True magnetic structure of the ferrimagnetic garnet Y3Fe5O12 and magnetic moments of iron ions

To examine and emphasize the difference between approximative and true magnetic structure, we choose ferrimagnetic garnet Y3Fe5O12 whose magnetic properties are well known. In order to study the magnetic structure, neutron diffraction experiments were done on powder sample of Y3Fe5O12 at 10 and 295 K at zero field and at 295 K in the applied held B = 0.2 T. Using these data the crystal and magnetic structures were first refined in the space group Ia3d. Beside the use of a cubic space group, for a magnetically ordered crystal, this solution suffers from unrealistic magnetic moment per formula unit, which are all significantly above 5 mu(B) and temperature factors at 10 K which lead to unreliable Debye temperature. To satisfy compatibility of the symmetry of the magnetic moments (oriented along the body diagonal in cubic system) and crystal symmetry, the crystal and magnetic structures were also refined in the trigonal space group R (3) over bar. In this group the magnetic moments orientation, along the principal axis, is compatible with the symmetry of local positions. The magnetic R factors are significantly lower than in cubic system. The magnetic moment per formula unit: 3.1 mu(B), from the diffraction measurements in the field, is close to the result obtained by classical measurements of magnetization. (C) 1999 Elsevier Science B.V. All rights reserved.