Effect of magnetic field on the 3d ordering and spin correlations of the linear antiferromagnet TMMC

We present an investigation of the effect of an applied external magnetic field on the low temperature magnetic properties of the linear Heisenberg antiferromagnet (CH3)4NMnCl3(TMMC). The proton and deuteron spin-lattice relaxation rate T−11 has been measured in the temperature interval 1.3–4.2 K and for applied field ranging from 1 kG to 88 kG. Large effects are observed both for the field parallel and perpendicular to the chain axis c. In particular, for H⊥c, a peak in T−11 occurs at the 3d ordering temperature TN. In this way the investigation of the field dependence of TN reported by Dupas and Renard up to 10 kG is extended to 88 kG. In this paper we limit the discussion to the effect of H on TN, for H⊥c. The results are explained by considering the effect of both the dipolar anisotropy and the magnetic field on the spin correlations. It is shown that for H⊥c the system should be described by an XY model for T<20 K and it crosses over to an Ising model for kT∼gμB[S (S+1) H]1/2. The corresponding changes in the correlation length are responsible for the strong field dependence of TN. At low fields we obtain theoretically the correct H2 dependence of TN. For high fields one suggests that the correlation length and hence TN are affected by the crystal defects.