Spin-related Transport Study Of Transition Metals

After the spectacular advances and successes of the Electronics, the spin of electron has attracted a great deal attention in the last two decades. The production, manipulation, and detection of pure spin current are at the heart of spintronics. On the heel of spintronics, we now have spin caloritronics where one exploits the interaction between heat transport and the charge or spin degree of freedom. It was found the thermal effects in Thermotics all have spin-dependence. Among which the spin Seebeck effect (SSE) is one of the most fascinating phenomenon recently discovered, and it denotes the the generation of spin current from temperature gradient. The spin Seebeck effect was discovered first in Permalloy, and later in ferromagnetic semiconductors (GaMnAs), electrically insulating yttrium iron garnet (YIG). Once generated, a spin current cannot be detected electrically but by the inverse spin Hall effect (ISHE) in heavy (high-Z) metals with strong spin-orbit coupling. In most cases, it involves Pt as the pure spin current detecter. However, Pt is near the Stoner ferromagnetic instability and exhibits induced moments when deposited on YIG substrate as confirmed by X-ray magnetic circular dichroism (XMCD). This is the magnetic proximity effect (MPE). The influences of MPE in Pt on both thermal and electric transport have drawn intensive attention in the spintronics community. And people even doubt the existence of the spin Seebeck effect. Based on this two critical questions, we carry on thermal and electronic transport measurements on Pt/YIG bilayer structure. It is found that the MPE has negligible thermal contribution, while has sizable effect on magnetoresistance and anomalous Hall voltage. And unitlize the longitudinal spin Seebeck effect, we study the inverse spin Hall effect in a ferromagnetic metal Py in the last section.Normally, the magnetic moment develops a transverse voltage from anomalous Nernst effect (ANE) under perpendicular temperature gradient. As a result, if the polarized Pt acquires ANE, it may compromise the longitudinal SSE for the same magnetic field direction dependence. However, this critical question remains to be clarified. In this work, by inserting Cu layer of different thicknesses between Pt and YIG, we have separated the contribution from induced moment and spin current. The thermal signal of Pt/YIG bilayer structure potentially has two sources:spin current from SSE in YIG and ANE from induced moment of Pt. After insertion of Cu, the thermal voltage of Pt/Cu/YIG decays quickly. Two possible reasons may account for this behavior:On one hand, the spin current diminishes a lot after penetrating through Cu; on the other hand, the ANE contribution from polarized Pt decays with Cu layer thickness. In order to clarify the underlying mechanism, similar experiments were performed on Au/Cu/YIG. Because Au is rather intrinsic and has no polarized moment when deposited on YIG. Thus, the decay should come from the loss of spin current only. Surprisingly, the decay rate for Au/Cu/YIG is exactly the same as Pt/Cu/YIG. Based on this critical behavior, we can draw the conclusion that the induced moment itself in Pt has negligible thermal contribution. The thermal signal of Pt/YIG mainly comes from SSE instead of ANE.On the other hand, YIG is a good insulator and accommodates no charge current. As a nonmagnetic metal, Pt in isolation shows no discernible MR. In contrast, a thin Pt layer in contact with ferrimagnetic insulator YIG exhibits a sizable MR whose magnitude decreases with increasing Pt thickness. The MR in Pt/YIG depends on the direction of the magnetization of the underlying YIG with respect to the current, with a unique angular dependence which is totally different from anisotropic magnetoresistance. Spin Hall MR (SMR) due to spin/charge current conversion has been proposed to account for the new MR. The SMR model, in which Pt remains non-magnetic, involves the conversion of charge/spin current propagating in the Pt layer due to the spin Hall effect (SHE) and the inverse spin Hall effect (ISHE), and the concurrent absorption and reflection of spin current at the surface of the ferrimagnetic insulator. Meanwhile, there are ample evidences indicate Pt acquires polarized moment when in contact with YIG substrate due to magnetic proxitmiy effect. However, the nature of the intriguing new MR remains to be resolved. In this work, by investigating the new MR in different systems and revealing its field dependence, we demonstrate that the new MR has two contributions from spin current and MPE. Both contributions give the unique angular dependence. The MR at low field is mainly related to spin current transmitted across the Pt-YIG interface, whereas at high field is due to MPE. The contribution of the spin current decreases, while that of MPE increases, with increasing magnetic field H. The feature of new MR can also be reproduced when Pt is in contact with a non-magnetic insulator doped with a few percent of Fe impurities. Through tuning the YIG surface and insertion between Pt and YIG, we are able to separate the two contributions. By inserting an Au layer thicker than 6 nm between Pt and YIG, the intrinsic spin current contribution can be isolated.In most spin-related phenomena, such as spin Seebeck effect and spin pumping etc., the inverse spin Hall effect is utilized to detect pure spin current. ISHE has been observed only in non-magnetic metals, such as Pt and Au, with a strong spin-orbit coupling. Harnessing the spin Seebeck effect in ferromagnetic insulator yttrium iron garnet (YIG), we utilize the pure spin current from temperature gradient to demonstrate the ISHE in a ferromagnetic Permalloy (Py). Through controlling the spin current injection by altering the Py/YIG interface, we have isolated the spin current contribution and demonstrated the ISHE in a ferromagnetic metal, the reciprocal phenomenon of anomalous Hall effect. Using Py of various thicknesses, we have determined spin Hall angle for Py with a value comparable to that of Pt. We note that Pt has one of the largest spin Hall angle in non-magnetic metals people have investigated. The realization of ISHE in ferromagnetic metals greatly expands the varieties of materials that can be exploited for spin current phenomena, including the use of inexpensive materials with exceptionally larger spin Hall angle.