Ferromagnetic Ordering, Anisotropy, and Spin Reorientation for the Cyano-Bridged Bimetallic Compound Mn₂(H₂O)₅Mo(CN)₇·4H₂O (α Phase)

The title compound has been synthesized by slow diffusion of aqueous solutions containing K 4 [Mo(CN) 7 ]·2H 2 O and [Mn(H 2 O) 6 ](NO 3 ) 2, respectively. The compound crystallizes in the monoclinic system, space group P 2 1 / c . The Mo site is surrounded by seven −C−N−Mn linkages in a distorted pentagonal bipyramid fashion. There are two distorted octahedral Mn sites, one with four and the other with three Mn−N−C−Mo linkages. The structure is three-dimensional and of low symmetry. It consists of ladders made of edge-sharing lozenge motifs (MoCNMnNC) 2 running along the a direction. These ladders are linked further along the b and c directions. Both temperature dependences of the magnetic susceptibility and field dependences of the magnetization have been investigated along the three crystallographic directions, a, b, and c *. The angular dependence of the magnetization in the ab plane as a function of the external field has also been measured. These single crystal magnetic measurements have revealed that the compound orders ferromagnetically at T c = 51 K, without hysteresis effect. Two additional magnetic phenomena have been detected. In zero field, another transition has been observed at 43 K, which disappears when applying a field higher than 100 Oe along the a direction. For a critical value of the field, H c, applied along the a direction, a field-induced spin reorientation has been observed. The b direction is the easy magnetization axis in low field; both the a and b directions are easy magnetization axes as the external field overcomes the critical value. From the magnetic data, a magnetic phase diagram has been established. Finally, it has been found that when the compound is partially dehydrated, the long-range ferromagnetic ordering was shifted up to 65 K. This partially dehydrated material presents a magnetic hysteresis loop with a coercive field of 0.85 kOe at 5 K. The origins of the magnetic anisotropy along with the mechanism of the ferromagnetic interaction between low-spin Mo 3+ and high-spin Mn 2+ ions through the cyano bridge have been discussed.