Europhys. Lett.
Volume 48, Number 2, October 1999
Page(s) 233 - 239
Section Condensed matter: electronic structure, electrical, magnetic and optical properties
Published online 01 September 2002
DOI: 10.1209/epl/i1999-00471-9

Europhys. Lett, 48 (2), pp. 233-239 (1999)

Spin density on ligands ${\rm O^{2-}}$ and covalency of ${\rm Fe^{3+}}$ions in octahedral sites of the ${\rm Ca_3Fe_2Ge_3O_{12}}$ garnet:
A polarised neutron diffraction study

V. P. Plakhty 1, A. G. Gukasov 2, R. J. Papoular 2 and O. P. Smirnov 1

1Petersburg Nuclear Physics Institute, RAS - Gatchina, St. Petersburg, 188350, Russia
2Laboratoire Leon Brillouin - CE-Saclay, 911191 Gif-sur-Yvette Cedex, France

(received 31 May 1999; accepted in final form 24 August 1999)

PACS. 75.50Ee - Antiferromagnetics.
PACS. 75.25${\rm +z}$ - Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies synchrotron-source X-ray scattering, etc.).
PACS. 75.30Et - Exchange and superexchange interactions.


By means of polarised neutron diffraction in an external magnetic field of $\rm 7.5\;T$ at $\rm 2\;K$ the covalent spin density is unambiguously observed on the ${\rm O^{2-}}$ligands of the ${\rm Fe^{3+}}$ ions that occupy only the octahedral 16a sites in the antiferromagnetic ${\rm Ca_3Fe_2Ge_3O_{12}}$ garnet. The maximum entropy method is used to reconstruct these spin density peaks. The moment value of the field-induced component is found by the least-squares refinement on the flipping ratios of the reflections affected only by oxygen. The ligand moment is determined to be $0.116(18) \;\mu_{\rm B}$ by extrapolation of this component to the spin-flip (exchange) field. The corresponding reduction of the ${\rm Fe^{3+}}$ moment is 14(2)%. An impurity moment of $0.0085(13) \;\mu_{\rm B}$ in 24c sites (probably 0.21% of $\rm
Fe^{2+}$) can explain the partial magnetic disorder of this material.


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