Electrical bias driven effects for spintronic applications : ferromagnetic and antiferromagnetic materials

Williamson, Morgan Cole
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The ever-present demand for computing technology advancement has not only compelled progress toward the refinement of conventional CMOS routes, but has also driven new, less orthodox schemes whose advantages lie in branching out to access more of a system’s degrees of freedom. Spintronics, one such alternative scheme now adopted by Intel and Samsung (among others), is being proven out for memory applications in mass production. Spintronics is a field of study that exploits electron spin as well as charge for encoding binary information. Although still using nanomagnets to store data, spintronics transcends previous hard disk drive technology by pursuing nonvolatile solid-state magnetic memory. In this work, we investigate voltage controlled magnetic anisotropy in magnetic tunnel junctions and study the transition metal oxide Sr₃Ir₂O₇ for use in antiferromagnetic spintronics, which aims at high speed, high density memory applications. In the antiferromagnetic Mott insulator Sr₃Ir₂O₇, we study the effects of an applied bias on its transport properties. For instance, we demonstrate that the bias can produce changes in the materials’ resistivity, including a reversible resistive switching, consistent with electric field driven lattice distortions. The strong spin-orbit coupling in Sr₃Ir₂O₇ locking the crystal structure to the magnetic moments suggests that any structural transition is important for controlling the magnetic order of AFM devices. We use time-based measurements to study the thermal activation behavior of the resistive switching process in Sr₃Ir₂O₇ and acquire information about the energy barrier associated with the transition, including dependence on applied bias and temperature. We also demonstrate that the high bias switching state displays an increased noise pattern indicative of a dynamical state. Our observations support the possibility of controlling magnetic order in TMOs using electrical bias. We also quantify the effects of VCMA in both perpendicular and in-plane MTJs using ferromagnetic resonance. In perpendicular MTJs, we observe a linear dependence of VCMA on applied voltage with a maximum virtual field of 40 mT and a saturation of the effect above 2 V. The effect of VCMA shows a quadratic dependence for in-plane MTJs with a maximum virtual field of 20 mT. With the mainstream adoption of MRAM, efforts studying VCMA and other electric field driven effects will be crucial to increasing the energy efficiency and viability of future spintronic memory devices