Showing papers on "Magnetoresistance published in 2007"

Magnetic effects at the interface between non-magnetic oxides

Abstract: The electronic reconstruction at the interface between two insulating oxides can give rise to a highly conductive interface. Here we show how, in analogy to this remarkable interfaceinduced conductivity, magnetism can be induced at the interface between the otherwise non-magnetic insulating perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the interface is found, together with a logarithmic temperature dependence of the sheet resistance.At lowtemperatures, the sheet resistance reveals magnetic hysteresis.Magnetic ordering is a key issue in solid-state science and its underlying mechanisms are still the subject of intense research. In particular, the interplay between localized magnetic moments and the spin of itinerant conduction electrons in a solid gives rise to intriguingmany-body effects such as Ruderman–Kittel–Kasuya–Yosida interactions3, the Kondo effect4 and carrier-induced ferromagnetism in diluted magnetic semiconductors5. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise non-magnetic materials.

Magnetic effects at the interface between nonmagnetic oxides

Abstract: The electronic reconstruction at the interface between two insulating oxides can give rise to a highly-conductive interface. In analogy to this remarkable interface-induced conductivity we show how, additionally, magnetism can be induced at the interface between the otherwise nonmagnetic insulating perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the interface is found, together with a logarithmic temperature dependence of the sheet resistance. At low temperatures, the sheet resistance reveals magnetic hysteresis. Magnetic ordering is a key issue in solid-state science and its underlying mechanisms are still the subject of intense research. In particular, the interplay between localized magnetic moments and the spin of itinerant conduction electrons in a solid gives rise to intriguing many-body effects such as Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, the Kondo effect, and carrier-induced ferromagnetism in diluted magnetic semiconductors. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise nonmagnetic materials.

Spin-diffusion lengths in metals and alloys, and spin-flipping at metal/metal interfaces: an experimentalist's critical review

Abstract: In magnetoresistance (MR) studies of magnetic multilayers composed of combinations of ferromagnetic (F) and non-magnetic (N) metals, the magnetic moment (or related ‘spin’) of each conduction electron plays a crucial role, supplementary to that of its charge. While initial analyses of MR in such multilayers assumed that the direction of the spin of each electron stayed fixed as the electron transited the multilayer, we now know that this is true only in a certain limit. Generally, the spins ‘flip’ in a distance characteristic of the metal, its purity, and the temperature. They can also flip at F/N or N1/N2 interfaces. In this review we describe how to measure the lengths over which electron moments flip in pure metals and alloys, and the probability of spin-flipping at metallic interfaces. Spin-flipping within metals is described by a spindiffusion length, l M sf , where the metal M = F or N. Spin-diffusion lengths are the characteristic lengths in the current-perpendicular-to-plane (CPP) and lateral non-local (LNL) geometries that we focus upon in this review. In certain simple cases, l N sf sets the distance over which the CPP-MR and LNL-MR decrease as the N-layer thickness (CPP-MR) or N-film length (LNL) increases, and l F does the same for increase of the CPP-MR with increasing F-layer thickness. Spinflipping at M1/M2 interfaces can be described by a parameter, δM1/M2 ,w hich determines the spin-flipping probability, P = 1 − exp(−δ). Increasing δM1/M2 usually decreases the MR. We list measured values of these parameters and discuss the limitations on their determinations.

Giant tunnel magnetoresistance in magnetic tunnel junctions with a crystalline MgO(0 0 1) barrier

Abstract: A magnetic tunnel junction (MTJ), which consists of a thin insulating layer (a tunnel barrier) sandwiched between two ferromagnetic electrode layers, exhibits tunnel magnetoresistance (TMR) due to spin-dependent electron tunnelling. Since the 1995 discovery of room-temperature TMR, MTJs with an amorphous aluminium oxide (Al–O) tunnel barrier have been studied extensively. Al–O-based MTJs exhibit magnetoresistance (MR) ratios up to about 70% at room temperature (RT) and are currently used in magnetoresistive random access memory (MRAM) and the read heads of hard disk drives. MTJs with MR ratios significantly higher than 70% at RT, however, are needed for next-generation spintronic devices. In 2001 first-principle theories predicted that the MR ratios of epitaxial Fe/MgO/Fe MTJs with a crystalline MgO(0 0 1) barrier would be over 1000% because of the coherent tunnelling of fully spin-polarized Δ1 electrons. In 2004 MR ratios of about 200% were obtained in MTJs with a single-crystal MgO(0 0 1) barrier or a textured MgO(0 0 1) barrier. CoFeB/MgO/CoFeB MTJs for practical applications were also developed and found to have MR ratios up to 500% at RT. MgO-based MTJs are of great importance not only for device applications but also for clarifying the physics of spin-dependent tunnelling. In this article we introduce recent studies on physics and applications of the giant TMR in MgO-based MTJs.

