. 290–291, 1524 (2005). [8] A. Vansteenkiste, J. Leliaert, M. Dvornik, AIP Adv. 4, 107133 (2014).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ab0568662cd1fd8eb2bea9c14dc8237e.jpg)
FIG. 1. XRD of 0.68 µm, 5.06 µm thick electrodeposited thin films having only the substrate peaks (i.e., Au and Si) confirms the amorphousness of the samples.

![](https://assets.underline.io/uploads/markdown_image/1/image/af142b7a2e9ace085c438741e63feae8.png)
FIG. 2. Hysteresis loop of the electrodeposited CoP thin films measured in XY plane, and here, dir-1+ 90° = dir-2. The thickness of the films is (a) 0.68 µm, (b) 11.54 µm. (c)-(d) The differential of the magnetization of the (a) 0.68 µm, (b) 11.54 µm thick CoP thin films, respectively. (e) Simulated Hysteresis Loop and different micromagnetic spin configurations at different coordinates of the hysteresis loop.

 Exchange Spring Co-Rich CoP Nanomagnetic Thin Films

Controlled manipulation of spin texture in magnetic materials by external means, e.g., magnetic and electric fields, temperature etc., is a prerequisite for the cutting-edge spintronic applications. Magnetic materials with perpendicular magnetic anisotropy (PMA) are of special interest because of their wide-ranging functionalities starting from applications in magnetoresistive random access memory (MRAM) devices to modulation of topological spin textures. Recently, there has been a growing interest in rare earth (RE)-transition metal (TM) alloy thin films due to their tuneable spin configurations as a function of temperature and RE content. For example, GdFeCo shows in-plane spin configuration at room temperature, but PMA occurs near the compensation temperature (TCO).[1] Such spin reorientation is generally attributed to competition between PMA and shape anisotropy. In order to understand the behaviour of magnetic anisotropy around the spin reorientation temperature, several indirect methods were employed to determine the temperature dependence of anisotropy fields, e.g., longitudinal and polar MOKE, FMR etc.[1] Here, we examined the spin reorientation in single layer ferrimagnetic alloy thin films of FeGd by dc magnetometry and radio frequency transverse susceptibility measurement, which is a direct method to determine the effective anisotropy fields. The FeGd system belongs to the RE-TM family and is particularly interesting as TCO of this system can be tuned by varying the chemical composition. We prepared the FexGd100-x(90 nm)/Ta(8 nm) films onto a Si substrate coated with a 10 nm Ta layer using RF and DC sputtering; we focus below on a nominal composition of Fe75.7Gd24.3. From the temperature dependence of the in-plane magnetization M(T) curve, TCO for the Fe75.7Gd24.3 film was estimated to be ~ 70 K (main panel of Fig. 1(a)). Inset of Fig. 1(a) depicts the in-plane M(H) loops for this film measured at T = 10 and 300 K. While the in-plane saturation magnetization has drastically dropped between 300 and 10 K, the coercivity is enhanced significantly. In addition, the ratio of remanent magnetization to the saturation magnetization is an order of magnitude higher at 10 K than at 300 K. These observations indicate the occurrence of spin reorientation between 10 and 300 K. In order to probe the spin reorientation, we performed magnetic field dependent transverse susceptibility (TS) measurements for both in-plane and out-of-plane configurations at different temperatures between T = 300 and 10 K. Our TS measurement set up exploits a self-resonant ultra-sensitive tunnel diode oscillator circuit with a resonant frequency of 13 MHz and is a highly sensitive technique to precisely determine the effective anisotropy fields.[2] The inset of Fig. 1(b) shows the magnetic field dependence of the relative change in TS at 40 K for the Fe75.7Gd24.3 for in-plane configuration. As the dc magnetic field is swept from positive to negative saturation, the first (second) peak to positive (negative) anisotropy field (HK). As shown in the main panel of Fig. 1(b), HK remains the same for in-plane and out-of-plane configurations but begins bifurcating below 250 K. At lower temperatures, the out-of-plane HK shows a broad maximum between 200-150 K and then decreases smoothly. On the other hand, the in-plane HK continues to decrease down to 100 K below which it shows a small plateau around TCO and then decreases gradually. These observations indicate that the spin configuration in the system changes from a combination of in-plane and out-of-plane spins with equal contributions above 250 K, to a combination of in-plane and out-of-plane spins with a major contribution by the out-of-plane spins below 250 K. The origin of such temperature-driven spin reorientation might be related to phase-segregation. With the help of scanning transmission electron microscopy equipped with energy dispersive x-ray spectroscopy (STEM-EDS) and x-ray reflectivity (XRR) measurements, a recent study shows that modulation of chemical composition occurs in FeGd films with thickness more than 20 nm where Fe is partially diffused to the adjacent Ta layer.[3] Since the thickness of our film is ~ 90 nm, it is possible that there are different Fe-deficient (Gd-rich) regions which are separated by regions with nominal chemical composition. In such a hybrid phase-segregated spin system, different regions have distinct spin configurations and hence, different orientations of magnetic anisotropy. We have also performed out-of-plane M(H) measurements on the Fe75.7Gd24.3 film that showed a double-step reversal behaviour below 250 K and hence, confirms the chemical phase-segregation in our system. The spin reorientation in our system is thus determined by the competition between local magnetic anisotropy energies of adjacent phase-segregated regions. Our TS technique determines the average behaviour of the local anisotropy field and hence effectively probes the spin reorientations in these micron-scale phase-segregated magnetic systems. Results for other Fe/Gd compositions will also be discussed.

