Articles you may be interested inMagnetotransport properties of c -axis oriented La 0.7 Sr 0.3 MnO 3 thin films on MgO-buffered SiO 2 / Si substrates J. Appl. Phys. 105, 07D711 (2009); 10.1063/1.3065974 Influence of structural disorder on magnetic and transport properties of ( La 0.7 Sr 0.3 ) 0.5 ( Pr 0.65 Ca 0.35 ) 0.5 MnO 3 films Low Temp.A La 0.7 Ca 0.3 MnO 3 thin film made by pulsed laser deposition ͑PLD͒ and another film of the same composition made by metal organic chemical vapor deposition ͑MOCVD͒, both on single crystal LaAlO 3 , were subject to a series of six, short, controlled anneals. The oxygen content was purposely not changed in the films from the first anneal to subsequent anneals. After each anneal, the film microstructures were characterized to determine average grain size, lattice constants, nonuniform strain, and crystalline mosaic spread, and these parameters were correlated with the magnetotransport properties. For both sets of films, the influence of annealing was to both increase the temperature at which the maximum in the magnetoresistance occurs (T m ) and the maximum magnetoresistance ͑MR͒ value. The improvement in film properties occurred in conjunction with stress relaxation and improved crystallinity, as a result of grain growth. The MOCVD films showed poorer grain coupling and poorer epitaxy compared to the PLD films. These features did not significantly influence the absolute values of the resistivity, but did produce spin canting in the MOCVD film, as seen in magnetization and resistivity versus field data. The canting resulted in a lower T m and depressed MR value for the MOCVD film which increased only marginally with annealing. The work highlights the importance of controlling microstructure for optimizing properties of colossal magnetoresistance films.

The structural changes and magnetoresistance (MR) properties of as-grown and post-annealed La0.7Ca0.3MnO3 films were investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The data for the films were compared to that for bulk La0.7Ca0.3MnO3 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 × ∼2ap × 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, a minor primitive pseudo-tetragonal structure evolved, of cell parameters . A total of four possible orientations of the two structures was observed by TEM, comprised of one orientation of the ∼ 2ap × ∼ 2ap × ∼ 2ap cell, i.e., the cube-on-cube epitaxy, giving rise to (00l) peaks in x-ray, and three orientations of the cell, giving rise to a single (00l)/(hk0) peak in x-ray. The bulk La0.7Ca0.3MnO3 sample also contains the × 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.

It has been found that the synthesis conditions and subsequent annealing treatments can significantly change the magnetic and transport behavior of the colossal magnetoresistive materials of the general formula La1−zAzMnO3, where A is a divalent ion. In order to clarify the role of vacancies within this structure, resistivity and magnetization measurements have been carried out on a series of samples derived from the parent compound LaMnO3, with La and Mn vacancies introduced by systematically varying the oxygen annealing conditions. Previous studies have shown that for a given carrier concentration, the Curie temperature of the paramagnetic to ferromagnetic transition (Tc) increases as the tolerance factor of the perovskite structure increases and then begins to decrease slowly for even higher tolerance factors. Generally, Tc also increases with the Mn4+ content, consistent with a double exchange mechanism responsible for ferromagnetism. In this study Tc was found to decrease as the vacancy concentration (and therefore the Mn4+ concentration) and tolerance factor both increase. The magnetic and transport data are discussed in the light of structural information obtained from neutron diffraction studies. In the present study it is found that La and Mn vacancies play a significant role in determining the physical properties of these materials: for high vacancy concentrations (x,y⩾0.080) the magnetic properties are analogous to those of spin glasses associated with disorder and frustration. Semiconductorlike resistivity was observed for all samples at all temperatures.