Critical behavior in the heat capacity of Fe[S2CN(C2H5)2]2Cl: Evidence for chiral universality

The heat capacity of Fe[S 2 CN(C 2 H 5 ) 2 ] 2 Cl has been measured in the region of the 2.5 K ferromagnetic transition with much better temperature resolution than in any previous set of data on this material. Analysis of data within 0.1 in reduced temperature t = (T- T c )/T c of the transition, both above and below T c . leads to a value T c = 2.442 4 K. This agrees well enough with the previously established value 2.457 K. considering likely uncertainties in absolute temperature among different instruments. Simple power law analysis of the magnetic heat capacity above T c , i.e., C(mag)t - α , yields α≃0.22 for reduced temperatures above 0.01. A few data at temperatures yet closer to T c suggest a larger value for α, and possible crossover; but the probability is substantial that these are transition rounding effects. A global analysis of data above and below the transition, and allowing for additional regular terms in the heat capacity, leads to the T c given above and α = 0.244 ′0.005, along with other parameters. The a is more consistent with the three-dimensional (3D) chiral Heisenberg model value 0.24′0.08 than with the 3D chiral XY model value 0.34′0.06. The leading amplitude ratio is A + /A - = 0.325′0.005, also consistent only with chiral model results.