Field-dependent anisotropic microwave responses in van der Waals ferromagnet Fe4GeTe2

The exploration of two-dimensional van der Waals (2D-vdW) ferromagnets with high Curie temperature (TC) is crucial for the development of high-performance, non-volatile, and low-power spintronic devices. 2D-vdW Fe4GeTe2 (F4GT) is considered a promising candidate material due to its nearly room-temperature TC, strong magnetization, high conductivity, and variable magnetic anisotropy. In this study, we investigate the multiple magnetic couplings in F4GT single crystals using electron spin resonance (ESR) technique. ESR lines associated with the change from perpendicular magnetic anisotropy to easy plane anisotropy are observed, indicating strong anisotropic responses to the microwaves. As the temperature increases, the resonance lines for H∥ab shift toward higher fields, while those for H∥c move toward lower fields. These opposite behaviors are attributed to the competition between the inner magnetic field and the demagnetization effect. In addition, the angle-dependent ESR spectra of F4GT single crystal exhibit a [3 cos2(π2−φ)−1]2-like behavior, which indicates a characteristic of 2D magnetic coupling attributed to the dominant effects of q→ 0 mode. These findings provide significant insights into the complex spin couplings in F4GT and pave the way for its potential applications in microwave-based spintronic devices.