Giant magnetoresistance in La0.67Ca0.33MnO3 granular system with CuO addition

We report on a heterogeneous precipitation method to modify the surfaces of La_0.67Ca_0.33MnO₃ (LCMO) grains with CuO. It is shown that such a modification causes transport and magnetoresistance (MR) properties of the composites largely different from that observed in pure LCMO granular system. Especially, a significant enhancement in MR is observed near the insulator-metal transition temperature (T_IM). The maximum MR reaches as high as ∼88 and ∼90% at a low magnetic field of 0.3 T for the modified samples of LCMO/xCuO with x = 4 and 15 mol%, respectively. Compared to pure LCMO, the CuO-modified samples have a substantial decrease in resistivity (ρ) at the temperature regions apart T_IM. Furthermore, for the x = 4% sample, a considerable thermal hysteresis is observed at the same temperature region where abnormal MR effect appears. On the basis of magnetization measurement and structural analysis, a possible interpretation for the experimental observations is presented.     

Preparation and colossal magnetoresistance in a trilayer La0.67Sr0.33MnO3/La0.75MnO3/La0.67Sr0.33MnO3 device by dc magnetron sputtering

Epitaxial trilayer films of La_0.67Sr_0.33MnO₃ (LSMO)/La_0.75MnO₃ (L0.75MO)/La_0.67Sr_0.33MnO₃ (LSMO) have been prepared on (0 0 1) oriented LaAlO₃ substrates by dc magnetron sputtering. The structure and MR are studied. All as-deposited trilayer films exhibit a semiconductor to metal transition at temperature ranging from 116 to 185 K. The MR is also shown to be dependent on the thickness of the middle oxide layer. A maximum MR value of 32% (ΔR/R₀) has been obtained at 132 K under 0.4 T magnetic field for a LSMO (300 nm)/L0.75MO (70 nm)/LSMO (300 nm) trilayer film. The MR of trilayer film prefers to that of both LSMO and L0.75MO single layer films.     

Electrical transport and magnetoresistance in La0.67Ca0.33MnO3/BaTiO3 composites

The results of the structure, electrical transport and magnetoresistance of a ferromagnet-ferroelectric-type La_0.67Ca_0.33MnO₃ (LCMO)/BaTiO₃ composites fabricated by the sol-gel method are presented. The structure and morphology characterization indicates no apparent variations in morphology and particle size in spite of the existence of BaTiO₃. The insulator-metal transition temperature (T_IM) is shifted to a higher temperature and resistivity decreases with the increase of low content BaTiO₃. Magnetoresistance (MR) of the composites is enhanced over the whole temperature range as a result of the introduction of BaTiO₃. By calculating in terms of a ferromagnetic grain coupling model, we attribute these transport properties to the enhancement of the ferromagnetic coupling between the neighboring grains, which could be explained by the increase of the carrier concentration at the grain boundary due to the introduction of BaTiO₃ and the associated magnetoelectric coupling effect.     

Effects of chemical stability of heterogeneous oxides on transport properties of La0.67Ca0.33MnO3

Two kinds of composites of La_0.67Ca_0.33MnO₃/ZrO₂ and La_0.67Ca_0.33MnO₃/YSZ have been investigated, in which the heterogeneous oxide YSZ represents yttria-stabilized zirconia (8 mol% Y₂O₃ + 92 mol% ZrO₂). Their transport properties are measured in a temperature range of 10-300 K and a magnetic field range of 0-3 T. With increase in ZrO₂ doping level for the range of 0-2%, the metal-insulator transition temperature, T_P, decreases, but T_P increases with the doping level increase further for higher than 2%. Meanwhile, in the composite of La_0.67Ca_0.33MnO₃/ZrO₂, besides the intrinsic metal-insulator (ferromagnetic-paramagnetic) transition, a new kink and a widen transition temperature region are observed both in the temperature dependence of resistivity and magnetization curves. Compared to pure La_0.67Ca_0.33MnO₃ and the composite of La_0.67Ca_0.33MnO₃/YSZ, we assume that the different effects between ZrO₂ and YSZ on transport behavior of La_0.67Ca_0.33MnO₃ result mainly from their different chemical stability.     

