Magnetic relaxation in a three-dimensional ferromagnet with weak quenched random-exchange disorder

Abstract

Isothermal remanent magnetization decay,M r(t), and 'in-field' growth of zero-field-cooled magnetization,M ZFC(t), with time have been measured over four decades in time at temperatures ranging from 0.25T _c to 1.25T _c (whereT _c is the Curie temperature, determined previously for the same sample from static critical phenomena measurements) for a nearly ordered intermetallic compound Ni₃Al, which is an experimental realization of a three-dimensional (d = 3) ferromagnet with weak quenched random-exchange disorder. None of the functional forms ofM r(t) predicted by the existing phenomenological models of relaxation dynamics in spin systems with quenched randomness, but only the expressions M_T(t) = M_o[M₁ exp(-t/T₁)+ (t/T₂)_-a] and M_zfc(t0 = M_o^'(1-{M'₁ exp(-t/T'₁)+ (t/T'₂) ^-a'}] closely reproduce such data in the present case. The most striking features of magnetic relaxation in the system in question are as follows: Aging effects are absent in bothM r t andM ZFC(t) at all temperatures in the temperature range covered in the present experiments. A cross-over in equilibrium dynamics from the one, characteristic of a pured = 3 ferromagnet with complete atomic ordering and prevalent at temperatures away from Tc, to that, typical of ad = 3 random-exchange ferromagnet, occurs asT ? Tc. The relaxation times t1(T)(t1 '(T)) and t2(T)(t2 '(T)) exhibit logarithmic divergence at critical temperatures T^T1_c(T_T'1c(H)) and T^T2_c(T_T2'c(H)); and T_T'1c and T_T2'c both increase with the external magnetic field strength,H, such that at any given field value, T_T'1c = T_T2'c. The exponent characterizing the logarithmic divergence in t 1 ' (T) and t 2 ' T possesses a field-independent value of ?16 for both relaxation times. Of all the available theoretical models, the droplet fluctuation model alone provides a qualitative explanation for some aspects of the present magnetic relaxation data Keywords Magnetic relaxation - spin dynamics - random-exchange ferromagnet - remanent magne-tization decay - time evolution of zero-field-cooled magnetization.