Magnetic relaxation in a three-dimensional ferromagnet with weak quenched random-exchange disorder
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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 Ni_{3}Al, 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$$M_r (t) = M_0 [M_1 \exp ( - t/\tau _1 ) + (t/\tau _2 )^{ - \alpha } ]$$ 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 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 T_{c}, to that, typical of ad = 3 random-exchange ferromagnet, occurs asT → T_{c}. The relaxation times τ_{1}(T)(τ_{1}^{′}(T)) and τ_{2}(T)(τ_{2}^{′}(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 τ_{1}^{′}(T) and τ_{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
S N Kaul^{1} ^{2} ^{} Anita Semwal^{1}
Volume 95, 2021
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