X-Ray Diffraction Studies on Yttrium-Doped La0.67Ca0.33MnO3

Polycrystalline samples of yttrium-doped La_0.57Ca_0.33MnO₃ (LCMO) have been prepared. Structure characterizations have been carried out at room temperature by X-ray diffraction with Rietveld refinements. The octahedral tilting distortion increases with the increase of yttrium content or the decrease of the average radius of La-site cations, La. The difference between the two crystallographically distinct Mn—O—Mn bond angle increases with increasing Y content. The average Mn—O bond distance is relatively constant across the entire series, but individual Mn-O bond distances and the Mn—O—Mn bond angles show the presence of the cooperative Jahn–Teller effect at room temperature in yttrium-doped LCMO.     

Microstructures of YBa2Cu3O7/La0.7Ca0.3MnO3 and La0.7Ca0.3MnO3/YBa2Cu3O7 bi-layers grown on (001)LaAlO3

Epitaxial YBa₂Cu₃O₇/La_0.7Ca_0.3MnO₃ (YBCO/LCMO) bi-layers and La_0.7Ca_0.3MnO₃/YBa₂Cu₃O₇ (LCMO/YBCO) bi-layers were grown on (001)LaAlO₃ by pulsed laser deposition, and their microstructures were compared by transmission electron microscopy investigation. In the YBCO(100 nm)/LCMO(150 nm) bi-layers, the LCMO layer consists of columnar grains of ~ 17 nm in diameter and contains mixed orientation domains of [100]_c, [010]_c and [001]_c. The YBCO layer is totally c-axis oriented and the YBCO lattices are tilted − 2.5° to + 2.5° as they grew on the rough surfaces of LCMO columnar grains. For the LCMO(140 nm)/YBCO(140 nm) bi-layers, the LCMO/YBCO interface is sharp and flat. The initial 12-nm thickness of the YBCO layer is composed of c-axis oriented domains, and the upper part of YBCO layer is [100] oriented. The LCMO layer was predominantly [001]_c oriented while [100]_c-oriented domains were occasionally observed.     

La0.67Ca0.33MnO3 thin films on Si (1 0 0) by DC magnetron sputtering technique using nanosized powder compacted target

La_0.67Ca_0.33MnO₃ (LCMO) thin films were successfully fabricated by a DC magnetron sputtering technique on Si (1 0 0) substrates from chemically synthesized compacted powders. Powders of proper stochiometry composites were synthesized by a novel chemical technique [D.R. Sahu, B.K. Roul, P. Pramanik, J.L. Huang, Physica B 369 (2005) 209] and were found to be nanosized (≈40-50 nm). The sinterability of the powders were improved significantly due to their large surface area with a reduction of sintering temperature (up to 500 °C) as compared to the powders prepared by other solid-state reaction route. Bulk LCMO targets were prepared and preliminary structural and magnetic properties of target were investigated for colossal magnetoresistance (CMR) properties. Films deposition parameters like DC power, gas flow rate, deposition time, etc., were critically optimized to achieve desired thickness of film using above LCMO target by DC magnetron sputtering. LCMO films fabricated on Si (1 0 0) substrates showed enhanced magnetoresistance (MR) at low temperature. Maximum MR of about 1000% was observed at 100 K. Paramagnetic to ferromagnetic transitions were observed in films below room temperature and were found at approximately 240 K. However, as compared to bulk target prepared by a chemical route, it was found that Curie temperature (T_c) and MR response of bulk target were higher than the thin films. Preliminary point chemical analysis revealed the deficiency of Ca²⁺ ions in CMR films.     

