Showing papers on "Magnetoresistance published in 2003"

CMR manganites: physics, thin films and devices

Abstract: The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

Nearly total spin polarization in La2/3Sr1/3MnO3 from tunneling experiments

Abstract: We have performed magnetotransport measurements on La2/3Sr1/3MnO3/SrTiO3/La2/3Sr1/3MnO3 magnetic tunnel junctions. A magnetoresistance ratio of more than 1800% is obtained at 4 K, from which we infer an electrode spin polarization of at least 95%. This result strongly underscores the half-metallic nature of mixed-valence manganites and demonstrates their capability as a spin analyzer. The magnetoresistance extends up to temperatures of more than 270 K. We argue that these improvements over most previous works may result from optimizing the patterning process for oxide heterostructures.

Magnetically engineered spintronic sensors and memory

Abstract: The discovery of enhanced magnetoresistance and oscillatory interlayer exchange coupling in transition metal multilayers just over a decade ago has enabled the development of new classes of magnetically engineered magnetic thin-film materials suitable for advanced magnetic sensors and magnetic random access memories. Magnetic sensors based on spin-valve giant magnetoresistive (GMR) sandwiches with artificial antiferromagnetic reference layers have resulted in enormous increases in the storage capacity of magnetic hard disk drives. The unique properties of magnetic tunnel junction (MTJ) devices has led to the development of an advanced high performance nonvolatile magnet random access memory with density approaching that of dynamic random-access memory (RAM) and read-write speeds comparable to static RAM. Both GMR and MTJ devices are examples of spintronic materials in which the flow of spin-polarized electrons is manipulated by controlling, via magnetic fields, the orientation of magnetic moments in inhomogeneous magnetic thin film systems. More complex devices, including three-terminal hot electron magnetic tunnel transistors, suggest that there are many other applications of spintronic materials.

Spin-dependent tunnelling in magnetic tunnel junctions

Abstract: The phenomenon of electron tunnelling has been known since the advent of quantum mechanics, but continues to enrich our understanding of many fields of physics, as well as creating sub-fields on its own. Spin-dependent tunnelling (SDT) in magnetic tunnel junctions (MTJs) has recently aroused enormous interest and has developed in a vigorous field of research. The large tunnelling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible applications in non-volatile random-access memories and next-generation magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of SDT. In this review article we present an overview of this field of research. We discuss various factors that control the spin polarization and magnetoresistance in MTJs. Starting from early experiments on SDT and their interpretation, we consider thereafter recent experiments and models which highlight the role of the electronic structure of the ferromagnets, the insulating layer, and the ferromagnet/insulator interfaces. We also discuss the role of disorder in the barrier and in the ferromagnetic electrodes and their influence on TMR.

Non-saturating magnetoresistance in heavily disordered semiconductors

Abstract: The resistance of a homogeneous semiconductor increases quadratically with magnetic field at low fields and, except in very special cases, saturates at fields much larger than the inverse of the carrier mobility, a number typically of the order of 1 T (refs 1, 2). A surprising exception to this behaviour has recently been observed in doped silver chalcogenides3,4,5, which exhibit an anomalously large, quasi-linear magnetoresistive response that extends down to low fields and survives, even at extreme fields of 55 T and beyond. Here we present a simple model of a macroscopically disordered and strongly inhomogeneous semiconductor that exhibits a similar non-saturating magnetoresistance. In addition to providing a possible explanation for the behaviour of doped silver chalcogenides, our model suggests potential routes for the construction of magnetic field sensors with a large, controllable and linear response.

Radiation-induced magnetoresistance oscillations in a 2D electron gas.

Abstract: Recent measurements of a 2D electron gas subjected to microwave radiation reveal a magnetoresistance with an oscillatory dependence on the ratio of radiation frequency to cyclotron frequency. We perform a diagrammatic calculation and find radiation-induced resistivity oscillations with the correct period and phase. Results are explained via a simple picture of current induced by photoexcited disorder-scattered electrons. The oscillations increase with radiation intensity, easily exceeding the dark resistivity and resulting in negative-resistivity minima. At high intensity, we identify additional features, likely due to multiphoton processes, which have yet to be observed experimentally.

