THE LOW-FIELD MAGNETIC RESPONSE OF A REENTRANT NI MN ALLOY

The a.c. susceptibility of a re-entrant Ni77.5Mn22.5 alloy has been measured as a function of temperature, 15 K < T < 250 K, and magnetic field, 0 < H(a) < 704 Oe, at excitation frequencies, omega, of 16 and 2400 Hz. While the zero-field susceptibility exhibits typical re-entrant characteristics, the application of small static magnetic fields reveals a triple-peaked structure in the temperature dependence of the susceptibility. The high-temperature peak exhibits a systematic suppression in amplitude and upward shift in temperature which is typical of ferromagnetic critical peaks. A critical analysis is performed which yields estimates for the Curie temperature, T(c), and the effective exponents gamma* and delta*. The remaining binary structure is localized in the vicinity of the re-entrant transition. Both peaks are suppressed in amplitude and shifted downward in temperature with increasing field. The upper peak displays the reversible behaviour characteristic of the Gabay-Toulouse (GT) transition, while the lower peak is strongly irreversible and exhibits a simple power-law dependence of peak temperature on applied field, which is consistent with the de Almeida-Thouless (AT) instability line. An analysis of susceptibility isotherms obtained at a variety of temperatures between 46 and 160 K reveals a non-divergent anomaly in the temperature dependence of the amplitude of the leading nonlinear term in the vicinity of the re-entrant temperature. A comparison of this anomalous structure with numerical simulations based on a simple effective-field model indicates that it may be the longitudinal response to transverse spin-glass freezing.