Femtosecond all-optical coherent control of spin polarization in altermagnets

Abstract

Altermagnets constitute an emerging materials platform for spintronic technologies by combining compensated magnetic order with ferromagnet-like spin-split electronic bands. Here, we investigate the proposed d-wave altermagnetic material RuO2 using circularly polarized ultrashort laser pulses. Time-resolved magneto-optical Kerr effect measurements, which are intrinsically sensitive to surface and interface states, reveal the ultrafast spin response of RuO2. In contrast to the demagnetization dynamics characteristic of conventional ferromagnets, we observe a distinct coherent contribution to the complex Kerr rotation that appears during the light-matter interaction and lasts for about 200 fs. Similar signatures have been associated with spin-momentum locking and directional band splitting in spin-split surface states of topological insulators as well as spin-orbit-coupled semiconductors. They are governed by a finite Raman coherence time. We interpret this coherent response as evidence for transient spin-polarized surface states in RuO2, consistent with the emergence of altermagnetic surface states that are directly relevant to spin-polarized transport at surfaces and interfaces.

Figures (5)

• Figure 1: a) Illustrates the most basic crystal structures with either up- or down-oriented, or zero magnetic moment, in orange, green, and gray, respectively, for ferromagnets, antiferromagnets, topological insulators, and altermagnets (from left to right). b) A cross-section through the Fermi surfaces for all four material systems. c) We apply selective pumping processes by utilizing circularly polarized photons to enhance specific electron transitions. d) This leads to material-distinct magnetization dynamics, measured by time-resolved magneto-optical Kerr-effect, apparent in the relation between the real $\Delta\theta_K$ and the imaginary $\Delta\epsilon_K$ parts of the complex Kerr rotation. Transient spin polarized states (e.g. topological insulators and altermagnets) produce characteristic "butterfly" structures in materials with $\vec{k}$ dependent electron spin within the first $200\,\mathrm{fs}$ after the excitation with ultrashort laser pulses $\sim 45\,\mathrm{fs}$. In (anti-) ferromagnets, the electron spin is, in first order, independent of $\vec{k}$, revealing a linear relation.
• Figure 2: The setup a) employs a double modulation time-resolved pump-probe MOKE scheme to extract dynamics on the femtosecond timescale. A photo-elastic modulator modulates the probe, and a mechanical chopper modulates the pump beam. The sample is rotated by the angle $\alpha$ indicated by the blue arrow. Crystal structure overview, cross-section b), and front view c). The grey arrows mark the crystal directions. The Néel vector points in the [001] direction. Oxygen atoms are colored red, Ru atoms in either sublattice are colored orange/green with their corresponding magnetic moment hinted by arrows. The oval and circular structures in orange and green indicate a cut through the Fermi surface.
• Figure 3: Time-resolved complex Kerr angle components data for a) RuO$_2$ on MgO, and b) RuO$_2$ on TiO$_2$ during the first $300\,\mathrm{fs}$ after excitation for all three excitation polarizations. The $\sigma^{\pm}$ spectra are shifted for clarity; the gray dashed lines represent the original zero line. The circular signal is on average $100\,\mathrm{fs}$ faster and 10 times stronger than the linear response. Additionally, a second peak arises. Dynamics data transferred to the complex plane (c) and d)), depicting a butterfly structure, which develops during the femtosecond scattering process between the transient spin polarized states e). Also observed in topological insulators f).
• Figure 4: RuO$_2$ on MgO: $\Delta\theta_K$ spectra for an a) left circular, b) linear, and c) right circular polarized excitation during the first 1 ps after excitation for selected sample rotation angles. d) and e), the first peak extracted from the corresponding dynamic curves for RuO$_2$ on MgO (twinned) and TiO$_2$ (single crystal), respectively, plotted as a function of sample rotation angle $\alpha$. The data show a clear twofold symmetry for a full sample rotation. The single-crystal data are shifted for clarity, with dashed lines denoting the zero-crossing. In the complex plane, f) twinned and g), h) single crystal, the butterfly structure is shown for both samples. In g) and h), the "switching" behavior shows in the sign change between the wings from $\alpha=60^{\circ}$ and $\alpha=135^{\circ}$.
• Figure 5: a) and b) X-ray diffraction spectra for both samples grown on MgO and TiO$_2$, respectively. c) and d) X-ray reflectivity spectra for both samples grown on MgO and TiO$_2$, respectively.