Monte Carlo Simulation of Kerr Effect and Giant Magnetoresistance in CoxAg（1-x） Granular Films

Based on the Monte Carlo simulation, we present the theoretical calculation of the Kerr loops and giant magnetoresistance (GMR) curves in CoxAg（1-x) granular films. The calculated results of both the Kerr loops and the GMR curves are in good agreement, with the experimental ones. The anisotropy energy and the dipolar interaction evidently influence the Kerr loops and the GMR effect and it is found that with the increasing Co content X, the value of anisotropy constant K decreases and the value of dipolar interaction constant D increases.The values of Kt for the CoxAg(1-x) granular films are calculated, which plays an important role in explaining the GMR effect of the granular films. Here Kt is a parameter related to the resistivity at zero field and to the scattering parameter constant.     

Photoinduced Reconstruction of Electronic Structure in Half-Metal CrO2

We investigate the photoinduced effects on the electronic structure for half-metallic ferromagnet CrO2 （Tc 390 K）, in which the conducting electrons are totally polarized, by using the LSDA＋U method. A significant change is found for the band structure and the density of states （DOS） for CrO2 under photo-excitation, especially for the Cr 3dt2g band： disappearance of the spin-split band, suggesting collapse of the half-metallic state. We ascribe the change of electronic structure under photo-excitation to the wider one-electron band W via the strong hybridization of the down-spin Cr 3d and O 2p states. Furthermore we discuss the magnetic properties under photo-excitation.     

Giant Positive Magnetoresistance in Magnetic Multilayer Film Prepared by Ion—Beam Sputtering

The magnetic multilayers Ni78Co22//Cu//Ni78Co22//Ni78Co22O//Ta were fabricated by ion-beam sputtering through applied magnetic field and treatment under high vacuum. Resistance against applied magnetic field was measured by the standard four-point probe method at room temperature. The giant positive magnetore-sistance has been observed. A maximum positive magnetoresistance at room temperature was obtained to be 280%.     

Intergranular Tunnelling and Field-Induced Percolation Fluctuation of Granular Composites （La1-zAgzMnO3）/（MnO2/Mn2O3）

We investigate the giant magnetoresistance of composites （La1-xAgzMnO3）1-υ（MnO2/Mn2O3）υ. The magnetoresistive property of the composites shows the characteristics of intergranular tunnelling. A conductivity leap has been observed around y = 0.7. The composite （La0.926Ag0.074MnO3）0.698（MnO2/Mn2O3）0.302 has been found to show the greatest magnetoresistance in the samples. Its room-temperature MR ratio reaches 24% under a field of 1.8T. These phenomena suggest a percolation transformation and a kind of field-induced fluctuation in percolation in the samples investigated.     

Spin-Polarized Tunnelling Magnetoresistance Effects in La0.833K0.167MnO3／SrTiO3 Polycrystalline Perovskite Manganites

A sample of La0.833K0.167MnO3-SrTiO3 (LKMO/STO) is fabricated by the sol-gel method. The microstructure,magnetic and transportation properties have been studied. X-ray diffraction patterns indicate that the structure of LKMO/STO is a homogeneous solid solution phase. The resistivity of LKMO/STO shows the insulator behaviour, which is different from La0.833K0.167MnO3 (LKMO) whose resistivity shows metal-insulator transition with decreasing temperature. The low-field (moH = 0.02 T) magnetoresistance decreases from 11% to 0.2% with the increasing temperature from 4 K‘to 220K for the LKMO/STO sample. The magnitude of magnetoresistance in a strong field (μoH = 5.5 T) almost increases linearly with decreasing temperature and reaches the maximum of 65% at the low temperature of 4.2K, which is much higher than that oE LKMO (40%). The enhanced low-field magnetoresistance effects are quantitatively explained by the spin-polarized tunnelling at grain boundaries.     

Magnetoresistance in the ferromagnet/insulator/ferromagnet tunnel junction

Recently experiments and theories show that the tunnel magnetoresistance （TMR） does not only depend on the ferromagnetic metal electrodes but also on the insulator. Considering the rough-scattering effect and spin-flip effect in the insulator, this paper investigates the TMR ratio in a ferromagnet/insulator/ferromagnet （FM/I/FM） tunnelling junction by using Slonczewsik＇s model. A more general expression of TMR ratio as a function of barrier height, interface roughness and spin-flip effect is obtained. In lower barrier case, it shows that the TMR ratio depends on the roughscattering effect and spin-flip effect.     

