Abstract
The magnetic properties of two orthopyroxenes FexMg1-xSiO3 with x=1.0 (synthetic sample) and x=0.87 (natural compound XYZ) have been investigated by means of susceptibility and magnetisation measurements, neutron diffraction and Mossbauer spectroscopy. A collinear antiferromagnetic ordering is observed below TN(x=1.0)=45K and TN(x=0.87)=26K characterised by a wave-vector k=(010) and by an alignment of the moments along the b axis. Inside one ribbon formed by two non-equivalent M1 and M2 sites the moments are parallel to each other resulting from a preponderant ferromagnetic nearest-neighbour coupling. The Fe2+ ions on M1 and M2 sites are quite distinct: large orbital contributions were found in FeSiO3 for the moments on the M1 site leading to m0(M1)=4.1 mu B (neutron result) and to low values of the hyperfine field Hhf(M1)=70 kOe (Mossbauer data), compared with m0(M2)=3.3 mu B and Hhf(M2)=315 kOe on the M2 site. For x=0.87 a distribution of Hhf around these mean values is present. The strongly anisotropic character of these systems is reflected by the large difference observed between the susceptibilities along the orthorhombic axes even at T=300K, and by the metamagnetic transition that occurs at Hc(x=1)=50 kOe and Hc(x=0.87)=35 kOe.
Published Version
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