Antiferromagnetic manganite La1/3Ca2/3MnO3 as barrier material in superconductor/insulator/ferromagnet tunnel junctions

Current transport through thin antiferromagnetic (AF) barriers of the perovskita manganite La_1/3Ca_2/3MnO₃ (LCMO) was studied with respect to its dependence on temperature and voltage. Planar-type La_2/3Ca_1/3MnO₃(~ 80 nm)/La_1/3Ca_2/3MnO₃(~ 7 nm)/YBa₂Cu₃O_7 − δ(~ 100 nm) heterojunctions were used as basic structures. The current-voltage (I-V) measurements were carried out on test junctions with a standard area of 20 × 40 µm² in a four-terminal configuration. In spite of the carefully controlled growth conditions, barriers with the same nominal thickness showed different electrical behavior varying from elastic tunneling to Mott variable range hopping (VRH) via localized states. The different transport characteristics seem to be related to intrinsic difference in microstructure as the average surface roughness of the constituent layers may already be larger than the thickness of the barrier itself.     

Electrical transport and magnetic properties in La0.67Ca0.33MnO3 and CuFe2O4 composites

The (1 − x) La_0.67Ca_0.33MnO₃/xCuFe₂O₄ (x = 0, 0.04, 0.07, 0.10 and 0.15) composites have been prepared and investigated systematically for their microstructure, electrical and magnetic properties. The X-ray diffraction and scanning electronic microscopy analysis reveal that both La_0.67Ca_0.33MnO₃ (LCMO) and CuFe₂O₄ phases are distributed in the composites. Compared with pure LCMO, an obvious enhancement of magnetoresistance is observed over a wide temperature range for the composites. Under 3 T field, the magnetoresistance rises from a base value 44.5% for pure LCMO, to a maximum value of 66.7% for x = 0.1 composite. Meanwhile, a weak temperature-dependent MR platform is observed in the temperature range from 210 K to 240 K. The MR enhancement is discussed in terms of spin-polarized intergrain tunneling due to enhanced magnetic disorder and magnetic coupling near boundaries between LCMO and CuFe₂O₄ grains.     

Grain boundary effects on the electrical and magnetic properties of Pr2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 manganites

Electrical and magnetic properties of orthorhombic Pr_2/3Ba_1/3MnO₃ (PBMO) and La_2/3Ca_1/3MnO₃ (LCMO) manganites with considerable difference in variance factors (σ²) are reported here. PBMO with higher variance exhibits distinct intrinsic (due to grains) and extrinsic (due to grain boundaries) transitions in the resistivity behaviour. Extrinsic effects, however, are not observed in the lower σ² LCMO system. Low field magnetoresistivity (LFMR) data also substantiate these results. Increase in the density of states obtained through Mott's 3-D variable range hopping mechanism in the paramagnetic insulating regime indicates the suppression of magnetic domain scattering with applied magnetic field. Ferromagnetic metallic regime below the extrinsic transition in PBMO seems to emanate from the electron-magnon scattering process. LFMR at 77 K also points towards the higher canting of spins in the vicinity of grain boundary regions in PBMO compared to that in LCMO.     

A modified synthesis method of manganese perovskite La0.67Ca0.33MnO3 by high energy ball milling and post heat treatment

A modified synthesis method of La_0.67Ca_0.33MnO₃ by high energy ball milling and post sintering is reported. The characteristics of samples from this modified method are also studied. The new synthesis method has some advantages including reducing the synthesis procedure into two simple steps, shortening the sintering time, and obtaining similar properties compared with conventional synthesis method. High energy ball milling was employed for 10 h to refine the starting materials into a sub-micron particle size (for comparison, samples milled for 30 and 50 h were also synthesized); during the process, ethanol was added as a milling medium for avoiding amorphization or crystallization, as well as suspending the starting powders for more profound milling. Finally, the as-milled powder was sintered in air to crystallize. The samples show metal-to-insulator transitions at around 260 K, with grain size from 200 nm to 8 μm, and exhibit large magnetoresistance up to 21% at a magnetic field of 3000 Oe near the transition temperature. The possible mechanism is discussed.     

The demonstration of epitaxial and electronic-magnetic properties for La0.67Sr0.33MnO3−δ films with (1 1 1) orientation

La_0.67Sr_0.33MnO_3−δ films, fabricated on (1 1 1) LaAlO₃ single-crystal substrates using a direct current magnetron sputtering technique, are demonstrated by X-ray diffraction patterns and pole figures to be high quality epitaxial films and there is a perfect matching relationship between the films and the substrates. We observed an obvious difference of the electronic-magnetic transportation properties among films sputtered on (1 1 1), (1 0 0) and (1 1 0) LaAlO₃ substrates, respectively. A mechanism for the difference is discussed briefly.     

