Irreversible response in spin-glasses: An experimental study in amorphous Fe-Mn

The equilibrium and nonequilibrium susceptibilities χₑ and χₙₑ of the spin-glass (Fe0.64Mn0.36)₇₅{P}₁₆B₆{Al}₃$ have been measured as functions of temperature ($4.2<~T<~80$ K) and a cooling field ($8 Oe<~H<~2 kOe$). A phase transition occurs at a temperature ${T}c(H)$ below which the irreversible response ${Δ}(T){∝}T({{χ}}ₑ{-}{{χ}}ₙₑ)$ appears. ${T}c(H)$ decreases from the freezing temperature (${T}f=41.6$ K) as ${H}^{0.75±{}0.1}$. For small ${τ}{≡}1{-}{T}{{T}c(H)}$, ${Δ}$ behaves as $A{τ}+B{{τ}}²$ with field-dependent coefficients $A$ and $B$ and $A(H=0)=0$. The results are compared with the predictions of the mean-field theory of spin-glasses.