A Torque on Conventional Magnetic Wisdom


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Physicists at the University of Illinois at Urbana-Champaign have observed a magnetic phenomenon called the "anomalous spin-orbit torque" (ASOT) for the first time. Professor Virginia Lorenz and graduate student Wenrui Wang, now graduated and employed as an industry scientist, made this observation, demonstrating that there exists competition between what is known as spin-orbit coupling and the alignment of an electron spin to the magnetization. This can be thought of as analogous to the anomalous Hall effect (AHE).

For a long time now, physicists have known about interesting phenomena such as the AHE in which spins of a certain species accumulate on a film edge. Their accumulations are detectable with electric measurements. This type of experiment requires the magnetization of the film to point perpendicular to the plane of the film. In fact, the Hall effect and similar experiments such as the AHE in the past all use an applied magnetic field (for non-magnetic samples) or the magnetization of the film (for magnetic samples), always perpendicular to the plane of the film.

Effects like the AHE had not been found for magnetizations that point in-plane, until now.

By taking advantage of the magneto-optic Kerr effect (MOKE), which can probe the magnetization near the surface of a magnetic sample, Wang and Lorenz demonstrated that an electrical current modifies the magnetization near the surface of a ferromagnetic sample to point in a direction different from the magnetization of the interior of the sample. It is not necessarily strange that the magnetization near the surface can differ from that in the interior, as evidenced by previous experiments in spin-orbit torque. However, the Illinois researchers used a purely ferromagnetic film, whereas past experiments in spin-orbit torque combined ferromagnets with metals that have a property called "spin-orbit coupling."

This discovery has implications for energy-efficient magnetic-memory technology.

Magnetism & conventional spin-orbit torque

Magnetism is ubiquitous--we use it every day, for example, to stick papers to a refrigerator door or to ensure that our phone chargers do not detach prematurely.

Microscopically, magnetism arises from a collection of electrons, which all have a property known as spin. Spin is one source of angular momentum for electrons and its "movement" can be likened to how toy tops spin--though in actuality, in quantum mechanics, the motion of spin does not resemble anything in classical mechanics. For electrons, spin comes in two species, formally called up spin and down spin. Depending on how the spins collectively point, a material might be ferromagnetic, having neighboring electron spins all pointing in the same direction, or antiferromagnetic, having neighboring electron spins pointing in opposite directions. These are just two of several types of magnetism. 

But what happens when magnetism is combined with other phenomena such as spin-orbit coupling?

Lorenz notes, "There is an entire family of effects that are generated from simply running an electric current through a sample and having the spins separate. The anomalous Hall effect occurs in thin ferromagnetic films and is seen as the accumulation of spins on the edges of the sample. If the magnetization points out of the plane of the film--that is, perpendicular to the plane of the sample surface--and a current flows perpendicular to the magnetization, then accumulations of spins can be seen. But this happens only if the ferromagnetic film also has spin-orbit coupling."

Spin-orbit coupling causes the spin species--up or down--to move strictly in certain directions. As a simplistic model, from the point of view of electrons moving through a film, they can scatter to the left or right if something interrupts their movement. Interestingly, the spins are sorted based on the direction that an electron moves. If the left-scattered electrons have spin up, then the right-scattered electrons must have spin down and vice versa.

Ultimately, this leads to up spins accumulating on one edge of the film and down spins accumulating on the opposite edge.

Conventional spin-orbit torque (SOT) has been found in bilayer structures of a ferromagnetic film adjacent to a metal with spin-orbit coupling.

Lorenz points out, "In the past, this has always happened with two layers. You need not just a ferromagnet, but also some source for the spins to separate to induce a change in the ferromagnet itself."

If a current flows through the spin-orbit coupled metal, the up and down spins separate like in the AHE. One of those spin species will accumulate at the interface where the ferromagnet and the metal meet. The presence of those spins affects the magnetization in the ferromagnet near the interface by tilting the spins there.

Lorenz continues, "It was always assumed--or at least not investigated heavily--that we need these metals with a strong spin-orbit coupling to even see a change in the ferromagnet."

The results of Wang and Lorenz's experiment now directly challenge this assumption. 

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