**References:**

[1] W. He, H. -L. Liu, H.-Y. Wu, J. -W. Cai, and Z. -H. Cheng, Appl. Phys. Lett. 106, 042401 (2015). [2] V. Kalappattil, R. Geng, R. Das, M. Pham, H. Luong, T. Nguyen, A. Popescu, L. M. Woods, M. Kläui, H. Srikanth and M. H. Phan, Mater. Horiz. 7, 1413-1420 (2020). [3] E. Kirk, C. Bull, S. Finizio , H. Sepehri-Amin, S. Wintz , A. K. Suszka, N. S. Bingham, P. Warnicke, K. Hono, P. W. Nutter, J. Raabe, G. Hrkac, T. Thomson and L. J. Heyderman, Phys. Rev. Mater. 4, 074403 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ad612b336fa97c19a15298613ec09cf9.png)
(a) Main panel: in-plane M(T) of Fe75.7Gd24.3 measured in 1 T field and inset: in-plane M(H) loops at 10 and 300 K, (b) main panel: in-plane and out-of-plane HK(T), inset: field dependent TS curve at 40 K for in-plane configuration.

 Competing Magnetic Anisotropies and Spin Reorientation in Phase-Segregated Single Layer Ferrimagnetic FeGd Films

Antiferromagnets (AFMs), due to their unique set of properties, are promising candidates for future spintronic materials. However, their low ordering temperature makes a wide group of AFM materials not applicable. Hence, tuning ordering temperature and spin configuration due to magnetic proximity effect in AFM/AFM systems has been recently identified as an important field of research [1]. In our studies we investigated how the proximity of antiferromagnetic layer with a higher ordering temperature (Néel temperature, TN) influences the TN of FeO in FeO/CoO bilayer. The layers were grown by molecular beam epitaxy on MgO(001) substrate coated with a 5 nm-thick homoepitaxial MgO buffer. CoO layer with a thickness of 2 nm was prepared by reactive deposition of Co in a molecular oxygen atmosphere under a pressure of 1×10-6 mbar. FeO layer was deposited by reactive deposition of 1 nm of Fe under oxygen pressure of 5×10-8 mbar, which corresponds to about 1.7 nm of stoichiometric wüstite (FeO). To enable Mössbauer spectroscopy (MS) studies we used 57Fe isotope during FeO deposition. The sample was capped with 3 nm of MgO. To compare magnetic properties of wüstite in FeO/CoO bilayer with a single FeO layer a MgO/FeO/MgO sample was prepared. The magnetic properties of FeO were studied with Conversion Electron Mössbauer Spectroscopy (CEMS). CEMS measurements were obtained in temperature controlled cryostat under UHV conditions with use of a channeltron detector connected to a standard Mössbauer spectrometer and a 57Co(Rh) source irradiating the sample at normal incidence. Figure 1 shows CEMS spectra collected at 240 K for MgO/57FeO/MgO (Fig. 1(a)) and MgO/57FeO/CoO/MgO (Fig. 1(b)). The temperature of 240 K is far above the TN of FeO (198 K) [2]. The spectrum of FeO (Fig. 1(a)) is characteristic for bulk wüstite in paramagnetic state [3]. It was fitted by two quadrupole sites, which describe the octahedrally coordinated Fe2+ (Q1) atoms and Fe2+ atoms neighbouring defects (Q2). Interestingly, proximity of CoO causes a serious modification of the spectrum, it becomes magnetically split. Similar spectrum was recorded for MgO/FeO/MgO under the TN (not shown). Systematic measurement of CEMS spectra recorded as a function of temperature for FeO/CoO revealed that the magnetic character of spectrum is preserved up to 260 K. Thus, proximity of CoO layer induces enhancement of the TN of FeO up to 260 K. To determine TN of CoO in FeO/CoO bilayer we performed X-ray Magnetic Linear Dichroism (XMLD) measurements [4]. X-ray absorption spectra (XAS) were collected using a linearly polarized x-rays for two grazing incident angles (0° and 60°) as a function of temperature. XMLD effect observed at the Co L3 edge can be attributed to the AFM ordering in CoO. Thus, RL3 difference (ΔRL3) described as ΔRL3 = RL30 - RL360, where RL30 and RL360 are defined as the ratio of the intensities of the XAS peaks at 777 eV and 779.6 eV for grazing incident angles of 0° and 60°, respectively is a measure of AFM ordering in CoO. The AFM contribution to ΔRL3(T) decreases with an increase of temperature and vanishes above TN. The inset in Figure 1 shows ΔRL3(T) dependence obtained for CoO in MgO/FeO/CoO/MgO multilayers. We noted positive ΔRL3 values, which indicates in-plane alignment of CoO AFM spins in FeO/CoO/MgO [5,6]. Dependence of ΔRL3 on temperature is negligible for T ≥ 290 K. Thus, the ordering temperature of CoO layer in FeO/CoO/MgO stack is comparable to the TN of bulk CoO (293 K). Enhancement of ordering temperature of FeO should result in increase in exchange interaction between ferromagnetic (FM) layer and FeO in FM/FeO/CoO multilayer. To elucidate how AFM proximity effect influence exchange interaction with FM a dedicated sample was prepared for which Fe(1 nm)/FeO bilayer was grown on wedge shaped CoO. Figure 2 presents the product of exchange bias field (defined as HEB = (HC1 - HC2)/2, where C1 and C2 represent respectively the coercive fields of ascending and descending branches of hysteresis loop) and CoO layer thickness dCoO as a function of temperature obtained for different CoO thicknesses. The values of HEB were determined from the magnetic hysteresis loops measured with longitudinal magnetooptic Kerr effect (LMOKE). While for Fe/FeO/CoO(0.7 nm) we noted vanishing of HEB * dCoO at around 220 K (so called blocking temperature, TB), for dCoO &gt; 2 nm TB was established at around 260 K, which is much greater than a blocking temperature of Fe/FeO bilayer (160 K). Our study demonstrates the feasibility of tailoring the magnetic properties of AFM layer with a proximity of other AFM. This work was supported by the “Antiferromagnetic proximity effect and development of epitaxial bimetallic antiferromagnets – two routes towards next-generation spintronics” project, which is carried out within the Homing programme of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund.

