Magnetic characteristics of a high-layer-number NiFe/FeMn multilayer

Abstract

We report the static and dynamic magnetic characteristics of a high-layer-number NiFe/FeMn multilayer test structure with potential applications in broadband absorber and filter devices. To allow fine control over the absorption linewidths and to understand the mechanisms governing the resonances in a tailored structure similar to that expected to be used in real world applications, the multilayer was intentionally designed to have layer thickness and interface roughness variations. Magnetometry measurements show that the sample has complex hysteresis loops with features consistent with single ferromagnetic film reversals. Characterisation by transmission electron microscopy allows us to correlate the magnetic properties with structural features, including the film widths and interface roughnesses. Analysis of resonance frequencies from broadband ferromagnetic resonance measurements as a function of field magnitude and orientation provide values of the local exchange bias, rotatable anisotropy, and uniaxial anisotropy fields for specific layers in the stack and explain the observed mode softening. The linewidths of the multilayer are adjustable around the bias field, approaching twice that seen at larger fields, allowing control over the bandwidth of devices formed from the structure. V C 2015 AIP Publishing LLC.

Isotropic FMR frequency enhancement in thin Py/FeMn bilayers under strong magnetic proximity effect

“…In contrast to the conventional approach to enhancing the FMR frequency by tailoring the magnetic anisotropy, e.g. by using exchange bias [3][4][5][6][7][8][9][10][11] , we show that the frequency can be significantly increased by employing coupled FMR modes in the proximity-magnetized regime of the AFM near its Néel temperature, where the frequency gap between the spin excitations in the two materials becomes sufficiently narrow. This approach can potentially result in a new class of ferromagnetic-like materials operating at sub-THz frequencies, important for a variety of high-speed applications.…”

“…In conclusion, our findings demonstrate an alternative way of achieving enhanced FMR properties, by employing nmthin AFM films subject to strong finite-size and FM-proximity effects. In contrast to the conventional method of enhancing the FMR frequency by tailoring the magnetic anisotropy via unidirectional exchange bias [3][4][5][6][7][8][9][10][11], this approach can yield negligibly small in-plane anisotropy, thereby broadening the range of possible applications. The unusually strong interlayer exchange coupling and the rotatable AFM-proximity-magnetization deduced from our experiments offer additional tuning parameters for designing novel magnetic multilayered materials.…”

“…The idea to enhance the FMR frequency by combining FM and AFM materials, e.g. in thin-film multilayers [3][4][5][6][7][8][9][10][11], meets difficulties in practice, which arise from typically weak exchange coupling at FM/AFM interfaces [2]. This problem is well known and well-studied in relation to exchange bias [12,13] widely used in spintronic applications for creating the so-called exchange pinning of FM layers.…”