Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

Structural characterization of La0.9Ba0.1MnO3/Y-ZrO2 film by X-ray diffraction

Perovskite manganite La_0.9Ba_0.1MnO₃(LBMO) films were deposited on (0 0 1)-oriented single crystal yttria-stabilized zirconia (YSZ) substrate by 90° off-axis radio frequency magnetron sputtering. The film thickness ranged from 10 nm to 100 nm. Grazing incidence X-ray diffraction technique and high resolution X-ray diffraction were applied to characterize the structure of LBMO films. The LBMO film mainly consisted of (0 0 1)-orientated grain as well as weakly textured (1 1 0)-orientated grain. The results indicated that an amorphous layer with thickness of about 4 nm was formed at the LBMO/YSZ interface. The strain in LBMO film was small and averaged to be about -0.14%. The strain in the film was not lattice mismatch-induced strain but residual strain due to the difference in thermal expansion coefficient between film and substrate.     

Large room temperature magnetoresistance in YSZ doped La0.67Ba0.33MnO3 composite

The temperature dependence of the resistivity for composite samples of (1−x)La_0.67Ba_0.33MnO₃+xYSZ(LBMO/YSZ) with different YSZ doping level of x has been investigated in a magnetic field range of 0-7000 Oe, where the YSZ represents yttria-stabilized zirconia (8 mol% Y₂O₃+92 mol% ZrO₂). With increasing YSZ doping level, the range of 0-10%, the metal-insulator transition temperature (T_P) decreases. However, the resistivity, specially the low temperature resistivity, increases. Results also show that the YSZ doping level has an important effect on a low field magnetoresistance (LFMR). In the magnetic field of 7000 Oe, a room temperature magnetoresistance value of 20% was observed for the composite with a YSZ doping level of 2%, which is encouraging for potential application of CMR materials at room temperature and low field.     

Surface and transport studies on La0.7Ba0.3MnO3:SnO2 bilayer

We report on study of morphology, optical contrast and transport characteristics of La_0.7Ba_0.3MnO₃ (LBMO) manganite thin films bilayered with SnO₂ on Si (0 0 1) substrate, synthesized using pulsed laser deposition system. X-ray diffraction study reveals that both LBMO and SnO₂ show polycrystalline growth over the substrate. Atomic force microscopy shows interesting pyramidal structures of LBMO of size ∼2 μm × 1 μm × 0.1 μm. On the other hand, SnO₂ grows in the form of close packed cylindrical clusters of ∼200 nm radius. Near-field optical microscopy (NSOM) study using 532 nm laser reveal that optical NSOM output intensity in LBMO is four times less than SnO₂ signal. Transport characterizations show that this bilayer configuration exhibit non-linear current-voltage characteristics from 300 upto 50 K. The nature becomes linear below this temperature. The results project the system as a promising candidate in non-conventional device category in the area of spintronics.     

Large magnetoresistance induced by surface ferromagnetism in A-type antiferromagnetic La0.4Sr0.6MnO3 nanoparticles

Grain size effects on magnetic and transport properties for heavily Sr-doped A-type antiferromagnetic La_0.4Sr_0.6MnO₃ ceramics were studied. It was observed that with decrease in grain size, surface ferromagnetism could be introduced due to bond-breaking at surfaces. With decrease in grain size, the surface ferromagnetism was enhanced, and the phase transition order distinguished from the Arrott plot was a second one. The surface-induced ferromagnetism was insulating as judged from transport properties. With decrease in grain size, magnetoresistance was largely improved for both high magnetic and low magnetic fields. Under a 500 Oe magnetic field, the magnetoresistance is improved from 0.2%, 0.1%, 0.03% and 0.02% for the sample with grain size of 150 nm at 10, 100, 200 and 300 K, respectively, to 3%, 2.3%, 0.43% and 0.12% for the sample with grain size of 20 nm at 10, 100, 200 and 300 K. It was interesting to find that large magnetoresistance could be induced due to the surface ferromagnetism in A-type antiferromagnetic La_0.4Sr_0.6MnO₃ nanoparticles, which suggested that it was possible to search for manganites with relatively high low-field magnetoresistance in nanostructured A-type antiferromagnetic materials.     

