Voltage controlled magnetism in 3d transitional metals

Summary form only given. Controlling the magnetic properties of solids by electric fields has been an interesting research subject, not only because of the intriguing correlation between the electric and magnetic orders in solid-state systems, but also the potential applications in ultra-low energy spintronic devices. In the past, research has mostly been carried out with multiferroic materials and magnetic semiconductors. Recently, more effort was focused on 3d transition ferromagnetic metals. Especially in heavy metal/ferromagnet/oxide (HM/FM/oxide) structures where the magnetic anisotropy has an interfacial origin, electric fields can cause a marked change in the magnetic anisotropy energy. This voltage-controlled magnetic anisotropy (VCMA) can be understood by the electric field induced charge transfer among different d orbitals of the FM. The order of this effect is around 100 fJ/Vm and it vanishes with the removing of the electric fields. Here we demonstrate another approach to alter the magnetism by electrically controlling the oxidation state of the 3d FM at the FM/oxide interface . The thin FM film sandwiched between a heavy metal layer and a gate oxide can be reversibly changed from an optimally-oxidized state with a strong perpendicular magnetic anisotropy to a metallic state with an in-plane magnetic anisotropy, or to a fully-oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields . This is a voltage controlled magnetism (VCM) effect, where both the saturation magnetization and anisotropy field of the 3d FM layer can be simultaneously controlled by voltage in a non-volatile fashion . Although at present the speed of this effect is slow, the magnitude of magnetic anisotropy change can reach > 10 pJ/Vm, much larger than that of previous VCMA effects . A very different time dependence on voltages with different polarities was observed, reflecting the asymmetric energy barrier at the FM/o- ide interface . We will also present the behavior of the VCM effect in different HM/FM/oxide systems, and its impact on magnetic tunnel junctions and spin Hall switching experiments . This work was supported in part by NSF (ECCS-1310338) and by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA . Work at Argonne National Laboratory was supported by the US Department of Energy .