STRUCTURAL AND MAGNETIC POWDER DIFFRACTION AS A PROBE OF ELECTRON-LATTICE COUPLING IN MANGANESE PEROVSKITES
P.G. Radaelli1,
L. Capogna2, G. Iannone2,
D.E. Cox3, D.N. Argyriou4,
H. Casalta2,
K. Andersen2, S-W. Cheong5,6,
J.F. Mitchell7 and M. Marezio8
1
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot,
OX11 0QX, UK
2 Institut Max Von Laue - Paul Langevin,
BP 156, 38042 Grenoble, FRANCE
3 Physics Department, Brookhaven
National Laboratory, Upton, NY 11973, USA
4 Los Alamos Neutron Scattering Center,
Los Alamos National Laboratory, Los Alamos, NM, 87545 USA
5 Bell Laboratories, Lucent
Technologies, Murray Hill, NJ 07974 USA
6 Department of Physics and Astronomy,
Rutgers University, Piscataway, NJ08855 US
7 Materials Science Division, Argonne
National Laboratory, Argonne, IL 60439 USA
8 MASPEC-CNR, via Chiavari 18A, 42100
Parma ITALY.
One of the most intriguing aspects of the physics of manganese perovskites (general formula: A1xA'xMnO3, A = La, Rare Earth, A' = Ca, Sr, Ba, Pb…) is the competition between electron-lattice coupling and double exchange. The first, promoted by the Jahn-Teller nature of Mn+3 and by the size difference between Mn+3 and Mn+4, favours, at high temperatures, the well-known polaronic behaviour, and, at low temperatures, the formation of stable charge- orbital- and magnetic-ordered arrangements, associated with localised electrons and large lattice distortions. On the contrary, double exchange favours the formation of ferromagnetic regions, associated with de-localised electrons and small lattice distortions. In the manganites, these competing interactions produce a surprising variety of exotic phenomena, which can be "tuned" by changing the doping level x, the electronic bandwidth, controlled by the average A-site ionic radius <rA>, and the A-site disorder. Since the discovery of these compounds in the early fifties and the pioneering work by E.O. Wollan and W.C. Koehler(1), structural and magnetic powder diffraction have given an invaluable contribution to the understanding of the physics of manganites, and continue to play a paramount role, which underlines the very significant progresses of the experimental techniques. Among the most important achievements is the discovery of charge, orbital and magnetic ordering, which occur for particular values of x and <rA>. Some of the most recent results on the charge-ordered phases, including the discovery of a 3-fold magnetic superstructure associated with a "stripe" phase for the composition La0.333Ca0.667MnO3, will be described. Crystallographic techniques have also contributed to the understanding of the "polaronic" behaviour of the paramagnetic phases, which is revealed by the anomalous temperature dependence of the cell volume(2) and of the Debye-Waller factors(3, 4), and, more recently, of the interplay between these local lattice distortions and short-range ferromagnetic ordering (FM "clusters") (5). In the case of Pr0.7Ca0.3MnO3, we will show that a very complex transition from dynamic to static FM clusters as a function of decreasing temperature, followed by an externally induced phase segregation process, could be hypothesised purely on the basis of high-resolution synchrotron x-ray and neutron diffraction data(6). Subsequently, quasi-elastic small-angle neutron scattering measurements have confirmed that the overall picture obtained from crystallography was correct. These results indicate that "physical" crystallography should increasingly find a place among other techniques more traditionally associated with physics, as one of the most important tools to probe the optical, electronic and magnetic properties of materials.