Large Magnetoresistance Effect Research Articles

The condition for a large magnetoresistance effect in perovskite cobaltites

The magnetic and magnetotransport properties of the anion-deficient perovskites La1–xBaxCoO3–δ are studied. It is shown that a gradual transition from the metallic to the semiconductor conduction and from ferromagnetism to antiferromagnetism takes place when the oxygen content decreases. The magnetoresistance is maximal at low temperatures near the concentration limit of the magnetic phase transition. A large value of the magnetoresistance is explained by the magnetic-field induced transition to a more conductive ferromagnetic phase, which occurs simultaneously with the change of a spin state of cobalt ions.

Spin-dependent resonant tunneling of multiferroic tunnel junction via head-to-head 180° domain wall

The extraordinary properties of the two-dimensional electron gas (2DEG) at oxide heterostructure interface such as LaAlO3/SrTiO3 have attracted considerable investigations. Recent work suggested that the 2DEG could be formed within the SrTiO3 tunnel barrier by replacing the central TiO2 atomic layer with LaO, and that such a tunnel junction exhibits resonant tunneling behavior with new intriguing properties and potential applications [J. D. Burton et al., Phys. Rev. B 80, 115408 (2009)]. Here we investigate an alternative approach providing the resonant transmission of which a 180° head-to-head (HH) domain wall in the middle of the tunnel barrier induces the 2DEG via the free electronic carriers screening the polarization bound charges in situ. Using first-principles calculations on the Fe/PbTiO3/Fe as a model system, we show that this strategy allows for the formation of a 2DEG within the barrier, and that the remarkable difference between the majority- and minority-spin channels results in large tunneling magnetoresistance (TMR) effect. The resonant tunneling and the large TMR effect are beneficial to magnetic recording applications. We also discuss the feasibility to obtain such a 180° HH domain wall in the practical magnetic tunnel junctions and the influences of FeO formation at the iron-complex oxide interface on the electronic structure and tunneling properties.

RETRACTED: Ultrahigh magnetoresistance at room temperature in molecular wires.

Systems featuring large magnetoresistance (MR) at room temperature and in small magnetic fields are attractive owing to their potential for applications in magnetic field sensing and data storage. Usually, the magnetic properties of materials are exploited to achieve large MR effects. Here, we report on an exceptionally large (>2000%), room-temperature, small-field (a few millitesla) MR effect in one-dimensional, nonmagnetic systems formed by molecular wires embedded in a zeolite host crystal. This ultrahigh MR effect is ascribed to spin blockade in one-dimensional electron transport. Its generic nature offers very good perspectives to exploit the effect in a wide range of low-dimensional systems.

Magnetotransport in Zn1−xMnxSe:Cl under hydrostatic pressure

Pressure dependent magnetotransport measurements have been carried out on a chlorine-doped Zn0.98Mn0.02Se epilayer with a carrier concentration below the critical concentration of the metal-insulator transition (MIT). The large positive magnetoresistance at low temperatures can qualitatively be related to the giant Zeeman splitting of the donor states. The increase of the resistivity and the magnetoresistance effects under hydrostatic pressure is mainly related to the increase of the electron effective mass in the conduction band. This increase of the effective mass is the reason for an increase of the donor depth and a corresponding shrinkage of the donor wavefunction, which in turn leads to a narrowing of the impurity band (IB). The splitting of this narrow IB, which results from an applied pressure, is the main reason for the enhancement of the large positive magnetoresistance effects under pressure.

Including fringe fields from a nearby ferromagnet in a percolation theory of organic magnetoresistance

Random hyperfine fields are essential to mechanisms of low-field magnetoresistance in organic semiconductors. Recent experiments have shown that another type of random field --- fringe fields due to a nearby ferromagnet --- can also dramatically affect the magnetoresistance. A theoretical analysis of the effect of these fringe fields is challenging, as the fringe field magnitudes and their correlation lengths are orders of magnitude larger than that of the hyperfine couplings. We extend a recent theory of organic magnetoresistance to calculate the magnetoresistance with both hyperfine and fringe fields present. This theory describes several key features of the experimental fringe-field magnetoresistance, including the applied fields where the magnetoresistance reaches extrema, the applied field range of large magnetoresistance effects from the fringe fields, and the sign of the effect.

