United States

Voltage-controlled spintronic devices utilizing the spin degree of freedom are desirable for future applications, and may allow energy-efficient information processing. A pure spin current can be created by thermal excitations in magnetic systems via the spin Seebeck effect (SSE). However, controlling such spin currents, by purely electrical means, has been a fundamental challenge. Here, we investigate voltage control of the SSE in an antiferromagnetic insulator. We demonstrate that the SSE response generated in this material can be effectively controlled by applying a bias voltage, owing to the sensitivity of the SSE to the orientation of the magnetic sublattices as well as the existence of magnetoelectric couplings in the present system. Our experimental results on the voltage-controlled switching of magnetic sublattices are explained using a model based on the magnetoelectric effect. Acknowledgments: all work at Argonne was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The use of facilities at the Center for Nanoscale Materials, an Office of Science user facility, was supported by the US Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-06CH11357.

MMM 2020

Voltage control of magnon spin currents in an antiferromagnetic insulator

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

Recently manipulation of antiferromagnetic (AFM) order has been gaining the attention of the spintronics community [1]. Magnetic switching has been reported in AFM thin films based on electronic transport methods, which provide only spatially averaged information on AFM states. Other techniques, such as x- ray magnetic linear dichroism (XMLD) microscopy, can provide local information on AFM domains. In this study, we report the direct observation of spin reorientation in response to current pulses in (0001)-oriented α-Fe2O3 films [2,3]. Our experiment combines the application of current pulses and spatially resolved photoemission electron microscopy (PEEM) with XMLD to detect AFM contrast. Pulses were applied in two different configurations A and B (Fig. 1a), which results in resistance changes (Fig. 1b), and XMLD images were taken before and after the application of a current pulse to reveal changes in the AFM domains (Fig. 1c). Our analysis shows that electrical pulses do not only reorient the Néel vector within the easy plane as previous work indicates [2,3]. Instead, we conclude that most of the change associated with the Néel vector is out of the film plane and different pulse directions can bring the AFM order in and out of the plane deterministically. This work is supported in part by the Air Force Office of Scientific Research under Grant FA9550- 19-1-0307.References: [1] V. Baltz et al., Rev. Mod. Phys. 90, 015005 (2018) [2] Y. Cheng et al., Phys. Rev. Lett. 124, 027202 (2020) [3] P. Zhang et al., Phys. Rev. Lett. 123, 247206 (2019)

X-ray Magnetic Linear Dichroism Studies of Electrical Switching of Antiferromagnetic Order in α-Fe2O3 Epitaxial Films

In this work, we study the canted antiferromagnet α-Fe2O3. At room temperature, the Dzyaloshinskii-Moriya interaction (DMI) induces canting of the two magnetic sublattices MA and MB giving rise to a weak ferromagnetic (FM) moment m. We employ spin Hall magnetoresistance (SMR) measurements to study electrical and magnetic field control of the Néel order in α-Fe2O3/Pt bilayers. The angle-dependent magnetoresistance (ADMR) shows the characteristic saturated negative SMR signal of AFMs (red curve in figure 1) agreeing with previous findings1–4. We note that the FM moment lifts the degeneracy of the sublattices and consequently the orientation of the Néel vector with respect to the applied magnetic field. The blue curve in figure 1 shows the remanent resistance as a function of the angle of the preceding saturating magnetic field. The data shows a simple scaling between the saturated ADMR signal and the remanent resistance. The lack of signatures of anisotropy in the remanent signal implies that the relaxation of the SMR signal is dominated by destressing effects favoring an equilibrium state with multiple domains distributing the orientation of the Néel vector along different directions. Applying the insights from investigating the remanence of α-Fe2O3 with an external magnetic field on our current-induced switching experiments, we find that the fundamental limit of the switching efficiency is given by the magnitude of the remanence Furthermore, we observe hysteresis in the SMR signal. The irreversibility upon switching the direction of the magnetic field in combination with investigating the transient effects of current-induced changes in the domain structure allows us to study the nature and formation of domain walls at low magnetic fields. Our work emphasizes the importance of the FM moment induced by DMI and the relaxation effects for the equilibrium domain structure in α-Fe2O3.References: 1. Lebrun, R. et al. Anisotropies and magnetic phase transitions in insulating antiferromagnets determined by a Spin-Hall magnetoresistance probe. Commun. Phys. 2, 50 (2019). 2. Cheng, Y., Yu, S., Zhu, M., Hwang, J. & Yang, F. Electrical Switching of Tristate Antiferromagnetic Néel Order in a-Fe2O3 Epitaxial Films. Phys. Rev. Lett. 124, 027202 (2020). 3. Fischer, J. et al. Large Spin Hall Magnetoresistance in Antiferromagnetic a-Fe2O3/Pt Heterostructures. Phys. Rev. Appl. 13, 014019 (2020). 4. Zhang, P., Finley, J., Safi, T. & Liu, L. Quantitative Study on Current-Induced Effect in an Antiferromagnet Insulator/Pt Bilayer Film. Phys. Rev. Lett. 123, 247206 (2019).

