THE MAGNETIC-STRUCTURE OF BRAUNITE MN2+MN63+O8 SIO4/

Neutron powder diffraction data of braunite Mn2+Mn63+O8/SiO4 (space gr oup I4(1)/acd) are presented, which have been collected with the diffr actometer DIA at ILL Grenoble and E6 and E2 at BENSE (HMI Berlin) at t emperatures between 300 K and 1.4 K. The chemical cell has a layer str ucture consisting of A sheets with two Mn3+ positions (M2, M3) and B s heets containing Si, Mn2+ (Mi) and a third Mn3+ position (M4). Below T N at 90 K additional reflections being caused by antiferromagnetic (af ) ordering occur The magnetic cell coincides with the chemical cell. T he magnetic structure is dominated by the magnetic ordering of the A l ayers thus reflecting the relations of the chemical cell: Within the ( 001) plane of the A sheets magnetic moments of the two nonequivalent M n3+-ions are ferromagnetically aligned, and neighbouring A sheets are af-coupled to each other. Magnetization experiments indicate that besi des the af-ordered Mn ions weakly coupled spins ordering at temperatur es below 2K exist. In accordance with that, additional neutron diffrac tion reflections arise at T < 3.0 K, allowing the refinement of the wh ole structure: Within the B sheets the Mn2+ ions are responsible for t he weakly coupled spins, furthermore, the spin arrangement is not coll inear. The spin directions of the Mn3+ ions (M4) are canted, forming a ngles of about 120 degrees with those of the Mn2+ ions (Mi). The overa ll magnetic ordering within each B sheet is antiferromagnetic thus res ulting in a zero magnetic moment for these sheets. Refined magnetic sa turation moments of all the manganese ions agree well to expected spin -only values. The long range order of the peculiar af-structure of pur e Mn-braunite is rapidly destroyed by a small incorporation of Fe3+ io ns, thus leading to spin glass behaviour.