Spurious re-entrant behaviour in a system with short-range order

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Abstract
The a.c. susceptibility of three samples of a Ni76Mn24 alloy has been measured as a function of temperature and magnetic field at two widely different excitation freauencies, and under a variety of auenching conditions. While rapid auenching yields a canonical spin glass, a reduced auenching rate generates an apparently typical re-entrant ferromagnet with seauential transitions. For the latter samples, the superposition of finite static magnetic fields reveals a distinct peak in the vicinity of the lower transition, which is systematically suppressed in amplitude and temperature, while magnetic isotherms exhibit a strongly-temperature-dependent anomalous structure in the nonlinear susceptibility. There is also evidence for extremely subtle structure in the neighbourhood of the upper transition, but no well-defined critical peaks. We conclude that, while the lower transition is definitely correlated with the onset of a spin-glass phase, the high-temperature portion of the re-entrant profile is a conseauence of short-range atomie (and magnetic) order, and no genuine long-range ferromagnetism is present in this system. These data provide a vivid illustration of the way in which metallurgical inhomogeneities can imitate re-entrant behaviour, and reinforce the need for a comprehensive asymptotic critical analysis in order to properly characterize a system as critically re-entrant.