This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For the first time, researchers have directly imaged the magnetization dynamics inside a spin Hall nano-oscillator—a nanoscale device that converts direct current into tunable microwave signals and is a promising building block for energy-efficient wireless communication and brain-inspired computing. A Swedish–German team led by the University of Gothenburg and Helmholtz-Zentrum Berlin (HZB) achieved this using time-resolved scanning transmission X-ray microscopy at the MAXYMUS instrument at BESSY II.
The results, published in Advanced Materials, reveal spin-wave features that had escaped previous indirect measurement techniques. Spin Hall nano-oscillators (SHNOs) are among the most versatile devices in spintronics: A direct current driven through a nanometer-sized constriction sets the local magnetization into steady precession, turning a DC input into a tunable radio-frequency output. Because SHNOs are easy to fabricate, CMOS-compatible and can be mutually synchronized in large arrays, they are considered a model platform for tunable microwave sources and neuromorphic, wave-based computing.
Yet what happens inside a single oscillator had never been observed directly—the dynamics unfold on nanometer-length scales and within fractions of a nanosecond. The team at the University of Gothenburg fabricated SHNOs based on ultrathin CoFeB layers and characterized their microwave dynamics. To look inside the devices, the group teamed up with researchers at HZB and colleagues from the Max Planck Institute for Intelligent Systems in Stuttgart.
At the scanning transmission X-ray microscope MAXYMUS, operated at the BESSY II electron storage ring in Berlin, the researchers filmed the magnetization dynamics stroboscopically, using X-ray magnetic circular dichroism as a contrast mechanism. They achieved a spatial resolution of a few tens of nanometers and a time resolution well below the period of the oscillations at about 6 GHz. The X-ray movies show that the spin-wave auto-oscillations concentrate at the two edges of the nanoconstriction—but with a pronounced asymmetry favoring one edge—and that the emitted spin waves propagate in a strongly anisotropic way, moving out perpendicular to the applied magnetic field.
Micromagnetic simulations could reproduce these observations only after three effects that are usually neglected were taken into account. First, the magnetic film is not perfectly uniform but consists of tiny crystal grains whose boundaries affect the spin waves. Second, nanofabrication slightly weakens the perpendicular magnetic anisotropy—the built-in tendency of the magnetization to point out of the film plane—which shifts the spots from which the waves are emitted.
Third, a subtle interface effect, the Dzyaloshinskii–Moriya interaction, makes spin waves traveling in opposite directions behave differently, explaining the one-sidedness of the emission. Only with all three ingredients did simulation and experiment agree—a clear sign that the physical models used to design SHNOs and their synchronized networks need to be refined. The measurements also produced an unexpected side observation: Under prolonged illumination with intense soft X-rays, the magnetic properties of the samples changed gradually and permanently.
Extract — continue reading at the source.
