Magnetism and Spin Dynamics in Emergent Two-Dimensional van der Waals Magnets

Abstract

The low Curie temperature of most two-dimensional (2D) van der Waals (vdWs) magnets makes it challenging to incorporate them into device applications. This thesis explores two intriguing materials: Fe5GeTe2, a 2D vdWs room temperature magnet, and Cu(1,3-bdc), a quasi-2D topological magnon insulator with low Curie temperature but peculiar magnetic properties. The materials were studied with various metrology, including X-ray diffraction, vibrating sample magnetometry, broadband FMR spectroscopy, thermal transport, etc. The magnetic measurements were performed with external magnetic fields applied in-plane and out-of-plane, and at different temperatures. We find that Fe5GeTe2 shows a record high Curie temperature of 332 K. Interestingly, for both magnets, a sizable Landé g-factor difference between the in-plane and out-of-plane cases was discovered, the Landé g-factor values deviate from g = 2, indicating a contribution of orbital angular momentum to the magnetic moment. The FMR measurements have revealed that Fe5GeTe2 has a damping constant comparable to Permalloy, and with reducing temperature, the linewidth has broadened. Our measurements not only demonstrate the room-temperature magnetization dynamics of Fe5GeTe2, but also provide evidence that Fe5GeTe2 transitions from ferromagnetic to ferrimagnetic at lower temperatures. In Cu(1,3-bdc), we have found that the interplay of topology, spin excitations, and orbital magnetism presents a playground for exploring topological spintronics. While the differences of in-plane and out-of-plane Landé g-factor (Δg) and saturation magnetization (ΔMs) in Cu(1,3-bdc) are well correlated at low temperatures, they diverge at higher temperatures (T>~4 K). Further theoretical analyses show that topological orbital moment induced by thermally excited spin chirality results in the g-factor anisotropy at higher temperatures. Our experiments have identified critical quantum phenomena in 2D magnets, highlighting them as ideal platforms for studying fundamental physics and building efficient spintronic devices.