Speaker: Andrei Slavin, Oakland University, Rochester, MI
Title: Current-induced THz-frequency Magnetization Dynamics in Antiferromagnettic Crystals
Antiferromagnetic (AFM) materials have natural resonance frequencies in the sub-THz to THz frequency range. Thus, it is tempting to use antiferromagnets as active layers in THz-frequency spin-torque nano-oscillators (STNOs). However, a familiar mechanism of spin-transfer torque (STT) damping compensation used in ferromagnetic (FM) STNOs  does not work for the AFM materials. In the AFM two magnetic sublattices are aligned anti-parallel to each other, so, when the STT compensates damping in one of the sublattices, it increases the damping in the other sublattice, resulting in a zero net effect. At the same time, the STT can cause a lattice instability in an AFM. For example, it has been shown in , that the STT can lead to the reorientation of the order vector l in the AFM with cubic anisotropy.
In this work we propose a novel approach to the excitation of oscillations in AFM materials . In the framework of this approach the STT is used to change the effective energy landscape of the AFM. We show theoretically, that in a bi-axial AFM (such as NiO ) the magnetic lattice can lose its stability under the action of STT, which results in a self-sustained precession of the order vector l of the AFM. We found that for NiO the lowest threshold of the self-sustained oscillations occurs for the STT directed along the hard axis of a single crystal NiO. The threshold of generation in this case is determined by the weak easy plane anisotropy (Ha1≈ 380 Oe in NiO) of the bi-axial AFM, and not by the Gilbert damping of the AFM. Above the generation threshold the AFM order vector l starts to precess in the AFM easy plane with the frequency defined by the magnitude of the STT and by the Gilbert damping in the AFM, see Fig. 2. The threshold of the self-sustained oscillations for the case of the STT directed along the easy axis of the AFM in several orders of magnitude higher, than in the case when the STT is directed along the AFM hard axis.
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Andrei Slavin received PhD degree in Physics in 1977 from the St.Petersburg Technical University, St. Petersburg, Russia. Dr. Slavin developed a state-of-the-art theory of spin-torque oscillators, which has numerous applications in the theory of current-driven magnetization dynamics in magnetic nanostructures. His current research support includes multiple grants from the U.S. Army, DARPA, SRC and the National Science Foundation. This research involves international collaborations with leading scientists in many countries, including Germany, Ukraine, France, Italy, and the United States. Dr. Slavin is a frequently invited speaker at magnetism conferences around the world.
Andrei Slavin is Fellow of the American Physical Society, Fellow of the IEEE, and Distinguished Professor and Chair of the Physics Department at the Oakland University, Rochester, Michigan, USA.