Spin Pumping in Magnetostrictive Ta/FeGaB/Ta Multilayer Thin Films

*Karampuri Yadagiri and Tao Wu*

### **Abstract**

The magnetostrictive multilayer thin film stack (Ta/FeGaB(t)/Ta) deposited/ sputtered, studied the surface morphology, static and dynamic magnetic properties. The static magnetic properties multilayer studied; the coercive field and squareness increased for increasing thickness of FeGaB. The systematic study of damping in Ta/FeGaB/Ta multilayer performed by use of broadband ferromagnetic resonance (FMR) spectroscopy in-plan geometry in the range of temperature from 300 K to 100 K. The data were fitted to obtain the inhomogeneous line width (∆H0) and the damping factor (α). The damping factor is enhanced for the increased thickness of FeGaB. The enhancement of damping is due to spin pumping at the interface of Ta and FeGaB. The spin mixing conductance (geff) was calculated for magnetostriction thin films FeGaB; which had been increased for lowering the temperature. At 0 K, the geff of thin-film stack has 0.141 × 1018 m−2. Therefore, the magnetostriction multilayer film stack can be used for magnonics, spin caloritronics, and spintronics applications.

**Keywords:** soft magnetic thin films, surface morphology of thin films, magnetization, ferromagnetic resonance, damping factor, spin pumping

### **1. Introduction**

The recently, several phenomena/theories have been proposed across the interface of heavy metal/ferromagnetic (FM) such as the spin Hall effect (SHE) [1], spin-orbit torques [2], Dzyaloshinskii-Moriya interaction [3], and spin pumping [4]. In a while, the spin current mechanism has developed for non-magnetic materials (NM), which is one of the key points of modern spintronics. In the ferromagnet-nonmagnetic metal layer, a precessional magnetization in ferromagnetic layer generates as oscillating spin density, which can source a spin-polarized current to flow into the normal metal. This phenomenon is known as spin pumping [5–7], which offers the most interest of spin current. This spin current flows and dissipates into nonmagnetic metal, due to the influence of spin-orbit interaction. Subsequently, the damping factor enhances in NM/FM system [8–10].

Spin pumping theory [11] illustrates the relaxation of spin current in the NM layer, which denotes in way of spin-mixing conductance (g). The mixing conductance has been assumed as a properly of the NM. According to the theoretical model [12], spin pumping is a complex picture; however, it has been explained experimentally through the enhancement of damping factors for different materials. The effective mixing

conductance (geff) consists of spin current across the interface of NM-FM, besides to the relaxation of spin current. The interface spin current characterizes as an effective specific interface spin resistance and relaxation associate with crossing the interface, called spin memory loss. The other model suggests the spin memory loss is due to interfacial spin-orbit interaction [10]. The interfacial spin resistance/spin memory loss, details of the FM-NM interface structure show an important role in determining the damping contribution due to spin pumping.

Magnetostrictive materials have been extensively utilized in vast applications such as sensors, actuators, micro-electrochemical-mechanical–systems (MEMS), and energy harvesters [13–16]. Among all the magnetostrictive materials, Terfenol-D has a large magnetostrictive constant (λ) 1600 ppm [17, 18], which is widely used in low-frequency devices, but the drawback of this material is hard to get saturation. The rare-earth free alloy, FeGa (Galfenol) shows great potential with high saturation magnetostriction of ~400 ppm for single crystal [19, 20] and ~280 ppm for directional solidified polycrystalline alloys [21]. It possesses a large saturation magnetization (~18 kG) at a low field (~100 Oe) [22]. However, the FeGa single-crystal films have been very lossy at microwave frequencies with a large line width of ferromagnetic resonance, which cannot be incorporated microwave magnetoelectric devices. The integration of metalloid element carbon into FeGa alloys are formed the D03 phase, which shows high saturation magnetostriction. This magnetostriction value is greater than that of FeGa binary alloys [22, 23]. Boron (B) is a well-known metalloid element, which is widely used in soft magnetic films for instance CoFeB thin films [24, 25]. Because of the B element inside CoFe thin film, the grain size of films refines and diminishes magneto crystalline anisotropy leading to excellent magnetic softness and microwave performance. In literature, the incorporation of B atoms in these FeGaB films can produce a nearly tripled saturation magnetostriction at a B content of 12 at.% [26]. The combination of soft magnetism, large magnetostriction constant, and excellent microwave magnetic properties make FeGaB film a potential candidate for magnetoelectric materials and other RF/microwave device applications. The magnetoelectric effects employ for creating electrostatically tunable [27] microwave resonators, phase shifters, and filters, which are important for applications in signal processing technologies [28, 29], and in schemes for performing logical processing operations using spin waves [30, 31].

The hybrid structure of ferromagnet (magnetostrictive ferromagnetic)/piezoelectric produce magnetoelectric effects [27]; which employs to construct efficient magnetic random access memory (MRAM) and spin-wave logical processing devices [32, 33]. The understanding of such nanoscale magnetoelectric devices requires the development of thin magnetic films with high magnetostriction constants. Therefore, the high value of magnetostriction of the film utilizes to reduce the line width and increase the magnitude of the magnetoelectric effect. The narrow resonant line widths and low damping are particularly important attributes of materials for microwave and spin-wave applications. So, we pick the FeGaB with 12 at.% of B; which has large magnetostriction constant and to investigate resonant linewidth and spin pumping across interface FeGaB and non-magnetic film.

In this work, we focus on the thickness dependence of magnetostrictive multilayer thin film stack (Ta/FeGaB(t)/Ta) deposition, surface morphology studies, static and dynamic magnetic properties. The thickness dependence of FMR shows the enhancement of the damping factor, which attributes the spin pumping across the interface of FeGaB and Ta. The spin-mixing conductance of the magnetostrictive multilayer thin film stack shows 0.081 × 1018 m−2 at 300 K, which is comparable with thin films of Si/ SiO2/Cu/Co(t)/Cu.
