A Torque on Conventional Magnetic Wisdom
July 23, 2019 | University of Illinois College of EngineeringEstimated reading time: 7 minutes

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.
Page 1 of 2
Testimonial
"The I-Connect007 team is outstanding—kind, responsive, and a true marketing partner. Their design team created fresh, eye-catching ads, and their editorial support polished our content to let our brand shine. Thank you all! "
Sweeney Ng - CEE PCBSuggested Items
Driving Innovation: Depth Routing Processes—Achieving Unparalleled Precision in Complex PCBs
09/08/2025 | Kurt Palmer -- Column: Driving InnovationIn PCB manufacturing, the demand for increasingly complex and miniaturized designs continually pushes the boundaries of traditional fabrication methods, including depth routing. Success in these applications demands not only on robust machinery but also sophisticated control functions. PCB manufacturers rely on advanced machine features and process methodologies to meet their precise depth routing goals. Here, I’ll explore some crucial functions that empower manufacturers to master complex depth routing challenges.
Polar Instruments Announces Additive Transmission Line Support for Si9000e
08/20/2025 | Polar InstrumentsTransmission lines embedded into the PCB surface are a feature of UHDI constructions. The 2025 fall release of Polar's Si9000e PCB impedance & insertion loss transmission line field solver incorporates eight new single ended, differential and coplanar transmission line structures.
Henniker Plasma Launches Stratus Turnkey Plasma Manufacturing Cell
08/13/2025 | Henniker PlasmaHenniker Plasma, a leading manufacturer of plasma treatment systems, proudly announces the launch of its Stratus Plasma Manufacturing Cell range — a fully integrated, turnkey solution that combines advanced atmospheric plasma surface treatment with robotic automation.
Trouble in Your Tank: Metallizing Flexible Circuit Materials—Mitigating Deposit Stress
08/04/2025 | Michael Carano -- Column: Trouble in Your TankMetallizing materials, such as polyimide used for flexible circuitry and high-reliability multilayer printed wiring boards, provide a significant challenge for process engineers. Conventional electroless copper systems often require pre-treatments with hazardous chemicals or have a small process window to achieve uniform coverage without blistering. It all boils down to enhancing the adhesion of the thin film of electroless copper to these smooth surfaces.
Designers Notebook: Basic PCB Planning Criteria—Establishing Design Constraints
07/22/2025 | Vern Solberg -- Column: Designer's NotebookPrinted circuit board development flows more smoothly when all critical issues are predefined and understood from the start. As a basic planning strategy, the designer must first consider the product performance criteria, then determine the specific industry standards or specifications that the product must meet. Planning also includes a review of all significant issues that may affect the product’s manufacture, performance, reliability, overall quality, and safety.