Mechanism of Spin Scattering in Ta investigated by Scanning Inverse Spin Hall Effect Meters

In this work, a scanning inverse spin Hall effect measurement system based on a shorted coaxial resonator has been built, which provides a high throughput method to characterize spin transport properties. The spin diffusion length of Ta at room temperature is determined via automatic measurements of Py/Ta bilayer strips with different thicknesses of Ta. The results show that the spin diffusion length is about 4 nm with conductivity about 7.5 × 105 Ω -1m-1, which lead to the conclusion that the intrinsic mechanism of spin relaxation of Ta is the Elliott-Yafet interactions. The setup developed in this work provides a convenient, efficient and nondestructive way to obtain the spin and electron transportation characteristics of the spintronic materials, which will fertilize this community by developing new materials and figuring out their mechanism. Spin pumping (SP), which transports polarized electron spins from a ferromagnet into the adjacent normal metal layer with assistance of microwaves, is one of the most efficient method 1, 2 for spin injection. It can avoid the conductance mismatch 3, 4 so that the spins can be injected from metals to semiconductors or vice visa. The injected spins are in nonequilibrium and can convert into charge currents during diffusion in the materials due to the spin orbital coupling, which is commonly known as the inverse spin Hall effect (ISHE)1. The coupling effects are essential to develop spintronic devices. There is one crucial parameter called spin diffusion length (λ), which measures the distance over which electron spins flipped 2. There are several ways to determine this parameter, such as Andreev spectroscopy 3, spin magnetoresistance 4, spin value 5, ISHE 6, and non-local method 7. In the non-local method, lithography with precision of nanometer is quite demanding. Among them, the SP-ISHE effect is widely used due to the high spin injection efficiency and low demanding of lithography. The experiments are usually measured by a resonator 8, coplanar waveguide 9 or transmission line 10. Tens of samples with different thickness are prepared and measured separately 8, 11, the spin diffusion length is determined by fitting the data with different film thickness 8. As proposed by Meinert et al. terahertz emission spectroscopy and broadband ferromagnetic resonance are the two lithography-free high-throughput ways to measure the ISHE 11. In this letter, we proposed a high through-put way to measure the ISHE with a shorted microwave probe. The shorted probe is already used to characterize the local ferromagnetic resonance (FMR) properties in a broadband frequency range, where the permeability mapping can be obtained 12, 13. In our experiments, the samples are prepared on a wafer and patterned with lithography or rigid masks. The measurements can be automatically performed with the help of motorized stage. By this a) Electronic mail: xwzhang@uestc.edu.cn 2 method, several parameters such as conductance, anisotropic magnetoresistance, FMR properties, photo-voltage from spin pumping can be measured simultaneously. The shorted microwave probe used in this experiment is a semirigid coaxial cable with length of 50 mm, which is terminated with a gold foil with width of 2 mm at one end, as shown in Fig. 1(a). The diameters of the inner and outer conductor of the probe are 3 mm and 6.35 mm, respectively. When the shorted probe is coupled to a RF generator, a microwave magnetic field in the y direction will be generated by the microwave current in the gold foil at the shorted-end. The magnitude of hy is about 20 A/m at 4 GHz according to the simulations [inset of Fig. 1(b)] with the Finite Element Method. The S-parameter S11 of the probe [Fig. 1(b)] was obtained by a VNA (Agilent N5234A). Stripes with permalloy/non-ferromagnetic metal (Py/Ta) bilayers are used to detect the ISHE voltages converted from the spins injected by the spin pumping. The geometries for the ISHE detection of the spin current is shown in Fig. 1(c). We use a Py/Ta bilayer film with interfaces normal to z. The width and the length of the sample are fixed to be 40 μm and 8 mm, respectively, to suppress the spin rectification effect (SRE) according to our previous work 14. The permalloy films are sputtered onto a thermally oxidized silicon wafer by magnetron sputtering. Two aluminum pads with 2×2 mm2 were sputtered on both ends of the bilayer strips to enhance the electrical connection with the metallic spring probes, which are fixed at the opposite sides of the coaxial cable to measure the DC voltages generated during the spin pumping. The block diagram of the automatic SP-ISHE measurement system is drawn in Fig. 1(d). Modulated microwaves generated by a signal generator are fed into the shorted probe after amplification by a power amplifier. Meanwhile, a reference signal with same frequency as the modulated wave is sent to a lock-in amplifier. The DC magnetic field is provided by an electromagnet. The sample can move in three directions (XYZ) through the motorized stage, thus the voltage signals in different area of a wafer sample can be obtained. Fig. 1. (a) Schematic drawing of the shorted probe. (b) S11 curve of non-load the shorted probe, the inset is the simulation result of microwave magnetic field. (c) Schematic drawing of the setup for measurement by the shorted