M. Sahana

Phase transformation and semiconductor-metal transition in thin films of VO2 deposited by low-pressure metalorganic chemical vapor deposition

Thin films of the semiconducting, monoclinic vanadium dioxide, VO2(M) have been prepared on ordinary glass by two methods: directly by low-pressure metalorganic chemical vapor deposition (MOCVD), and by argon-annealing films of the VO2(B) phase deposited by MOCVD. The composition and microstructure of the films have been examined by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Films made predominantly of either the B or the M phase, as deposited, can only be obtained over a narrow range of deposition temperatures. At the lower end of this temperature range, the as-deposited films are strongly oriented, although the substrate is glass. This can be understood from the drive to minimize surface energy. Films of the B phase have a platelet morphology, which leads to an unusual microstructure at the lower-deposition temperatures. Those grown at ∼370u2009°C convert to the metallic, rutile (R) phase when annealed at 550u2009°C, whereas those deposited at 420u2009°C transform to the ...

Colossal magnetoresistance in epitaxial La(1−x−y)NayMnO3 thin film

We have synthesized La0.83Na0.11MnO2.93 by heating La2O3 and MnCO3 in NaCl melt at 900u2009°C. The exact composition was arrived by analyzing each ion by an independent chemical method. The compound crystallized in a rhombohedral structure and showed an insulator-to-metal transition at 290 K. Epitaxial thin films were fabricated on LaAlO3 (100) using a pulsed laser deposition technique. The film also showed an insulator-to-metal transition at 290 K. Magnetoresistance [ΔR/R0=(RH−R0)/R0] was −71% near the insulator-to-metal transition temperature of 290 K at 6 T magnetic field.

Electrical resistivity and enhanced magnetoresistance in (La0.6Pb0.4MnO3/La0.85MnO3−δ)n superlattices

Resistivity and magnetoresistance studies have been carried out on (La0.6Pb0.4MnO3/La0.85MnO3−δ)n (n=30 periods) ferromagnetic/paramagnetic oxide multilayers fabricated by pulsed laser deposition. An epitaxial film of La0.6Pb0.4MnO3 is ferromagnetic and metallic below 320 K, whereas that of La0.85MnO3−δ is a paramagnetic (and antiferromagnetic at low temperatures) insulator. The multilayers showed an insulator-to-metal transition at 285±5u2009K. The magnetoresistances of these multilayers were about 60%–75% at the transition temperature in 6 T magnetic field. The magnetoresistance is also shown to be dependent on the thickness of the ferromagnetic oxide layer. A maximum magnetoresistance of 75% was observed for a [La0.6Pb0.4MnO3(50u2009A)/La0.85MnO3−δ(20u2009A)]30 superlattice as against 25% in a single La0.6Pb0.4MnO3 film at 285 K.

Magnetic structure of sodium and potassium doped lanthanum manganites

Abstract Neutron diffraction, depolarization, and magnetization measurements were carried out on Na (La 0.82 Na 0.13 MnO 2.93 ) and K (La 0.82 K 0.08 MnO 2.89 ) doped lanthanum manganites. The study shows that both the compounds are ferromagnetic below 300 K. Low-temperature magnetic structure refinement together with the magnetization (M) measurements suggest that both the compounds have a canted ferromagnetic structure at 14 K. Smooth variation of the magnetic peak intensity(∝ M 2 ) as a function of temperature could be fitted to the mean field approximation relationship, which indicates that, there is no abrupt change in the canting angle with temperature. Magnetic domain size (which is restricted by the grain size in the polycrystalline samples) obtained from the neutron depolarization measurements was found to be ∼1–2 μm in a magnetic field of 1.2 kOe for both samples.

