Magnetic anisotropy: FMR against VSM in interacting magnetic nanowires

JOAQUÍN ALMEIRA; CARLOS A. RAMOS; JULIÁN MILANO; FERNANDO MENESES; SILVIA E. URRETA; PAULA G. BERCOFF

Bariloche

Workshop; International Workshop on Spintronics 2022 (SPIN Argentina); 2022

Magnetic Resonance Laboratory at the Balseiro Institute? Centro Atómico Bariloche ? Comisión Nacional de Energía Atómica in Bariloche.

In this work, different techniques were applied to determine the magnetic anisotropy ofFe x Ni 100-x (x = 15, 25 and 38) nanowire (NW) arrays, grown into ordered nanoporous aluminatemplates with porosity P~0.35. The effective magnetic anisotropy was measured usingferromagnetic resonance (FMR) and also from DC hysteresis loops performed in a vibratingsample magnetometer (VSM). The FMR experiments were conducted at 34 GHz as afunction of the angle between the NW axis and the applied magnetic field H. At thisfrequency, the resonance fields are well above the saturation field. Magnetizationmeasurements, M(H), show the largest hysteresis when the external field is applied parallel tothe wire axis (the easy direction) and almost no hysteresis along the normal hard axis. Thislast characteristic is expected for uniaxial shape anisotropy directions well-alignedthroughout the array (Figure 1a). From the difference of areas of the M(H) demagnetizingcurves in the first quadrant, corresponding to the applied field parallel and perpendicular tothe easy-axis respectively, the magnetic anisotropy energy was determined, and from it aneffective anisotropy field, Ha was calculated. Comparing FMR and VSM results, similartrends with the iron content x are found; however, the magnitude of the magnetic anisotropyobtained is quite different, as illustrated in Figure 1b. This contrasts with previous results [1,2] that show a closer match between the two experimental determinations, suggesting that inthe present case the magnetization mechanisms considered may not follow a Stoner-Wolhfarth-like model and new internal degrees of freedom need to be considered in modelingboth the DC M(H) and the FMR behavior.[1] L. Forzani, A.M. Gennaro, R. Koropecki, and C. A. Ramos, “Sensing anisotropic strains with ferromagneticnanowires” APL 116, 013104 (2020); https://doi.org/10.1063/1.5132539.[2] J. De la Torre Medina, M. Darques, L. Piraux, and A. Encinas, “Application of the anisotropy fielddistribution method to arrays of magnetic nanowires” JAP, 105, 023909 (2009);https://doi.org/10.1063/1.3067773