**References:**

 [1]. S. Fukami et al., Journal of Applied Physics 128 070401 (2020) [2] K. Koike, T. Furukawa, Physical Review Letters 77 3921 (1996) [3] A. Koziol-Rachwal et al., Applied Physics Letters 108 041606 (2016) [4] https://synchrotron.uj.edu.pl/en_GB/linie-badawcze/peem-xas [5] J. Zhu et al., Physical Review B 90 054403 (2014) [6] Q. Li et al., Scientific Reports 6 22355 (2016)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e8177b448bb39166aa1dc962682247fd.jpg)
Fig. 1 CEMS measurements of FeO (black points) in MgO/FeO/MgO and MgO/FeO/CoO at 240 K. Inset represents ΔRL3(T) dependence for CoO in FeO/CoO/MgO(001).

![](https://assets.underline.io/uploads/markdown_image/1/image/b63fa16fb0795eb0b8aa4752326fde37.jpg)
Fig. 2 HEB*dCoO(T) obtained from LMOKE measurements for MgO/Fe/FeO/CoO/MgO(001) for different CoO thicknesses. Lines are guides for the eye.

 Beating the Limit of Ordering Temperature of FeO with Antiferromagnetic Proximity in FeO/CoO

Artificial spin ice (ASI) consists of a lithographically manufactured two-dimensional array of ferromagnetic nanoislands with a strong shape anisotropy resulting in single-domains that behave like giant Ising spins. In natural spin ices, such as, for example, the rare earth pyrochlore Ho2 Ti2 O7, the local ordering of magnetic moments is particularly difficult to measure [1]. Despite the extraordinary advances in imaging techniques in the last decade, which allows us now to access the three-dimensional magnetic nanostructure of micrometric bulk samples [2], we still need to work together with simplified models to understand more about the underlying physics that governs these highly complex systems. In that direction, artificial frustrated magnetic systems allow us to test and reproduce theoretical models as well as to understand more about their three-dimensional analogs. But even more importantly, it allows us to design a system rather than discover it [3]. Here we study an squared artificial spin ice, with dipolar interactions, where there is a height separation h between the magnetic islands which are oriented perpendicularly [3, 4] (see Fig. 1). In a tetrahedrical lattice, such as the one found in natural spin ices, the distance between any pair of spins (belonging to the same tetrahedron) is the same, and therefore, the dipolar energy has the same value for any pair. On the contrary, in the square ASI, the distance between collinear spins is different from the distance between perpendicular spins. Hence, the need to add this height parameter. In this way, this system is halfway between three-dimensional (natural) and two-dimensional (artificial) spin ices. The effect of disorder is still an open question and a field to study in ASI [1,4]. Budrikis et al (2012) study from the point of view of theory, simulations and experiments the influence of disorder on the response of the system to an external field [5]. To do this, in the simulations, they propose that each spin had an internal (coercive) field given by a Gaussian distribution. With this model, they manage to estimate the strength of disorder in a sample. There are several works that quantify the disorder, what is new in Budrikis’ work is that they also study how disorder affects the dynamics of ASI. Chern et al (2014) take this same idea to model disorder and study the avalanches and critical behavior in square and kagome ASI when a magnetic field is applied in-plane [6]. This system shows a phase transition out-of-equilibrium induced by disorder strength. Reichhardt et al (2015) show that the avalanche distributions of this process follow a power law [7]. Another way to include disorder in the system is to disconnect the islands by eliminating