Magnetic transitions and isotropic versus anisotropic magnetic behaviour of [CH3NH3][M(HCOO)3] M = Mn2+, Co2+, Ni2+, Cu2+ metal-organic perovskites

Here we present an in-depth study of the magnetic properties of a family of metal organic perovskites ABX(3), [CH3NH3][M(HCOO)(3)] in which A = CH3NH3+ is the methylammonium cation, B = M is a divalent metal cation (Mn2+, Co2+, Ni2+ or Cu2+), and X is the formate anion (HCOO-). The magnetic properties have been measured on powdered samples and along the different orientations of mm-sized single crystals. They display spin-canted weak ferromagnetism with Neel temperatures of 8.0 K (Mn2+), 15.7 K (Co2+) and 34 K (Ni2+), which are inversely proportional to the ionic radii of the metal cations. The Cu2+ member displays low-dimensional magnetism as a result of orbital ordering of the Cu2+ d orbitals originating from a Jahn-Teller distortion. Pulsed-field magnetization experiments (fields of up to 60 T at temperatures down to 0.6 K) show that Mn2+, Co2+ and Ni2+ formates display cation-characteristic spin flop transitions. A saturation magnetization value of 5 mu(B) (at 12.5 T) was observed for Mn2+, meanwhile the Co2+ formate shows an orientation dependent quasi saturation (5.1 mu(B) at 21 T along [101] vs. 5.8 mu(B) at 26 T along [010]). The different isotropic/anisotropic behaviour can be explained by the orbital contribution to the magnetic response.