Dynamic exchange via spin currents in acoustic and optical modes of ferromagnetic resonance in spin-valve structures

Abstract

Two ferromagnetic layers magnetically decoupled by a thick normal metal spacer layer can be, nevertheless, dynamically coupled via spin currents emitted by the spin-pump and absorbed through the spin-torque effects at the neighboring interfaces. A decrease of damping in both layers due to a partial compensation of the angular momentum leakage in each layer was previously observed at the coincidence of the two ferromagnetic resonances. In case of non-zero magnetic coupling, such a dynamic exchange will depend on the mutual precession of the magnetic moments in the layers. A difference in the linewidth of the resonance peaks is expected for the acoustic and optical regimes of precession. However, the interlayer coupling hybridizes the resonance responses of the layers and therefore can also change their linewidths. The interplay between the two mechanisms has never been considered before. In the present work, the joint influence of the hybridization and non-local damping on the linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve multilayers. It has been found that the dynamic exchange by spin currents is different in the optical and acoustic modes, and this difference is dependent on the interlayer coupling strength. In contrast to the acoustic precession mode, the dynamic exchange in the optical mode works as an additional damping source. A simulation in the framework of the LandauLifshitz-Gilbert formalism for two ferromagnetic layers coupled magnetically and by spin currents has been done to separate the effects of the non-local damping from the resonance modes hybridization. In our samples both mechanisms bring about linewidth changes of the same order of magnitude, but lead to a distinctly different angular behavior. The obtained results are relevant for a broad class of coupled magnetic multilayers with ballistic regime of the spin transport.

Anisotropic magnetization relaxation in ferromagnetic multilayers with variable interlayer exchange coupling

“…For the case of d = 6 nm, the obtained α 1 and α 2 values are ∼ 0.02 and ∼ 0.05, which are close to those reported in the literature for single Py and CoFe films (α 1 ∼ 0.006 ÷ 0.02 and α 2 ∼ 0.05). 15,25,46 The increase of the interlayer coupling affects the values of α 1 and α 2 , but leaves the ratio α 2 /α 1 almost unchanged.…”

“…This imbalance enhances damping of the magnetization precession. 9,15,25 The above spin relaxation effects, being of wide fundamental and applied interest, can be studied most suitably using ferromagnetic resonance (FMR). This powerful method for characterizing magnetic materials relates the measured FMR linewidth to the spin relaxation mechanisms outlined above.…”

Magnetization dynamics in an exchange-coupled NiFe/CoFe bilayer studied by x-ray detected ferromagnetic resonance

“…In the past, ferromagnetic resonance (FMR) has been widely used to study magnetization dynamics of multilayer systems [11][12][13][14][15] and exchange-coupled bi-and trilayer systems [16][17][18]. Both broadband coplanar waveguide (CPW) and resonant cavity FMR techniques are commonly employed.…”

Ferromagnetic resonance of exchange-coupled perpendicularly magnetized bilayers

“…However, as soon as the layers are coupled, each eigenmode involves magnetization motion in both layers, such that the damping of both layers matters to set the linewidth of a given eigenmode 19 even at low coupling when the mode can be considered to belong preferentially to one of the layers. We have illustrated in Fig.…”

“…Such a configuration-dependent linewidth has been found in the past in various in-plane magnetized systems including Fe/Au/YIG bilayers 20 and all-metallic spin-valves 21,22 ; in these weakly exchange-coupled systems, the configuration dependence of the linewidth was attributed to a configurationdependent contribution of spin pumping coupling 23,24 , but our results suggest that it could also be partly attributed to the interlayer exchange coupling, in line with the conclusions drawn for in-plane magnetized layers in ref. 19. A way to discriminate between spin-pumping-induced and interlayer-exchangeinduced configuration-dependence of the linewidth is that the first one is substantially enhanced when near a crossing of the acoustical and optical eigenexcitation frequencies, while the second one has a broadband impact i.e.…”