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Antiferromagnets (AFMs) have regained strong attention from the magnetism community especially with the advent of antiferromagnetic spintronics [1]. The key operational element of spintronic devices is represented by information carriers, such as domain walls (DWs) and skyrmions. The simplest AFM DW separates two regions with the opposite orientation of the Neel order parameter. Although highly relevant, the experimental studies of AFM DWs (visualization, dynamics, mechanics) are challenging because of strict requirements on measurement techniques to access their properties.&lt;br/&gt;&lt;br/&gt;Here, we overcome these limitations and conduct detailed, quantitative studies of the mechanics and the nanoscale properties of individual, antiferromagnetic DWs in a single crystal Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; – a room-temperature, magnetoelectric, insulating AFM [2]. Our results reveal a remarkably pristine DW behaviour, which is governed by DW energy minimization and boundary conditions, but largely unaffected by pinning or disorder – a “textbook example” of antiferromagnetic DW physics.&lt;br/&gt;&lt;br/&gt;In our experiment, the crystal’s (0001) surface is patterned by a grid of mesas with mean thickness and width t=166 nm and w=2400 nm, respectively. The DW is nucleated by means of magnetoelectric field cooling by inverting the electric bias field over opposite halves of the sample. The DW may also be dragged through the mesa pattern by a focused laser spot. The magnetic texture is imaged using Nitrogen Vacancy (NV) magnetometry [3]. We find that the DW mimics an elastic surface with specific mechanical properties, determined by the interaction with the topographic features of the sample where the DW is (i) deflected from the straight plane crossing the mesa; (ii) bent around mesa corners.&lt;br/&gt;&lt;br/&gt;To address the DW behaviour theoretically, we perform large-scale spin-lattice simulations with GPU speed-up [4]. The analytical Ansatz is developed based on the numerically-obtained, three-dimensional DW profile. All main features of the DW behaviour can be determined taking into account the nearest-neighbour exchange and uniaxial anisotropy for a general model of a bipartite AFM. Crossing the mesa, the DW experiences an S-shaped distortion observed at the mesa surface, which is the result of exchange-driven boundary conditions at the side faces of the mesa, see Fig. 1. Below the top surface, the DW possesses a twist to match this distortion with the straight plane far below the sample’s surface. We find that the DW surface is deflected from the plane over a characteristic depth of 0.34w. Comparison of the equilibrium DW direction in bulk and at the mesa’s top surface allows us to derive an effective Snell’s law for the DW behaviour at the sample’s surface with the given incidence and refraction angles θ&lt;sub&gt;1&lt;/sub&gt; and θ&lt;sub&gt;2&lt;/sub&gt;. The effective refraction coefficient is determined by the analytical energy minimization and reads&lt;br/&gt;&lt;br/&gt;n&lt;sub&gt;eff&lt;/sub&gt; = 1 + 3.1t/w + O(θ&lt;sub&gt;1&lt;/sub&gt;).&lt;br/&gt;&lt;br/&gt;Controlled manipulation via laser not only enables DW dragging through the mesa, but also pinning at mesa corners. The shape of the DW surface is governed by its intrinsic elasticity. In terms of mechanics of an elastic ribbon, the corresponding tension coefficient is determined by the temperature-dependent exchange stiffness and anisotropy coefficient. This allows for curved DW states in which it is pinned at the opposite mesa sides. Using mesas as bistable pinning sites, we propose a potential DW-based AFM memory concept. Here, the memory state “0” or “1” is associated with the direction of the Neel order parameter at the mesa surface. We have realized such pinning sites experimentally and have shown manipulation of the state via laser dragging.&lt;br/&gt;&lt;br/&gt;In summary, we realize engineered DW potentials and control over DW trajectories by topographic structuring and manipulation of the DW position by means. The physics of AFM DWs in a single crystal with non-trivial surface topography is described theoretically by means of spin-lattice simulations and analytical model. A novel nanoscale AFM memory architecture is suggested.

**References:**

[1] T. Jungwirth, J. Sinova, A. Manchon et al, Nat. Phys. Vol. 14, p. 200 (2018); T. Jungwirth, X. Marti, P. Wadley et al, Nat. Nano. Vol. 11, p. 231 (2016); V. Baltz, A. Manchon, M. Tsoi et al, Rev. Mod. Phys. Vol. 90, p. 015005 (2018); O. V. Pylypovskyi, D. Y. Kononenko, K. V. Yershov et al, Nano Lett. Vol. 20, p. 8157 (2020)&lt;br/&gt;[2] N. Hedrich, K. Wagner, O. V. Pylypovskyi et al, arXiv:2009.08086&lt;br/&gt;[3] L. Rondin, J.-P. Tetienne, T. Hingant et al, Rep. Prog. Phys. Vol. 77, p. 056503 (2014); N. Hedrich, D. Rohner, M. Batzer et al, Phys. Rev. Applied, Vol. 14, p. 064007 (2020)&lt;br/&gt;[4] SLaSi simulation package, http://slasi.knu.ua; O. V. Pylypovskyi, D. D. Sheka, Book of Abstracts, EUROPT Workshop, p. 11 (2013)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/53f177df2920dadc9b01b189e65f13fb.jpg)
Fig. 1. The deflection of the DW from the straight path and distortion at the mesa surface shown by the stray magnetic field image taken using Nitrogen Vacancy microscopy. Inset shows the DW trajectory in spin-lattice simulations.

![](https://assets.underline.io/uploads/markdown_image/1/image/468d98b21c2bc9cd432573e88f7fb894.jpg)
Fig. 2. Concept of DW-based AFM memory with bits defined by the direction of the Neel order parameter at the mesa top surface (red or blue color, respectively).

INTERMAG 2021

 Domain walls in antiferromagnetic samples with non-trivial surface topography

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AAMAS (International Conference on Autonomous Agents and Multiagent Systems) is the largest and most influential conference in the area of agents and multiagent systems. The aim of the conference is to bring together researchers and practitioners in all areas of agent technology and to provide a single, high-profile, internationally renowned forum for research in the theory and practice of autonomous agents and multiagent systems. AAMAS is the flagship conference of the non-profit International Foundation for Autonomous Agents and Multiagent Systems (IFAAMAS).

