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Antiferromagnets (AFMs) possess great potential in spintronics because of their immunity to external magnetic disturbance, the absence of a stray field, or the resonance in the terahertz range. The coupling of insulating AFMs to spin-orbit materials enables spin transport via AFM magnons. Particularly, spin transmission over several micrometers occurs in some AFMs with easy-axis anisotropy [1]. Easy-plane AFMs with two orthogonal, linearly polarized magnon eigenmodes own unique advantages for low-energy control of ultrafast magnetic dynamics. However, it is commonly conceived that these magnon modes are less likely to transmit spins because of their vanishing angular momentum. In this work, we report experimental evidence that an easy-plane insulating AFM, a α-Fe2O3 thin film, can efficiently transmit spins over micrometer distances (Fig. 1) [2]. Using nonlocal transport device geometry, we find that the spin transmission reaches maximum (zero) when the injected spin polarization is collinear with (perpendicular to) the Néel vector of the AFM, indicating that spins are carried by the excitations of the Néel vector rather than the residual magnetic moment. The spin decay length measured by varying the injector-detector distances shows an unconventional temperature dependence which cannot be captured considering solely thermal magnon scatterings. We interpret our observations in terms of an interference of two linearly polarized, propagating magnons in analogy to the birefringence effect in optics. Furthermore, our devices can realize a bi-stable spin-current switch with a 100% on/off ratio under zero remnant magnetic field. These findings provide additional tools for nonvolatile, low-field control of spin transport in AFM systems.References: 1. Lebrun, R. et al. Tunable long-distance spin transport in a crystalline antiferromagnetic iron oxide. Nature 561, 222 (2018). 2. Han, J. et al. Birefringence-like spin transport via linearly polarized antiferromagnetic magnons. Nat. Nanotechnol. https://doi.org/10.1038/s41565-020-0703-8 (2020).

MMM 2020

Birefringence-like spin transport via linearly polarized antiferromagnetic magnons (B)

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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technical paper

Magnetic rogue waves, like ocean rogue waves, are large deviations from equilibrium which are localized in time and space. They can be created in thin magnetic films by a time-reversal process consisting of a recording step and a reconstruction step. During the recording step, an initial rogue wave is allowed to decay and the microwave fields at various sites are recorded. In reconstruction step, these fields are played backwards in time at the recording sites, ultimately recreating the initial rogue wave as seen in Fig. 1. This technique gives the ability to control the magnitude and position of the rogue waves, which may make them suitable for application in spin wave logic devices and for encryption and secure communications. We study the efficiency of constructing rogue waves. For a Permalloy film which is 30 nm thick, the peak to noise ratio (PNR) of the reconstructed rogue waves can vary by a factor of 5 depending on the location of sensors with respect to the magnetic field. This is caused by the emission of caustic beams as the initial wave evolves in time. The best location is with the sensors perpendicular to the applied field. We also study the relative importance of dipolar and exchange coupling in creating rogue waves. We explore a geometry of a square magnetic film (side length 300 nm) separated from a surrounding magnetic rim by a gap of 10 nm. This gap prevents exchange coupling between the two regions. The initial rogue wave, a small square (25 nm on a side) where the magnetization is raised out of plane by a maximum of 2 degrees, is established in the inner region, but all the recording sensors are in the outer region. It is clear in Fig. 2 that the system in the middle of the recording phase is dominated by short wavelength modes, where exchange is important. However, in the playback, it is the long wavelength dipolar fields which are important in reconstructing the rogue wave.

Theoretical Studies of Dipolar and Exchange Effects in Creating Magnetic Rogue Waves

