Superexchange in manganese formate dihydrate, studied by polarized-neutron diffraction

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The crystalline structure of manganese formate dihydrate Mn(HCCO) 2 .2H 2 O may be described by stacking along the a axis two parallel layers of non-equivalent manganese Mn 2+ ions, consisting of antiferromagnetically coupled A ions and weakly interacting B ions. The A layers are characterized by formate bridges related each A ion to the four neighbouring ions, while the B ions are isolated from each other. On the other hand, the A layers are related to the B layers by formate bridges. The induced magnetization density has been determined by polarized-neutron diffraction, slightly above (and below) the antiferromagnetic (AF) ordering temperature T N =3.7 K. A field of 0.8 T was applied to the single crystal along b. The experimental conditions studied, below T N the AF moments of the A ions are canted, with a weak ferromagnetic component along b. Magnetic moments are observed on the carbon and oxygen atoms of the A-A and A-B formate bridges, demonstrating the role of the formate group in the indirect exchange magnetic interaction between A ions. At 5.5 K, alternate positive and negative signs are found on the O-C-O pathway between two A ions and between the A and B ions: O(1)(0.022(6) mu B )-C(1)(-0.017(7) mu B )-O(2)(0.015(6) mu B ) and O(3)(0.024(7) mu B )-C(2)(-0.011(9) mu B )-O(4)(0.013(7) mu B ). The induced moments on A and B are equal to 0.38(2) mu B and 1.73(2) mu B , respectively. At 2.5 K, the measurements have been confined to the reflections with k+l even, which do not contain any contribution from the AF components. The magnetization density observed around the A ions is then the sum of the spontaneous weak ferromagnetic component (0.09 mu B ) and of the component induced by the field (0.19 mu B ). The spin distribution on the bridges is perturbed in the magnetically ordered phase; in particular, the alternation of signs is no longer observed.