Critical exponents at the ferromagnetic phase transition of Fe100-xPtx single crystals.

Abstract

We study the ferromagnetic phase transitions of single crystals from the Invar system ${\mathrm{Fe}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pt}}_{\mathit{x}}$ for different concentrations x and metallurgical order parameters S by measurements of the ac susceptibility and the magnetization. Well-defined phase transitions only exist within a limited range of the (S-x) plane---outside of this range the phase transitions are smeared. The ferromagnetic critical behavior of ordered ${\mathrm{Fe}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pt}}_{\mathit{x}}$ is described by a universal fixed point for a three-dimensional magnet with long-range magnetic interactions with the critical exponents \ensuremath{\beta}=0.46, \ensuremath{\gamma}=1.22, and \ensuremath{\delta}=3.6 for the spontanous magnetization, the susceptibility, and the isothermal magnetization exponent, respectively. Random metallurgical disorder changes the numerical values of the exponents, especially close to the stoichiometric composition ${\mathrm{Fe}}_{75}$${\mathrm{Pt}}_{25}$.