高度取向石墨的巨磁电阻效应

石墨是准二维半金属材料，然而在通常细晶粒、无取向的石墨中并没有发现很大的磁电阻效应.在高度取向的石墨中发现了巨大的正磁电阻效应，在8.15 T的外磁场中，4.2，300 K温度下的磁电阻分别高达85300%和4950%.生产这一巨磁电阻效应的机制除正常磁电阻效应外，可能源于磁场诱导的类金属-绝缘体的转变 关键词： 磁电阻效应 石墨     

Magnetoresistive remanence curves in Co/Cu multilayers

We report measurements of magnetoresistive remanence curves and initial magnetoresistance curves on Co/Cu multilayers. The initial magnetoresistance curve shows an enhance magnetoresistance change over that of the hysteretic magnetoresistance curve. Using remanence curves we are able to separate out the irreversible and reversible components contributing to the magnetoresistance. We show that it is irreversible changes of resistance within the multilayers from an initial low energy demagnetised state that is responsible for this ‘extra’ magnetoresistance.     

Electrical and magnetic properties of lead-substituted lanthanum ferrimanganites

The temperature dependences of the electrical conductivity, thermopower, and magnetoresistance for single crystals of lead-substituted lanthanum ferrimanganites are investigated. The data on the magnetic microstructure obtained by Mössbauer spectroscopy are analyzed. An inversion of the magnetoresistance sign with an increase in temperature and the giant positive magnetoresistance are found for one of the samples. The magnetoresistance quadratically depends on the field, and its temperature dependence exhibits a maximum.     

Effect of thickness on magnetic phase coexistence and electrical transport in Nd0.51Sr0.49MnO3 films

We present the impact of the film thickness on the coexistence of various magnetic phases and its link to the magnetoresistance of Nd_0.51Sr_0.49MnO₃ thin films. These epitaxial films are deposited on LaAlO₃ (001) substrates by DC magnetron sputtering. Films with thicknesses of approximately 30 nm are found to be under full compressive strain while those with thicknesses ∼100 nm and beyond exhibit the presence of both strained and relaxed phases, as evidenced from X-ray diffraction studies. Both films exhibit multiple magnetic transitions controlled by strong electron correlations and phase coexistence. These films also display insulator–metal transitions (IMT) and colossal magnetoresistance (CMR) under moderate magnetic fields. Among the two set of films, only the 30-nm films show a weak signature of charge ordering at T≈50 K. Even at temperatures much lower than the IMT, the 30-nm films show huge magnetoresistance (MR) ∼80%. This suggests presence of softened charge-ordered insulating (COI) clusters that are transformed into ferromagnetic metallic (FMM) ones by the external magnetic field. In the 100-nm films, the corresponding MR is suppressed to less than 20%. Our study demonstrates that the softening of the COI phase is induced by the combined effect of the in-plane compressive strain and a slight reduction in Sr concentration.     

Spin polarization and magnetoresistance through a ferromagnetic barrier in bilayer graphene

We study spin dependent transport through a magnetic bilayer graphene nanojunction configured as a two-dimensional normal/ferromagnetic/normal structure where the gate voltage is applied on the layers of ferromagnetic graphene. Based on the four-band Hamiltonian, conductance is calculated by using the Landauer-Buttiker formula at zero temperature. For a parallel configuration of the ferromagnetic layers of bilayer graphene, the energy band structure is metallic and spin polarization reaches its maximum value close to the resonant states, while for an antiparallel configuration the nanojunction behaves as a semiconductor and there is no spin filtering. As a result, a huge magnetoresistance is achievable by altering the configurations of ferromagnetic graphene around the band gap.     

Orbital and spin effects in the low-temperature behavior of the magnetoresistance of doped CdTe crystals

An observation of the suppression of negative magnetoresistance in samples of doped CdTe that are far from the metal-insulator transition as the temperature is lowered in the temperature range 3–0.4 K was previously reported [N. V. Agrinskaya, V. I. Kozub, and D. V. Shamshur, JETP 80, 1142 (1995)]. The results of an investigation of samples that are closer to the transition in the low-temperature region below 36 mK are presented. It is discovered that the samples investigated (which do not exhibit the suppression of negative magnetoresistance at comparatively high temperatures) display this effect at low temperatures and that, as previously, the suppression of the negative magnetoresistance correlates with the transition to conduction via Coulomb-gap states. A plateau-like magnetoresistance feature is displayed at low temperatures for the sample that is closest to the metal-insulator transition. The results obtained are analyzed within existing theoretical models that take into account the role of both the orbital and spin degrees of freedom. In particular, the low-temperature feature indicated is interpreted as a manifestation of positive magnetoresistance caused by spin effects. Nevertheless, it is shown within a detailed analysis supplemented by numerical calculations that the observed suppression of the negative magnetoresistance cannot be attributed only to the appearance of spin positive magnetoresistance. Moreover, the possibility of observing spin positive magnetoresistance is determined to a certain extent specifically by the suppression of the negative magnetoresistance competing with it. Zh. éksp. Teor. Fiz. 111, 1477–1493 (April 1997)     