a given percentage of them at random. Greenberg et al (2018) show through simulations that this system changes its thermal behavior as the percentage of holes increases [8]. These results coincide with what was observed experimentally in diluted spin ice, once again showing that the study of ASI allows modeling and better understanding of the behavior of spin ice. Our aim is to study, through simulations and an energetic analysis, the effect of a geometrical quenched disorder by design on an square ASI. To do this, we propose to include disorder in the length of the magnetic islands. In this way, the strength of disorder could be easily controlled experimentally. As the system behavior depends on its geometrical parameters (a, h and d in Fig. 1), we focus on studying it in the proximity of the ice regime, where Type I and II vertices are equally likely (see Fig. 2). This regime is quite difficult to thermalize both in experiments and simulations. In this work, we show how length disorder affect the antiferromagnetic (all Type I vertices) and locally ferromagnetic (all Type II vertices) ordering, by inducing the system, in the case of weak disorder, to intermediate or mix states. An important result is that this means that disorder strength can be chosen and tuned to thermalize the ice regime. Moreover, in the case of strong disorder, ferromagnetic vertices prevail regardless of whether the mean length of the islands corresponds to an antiferromagnetic ordering. The energetic analysis let us understand the intermediate regimes that we found and also why disorder does not permit defects (Type III and IV vertices) at low temperatures in this model, contrary to expectations. Also, with this analysis, we show in detail how the ferromagnetic–antiferromagnetic transition with h is rounded by quenched disorder. Finally, we found that disorder in the geometry causes a slowdown in the dynamics of the system which increases with disorder.

**References:**

[1] C. Nisoli, R. Moessner and P. Schiffer, Rev. Mod. Phys., Vol. 85, p. 1473 (2013) [2] C. Donnelly, M. Guizar-Sicairos, V. Scagnoli et al, Nature, Vol. 547, p. 328–331 (2017) [3] R. F. Wang, C. Nisoli, R. S. Freitas et al, Nature, Vol. 439, p. 303 (2006) [4] G. Möller and R. Moessner, Phys. Rev. Lett., Vol. 96, p. 237202 (2006) [5] Z. Budrikis, J. P. Morgan, J. Akerman et al, Phys. Rev. Lett., Vol. 109 p. 037203 (2012) [6] G. W. Chern, C. Reichhardt and C. Nisoli, Appl. Phys. Lett., Vol. 104, p. 013101 (2014) [7] C. J. O. Reichhardt, G. W. Chern, A. Libál and C. Reichhardt, J. Appl. Phys., Vol. 117, p. 172612 (2015) [8] N. Greenberg and A. Kunz, AIP Adv., Vol. 8, p. 055711 (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/40c1929d56323d1f53e49ae6ec1ca273.jpg)
Fig. 1: Square artificial spin ice array with dipolar interactions: a is the lattice spacing and d is the island length; in this model, the width is considered negligible. There is a height separation h between the position of islands which are oriented in perpendicular directions; this means that dark-gray shaded islands are located at height z = h while light-gray shaded islands are located at height z = 0.

![](https://assets.underline.io/uploads/markdown_image/1/image/403dd6ed545da510cecae93a68da98cd.jpg)
Fig. 2: The point where four islands concur is called a vertex. There are 16 possible configurations for each vertex of an squared ASI, according to the orientation of its spins. Furthermore, these can be classified into four different topological groups. Type I and II satisfy the so called ice rule (two spins point inwards the vertex and two point outwards), while Type III and IV do not. Type II and III vertices have a non-trivial net magnetization.