Microstructure and magnetic properties of nanocomposite FePt/MgO and FePt/Ag films

An in-plane magnetic anisotropy of FePt film is obtained in the MgO 5 nm/FePt t nm/MgO 5 nm films (where t=5, 10 and 20 nm). Both the in-plane coercivity (H_c∥) and the perpendicular magnetic anisotropy of FePt films are increased when introducing an Ag-capped layer instead of MgO-capped layer. An in-plane coercivity is 3154 Oe for the MgO 5 nm/FePt 10 nm/MgO 5 nm film, and it can be increased to 4846 Oe as a 5 nm Ag-capped layer instead of MgO-capped layer. The transmission electron microscopy (TEM)-energy disperse spectrum (EDS) analysis shows that the Ag mainly distributed at the grain boundary of FePt, that leads the increase of the grain boundary energy, which will enhance coercivity and perpendicular magnetic anisotropy of FePt film.     

Characterizations of n-type ferromagnetic GaMnN thin film grown on GaN/Al2O3 (0 0 0 1) by metal-organic chemical vapor deposition

A high-quality ferromagnetic GaMnN (Mn=2.8 at%) film was deposited onto a GaN buffer/Al₂O₃(0 0 0 1) at 885 °C using the metal-organic chemical vapor deposition (MOCVD) process. The GaMnN film shows a highly c-axis-oriented hexagonal wurtzite structure, implying that Mn doping into GaN does not influence the crystallinity of the film. No Mn-related secondary phases were found in the GaMnN film by means of a high-flux X-ray diffraction analysis. The composition profiles of Ga, Mn, and N maintain nearly constant levels in depth profiles of the GaMnN film. The binding energy peak of the Mn 2p_3/2 orbital was observed at 642.3 eV corresponding to the Mn (III) oxidation state of MnN. The presence of metallic Mn clusters (binding energy: 640.9 eV) in the GaMnN film was excluded. A broad yellow emission around 2.2 eV as well as a relatively weak near-band-edge emission at 3.39 eV was observed in a Mn-doped GaN film, while the undoped GaN film only shows a near-band-edge emission at 3.37 eV. The Mn-doped GaN film showed n-type semiconducting characteristics; the electron carrier concentration was 1.2×10²¹/cm³ and the resistivity was 3.9×10⁻³ Ω cm. Ferromagnetic hysteresis loops were observed at 300 K with a magnetic field parallel and perpendicular to the ab plane. The zero-field-cooled and field-cooled curves at temperatures ranging from 10 to 350 K strongly indicate that the GaMnN film is ferromagnetic at least up to 350 K. A coercive field of 250 Oe and effective magnetic moment of 0.0003 μ_B/Mn were obtained. The n-type semiconducting behavior plays a role in inducing ferromagnetism in the GaMnN film, and the observed ferromagnetism is appropriately explained by a double exchange mechanism.     

Quasi-two-dimensional conductivity and magnetoconductivity of graphite-like nanosize crystallites

The temperature dependence of electrical conductivity and magnetoconductivity of new type of carbon films composed of nanosize thin graphite-like crystallites were investigated at temperature interval of 4.2-300 K and in the magnetic field range of 0-12 kG at 4.2 K, respectively. The crystallites consist of several (5-50) graphene layers which have predominant orientation perpendicularly to a film surface. At temperature ≤30 K the logarithmic conductivity decreases linearly with temperature. The positive magnetoconductivity of the films was observed in a magnetic field directed perpendicularly to the film surface in all intervals of field values. In magnetic field B≥4 kG the logarithmic asymptotic of conductivity from magnetic field was observed. That is characteristic of the systems with two-dimensional quantum corrections to magnetoconductivity. In a magnetic field directed along a film surface, the crossover from negative to positive magnetoresistivity is observed at B≥8 kG.     

