FMR and Thermal Spin Pumping in Garnets|Pt Bilayers

Abstract

The Y3Fe5O12 (YIG) ferrimagnetic insulator is recognized as a model system for spintronic and magnonic phenomena, including spin pumping, spin Seebeck, proximity effects, and spin wave propagation. In these materials, the spin current is produced by spin-waves or magnons. Magnons can be electrically excited and sensed via the Spin Hall Effect (SHE) and Anomalous Spin Hall Effect (ASHE) or they can be thermally excited through the Spin Seebeck Effect (SSE). To detect the spin current, a thin non-magnetic layer such as Pt is deposited on top of the material of interest. This transforms the spin current into an observable thermoelectric voltage (VSSE) via the Inverse Spin Hall Effect (ISHE). Pt/YIG systems allow efficient spin-charge conversion and pure detection of spin-current effects, respectively, due to their unique magneto-optical and spin dynamic properties [1]. These properties allow a clean detection of the pure spin current thanks to the spin-orbit effects in the Pt/YIG microstructures [2]. Additionally, Rare Earth (RE) garnet films are magnetic and magnetoelastic, and their properties can be manipulated through choice of composition and substrate. When a proportion of bismuth (Bi) is added, the magneto-optical properties of YIG increase according to the concentration of bismuth (Bi) at the yttrium site [3]. But there are few studies on how the spin dynamic properties are modified due to this substitution. In this work we study different parameters in the manufacture of ferrimagnetic/non-magnetic bilayers to raise the thermoelectric spin voltage VSSE.Samples with different thicknesses of pure Y3Fe5O12 (YIG) films and a 5% Bi substitution, that is, thin films of BiY2Fe5O12 (BiYIG), were grown on Gd3Ga5O12 (111) and Gd3Sc2Ga3O12 (111) substrates by pulsed laser deposition (PLD). XRD, HR-TEM / STEM-HAADF and GPA measurements revealed high quality crystalline films as it can be seen in Figure 1. Ferromagnetic resonance (FMR) measurements were performed at room temperature for all heterostructures. A resonance field (Hres) and a resonance linewidth (FWHM) were extracted and are shown in Figure 2 as a function of frequency. These values allow the calculation of a Gilbert damping coefficient that is directly correlated with the SSE according to Chang et. Al [4]. In our case, the Gilbert damping coefficient is (5-14)x10-4 for the YIG//GGG samples and (18-19) x10-4 for the YIG//SGGG samples. The linewidth of BiYIG samples is much wider. After growth of an 8 nm-Pt coating layer by sputtering, the damping in all the samples increases due to the contribution due to spin pumping from the YIG or BiYIG to the metal film. The difference between the Heff result and the bulk value of 4πMS reported (1750 G [5]) demonstrates the existence of a perpendicular anisotropy.With a maximum 20K thermal difference across the entire structure, we measure longitudinal SSE voltages in both YIG and BiYIG samples with sputtered Pt. The YIG//SGGG combination reported to have the best voltage signal from the Inverse spin-Hall effect (VISHE) and Spin Seebeck Effect (VSSE), along with the co-occurrence of a positive perpendicular anisotropy. In the case of the BiYIG samples, the voltages induced in the BiYIG//SGGG bilayers were lower with respect to YIG//SGGG but higher with respect to YIG//GGG. The samples with the lowest perpendicular magnetic anisotropy showed the highest VSSE in both cases, YIG and BiYIG.This work is funded thanks to the SPICOLOST (GA 734187) project framed within “Horizon 2020 Funding” of the European Union with the MARIE SKLODOWSKA-CURIE - RISE Actions (Research and Innovation Staff Exchanges) program coordinated by the University of Zaragoza. **