J. Phys. I France
Volume 1, Number 12, December 1991
Page(s) 1809 - 1821
DOI: 10.1051/jp1:1991241
J. Phys. I France 1 (1991) 1809-1821

The low field response of a reentrant NiMn ferromagnet close to the tricritical point

R. 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 $T_{\rm c}=(104\pm 2)$ K and an effective critical isotherm exponent $\delta$ with both low field and high field regimes, $\delta_{\rm LF}=5.6 \pm 0.3$ and , while the effective Kouvel-Fisher exponent $\gamma^{*}$, 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 $\gamma=1$ for $T \gg T_{\rm c}$. 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 $\ll T \ll 104$ 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 $\tau$ indicative of canonical spin glass behaviour.

© Les Editions de Physique 1991