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

Abstract

Exchange-coupled hard and soft magnetic layers find extensive use in data storage applications, for which their dynamical response has great importance. With bulk techniques, such as ferromagnetic resonance (FMR), it is difficult to access the behaviour and precise influence of each individual layer. By contrast, the synchrotron radiation-based technique of x-ray detected ferromagnetic resonance (XFMR) allows element-specific and phase-resolved FMR measurements in the frequency range 0.5-11 GHz. Here, we report the study of the magnetization dynamics of an exchange-coupled Ni 0.81 Fe 0.19 (43.5 nm)/Co 0.5 Fe 0.5 (30 nm) bilayer system using magnetometry and vector network analyser FMR, combined with XFMR at the Ni and Co L 2 x-ray absorption edges. The epitaxially grown bilayer exhibits two principal resonances denoted as the acoustic and optical modes. FMR experiments show that the Kittel curves of the two layers cannot be taken in isolation, but that their modelling needs to account for an interlayer exchange coupling. The angular dependence of FMR indicates a collective effect for the modes of the magnetically hard CoFe and soft NiFe layer. The XFMR precessional scans show that the acoustic mode is dominated by the Ni signal with the Co and Ni magnetization precessing in phase, whereas the optical mode is dominated by the Co signal with the Co and Ni magnetization precessing in anti-phase. The response of the Co signal at the Ni resonance, and vice versa, show induced changes in both amplitude and phase, which can be ascribed to the interface exchange coupling. An interesting aspect of phase-resolved XFMR is the ability to distinguish between static and dynamic exchange coupling. The element-specific precessional scans of the NiFe/CoFe bilayer clearly have the signature of static exchange coupling, in which the effective field in one layer is aligned along the magnetization direction of the other layer.

Ferromagnetic resonance of MBE-grown FeRh thin films through the metamagnetic phase transition

“…On the other hand, the FMR response for two exchange‐coupled ferromagnetic thin films has been studied in Refs. . As was pointed out, ferromagnetic coupling between the two layers can lead to a modulation of the intensity ratio between the two modes depending on the coupling strength.…”

Magnonic crystals for data processing

“…Major opportunities for fundamental studies [193] Other techniques Nuclear resonant scattering Detection Highest spatial resolution currently achievable [194] X-ray-detected ferromagnetic resonance (XFMR) Detection High spatial resolution, access to layer resolution [195][196][197] Electron-magnon scattering approach Detection Suitable for the detection of domain wall position [198,199] and temporal resolution. For spin-wave excitation techniques the efficiency of excitation, its coherency and the wavenumber range are of primary importance [155-157, 159, 160, 163-165, 168-172, 174, 177-189, 191-199].…”

Intensity enhancement of ferromagnetic resonance modes in exchange coupled magnetic multilayers

“…Similarly, a standard value of g i =0.01 is used for all layers. The cubic anisotropy of the NiFeCu is on the same order than that of the NiFe [50,70] and thus both are taken equal.…”

“…In some cases, these requirements can also be fulfilled by using structures composed of multiple materials. For example, in a bilayer with an effective ferromagnetic exchange coupling (which tends to align the magnetization of the layers parallel to each other), two resonance modes with distinguishable frequencies are observed: the low frequency acoustic (both layers are in phase) and the high frequency optical (both layers are in antiphase) modes [50,51]. The acoustic mode is of particular importance, as it has an enhanced intensity due to the additive contribution of each layer to the frequency signal.…”