Possible mechanism for tunneling magnetoresistance in La0.9Ba0.1MnO3/Nb-doped SrTiO3 p-n junctions

The observed tunneling magnetoresistance (TMR) effect in La_0.9Ba_0.1MnO₃ (LBMO)/Nb-doped SrTiO₃ (Nb-STO) p⁺-n junctions is investigated and a possible mechanism responsible for the TMR generation is proposed by taking into account the dynamic spin accumulation and paramagnetic magnetization in the Nb-STO layer. Because of carrier diffusion across the dynamic domain boundaries in the Nb-STO layer and spin disordering in the LBMO layer, the tunneling resistance through the junction is high at zero magnetic field. The spin disordering is suppressed upon applying a non-zero magnetic field, which results in the spin-polarized tunneling in this ferromagnetic/depletion layer/dynamic ferromagnetic sandwiched structure and thus the observed TMR effect. The dependence of the TMR effect on the domain size in the LBMO layer, the tunneling current and temperature as well is explained, qualitatively consistent with the experimental observation.     

Van der Pauw resistivity measurements on evaporated thin films of cadmium arsenide, Cd3As2

Cadmium arsenide is a II-V semiconductor, exhibiting n-type intrinsic conductivity with high mobility and narrow bandgap. It is deposited by thermal evaporation, and has shown the Schottky and Poole-Frenkel effects at high electric fields, but requires further electrical characterisation. This has now been extended to low-field van der Pauw lateral resistivity measurements on films of thickness up to 1.5 μm. Resistivity was observed to decrease with increasing film thickness up to 0.5 μm from about 3 × 10⁻³ Ω m to 10⁻⁵ Ω m, where the crystalline granular size increases with film thickness. This decrease in resistivity was attributed to a decrease in grain boundary scattering and increased mobility. Substrate temperature during deposition also influenced the resistivity, which decreased from around 10⁻⁴ Ω m to (10⁻⁵ to 10⁻⁶) Ω m for an increase in substrate deposition temperature from 300 K to 423 K. This behaviour appears to result from varying grain sizes and ratios of crystalline to amorphous material. Resistivity decreased with deposition rate, reaching a minimum value at about 1.5 nm s⁻¹, before slowly increasing again at higher rates. It was concluded that this resulted from a dependence of the film stoichiometry on deposition rate. The dependence of resistivity on temperature indicates that intercrystalline barriers dominate the conductivity at higher temperatures, with a hopping conduction process at low temperatures.     

Magnetotransport properties of epitaxial Pr0.5Ca0.5MnO3 films grown by a solution technique

Epitaxial Pr_0.5Ca_0.5MnO₃ films have been synthesized on (0 0 1) SrTiO₃ substrate using a chemical solution deposition technique and two-step post-annealing process. The zero field resistivity of the films shows semiconducting behavior and a characteristic of charge ordering is observed at 230 K. The resistivity of the 10 nm film did not show any effect with the magnetic field. However, melting of charge ordering was observed for the 120 nm film at an applied magnetic field of 4 T. Large decrease in the resistivity of the 120 nm film (<100 K) resulted in magnetoresistance of nearly −100% at 75 K.     

Half Ca-doped LaMnO3 films on (0 0 1) SLAO and STO substrate studied by magnetization and transport measurements

In this study, La_0.5Ca_0.5MnO₃ (LCMO) films, at the boundary between ferromagnetic metallic and charge-ordered antiferromagnetic insulator according to the bulk phase diagram, were epitaxially grown on (0 0 1) SrTiO₃ (STO) and SrLaAlO₄ (SLAO) substrates by pulsed laser deposition technique. The films were analyzed by X-ray diffraction, magnetization and magnetoresistance measurements. A considerably higher magnetization was measured for 290-nm-thick film on SLAO substrate compared to the film on STO substrate, although both films have the same chemical composition, thickness and epitaxial orientation. The film on SLAO shows a metal-insulator (MI) transition, which occurs at higher temperatures with increasing applied magnetic field, whereas only insulating behavior was observed for the 290-nm-thick film on STO except for the highest applied magnetic field (7 T). In addition, transport measurements were performed and analyzed by Mott's variable range hopping (VRH) model to correlate the resistivity of the films with the Jahn-Teller strain (ε_J−T) in the structure.     