The effect of Mn doping of a Cr84Re16 alloy are reported, as well as the possible existence of a spin glass (SG) state which were investigated using electrical resistivity and magnetisation measurements. Mn is an electron donor resulting in a concomitant increase in the size of the electron sheet and a subsequent increase in Néel temperature, TN [1]. The temperature dependence of magnetisation, M(T), for the (Cr84Re16)100−xMnx alloys with x = 0.4, 0.6 and 3.1 showed bifurcation of the M(T) data at the pinning temperature, Tp, in the zero field cooled (ZFC) and field cooled (FC) curves. The splitting of the ZFC and FC curves is not in itself a signature of SG but is an indication of magnetic anisotropy in the system [2]. In the (Cr84Re16)100−xMnx alloy system, there is a sharp increase in magnetisation on increasing T after ZFC. A prominent peak is observed close to 40 K above which the behaviour is identical to that observed in the FC state as shown in the figure for the x = 0.4 alloy. Other signatures associated with canonical SG namely the time dependence of isothermal remanent magnetisation (IRM) and the increase of magnetic stiffness upon increasing the wait time in thermoremanent magnetisation (TRM) were also observed in the (Cr84Re16)100−xMnx alloys [3]. The frequency dependence of the real component of AC-susceptibility shows the SG characteristic cusp close to Tp [4]. However, there is no dependence of the maximum value of cusp or the temperature corresponding to the cusp on frequency which is contrary to the SG state [5]. As only some of the SG signatures were observed in the (Cr84Re16)100−xMnx alloy system, it cannot be categorised as canonical SG. These alloys can be classified as a new type of SG with frustration of itinerant spins at the interfaces between antiferromagnetic (AFM) domains established around each Mn local magnetic moment.References: [1] E. Fawcett, H.L. Alberts, V.Y. Galkin, D.R. Noakes and J.V. Yakhmi, Rev. Mod. Phys., Vol. 66, p.25 (1994) [2] P.A. Joy, P.S. Anil Kumar and S.K. Date, J. Phys. Condens. Matter., Vol. 10, p.11049 (1998) [3] B. Li, H.L. Alberts, A.M. Strydom, B.M. Wu, A.R.E. Prinsloo and Zh.J. Chen, J. Magn. Magn. Mater., Vol. 321, p.61 (2009) [4] C.A.M. Mulder, A.J. Van Duyneveldt and J.A. Mydosh, Phys. Rev. B., Vol. 25, p.515 (1982) [5] J.A. Mydosh, “Spin glasses: an experimental introduction”, Taylor and Francis, London (1993)

Spin Glass Behaviour in (Cr84Re16)100−xMnx Alloys

Authors: A. Solyom, M. Caouette-Mansour, B. Ruffolo, L. Childress, J. Sankey, Physics, McGill University, Montreal, Quebec, CANADA|P.M. Braganca, Western Digital Corp, San Jose, California, UNITED STATES|M. Pioro-Ladrière, Institut Quantique and Département de Physique, Universite de Sherbrooke, Sherbrooke, Quebec, CANADA|M. Pioro-Ladrière, Quantum Information Science Program, Canadian Institute for Advanced Research, Toronto, Ontario, CANADA|

Abstract Body: We discuss our latest efforts toward using a single, optically active nitrogen-vacancy (NV) spin sensor (implanted in a single-crystal diamond substrate) to measure magnetic thermal noise modified by spin Hall torques [1] in a Py/Pt nanowire [2]. In this poster, we first present our subtractive method for fabricating magnetic nanocircuits on diamond, and initial characterization of working Py(5 nm)/Pt(5 nm) nanowires. We reliably achieve contacts with few-ohms of resistance and 200 nm of overlap at each end of the 2-μm-long, 400-nm-wide wire, and we observe an anisotropic magnetoresistance of 0.4%. Importantly, we observe that the subtractive patterning process (masked ion milling) used to define the devices reduces the NV spin resonance contrast by a factor of ∼5, but that subsequent exposure to an oxygen plasma returned the contrast to its nominal level without affecting the nanowire behavior. Finally, we present progress toward using transport measurements to estimate thermal time scales in these systems (while calibrating the RF current in the nanowire), as well as preliminary NV readout of spin-transfer-controlled magnetic thermal noise.References: [1] C. Du, T. van der Sar, et al., Science, 357, 195-198 (2017) [2] A. Solyom, Z. Flansberry, et al., Nano Lett., 18, 6494-6499 (2018)

GMW Best Poster Award 2 - Sensing Stray Fields From Magnetic Nanocircuits With Nitrogen-Vacancy Defects