Spin-transfer torque switching in magnetic tunnel junctions and spin-transfer torque random access memory

Abstract: We present experimental and numerical results of current-driven magnetization switching in magnetic tunnel junctions. The experiments show that, for MgO-based magnetic tunnelling junctions, the tunnelling magnetoresistance ratio is as large as 155% and the intrinsic switching current density is as low as 1.1 ? 106?A?cm?2. The thermal effect and current pulse width on spin-transfer magnetization switching are explored based on the analytical and numerical calculations. Three distinct switching modes, thermal activation, dynamic reversal, and precessional process, are identified within the experimental parameter space. The switching current distribution, write error, and read disturb are discussed based on device design considerations. The challenges and requirements for the successful application of spin-transfer torque as the write scheme in random access memory are addressed.

Double perovskites with ferromagnetism above room temperature

Abstract: We review the structural, magnetic and transport properties of double perovskites (A2BB'O6) with ferromagnetism above room temperature. Ferromagnetism in these compounds is explained by an indirect B?O?B'?O?B exchange interaction mediated by itinerant electrons. We first focus on the BB' =?FeMo-based double perovskites, with Sr2FeMoO6 (TC = 420?K) being the most studied compound. These compounds show metallic behaviour and low magnetic coercivity. Afterwards, we will focus on B' = Re compounds, where the significant orbital moment of Re plays a crucial role in the magnetic properties, for example in the large magnetic coercivity and magnetostructural coupling. More specifically, we first discuss the A2FeReO6 series, with maximum TC = 520?K for Ca2FeReO6, which shows a tendency to semiconducting behaviour. Finally, we describe the Sr2(Fe1?xCrx)ReO6 series, with maximum TC = 625?K for Sr2CrReO6, which is the highest TC in an oxide compound without Fe. This compound is metallic. We discuss the impact of these materials for spin electronics in the light of their high spin polarization at the Fermi level and metallicity. In particular, we focus on the large intergrain magnetoresistance effect observed in polycrystalline samples and the possible implementation of these materials as electrodes in magnetic tunnel junctions.

Bipolaron Mechanism for Organic Magnetoresistance

Abstract: We present a mechanism for the recently discovered magnetoresistance in disordered $\ensuremath{\pi}$-conjugated materials, based on hopping of polarons and bipolaron formation, in the presence of the random hyperfine fields of the hydrogen nuclei and an external magnetic field. Within a simple model we describe the magnetic field dependence of the bipolaron density. Monte Carlo simulations including on-site and longer-range Coulomb repulsion show how this leads to positive and negative magnetoresistance. Depending on the branching ratio between bipolaron formation or dissociation and hopping rates, two different line shapes in excellent agreement with experiment are obtained.

Tuning magnetoresistance between positive and negative values in organic semiconductors.

Abstract: Magnetic-field-dependent injection current, namely magnetoresistance, is readily observable in organic semiconductor devices. This provides a non-contact approach to tune organic optoelectronic properties by using a magnetic field. Here, we demonstrate that this magnetoresistance can be changed between positive and negative values by adjusting the dissociation and charge reaction in excited states through changing the bipolar charge injection in organic light-emitting diodes. This finding reveals that the magnetic-field-dependent generation of secondary charge carriers from the dissociation and charge reaction affects the injection current by forming further space charges at the organic–electrode interfaces and therefore accounts for the tunable magnetoresistance. Furthermore, the dissociation and charge reaction have opposite dependences on magnetic field in the generation of secondary charge carriers, consequently leading to negative and positive magnetoresistance, respectively. As a result, adjusting the dissociation and charge reaction in excited states provides a convenient pathway to tune the magnetoresistance in organic semiconductors.

Magnetoresistance and efficiency measurements of Alq 3 -based OLEDs

Abstract: Magnetoresistance and efficiency measurements of indium tin oxide/$N,{N}^{\ensuremath{'}}$-diphenyl-$N,{N}^{\ensuremath{'}}$ bis(3-methylphenyl)-($1,{1}^{\ensuremath{'}}$-biphenyl)-$4,{4}^{\ensuremath{'}}$ diamine/aluminum tris(8-hydroxyquinoline)/cathode organic light-emitting diode structures have been made as a function of magnetic field and cathode type. It has been found that magnetoresistance occurs only when there is light emission from the devices, which suggests that the magnetoresistance is related to exciton formation. Comparison of the effects of applied field on device efficiency and magnetoresistance shows that the magnetoresistance cannot be due to the recombination current. We suggest that the effect may be due to trapping of charge carriers at triplet excitons within the device.

