Spin Hall magnetoresistance in metal/ferromagnetic insulator heterostructures

The magnon or spin wave, which is one type of pure spin currents, can transport in magnetic insulators including ferromagnetic and antiferromagnetic insulators. Owing to the absence of the current and the accompanied Joule heating in magnetic insulators, it provides a possible route to develop extremely low energy consumption spintronic devices. On the other hand, the electron type of the pure spin current, in which the electrons with the opposite spins flow in the opposite directions, and the charge current can be interconverted via the spin Hall effect (SHE) and inverse spin Hall effect (ISHE) in heavy metals. Since the pure spin current can transport across the metal/magnetic insulator interface, we can detect or manipulate the pure spin current in magnetic insulators electronically. A variety of interesting physical phenomena were discovered in the metal/magnetic insulator heterostructures. The spin Hall magnetoresistance (SMR) is one of the most studied phenomena. SMR originates from the pure spin current reflection rate at the metal/magnetic insulator interface depending on the relative orientation of the electron spin and the magnetic moment of the ferromagnets (or the Neel vector of the antiferromagnets). Owing to the simple magneto-transport measurement for SMR, it is convenient to determine the direction of the magnetic moment or the Neel vector and estimate several important parameters such as the spin mixing conductance, spin diffusion length and spin Hall angle. SMR was initially discovered in Pt/YIG bilayer and then observed in metallic systems and metal/antiferromagnetic insulator heterostructures. SHE and ISHE originate from the spin orbit interaction (SOI) in the heavy metals, meaning that the heavy metal is essential. However, in addition to SMR. we found a magnetoresistance effect in Cu/Pt/YIG, in which Pt is very thin to form the separated islands. Due to the negligible SOI in Cu and the negligible spin current generated by SHE in Pt, the resultant SMR from either Cu or Pt is negligible. This magnetoresistance disappears when the Pt islands are absent or located away from the Cu/YIG interface, indicating the indispensable role of the Pt-decorated interface. The observed magnetoresistance is attributed to the Rashba interaction induced by Pt. The spin Hall angle, measuring the efficiency of the spin-charge interconversion, varies with materials and can even have opposite signs. Intuitively, one expects that the spin current would be reduced if combining two metallic layers with opposite sign spin Hall angles. However, we studied the SMR in W/Pt/YIG trilayers, where W and Pt have opposite spin Hall angles. We found that the SMR ratio, thus the spin accumulation at the Pt/YIG interface, is enhanced with the additional ultrathin W layer, contrary to the intuitive viewpoint above. Our theoretical simulation based on the spin drift-diffusion model also confirms the experimental results. A negative SMR was observed in Pt/NiO/YIG trilayers. The mechanism of this phenomenon is under debate. One of the key issues to distinguish the mechanisms is to detect the magnetic coupling between NiO and YIG. We investigated the magnetic moment distribution in Pt/NiO/YIG using polarized neutron reflectivity. Our results directly show the spin-flop coupling between NiO and YIG therby provide an important clue to reveal the mechanism of the negative SMR. The spin rectification effect (SRE) rectifies a microwave current into a dc voltage via the magnetoresistance effect. It is a useful approach for studying spin dynamics and can be used in microwave applications such as spintronic microwave detection. Here, we provide several experimental evidences and theoretical analyses to demonstrate the SMR-induced SRE.