Exotic Temperature Dependence of Uniaxial Magnetocrystalline Anisotropy in a Two-Dimensional Ferromagnet

Uniaxial magnetocrystalline anisotropy (UMA) is the foundation for data storage technology as well as for realizing topological and quantum phases. Combined with the confined electrons and distinct lattice dynamics of two-dimensional (2D) materials, exploring UMA, especially its temperature dependence in 2D ferromagnets, is significant. Here, we report an unprecedented exotic behavior of temperature dependence of UMA in the 2D van der Waals ferromagnet ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$. Below 20 K, the temperature-dependent UMA constant ${K}_{\mathrm{u}1}(T)$ exhibits a canonical Akulov-Zener-Callen-Callen-type ${K}_{\mathrm{u}1}(T)$ with rapidly decreasing trend. Strikingly, above 20 K, it transitions to an anomalous Carr-type one with rapidly increasing trend. Temperature-dependent x-ray diffraction measurements combined with first-principles calculations reveal that the enhanced ferromagnetic exchange coupling caused by the lattice variation preserves the two-ion anisotropy at high temperature. Therein, both the itinerant nature and lattice thermal expansion are altered, thus triggering such exotic ${K}_{\mathrm{u}1}(T)$. Our findings not only deepen the basic understanding of the temperature dependence of UMA but also favor optimizing the ultra-high-density heat-assisted magnetic recording technology by considering the cooperation between lattice and ferromagnetic exchange coupling.