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 Fe 4 GeTe 2 as the temperature decreases. Such a transition occurs at T SR = 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 Fe 4 GeTe 2 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.