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

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) (where T-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 of M-r(t) predicted by the existing phenomenological models of relaxation dynamics in spin systems with quenched randomness, but only the expressions Mr(t) = M-0[M-l exp(-t/tau(l)) + (t/tau(2))(-alpha)] and M-ZFC(t) = M'(0)[1 - {M'(1) exp(-t/tau'(1)) + (t/tau'(2))(-alpha')}] 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 both M-r(t) and M-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 pure d = 3 ferromagnet with complete atomic ordering and prevalent at temperatures away from T-c, to that, typical of a d = 3 random-exchange ferromagnet, occurs as T --> T-c,. The relaxation times tau(1)(T)(tau'(1)(T)) and tau(2)(T)(tau'(2)(T)) exhibit logarithmic divergence at critical temperatures T-c(tau1)(T-c(tau'1)(H)) and T-c(tau2) (T-c(tau'2)(H)); T-c(tau'1) and T-c(tau'2) both increase with the external magnetic field strength, H, such that at any given field value, T-c(tau'1) = Tc-tau'2. The exponent characterizing the logarithmic divergence in T-c(tau1)(T-c(tau1)) and tau'(2)(T) possesses a field-independent value of similar or equal to16 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.