The phenomena of spin-filter tunnelling

Abstract: The spin filtering phenomenon allows one to obtain highly spin-polarized charge carriers generated from nonmagnetic electrodes using magnetic tunnel barriers. The exponential dependence of tunnel current on the tunnel barrier height is operative here. The magnetic, semiconducting europium chalcogenide compounds have strikingly demonstrated this effect. The possibility of employing ferrites and other methods opens the potential for display of this phenomenon at room temperature, which can be expected to lead to huge progress in spin injection and detection in semiconductors. But first, extremely challenging material-related issues have to be addressed. This review covers the field.

Magnetic field-induced martensitic transformation and large magnetoresistance in NiCoMnSb alloys

Abstract: Magnetic field-induced martensitic transformation was realized in Ni50−xCoxMn39Sb11 alloys. The partial substitution of Co for Ni has turned the antiferromagnetically aligned Mn moments in the starting material Ni50Mn39Sb11 into a ferromagnetic ordering, raising the magnetization at room temperature from 8emu∕g for NiMnSb to ∼110emu∕g for Ni41Co9Mn39Sb11. In the same quaternary sample, a magnetization difference up to 80emu∕g was measured across the martensitic transformation, and the transformation temperature (T0=259K) could be lowered by 35K under a field of 10T. Also a magnetoresistance over 70% was observed through this field-induced transformation.

Magnetic molybdenum disulfide nanosheet films.

Abstract: Bulk molybdenum disulfide is known to be a nonmagnetic material. We have synthesized edge-oriented MoS2 nanosheet-like films that exhibit weak magnetism (∼1−2 emu/g) and 2.5% magnetoresistance effects with a Curie temperature of 685 K. The magnetization is related to the presence of edge spins on the prismatic edges of the nanosheets. Spin-polarized calculations were performed on triangular-shaped cluster models in order to provide insight into the origin of magnetism on the edges as well as the size-property correlation in these MoS2 nanosheets. Our results imply that nanostructured films with a high density of edge spins can give rise to magnetism even though the bulk material is nonmagnetic.

Spin transfer switching in dual MgO magnetic tunnel junctions

Abstract: Dual magnetic tunnel junction (MTJ) structures consisting of two MgO insulating barriers of different resistances, two pinned reference layers aligned antiparallel to one another, and a free layer embedded between the two insulating barriers have been developed. The electron transport and spin dependent resistances in the dual MTJ structures are accounted for by sequential tunneling with some spin-flip relaxation in the central electrode (the free layer). With a tunneling magnetoresistance ratio of 70%, a switching current density Jc (at 30ms) of 0.52MA∕cm2 is obtained, corresponding to an intrinsic value of Jc0 (at 1ns) of 1.0MA∕cm2. This value of Jc0 is 2–3 times smaller than that of a single MgO insulating barrier MTJ structure and results from improvements in the spin-transfer torque efficiency. The asymmetry between JcAP→P and JcP→AP is significantly improved, which widens the read-write margin for memory device design. In addition, the experimental results show that the switching current density can be further reduced when an external field is applied along the hard axis of the free layer.

Separating positive and negative magnetoresistance in organic semiconductor devices.

Abstract: We study the transition between positive and negative organic magnetoresistance (OMAR) in tris-(8 hydroxyquinoline) aluminium (${\mathrm{Alq}}_{3}$), in order to identify the elementary mechanisms governing this phenomenon. We show how the sign of OMAR changes as function of the applied voltage and temperature. The transition from negative to positive magnetoresistance (MR) is found to be accompanied by an increase in slope of $\mathrm{log} (I)$ versus $\mathrm{log} (V)$. ac admittance measurements show this transition coincides with the onset of minority charge (hole) injection in the device. All these observations are consistent with two simultaneous contributions with opposite sign of MR, which may be assigned to holes and electrons having different magnetic field responses.

Spin injection into a graphene thin film at room temperature

Abstract: We demonstrate spin injection into a graphene thin film with high reliability by using non-local magnetoresistance (MR) measurements, in which the electric current path is completely separated from the spin current path. Using these non-local measurements, an obvious MR effect was observed at room temperature; the MR effect was ascribed to magnetization reversal of ferromagnetic electrodes. This result is a direct demonstration of spin injection into a graphene thin film. Furthermore, this is the first report of spin injection into molecules at room temperature.