Magnetoresistance measurement of unpatterned magnetic tunnel junction wafers by current-in-plane tunneling

Abstract: We demonstrate a method for measuring magnetoresistance (MR) and resistance area product (RA) of unpatterned magnetic tunnel junction film stacks. The RA is measured by making a series of four point probe resistance measurements on the surface of an unpatterned wafer at various probe spacings. The key to this technique is in placing the probes at the appropriate spacings, on the order of microns for typical applications. The MR is obtained by repeating the measurement at different magnetic fields. A simple conceptual model and an exact analytical solution in good agreement with experimental data are presented. The current-in-plane tunneling method requires no processing, is fast, and provides reliable data which are reflective of the deposition only.

Detection of single micron-sized magnetic bead and magnetic nanoparticles using spin valve sensors for biological applications

Abstract: We have fabricated a series of highly sensitive spin valve sensors on a micron scale that successfully detected the presence of a single superparamagnetic bead (Dynabeads M-280, 2.8 μm in diameter), and thus showed suitability for identifying biomolecules labeled by such magnetic beads. By polarizing the magnetic microbead on a spin valve sensor with a dc magnetic field and modulating its magnetization with an orthogonal ac magnetic field, we observed a magnetoresistance (MR) signal reduction caused by the magnetic dipole field from the bead that partially cancelled the applied fields to the spin valve. A lock-in technique was used to measure a voltage signal due to the MR reduction. A signal of 1.2 mV rms or 5.2 mΩ of resistance reduction was obtained from a 3 μm wide sensor and a signal of 3.8 mV rms or 11.9 mΩ from a 2.5 μm wide sensor. Micromagnetic simulations were also performed for the spin valve sensors with a single bead and gave results consistent with experiments. Further experiments and simula...

A coherent three-dimensional Fermi surface in a high-transition-temperature superconductor

Abstract: All conventional metals are known to possess a three-dimensional Fermi surface, which is the locus in reciprocal space of the long-lived electronic excitations that govern their electronic properties at low temperatures. These excitations should have well-defined momenta with components in all three dimensions. The high-transition-temperature (high-T(c)) copper oxide superconductors have unusual, highly two-dimensional properties above the superconducting transition. This, coupled with a lack of unambiguous evidence for a three-dimensional Fermi surface, has led to many new and exotic models for the underlying electronic ground state. Here we report the observation of polar angular magnetoresistance oscillations in the overdoped superconductor Tl2Ba2CuO6+delta in high magnetic fields, which firmly establishes the existence of a coherent three-dimensional Fermi surface. Analysis of the oscillations reveals that at certain symmetry points, however, this surface is strictly two-dimensional. This striking form of the Fermi surface topography, long-predicted by electronic band structure calculations, provides a natural explanation for a wide range of anisotropic properties both in the normal and superconducting states. Our data reveal that, despite their extreme electrical anisotropy, the high-T(c) materials at high doping levels can be understood within a framework of conventional three-dimensional metal physics.

Radiation-induced magnetoresistance oscillation in a two-dimensional electron gas in Faraday geometry.

Abstract: Microwave-radiation induced giant magnetoresistance oscillations recently discovered in high-mobility two-dimensional electron systems are analyzed theoretically. Multiphoton-assisted impurity scatterings are shown to be the primary origin of the oscillation. Based on a theory which considers the interaction of electrons with electromagnetic fields and the effect of the cyclotron resonance in Faraday geometry, we are able not only to reproduce the correct period, phase, and the negative resistivity of the main oscillation, but also to predict the secondary peaks and additional maxima and minima observed in the experiments. These peak-valley structures are identified to relate, respectively, to single-, double-, and triple-photon processes.

Intergranular magnetoresistance in nanomanganites

Abstract: In this paper we present some of the most important magnetic and transport properties of mixed-valence manganite nanoparticles. The samples were prepared by a sol–gel method, which allows us to control particle size and, in this way, to obtain new properties of the archetypal ferromagnetic- metallic compound La2/3Ca1/3MnO3. Magnetic properties allow us to present a model for the nanoparticles based on an ideal inner core and an outer shell in which the magnetism is modified by oxygen non-stoichiometry, vacancies and stress. The experimental results obtained from the electrical transport properties, namely increasing intergranular magnetoresistance (MR) with reducing particle size, tuning of intrinsic colossal MR and low-temperature electrostatic blocking effects, seem to support the proposed model.

Domain wall pinning in narrow ferromagnetic ring structures probed by magnetoresistance measurements.

