Analysis of colossal magnetoresistance effect in perovskite oxide heterostructures

A systemic study of magnetoresistance (MR) in manganite perovskite oxide p-n junction is performed with experiment and theoretical calculation. The spin-dependent tunneling current is calculated with a model of double-band barrier and MR with reverse bias is explained as a result of competition between tunneling currents with different spins. The reduction of recombination rate at the interface of heterojunction with magnetic field is proposed to explain positive MR at forward bias. Furthermore, negative MR is predicted to be observed in oxide heterostructure without electron filling in t2g↓ band of manganite at the interface region with both forward and reverse bias.     

Non-saturating linear magnetoresistance in phase separated amorphous Ag10Ge15Te75 films

We report non-saturating linear magnetoresistance (LMR) in silver-poor composite thin films of Ag₁₀Ge₁₅Te₇₅ (AGT). The LMR increases with decreasing temperature and reaches a value of 110% at 200 K under a magnetic field of 70 kOe. The observed magnetoresistance is proportional to carrier mobility due to identical temperature dependencies. The microstructures of AGT films annealed at different temperature are investigated by TEM observations. The observed LMR effect is resulting from the inhomogeneous distribution of conductivity caused by highly conductive Ag₂Te and GeTe₄ precipitates, which generate a current distortion in the surrounding amorphous silver-poor Ag–Ge–Te matrix.     

Ballistic magnetoresistance in perovskite magnetonanocontacts under high-bias voltages

Ballistic magnetoresistance is studied in point contacts prepared by a break junction technique. The La_0.7Sr_0.3MnO₃ ceramic is fixed to an elastic substrate and a point contact is obtained by a fracture creation using the three point bending mechanism. The percentage change of the resistance in a field switch between 0 and 3.8 k Oe is of the order of 1000% at room temperature.     

Universal relationship between giant magnetoresistance and anisotropic magnetoresistance in spin valve multilayers

We measure the giant magnetoresistance (GMR) with the current both parallel and perpendicular to the direction of the magnetization in the ferromagnetic (FM) layers and thus probe the anisotropy of the effective mean free paths for the spin-up and spin-down electrons, seen in the anisotropic magnetoresistance. We find that the difference of the GMR in the two configurations, when expressed in terms of the sheet conductance, displays a nearly universal behavior as a function of GMR. On interpreting the results within the Boltzmann transport formalism we demonstrate the importance of bulk scattering for GMR.     

Perpendicular giant magnetoresistance in magnetic multilayered nanowires

We present giant magnetoresistance (GMR) measurements performed on electrodeposited Co/Cu multilayered nanowires. The variation of the GMR with the thicknesses of the Cu and Co layers over wide ranges is discussed in the framework of the Valet-Fert model for perpendicular GMR. The interface and bulk spin-dependent scattering parameters as well as the spin diffusion lengths in the nonmagnetic and ferromagnetic layers are extracted from this analysis.     

Intergranular magnetoresistance in Sr2FeMoO6 from a magnetic tunnel barrier mechanism across grain boundaries

We present magnetization (M) and magnetoresistance (MR) data for a series of Sr2FeMoO6 samples with independent control on antisite defect and grain-boundary densities, which reveal several unexpected features, including a novel switching-like behavior of MR with M. These, in conjunction with model calculations, establish that the MR in Sr2FeMoO6 is dominantly controlled by a new mechanism, derived from the magnetic polarization of grain-boundary regions acting like spin valves, leading to behavior qualitatively different from that usually encountered in tunneling MR. We show that a simple and useful experimental signature for the presence of this spin-valve-type MR (SVMR) is a wider hysteresis in MR compared to that in M.     

Field-induced phase transitions and giant magnetoresistance in Dy D{y_3}CoCo single crystals

Electrical resistivity and calorimetric measurements on Dy ₃ Coshow that below the Néel temperature (T N =44 K) the non-collinear antiferromagnetic structure exhibits field-induced magnetic phase transitions of a first-order type along all principal axes, accompanied by a strongly anisotropic giant magnetoresistance and by a change of the Sommerfeld coefficient of the specific heat. Quantum tunnelling of the magnetization appears to be possible for T < 0.6 K. Received 15 July 1999 and Received in final form 6 December 1999     

Resonant magnetoresistance in the vicinity of a phase transition

The change in the electrical conductivity of manganite films upon microwave pumping in the magnetic resonance conditions is investigated. The temperature dependence of the effect correlates with the temperature variation of colossal magnetoresistance (CMR), passing through a maximum at the Curie point. The results are interpreted using a model that assumes a decrease in the absolute value |M| of the magnetic moment of the sample under the action of magnetoresonant saturation, which leads to an increase in resistance in accordance with the CMR mechanism. Theoretical analysis based on the Landau-Lifshitz-Bloch equation confirms the correctness of this model and ensures good agreement with experiment.     