Importance of the Ferromagnetic Moment Induced by Dzyaloshinskii-Moriya Interaction in a Canted Antiferromagnet for Electrical and Magnetic Field Control of the Néel Order

Magnons and phonons are two elementary excitations associated with spin waves and lattice vibrations in solids. In the presence of magneto-elastic interactions, the coupling of the spin and lattice degrees of freedom leads to the formation of a new type of hybridized excitation, i.e., magnon-polarons. Manifested as anomalies in the spin Seebeck effect (SSE) in a ferrimagnet YIG, magnon-polarons have profound consequences in modifying the magnon and phonon dispersion relations and affecting spin transport properties in ferrimagnetic insulators. In antiferromagnetic insulators, the magnon spectrum is distinctly different due to the existence of two equivalent, antiferromagnetically coupled spin sub-lattices. Here we report an observation of antiferromagnetic magnon-polarons in Cr2O3, a prototypical uniaxial antiferromagnet, featured as anomalies in the SSE signals of Cr2O3/heavy metal heterostructures. Despite the relatively high magnon energy in the absence of an external magnetic field, near the spin-flop transition of ~ 6 T, the left-handed magnon spectrum shifts downward to intersect those of acoustic phonons (both longitudinal and transverse) to cause hybridization. We have measured the temperature dependence of the SSE magnon-polaron anomalies in Cr2O3 and revealed the strength of the magneto-elastic coupling. Our study indicates that the hybridized excitations can be employed to manipulate spin transport in antiferromagnetic systems. This work was supported as part of the SHINES, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award No. SC0012670.

Observation of magnon-polarons in a uniaxial antiferromagnetic insulator Cr2O3

Searching for the efficient manipulation of antiferromagnets for spintronic applications, we study the properties of the magnetic domain walls with due account of magnetoleastic effects. We show that magnetoelastic coupling induces anisotropy of the domain wall orientation which affects the shape and distribution of the domain walls in a sample. In case of non-1800 domains such anisotropy favours formation of the plane walls oriented along or perpendicular the hard magnetic axis. We study the pinning effects caused by magnetoelasticity and calculate the frequency of the localized oscillations and depinning field for an easy-plane antiferromagnet. We further consider dynamics of excitations of magnetoelastic domain wall and show that such a wall effectively scatters the long-wave magnons. The scattering and pinning effects are more pronounced in case of non-1800 domains, due to the strain-induced anisotropy inside domains. However, 1800 domain wall can also scatter the magnons due to the magnetoelastic strains inside the wall.To illustrate possible applications of our results we discuss the magnon propagation in multidomain hematite and NiO.

