Colossal magnetoresistive and ferroelectric thin films deposited by excimer laser induced plasma

We prepared the colossal magnetoresistive La_0.8Sr_0.2MnO₃ thin films on single crystal MgO, SrTiO₃ and LaAlO₃ substrates by KrF excimer pulsed laser deposition technique. The thickness dependence of the structural, electric and magnetic properties of the LSMO films on different substrates is reported. The integration of the ferroelectrics with the colossal magnetoresistance thin film was achieved. The plasma plume ablated from target was investigated by intensified charge coupled device and optical emission spectroscopy.     

Electrical transport properties and room-temperature positive magnetoresistance of Fe3O4/a-C/n-Si junctions

Fe₃O₄/a-C/n-Si junctions were formed by sequent depositing a-C and Fe₃O₄ layers on Si substrates by facing-target magnetron sputtering. The junctions exhibit nonlinear and slightly asymmetric current-voltage behaviors. For the junctions with thinner a-C layers, high value of the ideality factors suggests that the current transport is primary dominated by thermionic field emission other than thermionic emission. The tunneling is found as the dominant mechanism for the junctions with thicker a-C layers. Compared to Fe₃O₄/n-Si junctions, the positive magnetoresistance in the Fe₃O₄/a-C/n-Si junctions was enhanced. Magnetoresistance is considered to originate from the interfacial scattering of carriers where the energy band of Fe₃O₄ is modified by the defect states. A possible mechanism for the enhancement of magnetoresistance is the heavier intermixing between Fe₃O₄ and a-C layer, which leads to a stronger scattering with the increase of a-C layer thickness.     

Structural,magnetic, and magnetotransport properties of LaFe0.5Ni0.5O3

The perovskite-type LaFe_0.5Ni_0.5O₃ belonging to the rhombohedral (space group R-3c) crystal structure has been synthesized for which we have identified a magnetic transition at T₁ =?8?K corresponding to the minimum observed in the derivative of temperature dependent magnetization. A bifurcation in the ZFC and FC curves is observed below T₁ that suggests a frustrated magnetic behavior. The non-zero moment above T₁ hints the possibility of the presence of a high-temperature magnetic transition in the material. The resistivity of LaFe_0.5Ni_0.5O₃ evolves as a function of temperature similar to that of a semiconductor. Mott's variable range hopping governs the conduction mechanism of the material. Presence of various anisotropy terms and inhomogeneous magnetic interactions lead to the presence of antiferromagnetic and ferromagnetic interfaces, which eventually causes a magnetic exchange bias and magnetic hysteresis in resistivity. We have also observed direction dependent magnetoresistance in the material.     

Colossal and constant magnetoresistance over a large temperature range between 230 and 4.2 K in La0.8Li0.2MnO3 prepared by a partial melting technique

La_0.8Li_0.2MnO₃ was prepared by a partial melting technique. The Li-doped LaMnO₃ compound has a rhombohedral perovskite structure and shows a wide semiconductor-to-metal transition between 230 and 175 K in resistance versus temperature. Nearly constant magnetoresistance, Δρ/ρ₀, above 20–60% was achieved with application of magnetic fields from 1 to 8 Tesla (T) over a very large temperature range from the ferromagnetic transition at 230–4.2 K. As a comparison, a partial melted La_0.7Ca_0.3MnO₃ shows a very narrow ferromagnetic transition around 230 K which is the same as that of single crystals and epitaxy thin films.     

Magnetoresistive conductive polymer-tungsten trioxide nanocomposites with ultrahigh sensitivity at low magnetic field

The effect of conductive polymer matrix including polyaniline (PANI) and polypyrrole (PPy) on the magnetoresistance (MR) behaviors in the variable range hopping (VRH) regime has been investigated in the disordered polymer nanocomposites containing tungsten trioxide (WO₃) nanoparticles. These nanocomposites have demonstrated ultrahigh MR sensitivity at low magnetic field regime. The observed positive MR has been well explained by the wave-function shrinkage model. The conductive polymer matrix has shown different effects on the MR behaviors of the nanocomposites. The WO₃/PANI nanocomposites have a lower localization length (a₀) and density of states at the Fermi level (N(E_F)), and higher average hopping distance (R_hop) and average hopping energy (W) compared with those of the WO₃/PPy nanocomposites.     

