Researchers at the University of Gothenburg, Helmholtz-Zentrum Berlin (HZB), the University of Cambridge and the Max Planck Institute for Solid State Research have, for the first time, directly imaged the magnetization dynamics inside a spin Hall nano-oscillator (SHNO). Using time-resolved scanning transmission X-ray microscopy, the team observed spin-wave behavior that earlier, indirect measurements had missed. The team also found that prolonged X-ray exposure permanently altered the devices' CoFeB/MgO interface.
In an SHNO, a direct current flowing through a heavy-metal layer generates spin-orbit torque in an adjacent ultrathin magnetic layer through the spin Hall effect. At a nanoscale constriction, the torque is strong enough to cancel magnetic damping and sustain steady precession of the magnetization, turning a DC input into a tunable microwave signal. SHNOs are simple to fabricate, CMOS-compatible and can mutually synchronize in large arrays, which makes them candidates for tunable microwave sources and for neuromorphic computing. The dynamics inside a single oscillator, however, play out over nanometers and fractions of a nanosecond. Until now they could only be inferred from indirect techniques.
The team fabricated SHNOs from tungsten-tantalum/CoFeB/MgO stacks, with a CoFeB layer just 1.4 nm thick. The devices were imaged at the MAXYMUS X-ray microscope at HZB's BESSY II synchrotron. To make stroboscopic imaging possible, the researchers locked the oscillator to an external signal at twice its 6.17 GHz oscillation frequency, which synchronized its dynamics with the microscope.
The images showed that the oscillations were concentrated at the two edges of the constriction, with a strong asymmetry favoring one edge. The emitted spin waves traveled mainly in one direction, perpendicular to the applied magnetic field. Micromagnetic simulations could only reproduce these observations once three usually neglected effects were included: the film's polycrystalline grain structure, a roughly 8% reduction in perpendicular magnetic anisotropy that the team attributes to nanofabrication, and the interfacial Dzyaloshinskii–Moriya interaction (DMI). According to the researchers, this is the first time DMI has had to be included explicitly to match SHNO experiments. They argue that the models used to design SHNOs and their synchronized networks need to be refined. "Effects like grain boundaries and the Dzyaloshinskii–Moriya interaction turned out to be essential, not optional," said Sebastian Wintz, who led the measurements at BESSY II.
The team also found that intense soft X-ray exposure changed the devices gradually and permanently. It shifted their oscillation frequency by hundreds of MHz and, in some cases, stopped them from oscillating at all. Because the changes depended on the total X-ray dose rather than on heating, the researchers attribute them to chemical changes at the CoFeB/MgO interface. They note that the effect concerns this particular material combination, not magnetic materials in general. CoFeB/MgO is, however, a standard building block of spintronic devices, including MRAM, and is often described as radiation-hard. The researchers say its behavior under high radiation doses deserves closer study, particularly for spacecraft and satellites.
Johan Åkerman, who leads the Gothenburg team, said the direct imaging provides "a much firmer microscopic basis" for understanding how spin waves set the coupling and phase relationships within synchronized SHNO networks.