Chong Bi

Reversible Control of Co Magnetism by Voltage-Induced Oxidation

We demonstrate that magnetic properties of ultrathin Co films adjacent to Gd2O3 gate oxides can be directly manipulated by voltage. The Co films 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 an oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields. Consequently, an unprecedentedly large change of magnetic anisotropy energy up to 0.73 erg/cm(2) has been realized in a nonvolatile manner using gate voltages of only a few volts. These results open a new route to achieve ultralow energy magnetization manipulation in spintronic devices.

Anomalous spin-orbit torque switching in synthetic antiferromagnets

We report that synthetic antiferromagnets (SAFs) can be efficiently switched by spin-orbit torques (SOTs) and the switching scheme does not obey the usual SOT switching rule. We show that both the positive and negative spin Hall angle (SHA)-like switching can be observed in Pt/SAF structures with only positive SHA, depending on the strength of applied in-plane fields. A new switching mechanism directly arising from the asymmetric domain expansion is proposed to explain the anomalous switching behaviors. Contrary to the macrospin-based switching model that the SOT switching direction is determined by the sign of SHA, the new switching mechanism suggests that the SOT switching direction is dominated by the field-modulated domain wall motion and can be reversed even with the same sign of SHA. The new switching mechanism is further confirmed by the domain wall motion measurements. The anomalous switching behaviors provide important insights for understanding SOT switching mechanisms and also offer novel features for applications.

Perpendicular magnetic tunnel junction with W seed and capping layers

C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program; MARCO; DARPA; National Science Foundation [ECCS-1310338]; Inston, Inc., through a Phase II Small Business Innovation Research award from the National Science Foundation

Voltage-controlled interlayer coupling in perpendicularly magnetized magnetic tunnel junctions

Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdOx tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.

Temperature dependence of interlayer coupling in perpendicular magnetic tunnel junctions with GdOX barriers

Perpendicular magnetic tunnel junctions with GdOX tunneling barriers have shown a unique voltage controllable interlayer magnetic coupling effect. Here we investigate the quality of the GdOX barrier and the coupling mechanism in these junctions by examining the temperature dependence of the tunneling magnetoresistance and the interlayer coupling from room temperature down to 11 K. The barrier is shown to be of good quality with the spin independent conductance only contributing a small portion, 14%, to the total room temperature conductance, similar to AlOX and MgO barriers. The interlayer coupling, however, shows an anomalously strong temperature dependence including sign changes below 80 K. This non-trivial temperature dependence is not described by previous models of interlayer coupling and may be due to the large induced magnetic moment of the Gd ions in the barrier.

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/oxide 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 .