MAGNETIC PHASE-TRANSITION IN THE METAL-RICH RARE-EARTH CARBIDE HALIDES GD(2)XC (X=BR,I)

The critical behavior of the ferromagnetic-paramagnetic phase transiti on of metal-rich rare-earth carbide halides Gd2BrC and Gd2IC is invest igated. These compounds exhibit a layered structure of Gd-C-Gd slabs w hich are sandwiched between layers of halogen atoms (Br,I). The critic al exponents beta, gamma', gamma, and delta are determined by several methods: modified Arrott plots, Kouvel-Fisher plots, scaling plots, an d ln J vs ln(mu(0)H) plots. The asymptotic critical exponents beta and gamma as well as the critical amplitudes J(0) and h(0)/J(0) were dete rmined from both the Kouvel-Fisher and the correction to scaling analy sis. The derived critical exponents are identical for all methods and in good agreement with the predictions of the three-dimensional Heisen berg model whereas the reduced critical amplitudes J(0)/J(s)(0), mu h( 0)/k(B) T-c, and D J(0)(delta)/h(0) cannot be related to a particular model. The temperature dependence of the Kouvel-Fisher exponent gamma( T) is discussed with respect to chemical disorder.