Giant Magnetoresistance Effect in the Metal–Insulator Transition of Pyrochlore Oxide Nd2Ir2O7

We investigated the magnetoresistance (MR) effect of the pyrochlore oxide Nd2Ir2O7, which shows a metal-insulator transition at T_MI =33 K. A small positive MR effect was observed in the metallic state above T_MI, while a large negative MR effect was observed in the insulating state below T_MI . MR effects exceeding 3000% were found at 1 K at a field of 9 T. As a result, we confirmed the crossover from the insulating state to a state with a small or partial band gap in a field up to 56 T. Furthermore, from the MR effect in Eu2Ir2O7 (T_MI = 120 K) and Gd$_2$Ir$_2$O$_7$ (T_MI = 127 K), we revealed that the large negative MR effect of the pyrochlore iridate Ln2Ir2O7 depends on the magnetism of the lanthanide Ln^{3+} ion. The d-f interaction plays a significant role in the large negative MR effect in the insulating state.

Domain-Wall Induced Large Magnetoresistance Effects at Zero Applied Field in Ballistic Nanocontacts

We determine magnetoresistance effects in stable and clean Permalloy nanocontacts of variable cross section, fabricated by UHV deposition and insitu electromigration. To ascertain the magnetoresistance (MR) effects originating from a magnetic domain wall, we measure the resistance values with and without such a wall at zero applied field. In the ballistic transport regime, the MR ratio reaches up to 50% and exhibits a previously unobserved sign change. Our results can be reproduced by recent atomistic calculations for different atomic configurations of the nanocontact, highlighting the importance of the detailed atomic arrangement for the MR effect.

A Large Magnetoresistance Effect in p–n Junction Devices by the Space‐Charge Effect

AbstractThe finding of an extremely large magnetoresistance effect on silicon based p–n junction with vertical geometry over a wide range of temperatures and magnetic fields is reported. A 2500% magnetoresistance ratio of the Si p–n junction is observed at room temperature with a magnetic field of 5 T and the applied bias voltage of only 6 V, while a magnetoresistance ratio of 25 000% is achieved at 100 K. The current‐voltage (I–V) behaviors under various external magnetic fields obey an exponential relationship, and the magnetoresistance effect is significantly enhanced by both contributions of the electric field inhomogeneity and carrier concentrations variation. Theoretical analysis using classical p–n junction transport equation is adapted to describe the I–V curves of the p–n junction at different magnetic fields and reveals that the large magnetoresistance effect origins from a change of space‐charge region in the p–n junction induced by external magnetic field. The results indicate that the conventional p–n junction is proposed to be used as a multifunctional material based on the interplay between electronic and magnetic response, which is significant for future magneto‐electronics in the semiconductor industry.

Magnetic order, field-induced phase transitions and magnetoresistance in the intercalated compound Fe0.5TiS2

Measurements of the magnetic susceptibility, magnetization, electrical resistivity and neutron diffraction have been performed for the compound Fe0.5TiS2 in which Fe atoms are intercalated between S–Ti–S tri-layers. It has been shown that this compound with a monoclinic crystal structure exhibits an antiferromagnetic (AF) ground state below the Néel temperature TN ≈ 140 K. Small deviations from the stoichiometry and some disordering effects caused by the additional low-temperature heat treatment do not affect substantially the AF state in Fe0.5TiS2. According to neutron diffraction data the magnetic structure at 2 K is described by the propagation vector k = (1/4,0,1/4). The Fe magnetic moments with a value of (2.9 ± 0.1) μB are directed at an angle of (78.5 ± 1.8)° to the layers. Application of the magnetic field at T < TN induces a metamagnetic phase transition to the ferromagnetic (F) state, which is accompanied by the large magnetoresistance effect (|Δρ/ρ| up to 27%). Below 100 K, the field-induced AF–F transition is found to be irreversible, as evidenced by magnetoresistance and neutron diffraction measurements. The magnetization reversal in the metastable F state is accompanied at low temperatures by substantial hysteresis (ΔH ∼ 100 kOe) which is associated with the Ising character of Fe ions.