Abstract: We present a magnetoresistance study of magnetization reversal and domain wall pinning effects in a mesoscopic narrow ferromagnetic Permalloy ring structure containing notches. The size and strength of the attractive pinning potential created by a notch is measured and the resistance minimum at remanence is found to occur when a single transverse domain wall is pinned at the notch, in agreement with the results of numerical simulations of the anisotropic magnetoresistance. When a field is applied in the direction corresponding to a potential well edge, a novel magnetic state with a very wide domain wall is stabilized, giving rise to a characteristic signature in the magnetoresistance at such angles.

Domain wall motion induced by spin polarized currents in ferromagnetic ring structures

Abstract: We present an experimental study of the influence of spin-polarized currents on the displacement of domain walls in submicrometer permalloy ring structures. Using magnetoresistance (MR) measurements with multiple nonmagnetic contacts, we can sense the displacement of a domain wall and, by injecting large dc current densities (1011 A/m2), we can increase or decrease the magnetic field needed to move a single domain wall, depending on the direction of the current with respect to the applied field direction. Using rings with and without notches and by measuring the MR with the magnetic field applied along different directions, we show that we can exclude the possibility that the dominating effect is a classical Oersted field. We conclude that our observations can be explained by a directional spin torque effect.

Resonant inversion of tunneling magnetoresistance.

Abstract: Resonant tunneling via localized states in the barrier can invert magnetoresistance in magnetic tunnel junctions. Experiments performed on electrodeposited Ni/NiO/Co nanojunctions of area smaller than 0.01 microm(2) show that both positive and negative values of magnetoresistance are possible. Calculations based on Landauer-Buttiker theory explain this behavior in terms of disorder-driven statistical variations in magnetoresistance with a finite probability of inversion due to resonant tunneling.

Strain-induced magnetic stripe domains in La0.7Sr0.3MnO3 thin films

Abstract: We have investigated magnetic microstructures of magnetoresistive La0.7Sr0.3MnO3 (LSMO) thin films. Magnetic images are strongly dependent on structural strain induced by the substrates. The LSMO film on SrTiO3 dominated by tensile stress effect displays a feather-like pattern, whereas LSMO films on LaAlO3 and NdGaO3 substrates under compressive stress show stripe domains. In particular, the magnetic image of the film on NdGaO3 reveals distinctive straight stripe domain patterns on the order of about 120 nm, suggesting the presence of a sizable out-of-plane magnetization. The ordering of the stripe domains is also sensitive to the field direction.

Nonequilibrium spintronic transport through an artificial Kondo impurity: conductance, magnetoresistance, and shot noise.

Abstract: We investigate the nonequilibrium transport properties of a quantum dot when spin flip processes compete with the formation of a Kondo resonance in the presence of ferromagnetic leads. Based upon the Anderson Hamiltonian in the strongly interacting limit, we predict a splitting of the differential conductance when the spin flip scattering amplitude is of the order of the Kondo temperature. We discuss how the relative orientation of the lead magnetizations strongly influences the electronic current and the shot noise in a nontrivial way. Furthermore, we find that the zero-bias tunneling magnetoresistance becomes negative with increasing spin flip scattering amplitude.

Very Large Magnetoresistance in Lateral Ferromagnetic (Ga,Mn)As Wires with Nanoconstrictions

Abstract: We have fabricated (Ga,Mn)As nanostructures in which domain walls can be pinned by sub-10 nm constrictions. Controlled by shape anisotropy, we can switch the regions on either side of the constriction to either parallel or antiparallel magnetization. All samples exhibit a positive magnetoresistance, consistent with domain-wall trapping. For metallic samples, we find a magnetoresistance up to 8%, which can be understood from spin accumulation. In samples where, due to depletion at the constriction, a tunnel barrier is formed, we observe a magnetoresistance of up to 2000%.

Observation of minority spin character of the new electron doped manganite La0.7Ce0.3MnO3 from tunneling magnetoresistance

Abstract: We report the magnetotransport characteristics of a trilayer ferromagnetic tunnel junction built of an electron doped manganite (La0.7Ce0.3MnO3) and a hole doped manganite (La0.7Ca0.3MnO3). At low temperatures the junction exhibits a large positive tunneling magnetoresistance (TMR), irrespective of the bias voltage. At intermediate temperatures below T(C) the sign of the TMR is dependent on the bias voltage across the junction. The magnetoresistive characteristics of the junction strongly suggest that La0.7Ce0.3MnO3 is a minority spin carrier ferromagnet with a high degree of spin polarization, i.e., a transport half-metal.

