Temperature-Dependent Spin Reorientation and Versatile Spin Valve Effect in Fe₄GeTe₂

Two-dimensional van der Waals ferromagnetic materials not only exhibit a plethora of magnetic phenomena under reduced dimensions but also enrich the realm of nanoscale spintronics devices. Here, we identify a spin reorientation (SR) transition from easy plane to uniaxial in Fe4GeTe2 as the temperature decreases. Such a transition occurs at TSR = 107 K for bulk samples determined through angle-resolved magnetization measurements and at 125 K for a 15-nm-thick nanoflake as evidenced by distinct responses of the residual resistance and coercive field in anomalous Hall measurements. Moreover, the spin valve, employing two exfoliated Fe4GeTe2 crystals as ferromagnetic electrodes without a spacer layer, exhibits a temperature-dependent magnetoresistance (MR) sign, displaying normal negative MR with in-plane magnetic anisotropy at elevated temperatures and inverse spin-valve-type MR with perpendicular magnetic anisotropy at lower temperatures. Our findings open up exciting possibilities for the development of functional nanoscale spintronics devices.