Transport properties and colossal magnetoresistance in epitaxial La0.67Cd0.33MnO3 thin film

An optimal composition of La0.67Cd0.33MnO3 was synthesized by ceramic route. The compound crystallized in a rhombohedral structure with lattice parameters a = 5.473(4)u200aA and α = 60°37′. Resistivity measurement showed an insulator-to-metal transition coupled with a ferromagnetic transition of around 255 K. Epitaxial thin films were fabricated on the LaAlO3 (100) substrate by a pulsed laser deposition technique. The psuedocubic lattice parameter a of the film is 3.873(4) A. The insulator-to-metal transition of the film was observed at 250 K which is comparable with the bulk value. The film was ferromagnetic below this temperature. Magnetoresistance defined as ΔR/R0 = (RH−R0)/R0 was over −86% near the insulator-to-metal transition temperature of 240 K at 6 T magnetic field and over-30% at relatively low fields of 1 T. No magnetoresistance was observed at low temperatures in the film unlike in the polycrystalline sample, where about a 40% decrease in resistance was observed on applying 6 T magnetic field due to the spin dependent scattering at the grain boundaries.

Magnetic properties and specific heat of LaMn1−xTixO3+δ (0<x⩽0.2)

Abstract We present a magnetic study of the insulating perovskite LaMn 1− x Ti x O 3+ δ (0 x ⩽0.2) including measurements of magnetization, susceptibility, and magnetic relaxation. The Curie temperature was found to decrease with increasing content of Ti. Two distinct magnetic transitions, irreversibility, non-exponential relaxation and aging effects confirm a reentrant spin–glass state for x =0.2. The time decay of the magnetization has an algebraic functional form for times up to 2xa0h. The specific heat also displays characteristic features of a spin–glass system by a linear low-temperature dependence and a broadened peak near the temperature of the reentrant transition.

GIANT MAGNETORESISTANCE IN La0.6Pb0.4Mn(1−x)TixO3 FILMS: ENHANCEMENT OF MAGNETORESISTANCE DUE TO Ti4+ SUBSTITUTION IN Mn4+ SITES†

Abstract Nonmagnetic Ti 4+ ion is substituted for Mn 4+ in La 0.6 Pb 0.4 MnO 3 . The resulting La 0.6 Pb 0.4 Mn (1−x) Ti x O 3 (0 ≤ × ≤ 0.3) crystallizes in rhombohedral structure. The insulator-to-metal transition temperature decreases from 320 K to 92 K as x is varied from 0 to 0.09. Epitaxial films for the compositions x = 0, 0.03, 0.05, and 0.07 are fabricated by pulsed-laser deposition. Magnetoresistance at 6 T is enhanced from 38% for x = 0 to 75% for x = 0.07. The presence of Ti 4+ (d 0 ) ion decreases the Mn-Mn interaction through oxygen and thereby affects the resistivity and magnetoresistance of La 0.6 Pb 0.4 MnO 3 .

Structure of the mixed oxides La0.6Pb0.4Mn1-yTiyO3±z (0 < y < 0.4) via Rietveld refinements

Abstract The crystal structure of several compositions in the system La 0.6 Pb 0.4 Mn 1− y Ti y O 3 (0 ≤ y ≤ 0.4) have been investigated by powder X-ray diffraction using the Rietveld refinement technique. The solid solutions of compositions y = 0.1, 0.2, and 0.3 crystallize in the rhombohedral structure (space group R 3 c ), while for the composition corresponding to y = 0.4, the crystal structure transforms from rhombohedral to orthorhombic system (space group Imma ). Profile refinements on all these compositions reveal that the MnO 6 octahedra are distorted and rotated from their ideal positions. The doping at the B-site results in the formation of the orthorhombic phase Imma at room temperature. The structural implications on the magnetic properties also correlate with previous observations.

Magnetic ordering in La0.85Pb0.15Mn1−xTixO3 (0⩽x⩽0.15)

Abstract Powder-neutron diffraction study has been carried out at 300 and 10xa0K in La 0.85 Pb 0.15 Mn 1− x Ti x O 3 (0⩽ x ⩽0.15). The samples crystallize in the rhombohedral phase. The magnetic moment reduces nonlinearly with increase in Ti and correlates well with the reported behavior of T C . The change in the moment and T C could not be related to change in the one electron bandwidth, W . The reduction is attributed to the effect of dilution and thereby reducing the double exchange ferromagnetic interaction.