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

Abstract

Here we present an in-depth study of the magnetic properties of a family of metal–organic perovskites ABX₃, [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 Néel 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 μ_B (at 12.5 T) was observed for Mn²⁺, meanwhile the Co²⁺ formate shows an orientation dependent quasi saturation (5.1 μ_B at 21 T along [101] vs. 5.8 μ_B at 26 T along [010]). The different isotropic/anisotropic behaviour can be explained by the orbital contribution to the magnetic response.