Magnetoelastic study of amorphous Fe_90+xZr_10-x alloys

Abstract

Young's modulus (E) of stress-relieved Fe-rich Fe-Zr amorphous alloys has been measured as a function of temperature in the range 80-350 K using the vibrating reed technique. The measured E(T) data are corrected for thermal expansion and non-magnetic background, E_bg(T), to arrive at the magnetic contribution to Young's modulus, ΔE/EE_bg=[E(T)-E_bg(T)]/E(T)E_bg(T), in the ferromagnetic state. A strikingly different behaviour of ΔE/EE_bg is observed in alloys whose Fe content differs barely by 1 at% in that ΔE/EE_bg decreases monotonously in amorphous (a-) Fe₉₀Zr₁₀, whereas after an initial decrease it increases steeply in a-Fe₉₁Zr₉ as the temperature is lowered from the Curie point (T_C) down to 80 K. Generalisation of the Landau theory of phase transitions leads to an expression for ΔE/EE_bg that includes both first- and second-order magnetoelastic contributions, which respectively are linear and quadratic in stress. This expression is shown to provide a straightforward explanation for the distinctly different behaviour of ΔE/EE_bg observed in a-Fe₉₀Zr₁₀ and a-Fe₉₁Zr₉ alloys. Furthermore, the present theoretical approach not only brings out clearly the role of exchange-enhanced local spin-density fluctuations in the thermal demagnetisation of spontaneous magnetisation but also permits an accurate determination of the pressure dependence of T_Cfrom the Young's modulus measurements on systems (which exhibit strong magnetoelastic effects) such as the alloys in question.