S. Pignard

Effect of crystallinity on the magnetoresistive properties of La0.8MnO3−δ thin films grown by chemical vapor deposition

We report here a study on the role of crystallinity on the magnetoresistive properties of self-doped La0.8MnO3−δ thin films. Films have been grown by a liquid-source metalorganic chemical vapor deposition technique on two different single crystals: SrTiO3 (001) and Al2O3 (012). Epitaxial films are obtained on strontium titanate and show a high magnetoresistance peak at low fields: maximum magnetoresistive effect in the vicinity of the magnetic transition temperature, and negligible 50 K below. Polycrystalline films are observed on sapphire; they exhibit a lower magnetoresistance of several percent which remains nearly constant over a wide temperature range. Two magnetoresistive regimes can be distinguished in these films: in low fields (up to 5 mT), a strong magnetoresistance is obtained with a sensitivity of 0.28%/mT at 78 K; in higher fields, the magnetoresistance is linear and lower than this obtained in epitaxial films. The role of grain boundaries on the magnetoresistive effect is discussed.

Magnetic and electromagnetic properties of RuZn and RuCo substituted BaFe12O19

Abstract Polycrystalline samples of M-type hexaferrites BaFe 12−2 x Ru x Zn x O 19 and BaFe 12−2 x Ru x Co x O 19 with 0⩽ x ⩽0.45 have been prepared by a classical sintering method. The evolutions with x of the cell parameters, the saturation magnetization and the magnetic transition temperature have been measured; in this range of small doping ratios, saturation magnetization and Curie temperature of substituted hexaferrites remain close to those of the undoped BaFe 12 O 19 . X-ray diffraction measurements on oriented powders show that a change of magnetocrystalline anisotropy from axial to planar occurs in both cases for a small substituting ratio x c =0.375. Microwave electromagnetic characteristics have been studied on the ceramic samples from 0.1 to 10xa0GHz. The behaviour of the magnetic losses ( μ ″) corroborates the anisotropy change when doping; a convolution of the dissipation mechanisms (domain wall motions and gyromagnetism) is obtained for x c . The level of the magnetic losses is discussed in relation with others substituted Ba-hexaferrites.

Epitaxial and polycrystalline BaFe12O19 thin films grown by chemical vapour deposition

Epitaxial and polycrystalline barium hexaferrite BaFe12O19 thin films were prepared by metalorganic chemical Vapour deposition (MOCVD). Films were grown by a liquid MOCVD technique which aim is to control precisely the precursor vapour pressures. Two kinds of substrates were used: sapphire (001) and silicon thermally oxidized. On Si/SiO2 films are polycrystalline and the magnetization is isotropic. On Al2O3 (001), structural studies reveal the films to be predominantly single phase, well crystallized without annealing procedure and with the c-axis perpendicular to the film plane; epitaxial relationships between the film and the substrate were determined. The magnetic parameters, deduced from vibrating sample magnetometer measurements, show a high dependence of the magnetization with the orientation of the field with respect to the surface of the film

Chemical vapor deposition and structural study of La0.8 MnO3−δ thin films

Abstract We report here the synthesis of lacunar lanthanum manganites La 0.8 MnO 3− δ by a liquid source metal organic chemical vapor deposition technique which has been developed recently in our laboratory. The role of the substrate temperature on the growth rate, on the molar ratio La/Mn and on the crystallinity is studied for films deposited on MgO (001) single crystals. A structural study by means of X-ray diffraction ( θ /2 θ scans, ω -scans, ϕ -scans) and scanning electron microscopy (channeling electron patterns) has been achieved on lanthanum manganites films with the composition La 0.8 MnO 3− δ grown on three kinds of substrates: MgO (001), LaAlO 3 (012) and SrTiO 3 (001). Films on LaAlO 3 and SrTiO 3 show an epitaxial quality with a small mosaicity in the plane of the substrate. Due to a bigger lattice mismatch with the substrate, films deposited on MgO present a worse crystalline quality with two types of epitaxially grown crystallites.