Giant room-temperature magnetoresistance in polycrystalline Zn(0.41)Fe(2.59)O4 with alpha-Fe2O3 grain boundaries

A tunneling-type magnetoresistance (MR) as large as 158% is observed at T = 300 K in a polycrystalline Zn0.41Fe2.59O4 sample, in which the Zn0.41Fe2.59O4 grains are separated by insulating alpha-Fe2O3 boundaries. The huge room-temperature MR is attributed to the high spin polarization of Zn(0.41)Fe(2.59)O4 grains and antiferromagnetic correlations between magnetic domains on both sides of the insulating alpha-Fe2O3 boundary. The MR exhibits strong temperature dependence below 100 K and its magnitude is enhanced to reach 1280% at 4.2 K, which may arise from the Coulomb blockade effect.     

Coulomb blockade anisotropic magnetoresistance effect in a (Ga,Mn)As single-electron transistor

We observe low-field hysteretic magnetoresistance in a (Ga,Mn)As single-electron transistor which can exceed 3 orders of magnitude. The sign and size of the magnetoresistance signal are controlled by the gate voltage. Experimental data are interpreted in terms of electrochemical shifts associated with magnetization rotations. This Coulomb blockade anisotropic magnetoresistance is distinct from previously observed anisotropic magnetoresistance effects as it occurs when the anisotropy in a band structure derived parameter is comparable to an independent scale, the single-electron charging energy. Effective kinetic-exchange model calculations in (Ga,Mn)As show chemical potential anisotropies consistent with experiment and ab initio calculations in transition metal systems suggest that this generic effect persists to high temperatures in metal ferromagnets with strong spin-orbit coupling.     

3000% high-field magnetoresistance in super-lattices of CoFe nanoparticles

We report on magnetotransport measurements on millimeter-large super-lattices of CoFe nanoparticles surrounded by an organic layer. Electrical properties are typical of Coulomb blockade in three-dimensional arrays of nanoparticles. A large high-field magnetoresistance, reaching up to 3000%, is measured between 1.8 and 10 K. This exceeds by two orders of magnitude magnetoresistance values generally measured in arrays of 3d transition metal ferromagnetic nanoparticles. The magnetoresistance amplitude scales with the magnetic field/temperature ratio and displays an unusual exponential dependency with the applied voltage. The magnetoresistance abruptly disappears below 1.8 K. We propose that the magnetoresistance is due to some individual paramagnetic moments localized between the metallic cores of the nanoparticles, the origin of which is discussed.     

Ordinary and extraordinary Coulomb blockade magnetoresistance in a (Ga, Mn)As single electron transistor

In the conventional Ohmic regime, magnetoresistance effects comprise the ordinary responses to the external magnetic field and extraordinary responses to the internal magnetization. Here we study magnetoresistance effects in the Coulomb blockade regime using a ferromagnetic (Ga, Mn)As single electron transistor. We report measurements of the magneto-Coulomb blockade effect due to the direct coupling of high external magnetic fields and the Coulomb blockade anisotropic magnetoresistance associated with magnetization rotations in the ferromagnet. The latter, extraordinary magnetoresistance effect is characterized by low-field hysteretic magnetoresistance which can exceed three orders of magnitude. The sign and size of this magnetoresistance signal is controlled by the gate voltage, and the data are interpreted in terms of anisotropic electrochemical shifts induced by magnetization reorientations. Non-volatile transistor-like applications of the Coulomb blockade anisotropic magnetoresistance are briefly discussed.     

基于赫斯勒合金Co2MnSi电极的磁性隧道结呈现巨磁阻效应

本文基于电子密度泛函理论计算和非平衡态格林函数技术研究了具有三明治结构的磁性隧道结构(非极化SrTiO₂薄层被夹在两个赫斯勒合金Co₂MnSi电极之间)的自旋极化输运特性. 理论计算结果清楚地表明磁平行组态的磁性隧道结呈现出几乎完美的自旋过滤效应. 磁反平行组态的隧穿系数比磁平行组态的隧穿系数小几个数量级,导致体系的磁阻比高达10⁶. 电子结构计算分析表明该磁性隧道结的巨磁阻效应源自赫斯勒合金Co₂MnSi电极内在的半金属性、以及阻挡层和电极之间界面处过渡金属原子3d电子的显著自旋极化.     

Spin-flip induced magnetoresistance in positionally disordered organic solids

A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up "spin-blocked" pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory.     

Magnetoresistance effect in vertical NiFe/graphene/NiFe junctions

For convenient and efficient verification of the magnetoresistance effect in graphene spintronic devices, vertical magnetic junctions with monolayer graphene sandwiched between two NiFe electrodes are fabricated by a relatively simple way of transferring CVD graphene onto the bottom ferromagnetic stripes. The anisotropic magnetoresistance contribution is excluded by the experimental result of magnetoresistance (MR) ratio dependence on the magnetic field direction. The spin-dependent transport measurement reveals two distinct resistance states switching under an in-plane sweeping magnetic field. A magnetoresistance ratio of about 0.17 % is obtained at room temperature and it shows a typical monotonic downward trend with the bias current increasing. This bias dependence of MR further verifies that the spin transport signal in our device is not from the anisotropic magnetoresistance. Meanwhile, the I—V curve is found to manifest a linear behavior, which demonstrates the Ohmic contacts at the interface and the metallic transport characteristic of vertical graphene junction.     

