Magnetoelastic study of amorphous Fe90+xZr10−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, Ebg(T), to arrive at the magnetic contribution to Young's modulus, ΔE/EEbg=[E(T)−Ebg(T)]/E(T)Ebg(T), in the ferromagnetic state. A strikingly different behaviour of ΔE/EEbg is observed in alloys whose Fe content differs barely by 1 at% in that ΔE/EEbg 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 (TC) down to 80 K. Generalisation of the Landau theory of phase transitions leads to an expression for ΔE/EEbg 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/EEbg 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 TC from the Young's modulus measurements on systems (which exhibit strong magnetoelastic effects) such as the alloys in question.