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

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.

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, 6, 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, 9, 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 (g_eff) 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, 14, 15, 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.

2. Experimental details

The tri-layer film stack (Ta/FeGaB/Ta) were deposited with DC & RF co-sputtering targets with base pressure < 1.0 × 10⁻⁷ Torr at room temperature onto fresh Si substrates. Fe₈₀Ga₂₀ and B targets are employed for FeGaB thin films. Here, the first Ta layer was employed as a buffer layer 25 nm, with different thickness (15, 25, 50, and 75 nm) of FeGaB film and 5 nm thick Ta film were deposited as the magnetic layer and the capping layer, respectively. The composition of the FeGaB film was characterized by using XPS and found as ~70, ~18, and ~12 at.% of Fe, Ga, and B, respectively. The surface morphologies images of samples were captured by Oxford Asylum MFP-3D AFM. The static magnetic properties of these samples were performed by employing Quantum Design® SQUID MPMS at room temperature. The dynamical magnetic properties studied using NanOsc Instruments Cryo-FMR in the VersaLab system with temperature variation 100–300 K and exciting frequency from 2 to 20 GHz.

3. Results and discussion

3.1 Surface morphology

Figure 1 shows the tri-layer film stack roughness is recorded by atomic force microscopy; all samples have shown roughness below 0.9 nm. This means that the high quality of films has been obtained.

Figure 1

(a) The RMS values versus of thickness of FeGaB layer of thin film stack Ta (25)/FeGaB (15, 25, 50, and 75)/Ta (5); inset display the 3D image of thin films Ta (25)/FeGaB (75)/Ta (5). (b) AFM image of Ta (25)/FeGaB (15)/Ta (5).

3.2 Magnetic properties

Figure 2 shows the static magnetization of Ta (25)/FeGaB (t)/Ta (5). Magnetic moments have been increasing with increasing thickness of FeGaB. This means that magnetic spins of FeGaB are aligned along the easy axis. The magnetic coercive field (Hc) and squareness (Mr/Ms) have measured for the thickness of FeGaB film as shown in Figure 2b. As the thickness of film increases, Hc has initially decreased, in a while gradually increased. At film thickness, 25 nm has shown the low value of Hc and squareness, due to the influence of the buffer layer on magnetic spins of FGB. The large value of Hc has been obtained for the 75 nm thickness of FGB.

Figure 2.

(a) M-H loop of Ta (25)/FeGaB (t)/Ta (5) (b) coercive field (Hc) and squareness (Mr/Ms) of Ta (25)/FeGaB (t)/Ta versus thickness of FeGaB.

3.3 Dynamic properties

Figure 3a and b shows the inhomogeneous of line width (∆H₀) and damping factor (α) as a function of temperature. The ∆H₀ and α have been derived from the resonance line width (∆H) – excitation frequency (GHz) by fitting ∆H = ∆H₀ + αf/γ [34, 35]. The thickness of FGB thin film increases, the (∆H₀) is increasing at a specific temperature. The large thickness of FGB thin film (75 nm) has shown a large value of the ∆H₀ among all thin films. The temperature lowering to 100 K from 300 K, ∆H₀ is decreasing for FGB thin film thickness of 15–50 nm. Whereas, the 75 nm thickness of FGB thin film has shown an increasing trend, due to the creation of defects or structural imperfections for lowering the temperature.

Figure 3.

(a) inhomogeneous of line width (∆H₀) as a function of temperature (b) damping factor (α) versus temperature (T(K)) for Ta (25)/FeGaB (t)/Ta (5).

Figure 3b shows the damping factor (α) as a function of temperature. The α has increased for lowering the temperature, except for the 75 nm thin film of FGB. The thickness of 50 nm FGB has shown a large damping value at 100 K. Because the 50 nm of FGB thin films shown a low value of ∆H₀ at 100 K. It means that the thin film has not produced any defects/structural imperfection, otherwise condensed the defects. Therefore, the damping factor of the spin-wave is increasing for a 50 nm thin film of FGB. Whereas, 75 nm thickness FGB has exhibited a low value of damping due to multiple scattering of spin-wave from the defect and imperfections.

To find out interface and surface damping, damping factor (α) multiply with the thickness of FGB film and plotted with the thickness of FGB as shown in Figure 4a. The plot (αt vs. t) is linearly fitted an obtained the surface or inherent damping (α₀) and interface damping (α_i) parameters [35]. These parameters are varying for different temperatures. The surface damping is initially increasing for 300–250 K as shown in Figure 4b, later on, it is decreasing for lowering the temperature. Whereas the interface damping is exhibiting the opposite nature to surface damping. Finally, we can conclude that surface damping is mostly dominant at near to RT, and interface damping is dominant at a lower temperature in this film stack.

