Mechanism of Néel Order Switching in Antiferromagnetic Thin Films Revealed by Magnetotransport and Direct Imaging

Abstract

We probe the current-induced magnetic switching of insulating antiferromagnet/heavy metals systems, by electrical spin Hall magnetoresistance measurements and direct imaging, identifying a reversal occurring by domain wall (DW) motion. We observe switching of more than one third of the antiferromagnetic domains by the application of current pulses. Our data reveal two different magnetic switching mechanisms leading together to an efficient switching, namely the spin-current induced effective magnetic anisotropy variation and the action of the spin torque on the DWs. 2 MANUSCRIPTElectrical read-out and writing of the antiferromagnetic state is crucial to exploit the properties of antiferromagnets in future spintronic devices. Antiferromagnetic materials have the potential for ultrafast operation [1], with spin dynamics in the terahertz range, high packing density, due to the absence of stray magnetic fields, and an insensitivity to magnetic fields [2,3]. Furthermore, low-power operation is possible in antiferromagnetic insulators (AFM-Is) due to long spin diffusion lengths [4] and the theoretical prediction of superfluid spin transport [5].Recently, the electrical reading of the Néel order (n) orientation in AFM-Is was demonstrated via spin Hall magnetoresistance (SMR) [6-10], a magnetoresistive effect depending on the mutual orientation of the magnetic order and an interfacial spin accumulation μs. However, one of the main challenges faced by AFM spintronics is the reliable electrical writing of the orientation of n. One possible approach exploits staggered Néel spin orbit torques [11], creating an effective field of opposite sign on each magnetic sublattice. However, these torques rely on special material requirements, which has limited their application to the conducting AFMs CuMnAs and Mn2Au [12][13][14][15][16]. Another approach would be to use the non-staggered, antidamping-like torque exerted by a spin accumulation at the interface of a heavy metal and an AFM-I. A charge current in the heavy metal layer can generate a transverse spin current via the spin Hall effect, creating antidamping-like torques in the antiferromagnet.The possibility of such switching was demonstrated in NiO(001)/Pt and Pt/NiO(111)/Pt [17,18], but the mechanisms are still debated. One of the possible mechanisms relies on spin-current induced domain wall (DW) motion [19], predicting that DWs with opposite chirality are driven in opposite directions, thus excluding the electrical signature of the switching when DWs with opposite chirality are equally probable. A second mechanism [18], based on the coherent rotation of n, predicts a current threshold ten times larger than that found experimentally. A third mechanism, based on field-like torques acting on uncompensated interfacial spins, requires perfectly flat interfaces [17]. Currently, none of these provides a consistent explanation of the effect.In this work we realize reliable current-induced switching in epitaxial antiferromagnetic NiO/Pt bilayers. We show that the magnetic state of ...

Electrical Switching of Tristate Antiferromagnetic Néel Order in α−Fe2O3 Epitaxial Films

“…for Ip along E3, E2, and E1, respectively, as expected from the angular dependence of the damping-like SOT induced SH-AHE. 6,9,32 To corroborate the results in Fig. 2d, we use an independent approach to control the Néel order by an applied field (H) which aligns n  H via the in-plane spin-flop (SF) transition once H exceeds the SF field.…”

“…During the switching of n from one easy axis to another, thermal fluctuation is expected 2,9 to help n overcome the potential barrier due to magnetocrystalline anisotropy. We measure the temperature (T) dependence of ΔRxy at Ip = 9 mA from 200 to 300 K in zero field as shown in Fig.…”

Quantitative Study on Current-Induced Effect in an Antiferromagnet Insulator/Pt Bilayer Film

“…Current induced magnetic switching has been recently reported in both metallic [1][2][3][4][5][6][7] and insulating [8][9][10][11] antiferromagnetic systems. Anisotropic magnetoresistance (AMR), spin Hall magnetoresistance (SMR) or related planar Hall resistance (PHR) has been generally employed to characterize the electrically induced 90° Né el vector switching.…”

“…However, unlike ferromagnetic systems, where the current induced spin torque can be calibrated by using external magnetic field as a standard [12][13][14] , it is much less straightforward to build a quantitative relationship between the change of resistance value and the magnitude of spin torque in antiferromagnets. So far X-ray based imaging techniques, which requires specialized facilities, have to be utilized to determine the ratio of switched magnetic domains 2,7,8,10 . Therefore, there is an urgent need for the development of an electrical measurement method that can be used to quantify the magnitude of current-induced effect in antiferromagnets.…”

The Heat in Antiferromagnetic Switching

“…The recent proposal of electrical switching via SOT of the antiferromagnetic (AFM) materials, with the potential of ushering in AFM spintronics with terahertz frequencies, has attracted much attention [13][14][15][16][17][18]. However, unlike FMs, AFMs have no net magnetization (M ¼ 0).…”