In:
Australian Journal of Physics, CSIRO Publishing, Vol. 52, No. 2 ( 1999), p. 279-
Abstract:
In this paper we describe a detailed neutron diffraction investigation of the
crystal and magnetic structure of two layered CMR manganites La1·2Sr1·8Mn2O7
(x = 0·4) and La1·4Sr1·6Mn2O7
(x = 0·3). In these materials of reduced dimensionality compared to the 3D perovskites, we find competing effects
between charge-lattice and spin degrees of freedom. These effects can be investigated by studying the behaviour of crystal and magnetic structure as a
function of temperature, composition and hydrostatic pressure. We find opposite lattice responses to the onset of charge delocalisation and magnetic
ordering in these two layered compounds. Below the insulator-to-metal transition (TIM), the lattice response suggests that charge is transferred to d3z2-r2
orbitals in La1·2Sr1·8Mn2O7
and to dx2-y2
orbitals in La1·4Sr1·6Mn2O7.
We argue that these changes are too large to be due to chemical differences. Instead we suggest that the orbital configuration of the Mn ion below TIM is
sensitive to electronic doping. In La1·2Sr1·8Mn2O7
we find that the lattice response at TIM to be driven by lattice displacements that relax below TIM, consistent with polaronic degrees of freedom. We also
note that the competition between super- and double-exchange to be significant in reduced dimensions. This is manifested in the change in the sign of the
apical Mn-O bond compressibilities above and below TIM. Finally, we describe the magnetic structure of these two different layered manganites. We find that
electronic doping also results in significant changes to the ordered arrangement of Mn spins. Interestingly the magnetism in reduced dimensions in
these materials can be varied from relative simple structures that show ferromagnetic inter-bilayer coupling as observed in
La1·2Sr1·8Mn2O7 to structures with antiferromagnetic inter-bilayer coupling as found in
La1·4Sr1·6Mn2O7.
Type of Medium:
Online Resource
ISSN:
0004-9506
Language:
English
Publisher:
CSIRO Publishing
Publication Date:
1999
detail.hit.zdb_id:
2178508-9
detail.hit.zdb_id:
1474434-X
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