Antiferromagnets (AFMs) have regained strong attention from the magnetism community especially with the advent of antiferromagnetic spintronics [1]. The key operational element of spintronic devices is represented by information carriers, such as domain walls (DWs) and skyrmions. The simplest AFM DW separates two regions with the opposite orientation of the Neel order parameter. Although highly relevant, the experimental studies of AFM DWs (visualization, dynamics, mechanics) are challenging because of strict requirements on measurement techniques to access their properties. Here, we overcome these limitations and conduct detailed, quantitative studies of the mechanics and the nanoscale properties of individual, antiferromagnetic DWs in a single crystal Cr2O3 – a room-temperature, magnetoelectric, insulating AFM [2]. Our results reveal a remarkably pristine DW behaviour, which is governed by DW energy minimization and boundary conditions, but largely unaffected by pinning or disorder – a “textbook example” of antiferromagnetic DW physics. In our experiment, the crystal’s (0001) surface is patterned by a grid of mesas with mean thickness and width t=166 nm and w=2400 nm, respectively. The DW is nucleated by means of magnetoelectric field cooling by inverting the electric bias field over opposite halves of the sample. The DW may also be dragged through the mesa pattern by a focused laser spot. The magnetic texture is imaged using Nitrogen Vacancy (NV) magnetometry [3]. We find that the DW mimics an elastic surface with specific mechanical properties, determined by the interaction with the topographic features of the sample where the DW is (i) deflected from the straight plane crossing the mesa; (ii) bent around mesa corners. To address the DW behaviour theoretically, we perform large-scale spin-lattice simulations with GPU speed-up [4]. The analytical Ansatz is developed based on the numerically-obtained, three-dimensional DW profile. All main features of the DW behaviour can be determined taking into account the nearest-neighbour exchange and uniaxial anisotropy for a general model of a bipartite AFM. Crossing the mesa, the DW experiences an S-shaped distortion observed at the mesa surface, which is the result of exchange-driven boundary conditions at the side faces of the mesa, see Fig. 1. Below the top surface, the DW possesses a twist to match this distortion with the straight plane far below the sample’s surface. We find that the DW surface is deflected from the plane over a characteristic depth of 0.34w. Comparison of the equilibrium DW direction in bulk and at the mesa’s top surface allows us to derive an effective Snell’s law for the DW behaviour at the sample’s surface with the given incidence and refraction angles θ1 and θ2. The effective refraction coefficient is determined by the analytical energy minimization and reads neff = 1 + 3.1t/w + O(θ1). Controlled manipulation via laser not only enables DW dragging through the mesa, but also pinning at mesa corners. The shape of the DW surface is governed by its intrinsic elasticity. In terms of mechanics of an elastic ribbon, the corresponding tension coefficient is determined by the temperature-dependent exchange stiffness and anisotropy coefficient. This allows for curved DW states in which it is pinned at the opposite mesa sides. Using mesas as bistable pinning sites, we propose a potential DW-based AFM memory concept. Here, the memory state “0” or “1” is associated with the direction of the Neel order parameter at the mesa surface. We have realized such pinning sites experimentally and have shown manipulation of the state via laser dragging. In summary, we realize engineered DW potentials and control over DW trajectories by topographic structuring and manipulation of the DW position by means. The physics of AFM DWs in a single crystal with non-trivial surface topography is described theoretically by means of spin-lattice simulations and analytical model. A novel nanoscale AFM memory architecture is suggested.

**References:**

[1] T. Jungwirth, J. Sinova, A. Manchon et al, Nat. Phys. Vol. 14, p. 200 (2018); T. Jungwirth, X. Marti, P. Wadley et al, Nat. Nano. Vol. 11, p. 231 (2016); V. Baltz, A. Manchon, M. Tsoi et al, Rev. Mod. Phys. Vol. 90, p. 015005 (2018); O. V. Pylypovskyi, D. Y. Kononenko, K. V. Yershov et al, Nano Lett. Vol. 20, p. 8157 (2020) [2] N. Hedrich, K. Wagner, O. V. Pylypovskyi et al, arXiv:2009.08086 [3] L. Rondin, J.-P. Tetienne, T. Hingant et al, Rep. Prog. Phys. Vol. 77, p. 056503 (2014); N. Hedrich, D. Rohner, M. Batzer et al, Phys. Rev. Applied, Vol. 14, p. 064007 (2020) [4] SLaSi simulation package, http://slasi.knu.ua; O. V. Pylypovskyi, D. D. Sheka, Book of Abstracts, EUROPT Workshop, p. 11 (2013)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/53f177df2920dadc9b01b189e65f13fb.jpg)
Fig. 1. The deflection of the DW from the straight path and distortion at the mesa surface shown by the stray magnetic field image taken using Nitrogen Vacancy microscopy. Inset shows the DW trajectory in spin-lattice simulations.

![](https://assets.underline.io/uploads/markdown_image/1/image/468d98b21c2bc9cd432573e88f7fb894.jpg)
Fig. 2. Concept of DW-based AFM memory with bits defined by the direction of the Neel order parameter at the mesa top surface (red or blue color, respectively).

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Spin-torque nano-oscillators (STNO) are nanoscale spintronic devices in which applied currents drive self-sustained magnetization oscillations. Heusler compounds are promising materials for spintronics as they exhibit low Gilbert damping for magnetization dynamics and large spin polarizations due to their (almost) half-metallic nature [1,2]. As such, they are ideal for devices such as spin-torque nano-oscillators, where critical currents can be reduced by the lower damping values and output signals enhanced by stronger magnetoresistances. This idea has been explored in vortex-based systems involving Co2FexMn1-xSi where large output powers and large quality factors have been reported [3]. A unique feature of nanocontact vortex oscillators is the onset of periodic core reversal above a critical current [4-6]. This reversal occurs concurrently with the steady-state gyration around the nanocontact, giving rise to various behaviors we have previously studied: chaotic core reversal [4,5], phase-locking or modulation phenomena [6] in permalloy-based spin valves. These dynamics are driven primarily by currents flowing in the film plane (CIP), resulting in spin-transfer torques of the Zhang-Li form, rather than the Slonczewski torques associated with currents perpendicular to the film plane in nanopillar geometries. Here, we will discuss how the dynamics changes when all Heusler spin valves are used instead. We fabricated 20-nm nanocontacts on Co2MnGe/Au/Co2MnGe films grown by molecular beam epitaxy, with a nano-indentation technique [7]. We investigated the vortex dynamics in the time and frequency domain at room temperature. An example of the measured current-dependence of the power spectral density is given in Fig. 1. We observe a rich harmonic content with oscillations that persist down to 4.5 mA, which is lower than previous cases [4-6]. Moreover, the power spectra comprise broad bands that are reminiscent of a chirped signal. Other frequency domains measurements show single frequency signal, broad spectral lines or extinction of the signal over current intervals. Time domain measurements enable us to observe other behavior, such as mode hopping, which can be seen on Fig. 2 for different currents. Other measurements have exhibit other behaviors, such as mode coexistence, intermittent signal or signal fluctuations. We attribute the rich dynamics to the coupled motion of vortices in the free and reference layers. This hypothesis is supported by some simulations.

**References:**

[1] T. Graf et al., IEEE Trans. on Magn., 47, 367-373 (2011) [2] C. Guillemard et al., Phys. Rev. Appl., 11, 064009 (2019) [3] T. Yamamoto et al., Phys. Rev. B 94, 094419 (2016) [4] S. Petit-Watelot et al., Nat. Phys., 8, 682-687 (2012) [5] T. Devolder et al., Phys. Rev. Lett. 123, 147701 (2019) [6] J. Létang et al., Phys. Rev. B 100, 144414 (2019) [7] J. Létang et al., arXiv:1906.08492 (2019)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ddb04ce76b79671646bb2ba3d9c34278.jpg)
Figure 1: Power spectral density of vortex oscillations with applied dc current. The frequency bands are suggestive of a chirped signal.

