Spin glassiness, ferromagnetism, and their coexistence in a Au-Fe alloy

Magnetization measurements were made on the alloy Au₈₅{Fe}₁₅$ quenched from two different annealing temperatures (${T}A=275 and 200^∘C). In both conditions, the alloy is ferromagnetic from{T}c(~95 and ~105 K, respectively) down to{T}fg{≈}30$ K, where it transforms to a reentrant spin-glass-like state, as evidenced by its irreversible time-dependent magnetic properties. However, the time-independent magnetizations ($M$) measured in various fixed fields applied during cooling (${H}cool$) reveal a significant thermodynamic difference between the reentrant states in the two cases. For ${T}A=275^∘C, theM({H}cool)$ isotherms below ${T}fg$ have a finite initial slope, as in a normal spin glass, whereas for ${T}A=200^∘C, the corresponding isotherms exhibit a spontaneous ferromagnetic moment, indicating that the state is mixed. In the magnetic phase diagram with{T}A^{{-}1}$ as abscissa, the boundary line between the mixed and nonmixed spin-glass-like states lies well away from the multicritical point, in apparent disagreement with theoretical expectation. The magnetization behavior near Tfg suggests that the transition from reentrant state to ferromagnetism is continuous and that some spin-glass order persists into the ferromagnetic state.