结晶速度对InSb－NiSb共晶复合材料磁致电阻性能R_b／R_o影响的研究

研究了单向结晶速度Ｖ对ＩｎＳｂ－ＮｉＳｂ共晶磁致电阻性能Ｒ＿ｂ／Ｒ＿ｏ的影响。结果发现不同Ｖ，其Ｒ＿ｂ／Ｒ＿ｏ也不同。并且在Ｖ－Ｒ＿ｂ／Ｒ＿ｏ关系中存在着一个Ｒ＿ｂ／Ｒ＿ｏ最大值。这个最大值可由半导体的物理磁阻效应和几何磁阻效应的综合作用来说明。     

Temperature-dependent magnetoresistance of ZnO thin film

A ZnO film was deposited, and the magnetic and the magnetoresistive (MR) properties were studied. The MR measurements reveal negative MR at 80, 50, 20, 10 and 6 K, which is supposed to be induced by the weak-localization effect, based on a logarithmic dependence of the electrical conductivity on temperature. When temperature was reduced to be 2 K, a positive MR was observed. We suggest that it is related to the spin splitting induced by exchange interaction between itinerant electrons and vacancy defects in ZnO. Through the magnetic measurement, it is found that ZnO shows ferromagnetism. It is suggested that the observed ferromagnetism is correlated with the exchange interaction.     

Reactively sputtered epitaxial γ′-Fe4N films: Surface morphology,microstructure, magnetic and electrical transport properties

Epitaxial γ′-Fe₄N films with (1 0 0) and (1 1 0) orientations have been fabricated by reactive sputtering; these films were characterized by X-ray θ–2θ and φ scans, pole figures and high-resolution transmission electron microscopy. The film surface is very smooth as the film is less than 58 nm thick. The films exhibit soft ferromagnetism, and the saturation magnetization decreases with an increase in temperature, following Bloch’s spin wave theory. The films also exhibit a metallic conductance mechanism. Below 30 K, magnetoresistance (MR) is positive and increases linearly with the applied field in the high-field range. In the low-field range, MR increases abruptly. Above 30 K, MR is negative, and its value increases linearly with the applied field.     

Effects of ferrites on magnetic transport properties in La2/3Sr1/3MnO3/ferrites composites

Effects of soft-magnetic MnZn ferrite (Mn0.5Zn0.5Fe2O4, MZF) and hard-magnetic Ba ferrite (BaO·6Fe2O3, BaM) on the structure and magnetic transport properties of [La2/3Sr1/3MnO3] (LSMO)/(x) [ferrites] (ferrites=MZF, BaM) composites have been investigated. It was found that the inclusion of MZF phase reduces magnetization and ferromagnetic-paramagnetic transition temperature (Tc) of the composites. With increasing the content of the dopants, the high-temperature magnetoresistance (MR) decreases, whereas low-...     

Preparation of submicron LaCaMnO submicron powder and LaCaMnO–alumina composites with CMR effects by a new method: Precursor sintering technique

We have successfully synthesized submicron LaCaMnO powder (with a nominal composition of La_0.67Ca_0.33MnO₃) and the LaCaMnO–Al₂O₃ composite powder by a newly invented precursor sintering technique. It is found that the precursor solution containing polyacrylamide and citric acid can facilitate the formation of LaCaMnO powders at a relatively lower sintering temperature because PAM and citric acid form a polymer network in the solution and the metal ions evenly distribute in the precursor solution. The critical sintering temperature was carefully studied based on X-ray diffraction patterns and scanning electron microscopy images. It turns out that a low-temperature sintering results in nano-sized powders with a particle size of 50–100 nm, but a high-temperature sintering leads to larger clusters of 1–3 μm. For the LaCaMnO–alumina composites system, secondary phases appear if the sintering temperature is relatively high (1200 °C). Magnetoresistance of the LaCaMnO wafer made from the obtained LaCaMnO powder has a maximum of 56.7% at 269 K. More appealing is that LaCaMnO–alumina composite powders have a very high MR, 82.5%, even in the same order of the MR of the epitaxial-grown LaCaMnO thin film. Besides, the advantages of the PST method include simple equipment, common chemical compounds and low-cost.