Magnetoelastic study of amorphous Fe 90+x Zr 10-x alloys

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-) Fe 90 Zr 10 , whereas after an initial decrease it increases steeply in a-Fe 91 Zr 9 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-Fe 90 Zr 10 and a-Fe 91 Zr 9 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 C from the Young's modulus measurements on systems (which exhibit strong magnetoelastic effects) such as the alloys in question.