A Study on Spin Transport in Paramagnetic Insulators

概要

1. Introduction
In spintronics, a flow of spin angular momentum is called a spin current, which drives spintronic functions in condensed matters. The discoveries of new materials which carry spin currents have stimulated progress in spintronics. After the demonstration of the long-range spin transport via magnons in ferrimagnetic Y3Fe5O12 (YIG), a magnetic insulator became an important platform in spintronics [1]. However, a paramagnetic insulator (PI) without magnetic ordering, has not attracted much attention so far, since it seems to be unlikely carrier of spin currents; its large charge gap prohibits electron conduction, and weak exchange interaction cannot facilitate magnons. This thesis investigates spin transport in a bulk PI and at an interface of a normal metal (NM)/PI. Our results unveil unique advantage of paramagnets and open a new research field: Paramagnetic spintronics.

We used a paramagnetic insulator Gd3Ga5O12 (GGG), which shows paramagnetic behavior above 2 K. Due to the large electron spin of Gd ion (S = 7/2), GGG shows larger magnetization than that of YIG by applying magnetic field B at low temperature T.

2. Observation of Long-range Spin Transport in Paramagnetic Insulators
We investigated spin transport through bulk GGG using a typical nonlocal device comprising two Pt wires on a GGG slab shown in Fig. 2a. We fabricated the device using e-beam lithography and a liftoff process. One Pt wire is a spin injector and the other is a spin detector. At the injector, an applied charge current induces the spin Hall effect (SHE), which injects a spin current into GGG. The injected spin current flows in GGG and is electrically detected via the inverse SHE (ISHE) at the detector as a nonlocal voltage V.

Figure 2b shows the magnetic field dependence of V at 300 K and 5 K. At 300 K, there is no V signal (red plots). However, a clear voltage signal appears (blue plots) at 5 K, which increases with increasing B. The shape of V is associated with the magnetization M of GGG which exhibits a large Brillouin-type M under the high B at low T. Our systematic measurement results show that the long-range spin transport through GGG. Comparison between the experimental results and theoretical model clarifies that the spin conductivity of GGG is much greater than that of the ferromagnetic YIG known as the best material for spin transport. We attribute this to the spin-wave formation via the long-range dipole interaction which is enhanced by the large M of GGG [2].

3. Observation of Paramagnetic Spin Hall Magnetoresistance
We studied the mechanism of spin transport at a Pt/GGG interface using the spin Hall magnetoresistance (SMR) that is a sensitive probe of the efficiency of interfacial spin transport for NM/magnet bilayers [3].

We found the resistivity of a Pt-Hall bar on GGG changes in response to the direction and amplitude of B. Figure 3 shows the B dependence of the resistivity of Pt. By applying in-plane (out-of-plane) B, we observed SMR [spin Hall anomalous Hall effect (SHAHE)] shown as blue (red) plots in Fig. 3. Both SMR and SHAHE saturate above 5 T, consistent with the saturation of paramagnetic M. Our theoretical model based on an interfacial exchange interaction well reproduces the observation and clarifies the mechanism of spin transport at the interface, the efficiencies of spin transfer and field torque between paramagnetic spins and conduction electron spins [4].

4. Summary
Through this thesis, we explored spintronic function with paramagnetic insulator and demonstrated the long-range spin transport, which upsets conventional wisdom, and spin Hall magnetoresistance in the Pt/GGG systems. This thesis develops paramagnetic spintronics and provides new material-design strategies for future spintronic devices.