Novel germanium-based magnetic semiconductors.

Abstract: Epitaxial synthesis and properties of novel Co and Mn-doped Ge magnetic semiconductors were studied. Epitaxial growth of high quality films with high doping concentrations has been stabilized by the use of two dopants. The magnetic and transport properties of the system exhibit high T(C) and large magnetoresistance effects that can be controlled systematically by the doping concentration. The maximum T(C) achieved in the semiconducting materials is approximately 270 K at a composition of Co0.12Mn0.03Ge0.85.

Magnetoresistive effect element and magnetic memory device

Abstract: A magnetoresistive effect element may be given satisfactory magnetic characteristics because a deterioration of a magnetoresistive changing rate by annealing can be suppressed and a magnetic memory device includes this magnetoresistive effect element to provide excellent write characteristics. A magnetoresistive effect element has a pair of ferromagnetic layers (magnetization fixed layer 5 and magnetization free layer 7 ) opposed to each other through an intermediate layer 6 to cause an electric current to flow in the direction perpendicular to the layer surface to obtain a magnetoresistive change. A magnetic memory device comprises the magnetoresistive effect element 1 in which at least one of the pair of ferromagnetic layers 5, 7 contains an amorphous ferromagnetic material whose crystallization temperature is higher than 623 K and bit lines and word lines sandwiching this magnetoresistive effect element and the magnetoresistive effect element in the thickness direction.

Hybrid write mechanism for high speed and high density magnetic random access memory

Abstract: A method of writing to a magnetic random access memory comprising: producing a magnetic field along a magnetically hard axis of a free layer of a magnetoresistive element; and passing current through the magnetoresistive element to change a direction of magnetization of the free layer by spin momentum transfer. A magnetic random access memory that operates in accordance with the method is also included.

Magnetoresistive element, magnetic head, magnetic memory and magnetic recording apparatus using the same

Abstract: A magnetoresistive element includes a multilayer film configuration including: a tunnel insulation layer; and a pair of magnetic layers that are laminated with the tunnel insulation layer interposed therebetween. A resistance value of the magnetoresistive element varies with a relative angle between magnetic orientations of both of the magnetic layers, and at least one of the magnetic layers includes a magnetic film having a thermal expansion coefficient not greater than a value obtained by adding 2×10 −6 /K to a thermal expansion coefficient of the tunnel insulation layer. The thus configured magnetoresistive element can exert excellent thermal stability. The use of such a magnetoresistive element can realize a magnetic head, a magnetic memory element and a magnetic recording apparatus with excellent thermal stability.

Magnetic and magnetotransport properties of La1−xBaxMnO3−x/2 perovskite manganites

Abstract: Crystal structure as well as magnetization and electrical transport vs. temperature and field for the La3+1−xBa2+xMn3+O2−3−x/2 (0 ≤ x ≤ 0.50) anion-deficient manganites have been studied. It is established that the samples in the region 0 ≤ x ≤ 0.05 are O/-orthorhombic perovskites and are rhombohedric with 0.10 ≤ x ≤ 0.25, whereas with 0.27 ≤ x ≤ 0.50 they are cubic. As the doping level increases the samples undergo a transition from a weak ferromagnetic (x = 0) to an inhomogeneous ferromagnetic (x ≥ 0.03) state. At x ≥ 0.12 samples show cluster spin glass properties with the temperature of magnetic moment freezing ∼45 K. All the reduced samples are semiconductors and show considerable magnetoresistance over a wide temperature range. The largest magnetoresistance ratio is observed for the x = 0.30 sample. Concentration dependences of spontaneous magnetization and the magnetic ordering temperature for the reduced La3+1−xBa2+xMn3+O2−3−x/2 manganites have been established by magnetic measurements and compared with those for stoichiometric ones. Magnetic data for the reduced La3+1−xBa2+xMn3+O2−3−x/2 manganites have been compared with those for the stoichiometric La3+1−xBa2+xMn3+1−xMn4+xO2−3. The magnetic state of the anion-deficient samples is interpreted on the basis of the superexchange interaction model.