Observation of large low‐field magnetoresistance in spinel cobaltite: A new half‐metal

Low‐field magnetoresistance is an effective and energy‐saving way to use half‐metallic materials in magnetic reading heads and magnetic random access memory. Common spin‐polarized materials with low field magnetoresistance effect are perovskite‐type manganese, cobalt, and molybdenum oxides. In this study, we report a new type of spinel cobaltite materials, self‐assembled nanocrystalline NiCo₂O₄, which shows large low field magnetoresistance as large as –19.1% at 0.5 T and –50% at 9 T (2 K). The large low field magnetoresistance is attributed to the fast magnetization rotation of the core nanocrystals. The surface spin‐glass is responsible for the observed weak saturation of magnetoresistance under high fields. Our calculation demonstrates that the half‐metallicity of NiCo₂O₄ comes from the hopping e_g electrons within the tetrahedral Co‐atoms and the octahedral Ni‐atoms. The discovery of large low‐field magnetoresistance in simple spinel oxide NiCo₂O₄, a non‐perovskite oxide, leads to an extended family of low‐field magnetoresistance materials. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The effect of spin-orbit coupling on magnetoresistance in nonmagnetic organic semiconductors

We investigated the effect of spin-orbit coupling on magnetoresistance in nonmagnetic organic semiconductors.A Lorentz-type magnetoresistance is obtained from spin-orbit coupling-dependent spin precession under the condition of a space-charge-limited current.The magnetoresistance depends on the initial spin orientation of the electron with respect to the hole in electron-hole pairs,and the increasing spin-orbit coupling slows down the change in magnetoresistance with magnetic field.The field dependence,the sign and the saturation value of the magnetoresistance are composite effects of recombination and dissociation rate constants of singlet and triplet electron-hole pairs.The simulated magnetoresistance shows good consistency with the experimental results.     

Disappearance of the resistivity peak and possibility of the conductivity peak in overdoped ferromagnets

At large charge carrier densities the resistivity peak close to the Curie point and colossal negative magnetoresistance common for degenerate ferromagnetic semiconductors disappear. Instead of them a conductivity peak and positive magnetoresistance may appear. Necessary conditions for this are an incomplete spin polarisation of charge carriers and a dominating role of the magnetoimpurity scattering.     

Transport in superlattices of magnetic nanoparticles: coulomb blockade, hysteresis, and switching induced by a magnetic field

We report on magnetotransport measurements on millimetric superlattices of Co-Fe nanoparticles surrounded by an organic layer. At low temperature, the transition between the Coulomb blockade and the conductive regime becomes abrupt and hysteretic. The transition between both regimes can be induced by a magnetic field, leading to a novel mechanism of magnetoresistance. Between 1.8 and 10 K, a high-field magnetoresistance attributed to magnetic disorder at the surface of the particles is also observed. Below 1.8 K, this magnetoresistance abruptly collapses and a low-field magnetoresistance is observed.     

Hopping magnetotransport via nonzero orbital momentum states and organic magnetoresistance

In hopping magnetoresistance of doped insulators, an applied magnetic field shrinks the electron (hole) s-wave function of a donor or an acceptor and this reduces the overlap between hopping sites resulting in the positive magnetoresistance quadratic in a weak magnetic field, B. We extend the theory of hopping magnetoresistance to states with nonzero orbital momenta. Different from s states, a weak magnetic field expands the electron (hole) wave functions with positive magnetic quantum numbers, m>0, and shrinks the states with negative m in a wide region outside the point defect. This together with a magnetic-field dependence of injection/ionization rates results in a negative weak-field magnetoresistance, which is linear in B when the orbital degeneracy is lifted. The theory provides a possible explanation of a large low-field magnetoresistance in disordered π-conjugated organic materials.     

Giant magnetoresistance of granular microwires: Spin-dependent scattering in integranular spacers

The anomalous behavior of magnetoresistance has been revealed in a number of granular microwires. In contrast to the giant magnetoresistance of granular alloys, which is associated with the spin-dependent scattering in the bulk of grains and at their surface, is linear in the square of the magnetization, and decreases with an increase in temperature, the magnetoresistance, for example, in Co₁₀Cu₉₀ microwires is negative, increases with an increase in temperature below the Curie temperature, and does not reach saturation in the field dependence in the high-field range. A simple mechanism of negative giant magnetoresistance due to scattering of spin-polarized charge carriers by impurity magnetic moments localized in the nonmagnetic intergranular spacers has been proposed taking into account that a considerable part of magnetic ions in microwires exhibiting this behavior is dissolved in the intergranular spacers. It has been shown that the corresponding contribution to magnetoresistance can reach 10–20%.