Magnetic anisotropy and metamagnetic behaviour of the bimetallic chain MnNi(NO₂)₄(en)₂ (en = ethylenediamine)

Single-crystal magnetization, heat capacity and neutron diffraction studies of the bimetallic regular chain Mn^IINi^II(NO₂)₄(en)₂ (en = ethylenediamine) are presented. This compound exhibits ferromagnetic interactions between the Mn and Ni ions within the chains. The onset of long-range antiferromagnetic ordering below T_N = 2.45 K is associated with weaker antiferromagnetic interactions between neighbouring chains. The magnetization studies reveal an easy axis anisotropy of the susceptibility at low T with a preferred orientation ∥c. This anisotropy results from the low local symmetry around the Mn ions. Below T_N, the susceptibility drop observed for H∥c is much larger than for H⊥c, pointing to a moment orientation parallel to the c axis in the ordered state. At 1.8 K, the metamagnetic transition is observed to proceed in two steps for H∥c, with H_c1 = 1.5 kOe and H_c2 = 2.5 kOe. This is associated with the familiar spin flop state at the intermediate field values. For H⊥c, at 1.8 K the ordered state is suppressed only in fields larger than 6 kOe. \mbox{Low-$T$} heat capacity data exhibit a pronounced anomaly at the antiferromagnetic transition. The reduced entropy at T_N points to significant short-range ordering effects above T_N associated with the ferromagnetic intrachain interactions. The ordered magnetic structure is an antiferromagnetic stacking of ferromagnetic sheets.