Critical magnetic susceptibility in the presence of long-range forces

Abstract

The relatively long spin-spin force range in the ferromagnet Gd${\mathrm{Ni}}_{2}$ renders it a prototypal system in which to compare critical exponents with calculations for a system where second-nearest-neighbor interactions are important. We have measured the low-field magnetic susceptibility of Gd${\mathrm{Ni}}_{2}$ in the reduced temperature interval ${10}^{\ensuremath{-}3}\ensuremath{\lesssim}\ensuremath{\epsilon}\ensuremath{\lesssim}4$, where $\ensuremath{\epsilon}\ensuremath{\equiv}\frac{(T\ensuremath{-}{T}_{C})}{{T}_{C}}$. The observation of mean-field behavior over an unusually large temperature interval has allowed experimental determination of the high-temperature parameters needed for a straightforward application of the series expansions. Hence, we approximate the long-range forces by including first- and second-nearest-neighbor interactions in our Hamiltonian. High-temperature series are calculated for four different models and the experimental results are compared in detail with the classical Heisenberg and spin-infinity Ising models. The combined experimental and theoretical study suggests that universality, if it holds for this material, does so in a temperature interval sufficiently narrow about the critical point to be both experimentally and theoretically inaccessible.