FERMI | TIMER Control of nanoscale spin waves with spatially tailored extreme ultraviolet pulses While the technology to transfer and control the flow of information using charges—i.e., electronics—has reached a level of maturity previously unimaginable and has led to drastic societal changes, the next generation of technologies hinges on the development of even smaller, faster, and more energy-efficient devices. To this end, scientists have turned to studying how information can be stored, transferred, and manipulated using electron spin, a quantum mechanical angular momentum intrinsic to subatomic particles. This idea has formed the basis of a nascent field termed spintronics. Unlike charge, spin quanta can, in theory, be transferred without energy loss and remain stable without continuous energy input. In magnetically ordered systems, spin angular momentum is propagated in the form of spin waves, or magnons—a collective, wave-like motion of spins with characteristic wavelengths that determine their properties. Informatics with spin waves constitutes its own subfield of spintronics, called magnonics. Spin waves with nanoscale wavelengths (<100 nm), though promising for magnonic applications due to their high frequencies and propagation speeds, are difficult to generate. Currently, this is achieved by fabricating nanoscale antennas onto sample surfaces. A noninvasive approach using laser radiation would enable more flexible control of nanoscale spin waves, but this is not possible with optical lasers, which operate at much longer wavelengths. Recently, a team of researchers from the US, Italy, and Germany demonstrated that crossed pulses of extreme ultraviolet (EUV) radiation, sourced from the FERMI free-electron laser, can be used to generate nanoscale, coherent spin waves. An overview of the experimental design is shown in Fig. 1 (a). When crossed, interference between the two pulses creates a periodic intensity pattern with a period in the tens-of-nanometers range, determined by the pump wavelength and the crossing angle. The “hot” regions of this pattern excite the sample, rapidly altering its magnetic landscape and generating coherent spin waves with the same periodicity as the interference pattern. These coherent spin waves were then probed by inelastic scattering of a time-delayed EUV probe pulse. This four-wave mixing method, commonly known as transient grating spectroscopy, has previously been used to study the properties of nanoscale acoustic phonons (vibrational excitations). Here, by tuning the probe wavelength to be resonant with a specific atomic transition of a magnetic element, in this case, gadolinium, the observed response is dominated by magnetic rather than vibrational dynamics. The response consists of an initial ultrafast component (inset of Fig. 1 c), followed by pronounced oscillations in the tens of GHz frequency range, visible in both the raw CCD images (Fig. 1 b) and the processed signal (Fig. 1 c). The initial femtosecond response results from periodic ultrafast demagnetization and was used to study the mechanism of demagnetization by varying the periodicity and observing changes in its timescale. The picosecond oscillations, on the other hand, are signatures of coherent spin waves. No evidence of acoustic dynamics was found, suggesting the resonant probe is an effective way to enhance sensitivity to magnetic dynamics. Furthermore, by varying the wavelength of the pump pulses, the researchers were able to specifically tune the wavelength of the coherent spin waves within the 50–90 nm range. This enabled the construction of a highresolution dispersion diagram (i.e., the relationship between frequency and wavelength), as shown in Fig. 1 (d). As an ultrafast time-domain spectroscopy technique, this method offers much higher frequency resolution compared to frequency-domain methods such as resonant inelastic x-ray and EUV scattering. The power of this approach is demonstrated in Fig. 1 (d), where the researchers compared the dispersion of two nearly identical samples, denoted PMA and CMA, that differed only slightly in their iron-to-gadolinium ratios. They found that, despite being nearly identical, the samples’ slight compositional differences resulted in significant shifts in frequencies and propagation velocities—shifts that would be unresolvable using frequency-domain scattering methods. In addition to higher frequency resolution, this approach allows for the study of nonequilibrium dynamics not accessible with existing spectroscopic techniques. Broadly, this work serves as an early demonstration of the potential for EUV, and even shorter-wavelength radiation, to control nanoscale magnetic dynamics. The transient grating technique offers a spectroscopic method with several desirable characteristics, including ultrahigh frequency resolution and access to nonequilibrium dynamics. From a fundamental perspective of light–matter interactions, this method represents a new way of mixing short-wavelength radiation mediated by coherent spin excitations. Moreover, similar techniques are being developed in the hard x-ray regime, opening the prospect of generating nanometer-scale spin waves. The outcome of this work will enable further studies of nanoscale spin waves and ultrafast magnetic dynamics, facilitating the development of smaller and faster magnonic devices. Elettra Highlights 2024–2025 90
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