 Disorder by Design in an Artificial Spin Ice with Dipolar Interactions: An Energetic Analysis

There have been reports for different systems that there is an asymmetry in the saturation magnetisation in exchange bias systems [1, 2]. This report contains the first comprehensive study of these effects in the CoFe/IrMn system which is used in the read head of almost all HDDs [3]. The magnetic behaviour of exchange bias systems has two quasi-independent contributions [3]. The first contribution derives from the ferromagnetic (F) layer which in device applications is in the range 2 to 5 nm. This thickness produces the maximum loop shift when the structure is field cooled. However, it has been shown that there can be a further significant contribution to the magnetisation from spins lying at the interface which can have their origin either from the F layer or the antiferromagnetic (AF) layer. It has been shown that ordering of interface spins increases the loop shift by increasing the coupling between the F and AF layers and that doping the interface in the CoFe/IrMn system with atomic layers of the high-moment Mn can increase the spin ordering significantly [3, 4]. In this work the asymmetry in the saturation moment (ms) and its temperature (T) dependence to &lt;4.2 K have been measured for sputtered polycrystalline multilayer Si/Ta (2)/Ru (2)/IrMn (10)/Mn (dMn)/CoFe (5)/Ru (5) (thicknesses in nm) thin films where 0≤ dMn ≤0.2 nm in a LakeShore 8600 Series VSM. The York Model of Exchange Bias was used to separate and interpret the effect of the F/AF interface [3]. Low-temperature measurements were undertaken to ensure that the bulk of the AF grains remained magnetically frozen. Thus there was no reorientation of the magnetic moments in the bulk of the AF grains in subsequent measurements. The magnetisation of an exchange bias system undergoes an athermal training effect whereby the first and second magnetisation loops exhibit different loop shifts after the initial field cooling, even when the temperature of measurements is such that the moment of the bulk of the AF grain does not vary. This effect on the coercivity, which is defined as half loop width, has been shown to originate in the interfacial spins [5]. Figure 1 shows the measured hysteresis loop, the structure of which is shown in the insert, at 5.5 and 300 K. For the system measured we established by separate measurements (not shown) that the loop shift for the system shown was thermally stable at T = 300 K by holding the magnetisation at reverse saturation for a period of 30 mins after which time the hysteresis loop show at 300 K reproduced. In figure 1 the difference between the loops measured was due to changes in the orientation and ordering of interfacial spin clusters. The increase in the measured loop shift comes from ordering of spins at the interface. Furthermore, the first loop training effect is very small at T = 300 K but becomes significant at T = 5.5 K. This shows that the training effect is caused by interfacial spins, as is the coercivity. There is an asymmetry in the magnetisation. This data was obtained for the same sample without it being removed from the magnetometer. A significant increase in ms occurs at 5.5 K, which is greater at negative saturation than is the case in a positive field. We designate this difference in ms in positive (ms+) and negative (ms–) fields as Δms = |ms+|–|ms–|. (1) Figure 2 shows the temperature dependence of Δms for samples where there is a variation in the thickness of the Mn interface doping layer (dMn). It has been found that the optimum loop shift for this structure is affected by dMn and has been found to be a layer of subatomic thickness of the order of 1/6 of an atom. The use of layers of an atomic thickness or greater were found to result a reduction in Δms [4]. Figure 2 shows Δms for the first (i) and second (ii) loops measured immediately after the field annealing process. There is a change in the interfacial spin order which gives rise to the interface training effect but also effects the change in Δms. For all films figure 2 shows that the increase in Δms is small at T &gt; 100 K. We attribute this to the interface spins not being ferromagnetically ordered and hence not responding to the modest fields used to measure the loops. Below 100 K the change in Δms with T becomes more rapid as it appears these spins become F ordered, probably in a spin cluster structure similar to a spin glass [6]. It appears that at &lt;6 K Δms goes through a peak for all samples which is consistent with a spin cluster structure. It would be expected that dMn would not be optimum for a layer of subatomic thickness. It is most likely that the Mn layer is making a compositional adjustment at the interface. In studies of a similar system by Baltz et al. using 3-D atom probe tomography on an exchange bias system containing Co and Mn significant diffusion was found to occur [7]. CoMn alloys have complex properties existing in both F, AF and superparamagnetic phases dependent on composition and temperature but no high resolution phase diagram has been determined [8]. In the production of an exchange bias system the field annealing requires up to 600 K which promotes interfacial diffusion [3, 7]. We conclude that the role of the Mn interface layer is to replace Mn deficiencies in the bulk of the AF.

