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Magnetismus Poster: Do., 13:00–15:30 D-P242
Neutron Investigation of the antiferromagnetic State of Co2SiO4 at HEiDi
Martin Meven 1 , Andrew Sazonov 1 , Gernot Heger 2
1 ZWE FRM-II TU München, Lichtenbergstraße 1, 85747 Garching – 2 Institut für
Kristallographie, Jägerstraße 17-19, 52056 Aachen
Olivine, (Mg,Fe)2SiO4, is an important mineral of the upper Earth’s mantle. Well
known natural olivine-type silicates are fayalite (Fe2SiO4, paramagnetic Fe 2+ ions),
forsterite (Mg2SiO4, diamagnetic Mg 2+ ions) and kirschteinite (CaFeSiO4, mixed diamagnetic/paramagnetic).
A remarkable feature of the orthorhombic olivine-type structure
(Pnma, no. 62) consists in two crystallographically non equivalent metal (Me)
positions which yield one-dimensional chains of edge-sharing Me(1)O6 octahedra which
are connected via Me(2)O6 octahedra.
The magnetic properties of olivine compounds are quite complex. They are driven
by different and competing super-exchange interactions. For instance, a complete replacement
of paramagnetic Me(2) ions by diamagnetic ions should result in a onedimensional
magnetic behaviour. Synthetic Co2SiO4 also crystallizes in the olivine
structure. Former investigations of the magnetization of Co2SiO4 single crystals in
dependence of crystal orientation and applied magnetic field were performed by W.
Lottermoser and H. Fuess [1, 2]. At Tc ≈50 K an antiferromagnetic phase transition
occurs. In the antiferromagnetic state at 20 K the Co(2) spins are collinear while
the Co(1) spins are canted. To continue the investigation on the magnetical properties
of Co2SiO4 a large single crystal (≈ 300 mm 3 ) was grown in a mirror furnace at
Aachen.
The single crystal diffractometer HEiDi at the hot source of the new neutron source
Heinz Maier-Leibniz (FRM-II) was developed for very accurate structural investigations
on single crystals and allows due to the short wavelengths between 1 ˚A and
0.3 ˚A to separate the magnetic contributions from nuclear contributions to determine
magnetic spin densities. The combination with a closed cycle cryostat in the Eulerian
cradle with a remarkable low Tmin of 2.2 K at the sample position makes the instrument
a very good choice for detailed magnetic investigations at very low temperatures.
A neutron diffraction data set of our Co2SiO4 sample was taken on HEiDi at room temperature
to characterize the sample quality. Cell parameters and atomic positions were
very precisely determined with good agreement to data from literature [1, 2]. Temperature
dependent measurements of magnetic reflections like (110) were performed from
above Tc down to 2.2 K to observe their evolution in the antiferromagnetic phase. At
2.2 K a complete Bragg data set with nuclear and magnetic contributions was taken. A
first result is the excellent agreement between the phase transition temperature found
in former investigations [1, 2] and our measurement. Further results of our investigations
of the antiferromagnetic state of Co2SiO4 will be presented on this conference.
[1] W. Lottermoser and H. Fuess, Phys. Stat. Sol. A 109, 589 (1988)
[2] W. Lottermoser and H. Fuess, Phys. Chem. Min. 19, 46 (1992)