Domain Walls in Antiferromagnets with Strong Magnetoelastic Coupling

The prospect of using antiferromagnets as the main component in logic devices has gained a lot of interest. Antiferromagnets offer many benefits over their ferromagnetic counterparts: terahertz dynamics, no device cross-talk, and robustness to external fields. However, the latter property makes them difficult to control and has hindered their potential for use in spintronics devices. Recently, electrical switching of antiferromagnets has been demonstrated using current-induced spin-orbit torques due to bulk crystal symmetry1,2,3, or spin-transfer torque from a neighbouring heavy metal layer4,5,6. Changes to the magnetic state are typically detected electrically in the transverse anisotropic magnetoresistance, which depends on the angle between the antiferromagnetic Néel vector and the direction of the probe current. This method is effective for measuring the average Néel vector orientation, but provides limited information on the microscopic magnetic texture. We study current-induced modification of the antiferromagnetic domain structure in CuMnAs microdevices using X-ray photoemission electron microscopy (XPEEM). Magnetic contrast is obtained due to X-ray magnetic linear dichroism, i.e. dependence on the angle between the magnetic moments and the incident X-ray polarisation. Our results show a current-induced motion of both 90° and 180° domain walls, in a direction which depends on the polarity of the current pulse (Fig.1). This is an important step towards the development of low power antiferromagnetic domain wall memory devices. Furthermore, 180° domain walls are topologically protected making them particularly good candidates for non-volatile applications.References: 1 P. Wadley et al. Electrical switching of an antiferromagnet. Science, 2016. 2 J. Godinho et al. Electrically induced and detected Néel vector reversal in a collinear antiferromagnet. Nature Communications, 2018. 3 S. Yu. Bodnar et al. Writing and reading antiferromagnetic Mn2Au by Néel spin-orbit torques and large anisotropic magnetoresistance. Nature Communications, 2018. 4 X. Z. Chen et al. Antidamping-torque-induced switching in biaxial antiferromagnetic insulators. Physical Review Letters, 2018. 5 M. Dunz et al. Spin-orbit torque induced electrical switching of antiferromagnetic MnN. Physical Review Research, 2020. 6 P. Zhang et al. Quantitative study on current-induced effect in an antiferromagnet Insulator/Pt bilayer film. Physical Review Letters, 2019.

Electrical control of antiferromagnetic domain walls

With high frequency spin dynamics, stability in the presence of external magnetic fields, and a lack of stray fields, antiferromagnetic materials are positioned to become key in future low power spintronic devices [1]. Here, we study the spin transport in high quality thin films of hematite (α-Fe2O3) (< 500 nm) of different orientations. Through measurements of the spin Hall magnetoresistance in hematite/Pt bilayers, the magnetic anisotropies of the thin films can be extracted, and the critical temperature of the Morin transition from the easy plane to the easy axis antiferromagnetic phase is electrically observed [2]. We find that the strain introduced by the growth process tilts the antiferromagnetic anisotropy axis, leading to complex signals across the full temperature range investigated [3]. Whilst a key part of antiferromagnetic spintronics is to encode and read information in the Néel vector, the efficient transfer of information is crucial for integration of antiferromagnets into devices. Recently, we demonstrated that a diffusive magnon current can be carried over micrometres in antiferromagnetic single crystals, but such crystals are not suitable for spintronic devices [4]. While mechanisms for the long-distance propagation of pure spin currents carried by the antiferromagnetic order [5, 6] have been developed for thin film antiferromagnets, experimentally the long distance transport of angular momentum by magnons has previoulsy not been achieved [7]. Making use of hematite thin films, a robust magnon current can propagate with intrinsic diffusion lengths of hundreds of nanometres [8]. The efficiency of the transport mechanisms can be tuned by the domain structure, the growth orientation, and the relative orientations of the magnetic field and magnetic anisotropies. Finally, we achieve room temperature magnon transport, where the films present an easy plane anisotropy, by dilute substitution of the ferric ions in order to alters the magnetic anisotropy axes of the thin films.References: [1] T. Jungwirth, J. Sinova, A. Manchon et al., Nat. Phys. 14, 200-203 (2018) [2] R. Lebrun, A. Ross, O. Gomonay et al., Comm. Physics 2 50 (2019) [3] A. Ross, R. Lebrun, C. Ulloa et al., arXiv:2001.03117 (2020) [4] R. Lebrun, A. Ross, S. Bender et al., Nature 561, 222-225 (2018) [5] S. Takei, B. Halperin, A. Yacoby et al., Phys. Rev. B, 90, 94408 (2014) [6] S. Bender, H. Skarsvåg, A. Brataas et al., Phys. Rev. Lett. 19, 056804 (2017) [7] H. Wang, C. Du, P. C. Hammel et al., Phys. Rev. B 91, 220410 (2015) [8] A. Ross, R. Lebrun, O. Gomonay et al., Nano Letters 20 1, 306-313 (2020)