![](https://assets.underline.io/uploads/markdown_image/1/image/07722c5e6c04a690cdd33ac59038bde8.jpg)
Figure 2: (a) Power spectral density computed from time-domain measurements. (b-d) Time-domain measurements for different dc current. Mode hopping between two frequencies is visible.

 Heusler-compound nanocontact vortex oscillators

Heusler magnetic compounds offer a wide variety of electronic properties very promising for spintronics and magnonics. Some of them exhibit a spin gap in their band structure at the Fermi energy. This peculiar electronic property was denominated as Half-Metal Magnetic (HMM) behavior [1]. This leads to two very interesting properties for spintronics, i.e. fully polarized current together with ultra-low magnetic damping, two key-points for spin-transfer-torque based devices. The full polarization was experimentally reported in Co2MnSi [2,3], and we measured a magnetic damping in the 10-4 range on this compounds. Recently, we reported magnetic damping in the same range for other Heusler compounds [4]. In this talk, we show that a key experimental point to get ultra-low magnetic damping (in the 10-4 range) is to control the compound stoichiometry as best as possible. Single-crystalline Co2MnZ thin films with Z=Al, Si, Ga, Ge, Sn and Sb were grown by using Molecular Beam Epitaxy [4]. We developed a process to control the Co, Mn and Z fluxes with accuracy lower than 1% [5]. The electronic properties of these Co2MnZ compounds were examined by using in situ techniques like electron diffraction during the growth, and spin resolved photoemission (SR-PES performed at SOLEIL synchrotron), and ex situ techniques like x-ray diffraction, transmission electron microscopy and ferromagnetic resonance. Electron diffraction during the growth (RHEED) allowed us to verify the stoichiometry after fluxes calibration [5]. This consists in recording diffracted intensity during deposition : if the growth is 2-dimensional, intensity oscillations are observed (called RHEED oscillations) due to interferences between electron scattered by the nth grown atomic plane, and n+1th plane that is growing (fig.1). We observed this behavior for the Co2MnZ series grown at 250C on a V or Cr buffer layers. Interestingly, a bilayer by bi layer growth mode was observed. Bilayer growth mode is well-known for MBE-grown semiconductors and is associated to the growth of basic blocks of the zinc blend or diamond structures. Similarly, the basic block for Co2MnZ Heusler compounds is a cube with Co atoms at its 8 corners, and Mn or Z in the center of the cube. These Heusler compounds thus want to develop these basic blocks during the growth, leading to bilayer growth. This is a clear experimental proof of chemical ordering during the growth [5]. Finally, the excellent agreement between the delay to complete 2 atomic planes measured by RHEED and the time deduced from fluxes calibration allows us to confirm stoichiometry accuracy around 1% for each element. The spin (half-) gaps obtained by SR-PES on Co2Mn-Si, -Ge and –Sn were found to be in very good agreement with ab initio calculations. The lowest magnetic damping values were obtained for these former HMM compounds, and are greater for Co2Mn – Al and Ga where no spin gap was observed (Table I). The control of the stoichiometry also offered us a robust experimental demonstration of the correlation between the spin polarization and the magnetic damping in Co2MnSixAl1-x epitaxial thin films [6]. Finally, we show that single crystalline thin films are not mandatory, since we observed spin gap and magnetic damping values as low as 10-3 in polycrystalline Co2Mn-Si and –Ge grown on glass [7]. This highlights the key role of the stoichiometry control for getting low damping, more importantly than structural defects like grain boundaries for instance.

**References:**

[1] New Class of Materials: Half-Metallic Ferromagnets, R. A. de Groot et al, Phys. Rev. Lett. 50, 2024 (1983) [2] Direct observation of half-metallicity in the Heusler compound Co2MnSi, M. Jourdan et al., Nat. Commun. 5, 3974 (2014) [3] Direct evidence for minority spin gap in the Co2MnSi Heusler compound, S. Andrieu et al, Phys. Rev. B 93, 094417 (2016) [4] Ultra-low magnetic damping in Co2Mn-based Heusler compounds promising materials for spintronic, C. Guillemard et al, Phys. Rev. Appl.. 11, 064009 (2019) [5] Issues in growing Heusler compounds in thin films for spintronic applications, C. Guillemard et al, Invited Tutorial, J.Appl. Phys., 128, 241102 (2020) [6] Engineering Co2MnAlxSi1-x Heusler compounds as a model system to correlate spin polarization, intrinsic Gilbert damping and ultrafast demagnetization, C. Guillemard et al, Adv. Mat. 1908357 (2020) [7] Polycrystalline Co2Mn-based Heusler thin films with high spin polarization and low magnetic damping, C. Guillemard et al, Appl. Phys. Lett. 115, 172401 (2019)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/18b04372cf1e5ed6b2c6e20c3c5b575f.jpg)
Fig.1 : Electron Diffraction Intensity oscilaltions during the growth of Co2MnGa on a V(001) buffer layer. The oscilaltions correspond to a bilayer by bilayer growth mode [5].

![](https://assets.underline.io/uploads/markdown_image/1/image/b0ed917652ad898d0b349c40287d4b7e.jpg)
Table I : reviiew of the structural and electronic properties observed on the Co2MnZ series. The crystalline structures were determined using transmission electron microscopy, spin polarization and gap using spin-resolved photoemission, and magnetic properties using feromagnetic resonance (FMR) [5].

 The Crucial Role of Stoichiometry to get Ultra-low Magnetic Damping in Heusler Compounds