Electric resistance and magnetoresistance of a two-layer epitaxial heterostructure (30 nm)La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/(30 nm)La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

Two-layer epitaxial heterostructures (30 nm)La_0.67Ca_0.33MnO₃/(30 nm)La_0.67Ba_0.33MnO₃ (LCMO/LBMO) have been grown by laser deposition on single crystal (001)LaAlO₃ (LAO) substrates. In this system, the upper (LCMO) layer occurs under the action of tensile stresses in the substrate plane, whereas the lower (LBMO) layer exhibits biaxial compression. The formation of a 30-nm-thick LCMO film on the surface of the 30-nm-thick LBMO layer leads to an increase in the level of mechanical stresses in the latter layer. The maximum electric resistivity ρ of the (30 nm)LCMO/(30 nm)LBMO/LAO structure was observed at a temperature 25–30 K below that corresponding to the maximum of the ρ(T) curve for a single (30 nm)LBMO film on the same LAO substrate.     

Photoluminescence properties of V-doped La0.67Ca0.33Mn1−xVxO3

The optical spectra have been investigated for a prototypical double exchange ferromagnetic La_0.67Ca_0.33MnO₃ with Mn substituted by V above the paramagnetic-magnetic transition temperature T_C. The excitation spectra under the probe wavelength of λ_em = 473 nm for all samples exhibit two activation bands around 360 and 294 nm, involving an electron transfer from oxygen 2p states to the Mn d states in MnO₆ octahedra. The photoluminescence spectra at λ_ex = 290 nm have the similar spectral features for all samples. The photoluminescence spectral peaks located at 400, 473, 534, 670, 738 and 770 nm, and the corresponding photon energy is in a broad range of 3.1-1.6 eV, indicating that the PL bands could have the different origin: the self-trapped excitons localized on MnO₆ octahedra; the interband transition between the O 2p and Mn 3d bands; the transition between the 3d electron states of Mn ions. So, it can be clearly seen that the electronic behavior above T_C is very complicated. Our results suggest that the charge transfer from O 2p to Mn 3d has the important effects on the electronic structure, and it not only contributes to the optical transition but is helpful and even important to understand the electric, magnetic and thermal properties etc. due to the strong correlation among charge, spin and lattice in perovskite manganites.     

Microwave assisted low temperature rapid synthesis of manganite system using La0.67Ce0.03Sr0.3MnO3 mini-cavity furnace

Amorphous La_0.7Sr_0.3MnO₃ (LSMO) and La_0.7Ca_0.3MnO₃ (LCMO) precursor powders synthesized by the citrate gel method at 673 K, have been found to crystallize by microwave irradiation in just 60 s using La_0.67Ce_0.03Sr_0.3MnO₃ (Ce-LSMO) as couplant. This is the lowest temperature treatment and synthesis time so far reported in literature for the formation of manganite systems. Using ceramic route, the same amorphous samples crystallize on heat treatment only at temperatures greater than 1000 K. The microwave heating through this method is novel and has tremendous potential for accelerating the evolution of the product phase in very shorter durations, with just low temperature processing of the precursors, which cannot be realized in normal process.     

Magnetocaloric Effect in Different Impurity Doped La0.67Ca0.33MnO3 Composite

In this paper, we investigate the effect of nonmagnetic YSZ and magnetic Fe₃O₄ addition on the magnetocaloric properties of the La_0.67Ca_0.33MnO₃ composite. The temperature dependences of the magnetization for pure LCMO and the impurity doped LCMO composite upon 0.1, 0.3, and 1 T magnetic field were simulated. By the help of the phenomenological model, magnetic entropy change and specific heat for magnetic field variation are predicted. The values of maximum magnetic entropy change, full-width at half-maximum, and relative cooling power were calculated at several magnetic fields. The ΔS _M value is 2.45, 2.39, and 5.58 J?kg^?1?K^?1 at 1 T for pristine LCMO, LCMO/YSZ, and LCMO/Fe₃O₄, respectively.     

