Photo-induced switching of magnetisation in the epsilon-near-zero regime

Abstract

The possibility of controlling spins using ultrashort light and strain pulses has triggered intense discussions about the mechanisms responsible for magnetic re-ordering. All-optical magnetisation switching can be achieved through ultrafast heat-driven demagnetisation or transient modifications of magnetic anisotropy. During the phononic switching of magnetic dielectrics, however, mid-infrared optical excitations can modify the crystal environment via both the thermal quenching of anisotropy and the generation of strain respectively, with the relative distinction between these thermal and non-thermal processes remaining an open question. Here, we examine the effect of mid-infrared pulses tuned to the frequency of optical phonon resonances on the labyrinthine domain structure of a cobalt-doped yttrium iron garnet film. We find that the labyrinthine domains are transformed into stable parallel stripes, and quantitative micromagnetic calculations demonstrate this stems predominantly from a partial quenching of the anisotropy. Contrary to conventional wisdom, however, we find that this heat-facilitated process of magnetisation switching is spectrally strongest not at the maximum of absorbed optical energy but rather at the epsilon-near-zero points. Our results reveal that the epsilon-near-zero condition provides an alternative pathway for laser-driven control of magnetisation, even when the underlying mechanism is primarily thermal.

Figures (4)

• Figure 1: Faraday images of the magnetic domain structure probed with wavelength $\lambda = 1064$ nm (a) before and (b) after illumination with a single burst of pulses (wavelength $\lambda =$ 13 $\mu$m). (a - inset) Schematic of the cubic crystal structure with the four equivalent [111] magnetocrystalline easy axes.
• Figure 2: (a) Faraday images obtained after illumination with the pump, for various initial magnetisation orientations described by the vector $\hat{m}_{ip}$. The orientation of the parallel stripes follows that of the initial in-plane projection of the magnetisation. (b) Faraday images obtained after illumination with the pump, with an external magnetic field applied along an easy axis.
• Figure 3: (a) Simulated magnetic state 50 ns after the strain pulse with a 75% change in magnetocrystalline anisotropy, for different initial magnetisation orientations. (b) Simulated magnetic state 100 ns after the stain pulse with a 70 % change in magnetocrystalline anisotropy and while an external magnetic field is applied along [111] or [11-1].
• Figure 4: (top panel) Real and imaginary parts of the dielectric function of Co:YIG. (bottom panel) Spectral dependence of the normalised switched area and absorbed optical energy.