Local distortions in the colossal magnetoresistive manganates La_0.70Ca_0.30MnO₃, La_0.80Ca_0.20MnO₃and La_0.70Sr_0.30MnO₃revealed by total neutron diffraction

Abstract

The manganates La0.80Ca0.20MnO3, La0.70Ca0.30MnO3 and La0.70Sr0.30MnO3 exhibit the colossal magnetoresistive (CMR) effect. Total neutron diffraction was employed to yield information on both the average and local atomic structure of these disordered crystalline materials as a function of temperature. The average structures were determined from Rietveld analysis of the Bragg scattering. Information on the local structures was obtained by Fourier transformation of the total diffraction pattern to yield the total correlation function, T(r). Particular attention is paid to changes in the Mn-O bond distances, which are widely believed to be important in the CMR effect. Jahn-Teller distortions of the MnO6 octahedra are absent at the lowest temperatures in the metallic phase. As the temperature is raised towards the paramagnetic-semiconducting to ferromagnetic-metallic transition at Tc, T(r) exhibits clear increases in the variance of Mn-O bond distances, which greatly exceed those expected from the increase in disorder due to atomic thermal motion. This is confirmed by comparing the behaviour of the three materials, which have different values for Tc. The advantage of studying the local structure directly by determining T(r) from total neutron scattering, rather than extrapolating from the average to local structure from Bragg scattering studies, is demonstrated. Comparisons are made with the results obtained for the ordered compound LaMnO3, which does not exhibit the CMR effect. The three CMR manganates studied here do not show a separation of the Mn-O distances into two well resolved sets above Tc as reported by other workers.

Understanding the Insulating Phase in Colossal Magnetoresistance Manganites: Shortening of the Jahn-Teller Long-Bond across the Phase Diagram ofLa1−xCaxMnO3

“…Our result disagrees with the canonical understanding from earlier experimental studies [15,17,18], though is consistent with the observations of a number of less well known studies [33,34]. The observation could be explained in a homogeneous picture where the electron density is uniformly distributed over all Mn sites in the insulating state.…”

Neutron powder diffraction studies of the La0.65Pb0.35MnO3 perovskite

“…Only one pure magnetic Bragg reflection was identified at 2h = 31.61j, attributed to the (MnMn) magnetic structure factor, which corresponds to the (110) orthorhombic (Pnma) structures, respectively. As can be clearly seen, the only possible fit to the data is obtained using the rhombohedral structure, in agreement with the results obtained for the La 0.7 Sr 0.3 MnO 3 compound [8]. The inset in Fig.…”

“…We find that the compound has a rhombohedral structure at all temperatures investigated, with the R3c space group. The goodness of each fit is given by the weighted profile factor R WP [8]. Because of the Mn magnetic form factor, magnetic contributions to Bragg reflections at 2u z 60j are essentially negligible, even at the lowest temperature, where the magnetic Bragg scattering is at its greatest.…”

Effect of biaxial strain on the electrical and magnetic properties of (001) La0.7Sr0.3MnO3 thin films

“…[6][7][8][9][10][11][12][13][14][15][16] In this work, we investigate the magnetotransport properties of fully commensurate epitaxial La 0.7 Sr 0.3 MnO 3 thin films grown by reactive molecular-beam epitaxy. Our films span a biaxial strain range from Ϫ2.3% to +3.2%, and we find excellent agreement between the measured Curie temperatures and the model of Millis et al 5 The films were grown on nine commercially available perovskite substrates: ͑100͒ p LaAlO 3 , ͑001͒ LaSrGaO 4 , ͑110͒ NdGaO 3 , ͑100͒ ͑LaAlO 3 ͒ 0.3 -͑SrAl 0.5 Ta 0.5 O 3 ͒ 0.7 ͑LSAT͒, ͑100͒ SrTiO 3 , ͑110͒ DyScO 3 , ͑110͒ GdScO 3 , ͑110͒ SmScO 3 , and ͑110͒ NdScO 3 , where the subscript p indicates pseudocubic indices.…”

“…The film deposited on LaAlO 3 ͑ xx = −2.3%͒ shows an insulating behavior over the entire temperature range, in agreement with previous work. 6,17 The films grown on GdScO 3 ͑ xx = +2.3%͒, SmScO 3 ͑ xx = +2.7%͒, and NdScO 3 ͑ xx = +3.2%͒ show room temperature resistivity values of Ϸ1 ⍀ cm. Due to their high resistivities these ͑T͒ curves have been measured over a reduced range of temperatures and are not reported in Fig.…”