Anomalous Hall effect in Cu and Fe codoped In2O3 and ITO thin films

We present a systematic study of the structure, magnetization, resistivity, and Hall effect properties of pulsed laser deposited Fe- and Cu-codoped In₂O₃ and indium-tin-oxide (ITO) thin films. Both the films show a clear ferromagnetism and anomalous Hall effect at 300 K. The saturated magnetic moments are almost the same for the two samples, but their remanent moments M_r and coercive fields H_C are quite different. M_r and H_C values of ITO film are much smaller than that of In₂O₃. The ITO sample shows a typical semiconducting behavior in whole studied temperature range, while the In₂O₃ thin film is metallic in the temperature range between 147 and 285 K. Analysis of different conduction mechanisms suggest that charge carriers are not localized in the present films. The profile of the anomalous Hall effect vs. magnetic field was found to be identical to the magnetic hysteresis loops, indicating the possible intrinsic nature of ferromagnetism in the present samples.     

High-frequency characteristics of CoFeVAlONb thin films

High-frequency characteristics of CoFeVAlONb thin films were studied. A thin film of Co_43.47Fe_35.30V_1.54Al_5.55O_9.93Nb_4.21 is observed to exhibit excellent magnetic properties; magnetic coercivity of 1.24 Oe, uniaxial in-plane anisotropy field of 66.99 Oe, and saturation magnetization of 19.8 kG. The effective permeability of the film is as high as 1089 and is stable up to 1.8 GHz, and with ferromagnetic resonance over 3 GHz. This film also has very high electrical resistivity of about 628 μΩ cm. These superior properties make it ideal for high-frequency magnetic applications.     

Low temperature resistivity minimum and its correlation with magnetoresistance in La0.67Ba0.33MnO3 nanomanganites

A systematic investigation of structural, magnetic and electrical properties of nanocrystalline La_0.67Ba_0.33MnO₃ materials, prepared by citrate gel method has been undertaken. The temperature-dependant low-temperature resistivity in ferromagnetic metallic (∼50 K) phase shows upturn behavior and is suppressed with applied magnetic field. The experimental data (<75 K) can be best fitted in the frame work of Kondo-like spin-dependant scattering, electron-electron and electron-phonon interactions. It has been found that upturn behavior may be attributed to weak spin disorder scattering including both spin polarization and grain boundary tunneling effects, which are the characteristic features of extrinsic magnetoresistance behavior, generally found in nanocrystalline manganites. The variation of electrical resistivity with temperature in the high temperature ferromagnetic metallic part of electrical resistivity (75K<T<T_P) has been fitted with grain/domain boundary, electron-electron and magnon scattering mechanisms, while the insulating region (T>T_P) of resistivity data has been explained based on adiabatic small polaron hopping mechanism.     

Low field magnetoresistance and conduction noise in layered manganite La1.4Ca1.6Mn2O7

Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La_1.4Ca_1.6Mn₂O₇ (DLCMO) are reported and compared with the infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). The double layered manganite was prepared using standard solid state reaction method and had a metal-insulator transition temperature (T_M-I) of 155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed voltage noise spectral density (S_V) shows 1/f^α type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic region (T<168 K), S_V/V² shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range 3D-ferromagnetic ordering at two different temperatures T_C2 and T_C1. In temperature range between T_C1 and T_C2, the magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].     

Large relative cooling power of Gd52.5Co16.5Al31 magnetic bulk metallic glass

The magnetocaloric effect and thermal stability have been investigated on the new bulk metallic glass (BMG) Gd_52.5Co_16.5Al₃₁ alloy. The extent of supercooled liquid region is 70 K, which is wider than that of any other Gd-Co-Al ternary BMGs. The magnetic entropy change (ΔS_M) and relative cooling power (RCP) of 9.8 J/kg K and 9.1×10² J/kg are obtained, respectively, under a field change of 5 T. The large ΔS_M and RCP values make Gd_52.5Co_16.5Al₃₁ BMG attractive potential candidate for the magnetic refrigeration application.     

