INDUCED NET SPONTANEOUS MAGNETIZATION BY NONMAGNETIC IMPURITIES IN THE QUASI-ONE-DIMENSIONAL ANTIFERROMAGNET (CH3NH3)MN1-XCDXCL3-CENTER-DOT2H(2)O

We report effects induced by the partial substitution of the Mn++ ions by Cd++ ones in the magnetic properties of the quasi-one-dimensional Heisenberg-like antiferromagnet compound (CH3NH3)Mn1-xCdxCl3. 2H(2)O a t very low fields. A three-dimensional (3D) ordering of the linear cha ins occurs at T-N=4.12 K (x=0). Samples with x similar to 0.01 were in vestigated. We observe the appearance of a remanent magnetization (M-r ) below T-N when the sample is cooled through T-N, in a small axial ma gnetic field (as low as few mOe) applied along the axis of antiferroma gnetic alignment. The magnitude of M-r is of the order of 1% of the su blattice magnetization of the antiferromagnet. Samples cooled in axial fields below 0.02 Oe may exhibit an inversion of the sign of M-r at a temperature below TN. In this case the susceptibility measurements sh ow two strong singularities: one at TN and the second at the temperatu re where the inversion point occurs. It was found that this inversion is due to a polarization of the bulk of the sample by its surface that orders at T-N=3.95 K and causes a demagnetization field in which the bulk polarizes at a lower temperature (T-p=3.55 K). These results are compared with those previously observed in some diluted three: dimensi onal antiferromagnets and it is shown that for applied fields above 0. 1 Oe, M-r obeys at temperatures below T-p the same universal behavior previously found for M-r in other low anisotropy 3D systems.