A Torque on Conventional Magnetic Wisdom


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Observation of an anomalous spin-orbit torque

Wang and Lorenz found that it was unnecessary to place a metal with spin-orbit coupling adjacent to the ferromagnetic film in order to generate a SOT and observe an out-of-plane magnetization.

Wang comments, "Our work reveals a long-overlooked spin-orbit phenomenon, the anomalous spin-orbit torque, or ASOT, in well-studied metallic ferromagnetic materials such as permalloy. The ASOT not only complements the physics picture of electrical current-induced spin-orbit effects such as the anomalous Hall effect, but also opens the possibility of more efficient control of magnetism in spin-based computer memories."

The researchers ran a current from one edge of the film to its opposite and additionally forced the magnetization of the film to point in the same direction.

The physics here is complicated by the fact that there are two phenomena that are competing--magnetization and spin-orbit coupling. Magnetization is working to align the spin with itself; the electron spins like a top, but over time it aligns with the magnetization and stops its precession. Without spin-orbit coupling, this would mean that the magnetization on all edges would point in the same direction. However, spin-orbit coupling is working to maintain the spin's direction with the movement of the electron. When spin-orbit coupling and magnetization compete, the outcome is a compromise: the spin is halfway between the two effects.

Professor David Cahill, who also collaborated on the experiments at the University of Illinois, explains: "Ultimately, spins that accumulate on the surface of the film end up pointing partially out of the surface plane and spins that accumulate on the oppositely facing surface point partially out of the surface plane in the opposite direction."

Unlike the AHE, the ASOT cannot be detected electrically, so Wang and Lorenz employed MOKE measurements, shooting lasers at two exposed surfaces to show that the magnetization pointed out of the plane of the surface.

Lorenz credits her collaborator, Professor Xin Fan of the University of Denver, with conceiving of this experiment.

Fan explains, "MOKE is an effect to describe the change in polarization as the light is reflected from the surface of a magnetic material. The polarization change is directly correlated to the magnetization and light has a small penetration depth into the sample, which makes it popular to use as a surface probe for magnetization."

But that's not all. The researchers noted that the exchange interaction can suppress the effects of ASOT, so they carefully chose a sample that was thick enough that the spins on the two sides of the sample could not force each other to point in the same direction.

Wang and Lorenz demonstrated that on the two surfaces of the film where spins accumulate, the same Kerr rotation is observed. Technically, the Kerr rotation refers to how the reflected light changes its polarization, which is directly correlated with how the magnetization is rotated out of the plane of the permalloy film. This is indisputable evidence of ASOT.

Additional confirmation of the research findings come from theoretical work. The researchers have run simulations using their phenomenological model to show that there is strong agreement with their data. Additionally, theorist collaborators have also used density functional theory--a type of modelling that looks microscopically at atoms rather than assuming the properties of objects--to show qualitative agreement with experiment.

Lorenz notes that Stanford University Adjunct Professor and Lawrence Lab Staff Scientist Hendrick Ohldag made seminal contributions to the conception of the experiment. Lorenz says the experiment also benefited from contributions of collaborators at the Illinois Materials Research Science and Engineering Center, the University of Denver, the University of Delaware, and the National Institute of Standards and Technology in Maryland and Colorado.

Lorenz emphasizes, "What we've shown now is that a ferromagnet can induce a change in its own magnetization. This could be a boon to the research and development of magnetic memory technology."

Fan adds, "While spin-orbit torque in ferromagnet/metal bilayers has been demonstrated to have great potential in future-generation magnetic memories, because of the electric control of magnetization, our result shows that the ferromagnet can generate very strong spin-orbit torque on itself. If we can properly harness the spin-orbit coupling of the ferromagnet itself, we may be able to build more energy-efficient magnetic memories."

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