Spin-dependent thermoelectric effects in graphene-based spin valves

Using first-principles calculations combined with non-equilibrium Green's function (NEGF), we investigate spin-dependent thermoelectric effects in a spin valve which consists of zigzag graphene nanoribbon (ZGNR) electrodes with different magnetic configurations. We find that electron transport properties in the ZGNR-based spin valve are strongly dependent on the magnetic configurations. As a result, with a temperature bias, thermally-induced currents can be controlled by switching the magnetic configurations, indicating a thermal magnetoresistance (MR) effect. Moreover, based on the linear response assumption, our study shows that the remarkably different Seebeck coefficients in the various magnetic configurations lead to a very large and controllable magneto Seebeck ratio. In addition, we evaluate thermoelectric properties, such as the power factor, electron thermal conductance and figure of merit (ZT), of the ZGNR-based spin valve. Our results indicate that the power factor and the electron thermal conductance are strongly related to the transmission gap and electron-hole symmetry of the transmission spectrum. Moreover, the value of ZT can reach 0.15 at room temperature without considering phonon scattering. In addition, we investigate the thermally-controlled magnetic distributions in the ZGNR-based spin valve and find that the magnetic distribution, especially the local magnetic moment around the Ni atom, is strongly related to the thermal bias. The very large, multi-valued and controllable thermal magnetoresistance and Seebeck effects indicate the strong potential of ZGNR-based spin valves for extremely low-power consuming spin caloritronics applications. The thermally-controlled magnetic moment in the ZGNR-based spin valve indicates its possible applications for information storage.

Metal–insulator–metal transition in Ti substituted antiferromagentic Ru2MnGe Heusler alloy

We report the composition-induced metal–insulator crossover transition in Ti substituted Ru-based metallic full Heusler alloy Ru2MnGe where the both end of materials shows a metallic properties. The samples at the middle composition show the large negative magnetoresistance effect. The experimental results of the resistivity and the magnetization indicate that this metal–insulator transition is induced by the Kondo effect because Ti atoms on antiferromagnetic Ru2MnGe have an important role in appearance of insulator states.

Control of spin-polarized electron magnetoresistance in Ta/NiFe/Ta films by intercalation of Au

Intercalation layers of Au were formed in Ta/NiFe/Ta films to control the interface properties of the films. It was found that small amounts of Au may enhance the magnetoresistance (MR) value up to 2.63%. The intercalation at different interfaces had different effects. Intercalation of Au at the top (bottom) interface with respect to the substrate exhibits weak (strong) diffusion of Au into NiFe. At weak diffusion, a homogeneous Au layer is formed which explains the large MR effect. At strong diffusion, since Au possesses a relatively strong spin–orbit interaction, the enhanced MR effect was explained by the spin–orbit scattering of polarized electrons in the NiFe layer.

High magnetoresistance in graphene nanoribbon heterojunction

We show a large magnetoresistance (MR) effect in a graphene heterostructure consisting of a metallic and semiconductor-type armchair-graphene-nanoribbon. In the heterostructure, the transmission across the first subband of the semiconducting armchair-graphene-nanoribbon and metallic armchair-graphene-nanoribbon is forbidden under zero magnetic-field, due to the orthogonality of the wavefunctions. A finite magnetic-field introduces the quantum hall-like effect, which distorts the wavefunctions. Thus, a finite transmission occurs across the heterojunction, giving rise to a large MR effect. We study the dependence of this MR on temperature and electron energy. Finally, we design a magnetic-field-effect-transistor which yields a MR of close to 100% (85%) at low (room) temperature.

Half-Metallic Ferromagnetism via the Interface Electronic Reconstruction in LaAlO3/SrMnO3 Nanosheet Superlattices

Spintronic devices are very important for future information technology. Suitable materials for such devices should have half-metallic properties with only one spin channel conducting. Nanostructures have played an important role in this aspect. Here, we report the realization of robust half-metallic ferromagnetism via the interface electronic reconstruction in artificial LaAlO(3)/SrMnO(3) nanosheet supperlattices. On the basis of first-principles density-functional calculations, we reveal an obvious electron transfer from the (LaO)(+) layer to the adjacent (MnO(2))(0) layer. And the partially occupied e(g) orbitals at the Mn sites can mediate a half-metallic state via a Zener double-exchange mechanism. On the other hand, for the superlattices with a (SrO)(0)/(AlO(2))(-) interface, hole transfer at the interface is identified. These transferred holes reside mainly at oxygen sites in SrMnO(3), leading to either the preserved G-type AFM ordering in pp-type superlattices or complex magnetic ordering in np-type superlattices. Interestingly, when these systems transit to ferromagnetic ordering by an external magnetic field, an obvious change of electronic state at the Fermi level is found, suggesting a large magnetoresistive effect therein. Our studies demonstrate the unique electric and magnetic properties arising from a magnetic ordering dependent charge transfer and electronic reconstruction at perovskite heterointerfaces, and their potential applications in spintronic devices.