Growth temperature dependence of the hysteretic behavior of Ni0.5Zn0.5Fe2O4 thin films

Herein, a discussion of the effect of deposition temperature on the magnetic behavior of Ni_0.5Zn_0.5Fe₂O₄ thin films. The thin films were grown by r.f. sputtering technique on (1 0 0) MgO single-crystal substrates at deposition temperatures ranging between 400 and 800 °C. The grain boundary microstructure was analyzed via atomic force microscopy (AFM). AFM images show that grain size (φ∼70-112 nm) increases with increasing deposition temperature, according to a diffusion growth model. From magneto-optical Kerr effect (MOKE) measurements at room temperature, coercive fields, H_c, between 37and 131 Oe were measured. The coercive field, H_c, as a function of grain size, reaches a maximum value of 131 Oe for φ ∼93 nm, while the relative saturation magnetization exhibits a minimum value at this grain size. The behaviors observed were interpreted as the existence of a critical size for the transition from single- to multi-domain regime. The saturation magnetization (21 emu/g<M_s<60 emu/g) was employed to quantify the critical magnetic intergranular correlation length (L_c≈166 nm), where a single-grain to coupled-grain behavior transition occurs. Experimental hysteresis loops were fitted by the Jiles-Atherton model (JAM). The value of the k-parameter of the JAM fitted by means of this model (k/μ_o∼50 A m²) was correlated to the domain size from the behavior of k, we observed a maximum in the density of defects for the sample with φ∼93 nm.     

Structure,magnetic properties and magnetostriction of Fe81Ga19 thin films

The structure, magnetic properties and magnetostriction of Fe₈₁Ga₁₉ thin films have been investigated by using X-ray diffraction analysis, scanning electron microscope (SEM), vibrating sample magnetometer and capacitive cantilever method. It was found that the grain size of as-deposited Fe₈₁Ga₁₉ thin films is 50–60 nm and the grain size increases with increase in the annealing temperature. The remanence ratio (M_r/M_s) of the thin films slowly decreases with increase in the annealing temperature. However, the coercivity of the thin films goes the opposite way with increase in the annealing temperature. A preferential orientation of the Fe₈₁Ga₁₉ thin film fabricated under an applied magnetic field exists along 〈1 0 0〉 direction due to the function of magnetic field during sputtering. An in-plane-induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the formation of in-plane-induced anisotropy results in 90° rotations of the magnetic domains during magnetization and in the increase of magnetostriction for the thin film.     

Conductivity behaviour of a cubic/tetragonal phase stabilized nanocrystalline La2O3-ZrO2

La₂O₃ doped nanocrystalline zirconia (ZrO₂) was prepared by chemical co-precipitation method for the 3, 5, 8, 10, 15, 20 and 30 mol.% concentrations of La₂O₃. Structural studies were performed using X-ray diffraction (XRD). All the as-synthesized samples were found to be in monoclinic phase. As-synthesized samples were given heat treatment at higher temperatures for tetragonal/cubic structural phase stabilization. Sintering the samples at temperature 1173 K stabilized the tetragonal and cubic phases. A slight shift in the 100% peak of the cubic phase was observed towards the low diffraction angle indicating the substitution of the bigger La³⁺ ion into the ZrO₂ lattice. Grain sizes were found to lie between 10 and 13 nm. Electrical conductivity studies were performed on the cubic phase stabilized La₂O₃-ZrO₂ by complex impedance spectroscopy. The conductivity increases up to the dopant concentration 10 mol.% and then decreases with further increase in La₂O₃ concentration. Initial increase in conductivity is correlated to the stabilization of the cubic phase and the subsequent decrease in the conductivity with the dopant content is interpreted on the basis of the oxygen-ion movement model. Electrical conductivity has contributions from grain and grain boundary regions. But the grain boundary conductivity is slightly higher than the corresponding grain conductivity. Higher grain boundary conductivity shows higher diffusion coefficient for the atoms on the surface of the ZrO₂ grains. The possible mechanism of the oxygen ion conduction in the La₂O₃ stabilized zirconia (LSZ) is reported. The Barton, Nakajima and Namikawa (BNN) relation has been applied to the conductivity data and found that the d.c. and a.c. conductions have been correlated to each other by the same mechanism.