Magnetic skyrmions and antiskyrmions are topologically protected non-collinear spin textures. They have opposite topological charges of +1 and – 1 owing the differently winding of the in-plane spin component about a fixed direction of the core1. The existence of them in a material system mostly depends on the symmetry of the underlying crystal structure. For example, cubic B20 compounds host Bloch skyrmion of isotropic spin texture2, whereas D2d symmetry compounds have anisotropic spin texture named 'antiskyrmion'3,4. Antiskyrmions comprise of alternating chiral boundaries of helicoids and cycloids. They found in two varieties of shape, namely round and square-shaped antiskyrmion5. Here for the first time, using the Lorentz microscopy technique, we have discovered elliptical shape skyrmions in the same material which comprises of Bloch walls only 5. We named them as "elliptical Bloch skyrmions". The elliptical skyrmion's major axis is oriented along the helical propagation directions, [100] and [010], and have opposite chiralities. They are found as spare and lattice states. Interestingly, both topological structure phases can also be stabilized in zero biased fields and below, over wide temperature ranges. It is found that in D2d long-range dipole-dipole interactions play a significant rule for the stabilization of those different opposite topological spin textures. Micromagnetic simulations also confirms these nano-objects. Antiskyrmions are mostly found in room temperature and above, and in lower temperatures, elliptical Bloch skyrmion is observed. At a critical temperature of 268 K, both these nano-objects are stabilized. We also performed experiment at room temperature on the nano-stripes. The transformation and density variation of antiskyrmions and elliptical Bloch skyrmions occur here that depend on the degree of tilting6. Notably, those nano-objects in the nano-tracks produce as a single chain form in the middle of the track even without giving an in-plane field that was indeed needed in the broad width lamellae. The novel and unanticipated finding of opposite topological charges of 1in one material system make the family of D2d material exceptional.References: 1. A. Bogdanov, U. Roeßler and K.-H. Mueller, Phys. Rev. B, 66, 214410 (2002) 2. S. Muehlbauer, B. Binz and C. Pfleiderer, Science, 323, 915-919 (2009) 3. A. K. Nayak, V. Kumar and S. S. P. Parkin, Nature, 548, 561 (2017) 4. J. Jena, R. Stinshoff and S. S. P. Parkin, Nano Lett., 20, 59−65 (2019) 5. J. Jena, B. Goebel and S. S. P. Parkin, Nat. Commun., 11, 1115 (2020) 6. J Jena, B. Goebel and S. S. P. Parkin, Submitted., XX,XXX (2020)

Observation of opposite topological charges in single D2d Heusler compound (B)

Electrical writing and readout of antiferromagnetic insulators is a key prerequisite in view of using this class of materials in applications. Only recently, this has been demonstrated in insulating AFM/heavy metal bilayers by utilizing the transverse spin Hall magnetoresistance effect (SMR) as the readout. In the electrical measurement, a steplike signal resulting from a magnetic SMR contribution was reported in NiO1 and α-Fe2O32, while non-magnetic contributions due to artifacts in the heavy metal layer were found to mimic the signal expected from the magnetic switching.3,4 These results have generated a debate on the magnitude of the magnetic contributions, the reliability of the SMR readout mechanism and even on the possibility of switching domains in AFMs electrically. In our work5 we use a table-top approach to demonstrate stable and reversible current-induced switching of large-area antiferromagnetic domains in NiO/Pt by direct imaging in a Kerr microscope.6 Concurrent transport and magneto-optical imaging measurements allow us to correlate the AFM domain switching fraction and magneto-transport signal response. We establish a procedure to subtract the non-magnetic contributions and extract the magnetic SMR response from the transverse resistance signal. The disentanglement of magnetic and non-magnetic contributions clarifies the presence of a significant SMR response, associated with the switching of the antiferromagnetic domains. We thus highlight the possibility to deduce details of the antiferromagnetic switching from simple transport measurements.References: 1 L. Baldrati, O. Gomonay, A. Ross, Phys. Rev. Lett. 123, 177201 (2019). 2 Y. Cheng, S. Yu, M. Zhu, Phys. Rev. Lett. 124, 027202 (2020). 3 A. Churikova, D. Bono, B. Neltner, Appl. Phys. Lett. 116, 022410 (2020). 4 C.C. Chiang, S.Y. Huang, D. Qu, Phys. Rev. Lett. 123, 227203 (2019). 5 F. Schreiber, L. Baldrati, C. Schmitt, Appl. Phys. Lett. 117, 082401 (2020). 6 J. Xu, C. Zhou, M. Jia, Phys. Rev. B 100, 134413 (2019).