Large room-temperature spin-dependent tunneling magnetoresistance in polycrystalline Fe3O4 films

Abstract: Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. Transmission electron microscopy images show that the films consist of quite uniform Fe3O4 grains well separated by grain boundaries. It was found that the tunneling of spin-polarized electrons across the antiferromagnetic coupled grain boundaries dominates the transport properties of the films. Magnetoresistance (MR) {=[ρ(H)−ρ(0)]/ρ(0)} shows linear and quadratic magnetic-field dependence in the low-field range when the field is applied parallel and perpendicular to film plane, which is similar to the behaviors observed in the epitaxial Fe3O4 films consisting of a large fraction of antiferromagnetic antiphase domain boundaries. At 300 K, the size of the MR reaches −7.4% under a 50-kOe magnetic field, which is a very large MR for polycrystalline Fe3O4 films.

Magnetoresistance effect element and magnetic memory

Abstract: PROBLEM TO BE SOLVED: To provide a magnetoresistance effect element which has a high resistance to heat fluctuation and has a low effective switching magnetic field in a magnetization free layer SOLUTION: The magnetoresistance effect element includes a ferromagnetic tunnel junction having a three-layer structure consisting of a first ferromagnetic layer, a tunnel barrier layer, and a second ferromagnetic layer with the first ferromagnetic layer having a larger coercive force than the second ferromagnetic layer; and has a tunnel conductance which changes due to a relative angle of magnetization of the two ferromagnetic layers Magnetization of ends of the two ferroelectric layers is fixed to a direction having a component perpendicular to an axis of easy magnetization of the two ferromagnetic layers COPYRIGHT: (C)2005,JPO&NCIPI

Field dependent permittivity of composite materials containing ferromagnetic wires

Abstract: A type of a composite material is proposed, the microwave permittivity of which changes under the effect of a dc magnetic field applied to the whole composite sample. The composite consists of short ferromagnetic wires embedded into a dielectric matrix. A strong field dependence of the permittivity is seen in the vicinity of the antenna resonance, where the dispersion behavior can experience a transformation from a resonant spectrum to a relaxation one under the effect of the field. This permittivity behavior is due to a high sensitivity of the ac surface impedance of a ferromagnetic wire to a magnetic field, known as the magnetoimpedance (MI) effect. If the resonance-like dispersion behavior is realized, the real part of the effective permittivity can be made negative past the resonance for wire inclusion concentrations well below the percolation threshold. Applying a magnetic field, the negative peak continuously decreases as the dispersion tends to become of a relaxation type. The effective permittivity is analyzed within a one-particle approximation, by considering a wire piece as an independent scatterer and solving the scattering problem with the impedance boundary condition. A magnetic field is assumed to be applied in parallel to the wire. A new integrodifferential equation for the current distribution in a wire is obtained, which is valid for the surface impedance matrix of a general form. This work demonstrates the possibility of using the MI effect to design field-controlled composites and band-gap structures.

Tunnel magnetoresistance in nanojunctions based on Sr2FeMoO6

Abstract: We report on the observation of magnetoresistance in a Sr2FeMoO6 (SFMO)-based tunnel junction This result is obtained by combining a three-step process for the growth of the Sr2FeMoO6 layer by pulsed laser deposition with a technology allowing the definition of nanometer-sized junctions A clear positive magnetoresistive signal of 50% is obtained at low temperature in a Sr2FeMoO6/SrTiO3/Co junction Since the SrTiO3/Co interface is known to have a negative spin polarization of about 20%, this result yields a negative spin polarization of SFMO, which we find to amount to more than 85% in our film This confirms the half-metallic character of this compound, predicted by band structure calculations

Exchange-biased spin valves with perpendicular magnetic anisotropy based on (Co/Pt) multilayers

Abstract: We have prepared spin valves exhibiting perpendicular magnetic anisotropy [perpendicular spin valves (PSVs)] by sputtering. These PSVs associate a “free” (Co/Pt) multilayer with a “pinned” (Co/Pt)/FeMn multilayer separated by various spacer materials (Pt, Cu, Al2O3). We carried out a comprehensive study of the magnetic and magnetotransport properties of the biased multilayers and of the complete spin valves. When the number of repeats in the (Co/Pt) exchange-biased multilayer is larger than 3, the samples present 100% remnant magnetization in the perpendicular configuration. The major hysteresis cycles exhibit two well-separated loops associated with the free and the exchange-biased (Pt/Co) multilayers. When optimized, the exchange-bias field can be larger than the coercivity of the pinned layer. Metallic PSVs with Cu spacers exhibit giant magnetoresistance but the amplitude is only of the order of 1% due to significant current shunting. In contrast, perpendicularly magnetized tunnel junctions are very promising.