Magnetism and Interface Physics

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Spintronics in Antiferromagnets

Antiferromagnets play a major role in condensed matter physics, not only as a specific class of magnetic materials, but also as a playground for investigating strongly correlated electron physics, terahertz spin dynamics, and exotic spin excitations. The fascination in these materials has taken an unexpected turn in the last decade, with the discovery of topological charge and spin Hall effects in compensated noncollinear magnetic systems on the one hand, and current-driven manipulation of antiferromagnetic order on the other. These phenomena have opened up a new field of research, in which antiferromagnetic order, topological band structure, and spintronic effects intersect with far reaching consequences.
Early measurements have been performed on CuMnAs, a collinear antiferromagnets in which the Mn atoms occupy inversion-partner lattice sites. Upon an injection of an electric current through CuMnAs, a local relativistic spin-orbit torque is generated which points in the opposite direction on magnetic atoms with opposite magnetic moments. The staggered relativistic torque can induce switching of the antiferromagnetic order parameter on an ultrafast timescale. High density electric currents also induce additional effects due to Joule heating, as shown, e.g., in Pt/NiO bilayers.

Obitronics Image
Figure 1. Schematic of an NV microscope tip scanning the surface of CuMnAs. The red/blue color scale indicates the intensity of the magnetic stray field produced by the domain walls of CuMnAs. From Wörnle et al. , arxiv:1912.05287 (2019).

A very interesting material is Mn3Sn, a Weyl metal with chiral antiferromagnetic order, which exhibits anomalous Hall and Nernst effects comparable to ferromagnets as well as efficient current-induced switching of the antiferromagnetic order parameter. Strong interest in this material is also motivated by the prospects of realizing low-power and ultrafast antiferromagnetic memories, which would integrate or replace thin film devices based on conventional ferromagnets. Yet, despite the increasing attention devoted to both fundamental and applied aspects of noncollinear antiferromagnets, little is known about the mechanisms and timescales that govern the reversal of chiral antiferromagnetic order.

Pt/Mn3Sn Bilayers
Figure 2. (a) Schematic of the time-resolved Hall effect measurements of Pt/Mn3Sn Bilayers. (b) TEM image of Mn3Sn grown by magnetron sputtering. (c) Time resolved switching traces of the magnetic octupole vector of Mn3Sn showing current-induced demagnetization. (d) Multistate switching depending on the fall time of the current pulses. Arrows indicate the possible orientation of the magnetic octupole moment in each state. From Krishnaswamy et al., Phys. Rev. Appl. 18, 024064 (2022).

Our work provided the first description of the current-driven reversal of chiral antiferromagnetic order in Pt/Mn3Sn bilayers in the time domain. Hall effect measurements for current pulses with rise and fall times ranging from μs to ns reveal that antiferromagnetic switching proceeds via a two-step demagnetization-remagnetization process that lasts several tens of nanoseconds and is explained by the timed interplay of current-induced heating and spin-orbit torques generated by a Pt or W layer adjacent to Mn3Sn. We further demonstrate the occurrence of tristable or bistable switching depending on the temporal structural of the current pulses. Time-resolved measurements indicate that chiral spin rotation is either damped or incoherent in Mn3Sn. These results provide unique insight into the switching timescale and dynamics of antiferromagnets, highlighting differences with respect to ferromagnets and collinear and noncollinear antiferromagnets.

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