Low Temperature Excitations in Low-Dimensional XY Ferromagnets CsNiF₃ and Graphite-CoCl₂

Abstract

The characteristic temperature dependence of magnetic heat capacities C_m of XY ferromagnetic spin systems in one and two dimensions has been observed in CsNiF₃ and graphite-CoCl₂, respectively. The value of C_m of CsNiF₃ has been shown to depend linearly on temperature T at low temperatures, and reproduced by the theoretical values for an XY ferromagnetic chain with the reported value of exchange constant J⁄k_B=11.8 K and with spin value S=1. While, in the CoCl₂ compound C_m has been found to obey C_m=m(J)T² which agrees with the spin wave theory for an XY ferromagnet in two dimensions with J⁄k_B=9.7 K. The thermal behaviour of these compounds reflects magnetic excitations intrinsic to the XY system, making a typical contrast with the case for the Heisenberg system.