Spin order and fluctuations in the EuAl4 and EuGa4 topological antiferromagnets: A μsR study

EuAl4 and EuGa4 are two candidate materials for studying the interplay between correlated-electron phenomena, topological spin textures, and topologically nontrivial bands. Both compounds crystallize in a centrosymmetric tetragonal BaAl4-type structure (space group I4/mmm) and show antiferromagnetic (AFM) order below TN=15.6 and 16.4 K, respectively. Here, we report on systematic muon-spin rotation and relaxation (μSR) studies of the magnetic properties of EuAl4 and EuGa4 single crystals at a microscopic level. In both cases, transverse-field μSR measurements, spanning a wide temperature range (from 1.5 to 50 K), show clear bulk AFM transitions, with an almost 100% magnetic volume fraction. Zero-field μSR measurements, covering both the AFM and the paramagnetic (PM) states, reveal internal magnetic fields Bint(0)=0.33 T and 0.89 T in EuAl4 and EuGa4, respectively. The transverse muon-spin relaxation rate λT, a measure of the internal field distribution at the muon-stopping site, shows a contrasting behavior. In EuGa4, it decreases with lowering the temperature, reaching its minimum at zero temperature, λT(0)=0.71 μs-1. In EuAl4, it increases significantly below TN, to reach 58 μs-1 at 1.5 K, most likely reflecting the complex magnetic structure and the competing interactions in the AFM state of EuAl4. In both compounds, the temperature-dependent longitudinal muon-spin relaxation λL(T), an indication of the rate of spin fluctuations, diverges near the onset of AFM order, followed by a significant drop at T<TN. In the AFM state, spin fluctuations are much stronger in EuAl4 than in EuGa4, while being comparable in the PM state. The evidence of robust spin fluctuations against external magnetic fields provided by μSR may offer valuable insights into the origin of the topological Hall effect and the possible magnetic skyrmions in the EuAl4 and EuGa4 compounds.