Magnetodynamic properties of FeGaB/Al2O3 multilayer thin film stack for microwave applications

Abstract

Integrated magnetic devices show great potential for applications of radiofrequency and microwave integrated circuits, such as chip-scale magnetic inductors, bandpass filters, phase shifters, etc. [1, 2]. Recently, many researchers have been vastly concentrating on ferromagnetic/insulator sandwich structure for high-frequency applications. This thin film stack reduces the eddy-current as well as the out-of-plane anisotropy [3, 4], reducing the energy dissipation in form of heat. The FeGaB has soft magnetic nature, a large magnetostriction coefficient, and saturated at lower magnetic fields. Due to the low loss property of Al2O3 in microwave frequency, we have attempted to obtain low damping (α) and narrow linewidth through the FeGaB/Al2O3 thin film stack and characterize the ferromagnetic resonance (FMR) by utilizing Cryo-FMR.In this article, Ta (5)/ [FeGaB (15)/ Al2O3 (3)] n/Ta (3) / Al2O3 (2) thin-film stack are deposited by DC and RF sputtering on Si substrate, where n indicates the number of multilayer (n=1, 5, and 10) and parenthesis indicates thickness in nanometers. Fig.1 (a) shows normalized magnetic loop of the thin film stack, which reveals that the magnetic saturation moment has increased for increasing layer thickness. The coercive filed (Hc) has a low value below 3 Oe as indicated in Fig.1 (b). The dynamical magnetic properties have been studied using NanOsc Instruments Cryo-FMR in VersaLab system with temperature variation 300-100 K and exciting frequency from 2 to 20 GHz. The ferromagnetic absorption spectrum is shown in Fig.2 (a). The absorption spectrum contains the perpendicular standing spin wave (PSSW) modes for n=5 and n=10 in addition to FMR mode. The linewidth of absorption spectrum is fitted with dH= dH0 + αf/γ, (d=delta) to obtain damping factor (α), and inhomogeneous line width (dH0)[5]. The thin film stack with n=1 has a narrow linewidth and a low damping factor compared to other two thin film stacks, as shown in Fig.2 (b). When the temperature decreases, the damping factor has decreased for n=1, but increased for n=5 and n=10 due to defects arise for lowering the temperature. The Fig.2 (c) shows temperature dependent inhomogeneous linewidth for thin film stacks. As the temperature lowering to 100 K, the dH0 has increased for n=1 and n=10 thin film stacks, whereas decreased for n=5. The dH0 is lower for n=1 and larger value for n=5. The frequency dependence of FMR fields (Hres) plot provides effective magnetization (Meff) and magnetic anisotropy, when the plot has fitted to the Kittle equation. The Meff has a lower value for n=10 than other two stacks. The Meff has increased in all thin film stack as the temperature decreases. Since the absorption of the spectrum shows the PSSW modes in additional to FMR mode, the exchange magnetic fields (Hex) have been calculated by the subtraction method [6]. The plot of Hex and number of PSSW mode has provided the spin-wave exchange stiffness (D). The D values are 13.6615 and 10.7371 x10-18 Tm-2 for n=5, and n=10, respectively at 300 K. As the temperature decreases to100 K, the D has decreased for n= 5 thin film stack whereas increased for n=10. The temperature-dependent stiffness is following the T2 model in a linear form, and it suggests the existence of the electron–magnon interaction in this thin film stack. In order to calculate the constant of spin-wave exchange stiffness (A), saturation magnetization of thin film stack has multiplied with the exchange stiffness. At 300K, the A values are 7.1108, and 5.3213 pJ/m, respectively.In summary, we have deposited multilayer film stack of Ta (5)/ [FeGaB (15)/ Al2O3 (3)] n/Ta (3) / Al2O3 (2) onto Si substrates, and studied magnetodynamic (ferromagnetic resonance) properties with multilayer thickness and temperature dependence. The magnetic properties of thin film stack reveal that saturation magnetic moment has enhanced for increasing thickness of thin film stack, and Hc has obtained below 3 Oe. Therefore, the multilayer thin film stacks have the most significant for microwave applications. The dynamic magnetic properties suggest that the lowest damping and narrow linewidth are obtained for n=1 thin film stack compared to other thin film stacks. The FMR absorption spectrum of thin film stack with n=5, and n=10 show the PSSW modes in addition to the FMR mode. Consequently, the spin-wave exchange stiffness has been extracted. The temperature-dependent stiffness has followed the T2 model, which provides the strong presence of electron–magnon interaction in materials. The constant of spin-wave exchange stiffness has calculated with help of saturation magnetic moment and the exchange stiffness. Hence, the multilayer thin film stacks can be employed for microwave device applications. **