Effect of sintering temperature on magneto-transport properties of La0.67Ba0.33MnO3/Ba0.7Sr0.3TiO3 composites

La_0.67Ba_0.33MnO₃-20 wt.%-Ba_0.7Sr_0.3TiO₃ composites were sintered at different temperatures in order to explore the possibility of improving the magneto-transport properties of the composites. Detail studies on the magnetic and electrical transport properties for the sintered composite samples have been performed. Results show that the sintered composites have identical ferromagnetic to paramagnetic transition temperature and filamentary feature of metallic phase. When sintering temperature higher than 1300 °C, the composites show Efros-Shklovskii-like variable-range hopping in the temperature range lower than Curie temperature. For samples sintered lower than 1100 °C, a dome-like resistance peak appears at a temperature well below the Curie temperature. Magnetoresistance behavior indicates the existence of spin polarized tunneling in the low temperature range. Considering the contributions from Efros-Shklovskii-like variable-range hopping and spin polarized tunneling, the resistance peak can be well fitted.     

Crystallization in rapid quenching of La0.33NbO3 melt

Transparent La_0.33NbO₃ flakes were fabricated by a rapid quenching technique. The quenched La_0.33NbO₃ phase takes a simple cubic perovskite structure and cation vacancies are randomly distributed on the A sites. The cubic La_0.33NbO₃ phase is a new modification which is different from the well-known orthorhombic phase with ordered A-site vacancies. Transparent flakes were found to show the predominant orientation of (110) and (220) planes.     

Temperature dependence of photovoltaic effect in La0.7Sr0.3MnO3/Si heterostructure

The photovoltaic effect and good rectifying behavior have been observed in a heterostructure fabricated by depositing the La_0.7Sr_0.3MnO₃ film on a Si substrate. The photovoltages increase quickly to the maximum values at about 266μs and then decrease gradually. The maximum photovoltage is about 0.220V at T = 90K. The maximum photovoltages decrease with increasing the temperature, which is attributed to the stronger thermal fluctuation. A local minimum in the photovoltages-temperature curve is observed at T = 143K, which is consistent with the metal-insulator transition temperature, and this might be caused by the thinner thickness of the depletion layer due to a change in the band structure of the LSMO layer. This result indicates that the photovoltaic effect of the manganite-based heterostructure can be changed by the intrinsic phase transition.     

Critical behaviour and magnetocaloric effect of nano crystalline La0.67Ca0.33Mn1−xFexO3 (x = 0.05, 0.2) synthesized by nebulized spray pyrolysis

The structure, critical exponents and magnetocaloric effect (MCE) of Nebulized Spray Pyrolysis (NSP) synthesized nano crystalline La_0.67Ca_0.33Mn_1−xFe_xO₃ (x = 0.05, 0.2) were investigated. The Reitveld refinement of XRD patterns show that the samples adopt an orthorhombic structure with Pnma space group. TEM inspection reveals that the average particle size is about 15 nm and 42 nm for NSP synthesized LCMFe_0.05 and LCMFe_0.2 samples respectively. The temperature and field dependent magnetization studies reveal the superparamagnetic state of La_0.67Ca_0.33Mn_0.95Fe_0.05O₃ and spin-glass-like state of La_0.67Ca_0.33Mn_0.8Fe_0.2O₃. The critical behaviour at the transition region studied using modified Arrott plot provides a second order nature of phase transition for both samples. The magnetocaloric studies show the maximum value of magnetic entropy change (ΔS_max) is in the range 2.3 J kg⁻¹ K⁻¹ at 158 K for LCMFe_0.05 and 0.3 J kg⁻¹ K at 92 K for LCMFe_0.2 respectively at 5 T field. The field dependence of the magnetic entropy changes are also analysed, which show a power law dependence (ΔS_M ∞ Hⁿ, n = 0.72 (2)) at transition temperature, T_C = 162 K for LCMFe_0.05 and n = 1.11(3) at 92 K for LCMFe_0.2.