X-ray imaging the dynamics of nanoscale spin structures

Magnetism is one of the dominant physical phenomena used in modern technology, such as for magnetic storage and sensing. Although known since ancient times, its fundamental principles are still not fully understood. At the beginning of the 20^th century, the electron spin (and its quantum-mechanical behavior) was identified as the basic quantity defining magnetism. Nanomagnetism aims to understand and tailor magnetic properties down to fundamental length and time scales, which are defined by the strengths of competing interactions between the spins (such as exchange and anisotropy). Spin electronics considers the spin as the new degree of freedom. Novel ways to manipulate spins on nanoscales, such as through spin currents, electric, or photonic control are currently being explored.¹ Synthesis of advanced magnetic materials involves elemental composition in heterostructures and laterally confined systems. To achieve a thorough understanding of the underlying fundamental principles, advanced analytical tools are required.

In most cases, the energetic ground state of a ferromagnetic system is characterized by a complex spin structure. Imaging these structures and their fast spin dynamics (which drive their functionality) is very appealing. To this end, a large variety of techniques are available that use, for instance, electrons or photons as probes. The grand challenge to magnetic microscopy is to achieve sub-10nm spatial resolution with femtosecond temporal resolution and elemental specificity to record snapshot images of the ultrafast dynamics in nanoscale spin structures. Soft x-ray microscopy holds the promise to deliver this.

We used a full-field soft x-ray microscope at the Advanced Light Source in Berkeley (California), where state-of-the-art Fresnel zone plates—see Figure 1(a)—provide spatial resolution down to 10nm.² The synchrotron's inherent bunch structure allows recording of time-resolved images with better than 100ps temporal resolution. However, the limited number of photons per pulse requires application of a stroboscopic pump-and-probe scheme. Therefore, we can only see the fully reproducible part of the spin dynamics, which has to be reset between consecutive pump-probe sequences. In the soft x-ray regime, large x-ray magnetic, circular dichroism effects occur in the vicinity of characteristic x-ray absorption edges, which enable a strong and inherently element-specific magnetic-contrast mechanism.

Figure 1. (a) Scanning-electron micrograph of the outermost region of a Fresnel zone plate with 12nm lines and spaces, which recently enabled 10nm spatial resolution. (b) X-ray microscopy image of the magnetic-domain structure of a 50nm-thin cobalt chromium platinum nanogranular alloy film with a perpendicular magnetic anisotropy, imaged with 15nm spatial resolution. Field of view: ~2×2μm². (c) Field-driven depinning of a magnetic-domain wall in a notched nanowire. Wire width: 450nm. (d) 3D representation of the inner part of a permalloy disk, showing the vortex core in the center. Field of view: ~250×250nm².

We studied current- and field-induced magnetic-vortex and domain-wall dynamics in confined magnetic microstructures in detail: see Figure 1(b)–(d). From the current-induced resonant-vortex-core motion in a permalloy disk, we unambiguously determined the current's polarization, which quantifies the strength of the spin-torque effect. We analyzed domain-wall oscillations upon a fast field pulse to find that we had to go beyond the harmonic-oscillator potential to fully describe our experimental data. Using high spatial resolution, we thoroughly investigated the stochastic character of nanoscale magnetic processes. In particular, we found a nondeterministic behavior in the nucleation process during field reversal. We also found an unexpected jump in the scaling exponent of the power-law behavior of Barkhausen avalanches in nucleation-mediated materials, which could be explained by proximity effects. We discovered that the proper choice of geometries in notched nanowires can significantly reduce the stochastic character of the pinning and depinning effect of domain-wall motion. These results are of both paramount fundamental scientific interest and high technological relevance.

While the spatial resolution obtained with Fresnel zone-plate-based soft x-ray microscopy is approaching fundamental magnetic-exchange lengths, the time resolution is still far from fundamental time scales but is limited by the currently available x-ray sources. At upcoming next-generation x-ray free-electron-laser sources, the accessible time scales are in the tens of femtosecond regime and the number of photons per x-ray pulse will be sufficient to take single-shot images. This will allow studying spin fluctuations on nanoscales and shed light on the origin of exchange interactions.

This work is supported by the director of the Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the US Department of Energy.