Correlation between magnetoresistive properties and growth morphology of La1−xMnO3−δ thin films deposited on SrTiO3, LaAlO3 and MgO

Abstract A new metal-organic chemical vapour deposition (MOCVD) technique was used for fabrication of epitaxial La-manganite thin films La 1− x MnO 3−δ with both lanthanum and oxygen deficiencies. Three kinds of substrates, i.e. SrTiO 3 (001), LaAlO 3 (012) and MgO (001), were chosen for comparison. Electrical and magnetic property measurements revealed that the amplitude of the colossal magnetoresistance (CMR) is dependent on the nature of the substrate whereas the electrical and magnetic transition temperatures are not sensitive to the substrate type. In order to correlate the physical properties of the films to their crystallization quality and their growth morphology, scanning electron microscopy (SEM) , transmission electron microscopy (TEM), as well as high-resolution transmission electron microscopy (HRTEM), were employed. Cross-sectional TEM investigations showed that the La 1− x MnO 3−δ is generally in good epitaxy with all three kinds of substrate. The orientation relationships between the film and substrate are rather well defined at the vicinity of the interface but not at the surface of the film. Deposition of La 1− x MnO 3−δ on SrTiO 3 produces the best interfacial match since these two have the least lattice misfit. This is consistent with the best magnetoresistive property measured for the investigated systems.

GMR thin films deposited by a new MOCVD-injection method. preparation, transport properties and magnetic behaviour of self-doped lanthanum manganites La1-xMnO3-δ (0 ≤ x ≤ 0.3)

Abstract A new liquid-source metal organic chemical vapour deposition process, using liquid injection, has been developed in order to control precisely the amount of precursor vapours to be produced. Films of self-doped La 1 − x MnO 3 − δ (0 ⩽ x ⩽ 0.3) have been deposited on MgO (1 0 0) and LaAlO 3 (0 1 2) substrates. X-ray diffraction measurements reveal an epitaxial growth. The as-deposited films exhibit both a ferromagnetic and metal—semiconductor transition at various temperatures depending on the x value. Post-annealing experiments greatly improve their magnetic and transport characteristics: the resistivity is considerably reduced and transition temperature substantially increased. For the x = 0.2 lacunar compound, the transition temperature raised up to 320 K and a magnetoresistance Δ ρ/ρ 0 = 20%/T is obtained at 300 K in the [0 T, 2 T] range.

La1 − x MnO3 − δ thin films grown by a new liquid source CVD process

Abstract A new liquid source CVD process has been developed in order to control precisely the amount of organometallic precursor vapours to be produced. Using the technique, thin films of self-doped La 1 − x MnO 3 − δ have been deposited on MgO(100) and LaAIO 3 (012) single crystals with various x values: x = 0, 0.06, 0.09, 0.20 and 0.25. X-ray diffraction measurements reveal an epitaxial growth on the two kinds of substrates. The as-deposited films exhibit ferromagnetic transitions at various temperatures depending on the x value. Different electrical behaviours are observed depending on x . A semiconductor/metal transition is obtained for samples with the highest x value. Magnetoresistance of the most lacunar as-deposited film has been measured: a GMR effect of 25% per tesla (T) is obtained at 190 K.

Hexaferrite and Manganite Films Obtained by Injection - MOCVD Process

We first present a new kind of CVD reactor using a new liquid delivery system which is called injection — CVD. Its principle is based on controlled injection of micro-amounts of solution containing the precursors and leads to precise control of the precursors vapour.

Relationships between crystal structure and magneto-transport properties in C.M.R. thin film of lacunar Lanthanum manganite before and after annealing

Abstract La 0.8 MnO 3− δ epitaxial thin films have been deposited by Injection-MOCVD on MgO. As-deposited film has a semiconducting-metal and para-ferromagnetic transition at about 220xa0K ; after annealing under air for 3xa0h at 700°C, this transition temperature is about 310xa0K. The crystal structure of the film has been studied by XRD and TEM, before and after annealing. XR data collections have been recorded from an 1xa0mm 2 thin film piece, using a conventional 4-circles-diffractometer designed for single crystals. In both cases the apparent symmetry is cubic with a cell parameter twice that of the classical perovskite. In fact, the true crystal structure is monoclinic with microdomain twinning. Atomic positions of all the atoms have been determined. Improvement of the magneto-transport properties after annealing is due to strengthening of the magnetic exchange interactions (Mn–O–Mn angles change from about 165° to 176°) and to increasing of the Mn 4+ /Mn 3+ ratio.