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Electrical Control of Magnetism in Metal/Oxide Heterostructures

Geoffrey Beach, MIT, Cambridge, USA

There has been great interest in using current-driven domain walls (DWs) in ferromagnetic nanotracks for high-performance memory and logic device applications [1]. Recent experiments show DWs in materials with perpendicular magnetic anisotropy (PMA) exhibit low critical currents for displacement, and can be driven at velocities of up to several hundred m/s by electric current alone [2,3]. These materials typically consist of ultrathin magnetic films sandwiched between nonmagnetic high-Z metals such as Pt, Au, or Pd, which generate PMA in the adjacent ferromagnet via interfacial spin-orbit coupling (SOC). As the ferromagnetic layers are usually only a few monolayers thick, interfacial SOC can facilitate new current-induced torques that dramatically enhance the efficiency of current-driven DW motion.

We first describe DW dynamics in Co/Pt nanotracks spanning eight decades in velocity, from deep in the thermally-activated creep regime up to the high-velocity flow regime [4]. We find that in Co/Pt multilayers with relatively thick Co layers, the effect of current on DW motion can be fully accounted for by Joule heating. However, when the Co layer thickness is reduced to just 3 Å, current-driven DW motion becomes surprisingly efficient, pointing to an interfacial origin to the current-induced torque. In asymmetric stacks of Pt/Co/GdOx, we find an even greater enhancement in current-induced DW motion, [5] explained in terms of Rashba spin-orbit coupling due to broken inversion symmetry [3].

We then show that magnetism at the metal/oxide interface can be directly controlled by an applied electric field, which modulates the interfacial electronic structure responsible for PMA. In Pt/Co/GdOx, DW velocity can be tuned by a gate voltage by modulating the activation energy barrier that governs DW creep dynamics [6]. The voltage-induced velocity enhancement scales linearly with the activation energy, which itself depends on the interfacial PMA. Finally, we show that functionally-active dielectric heterostructures can significantly enhance the magnetoelectric effect and introduce new device functionalities. By integrating a charge-trapping layer into the gate dielectric, we achieve the largest voltage-induced change to interfacial magnetic anisotropy yet demonstrated for a metallic thin film [7], and achieve long-term retention of the voltage-controlled magnetic state. In this system we can further exploit optically-induced charge trapping, which offers the unique opportunity to optically imprint the magnetic state in a continuous metal film [7].

Supported in part by NSF grant ECCS-1128439. The contributions of U. Bauer, S. Emori, and E. Rapoport (MIT) ; and M. Przybylski and J. Kirschner (Max Planck Institute, Halle, Germany) are gratefully acknowledged.

1. D.A. Allwood, et al., Science 309, 1688 (2005) ; S.S.P. Parkin, et al., Science 320, 190 (2008) ; J. Currivan et al., Magnetics Letters, IEEE 3, 3000104 (2012).

2. T. Koyama, et al., Nat Mater 10, 194 (2011).

3. I.M. Miron, et al., Nat Mater 10, 419 (2011).

4. S. Emori and G. S. D. Beach, J. Physics : Condens. Matter 24, 024214 (2012).

5. S. Emori, D. C. Bono, and G. S. D. Beach, submitted (2012).

6. U. Bauer, S. Emori, and G. S. D. Beach, Appl. Phys. Lett. 100, 192408 (2012).

7. U. Bauer, M. Przybylski, J. Kirschner, and G. S. D. Beach, Nano Lett. 12, 1437 (2012).