J. Phys. I France 1 (1991) 1809-1821
The low field response of a reentrant NiMn ferromagnet close to the tricritical pointR. M. Roshko and W. Ruan
Department of Physics, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
(Received 9 July 1991, accepted 5 September 1991)
Measurements of the low field complex dynamic susceptibility and of the relaxation of the thermoremanent magnetization of a reentrant Ni-23.6 at.% Mn ferromagnet, located very close to the tricritical point, are presented as a function of temperature, magnetic field, and observation time. The temperature dependence of the real part of the dynamic response in fixed static biasing field exhibits a triple-peaked structure, consisting of a "high" temperature ferromagnetic critical peak, and two lower temperature reentrant peaks. A critical analysis of the ferromagnetic peak yields a Curie tempearture K and an effective critical isotherm exponent with both low field and high field regimes, and , while the effective Kouvel-Fisher exponent , extracted from the temperature dependence of the zero field susceptibility, exhibits a maximum as a function of reduced temperature and approaches the mean field limit for . The decay of the low field thermoremanent magnetization, observed over four decades of time , provides direct evidence that the ferromagnetic state evolves into a spin glass-like configuration at low temperatures: within the ferromagnetic phase (65 K K), the relaxation is described by the superposition of a weak power law and a constant term, , while, within the reentrant phase ( T < 65 K), the decay abrubtly acquires typical spin glass charcteristics, and the functional representation changes to the superposition of a constant term and a stretched exponential, , with temperature dependent parameters n and indicative of canonical spin glass behaviour.
© Les Editions de Physique 1991