Figure 4.

(a) The thickness dependence of damping fitted with linearly to obtain surface/ inherent damping (α₀) and interface damping (α_i) (b) the plot of the obtained α₀ and α_i temperature.

The effective magnetization has been found out from Kettle equation [35, 36, 37] fitted with frequency-resonance magnetic fields spectrum. Magnetic anisotropy (H_k) has been found along with effective magnetization, which shows a positive value and small value compare to effective magnetization. It means that no perpendicular anisotropy generates at the interface. Therefore, the effective magnetization is considering as surface magnetization (Ms). The parameter Ms. has been utilized for further calculation.

3.4 Spin pumping at the interface of Ta/ FGB

The spin pumping associate with the real part of the spin-mixing conductance (g_eff). The g parameter is proportional to the flux of angular momentum in the form of spin-polarized carriers the ferromagnet/nonmagnetic interface. This is seen by gyromagnetic precession in a ferromagnet. The enhanced damping factor has been found out by the subtraction of damping factor (α), inherent/surface damping (α₀). The inherent/surface, interface damping obtained from the plotting of damping-thickness of FGB as shown in Figure 4b. The enhanced damping is related to spin-mixing conductance and thickness of FGB [7, 11, 12, 38, 39].

$$∆\alpha =\alpha -{\alpha }_0=g{\mu }_B\frac{g_{\mathit{eff}}}{4\mathit{\pi Ms}{t}_{\mathit{FGB}}}$$E1

where g is Lande factor, μ_B is Bohr constant, Ms is saturation magnetization and t_FGB is the thickness of FeGaB magnetic film.

To investigate the spin mixing conductance (g_eff), the enhanced damping factor has multiplied with saturation magnetization and thickness of magnetic films FeGaB. The product term has plotted with the thickness of FGB thin film, as shown in Figure 5a. The extraction of the spin mixing conductance (g_eff) has been obtained by linear fit as shown in Figure 5b. The mixing conductance value of the Ta/FeGaB/Ta thin film stack is 0.082 × 10¹⁸ m⁻² at 300 K, which has been increased gradually with lower temperature, as shown in Figure 6. This values are comparable with Si/SiO/Ta/Co(t)/Cu/Ta [40, 41], the Co/Cu films has mixing conductance 0.41 × 10¹⁸ m⁻². The conductance order is the same value; magnitude is different values. FeGaB thin film has a lower magnitude than that of the Co(t)/Cu. We can understand that spin pumping at the interface has reduced, due to the large thickness of FeGaB films and the large value of magnetostriction constant FeGaB.

Figure 5.

(a) thickness depended on 4π × (α₋α₀) × Ms × t at different temperature 300–100 K for calculating the spin mixing conductance (g_eff) at the interface, (b) At 200 K, a linear fit of thickness depended on 4π × (α₋α₀) × Ms × t provided the g_eff as 0.0962 × 10¹⁸ m⁻².

Figure 6.

(a) spin mixing conductance (g_eff) as a function of temperature (T (K), (b) shows the linear fit of g_eff-T plot, the intercept of the plot provides spin mixing conductance at 0 K.

Figure 6a shows the spin mixing conductance as a function of temperature. The mixing conductance has enhanced for lowering the temperature. At 100 K, the mixing conductance is a large value, 0.1245 × 10¹⁸ m⁻². It means that spin diffusion at the interface of Ta and FeGaB is increasing for low temperatures. We can conclude that spin mixing conductance depends on temperature. The linear fit of mixing conductance provides OK spin mixing conductance, as shown in Figure 6b. This value is 0.1417 × 10¹⁸ m⁻².

4. Conclusion

The multi-layer thin film stack Ta (25)/FeGaB (15,25,50 and 75)/Ta (5) deposited using the sputter. These film’s surface morphology is found by AFM; the results suggested that good quality of thin films obtained. The static magnetization of the multi-layer thin film stack resulted as the magnetic moment increased for the increased thickness of FeGaB. The coercive field and squareness increased for a large thickness of FeGaB film. The dynamic magnetization of the multi-layer thin film stack informed that the inhomogeneous line width is increased for increasing the thickness of FeGaB; which is decreased for lowering the temperature. The damping factor decreased for lower temperatures. The thickness dependence of damping showed enhancement for increasing thickness of FeGaB and provided the inherent/surface and interface damping. The spin mixing conductance (g_eff) was calculated and increased for decreasing the temperature. The spin mixing conductance of Ta/FeGaB(t)/Ta is comparable with Co/Cu thin films. All results suggest the magnetostriction based thin film stack can be employed for magnonics, spin caloritronics, and spintronics applications.