Change in character of the paramagnetic to helimagnetic transition in CrAs upon MnAs substitution

Abstract

The structural and magnetic characteristics of the para- (P) to helimagnetic (H c ) transition in Cr 1- t Mn t As (0.00 ⩽ t ⩽ 0.67 ±0.01) have been studied by X-ray diffraction, differential scanning calorimetry and magnetic susceptibility measurements. The distinctly first order character of the P to H c transition is retained at low substitution levels of Mn for Cr, as manifested by the observation of latent heat, temperature hysteresis and discontinuous jumps in the unit cell dimensions. All these effects decrease with increasing t, and in the vicinity of a pseudo tricritical point at t = 0.17 ± 0.02 the discontinuities in the a and c axes ( Pnma setting of the MnP type unit cell) and the temperature hysteresis are no longer unambiguously detactable. However, the gradually diminishing discontinuity in the b axis is maintained, and, together with the behaviour of the heat capacity, this shows that the first order character of the P to H c transition persists up to t = 0.32 ± 0.02. These unusual findings are discussed in terms of the concept of pseudo tricritical point, predicted by Bergman and Halperin for a compressible Ising system.