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Magnetismus Poster: Do., 13:00–15:30 D-P261
Neutron Diffraction Study of the Manganites Heavily Doped By Fe and Cr
Iions
I. O. Troyanchuk 1 , M. V. Bushinsky 1 , K. Baerner 2 , H. Szymczak 3
1 Joint Institute of Solid State and Semiconductor Physics, National Academy of Sciences
– 2 IV. Physikalisches Institut Bunsenstrasse 13, D37073 Göttingen, Germany
– 3 Institute of Physics, National Academy of Science, al. Lotnikov 32/36, Warsaw,
Poland
At present there are many manganite systems studied in which manganese ions are
substituted for other 3d- or diamagnetic ones. It is found that Cr and Fe ions have
the strongest effect on magnetic properties, small additions of them can result in a
change of the ground magnetic state. However the magnetic structure of manganites
heavily doped by Cr and Fe is unknown. So we have performed neutron diffraction
and magnetic studies for NdMn1−xFexO3 and NdMn1−xCrxO3 manganites in a wide
range of Fe and Cr dopants. Crystal structure of compound with x=0.5 has been
refined in space groups Pnma and P21/n. However, the refinement in P21/n group
did not improve significantly the reliability factors in comparison with the space group
Pnma what points the absence of the Mn and Cr ions ordering. Oxygen octahedron
around 3d-ions is rather elongated along one of the axes. This may be expected in
the case of Jahn-Teller stabilization of dz2 orbitals of manganese ions. The angles
O-Mn/Cr-O and O-Mn/Fe-O are close to 1520, what is much less than in manganites
of lanthanum doped with alkaline earth. In NdMn0.5Cr0.5O3 the magnetic structure
has been revealed consisting of both ferromagnetic and G-type antiferromagnetic components
resulting from a 3d-ions magnetic moments ordering. Magnetic moments of
Nd-ions are parallel to the ferromagnetic component. In NdMn0.5Fe0.5O3 only the
antiferromagnetic G-type structure has been revealed whereas Nd-sublattice is not ordered.
According to spectroscopic data both Cr and Fe ions in manganites adopt 3+
oxidative state. The magnetic moments of 3d-ions calculated from neutron diffraction
data are much less than one can expect in the ionic model where the magnetic moments
should be definitely localized. One can suppose the significant part of 3d-ions magnetic
moments to be in the spin glass state and not contribute therefore into the coherent
magnetic scattering of neutrons. Magnetic phase diagrams have been constructed for
the both systems, interpreted assuming superexchange interactions Mn 3+ -O-Cr 3+ to
be positive and Mn 3+ -O-Fe 3+ negative and taking into account the disordered arrangement
of Mn and Cr ions in the crystal structure sublattice. The sign of Mn 3+ -O-Mn 3+
superexchange interaction is governed with a dynamic nature of Jahn-Teller effect.
Magnetic phase separation phenomenon has been ascribed to the intrinsic chemical
inhomogeneity of solid solutions.