Bulk—like contribution of tunnel magnetoresistance in magnetic tunnel junctions

We have demonstrated that the bulk-like contribution to tunnelling magnetoresistance (TMR) exists in the magnetic tunnel junctions, and is determined by the tunnelling characteristic length of the ferromagnetic electrodes. In the experiment, a wedge-shaped CoFe layer is inserted at the interface between the insulating barrier and the reference electrode. It is found that TMR ratio increases from 18% without CoFe layer to a saturation value of 26.5% when the CoFe thickness is about 2.3 nm. The tunnelling characteristic length, l_{tc}, can be obtained to be about 0.8 nm for CoFe materials.     

Extra-Large Magnetoresistance in Fe/In2O3 Granular Films

Nanogranular Fe0.35/(In2O3)0.65 films with complex magnetic structure were prepared by the rf sputtering method.An extra-large magnetoresistance up to 506% was obtained at 2.2K,which is two orders of magnitude larger than that obtained at room temperature.This is related to the interaction with the impurities influencing the local magnetization,which is quite different from the spin-dependent tunnelling effect at room temperature.The interspacing Fe atoms dispersed in the In2O3 matrix play an important role in the transportation properties of carriers at low temperature.     

Giant Positive Magnetoresistance in Grain-Oriented CxCo1-x Alloys

The grain-oriented CxGo1-z (x = 0.9, 0.5) samples were fabricated by the hot.pressing method. The microstruc-ture was observed by an x-ray diffractometer and a scanning electron microscope. The resistance against the applied magnetic field was measured by a standard four-polnt probe method at different temperatures. The magnetoresistance and the magnetization ratio were studied as a function of magnetic field in the range of -1800 kA/m-1800 kA /m at different temperatures from 50 K to 300 K. The magnetoresistance of grain-oriented GxGo1-x is positive. The maximum positive MR of 98% at 50 K and 34% at 300 K was obtained under 1800 kA/m magnetic field in the C0.9Go0.1 sample.     

Unified Understanding of Giant Magnetoresistance Effect and Magnetization in Granular Films with Two-Particle Size Distribution

The giant magnetoresistance (GMR) effect and magnetization curves of Cu80 Co20 granular thin films are studied and measured in a superparamagnetism temperature (180K). The correlation between the GMR effect and the magnetization is analysed in a unified framework. These two independent properties are fitted by assuming that there are two size distributions in the Co nano-particle population. Under an assumption, good fittings are achieved for both the GMR and the magnetization, using the minimal number of parameters. The obtained average particle sizes for the smaller and larger particles are 1.0hm and 2.8nm, respectively. In fitting the magneto-transport, a power scaling relationship with the particle size for each size population is proposed, and the fitted results reveal that a certain degree of magnetic bulk scattering is present in larger particles.     

Electrical Transport and Magnetoresistance in the Sol-Gel Prepared La2／3Ca1／3Mn1-xTExO3 （x=0 and 4%, TE=Cu and Zn） Polycrystalline Samples

A comparative st udy of electrical transport and magnetoresistance (MR) is performed for sol-gel prepared samples of nominal composition La2/3 Cal/3MnO3, La2/3 Cal/3Mnl-x CuxO3 and La2/3 Ca1/3Mn1-x ZnxO3 with x = 4%.Compared with La2/3 Ca1/3MnO3, the introduction of Cu or Zn causes a significant downward shift of insulatormetal transition and a sharp MR peak near the transition. The sharp MR peak becomes observable even upon the application of low magnetic fields. It is also shown that although the same behaviour of transport and MR is observed, the sample containing Cu stands out clearly from the sample containing Zn for a substantial enhancement in MR near the transition.     

