Freely Controlling a Precessing Magnet: Spindynamics

Yukio Nozaki (Associate Professor, Department of Physics)

Electrons in magnetic materials orbit the atomic nucleus while also spinning on their own axis, much like the Earth orbits the sun while rotating. This self-rotation of the electron, called "spin," is the source of the north and south poles of a permanent magnet, making it the world's smallest magnet. Now, as you probably know, when you bring a permanent magnet near a compass, the north pole, which was pointing north, rotates to be attracted to the south pole of the permanent magnet, aligning the needle with the magnetic field lines around the magnet. This is a familiar phenomenon studied in elementary school science. So, does an electron spin rotate in the same way, aligning with the magnetic field lines when a magnet is brought near? Actually, it moves differently from a compass needle. If you stand a spinning top on a desk and then release it, it will immediately fall over due to gravity. However, if you give the top angular momentum by spinning it, it does not fall immediately but instead slowly wobbles as it gradually tips over. Because electrons also possess angular momentum, called spin, they behave similarly to a top, wobbling as they gradually align with the direction of the magnetic field lines (Figure 1).

However, unlike a top, an electron spin rotates perpetually, so any attempt to change its direction will inevitably cause a wobbling motion. This wobbling motion is called "precession," and taming it is crucial for freely controlling the direction of a magnet's north and south poles. Incidentally, the precession of electron spins within a magnet can only be observed for an extremely short time, about one-billionth of a second (one nanosecond), after a magnet is brought near. The precession is converted into thermal energy through friction, and after about one nanosecond, the spin aligns with the direction of the magnetic field lines. Conventional magnetic devices (devices and components that use magnets for functionality) had operating times longer than one nanosecond, so it was safe to assume that the magnetization (the direction of the north and south poles) was always aligned with the direction of the magnetic field (the direction of the magnetic field lines). However, with the recent demand for higher speeds in electronic equipment, it has become necessary to complete functional operations within the time frame that precession occurs. To freely control precession, it is necessary to control the four moments of force (torques) that are orthogonal to the spin angular momentum (Figure 1). Previously, precession was controlled by magnetic fields, but recently it has been discovered that the four torques can be controlled by electric fields, heat, light, microwaves, spin currents (a flow of aligned electron spins), and mechanical vibrations. However, many aspects of the relationship between the generation efficiency of each torque and the material remain unexplained, and research to clarify these points is being actively pursued. Furthermore, in magnetic materials, there are numerous electron spins, and each spin moves while influencing the others. This gives rise to a wide variety of motional modes (spin waves) that do not appear with a single electron spin. If the excitation of spin waves can be controlled using the four torques acting on the spin, it is expected that high-performance next-generation magnetic devices, such as 3D magnetic recording (Figure 2) and high-speed, low-power logic circuits, can be realized.