CRITICAL EXPONENTS AT THE FERROMAGNETIC PHASE-TRANSITION OF FE100-XPTX SINGLE-CRYSTALS

We study the ferromagnetic phase transitions of single crystals from the Invar system Fe100-xPtx 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 Fe100-xPtx is described by a universal fixed point for a three-dimensional magnet with long-range magnetic interactions with the critical exponents beta = 0.46, gamma = 1.22, and 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 Fe75Pt25.