Probing the Spin Pumping Mechanism: Exchange Coupling with Exponential Decay inY3Fe5O12/Barrier/PtHeterostructures

Abstract

Ferromagnetic resonance (FMR) driven spin pumping of pure spin currents from a ferromagnet (FM) into a nonmagnetic material (NM) promises new spin-functional devices with low energy consumption [1-6]. The mechanism of spin pumping is under intense investigation and it is widely believed that exchange interaction between the FM and NM is responsible for this phenomenon [2-5, 7]. We observe a thousand-fold exponential decay of the spin pumping from 20-nm thick Y 3 Fe 5 O 12 (YIG) films to platinum across insulating barriers, from which the exponential decay lengths of 0.16 and 0.23 nm are extracted for oxide barriers with band gaps of 4.93 eV and 2.36 eV, respectively. This prototypical signature of quantum tunneling through a barrier underscores the importance of exchange coupling for spin pumping and reveals its dependence on the characteristics of the barrier material. Generation and manipulation of spin currents is key to spintronic applications [1]. FMR driven spin pumping has been demonstrated to inject a pure spin current through angular 2 momentum transfer from an FM to an adjacent NM [2-6]. General belief assumes a dynamic coupling via exchange interaction between the precessing magnetization (M) of the FM and the conduction electrons of the NM at the NM/FM interface. This mechanism will lead to a short, atomic-scale coupling that decays exponentially with separation between the FM and NM. However, this has not been experimentally confirmed, partially due to the large dynamic range needed to measure such rapidly decaying spin pumping signal [6, 8]. Our recent demonstration of large spin pumping in Pt/YIG bilayers with mV-level inverse spin Hall effect (ISHE) voltage, V ISHE , offers a material platform with signal-to-noise ratio sufficient to quantitatively characterize the coupling range, enabling detailed insights into the spin pumping mechanism. We use three different barrier materials, including two oxide insulators and Si, to systematically investigate the barrier thickness t dependence of spin pumping in Pt/barrier(t)/YIG heterostructures. The clear exponential decays of ISHE voltage with characteristic lengths of ~0.2 nm for the oxide barriers provide decisive evidence for the predicted exchange coupling model for spin pumping. Our experiments utilize YIG thin films grown on (111)-oriented Gd 3 Ga 5 O 12 (GGG) substrates by an off-axis sputtering technique we developed for epitaxial film growth of complex materials [9-12]. Figure 1a shows a high resolution x-ray diffraction (XRD) scan of a 20-nm YIG film near the YIG (444) peak with clear Laue oscillations, indicating high uniformity throughout the film. The XRD rocking curve in the inset to Fig. 1a gives a full width at half maximum (FWHM) of 0.0185, demonstrating excellent crystalline quality. Figure 1b shows a representative FMR derivative spectrum for a 20 nm YIG film taken at radio-frequency (rf) f = 9.65 GHz and microwave power P rf = 0.2 mW with an in-plane magnetic field, from which the peak-to-peak linewidth (H) of 10 Oe is obtained [13]...

Spin current and inverse spin Hall effect in ferromagnetic metals probed by Y3Fe5O12-based spin pumping

“…[14][15][16] In this Letter, we report investigation of FMR spin pumping from YIG epitaxial films to Py, Fe, Co, and Ni, either in direct contacts or through a Cu spacer. The large spin pumping signals enabled by our YIG films 8,9 allow for quantitative understanding of pure spin currents in various FM metals and detection of the spin currents by inverse spin Hall effect (ISHE).…”

Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator

“…The spin current injected into the NM layer is J S ¼ ft NM σ NM =½Θ SH λ NM tanhðt NM =2λ NM ÞgðV ISHE =LÞ, where σ NM , t NM , Θ SH , λ NM are the conductivity, the thickness, the spin Hall angle, and the spin diffusion length of the NM layer, respectively [21]. Then, we have J S ¼ ft NM σ NM = ½Θ SH λ NM tanhðt NM =2λ NM ÞgðSΔT=t YIG Þ.…”

Strain-tunable extraordinary magnetocrystalline anisotropy inSr2CrReO6epitaxial films

“…We use off-axis sputtering [9,[14][15][16][20][21][22][23]] to deposit 90 nm thick Sr 2 CrReO 6 (001) films on (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT), SrTiO 3 (STO), and a relaxed SrCr 0.5 Nb 0.5 O 3 (SCNO) buffer layer on LSAT with lattice constants a = 3.868Å, 3.905Å and 3.946Å, respectively (see Supplemental Material for growth parameters). As a comparison, the pseudocubic lattice constants of bulk Sr 2 CrReO 6 are a p = 3.907Å and c p = 3.905Å [10].…”