Temperature dependence of microwave absorption phenomena in single and biphase soft magnetic microwires

The microwave absorption phenomena of single and biphase magnetic microwires with soft magnetic behavior have been investigated as a function of DC applied magnetic field using two alternative techniques: (i) absorption measurements in the temperature range of 4-300 K using a spectrometer operating at X-band frequency, at 9.5 GHz, and (ii) room-temperature, RT, ferromagnetic resonance measurements in a network analyzer in the frequency range up to 20 GHz. Complementary low-frequency magnetic characterization was performed in a Vibrating Sample Magnetometer. Studies have been performed for 8 μm diameter small-magnetostriction amorphous CoFeSiB single-phase microwire, coated by micrometric Pyrex layer, and after electroplating an external shell, 2 μm or 4 μm thick, of FeNi alloys. For single phase CoFeSiB microwire, a single absorption is observed, whose DC field dependence of resonance frequency at RT fits to a Kittel-law behavior for in-plane magnetized thin film. The temperature dependence behavior shows a monotonic increase in the resonance field, H r, with temperature. A parallel reduction of the circular anisotropy field, HK, is deduced from the temperature dependence of hysteresis loops. For biphase, CoFeSiB/FeNi, microwires, the absorption phenomena at RT also follow the Kittel condition. The observed opposite evolution with temperature of resonance field, Hr, in 2 and 4 μm thick FeNi samples is interpreted considering the opposite sign of magnetostriction of the respective FeNi layers. The stress-induced magnetic anisotropy field, H K, in the FeNi shell is deduced to change sign at around 130 K.The support of the Spanish Ministerio de Economia y Competitividad under Project MAT2010-02798-C05-01 is acknowledged