Magnetoresistive reading of perpendicular magnetization in ferrimagnetic insulators enhanced through proximity coupling

Ferrimagnetic insulators with perpendicular magnetic anisotropy exhibit unique properties that offer potential for advanced spintronic devices. One of the challenges in exploiting these materials in spintronics lies in the efficient electrical detection of their magnetization. Herein we report the magnetoresistive detection of perpendicular magnetization in insulating ferrimagnetic garnets, enhanced through proximity coupling with an adjacent ultrathin ferromagnetic layer. We design and characterize a hybrid spin-valve device consisting of terbium iron garnet ($\mathrm{Tb}\mathrm{IG}$)$/\mathrm{Co}/\mathrm{Cu}/\mathrm{Tb}$-$\mathrm{Co}$ multilayers where $\mathrm{Tb}\mathrm{IG}$ and $\mathrm{Tb}$-$\mathrm{Co}$ are perpendicularly magnetized soft and hard layers, respectively, while $\mathrm{Co}$ is in-plane magnetized. We reveal through anomalous Hall effect and x-ray magnetic circular dichroism measurements that the interfacial exchange coupling between $\mathrm{Tb}\mathrm{IG}$ and $\mathrm{Co}$ induces a perpendicular magnetization component in the $\mathrm{Co}$ layer, which increases the magnetoresistance readout signal by more than 2 orders of magnitude---reaching 0.2%---compared with the reference system without the $\mathrm{Co}$ layer. A notable but less pronounced enhancement is observed in the $\mathrm{YIG}/\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}/\mathrm{Cu}/\mathrm{Tb}$-$\mathrm{Co}$ spin-valve device compared with the stack without the $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$ insertion, demonstrating the applicability of the approach to similar structures. Finally, we show that the magnetoresistive output signal can be effectively used to probe the multidomain state of $\mathrm{Tb}\mathrm{IG}$, opening a path for future insulating spintronic device applications.