Magnetic and optical properties of the layer type magnets (CH 2) 2(ND 3) 2MnCl 4 and (CH 2) n(NH 3) 2CuCl 4, n = 2, 3, 5

Abstract

The magnetic and optical properties of the layer-type magnets (CH 2) 2(ND 3) 2MnCl 4 and (CH 2) n (NH 3) 2CuCl 4, n = 2, 3, 5, have been investigated. These compounds consist of layers of corner sharing chlorine octahedra which surround the magnetic metal ions. Adjacent layers are connected by covalent bonds. A complete picture of the magnetic properties could be obtained by the combination of magnetization and susceptibility measurements together with measurements of the optical absorption and the linear birefringence, in particular the linear magnetic birefringence (LMB). For (CH 2) 2(ND 3) 2MnCl 4 which has an antiferromagnetic interaction within the layers, the two dimensional (2-d) Heisenberg S = 5 2 antiferromagnet represents a very good approximation. The magnetic specific heat (as measured by the LMB) did not show any deviations from the ideal model. The Cu compounds consist of antiferromagnetically coupled ferromagnetic layers and show an entirely different behaviour. Very accurate measurements of the linear birefringence for (CH 2) 2(NH 3) 2CuCl 4 indicate a pronounced anomaly in the magnetic specific heat. This points to a 3-d magnetic behaviour also above the Néel temperature TN ≈ 34 K, which is corroborated by the magnetic measurements. This contrasts markedly with the behaviour previously found for the compound (CH 3NH 3) 2CuCl 4, which has pronounced 2-d properties. The reason for this “anomalous” behaviour could be found in a different superexchange path connecting Cu ions in adjacent layers in the di-ammonium salts, which involves two Cl anions (Cu-Cl-Cl-Cu), and is therefore more favorable than in the mono-ammonium salts. This exchange path implies a very strong dependence of the interlayer coupling on the distance between adjacent layers. In fact we have measured critical temperatures of 14.9 K and 7.6 K for the compounds (CH 2) 3(NH 3) 2CuCl 4 and (CH 2) 5(NH 3) 2CuCl 4, respectively. For the first time the LMB was used to measure a magnetic phase diagram, namely the one of (CH 2) 3(NH 3) 2CuCl 4. The critical exponent of the sublattice magnetization could be determined from the phase boundary at which the system enters the paramagnetic phase from the spin flop phase. Its “three dimensional” value β = 0.31 ± 0.02 is in accordance withthe theory of lattice dimensionality crossovers. Such crossovers are also observed by comparing the susceptibility curves of the different Cu salts.