Effects of Ag Doping on Magnetoresistance of La0.833 K0.167MnO3 Polycrystalline Perovskite Manganites

The microstructure,magnetic and transport properties of the heterogeneous system with a nominal composi-tion (1/m)Ag2O-La0.833K0.167MnO3 (1/m is molar ratio with m=32,16,8,4,and 2) have been studied.The x-ray diffraction patterns show that all the samples are two-phase composite and consist of a magnetic La0.833K0.167MnO3 (LKMO) perovskite phase and a nonmagnetic metal Ag phase.The room-temperature magnetoresistance (MR) effect is enhanced significantly due to the addition of Ag.For the m=4 sample,we obtain MR values up to 32％ under a lower field of 0.5 T and 64％ under a higher field of 5.5 T at the temperature of 300K,which are much larger than the corresponding MR values (10％and 35％) of the LKMO sample withoutAg.In the low-temperature range from 4.2K to 250K,the MR values of the Ag-added samples however decrease with the increasing Ag content in the samples.This effect is discussed qualitatively by using the relative contribution from the intrinsic MR effect and the extrinsic MR effect including the spin-polarized-tunnelling and spin-dependent scattering effects in different temperature regions.     

Phase Separation and Magnetoresistance in Nd0.52Sr0.48MnO3

Polycrystalline Nd0.52Sr0.48MnO3 ceramic is prepared by a Pechini process. Its electron spin resonance spectra,magnetic and transport properties have been investigated experimentally. At temperature above 270K, the compound is paramagnetic insulator, while in temperature between 160 K and 270 K, the compound is separated into a paramagnetic insulator and a ferromagnetic metal coexisting state. Below 160 K, the ferromagnetic phase coexists with an antiferromagnetic state, but the ferromagnetic phase remains to be dominant. This makes the compound exhibiting a metallic character even at temperature far below TN.     

Magnetointersubband Oscillations of the Two-Dimensional Electron Gas in AlxGa1-xN／GaN Heterostructures

Shubnikov-de Haas (SdH) measurements are performed over a temperature range of 1.5-20 K in Al0.22 Ga0.78N/GaN heterostructures with two subbands occupied. In addition to an intermodulation between two sets of SdH oscillations from the first and second subbands, a beating in oscillatory magnetoresistance at 12K is observed, due to the mixing of the first subband SdH oscillations and ‘magnetointersubband‘ (MIS) oscillations. A phase shift of π between the SdH and MIS oscillations is also clearly identified. Our experimental results, i.e. that the SdH oscillations dominate at low temperature and MIS oscillations dominate at high temperature, fully comply with the expected behaviour of MIS oscillations.     

Effect of Barrier Width on Bias-Dependent Tunnelling in Ferromagnetic Junctions

We present a finite temperature theory for bias-dependent tunnelling in ferromagnetic tunnelling junctions. The effects of the barrier width d on the tunnelling magnetoresistance (TMR) and its sign change behaviour are discussed with this theory. Numerical results show that both the zero-bias TMR and the critical voltage Vc at which the TMR changes its sign decrease with the increasing barrier width for a considerably thick barrier junction. Furthermore, it is found that a minimum exists in the curve of Vc versus d if a composite junction is under oxidized.     

Magnetic Properties and Magnetoresistance of CdMnS：Au Based Structures Prepared by Co-evaporation

Polycrystalline CdMnS and CdMnS：Au films with hexagonal structure on Si（111） substrates are prepared by co-evaporation, and exhibit ferroelectric and ferromagnetic properties, respectively. Under optimized growth conditions, CdMnS：Au samples with an average crystallite size of 90nm and Mn concentration of 5.0at.% are obtained, and an all-semiconductor spin valve device of Co/Au/CdMnS：Au/CdMnS/Pt is fabricated. Electrical measurement of the device reveals the clear dependence of resistance on applied magnetic field, with a relative magnetoresistance of 0.06% and a switching field of 100 Oe at 77K.     

Rotation of ferromagnetic clusters induced magnetoresistance in the junction composed of La0.9Ca0.1MnO3 +δ and 1 wt.% Nb-doped SrTiO3

A junction composed of ultrathin La0.9Ca0.1MnO3＋δ （LCMO） film and 1 wt.% Nb-doped SrTiO3 was fabricated and its magnetoresistance （MR） was studied and compared with LCMO film. It was found that the resistance of the junction has a similar dependence on magnetic field as that of the LCMO film： the curvature of R-H curves is upward above Curie temperature （Tc） and downward below TC. These behaviours strongly suggest that the rotation of ferromagnetic clusters in manganite also causes MR in the corresponding junction. This MR can be qualitatively understood by the change of the width of the barrier induced by the rotation of ferromagnetic clusters. These results suggest a possibility to obtain junctions with large low-field MR.