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 (MnNiII)-Ni-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, 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.