First-order reversal curve analysis of spin-transition thermal hysteresis in terms of physical-parameter distributions and their correlations

We investigate the thermal hysteresis of spin-crossover compounds by using the first-order reversal curve (FORC) method. By magnetic measurements we have recorded the FORC data for the pure Fe- and Zn-diluted spin transition system [FeₓZn_1-x(btr)₂(NCS)₂].H₂O, where x governs, through cooperative interactions, the width of the thermal hysteresis loop. The wiping-out and congruency properties are obeyed and support the description of the system by independent spin-like domains. The FORC analysis show, for increasing dilution parameter 1-x, almost monotonous trends: (i) increasing width of the bias distribution, (ii) decreasing width of the coercivity distribution, (iii) increasing correlation between the bias and coercivity distribution. The Preisach distributions finally are expressed in terms of P(Δ,J), where Δ=energy gap and J=intra-domain interaction parameter are the major physical parameter quantities involved in the two-level (e.g., Ising-like) standard description of interacting spin-crossover units. The physical origin of the distributions is discussed and the eventual Δ-J correlation is determined. The pure compound exhibits a negligible Δ-J correlation and therefore can be considered as made of independent spin domains. The diluted compounds exhibit a sizeable Δ-J correlation, which can merely be explained by a small spreading of the composition parameter.