Interfacial orbital torques excite nanoscale terahertz magnons

Inside the smallest electronic components, where data moves at the speed of light, there exists a hidden world of magnetic vibrations. These are not the slow, steady shifts of a compass needle, but rapid, high-frequency ripples traveling through the magnetic material itself. Scientists call these ripples magnons. When these ripples move slowly, they are easy to control with standard magnetic fields. However, when they are forced to vibrate at incredibly fast rates—reaching the terahertz range, which is a trillion cycles per second—they become extremely difficult to reach. At these speeds, the waves are so short that they fit only a few times across the thickness of a nanometer-scale film. To make them move, researchers need a driving force that acts right at the surface, a localized push that can shake the entire magnetic layer without needing a massive, uniform field. For years, the leading theory suggested that the key to unlocking these ultrafast vibrations lay in the flow of electron spin, a fundamental property of electrons that makes them act like tiny magnets.

A team of researchers at the University of Konstanz and Uppsala University has now looked deeper into this mechanism, revealing that the story is more complex than previously thought. By building a detailed computer model of a thin magnetic film sandwiched between two non-magnetic layers, they simulated how these materials respond to a burst of terahertz light. Their work challenges the long-held assumption that the flow of electron spin is the primary driver. Instead, they found that a different, often overlooked property of electrons—their orbital motion, which describes how they swirl around the atomic nucleus—is the true engine behind these high-speed vibrations. The researchers discovered that when an electric field hits the interface between the layers, it generates a specific type of torque, or twisting force, that is dominated by this orbital motion. This force is far more effective at exciting the desired high-frequency waves than the traditional spin-based forces, offering a new path for designing faster and more efficient spintronic devices.

The researchers focused on a specific setup: a thin layer of cobalt, a magnetic metal, placed between layers of platinum, a non-magnetic metal. This arrangement creates two distinct interfaces where the magnetic and non-magnetic materials meet. In recent experiments, scientists observed that firing a single-cycle pulse of terahertz light at this structure could excite these high-frequency magnetic waves. However, the microscopic origin of the force causing this excitation remained a mystery. The prevailing view was that the electric current generated by the light pulse caused electrons to accumulate spin angular momentum at the interface, which then transferred to the magnetic layer to start the vibration. This process is known as spin-orbit torque. Yet, recent theoretical work suggested that the same electric current also generates a significant accumulation of orbital angular momentum, a phenomenon that had been largely ignored in the context of driving these specific magnetic waves.

To untangle these competing effects, the team constructed a sophisticated atomistic model. This model allowed them to simulate the behavior of every single atom in the cobalt layer, tracking how the spins and orbital moments of the electrons responded to the incoming terahertz pulse. They could then separate the total twisting force into its constituent parts: the force coming from electron spin and the force coming from electron orbital motion. They also distinguished between two different types of twisting forces. One type acts like a magnetic field, pushing the magnetization in a specific direction regardless of how the magnet is currently oriented. The other type acts like a damper, either helping or hindering the motion depending on the direction of the magnetization. By running simulations where they could turn these different forces on and off independently, the researchers were able to see exactly which one was responsible for the observed vibrations.

The results were clear and decisive. When the researchers simulated the system using only the spin-based torque, the high-frequency magnetic waves were barely excited, even with strong electric fields. However, when they included the orbital torque, the waves appeared with strong, clear amplitudes. The orbital torque proved to be the dominant driver, responsible for the efficient excitation of these exchange-dominated terahertz magnons. Furthermore, the study showed that the specific type of orbital torque that acted like a magnetic field, independent of the magnetization's direction, was the most effective component. This finding explains why the vibrations are so efficiently generated in these thin films. The orbital torque is highly localized right at the interface, creating a sharp, non-uniform push that perfectly matches the symmetry required to launch these short-wavelength standing waves. In contrast, the spin-based torque, which depends heavily on the direction of the magnetization, becomes less effective as the magnetization is shaken rapidly by the terahertz pulse.

The researchers also explored how the thickness of the magnetic film influenced these results. They simulated films with different numbers of atomic layers, ranging from very thin to slightly thicker. In the thicker films, they observed a rich spectrum of different vibrational modes, with frequencies extending up to 3.7 terahertz. As the film became thinner, the spacing between these frequencies increased, pushing the energy of the waves even higher. Crucially, the pattern of which waves were excited depended on the symmetry of the interfaces. When the two interfaces of the film were identical, the system favored certain types of waves, while different interfaces favored others. This behavior matched perfectly with recent experimental observations, confirming that the model accurately captured the physical reality. The simulations showed that the orbital torque was the key to unlocking these specific modes, acting as the primary channel for transferring angular momentum from the electric field to the magnetic layer.

This discovery reshapes the understanding of how ultrafast magnetic dynamics work in modern materials. For a long time, the focus in spintronics has been almost exclusively on the spin of the electron. This study demonstrates that the orbital motion of the electron plays an equally, if not more, critical role in driving high-frequency magnetic phenomena. The orbital torque is not only stronger in this context but also more robust, maintaining its effectiveness even when the magnetic environment is changing rapidly. This suggests that future devices designed to operate at terahertz speeds should be engineered to maximize these orbital effects. By carefully choosing the materials and designing the interfaces to enhance the orbital angular momentum transfer, scientists can create more efficient ways to generate and control these ultrafast magnetic waves. The work provides a microscopic foundation for a new approach to controlling spin dynamics, moving beyond the limitations of spin-only models and opening the door to a new generation of ultrafast magnetic technologies.