Non-Mean-Field Interpretation of the Irreversibility Line in Spin-Glasses

Susceptibility data on an amorphous GdAl spin-glass show an irreversibility onset line $t=-A{}_{-}{}^{}{}_{}{}^{}H_{}^{\frac{2}{{\phi }_{-}}}{}_{}{}^{}$ and a susceptibility crossover line $t=+A{}_{+}{}^{}{}_{}{}^{}H_{}^{\frac{2}{{\phi }_{+}}}{}_{}{}^{}$ ($t$ is reduced temperature; $H$ is field). The exponents $\phi$ are both close to 3, and the amplitudes $A$ differ only by a factor of 2. In contrast to earlier explanations of the irreversibility in terms of a de Almeida-Thouless mean-field instability, here both crossovers are explained in terms of a non-mean-field scaling theory.