钙钛矿氧化物异质结的光电特性研究进展

光与物质相互作用可以产生各种光学现象,其中光电效应是非常重要的现象之一.文中集中回顾了文章作者在钙钛矿氧化物异质结的光电效应研究中的进展.在钙钛矿氧化物异质结中,分别观测到了传统的纵向光电效应和反常的横向光电效应,并通过对含时的漂移-扩散方程的自洽求解,从理论上分别揭示了钙钛矿氧化物异质结纵向和横向光电效应的动态过程.文章首先介绍了钙钛矿氧化物异质结纵向光电效应的研究进展,接着概述了钙钛矿氧化物异质结横向光电效应研究的进展.最后对氧化物异质结的纵向和横向光电效应的潜在应用前景进行展望.     

钙钛矿氧化物异质结的光电特性研究进展

光与物质相互作用可以产生各种光学现象,其中光电效应是非常重要的现象之一.文中集中回顾了文章作者在钙钛矿氧化物异质结的光电效应研究中的进展.在钙钛矿氧化物异质结中,分别观测到了传统的纵向光电效应和反常的横向光电效应,并通过对含时的漂移-扩散方程的自洽求解,从理论上分别揭示了钙钛矿氧化物异质结纵向和横向光电效应的动态过程.文章首先介绍了钙钛矿氧化物异质结纵向光电效应的研究进展,接着概述了钙钛矿氧化物异质结横向光电效应研究的进展.最后对氧化物异质结的纵向和横向光电效应的潜在应用前景进行展望.     

Anisotropic giant magnetoresistance originating from the π-d interaction in a molecule

We synthesized TPP[Fe^III(Pc)(CN)₂]₂, PTMA_x[Fe^III(Pc)(CN)₂]·y(MeCN), and PXX [Fe^III(Pc)(CN)₂], a new series of charge-transfer salts containing the axially-substituted phthalocyanine (Pc), [Fe^III(Pc)(CN)₂]⁻. In this molecular unit, the π conduction electron derived from the Pc-ring coexists with the d electron which is a potential source of a local magnetic moment. Therefore various phenomena associated with the interplay between local magnetic moments and conduction electrons are expected. We observed the giant negative magnetoresistance (GNMR) in all the three salts. The GNMR is highly anisotropic for the magnetic-field direction, and reflects the g-tensor anisotropy of the local magnetic moment in the [Fe^III(Pc)(CN)₂]⁻ unit. This indicates that the GNMR in these salts originates from the strong π-d interaction in the [Fe^III(Pc)(CN)₂]⁻ unit.     

Spin glass-like behavior,giant magnetocaloric and giant magnetoresistance effect in PrPb manganites

The Pr_1−xPb_xMnO₃ (x=0.1–0.5) perovskites have been fabricated by solid-state reaction. The X-ray diffraction patterns show that the samples are of single phase with orthorhombic structure. The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic curves measured at low field and low temperatures exhibit the spin glass-like state. The Curie temperature of samples increased with increase in Pb content. The maximum magnetic entropy change |ΔS_m|_max reaches the giant values of 3.91 and 3.68 J/kg K for quite low magnetic field change of 1.35 T for the samples x=0.1 and 0.4, respectively. The resistance measurements show that there is insulator–metal phase transition on the R(T) curves for samples with x?0.3. The giant magnetoresistance effect is also observed for all samples studied.     

Concentration dependence of giant magnetoresistance in magnetic granular composites

We combine effective medium theory (EMT) with the two-channel conducting model to study the magnetic granular concentration dependence of a giant magnetoresistance (GMR) in magnetic granular composites. The composite is composed of small magnetic granules (such as Co) embedded in an immiscible nonmagnetic metallic matrix (such as Ag). We present a model for the composite in which the magnetic metallic granules are spherical in shape and have a distribution in sizes, and in which there are different contributions of superparamagnetic and ferromagnetic granules to conductance. The calculated result about the concentration dependence of GMR is in agreement with the experimental data.     

Temperature dependence of giant magnetoresistance in CoAg granular composite

The temperature dependence of the giant magnetoresistance (GMR) of CoAg composite is studied phenomenologically in this paper. It is considered that the composite contains both large magnetic grains and tiny clusters. The tiny clusters, composing of several atoms, lead to an almost linear increase of the giant magnetoresistance with the temperature reduction. Our calculations are in good agreement with recent experimental data for a nanogranular CoAg films.