Microwave photovoltage and photoresistance effects in ferromagnetic microstrips

Abstract: We investigate the dc electric response induced by ferromagnetic resonance in ferromagnetic Permalloy $({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$ microstrips. The resulting magnetization precession alters the angle of the magnetization with respect to both dc and rf current. Consequently the time averaged anisotropic magnetoresistance (AMR) changes (photoresistance). At the same time the time-dependent AMR oscillation rectifies a part of the rf current and induces a dc voltage (photovoltage). A phenomenological approach to magnetoresistance is used to describe the distinct characteristics of the photoresistance and photovoltage with a consistent formalism, which is found in excellent agreement with experiments performed on in-plane magnetized ferromagnetic microstrips. Application of the microwave photovoltage effect for rf magnetic field sensing is discussed.

Magnetism of Co-doped ZnO thin films

Abstract: We have investigated magnetic and transport properties of 5% Co-doped and undoped ZnO thin films deposited on $r$ plane ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates by pulsed laser deposition. The Co doped films showed paramagnetic and ferromagnetic behavior as well as a high magnetoresistance and a small anomalous Hall effect. In a range of $0\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ at low temperatures we observed a double sign change of the magnetoresistance. For undoped ZnO films, prepared by the same conditions, only a negative MR was observed, but surprisingly also a very small anomalous Hall effect. We explain our results by applying a semiempirical fit consisting of a positive and a negative contribution to the magnetoresistance. Using x-ray magnetic circular dichroism we investigated element specific magnetic moments in Co-doped laser ablated ZnO films. As the Co atoms show a paramagnetic behavior, we attribute the ferromagnetism to a spontaneously spin impurity band induced by oxygen vacancies and defects due to the transition metal doping and/or interface stress to the substrate.

Room temperature spin filtering in epitaxial cobalt-ferrite tunnel barriers

Abstract: The authors report direct experimental evidence of room temperature spin filtering in magnetic tunnel junctions (MTJs) containing CoFe2O4 tunnel barriers via tunneling magnetoresistance (TMR) measurements. Pt(111)∕CoFe2O4(111)∕γ-Al2O3(111)∕Co(0001) fully epitaxial MTJs were grown in order to obtain a high quality system, capable of functioning at room temperature. Spin-polarized transport measurements reveal significant TMR values of −18% at 2K and −3% at 290K. In addition, the TMR ratio follows a unique bias voltage dependence that has been theoretically predicted to be the signature of spin filtering in MTJs containing magnetic barriers. CoFe2O4 tunnel barriers therefore provide a model system to investigate spin filtering in a wide range of temperatures.

Tunneling anisotropic magnetoresistance and spin-orbit coupling in Fe/GaAs/Au tunnel junctions.

Abstract: We report the observation of tunneling anisotropic magnetoresistance effect in the epitaxial metal-semiconductor system Fe/GaAs/Au. The observed twofold anisotropy of the resistance can be switched by reversing the bias voltage, suggesting that the effect originates from the interference of the spin-orbit coupling at the interfaces. Corresponding model calculations reproduce the experimental findings very well.

Room temperature spin filtering in epitaxial cobalt-ferrite tunnel barriers

Abstract: We report direct experimental evidence of room temperature spin filtering in magnetic tunnel junctions (MTJs) containing CoFe2O4 tunnel barriers via tunneling magnetoresistance (TMR) measurements. Pt(111)/CoFe2O4(111)/gamma-Al2O3(111)/Co(0001) fully epitaxial MTJs were grown in order to obtain a high quality system, capable of functioning at room temperature. Spin polarized transport measurements reveal significant TMR values of -18% at 2 K and -3% at 290 K. In addition, the TMR ratio follows a unique bias voltage dependence that has been theoretically predicted to be the signature of spin filtering in MTJs containing magnetic barriers. CoFe2O4 tunnel barriers therefore provide a model system to investigate spin filtering in a wide range of temperatures.

Semiconductors Between Spin-Polarized Sources and Drains

Abstract: Injecting spins into a semiconductor channel and transforming the spin information into a significant electrical output signal is a long-standing problem in spintronics. This is the prerequisite of several concepts of spin transistor. In this paper, we discuss the general problem of spin transport in a nonmagnetic channel between source and drain. Two problems must be mastered: 1) In diffusive regime, the injection/extraction of a spin-polarized current into/from a semiconductor beyond the ballistic zone at the interface with a magnetic metal requires the insertion of a spin-dependent and large enough interface resistance. 2) In both the diffusive and ballistic regimes and whatever the metallic or semiconducting character of the source/drain, a small enough interface resistance is the condition to keep the dwell time shorter than the spin lifetime and, thus, to conserve the spin-accumulation-induced output signal at an optimum level (DeltaR/Rap1 or larger). Practically, the main difficulties come from the second condition. In our presentation of experimental results, we show why the transformation of spin information into a large electrical signal has been more easily achieved with carbon nanotubes than with semiconductors, and we discuss how the situation could be improved in the later case