The control of magnetization by electric current plays an important role in the applications of magnetic recording, nonvolatile memories, etc. As a key way to realize the manipulation of magnetization efficiently, the application of spin transfer torque (STT) and spin-orbit torques (SOT) requires materials with high spin polarization in electron transport.1-3 Besides, magnetic materials with low magnetization could reduce the critical switching current effectively due to the switching current is proportional to the saturation magnetization.4 Antiperovskite type Mn4N films have attracted a lot of attention due to the perpendicular magnetic anisotropy (PMA) and low saturation magnetization.5,6 However, the details about electronic transport properties in Mn4N films are not yet clear. Thus, the electronic transport properties of the Mn4N films with different thicknesses on MgO substrates have been investigated systematically. Fig. 1(a) presents the ρxx(T)/ρxx(300 K) of Mn4N films with different thicknesses on MgO(001) substrates. It is obvious that the resistivity continuously increases as the temperature increases in most films, showing a metallic conductance mechanism for the Mn4N films. However, there is an abnormal trend in the 4.7-nm-thick Mn4N films whose resistivity decreases as the temperature increases. In order to evaluate the purity and integrity of the Mn4N films, the residual resistivity ρxx0 at 0 K originated from impurity scattering is extracted from the ρxx-T curves.7 Fig. 1(b) shows the ρxx-T and fitting curve of the 78.8-nm-thick Mn4N film. The ρxx0 is obtained by fitting the ρxx-T at low temperatures using the expression as ρxx(T) = ρxx0+αTβ, where ρxx0 and T are the residual resistivity and temperature, and the coefficient α and β will change with the scattering model. The calculated ρxx0 of the 78.8-nm-thick Mn4N film is 30.56 μΩ cm, which is slightly less than ρxx(2 K) = 30.95 μΩ cm. The calculated β is 2.014 below 85 K, which is closely proportional to 2 for the ferromagnets. And the ρxx0 of other thicknesses were obtained by fitting the ρxx-T curves with the thickness from 7.8 to 47.3 nm in the same way. The thickness-dependent ρxx0(t) curve is shown in the inset of Fig. 1(b). It is obvious that the ρxx0 increases with the decrease of the film thickness, which imply the enhancement of the electron-electron interaction effects and interfacial scattering.8 Anomalous Hall effect of the Mn4N films were measured in the temperature range of 5-300K and in magnetic fields to 7 T. The Hall bars were used for measuring the Hall resistivity ρxy of Mn4N films with the thickness of 4.7-78.8 nm. The AHE is analyzed by the expression as ρxy=ρOH+ρAH=R0H+RsM, where R0 and Rs are ordinary and anomalous Hall coefficients, respectively. Fig. 2(a)-(h) shows the hysteretic property of the Hall resistivity ρxy for the different thickness as a function of applied magnetic field H at the temperatures of 5-300 K. The hysteretic property of ρxy due to the AHE caused by quantum mechanical spin-orbit interaction, and the linear property at the high-field regions due to the ordinary Hall effect (OHE) originated from the Lorentz force.9 The definition of MR is [ρxx(H)-ρxx(0)]/ρxx(0)×100%, where ρxx(0) is the resistivity of the film at 0 T. MR of the Mn4N film displays a negative signal and butterfly shape. The films with the thickness of 47.3 nm and 23.6 nm show a sudden lift as the magnetic field increases from 0 to 0.5 T, and the sign of MR is positive. Then, MR gradually decreases and sign turns negative with the increasing magnetic field above 0.5 T. The MR of 11.0 nm and 7.8 nm thick films shows the same trend except for the turning point at about 1 T. The upturn of MR in the low fields is mainly due to the Lorentz force effect, which causes a decline of mean free paths and a rise of the scattering rates.10 And the downturn of MR at high fields could be attributed to the enhancement of ferrimagnetic order and the reduction of electron scattering by spin-wave excitations and local magnetic anisotropy.11 At a certain temperature, MR of the Mn4N films increases with the increase of film thickness. These results provide experimental basis for application of SOT devices. This work is supported by National Natural Science Foundation of China (51871161 and 52071233).

**References:**

[1] S. Picozzi, A. Continenza, and A. J. Freeman, Phys. Rev. B 66, 094421 (2002). [2] Y. Takahashi, A. Srinivasan, B. Varaprasad, et al, Appl. Phys. Lett. 98, 152501 (2011). [3] A. Schmehl, V. Vaithyanathan, A. Herrnberger, S. Thiel, et al, Nature Mater. 6, 882-887 (2007). [4] K. Yamada, K. Oomaru, S. Nakamura, et al, Appl. Phys. Lett. 106, 042402 (2015). [5] Y. Yasutomi, K. Ito, T. Sanai, K. Toko, et al, J. Appl. Phys. 115, 17A935 (2014). [6] K. Ito, Y. Yasutomi, K. Kabara, et al, AIP Adv. 6, 056201 (2016). [7] G. Su, Y. Li, D. Hou, et al, Phys. Rev. B 90, 214410 (2014). [8] A. Cottey, Thin Solid Films 1, 297-307 (1968). [9] S. Onoda, N. Sugimoto, and N. Nagaosa, Phys. Rev. B 77, 165103 (2008). [10] X. Feng, W. Mi, and H. Bai, J. Appl. Phys. 110, 053911 (2011). [11] B. Raquet, M. Viret, E. Sondergard, O. Cespedes, and R. Mamy, Phys. Rev. B 66, 024433 (2002).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/33b2abbb653ccc1887899b2f70a1a699.jpg)
Figure 1. (a) Temperature-dependent ρxx(T)/ρxx(300 K) of Mn4N films with different thicknesses on MgO substrates. (b) The longitudinal resistivity ρxx(T) of the 78.8-nm-thick Mn4N film and the fitting curve obtained below 85 K (blue line). The inset shows the thickness-dependent ρxx0

![](https://assets.underline.io/uploads/markdown_image/1/image/10105250aa2ccde783d9f5ac35cb2723.jpg)
Figure 2. Magnetic field-dependent Hall resistivity ρxy(H) of the (a) 78.8 nm, (b) 47.3 nm, (c) 31.5 nm, (d) 23.6 nm, (e) 15.8 nm, (f) 11.0 nm, (g) 7.8 nm and (h) 4.7 nm thick Mn4N films at different temperatures

 Electronic transport properties of antiperovskite Mn4N epitaxial films

Co-based Heusler compounds are promising candidates for many emerging spintronic applications regarding the potential full spin polarization and ultra-low magnetic damping, together with their high Curie temperature (900 – 1000 K) and tunable electronic properties [1]. In particular, the existence of a minority spin gap and 100% spin polarization has been demonstrated by spin-resolved photoemission spectroscopy (SR-PES) for Co2MnSi thin films, and an ultra-low magnetic damping coefficient of 4.6*10-4 was obtained for this compound, which is the lowest value reported so far for a conductive material [2]. Tunnel magnetoresistance (TMR) ratios as high as 1995 % and 354 % has been reported for epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions at low and room temperature, respectively [3]. Large magnetoresistance ratios over 30%, at room temperature, and over 60 %, at low temperatures, have been also reported for current perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices, with Co2MnSi as ferromagnetic electrode [15]. However, the electronic properties and the half-metallicity seem to be very sensitive to the structure and composition of the films [1], which can negatively impact the physical properties and the performance of these films when implemented into GMR or TMR magnetoresistive devices. In this work, we report on the transport properties of Co2MnSi Heusler compounds fabricated by molecular beam epitaxy, and patterned into Hall bars by standard UV-photolithography. The formation of the chemically-ordered L21 phase was verified by X-ray diffraction, reflection high-energy electron diffraction (RHEED) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (see Figure 1). A considerable increase in the resistivity of the films was observed with the decrease of thickness as a result of the increase in the inelastic electron scattering at the film surface. Fitting the film’s resistivity versus thickness using Fuchs and Sondheimer model allows us to estimate the electron mean free path for Co2MnSi equal to 13 ± 5 nm, a key parameter to consider when employing these alloys for current-in-plane (CIP) magnetoresistive devices. Furthermore, by fitting the resistivity of the films versus temperature, we access to the distance between the Fermi energy and the minority-spin conduction band minimum (kB * Δ), as shown in Figure 2. These values in combination with spin-resolved photoemission spectroscopy (SR-PES) measurements, performed to a 20-nm Co2MnSi film, permit to estimate the spin gap of Co2MnSi between 0.55 - 0.7 eV, in good agreement with ab initio calculations [6]. A negative anisotropy magnetoresistance (AMR) ratio was measured for all the samples, which is a signature of the Half-metallicity of the Co2MnSi films. Moreover, AMR values ranging from -0.15 to - 0.24 were obtained, which are in good agreement with the highest values reported in the literature for Co2MnSi. The carrier’s concentration and carrier’s mobility of the films and their dependency with temperature was determined from Hall effect measurements. Finally, the evolution of films resistivity with thickness and temperature provided the ideal scenario to study the scaling of Hall resistivity with longitudinal resistivity. The linear trend observed indicates that screw scattering is the dominant temperature-dependent scattering mechanism taking place in Co2MnSi. The electrical parameter obtained here were measured on high-quality Co2MnSi films, with ultra-low magnetic damping and 100% spin polarization, which can be very useful in views of the implementation of Co2MnSi films in magnetoresistive devices. Moreover, we demonstrate the feasibility of the electrical transport measurements as a mean to verify the half-metallicity of Heusler compounds, and to estimate their spin gap, as a complement to SR-PES measurements.

**References:**

[1] C. Guillemard, S. Petit-Watelot, S. Andrieu et al., J. Appl. Phys., Vol. 128, p.241102 (2020) [2] C. Guillemard, S. Petit-Watelot, S. Andrieu et al., Phys. Rev. Appl., Vol. 11, p.064009 (2019) [3] H. Liu, Y. Honda, T. Taira et al., Appl. Phys. Lett., Vol. 101, p.132418 (2012) [4] Y. Sakuraba, K. Izumi, T. Iwase et al., Phys. Rev. B., Vol. 82, p.094444 (2010) [6] S. Picozzi, A. Continenza, and A.J. Freeman, Phys. Rev. B., Vol. 66, p.094421 (2002)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e7063f455e3b8802939f7f8fea566be9.jpg)
Fig. 1. Reflection high-energy electron diffraction (RHEED) patterns for a 15-nm Co2MnSi film (noted CMS) along the [110] (left column) and [100] (right column) azimuths (top). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) micrograph of the CMS layer along the [110] zone axis (bottom).

![](https://assets.underline.io/uploads/markdown_image/1/image/83bb671dcb7c30a1a9ed3ee3e7bc79dd.jpg)
Fig. 2. Resistivity of the Co2MnSi films versus temperature from 300 to 5 K (top). Distance between the Fermi energy and the bottom of the minority spin band (kb * Δ) versus films thickness (bottom).

 Transport Properties of Co2MnSi Heusler Compounds

I. INTRODUCTION Thermoelectric generators (TEG) that convert waste heat to electricity is regarded as alternative and environment-friendly technology for harvesting and recovering heat [1]. Although the conventional thermoelectric devices based on the Seebeck effect (SE) are promising, SE-based TEGs suffer from the high fabrication cost due to their complex structure and the usage of toxic, rare, and expensive elements. Furthermore, the one-dimensional relationship between the heat and carrier transports defined in SE makes it very challenging to achieve a high energy conversion efficiency and be only limited to a flat heat source surface [1, 2]. Recently its counterpart, the anomalous Nernst effect (ANE) in ferromagnetic materials, has gained increasing interests with its potential merits to further improve the figure-of-merit, ZT, simplify a thermopile structure with low-cost materials, and be applicable for a non-flat surface [2, 3]. However, to date, the reported thermoelectric conversion efficiency of the ANE is too small compared with the SE, which hampers its application as TEG. Thus, it is quite meaningful to explore valid approaches to achieve larger ANE output. In this study, we focus on the multilayer structure with full Heusler alloy Co2MnGa which is a famous ferromagnet exhibiting the large ANE thanks to its characteristic electronic band structure [3, 4]. In addition to the ANE originating from the bulk property of Co2MnGa, we expect the enhancement of ANE in the multilayer structures as reported previously [5, 6]. Here, polycrystalline Co2MnGa/AlN multilayer films were prepared on the amorphous substrates, the layer thickness dependence of ANE was investigated. We found that the large anomalous Nernst thermopower (SANE) of 4.9 ± 0.4 µV K-1 can be obtained even for the polycrystalline Co2MnGa. II. EXPERIMENTAL METHOD The film stacking structure is Si/SiO2/AlN(20)/[Co2MnGa(t)/AlN(5)]25/t (t = 2.5, 5.0, 12.5, and 25.0, thickness in nm), which was fabricated by DC magnetron sputtering at room temperature. Then all the films were post-annealed at 500 oC for 3 hours. For the ANE measurement, the thin films were first patterned into Hall-cross shapes through the use of photolithography and Ar ion milling. After milling, the on-chip thermometers made of the 100-nm-thick sputtered Pt were prepared by the lift-off process. The in-plane temperature gradient was generated by an on-chip heater, and the temperature on the cold and hot sides of the sample was monitored by two Pt on-chip thermometers. An external magnetic field (± 2 T) was applied perpendicular to the sample plane. III. RESULT AND DISCUSSION The structural characterization was carried out using the x-ray diffraction. For all samples, the Co2MnGa layers showed the (110)-oriented textures on the SiO2 substrate. The 25-nm-thick Co2MnGa film (single layer) exhibited the weak Co2MnGa 220 peak. On the other hand, [Co2MnGa (12.5)/AlN (5)]2 multilayer film (multilayer) with the Co2MnGa layers sandwiched by dielectric AlN layers shows a much stronger (110) texture. Moreover, the satellite peaks were observed around 2θ = 44.6° of the multilayer sample, indicating shape and abrupt Co2MnGa/AlN interfaces. Furthermore, the largest SANE of 4.9 ± 0.1 µV K-1 was achieved for the multilayer film while the value of SANE was 3.8 ± 0.4 µV K-1 for the single layer film. Although the present SANE of 4.9 ± 0.1 µV K-1 is smaller than the record-high value of SANE which is ~ 8.0 μV K−1 for single crystal Co2MnGa bulk [7], it is still promising one order of magnitude larger than those for other ferromagnets with similar magnetizations [8]. The results indicate that the large ANE output can be obtained even for the polycrystalline Co2MnGa which quite meaningful for practical application. This fact also implies that multilayers and/or superlattices with low dimension and sharp interfaces are promising platform/candidates to achieve large ANE. The present results provide new insight into the correlation between microstructure and ANE. We consider that the size and interface effect are the primary origins for the enhancement of ANE in the proposed Co2MnGa/AlN multilayer films. There are three possible scenarios for explaining the enhancement of ANE in the multilayer samples. First, the distinct thermal expansion coefficient, elastic modulus, and lattice mismatch between metals (Here Co2MnGa) and ceramics (here AlN) would introduce tremendous interface stress during the annealing, i.e. heating and cooling processes. The interface stress in the Co2MnGa layer may be a driving force to promote the chemical ordering of Co2MnGa. Second, the lattice distortion in Co2MnGa due to the interface stress will modify its electronic band structures. Third, the composition variation in the Co2MnGa layers may occur via the interdiffusion through the interface. The detailed structural analyses will be addressed in the presentation. This work successfully demonstrated a way to achieve giant ANE thermopower for the polycrystalline Co2MnGa layer on the amorphous substrate by exploiting the dielectric AlN layers. This is an important finding for practical applications. This work was supported by KAKENHI (18H05246) and CSRN.

**References:**

[1] S. Twaha, et al., Renew Sustain Energy Rev, 65, 698 (2016) [2] Y. Sakuraba, Scr. Mater. 111, 29 (2016) [3] A. Sakai, et al., Nat Phys. 14, 1119 (2018) [4] K. Sumida et al., Commun Mater 1, 89 (2020) [5] K. Uchida et al., Phys. Rev. B 92, 094414 (2015) [6] C. Fang et al., Phys. Rev. B 93, 054420 (2016) [7] Xu, L. et al., Phys. Rev. B 101, 180404 (R) (2020) [8] S. N. Guin et al., NPG Asia Mater. 11, 16 (2019)

**Images:**

 Enhancement of the Anomalous Nernst Effect in Polycrystalline Co2MnGa/AlN Multilayers

Half-metallic ferromagnets (HMFs) have attracted much interest as promising materials in spintronics devices since they generate highly spin-polarized current due to the absence of one spin-channel at the Fermi level (EF). Among the known HMFs, the use of Co2-based Heusler alloys enables enhancing the giant-magnetoresistance, tunnel-magnetoresistance, and spin-accumulation effects, thereby improving the performance of spintronic devices [1-3]. Despite the large magnetoresistance (MR) ratio observed in Heusler based devices, the structural disorder is still one of the remaining issues which lower the P. Therefore, it is essential to have a highly ordered Heusler structure to realize the half-metallicity. The structural order (B2→L21) of the Heusler alloys e.g., Co2MnSi [4] and Co2FeGe0.5Ga0.5 [1] have been promoted by annealing at high temperatures (&gt;500°C) resulting in an enhanced MR ratio. However, for various applications, the permissible maximum annealing temperature is restricted, e.g., for a magnetic read head in a hard disk drive (HDD) the annealing temperature is limited to 300°C because of the temperature tolerance of the NiFe shield. Therefore, it is desirable to search for other Heusler alloys that crystallize in L21-order below 300°C. The present work is motivated by Okubo et al. [5]’s report, where the L21 to B2-order transition temperature of Co2MnZ (Z = Ge, Sn) Heusler alloys is very high (&gt;1500K) suggesting high energetical stability of L21-structure; and hence L21-order is expected to appear even in the as-deposited state or by low-temperature annealing. Co2MnZ (Z = Ge, Sn) and reference Co2FeGe0.5Ga0.5 films were deposited on MgO (001) single crystal substrate using an ultra-high vacuum magnetron sputter system at room temperature and then post-annealed in-situ at Tann = 200-700°C to induce the atomic-ordering. Here, Co50Fe50 (3nm) was used as a buffer layer to deposit Co2MnZ films to suppress a phase separation. Co2MnSn films with two different compositions Co:Mn:Sn = 53.4:29.3:17.3 and 47.4:25.5:27.1 were deposited, which we refer to as Sn-poor and Sn-rich films, respectively. The crystal structure of the films was analyzed using a high-resolution X-ray diffractometer with Cu-Kα radiation. Finally, to quantify the P of the conduction electron in the bulk region, Co2MnZ and Co2FeGe0.5Ga0.5 films were microfabricated into a non-local spin valve (NLSV) device using electron-beam lithography, Ar-ion milling, and the lift-off techniques. X-ray diffraction revealed that Co2MnZ and Co2FeGe0.5Ga0.5 films are epitaxially grown along (001) direction. The degree of L21-ordering (SL21) as a function of Tann of Co2MnZ, Co2FeGe0.5Ga0.5, and Co2MnSi [4] films are shown in Fig.1(a). In the case of Co2FeGe0.5Ga0.5 and Co2MnSi, L21-ordering was observed above 500°C. Interestingly, in Co2MnZ films, L21-ordering appeared even in the as-deposited state and it enhances systematically with increasing Tann. Figure 2(a) shows a scanning electron microscopy (SEM) image of a typical NLSV device using Co2MnGe and Cu and the schematic of non-local measurement. We observed a clear spin-accumulation signal (△RS) in all our NLSV devices, which decays with increasing the gap-length (d); as Fig.2(b) shows a representative △RS-d curve of Co2MnGe film (Tann = 300°C). To evaluate P, the measured △RS in the Cu channel was analyzed by one-dimensional spin diffusion model [6]. The evaluated P of the films are displayed in Fig.1(b), which varies systematically with increasing Tann. The spin-polarization P of Co2MnGe at Tann = 300°C is higher (~0.65) than that of Co2MnSn and Co2FeGe0.5Ga0.5 due to a higher SL21, suggesting that Co2MnGe is a promising material for various practical applications. For Co2MnSn, however, low P was found in both compositions, despite high SL21. Because first-principle calculations predict highly spin-polarized electronic structure in the Sn-rich composition, it is desirable to check the microstructure by transmission-electron microscopy to understand the origin of low P. In conclusion, we elucidated that Co2MnGe is a promising material to obtain high atomic order and large P in the sputtered film without high-temperature annealing. Since the present Co2MnGe film has a slightly Co-rich composition which is not suitable to obtain high P due to the formation of the Co antisite disorder, optimization of the composition ratio is expected to lead to higher P.

**References:**

[1] S. Li et al., Appl. Phys. Lett. 103, 042405 (2013). [2] H. Liu et al., Appl. Phys. Lett. 101, 132418 (2012). [3] Ikhtiar et al., J. Appl. Phys. 115, 173912 (2014). [4] Y. Sakuraba et al., Phys. Rev. B 82, 094444 (2010). [5] A. Okubo et al., Appl. Phys. Lett. 96, 222507 (2010). [6] S. Takahashi and S. Maekawa, Phys. Rev. B 67, 052409 (2003).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8902ea3d787672d1d6b6265fa50d0219.jpg)
Fig.1. Tann dependence of (a). SL21, and (b). bulk spin-polarization P measured in NLSV devices. SL21 exceeding 1 is expected to be caused by off-stoichiometry in the films.

![](https://assets.underline.io/uploads/markdown_image/1/image/1c520a3f468d3d48a7c2ba59af666eb7.jpg)
Fig.2. (a). An SEM image of a typical NLSV device using Co2MnGe and Cu, and the schematic of non-local measurement. (b). The gap-length (d) dependence of △RS of NLSV based on Co2MnGe film (Tann = 300°C). The solid line indicates the fitting curve using one-dimensional spin diffusion model [6].

 Enhancement of L21-atomic order and spin-polarization in Co2MnZ (Z = Ge, Sn) Heusler thin films by low-temperature annealing process

Room temperature magnetoelectric (ME) compounds are rare and high-quality artificially multilayered ones are in general difficult and costly to produce. In this respect, magnetic Naturally Layered Perovskite structures (NLP) with improper ferroelectricity [1, 2], such as the Ruddlesden-Popper (RP) calcium manganite compound Ca3Mn2O7 [2], offer an alternative route to achieve non-expensive and high-performance room temperature MEs for information storage, sensors, and actuators or low power energy-efficient electronics. They allow exploring oxygen octahedra nonpolar rotations and cation site displacement to attain non-centrosymmetry. Additionally, due to their high sensitivity to lattice-distortions, their preparation in thin film form over crystalline substrates allows the manipulation of acentricity and enables the tuning of lattice, electric and magnetic interactions. However, crystallizing the desired layer with RP structure is challenging. Thus, the preparation conditions to obtain the envisaged Can+1MnnO3n+1 phases, with different n, need to be optimized. The objective of this study is to identify the deposition conditions that lead to different phases of (CaO)(CaMnO3)n with n = 2 (improper ferroelectric Ca3Mn2O7) and/or with n = ∞ (CaMnO3). Here we adopted the Laser ablation deposition method for developing (CaO)(CaMnO3)n thin films. As such, thin films of calcium manganate have been prepared over SrTiO3 substrates using Ca3Mn2O7 target by laser ablation. Polycrystalline Ca3Mn2O7 was synthesized using a conventional high-temperature ceramic route. The phase purity of the target was checked using XRD. The deposition parameters were studied, specifically oxygen partial pressures (from 10-3 to 10-1 mbar), laser fluences (10-5 J/cm2), substrate temperatures (from room temperature to 730 oC) and post-annealing influence. Further, a detailed structural study using X-ray diffraction (XRD), microstructural analysis by scanning electron microscopy (SEM), quantification of composition with energy-dispersive X-ray spectroscopy (EDS), magnetic property using SQUID magnetometer, and dielectric analysis by impedance analyzer were carried out. The XRD results show that for oxygen pressures in the range 10-2 - 10-1 mbar, deposition temperatures of 600 oC to 730 oC and laser fluences from 1.7 J/cm2 to 2.9 J/cm2, the films are polycrystalline, stabilizing in the CaMnO3 orthorhombic Pnma phase. The SEM measurements show dense films with smooth surfaces (Fig.1). The corresponding EDS measurements indicate a 1:1 atomic proportion in these samples. On the other hand, by decreasing the oxygen pressure, increasing the temperature and increasing the laser fluence, the Ca/Mn proportion increases, so that for films prepared on SrTiO3, at 730 oC, with 4 J/cm2 laser fluence, ~10-3 mbar oxygen pressure and with a post-annealing process, the Ca2Mn3O7 (n = 2) phase is stabilized, as confirmed by XRD (Fig.2). The corresponding EDS analysis further gives a Ca/Mn atomic ratio of ~1.5:1, consistent with the presence of this phase. The corresponding magnetic properties of the films, that are correlated with the stabilized phases and preparation conditions, will be present as well as the dielectric properties that were characterized by impedance spectroscopy.

**References:**

[1] Ghosez, P., Triscone, J.-M., Coupling of three lattice instabilities: Multiferroics, Nature Materials, vol. 10, n. 4, 2011, 269–270. [2] Rocha-Rodrigues, P., Santos, S. S. M., Miranda, I. P., Oliveira, G. N. P, Correia, J. G., Assali, L. V. C., Petrilli, H. M., Araújo, J. P., Lopes, A. M. L., Ca3Mn2O7 structural path unraveled by atomic-scale properties: A combined experimental and ab initio study, Physical Review B, 101, 064103, 2020.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/0bee090bfd5ee13876d09318b47cab4d.jpg)
The cross-section image from SEM shows a dense film with a thickness of about 300 nm.

![](https://assets.underline.io/uploads/markdown_image/1/image/ff9a5f8b7a402acdcaca185e32eb98fb.jpg)
The XRD diffractogram confirms the stabilization of the Ca2Mn3O7 phase (n = 2).

 Growth of Ruddlesden–Popper (CaO)(CaMnO3)n Thin Films by Laser Ablation Deposition

Complex oxide thin films have highly tunable magnetic properties, not only due to the correlated degrees of freedom within these materials, but also due to the constraints imposed by choice of substrate material. For example, the magnetic properties of manganite materials depend heavily on the bond lengths and angles, resulting in a dead layer at the substrate/film interface [1,2,3]. Recently, techniques have been developed to separate oxide thin films from the substrate after growth, decoupling structural and chemical degrees of freedom at the interface [4]. This technique therefore may allow us to reduce dead layers at the interface of manganite thin films. Thin films of (001)-oriented LaMnO3+δ (LMO) were grown using pulsed laser deposition on a water-soluble Sr3Al2O6 (SAO) film on top of a (001) SrTiO3 substrate. After coating the film with a 1μm thick support layer of PMMA polymer, the SAO was dissolved in water, allowing the film to be separated from the substrate; this freestanding film could then be transferred to a different substrate, in this case, oxidized Si or LSAT. X-ray diffraction measurements show that freestanding films remain highly crystalline and oriented, but are no longer strained to the substrate. Magnetometry measurements indicate very similar TC, magnetic anisotropy, and saturation magnetization for both freestanding and epitaxial films, and cannot discern any difference in dead layer. Therefore, polarized neutron reflectometry (PNR) was performed at multiple temperatures to understand the depth profile of both the nuclear and magnetic structure of these freestanding films. As shown in Fig. 1, the film transferred onto the LSAT substrate is very smooth and has an expected structure. Between the substrate and freestanding film is a region with slightly lower nuclear scattering length density (SLD); from STEM and EELS, we find that this region is caused by intermixing with or residue from the water-soluble layer. In contrast, the film transferred onto silicon has a much more complicated structure, as shown in Fig. 2. The PNR data can only be explained with a long-range corrugation or rumpling of the film when placed on the Si substrate, with water incorporated into the interfacial region. After chemical removal of the PMMA support, this corrugation remains. Regardless of the substrate material, the magnetic profile within the freestanding manganite film is uniform, and there is no evidence of the magnetically dead layers at the substrate/film interface seen in epitaxial films. Thus, although choice of substrate can still affect freestanding films through chemical effects – which may have implications for the integration of these freestanding films with silicon-based devices – removing the structural constraints imposed by epitaxial growth is a promising avenue to reduce the dead layer in manganites.

**References:**

[1] Zhai, X., Cheng, L., Liu, Y., et al. Nat. Commun. 5, 4283 (2014). [2] Moon, E. J., Balachandran, P. V., et al. Nano Lett. 14, 2509-2514 (2014). [3] Liao, Z., Zhang, J. Appl. Sci. 9, 144 (2019). [4] Lu, D., Baek, D. J., Hong, S. S., et al. Nat. Mater. 15, 1255–1260 (2016).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/bd954455d1393d7baa1acecb08ea8393.jpg)
(a) PNR data and (b) spin asymmetry of a 20 nm LMO freestanding film transferred onto an LSAT substrate, taken at 50K in an in-plane magnetic field of 3T. (c) The nuclear and magnetic depth-profile used to generate the best-fit model.

![](https://assets.underline.io/uploads/markdown_image/1/image/f23f5b70115419a383466b864d2fe667.jpg)
(a) PNR data and (b) spin asymmetry of a 40 nm LMO freestanding film transferred onto a Si/SiOx substrate, taken at 7K in an in-plane magnetic field of 1.5T. (c) The nuclear and magnetic depth-profile used to generate the best-fit model.

 Interfacial Corrugation and Magnetism of Freestanding LaMnO3+δ Thin Films

Energy gap parameters can vary significantly in X2YZ Heusler alloys [1], where Y-elements are 3d-transition metals and Z-components are s- and p-elements. Therefore, changes in different physical properties, especially, in electronic and optical characteristics are observed. Some features may indicate conditions close to half-metallic ferromagnets (HMFs) [2] and spin gapless semiconductors (SGSs) [3]. Such compounds are promising materials for spintronics because they can possess almost 100% spin polarization of charge carriers. HMFs hold an energy gap near the Fermi level for the current carriers with spin down, while its absence for current carriers with spin up. In turn, SGSs are like HMFs for the spin projection down, however, for the opposite spin direction, the gap is being zero [2, 3]. Thus, the aim of the work is to follow the changes in the electronic transport, optical properties of the Co2YSi (Y = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys system experimentally, and to compare the results obtained with the electronic band structure calculation, to establish their interconnection and suggest possible proximity to HMF- and/or SGS-states in the Co2YSi system. According to the data of X-ray structural analysis, all compounds are found to be ordered in the L21 structure. The electrical resistivity was measured in a wide temperature range using a PPMS setup. The field and the temperature dependences of the magnetic properties being studied too on the SQUID magnetometer. The coefficients of the normal and anomalous Hall Effect were found. The values of current carrier concentration and their mobility were estimated. Results of optical measurements showed good agreement with electronic band structure calculation. Besides, the correlation between the changes in these electronic, optical characteristics, and the coefficient of the current carrier spin polarization depending on the number of valence electrons are observed, which may point to the possible states of the HMF or SGS in this system of Heusler alloys. The work was performed within the framework of the state assignment of the Ministry of Science and High Education of Russia (the themes “Spin”, No. AAAA-A18-118020290104-2-2 and “Electron” No. AAAA-A18-118020190098-5) with partial support from the RFBR (project No. 20-32-90065), and the Government of the Russian Federation (Decree No. 211, Contract No. 02.A03.21.0006).

**References:**

[1] T. Graf, C. Felser, and S.S.P. Parkin, Prog. Solid State Chem., Vol. 39, p. 1 (2011) [2] M.I. Katsnelson, V.Yu. Irkhin, L. Chioncel et al., Rev. Mod. Phys., Vol. 80, p. 315 (2008). [3] X.L. Wang, Phys. Rev. Lett. Vol. 100, p. 156404 (2008).

**Images:**

 Electronic properties and electronic structure of Co2YSi (Y = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys

Voltage-controlled magnetism, where magnetic properties are controlled via an applied electric field instead of current, could represent a significant increase in energy savings in future magnetically actuated devices. Practically, however, this approach faces several important obstacles, such as thickness limitations in electrically charged metallic films, mechanical failure in strain-mediated piezoelectric/magnetostrictive devices, and a lack of room-temperature multiferroics. Voltage-driven ionic motion (magneto-ionics) may provide a path forward by avoiding many of these drawbacks. Nevertheless, translating magneto-ionics into real world devices requires significant improvements in magneto-ionic rates, cyclability, and magnetization. Here, we report on the development of magneto-ionics in single-layer, semiconducting transition metal oxides and nitrides, and the subsequent enhancements in their performance. We first present electrolyte-gated and defect-mediated O transport in single-layer, paramagnetic Co3O4 at room temperature (i.e. without thermal assistance), which allows voltage-controlled magnetic switching (referred to here as ON-OFF ferromagnetism: Fig. 1) via internal reduction/oxidation processes[1]. Negative bias partially reduces Co3O4 to Co, resulting in films with Co- and O-rich areas (ferromagnetism: ON). Positive bias re-oxidizes Co back to Co3O4 (paramagnetism: OFF). We show that the bias-induced motion of O is caused by mixed vacancy clusters, with O motion promoted at grain boundaries and assisted by the development of O-rich diffusion channels. The generated ferromagnetism is shown to be stable, and easily erased by sufficient positive bias. This voltage-induced process is demonstrated to be compositionally, structurally, and magnetically reversible and self-contained. We then show that room-temperature magneto-ionic effects in electrolyte-gated paramagnetic Co3O4 films can be significantly increased, both in terms of generated magnetization (6 times larger) and speed (35 times faster), if the electric field is applied using an electrochemical capacitor configuration (utilizing an underlying conducting buffer layer: Fig. 2) instead of electric-double-layer transistor-like configuration (placing the electric contacts at the side of the semiconductor)[2]. In addition to gains in speed, magnetization measurements show a marked increase in the squareness ratio and a decrease in the switching field distribution of the hysteresis loops in Co3O4 biased in the capacitor configuration, the result of the formation of more uniform ferromagnetic regions. These results are attributed to the uniform electric field applied throughout the film, as confirmed by COMSOL simulations. As the measured films are quite thick, further miniaturization promises even greater magneto-ionic rates. We then demonstrate room-temperature voltage-driven nitrogen magneto-ionics (i.e., N transport) by electrolyte-gating of a CoN film[3]. Nitrogen magneto-ionics in CoN is compared to oxygen magneto-ionics in Co3O4, in films using an electrochemical capacitor configuration. Both materials are shown to be nanocrystalline (face-centered cubic structure), and show reversible voltage-driven ON-OFF ferromagnetism (Fig. 1). Nitrogen transport is found to occur uniformly throughout the film, creating a plane-wave-like migration front, without assistance of diffusion channels. Nitrogen magneto-ionics also requires lower threshold voltages and exhibits enhanced rates and cyclability, due to the combination of a lower critical electric field required to overcome the energy barrier for ion diffusion and the lower electronegativity of nitrogen with respect to oxygen, consistent with ab initio calculations contrasting N vs. O motion in cobalt stacks. These results place nitrogen magneto-ionics as a robust alternative for efficient voltage-driven effects and, along with oxygen magneto-ionics, may enable the use of magneto-ionics in devices that require endurance and moderate speeds of operation, such as brain-inspired/stochastic computing or magnetic micro-electro-mechanical systems.

**References:**

[1] A. Quintana, E. Menéndez, M. O. Liedke et al., ACS Nano, Vol. 12, p. 10291 (2018) [2] J. de Rojas, A. Quintana, A. Lopeandía et al., Advanced Functional Materials, Vol. 30, p. 2003704 (2020) [3] J. de Rojas, A. Quintana, A. Lopeandía et al., Nature Communications, Vol. 11, p. 5871 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/2bda3c2e7c2cf2043f08b607447b7fbe.jpg)
Fig. 1. Hysteresis loops (M vs. H) of as-prepared, negatively-biased, and positively-biased CoN films at magneto-ionic activation voltages.

![](https://assets.underline.io/uploads/markdown_image/1/image/12893d420d32ae333459d12c5c0e4b71.jpg)
Fig. 2. A schematic of the electrochemical capacitor configuration used to bias cobalt-oxide (Co3O4) and cobalt-nitride (CoN) films.

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