Concurrent magneto-optical imaging and magneto-transport readout of electrical switching of insulating antiferromagnetic thin films (B)

Antiferromagnets have considerable potential as spintronic materials [1]. Notably, they manifest resonant modes at frequencies higher than can be observed in conventional ferromagnetic materials. Synthetic Antiferromagnets (SAFs), engineered structures of antiferromagnetic (AF) RKKY exchange coupled ferromagnetic(FM)/non-magnetic(NM)/ferromagnetic trilayers, mimic the properties of single phase antiferromagnets whilst possessing greater compatibility with current device technologies such as those used for STT-MRAM production [2,3]. In addition to the conventional acoustic (Kittel) mode, SAFs also possess a higher order resonant (optic) mode [3]. The structure of a SAF is shown in Fig.1a. We report the dynamic properties of SAFs with structure Ta(2 nm)/CoFeB(5 nm)/Ru(tRu)/CoFeB(5 nm)/Pt(4 nm) where tRu is the thickness of the Ru layer, acting as the NM spacer layer. The samples were fabricated using magnetron sputtering and characterized by X-ray Reflectivity (XRR) to measure layer thickness, vector Vibrating Sample Magnetometry (VSM) to verify AF interlayer coupling and a Vector Network Analyser – Ferromagnetic Resonance (VNA-FMR) setup to explore dynamic properties. Fig.1b shows the resonant dynamics of a single CoFeB layer and a CoFeB SAF. The data demonstrate the highest zero field frequencies observed in SAFs to date [4] with resonances of 18 GHz and 21 GHz at the 1st and 2nd instances of AF RKKY coupling respectively. A three part analytical model can reproduce the major features of the measured spectra.References: [1] T. Jungwirth, X. Marti, P. Wadley et al., Nat. Nanotechnol., Vol. 11, p. 231–241 (2016) [2] R. Hao, L. Wang and T. Min, IEEE Trans. Magn., Vol. 55, p. 1-6 (2019) [3] S. Li, C. Wang, X. Chu et al., Adv. Funct. Mater., Vol. 26, p.3738-3744 (2016) [4] H. Waring, N.A.B. Johansson, I. Vera-Marun et al., Phys. Rev. Appl., Vol. 13, p. 034035 (2020)


Zero Field Optic Mode Beyond 20 GHz in a Synthetic Antiferromagnet (B)

Interface Characterisation on a MTJ Using TEM and SEM

Weyl Semimetals for Spintronics

Advanced Static and Dynamic Spin Depth Profiling

Structural and magnetic properties of a quasi-one-dimensional spin-1/2 compound NaVOPO4 are explored by x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, electron spin resonance, and 31P nuclear magnetic resonance measurements, as well as complementary ab initio calculations. Whereas magnetic susceptibility of NaVOPO4 may be compatible with the gapless uniform spin chain model, a detailed examination of the crystal structure reveals a weak alternation of the exchange couplings with the alternation ratio α ~ 0.98 and the ensuing zero-field spin gap Δ0/kB ~ 2.4 K directly probed by field-dependent magnetization measurements. No long-range order is observed down to 50 mK in zero-field. However, applied fields above the critical field Hc1 ~1.6 T give rise to a magnetic ordering transition with the phase boundary TN ∝ (H − Hc1 )1/φ, where φ ~1.8 is close to the value expected for Bose-Einstein condensation of triplons. With its weak alternation of the exchange couplings and small spin gap, NaVOPO4 lies close to the quantum critical point.References: [1] P. K. Mukharjee et. al., Phys. Rev. B 100, 144433 (2019) [2] S. Sachdev, Nat. Phys. 4, 173 (2008) [3] Mukhopadhyay et. al, Phys. Rev. Lett. 109, 177206 (2012) [4] Zapf et. al., Rev. Mod. Phys. 84, (2014)

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