Giant magnetothermal conductivity in the Ni–Mn–In ferromagnetic shape memory alloys

Abstract: In this letter the authors present the observation of giant magnetothermal conductivity in NiMnIn single crystals. Upon cooling, a martensitic transformation is accompanied by a ferromagnetic metal→ferrimagnetic poor-metal transition. Most strikingly, this transition can be shifted to lower temperature and even totally suppressed by a magnetic field. The magnetic field-induced phase transition leads to a large magnetoresistance and a large magnetothermal conductivity up to 70% and 120%, respectively. The specific heat measurements indicate that the large magnetotransport properties are due to the increasing the density of free electrons, suggesting existence of superzone gap in the low-temperature, ferrimagnetic martensite.

Magnetoresistance in organic light-emitting diode structures under illumination

Abstract: We have investigated the effect of illumination on the organic magnetoresistance (OMR) in organic light-emitting diode (OLED) structures. The results show that it is possible to obtain OMR at voltages below ``turn-on,'' where no OMR was visible for devices operated in the dark. The photoinduced OMR has a field dependence that is identical to that obtained for OLEDs containing very thin layers, where triplet dissociation at the cathode was a major component of the OMR. At voltages around the open circuit voltage, where the current through the device is very small, very large OMRs of $\ensuremath{\sim}300%$ can be observed. The results support our proposed model for organic magnetoresistance as being caused in part by the interaction of free carriers with triplet excitons within the device. The results suggest that the introduction of a low field magnet could provide a simple means of improving the efficiency of organic photovoltaic cells.

Magnetic tunnel junctions using amorphous materials as reference and free layers

Abstract: Magnetic tunnel junctions are constructed from a MgO or Mg—ZnO tunnel barrier and amorphous magnetic layers in proximity with, and on respective sides of, the tunnel barrier The amorphous magnetic layer preferably includes Co and at least one additional element selected to make the layer amorphous, such as boron Magnetic tunnel junctions formed from the amorphous magnetic layers and the tunnel barrier have tunneling magnetoresistance values of up to 200% or more

Magnetoresistance due to edge spin accumulation.

Abstract: Because of spin-orbit interaction, an electrical current is accompanied by a spin current resulting in spin accumulation near the sample edges. Due again to spin-orbit interaction this causes a small decrease of the sample resistance. An applied magnetic field will destroy the edge spin polarization leading to a positive magnetoresistance. This effect provides means to study spin accumulation by electrical measurements. The origin and the general properties of the phenomenological equations describing coupling between charge and spin currents are also discussed.

Tunneling magnetoresistance observed in La0.67Sr0.33MnO3/organic molecule/Co junctions

Abstract: Tunneling magnetoresistance has been observed in organic based spintronic devices using the organic semiconductors tetraphenyl porphyrin (TPP) and aluminum tris(8-hyroxyquinoline) (Alq3) as the spacer layer between La0.67Sr0.33MnO3 (LSMO) and Co films. The evidence for tunneling is twofold: (1) nonlinear current and conductance versus voltage curves and (2) an increasing junction resistance with decreasing temperature. In general, the magnetoresistance is found to decrease with increasing bias voltage and increasing temperature in both Alq3 and TPP junctions. These results demonstrate that organic molecules can form tunnel barriers that perform as well as most inorganic barrier materials on LSMO.

Room temperature exchange bias and spin valves based on BiFeO3∕SrRuO3∕SrTiO3∕Si (001) heterostructures

Abstract: We report the growth and characterization of exchange bias and spin valve heterostructures based on the multiferroic antiferromagnet BiFeO3 on Si (001) substrates. Using Co0.9Fe0.1 as the ferromagnet, we demonstrate heterostructures with large negative exchange bias and negligible training (or a decrease in exchange bias field as a function of repeated magnetic cycling) at room temperature. We additionally report the manufacture of spin valve structures that have been found to have current in-plane magnetoresistance of over 2.25% at room temperature.

Origin of ferromagnetism in Zn 1-x Co x O from magnetization and spin-dependent magnetoresistance measurements

Abstract: In order to elucidate the nature of ferromagnetic signatures observed in (Zn,Co)O, we have examined experimentally and theoretically magnetic properties and spin-dependent quantum localization effects that control low-temperature magnetoresistance. