Ferromagnetic resonance study of magnetic order-disorder phase transition in amorphous Fe<SUB>90-x</SUB>Co<SUB>x</SUB>Zr<SUB>10</SUB> alloys

The utility of the ferromagnetic-resonance (FMR) technique to determine accurately the spontaneous magnetization and initial susceptibility critical exponents &#946; and &#947; , which characterize the ferromagnetic (FM) -paramagnetic (PM) phase transition at the Curie temperature TC for ferromagnetic materials is demonstrated through a detailed comparative study on amorphous Fe90Zr10 alloy, which involves bulk magnetization and FMR measurements performed on the same sample in the critical region. Magnetization data deduced from the FMR measurements taken on amorphous Fe90-xCoxZr10 alloys with x=0, 1, 2, 4, 6, and 8 in the critical region satisfy the magnetic equation of state characteristic of a second-order phase transition. Contrary to the anomalously large values of the exponents &#946; and &#947; reported earlier, the present values, &#946; =0.38&#177; 0.03 and &#947; =1.38&#177; 0.06, are composition-independent and match very well the three-dimensional Heisenberg values. The fraction of spins that actually participates in the FM-PM phase transition, c, is found to increase with the Co concentration as c(x)-c(0)&#8773; ax2 and possess a small value of 11% for the alloy with x=0. The "peak-to-peak" FMR linewidth (&#916;Hpp) varies with temperature in accordance with the empirical relation &#916;Hpp(T)=&#916; H(0)+[A/Ms(T)], where Ms is the saturation magnetization. Both the Lande splitting factor g as well as the Gilbert damping parameter &#955; are independent of temperature, but, with increasing Co concentration (x), &#955; decreases slowly while g stays constant at a value 2.07&#177; 0.02. are independent of temperature, but, with increasing Co concentration (x), λ decreases slowly while g stays constant at a value 2.07± 0.02