**References:**

[1] U. Nowak et al., Phys. Rev. B, Vol. 66(1), p. 014430 (2002) [2] J. Keller et al., Phys. Rev. B, Vol. 66(1), p. 014431 (2002) [3] K. O'Grady et al., J. Magn. Magn. Mater., Vol. 322(8), p. 883-899 (2010) [4] R. Carpenter et al., IEEE Trans. Magn., Vol. 48(11), p. 4351–4354 (2014) [5] R. Carpenter et al., J. Appl. Phys., Vol. 115(17), p. 17D715 (2014) [6] J. L. Tholence and R. Tournier, Phys. B+C, Vol. 86-88, p. 873-874 (1977) [7] F. Letellier, et al., J. Appl. Phys., Vol. 116, p. 203906 (2014) [8] A. Z. Men’shikov, et al., Sov. Phys. JETP, Vol. 62, p. 734 (1985)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c4880f4f4f141c9f2e76b5ec7dfed9c9.jpg)
Figure 1: The first (i) and second (ii) hysteresis loops at 5.5 and 300 K for a film with structure shown inset.

![](https://assets.underline.io/uploads/markdown_image/1/image/b484480c5ac00e6406cf86b341a6f8a8.jpg)
Figure 2: The temperature dependence of the magnetisation offset (Δms) measured at ± 5 kOe shown for the first (i) and second (ii) magnetisation reversal for the thin films shown inset where 0≤ dMn ≤0.2 nm.

 Magnetisation Asymmetry in Exchange Bias Systems

Here we present an X-ray, magnetometry, and electron microscopy study of novel 40 nm FeCr spinel oxide nanoparticles (NPs) containing ~4 nm FeO clusters occupying up to 20% of the volume. The unique structure provides a large effective interface area between the antiferromagnetic (AFM) FeO clusters and the ferrimagnetic (FiM) spinel structure, leading to one of the largest exchange bias (EB) observed in FeO-based NP systems. Nanoparticles composed of Wüstite FeO and Magnetite Fe3O4 have been studied extensively and exhibit a wide range of magnetic properties [1-3]. Both oxides share the same fcc oxygen lattice and differ only in the occupancy and valence of the Fe cations in the tetrahedral (A) and octahedral (B) interstitial sites. Wüstite has a rock salt (RS) structure with Fe2+ occupying B-sites in the fcc O-lattice. In bulk, FeO is not stoichiometric and thermodynamically stable under ambient conditions, which results in a defect FexO RS structure. The Fe2+ ions in the RS structure can diffuse and oxidize into Fe3+ at the surface, forming the inverse spinel structure of Magnetite, where Fe3+ occupies the A-lattice while the B-lattice is equally shared by the Fe2+ and Fe3+ cations. At the nanoscale, the transformation of Wüstite into Magnetite leads to FexO/Fe3O4 core/shell (CS) NPs where exchange coupling between the AFM core and the FiM shell leads to the emergence of EB [1-6], which manifests as a horizontal shift of the hysteresis and coercivity increase. Here, NPs with an average diameter of 40 nm are generated using an aerosol technique based on spark ablation [7-9] where the material is evaporated from stainless steel grade 430 electrodes containing 97.4% FeCr with a 4:1 ratio of Fe:Cr [9]. The evaporated material is transported where adiabatic expansion and mixing with the carrier gas cool down the vapor, which condensates into sub 10 nm primary particles that form larger agglomerates as they collide. The agglomerates are transported through a furnace where they are compacted at 1200 oC before being size-selected and deposited onto a substrate using an electric field. The structural characterization was performed by high-resolution electron microscopy (HRTEM) and the compositional analysis was carried out with energy dispersive X-ray spectroscopy (EDXS) in scanning transmission electron microscopy (STEM) mode, see Figure 1 (a and b), along with X-ray photoelectron spectroscopy (XPS) performed at the Species beamline, MAX IV [10]. The NPs appear to have a single crystalline phase with a spinel oxide structure, and homogeneously distributed Fe and Cr within the NP with the expected Fe:Cr ratio of 4:1. However, X-ray diffraction (XRD) acquired at the 11-ID-B beamline of the Advanced Photon Source [11] reveals the co-existence of up to 20% RS crystallites with a size (~4 nm) about one order of magnitude smaller than the spinel phase (~40 nm). The significant volume fraction and the small size of the RS phase rule out a CS structure and point to FexO RS clusters embedded in the spinel structure as schematically illustrated in Fig. 1 c). In stainless steel, Cr prevents the corrosion of metallic Fe by forming an oxide that blocks further diffusion and oxidation of Fe at the surface. We propose that Cr plays a similar role here, but now preventing the full transformation from FexO → Fe3O4, both in the primary particles and during compaction. X-ray magnetic circular dichroism (XMCD) data, acquired at the X-treme beamline of the Swiss Light Source [12] shows the FM interaction between the Fe2+/Fe3+ and Cr3+ ions in the B-lattice, and an AFM coupling to the Fe3+ ions in the A-lattice. Differences observed in XMCD spectra recorded at 2 K and 300 K at the Fe L3-edge indicate an AFM to paramagnetic transition, further supporting the presence of FexO in the NPs. Element and site specific m(H) curves were recorded and the differences between Fe and Cr will be discussed, along with a more precise determination of the individual ions’ magnetic contribution by means of multiplet simulations. Temperature-dependent magnetization measurements also indicate the presence of FexO, where a rapid increase in magnetization is observed around the Neél temperature of FeO (TN ~ 200 K - Fig.2 left-panel). The magnetic moment as a function of applied magnetic field at a temperature of 2 K after cooling in a field of 1 T is shown in the right-panel of Figure 2. As can be seen, the hysteresis is significantly shifted with respect to the field axis. The observed shift of µ0H = 460 mT is among the highest values ever reported for Fe-oxide NPs, and significantly larger than the typical values for FexO/Fe3O4 NPs [13] (a record high of 860 mT was reported by Lottini et al [14] after Co doping). Furthermore, a 38% increase in the coercivity at 2 K (580 mT) after cooling in a magnetic field is also observed. That the EB is the result of interactions with the AFM FexO clusters is also corroborated by the temperature dependence of the HEB which vanishes above the FeO TN, immediately followed by a rapid coercivity decrease. The large EB is attributed to the unique structure in which the embedded AFM clusters have an increased effective AFM-FiM interface area compared to AFM/FiM core/shell structures.