Efficient Magnon Transport in Insulating Antiferromagnets Governed by Domain Structures and Doping

This work aims at designing a closed-loop magnetic circuit to enhance the uniformity and intensity of the magnetic field based on an internal conductive square Magnetohydrodynamics (MHD) thruster. Figure 1(a) shows the diagram of the square MHD thruster consisting of a pair of NdFeB magnets and electrodes. Figure 1(b) shows the design of the permanent magnet set comprising two permanent magnet pieces and an iron shell. The magnetization of the magnet pair is along the +z axis, and the iron shell formed a closed-loop magnetic circuit which reduces the magnetic leakage and enhances the magnetic field at the central region i.e. z=0 mm. Theoretically, the field strength decreases when the distance from the magnet is increased. The accurate measured magnetic flux density along the y-axis on the central cross-section shown in figure 1(c) reveals that the maximum field strength increases from 0.116 T to 0.142 T after installing the 2-mm thick iron shell, which reduces the magnetic leakage and increases the uniformity of the field. We performed a systematic experiment to measure the field distribution under various width and thickness of iron casing in this work. It is noticed the thicker shell results in stronger magnetic strength inside the thruster, and the field strength becomes stronger as the width of the iron shell decreases. The thruster with an iron shell is fully submerged in the saline water and hanged on a sliding track with negligible friction to verify its performance, as shown in figure 1(d). The moving speed is increased from 0.0826m/s to 0.12 m/s when the iron shell is installed. In this paper, the effect of the material of the shell is also investigated to evaluate the optimal design to enhance the uniformity and strength of the magnetic field at the central region of square MHD thruster.

Enhancement for magnetic field strength of a magnetohydrodynamic thruster consisting of permanent magnets

Magnetic Josephson junctions, where a magnetic material is sandwiched by superconductors, show peculiar phenomena such as π junction and spin triplet due to the exchange energy of the magnetic material. Among the magnetic Josephson junctions, ferromagnetic materials have been commonly used, and the π state has been reported in ferromagnetic CuNi1). However, ferromagnet has stray field, which would affect to superconductivity. To avoid the effect of stray field, we focus on antiferromagnet Mn3GaN (MGN). MGN is a non-collinear antiferromagnetic material in which magnetic moments cancel each other out noncollinearly and has characteristic properties not found in collinear antiferromagnetic materials.2) By using MGN in the Josephson junction, not only suppress the deterioration of the superconducting characteristics, but also unconventional magnetic junction characteristics is expected. In this study, we fabricated NbN/MGN/NbN hetero-stacked structures and evaluated the crystal structure and junction characteristics. NbN and MGN layers were deposited on a MgO(001) substrate by reactive magnetron sputtering. Figure 1 shows the out-of-plane XRD pattern of NbN/MGN bilayers. Both NbN and MGN(002) and (004) peaks were observed, indicating that (001)-oriented NbN/MGN bilayers grow on the MgO(001) substrates. Square mesa junctions of NbN/MGN/NbN with in-plane size L = 20 - 100 μm were fabricated by a conventional photolithographic and liftoff process. Figure 2 shows the current-voltage properties of the junction with 2.5 nm MGN layer. Thanks to the suppression of deterioration of superconducting properties at the junction interface, IcRN is as high as 3.1 mV, which is on the same order as the theoretical value, indicating that the Josephson junctions using the non-collinear antiferromagnetic material MGN are feasible and good junction characteristics can be obtained. Our results show the possible further improvement of the magnetic junction characteristics by using non-collinear antiferromagnet and potential of non-collinear antiferromagnetic Josephson junctions.References: 1) T. Yamashita et al., Phys. Rev. Appl. 8, 054028 (2017) 2) T. Hajiri et al., Appl. Phys. Lett. 115, 052403 (2019)

Fabrication and characterization of NbN magnetic Josephson junctions using non-collinear antiferromagnet Mn3GaN

