The magnetic structure of braunite Mn2+Mn3+6O8/SiO4

Neutron powder diffraction data of braunite Mn2+Mn3+6O8/SiO4(space groupI41/acd) are presented, which have been collected with the diffractometer D1A at ILL Grenoble and E6 and E2 at BENSE (HMI Berlin) at temperatures between 300 K and 1.4 K. The chemical cell has a layer structure consisting of A sheets with two Mn3+positions (M2, M3) and B sheets containing Si, Mn2+(M1) and a third Mn3+position (M4). BelowTNat 90 K additional reflections being caused by antiferromagnetic (af) ordering occur. The magnetic cell coincides with the chemical cell. The magnetic structure is dominated by the magnetic ordering of the A layers thus reflecting the relations of the chemical cell: Within the (001) plane of the A sheets magnetic moments of the two nonequivalent Mn3+-ions are ferromagnetically aligned, and neighbouring A sheets are af-coupled to each other. Magnetization experiments indicate that besides the afordered Mn ions weakly coupled spins ordering at temperatures below 2 K exist. In accordance with that, additional neutron diffraction reflections arise atT< 3.0 K, allowing the refinement of the whole structure: Within the B sheets the Mn2+ions are responsible for the weakly coupled spins, furthermore, the spin arrangement is not collinear. The spin directions of the Mn3+ions (M4) are canted, forming angles of about 120° with those of the Mn2+ions (M1). The overall magnetic ordering within each B sheet is antiferromagnetic thus resulting in a zero magnetic moment for these sheets. Refined magnetic saturation moments of all the manganese ions agree well to expected spin-only values.