Detailed ferromagnetic resonance study of amorphous Fe-rich Fe<SUB>90-x</SUB>Co<SUB>x</SUB>Zr<SUB>10</SUB> alloys. I: re-entrant behaviour and low-lying magnetic excitations

Results of exhaustive ferromagnetic resonance (FMR) measurements performed on amorphous (a-) Fe90-xCoxZr10 (0 &#8804; x &#8804;18) alloys at temperatures ranging from 10 to 500 K are presented and discussed in the light of existing theoretical models. The peak-to peak FMR linewidth &#916;Hpp consists of two contributions: one originating from two-magnon and multiple-magnon scattering processes and the other arising from the Laundau - Lifshitz - Gilbert damping mechanism. &#916;Hpp for the alloys with , as in other re-entrant (RE) systems, increases exponentially as the temperature is lowered through the RE transition temperature TRE. The Lande splitting factor is independent of both temperature and Co concentration, whereas the Gilbert damping parameter is temperature-independent but decreases with increasing x. For all the alloys in question, thermal demagnetization is mainly due to spin-wave (SW) excitations and the SW stiffness coefficient D renormalizes with temperature in accordance with the predictions of the itinerant-electron model. SW modes in the alloys with x &#8804; 2 soften at T &#8804; TRE and D possesses a reduced but finite value in the RE state. Competing interactions present in the parent alloy a- Fe90Zr10 confine the direct exchange interactions (DEIs) to the nearest neighbours only and partial replacement of Fe with Co progressively tilts the balance in favour of ferromagnetic interactions between Fe spins with the result that DEIs now involve next-nearest neighbours too and the divergence in &#916;Hpp(T)at low temperatures is completely suppressed for x &gt; 4