Ferromagnetic resonance investigation of magnetism in Fe-rich sputtered amorphous alloys

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Abstract
Fe-rich alloys in the Ti concentration range have been successfully prepared in the amorphous state for the first time using the DC co-sputtering method. Exhaustive ferromagnetic resonance (FMR) measurements were performed on these alloys at a fixed microwave-field frequency of about 9.23 GHz in the temperature range 77 - 300 K. In this temperature range, a single resonance is observed for the alloys with x = 10 and 11 as against two resonances (primary and secondary), having different properties, in the case of a-. For the primary (single) resonance in a- (the alloys with x = 10 and 11), the `in-plane' uniaxial anisotropy field scales with the saturation magnetization . This result suggests that the anisotropy energy is of dipolar origin. By contrast, the relation breaks down in the case of secondary resonance. The primary and secondary resonances thus characterize two distinctly different amorphous magnetic phases which widely differ in their chemical as well as geometrical short-range orders. For all the alloys in question, thermal demagnetization is mainly due to spin-wave (SW) excitations, and the SW stiffness D decreases with increasing x. SW modes soften at (the so-called re-entrant transition temperature) and D possesses a reduced value for . The existence of a re-entrant state at low temperatures is also vindicated by the rapid increase in the magnitude of the peak-to-peak FMR linewidth below a certain temperature. The temperature at which an upturn occurs in shifts to lower temperatures as the Fe concentration is decreased, indicating that the re-entrant behaviour is progressively suppressed with increasing x.