CONCENTRATION-DEPENDENCE OF THE RANDOM-FIELD-CROSSOVER SCALING IN FEXZN1-XF2

The proportionality between the field-induced shifts DELTA-T(c)(H) and DELTA-T(eq)(H) of the transition and equilibrium temperatures T(c)(H) and T(eq)(H) and the root-mean-square random field h(RF) was studied over a wide range of concentration (0.31 less-than-or-equal-to x less-than-or-equal-to 0.84) in the ideal random-field Ising model (RFIM) system Fe(x)Zn1-xF2. T(c)(H) and T(eq)(H) were determined experimentally from the optical birefringence DELTA-n, for which d(DELTA-n)/dT exhibits a peak at T(c)(H) proportional to the magnetic specific heat. T(eq)(H) was determined from the point above which zero-field-cooled and field-cooled measurements gave identical results. For all x studied, T(c)(H) and T(eq)(H) were found to shift from the H = 0 transition at T(N) as [DELTA-T(c)(H), DELTA-T(eq)(H)] = [c,c(eq)]T(N)h(RF)2/phi as predicted, with phi = 1.42 +/- 0.03, the universal random exchange to RFIM crossover exponent, after a mean-field correction bH2. The nonuniversal quantities c and c(eq) were found to be of order unity as expected, but slightly x dependent. An empirical x dependence for c and c(eq) is suggested. This analysis is applied to existing data on MnxZn1-xF2, and leads to very similar behavior for c, with essentially equal numerical values.