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

Abstract

Isothermal remanent magnetization decay,Mr(t), and ‘in-field’ growth of zero-field-cooled magnetization,MZFC(t), with time have been measured over four decades in time at temperatures ranging from 0.25Tc to 1.25Tc (whereTc 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 ofMr(t) predicted by the existing phenomenological models of relaxation dynamics in spin systems with quenched randomness, but only the expressions\(M_r (t) = M_0 [M_1 \exp ( - t/\tau _1 ) + (t/\tau _2 )^{ - \alpha } ]\) 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 bothMrt andMZFC(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 τ1(T)(τ1′(T)) and τ2(T)(τ2′(T)) exhibit logarithmic divergence at critical temperatures\(T_C^{\tau _1 } (T_C^{\tau '_1 } (H))\) and\(T_C^{\tau _2 } (T_C^{\tau '_2 } (H))\);\(T_C^{\tau '_1 } \) and\(T_C^{\tau '_2 } \) both increase with the external magnetic field strength,H, such that at any given field value,\(T_C^{\tau '_1 } = T_C^{\tau '_2 } \). The exponent characterizing the logarithmic divergence in τ1′(T) and τ2′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