Anomalous temperature dependence of uniaxial magnetocrystalline anisotropy in the van der Waals ferromagnet Fe3GeTe2

Uniaxial magnetocrystalline anisotropy (UMA) is vital for fundamental research, such as maintaining two-dimensional ferromagnetic order and realizing topological phases. However, in most cases, UMA rapidly decreases with increasing temperature and finally vanishes approaching the Curie temperature (T₂). The increasing UMA with increasing temperature is very rare in almost all traditional ferromagnetic materials and in emerging van der Waals (vdW) ferromagnets, which generally have relatively low T₂. Here, we experimentally unveil the anomalous temperature dependence of the UMA constant Kₔ₁ (T) in the vdW ferromagnet Fe₃GeTe₂. Surprisingly, the Kₔ₁ (T) first anomalously increases and then slowly decreases. We found that the anomalous Kₔ₁ (T) can be perfectly fitted by Carr's model. Further analysis and temperature-dependent x-ray diffraction measurements suggest that the partial localization of 3d electrons and considerable lattice expansion are crucial for anomalous Kₔ₁ (T). We propose that the complex competition between the two-ion mechanism and the itinerant-electron mechanism leads to the anomalous behavior of Kₔ₁ (T) in Fe₃GeTe₂. Our findings from this unusual case help deepen the understanding of the temperature dependence of UMA.