J. L. MacManus-Driscoll

Influence of strain and microstructure on magnetotransport in La0.7Ca0.3MnO3 thin films

A La0.7Ca0.3MnO3 thin film made by pulsed laser deposition (PLD) and another film of the same composition made by metal organic chemical vapor deposition (MOCVD), both on single crystal LaAlO3, 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 (Tm) and the maximum magnetoresistance (MR) 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. T...

Specific heat of La1−xCaxMnO3−δ

Abstract Detailed specific heat measurements from 4.5 to 300xa0K are reported on a series of polycrystalline La1−xCaxMnO3−δ samples, with x=0.10, 0.33 and 0.62. The samples were also investigated using electrical transport, magnetic measurements and low temperature NMR. The x=0.10 composition does not show a well defined transition in the specific heat at the ferromagnetic ordering temperature, but the low temperature specific heat shows a ferromagnetic T3/2 spin wave term, and no electronic linear term as expected for an insulator. The x=0.33 composition undergoes a transition to a ferromagnetic metal with a well defined peak in the specific heat, suggestive of an underlying first order transition at Tc; at low temperatures a linear electronic specific heat term is observed but no spin wave ferromagnetic term. High temperature specific heat data on the x=0.62 sample shows only one anomaly, suggesting simultaneous onset of charge ordering and antiferromagnetism. Low temperature data indicates that the magnetic phase is an A-type antiferromagnetic insulator with no electronic contribution. The NMR data at 4.2xa0K on this compound shows a motionally narrowed peak indicating short range ferromagnetic interactions or possibly magnetic polarons at 4.2xa0K.

Non-stoichiometry, structural defects and properties of LaMnO3+δ with high δ values (0.11≤δ≤0.29)

Strongly oxygenated LaMnO3+δperovskites, with nominal MnIV contents up to 58%, have been prepared by thermal decomposition of metal citrates followed by annealings either in air or under high oxygen pressure (200 bar). A high-resolution neutron powder diffraction study of four representative samples with 0.11≤δ≤0.29 reveals the presence of both La and Mn vacancies. Contrary to previous studies, it is found that there are a substantially higher proportion of Mn vacancies, depending rather sensitively on the oxidation conditions. The oxidation state for Mn calculated for the refined stoichiometry La1–xMn1–yO3 is in good agreement with the δ values previously determined by thermal analysis. Further to this, it is also found that as δ increases the Mn–O bond lengths shorten, the Mn–O–Mn angles progressively increase and the perovskite structure becomes more regular, which is consistent with the incorporation of MnIV cations. The presence of Mn vacancies (as much as 13% in samples prepared under high oxygen pressure) perturbs the conduction paths for the transport of holes across Mn–O–Mn, weakening the double-exchange interaction. This structural disorder explains the observed decrease of the ferromagnetic Curie temperature (TC) as δ increases.

Influence of oxygen vacancies on magnetoresistance properties of bulk La0.67Ca0.33MnO3-δ

Abstract Bulk polycrystalline samples of La 0.67 Ca 0.33 MnO 3− δ were annealed in controlled oxygen atmospheres. Two types of conditions were chosen, firstly with the aim of taking oxygen from the grain boundaries and secondly with the aim of removing oxygen uniformly from the sample. Reducing conditions increase the overall resistivity and suppress the Curie–Weiss temperature ( T c ). A second peak at temperatures below T c appears in the resistivity and magnetoresistivity when the samples are deoxygenated. However, the presence of this peak does not strongly influence the low-temperature magnetoresistance.

TRANSMISSION ELECTRON MICROSCOPY AND X-RAY STRUCTURAL INVESTIGATION OF LA0.7CA0.3MNO3 THIN FILMS

The structural changes and magnetoresistance (MR) properties of as-grown and post-annealed La 0.7 Ca 0.3 MnO 3 films were investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The data for the films were compared to that for bulk La 0.7 Ca 0.3 MnO 3 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 ∼2 a p × ∼2 a p × 2 a p , where a p is the single perovskite parameter. The phase showed a cube-on-cube epitaxy with the underlying LaAlO 3 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 ∼ 2 a p × ∼ 2 a p × ∼ 2 a p 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 )/( hk 0) peak in x-ray. The bulk La 0.7 Ca 0.3 MnO 3 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.

Effects of high vacancy concentrations on the magnetic properties of La1−xMn1−yO3 (0.02⩽x, y⩽0.13)

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...

Influence of grain size on magnetoresistance properties of bulk La0.67Ca0.33MnO3−δ

Abstract The magnetic and magnetoresistive properties of six polycrystalline bulk samples of nominal chemical composition La 0.67 Ca 0.33 MnO 3− δ and varying grain size (3.3xa0μm–20.4xa0μm), were investigated. Although the Curie temperature, T c , is independent of grain size, the variation of the saturation magnetisation with grain size depends on the magnetic history. At 20xa0K and 8xa0T applied field, the magnetisation decreases as the grain size increases. An approximate linear variation of resistivity and magnetoresistivity with grain size allows the use of a simple 1D model to extract the intrinsic grain magnetoresistance for these materials at T c and to separate the grain and grain boundary resistivity contributions. The relationship between resistivity and magnetisation is discussed.

Influence of oxygen stoichiometry on Raman phonon spectroscopy, lattice parameters and physical properties of La0.7Ca0.3MnO3 thin films

Abstract In this study the influence of annealing at reduced oxygen pressure on electrical transport, Raman active phonon modes and structural properties of several La 0.7 Ca 0.3 MnO 3 thin films has been determined. The out-of-plane lattice parameters and some of the Raman phonon modes were found to be insensitive to oxygenation condition. The temperature at which the peak magnetoresistance occurs and other of the Raman phonon modes, are found to change continuously as a function of decreasing oxygen partial pressure. The Raman studies indicate that small variations of oxygen content induce a structural change.

The effect of low oxygen partial pressure and high Pb-doping on Bi-2212 phase formation and flux pinning

Abstract Bi 2.2− x Pb x Sr 2 Ca 1 Cu 2 O 8+ δ ((Bi,Pb)-2212) was prepared under different oxidising conditions in a coulometric titration (CT) system. The effect of oxygen partial pressure ( p O 2 ) and Pb doping on (Bi,Pb)-2212 phase formation has been explored using X-ray diffraction (XRD), electron dispersive analysis (EDS) and transition electron microscopy (TEM). Under reducing conditions, the Pb solubility in Bi-2212 was found to increase. For 0.4 x p O 2 dependence of Pb solubility within the (Bi,Pb)-2212 phase is discussed with reference to solubility limits of Pb and possible spinodal decomposition of (Bi,Pb)-2212. The effect of Pb-doping on the intragranular critical current within the bulk samples was explored using a vibrating sample magnetometer at a range of temperatures and J c was found to be enhanced by a factor of 4 above 40 K.

Highly aligned, spin polarized thin films of Sr2FeMoO6 by a chemical vapor process

Highly oriented films of Sr2FeMoO6 were fabricated by ultrasonic spray pyrolysis. A combined organic/inorganic solution was developed and the films were deposited on single-crystal LaAlO3 between 600u200a°C and 900u200a°C followed by postannealing at 850u200a°C or 1200u200a°C in Ar/5% H2. Optimum films showed a metallic resistivity behavior with less than a 0.25% magnetoresistive effect at 50 K, 1 T, indicative of highly quality intra- and intergranular material. At 4.2 K, the lower bound saturation magnetization, Ms was ∼2.5u2009μB/f.u. and the transport spin polarization was 60±3%.