Analysis of TbZn resistivity temperature derivative above the Curie point using singular fitting equations

New analyses were carried out of previously published TbZn resistivity temperature-derivative data just above the Curie temperature, Tc. Fitting was accomplished using a weighted nonlinear least-squares program, GENLS, which allows one to take proper account of errors present in both the independent and the dependent variables. The utility of fitting with equations of either logarithmic or power-law form (involving a critical exponent λ), both with a pole at the Curie point, was investigated in detail. Fitting results were uncertain for the four-parameter power-law choice but, with one parameter fixed, led to estimates of Tc of 199.61 ± 0.02 K and of λ of 0.16 ± 0.01. The latter value is rather far removed from the value of 0.104 which is expected for a three-dimensional (n = 1) Ising model and from the value of 0.014 for a three-dimensional (n = 2) XY model. By contrast, fitting with a three-parameter logarithmic law, corresponding to a three-dimensional (n = 2) XY model, led to estimated standard deviations of all three free parameters much smaller than the corresponding ones of the four-parameter power-law fit and to a Tc estimate of 199.56 ± 0.03 K. The present results thus indicate the presence of marked XY or even Ising anisotropy. Quadrupolar effects seem irrelevant.