Magnetic anisotropy in the near-stoichiometric van der Waals ferromagnet Fe${}_3$GeTe${}_2$

Abstract

Quasi-two-dimensional (2D) van der Waals (vdW) ferromagnets such as the series Fe${}_{3-x}$GeTe${}_2$, with a relatively high Curie temperature and robust metallicity, offer an ideal platform for investigating itinerant magnetism in reduced dimensions. Here, we present a comprehensive electron spin resonance (ESR) investigation of single-crystalline almost-stoichiometric Fe${}_{3.03\pm 0.03}$GeTe${}_2$ across wide ranges of frequencies, temperatures, and magnetic fields to gain quantitative insights into its magnetic anisotropy and spin dynamics. Frequency-dependent ESR measurements establish Fe${}_3$GeTe${}_2$ as an easy-axis ferromagnet. Temperature-dependent high-field ESR reveals a large internal field that gradually decreases at higher temperatures. Remarkably, this internal field persists even above $T_{\mathrm{C}}$, evidencing short-range spin correlations in Fe${}_3$GeTe${}_2$. Analysis of spin-wave modes yields a strong uniaxial magnetocrystalline anisotropy $K_{\text{int}}\approx -5\times 10^6$ erg cm${}^{-3}$ at 3 K and a large magnon gap $\Delta (3\mathrm{K})$ $\approx$ 87.8 $\pm$ 13.7 GHz ($\approx$ 0.363 $\pm$ 0.057 meV). Our study highlights that small variations in Fe content in Fe${}_3$GeTe${}_2$ lead to substantial changes in the magnon gap at low temperatures, indicating the extreme sensitivity of spin dynamics to the chemical composition. These results establish Fe${}_3$GeTe${}_2$ as a model vdW ferromagnet for exploring tunable anisotropies and magnon excitations in metallic 2D magnets.

17 pages, 16 figures