Magnetic and electrical transport properties of nanostructured La0.67Ca0.33MnO3 networks

Contiguously nanostructured networks of La_0.67Ca_0.33MnO₃ (LCMO) are fabricated successfully by pulsed electron deposition (PED) onto the surface of porous anodic alumina oxide (AAO). The Curie temperature (T_C) is about 250 K. The metal–insulator transition temperature (T_p) is about 145 K without the magnetic field. The magnetoresistance can reach 84% near the peak temperature, which suggesting a strong magnetic correlated electronic transportation process. Zero field cooling (ZFC) and field cooling (FC) are split below the paramagnetic–ferromagnetic transition. The ZFC curves exhibit a typical blocking process. The observed spin glass and weak localization phenomena are due to the size effect, and it is found that the resistance dependent temperature curve above metal–insulator transition temperature (T_p) is more suitable to small polaron hopping (SPH) model.     

Spin-dependent tunnelling in La0.7Sr0.3MnO3/SrTiO3 superlattices

La_0.7Sr_0.3MnO₃ is predicted to show half-metallic behaviour at low temperature, which gives rise to a metallic character for one spin direction and an insulating character for the other. This 100% polarisation of the conduction band should enhance the spin dependent tunnelling in manganite-based tunnel junctions. La_0.7Sr_0.3MnO₃/SrTiO₃ epitaxial superlattices were grown on LaAlO₃(001) substrates by metal–organic chemical vapour deposition (MOCVD). These multilayers consist of 15 epitaxial bilayers of La_0.7Sr_0.3MnO₃ and SrTiO₃. The junctions were patterned using UV lithography and Ar ion milling to carry out transport measurements in the current perpendicular-to-plane geometry (CPP). A temperature-independent non-linear I–V curve, which is characteristic of a tunnelling conduction mechanism, was observed below 50 K. At higher temperatures, the I–V curves are found to become linear and temperature-dependent. Up to 30 K, a constant tunnel magnetoresistance (TMR) (3%) is measured. The switching field is consistent with the film coercive field (a few 10s of mT). At higher temperatures, the TMR decreases rapidly. This temperature dependence is compared to the expected behaviour of a spin tunnel junction with half-metallic electrodes, with thermal activation or the loss of spin polarisation taken into account.     

Low-Magnetoresistance RuO2–Al2O3 Thin-Film Thermometer and its Application

Thermometers consisting of RuO₂–Al₂O₃ composite thin films were prepared by RF sputtering. It was found that different electrode-patterning techniques have dissimilar effects on the magnetoresistance (MR) and the temperature coefficient of resistance (TCR). In general, the thermometers with electrodes fabricated by photo-resist lithography exhibit superior performance compared to those with electrodes prepared using a metal mask. By adjusting the relative compositions of RuO₂ and Al₂O₃, the thermometers can be applied to a wide temperature range from 60 mK to 500 K. In a pulsed magnetic field up to 55 T, the MR at 4.2 K of a typical thermometer for the temperature range from 1.4 K to 300 K increases linearly with magnetic field to a maximum of ~15 %, corresponding to a temperature deviation of ~−4 %. As frequency increases from dc to 1.9 MHz, the MR decreases from  −13 % to ~ − 0.5 % at T = 1.3 K and H = 55 T. By integrating the thermometer with a heater on a sapphire chip, a micro-calorimeter can be developed and successfully used to measure the heat capacity of small mg-sized sample. The RuO₂–Al₂O₃ composite film can also be employed as an infrared bolometer operated at room temperature.     

Photoconductivity in BiFeO3/La0.7Sr0.3MnO3 heterostructure

In this letter, an oxide heterostructure has been fabricated by successively growing La_0.7Sr_0.3MnO₃ (LSMO) and BiFeO₃ (BFO) layers on LaAlO₃ (100) by pulsed laser deposition. Analysis of the leakage current at different temperature demonstrated that the Poole-Frenkel dominated the leakage current mechanism. Additionally, the BiFeO₃/La_0.7Sr_0.3MnO₃ heterostructure exhibits a positive colossal magnetoresistance (MR) effect over a temperature range of 50-320 K. The maximum MR values are determined to be about 45.32% at H = 0.5 T and 28.34% at H = 0.3 T. At last, we report photoconductivity in BiFeO₃/La_0.7Sr_0.3MnO₃ film under illumination from 160 mW/cm² and 200 mW/cm² green-light source, and photoconductivities increase with the intensity of light enhanced.     

磁性多层膜研究进展:理论和实验

详细叙述了磁性多层膜研究的理论和实验进展，在不同的磁性多层膜材料中，大量实验事实证明和理论预示磁性多层膜具有独特的物理性质，如维度磁性、界面各向异性、耦合作用、巨磁阻、层间交换作用振荡和磁性量子隧道效应等等，因而，磁性多层膜是理论研究和技术应用的重要课题。     

Giant negative magnetoresistance in the vicinity of the ferromagnetic phase transition TC in Sm0.55Sr0.45MnO3

The temperature dependence of the resistance in Sm_0.55Sr_0.45MnO₃ at zero magnetic field and its first derivative served for the determination of T_C which turned out to be equal to 129.1 K. Then, the resistance measurements were carried out as functions of the stationary magnetic fields up to 14 T at the following temperatures: 118.6, 124.6, 133.6 and 139.7 K. Two of them are below and two of them are above T_C. It turned out that at all the four temperatures the resistance reveals a hysteresis. It turned also out that the closer the temperature to T_C the greater this hysteresis. At all the four temperatures the magnetoresistance is negative, its absolute value depends on the magnetic field and reaches 100% at 12 T. At lower fields the absolute value of the negative magnetoresistance increases monotonically with the decrease of temperature starting from 139.7 K. On the other hand, a maximum observed on all the primary and secondary curves above T_C moves with the decrease of temperature towards the lower magnetic fields and disappears on all the curves below T_C. On the secondary curves above T_C this maximum is flat.     

The transition from bipolaron to triplet-polaron magnetoresistance in a single layer organic semiconductor device

We measure the current–voltage–luminescence (I–V–L) and Magneto-Conductance (MC) response of a poly(3-hexyl-thiophene) (P3HT) based device (Au/P3HT(300 nm)/Al) in forward and reverse bias. In reverse bias (<1 V), the negative MC is described by a single non-Lorentzian function, consistent with the bipolaron theory. In forward bias, the transition from negative saturation MC (low bias) to positive saturation MC (high bias) occurs when the current density exceeds ∼10⁻² A cm⁻² and coincides with electroluminescence. Under these conditions the triplet density (∼10¹⁵ cm⁻³) becomes comparable to the hole density (∼10¹⁶ cm⁻³), consistent with the triplet-polaron interaction theory. From the current density dependence of the MC we conclude that in forward bias both mechanisms must be occurring simultaneously, within a given device, and that the overall sign of the MC results from competition between the two mechanisms.