Magneto-Thermal Microscopy of Spin-Torque Switching and Uncompensated Moments in Antiferromagnetic Materials

Gregory Fuchs, Cornell University

May 12, 2020

A key challenge for the emerging field of antiferromagnetic spintronics is the measurement of quantities that characterize order in antiferromagnetic materials.  These include the orientation of staggered spin (Néel) order and the orientation and pinning of uncompensated moments (UMs) that ubiquitously accompany and interact with Néel order in real materials.  Imaging these quantities is especially valuable for understanding the microscopic mechanisms of spin-orbit torque manipulation Néel order, which is often spatially nonuniform and can proceed via multiple mechanisms.  Here I will discuss magneto-thermal microscopy as a table-top tool for antiferromagnetic spintronics.  First we demonstrate the interfacial spin Seebeck effect in Pt/NiO and Pt/NiO/Pt thin film heterostructures. Using spin Seebeck microscopy we image changes in Néel order in response to current pulses, revealing both domain rotation and domain wall motion [1].  Next we apply anomalous Nernst microscopy to investigate the micron-scale UMs in FeRh thin films.  This material is antiferromagnetic at room temperature, but exhibits a first-order phase transition to a ferromagnetic state near 400 K that can be tuned via stoichiometry. FeRh provides the unique opportunity to directly study uncompensated moments in the antiferromagnetic phase and the interaction of UMs with emergent ferromagnetism near the transition temperature.  We find that the UMs cause local vertical exchange bias, and create emergent FM order that is a direct probe of UMs [2].  We also demonstrate the picosecond photothermal writing and re-writing of stable ferromagnetic structures in an antiferromagnetic background [3].

[1] Isaiah Gray, Takahiro Moriyama, Nikhil Sivadas, Gregory M. Stiehl, John T. Heron, Ryan Need, Brian J. Kirby, David H. Low, Katja C. Nowack, Darrell G. Schlom, Daniel C. Ralph, Teruo Ono, and Gregory D. Fuchs, Phys. Rev. X 9, 041016 (2019).
[2] Isaiah Gray, Gregory M. Stiehl, John T. Heron, Antonio B. Mei, Darrell G. Schlom, Ramamoorthy Ramesh, Daniel C. Ralph, and Gregory D. Fuchs, Phys. Rev. Mater. 3, 124407 (2019).
[3] Antonio B. Mei, Isaiah Gray, Yongjian Tang, Jurgen Schubert, Don Werder, Jason Bartell, Daniel C. Ralph, Gregory D. Fuchs, and Darrell G. Schlom, arXiv: 1906.07239 (2019).