Large magnetoresistance and Jahn-Teller effect in Sr2FeCoO6

Neutron diffraction measurement on the spin glass double perovskite Sr$_2$FeCoO$_6$ reveals site disorder as well as Co$^{3+}$ intermediate spin state. In addition, multiple valence states of Fe and Co are confirmed through M\"{o}ssbauer and X-ray photoelectron spectroscopy. The structural disorder and multiple valence lead to competing ferromagnetic and antiferromagnetic interactions and subsequently to a spin glass state, which is reflected in the form of an additional $T$-linear contribution at low temperatures in specific heat. A clear evidence of Jahn-Teller distortion at the Co$^{3+}$-O$_6$ complex is observed and incorporating the physics of Jahn-Teller effect, the presence of localized magnetic moment is shown. A large, negative and anomalous magnetoresistance of $\approx$ 63% at 14K in 12T applied field is observed for Sr$_2$FeCoO$_6$. The observed magnetoresistance could be explained by applying a semi-empirical fit consisting of a negative and a positive contribution and show that the negative magnetoresistance is due to spin scattering of carriers by localized magnetic moments in the spin glass phase.

Thickness effects on the magnetic and electrical transport properties of highly epitaxial LaBaCo2O5.5+δ thin films on MgO substrates

The transport properties of double perovskite LaBaCo2O5.5+δ thin films with different thicknesses were systemically studied. A thin (7 nm in thickness), disordered LaBaCo2O5.5+δ layer was formed at the interface between the film and substrate. The films had a typical semiconductor behavior with antiferromagnetic and ferromagnetic behavior coexisting at low temperature. Although the Curie temperature was independent of the film thickness, the coercive fields and magnetizations increase with increasing the film thickness. An ultra large magnetoresistance effect value of about 44% was obtained at 60 K for the film of 82 nm.

Magnetic-field-induced reverse martensitic transformation and large magnetoresistance in Ni50−xCoxMn32Al18 Heusler alloys

We have studied the magnetic and magnetoresistance properties in polycrystalline Ni50−xCoxMn32Al18 Heusler alloys. A unique martensitic transformation (MT) from the ferromagnetic austenite to the weak-magnetic martensite phase was obtained and magnetic-field-induced reverse MT was confirmed in these alloys. A magnetic difference up to 45 emu/g was measured across the MT for Ni42Co8Mn32Al18 alloy. In the same quaternary alloy, the large magnetoresistance effect of 67% at 225 K was observed through this field-induced magnetic phase transition.

Large piezoresistance and magnetoresistance effects on Ni45Co5Mn37.5In12.5 single crystal

We report resistivity measurements as a function of magnetic field or uniaxial stress across the martensitic transformation on Ni45Co5Mn37.5In12.5 Heusler single crystal. The resistivity and transformation temperatures are strongly affected by the application of an uniaxial stress or a magnetic field and consequent magnetoresistance and piezoresistance that can reach up to 60% or 122%, respectively, are measured. This behavior opens up a large range of possible applications with a relatively small pressure or magnetic field as a control parameter to tune the resistivity.

Large Magnetoresistance in Few Layer Graphene Stacks with Current Perpendicular to Plane Geometry

A large magnetoresistance (MR) effect of few-layers graphene between two non-magnetic metal electrodes with current perpendicular to graphene plane is studied. A non-saturation and anisotropic MR with the value over 60% at 14 T is observed in a two-layer graphene stack at room temperature. The resistance of the device is only tens of ohms, having the advantage of low power consumption for magnetic device applications.

Magnetocaloric effect and refrigerant capacity in Sm1−xSrxMnO3 (x = 0.42, 0.44, 0.46) manganites

Sm1−xSrxMnO3 (SSMO) with x ∼ 0.44 shows a multi-critical phase diagram which includes antiferromagnetic, charge ordering, and ferromagnetic transitions that give rise to large magnetoresistive and magnetocaloric effects. In this study, we systematically investigated the magnetic transitions, magnetocaloric effect (MCE), and refrigerant capacity (RC) in polycrystalline Sm1−xSrxMnO3 (x = 0.42, 0.44, 0.46) samples prepared by a standard ceramic method. Magnetization measurements and Arrott plot analyses reveal a first-order ferromagnetic transition for all samples investigated, with TC = 130 K, 134 K, and 133 K for x = 0.42, 0.44, and 0.46, respectively. The largest peak entropy change of −ΔSM = 4.61 J/kg K for a field change of μ0ΔH = 5 T is observed for the x = 0.44 composition around its Curie temperature of 134 K. After correcting for hysteretic losses of ∼15% – 30% engendered by the first order nature of the transition, sizable RC values of 151.42 J/kg, 140.15 J/kg, and 135.91 J/kg are still observed for x = 0.42, 0.44, and 0.46, respectively.