THE QUASI-1D ANTIFERROMAGNET CSMNCL3 2H2O .2. A MN-54 NUCLEAR ORIENTATION STUDY

An in situ Mn-54 nuclear orientation (NO) study of a quasi-1D antiferromagnet CsMnCl3 . 2H2O is presented and discussed. From a comparison between CsMnCl3 . 2H2O, and the 3D antiferromagnet MnCl2 . 4H2O, it is shown that there are significant differences in the NO behaviour of quasi-1D and 3D antiferromagnets, in the milliKelvin regime. For example, in zero applied field, the Mn-54 nuclei in CsMnCl3 . 2H2O can be cooled to much lower temperatures than those in MnCl2 . 4H2O. In one crystal, a base temperature of congruent-to 10 mK was reached. However, in the presence of applied magnetic fields, a strong reduction in the NO signal is observed in the quasi-1D antiferromagnet CsMnCl3 . 2H2O. This is in marked contrast to MnCl2 . 4H2O in which case the application of magnetic fields leads to a rapid decrease in the nuclear spin-lattice relaxation time T1, and hence to large NO signals. It is argued that many of the unexpected features observed in the quasi-1D antiferromagnet can be understood, at least qualitatively, in terms of both 'solitons' and a large zero-point motion of the Mn2+ spins. NO measurements taken in both the axial and equatorial directions reveal that there are about 12.5% spin flopped domains (solitons), in zero applied field. On the other hand, thermometric methods have been used to show that the hyperfine splitting of Mn-54 nuclei in CsMnCl3 . 2H2O is 356(15) MHz. This result implies that the Mn2+ spins in CsMnCl3 . 2H2O are characterized by a large zero-point motion of some 30%. In applied fields, it is argued that the observed reduction in the gamma-ray anisotropies can be explained in terms of a substantial increase in zero-point motion, as the spin flop field is approached.