Spin dynamics of room temperature van der Waals (vdW) ferromagnets and their usage in microwave devices

Abstract

Quasi-two-dimensional van der Waals (vdW) materials exhibiting room-temperature (RT) long-range ferromagnetic nature have emerged as a significant research field to explore fundamental condensed matter physics due to their intriguing physical properties. These vdW materials enable a futuristic platform for implementing novel spintronics devices. Here, we examined the spin dynamics of polycrystalline Fe5GeTe2 and Fe4.8Co0.2GeTe2 vdW materials using ferromagnetic resonance (FMR) spectroscopy. Vibrating Sample Magnetometer (VSM) study reveals that both materials have a soft ferromagnetic character at room temperature. From room temperature FMR measurements, the effective magnetization of Fe5GeTe2 and Fe4.8Co0.2GeTe2 derived ∼0.54 ± 0.056 and 0.50 ± 0.017 kOe, respectively. These results are consistent with reported VSM data. Fe5GeTe2 and Fe4.8Co0.2GeTe2 exhibit broad FMR linewidths of 0.697 ± 0.036 and 0.748 ± 0.056 kOe, respectively, which can be due to inhomogeneous line broadening. Besides its intrinsic contribution to linewidth, it is also affected by extrinsic Gilbert damping (αext). The value of αext is influenced by conflicting intra-band and inter-band electronic transitions, according to Modified Kambersky's theory. Furthermore, the effective Gilbert damping constant (α) obtained is 0.0513 ± 0.0046 for Fe5GeTe2 and 0.0526 ± 0.0031 for Fe4.8Co0.2GeTe2 at RT. Additionally, we developed microwave signal processing devices using these materials and evaluated their functionality both as a microwave band-reject filter and an adjustable phase shifter. The stop-band response was studied across the 5 to 25 GHz frequency range under an applied magnetic field as high as 7 kOe. For these flip-chip-based devices, attenuation is −5 dB/cm for the Fe5GeTe2-based filter and −3.2 dB/cm on sample Fe4.8Co0.2GeTe2 at 6.95 and 5.37 kOe, respectively. The same micro-strip filter was used as a tunable phase shifter in the off-resonance region. The optimal differential phase shift studied for Fe5GeTe2 and Fe4.8Co0.2GeTe2-based phase shifters in the high-frequency region (22 GHz for Fe5GeTe2 and 18 GHz for Fe4.8Co0.2GeTe2) is 23°/cm and 14°/cm, respectively, at high magnetic fields. These versatile devices find integration across a wide spectrum of applications, such as phased-array antennas, radar systems, and wireless communication systems, offering their benefits to diverse fields.