**References:**

[1] Sun, Xiaolian, et al. "Tuning exchange bias in core/shell FeO/Fe3O4 nanoparticles." Nano letters 12.1 (2012): 246-251. [2] Estrader, Marta, et al. "Origin of the large dispersion of magnetic properties in nanostructured oxides: Fe x O/Fe 3 O 4 nanoparticles as a case study." Nanoscale 7.7 (2015): 3002-3015. [3] Wetterskog, Erik, et al. "Anomalous magnetic properties of nanoparticles arising from defect structures: topotaxial oxidation of Fe1–x O| Fe3− δO4 core| shell nanocubes to single-phase particles." ACS nano 7.8 (2013): 7132-7144. [4] Iglesias, Oscar, Amilcar Labarta, and Xavier Batlle. "Exchange bias phenomenology and models of core/shell nanoparticles." Journal of nanoscience and nanotechnology 8.6 (2008): 2761-2780. [5] Meiklejohn, William H., and Charles P. Bean. "New magnetic anisotropy." Physical review 102.5 (1956): 1413. [6] Nogués, Josep, et al. "Exchange bias in nanostructures." Physics reports 422.3 (2005): 65-117. [7] R. Hallberg, L. Ludvigsson, C. Preger, B. Meuller, K. Dick, and M. Messing, "Hydrogen-assisted spark discharge generated metal nanoparticles to prevent oxide formation," Aerosol Science and Technology, vol. 52, no. 3, pp. 347-358, 2018. [8] B.O. Meuller, M.E. Messing, D.L.J. Engberg, et al. ”Review of spark discharge generators for production of nanoparticle aerosols” Aerosol Sci. Technol., 46, 1256, 2012. [9] C. Preger et al., "Controlled Oxidation and Self-Passivation of Bimetallic Magnetic FeCr and FeMn Aerosol Nanoparticles", The Journal of Physical Chemistry C, vol. 123, no. 26, pp. 16083-16090, 2019. [10] S. Urpelainen, C. Såthe, W. Grizolli, M. Agåker, A. R. Head, M. Andersson, S.-W. Huang, B. N. Jensen, E. Wallén, H. Tarawneh, et al., "The species beamline at the max iv laboratory: a facility for soft x-ray rixs and apxps", Journal of synchrotron radiation, vol. 24, no. 1, pp. 344-353, 2017. [11] U. Rütt, M. Beno, J. Strempfer, G. Jennings, C. Kurtz, and P. Montano, "Diffractometer for high energy x-rays at the APS", Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 467, pp. 1026-1029, 2001. [12] C. Piamonteze, U. Flechsig, S. Rusponi, J. Dreiser, J. Heidler, M. Schmidt, R. Wetter, M. Calvi, T. Schmidt, H. Pruchova, et al., “X-treme beamline at sls: X-ray magnetic circular and linear dichroism at high field and low temperature", Journal of synchrotron radiation, vol. 19, no. 5, pp. 661-674, 2012. [13] Phan, Manh-Huong, et al. "Exchange bias effects in iron oxide-based nanoparticle systems." Nanomaterials 6.11 (2016): 221. [14] Lottini, E., et al. "Strongly exchange coupled core| shell nanoparticles with high magnetic anisotropy: a strategy toward rare-earth-free permanent magnets." Chemistry of Materials 28.12 (2016): 4214-4222.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/d2fd8d2c49ec21852eb227573753590a.jpg)
Fig 1. a) left: HRTEM image of a NP; right: FFT indicating spinel; b) EDXS showing Fe, Cr and O distribution; c) Schematic model of a spherical spinel NP containing cubic FeO clusters (brown).

