Magnetic structure and two-dimensional nehavior of Rb2MnCl4 and CS2MnCl4

A neutron-diffraction study of powder samples of Rb2MnCl4 and Cs2MnCl4 was performed. These compounds are paramagnetic at room temperature with K2NiF4-type structure (D-4h(17)-I4/mmm). It was found that both compounds order antiferromagnetically at T-N similar to 55 degrees K. In both compounds the magnetic structure below T-N is collinear with antiferromagnetic axis parallel to z, isomorphic with that of low-temperature K2NiF4 (F(A)m'm'm). Calculations of the dipolar energy show that z is indeed a direction of lower dipolar energy than x. In addition, the diffraction patterns exhibit a "ridge" - a broad reflection below about 180 degrees K which indicates the existence of two-dimensional spin correlations. Such correlations in the planes parallel to (001) are expected and have already been found in K2NiF4 and isomorphic compounds.