Magnetism of the Nearly One-Dimensional Helical Spin System

Magnetic properties of the nearly one-dimensional helical magnet VF 2 are studied by the two-time Green function method. The Néel temperature, T N , sublattice magnetization, and susceptibility along the c axis, χ c , below T N are calculated by using the Tyablikov, Callen, and modified Callen decouplings. The results for T N are much lower than the molecular-field estimate and found to be fairly close to that observed. The measured temperature dependence of χ c is qualitatively consistent only with the Callen decoupling prediction. Then, by using this decoupling, various spin-correlation functions, magnon energies renormalized by a k -dependent factor, and χ c above T N are calculated. The temperature dependence of the turn angle is also studied, and the ratio of the exchange constants between and within linear chains is found to be about 0.156. For comparison a similar but three-dimensional magnet MnO 2 is treated.