![](https://assets.underline.io/uploads/markdown_image/1/image/04541974969216210d0c73ac8ae0bc2a.jpg)
Fig 2. Left: temperature susceptibility in a 10 mT field in ZFW and FCC cases; right: 1 T FC m(H) showing the horizontal hysteresis shift.

 Large Exchange Bias in FeCr Spinel Oxide Nanoparticles with Embedded FexO Clusters

The research on hybrid systems combining dissimilar materials with artificial interfaces is among the major challenges of the nanotechnology during the last decades. Different functionalities may be engineered by combining dissimilar materials. To this regard, bilayers composed of a ferromagnetic (FM) layer interfaced with a transition-metal-oxide (TMO) layer can be considered as model hybrid systems V2O3 is an archetypical TMO system exhibiting a first-order metal insulator transition (MIT) when the temperature is decreased below TMIT= 170 K [1], from a paramagnetic metallic state to an antiferromagnetic (AFM) insulating phase, accompanied simultaneously by a structural phase transition [2]. We are provided with an additional degree of freedom in order to control the FM properties at the FM/TMO interface. Below TMIT, several interfacial exchange-coupling effects have already been found in similar FM/V2O3 bilayers [3], ranging from moderate to negligible exchange bias fields, enhanced giant coercive fields and unique dynamical effects. To understand these interesting effects it is important to investigate in detail the temperature dependence of the magnetic anisotropies of this system. Here we present a detailed angular and temperature dependent angle-resolved vector magnetometry study of the model FM/TMO system mentioned across the MIT. This provides fundamental insights on how to tailor and control the interfacial exchange effects. More specifically, we investigated the magnetic anisotropy landscape of Co/V2O3 bilayers, where the Co layer has a well-defined uniaxial magnetic anisotropy at room temperature (see Fig.1a1 y Fig.1b1). We have studied the effects using different magnetic anisotropy configurations by setting a magnetic orientation of the FM layer across the MIT transition. Different field cooling (FC) procedures strongly affect the magnetic configuration of the hybrid bilayer. Just after the FC, the low temperature (50 K) loops display strong interfacial exchange coupling effects (see Fig.1a2 y Fig.1b2), i.e., enhanced coercive field, clear exchange bias, and asymmetric magnetization reversal [4]. In general, depending on the FC procedure, different angle-dependent and temperature evolutions are found, which are ascribed to the modifications of the anisotropy configuration. Our findings demonstrate that we have an effective control of the interfacial exchange phenomena in the FM/TMO heterostructures, especially across the MIT, which enables the development of novel magnetic devices with specific functionalities. This research was partially supported by the Spanish Ministry of Economy and Competitiveness (MINECO) (grant numbers FIS2016-78591-C3-1-R, RTI2018-097895-B-C42, and SEV-2016-0686) and by the Regional Government of Madrid (through project NANOMAGCOST-CM). JMD ackowledges support from MINECO through FPI program (BES-2017-080617). Work at UCSD supported by the Office of Basic Energy Science, U.S. Department of Energy, BES-DMS funded by the Department of Energy’s Office of Basic Energy Science, DMR under grant DE FG02 87ER-45332. We thank C. Urban, and J. Trastoy for initial work and discussions on this problem.

**References:**

[1] Imada, M., Fujimori, A. and Tokura, Y. Rev. Mod. Phys. 70, 1039-1263 (1998). [2]De La Venta, J., Wang, S., Ramirez, J. G., and Schuller, I. K. Applied Physics Letters, 102(12), 122404 (2013). [3] E. Jiménez, J. Camarero, J. Sort, J. Nogués, A. Hoffmann, N. Mikuszeit, J. M. Garcia-Martín, B. Dieny, and R. Miranda. Phys. Rev. B. 80,014415 (2009). [4] J. Camarero, J. Sort, A. Hoffmann, J. M. Garciaa-Martín, B. Dieny,R. Miranda, and J. Nogués. Phys. Rev. Lett. 95, 057204 (2005)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/84121644807d7ea9c1108a6365c3b517.jpg)
Representative v-MOKE loops of a V2O3 (100 nm)/Co (15 nm) heterostructure above (a1,b1) and below (a2,b2) the MIT transition using different field-cooling (FC) conditions: a) Negative-FC along the easy axis and b) Positive-FC around 50° off the easy axis. Note that after FC, the hysteresis field-shift (indicated with arrows) is opposite to the FC direction, while the reversal pathways are very different [notoriously in M⊥(H)].

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