We present results of Monte Carlo study of non-equilibrium critical behavior in three-dimensional Heisenberg model in isotropic case and with anisotropy of easy axis type. We investigate relaxation Glauber-like dynamics of these models with evolution from different initial states. Usually, we can distinguish the high-temperature initial states created at temperatures T0 > Tc with initial magnetization m0 = 0 and low-temperature initial states with T0 < Tc and m0 ≠ 0. Initial states with m0 ≠ 0 give rise to a new time scale tm ~m0-k with the exponent k > 0. This time scale produces a pronounced effect on the temporal behavior of magnetization, autocorrelation function and response function [1-4]. Study of influence of initial states with 0 < m0 ≤ 1 on dynamical properties of model has been carried out. It is established realization of scaling relation for time dependence of magnetization. We calculated critical exponents for isotropic and anisotropic models with values β/zν= 0.2565(4) and z=2.035(4) for the anisotropic Heisenberg model which agree well with the exponents for the 3D Ising model. Aging has been revealed during study of two-time dependence of the autocorrelation function and dynamical susceptibility on different initial states which are characterized by the slowing down with growth of its age tw. We show that the slowing down of the correlation function during evolution from high-temperature initial state is considerably slower than in case of low-temperature initial state. Analysis of parametrical dependence of the dynamical susceptibility on the correlation function gave possibility to determine asymptotical values of the fluctuation-dissipation ratio (FDR) for isotropic X∞=0.383(6) and anisotropic X∞=0.392(7) Heisenberg models with evolution from high-temperature initial states. The obtained value of FDO for anisotropic model agrees very well with X∞=0.391(12) for the 3D Ising model calculated in [4] by the same method. This study is supported by RFBR (projects 18-42-550003, 20-32-70189), the Ministry of Education and Science of Russian Federation (state assignment 0741-2020-0002) and grant MD-2229.2020.2 of the President of the Russian Federation.References: 1. V.V. Prudnikov, P.V. Prudnikov and M.V. Mamonova, Phys. Usp., 60, 762 (2017). 2. V.V. Prudnikov, P.V. Prudnikov and E.A. Pospelov, J. Stat. Mech., 2016, 043303 (2016). 3. V.V. Prudnikov, P.V. Prudnikov, E.A. Pospelov and A N. Vakilov, Phys. Lett. A 379, 774 (2015). 4. P.V. Prudnikov, V.V. Prudnikov, E.A. Pospelov and P.N. Malyarenko, PTEP, 2015, 053A01 (2015).

Nonequilibrium Critical Behavior of the 3D Isotropic and Anisotropic Heisenberg Models

So far, finding Majorana fermion is one of the important research in condensed matter physics. In theory, zero energy Majorana modes resides on the edge or vortex core in topological superconductor, characterizing the topological phase of matter protected by particle hole symmetry. Zero bias conductance peak is one of the phenomena from Andreev reflection process in normal metal/topological interface. In this work, we use the non-equilibrium Green's function method to study the interface between the normal metal and two dimensional noncentrosymmetric superconductor which lack a center of inversion symmetry and exhibit spin orbital coupling. The tight binding hamiltonian[1] for noncentrosymmetric superconductor is used. Then, we simulate the selective equal spin Andreev reflection[2]. Equal spin Andreev reflection means if the electrons in normal metal has the same spin polarization with the electrons on the edge state of superconductor, they are reflected as holes with the same spin. Moreover, with opposite spin polarization between electrons in normal metal and electrons in the edge of superconductor, they are reflected as electrons with same spin. As the result, we can get the conductance as the function of the spin polarized angle of electron in normal metal to realize the properties of Majoranana edge state. In addition, the spin current can be injected to the noncentrosymmetric superconductor which allow both singlet and triplet cooper pairs, generating the new spintronic device.References: 1. M. Biderang, H.Yavari, Mohammad-Hossein Zare, PHYSICAL REVIEW B 98, 014524(2018) 2. PA. Lee, PHYSICAL REVIEW LETTERS 112, 037001(2014)

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X-ray Magnetic Linear Dichroism Studies of Electrical Switching of Antiferromagnetic Order in α-Fe2O3 Epitaxial Films

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