Ferromagnetic-resonance-induced spin pumping in Co₂₀Fe₆₀B₂₀/Pt systems: damping investigation

Abstract

Microstrip line ferromagnetic resonance (MS-FMR) has been used to investigate the dependence of the magnetic damping enhancement on the thickness of Co20Fe60B20 and Pt due to spin pumping. Samples with variable thicknesses of Co20Fe60B20 (Pt) are used to determine the spin mixing conductance (spin diffusion length) via the thickness dependence of the Gilbert damping parameter α of Co20Fe60B20/Pt heterostructures. The results obtained from the analysis of the MS-FMR measurements reveal that α increases linearly with the inverse thickness of Co20Fe60B20 for films capped with a 10 nm thick Pt layer, while the variation of α versus the Pt thickness of the 4 nm thick Co20Fe60B20 layer has an exponential behaviour. The experimental data was analysed using a ballistic or a diffusive spin transport model for spin pumping, which includes the effective spin mixing conductance of the CoFeB/Pt interface and the spin-diffusion length of Pt. The estimated values are 37.5 nm−2 (spin mixing conductance) and 1.7 nm when the ballistic transport model is used, and 56.75 nm−2 and 2.2 nm when the experimental data are analysed using a more realistic model based on a diffusive transport. Moreover, MS-FMR measurements reveal that the effective magnetization varies linearly with the Co20Fe60B20 inverse thickness due to the perpendicular interface anisotropy. This anisotropy, estimated to be 1.31 erg cm−2, reinforces the perpendicular magnetization easy axis.

Spin transport across the interface in ferromagnetic/nonmagnetic systems

“…The effect of including a thickness-dependent λ sf in the analysis is shown in Table I. Fitting with a constant λ sf gives values typical of previous FMR analyses between 1 and 2 nm [13,18], but here, the inclusion of a NM layer thickness dependence gives bulk λ sf close to 10 nm, as reported elsewhere [24,29,60]; this resolves the discrepancy between values of λ sf from different methods. The magnitude of for Pt supports reports that the EY mechanism is the dominant source of spin-flip scattering [41] and is of a similar order of magnitude (10 −1 ) to reported values [2].…”

“…The transport properties of such systems have been probed with several techniques, including magnetoresistance [5,9,10], magnetooptics [11], spin Hall angle [12][13][14], ferromagnetic resonance (FMR) [15][16][17][18][19][20], and combinations of these techniques [6,[21][22][23]. A range of values for the spin-diffusion length λ sf have been reported for Pt at room temperature from 1.2 nm up to 11 nm [11,13] from these different methods, with FMR measurements giving lower values for λ sf than spin Hall measurements [13,18,24,25 The physical basis of the interfacial contribution is ascribed to additional spin-flip scattering associated with an effective interface thickness due to disorder [6], however, it was shown theoretically for an atomically flat FM/heavy metal interface that d-d hybridization enhances the damping [26], which effectively increases g ↑↓ eff . Furthermore, the influence on spin pumping and the spin-mixing conductance of crystallographic structuring at the interface has recently been demonstrated [27][28][29].…”

Resonant dynamics of skyrmion lattices in thin film multilayers: Localised modes and spin wave emission

“…1(a). The choice of Fe rich FeCoB and the aforementioned optimized thicknesses of Pt and FeCoB allow to have sufficient DMI and PMA [50][51][52] suitable to stabilise skyrmions, while limiting spin pumping effects which would otherwise lead to an increased damping coefficient 53 . We estimate by FMR a Gilbert damping constant of α = 0.022 (see Supplementary Figure 1) for our samples which is relatively low for such multilayer systems 53 .…”

“…The choice of Fe rich FeCoB and the aforementioned optimized thicknesses of Pt and FeCoB allow to have sufficient DMI and PMA [50][51][52] suitable to stabilise skyrmions, while limiting spin pumping effects which would otherwise lead to an increased damping coefficient 53 . We estimate by FMR a Gilbert damping constant of α = 0.022 (see Supplementary Figure 1) for our samples which is relatively low for such multilayer systems 53 . The overall magnetic volume is enhanced with the 20 repetitions of the trilayer, which not only increases the thermal stability of the skyrmions 28 but also provides a larger signalto-noise ratio for inductive measurements.…”

Proximity-induced magnetism and the enhancement of damping in ferromagnetic/heavy metal systems

“…Another concluded that spin memory loss was unaffected by PIM 29 . However a ferromagnetic resonance (FMR) study reported that a reduction in Pt PIM resulted in a decrease in the interfacial contribution to damping 20,30 . The controversy is not limited to transition metal/HM systems, with studies of PIM and spin transport in ferrimagnetic YIG/Pt reporting that PIM has either no effect, as determined from FMR measurements 31 , or a significant effect, from temperature-dependent spin Hall effect measurements 32 and angular-dependent FMR analysis 33 .…”