Spin electronics, or spintronics for short, aims to use the spin of electrons to interconvert electrical and magnetic signals, and offers promising routes for efficient memory, logic, and computing technologies. Although the active elements of spintronics devices are traditionally made of ferromagnetic metals, magnetic insulators are drawing growing attention due to their intrinsic favorable properties. Incorporating such materials in devices would, for example, allow for producing pure spin currents without the associated Joule heating, which is one of the leading losses in electronics. Moreover, the intrinsic low damping and long magnon diffusion length in insulators can enable the development of novel device functionalities. This poses a new challenge for research, as the electrical readout of magnetization in insulators is tricky. While the magnetization vector detection in conventional spintronic devices is realized by substantial electrical signals provided by tunnel and giant magnetoresistances (GMR), insulator detection relies on minute signals induced by inverse spin Hall voltages in an adjacent heavy metal, such as Pt.

 In our recent paper published in Communications Physics (link), we demonstrate a simple magnetoresistive way of detecting perpendicular magnetization reversal in an electrically insulating ferrimagnetic oxide, terbium iron garnet (TbIG). The detection does not require a heavy metal layer or spin Hall effect but relies on current-in-plane magnetoresistance measurements in a trilayer system, as shown in Fig.1a. Here, we engineered TbIG to be the free layer, displaying low coercivity and perpendicular magnetic anisotropy (PMA). We chose TbCo, a ferrimagnetic alloy, to act as the reference magnetic layer and spin polarizer, due to its easily tunable magnetic properties. During an out-of-plane magnetic field sweep, we observe abrupt longitudinal resistance changes upon crossing the coercive fields of the respective layers, analogous to the GMR in spin valves, albeit with a smaller amplitude with respect to a fully metallic stack. Figures 1b and 1c show the minor loops corresponding to the reversal of the magnetization (M) of TbIG when the reference TbCo layer is pre-set in the up and down direction, respectively. The material and temperature dependence of the magnetoresistance collectively pinpoints the spin-dependent scattering at the TbIG|Cu interface as the underlying cause of the observed phenomenon. Moreover, we found that the amplitude of the magnetoresistance decreases with increasing Cu spacer layer thickness, consistent with fully metallic spin valves. Theoretical calculations based on layer-resolved Boltzmann transport equations also agree with our experimental findings and further consolidate the presumed origin of the magnetoresistance effect.

To the best of our knowledge, this work constitutes the first demonstration of the spin valve effect using ferrimagnetic garnets and an in-depth study of the GMR in insulator-based spin valves. The simple two-terminal magnetoresistive reading reported in our paper may enable non-volatile binary memory cells where a magnetic insulator can be used as an active component instead of conventional magnetic conductors. This would bring about a series of advantages, among others, higher structural stability, broader magnetic tunability, ultrafast switching times, and low power consumption. The implications for future research in the field of spintronics are therefore significant. While the effect is relatively small for any microelectronic applications as of yet, it can be enhanced by orders of magnitude by materials and device engineering and support from theory. We expect that our paper will stimulate research into a wide spectrum of spacer and spin polarizer layer combinations and enable new device ideas and architectures based on magnetic insulators.