Including the magnetic field dependence on the superparamagnetic relaxation time $\tau$, "transition" lines in the $H-T$ plane are obtained for constant $\tau$. These lines follow the relation $T_H-T_0\propto H^{\nu }$ where $\nu \sim \frac{2}{3}$ except for $H\to 0$ which shows a crossover to $\nu =2\mathrm{}$. Thus a power law similar to that derived from mean-field models of spin-glasses is obtained, based strictly on a superparamagnetic relaxation-time approach. This questions the conclusion that experimental observations of $H-T$ lines are solely the result of a mean-field phase transition.

• Received 23 December 1983

DOI:https://doi.org/10.1103/PhysRevB.29.4156