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

Abstract

Single-crystal magnetization, heat capacity and neutron diffraction studies ofthe bimetallic regular chain MnIINiII(NO2)4(en)2 (en = ethylenediamine) are presented. This compoundexhibits ferromagnetic interactions between the Mn and Ni ions within thechains. The onset of long-range antiferromagnetic ordering belowTN = 2.45 K is associated with weaker antiferromagnetic interactionsbetween neighbouring chains. The magnetization studies reveal an easy axisanisotropy of the susceptibility at low T with a preferred orientation∥c. This anisotropy results from the low local symmetry around theMn ions. Below TN, the susceptibility drop observed for H∥c ismuch larger than for H⊥c, pointing to a moment orientation parallel tothe c axis in the ordered state. At 1.8 K, the metamagnetic transition isobserved to proceed in two steps for H∥c, with Hc1 = 1.5 kOe and Hc2 = 2.5 kOe. This is associated with the familiar spinflop state at the intermediate field values. For H⊥c, at 1.8 K theordered state is suppressed only in fields larger than 6 kOe. \\mbox{Low-$T$} heatcapacity data exhibit a pronounced anomaly at the antiferromagnetictransition. The reduced entropy at TN points to significant short-rangeordering effects above TN associated with the ferromagnetic intrachaininteractions. The ordered magnetic structure is an antiferromagnetic stackingof ferromagnetic sheets.