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MATERIALS RESEARCH

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Transmission electron microscopy and x-ray structural investigation of La0.7 Ca0.3 MnO3 thin films Y. H. Li and K. A. Thomas Department of Materials, Imperial College, Prince Consort Road, London, SW7 2BP, United Kingdom

P. S. I. P. N. de Silva and L. F. Cohen Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BZ, United Kingdom

A. Goyal Department of Materials Science, University of Cambridge, Cambridge CB2 3QZ, United Kingdom

M. Rajeswari Department of Physics and Electrical Engineering, University of Maryland, College Park, Maryland

N. D. Mathur, M. G. Blamire, and J. E. Evetts Department of Materials Science, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom

T. Venkatesan Department of Physics and Electrical Engineering, University of Maryland, College Park, Maryland

J. L. MacManus-Driscoll Department of Materials, Imperial College, Prince Consort Road, London SW7 2BP, United Kingdom (Received 28 April 1997; accepted 4 September 1997)

The structural changes and magnetoresistance (MR) properties of as-grown and post-annealed La0.7 Ca0.3 MnO3 films were investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The data for the films were compared to that for bulk La0.7 Ca0.3 MnO3 post-annealed under the same conditions. The main structure of the as-grown films was face-centered pseudo-cubic with a doubled perovskite unit cell, of size ,2ap 3 ,2ap 3 2ap , where ap is the single perovskite parameter. The phase showed a cube-on-cube epitaxy with the underlying LaAlO3 substrate. Upon annealing to a saturation point,pa minor primitive pseudo-tetragonal structure evolved, of p cell parameters , 2ap 3 , 2ap 3 , 2ap . A total of four possible orientations of the two structures was observed by TEM, comprised of one orientation of the , 2ap 3 , 2ap 3 , 2ap cell, i.e., the cube-on-cube p p epitaxy, giving rise to (00l) peaks in x-ray, and three orientations of the , 2ap 3 , 2ap 3 ,2ap cell, giving rise to a single p (00l)y(hk0) peak in x-ray. The bulk La0.7 Ca0.3 MnO3 sample also contains the , 2ap 3 p , 2ap 3 ,2ap structure. The difference between the bulk and the film and the effects of annealing on films can be ascribed to the influence of strain between the film and substate, induced by lattice mismatch.

I. INTRODUCTION

The mixed valence manganite perovskites and layered structures have attracted a lot of attention recently because of their magnetoresistive (MR) behavior.1–5 However, the so-called colossal magnetoresistive (CMR) effect is found only at the Curie temperature (TC ) and in fields of a few tesla. Before any significant practical applications are realized, it will be necessary to optimize the MR properties near room temperature, and in small fields. The MR change that occurs at TC can be qualitatively understood in terms of a decrease of the spin disorder scattering when the material undergoes a paramagnetic to canted-ferromagnetic transition. However, the magnitude of the MR transition cannot be qu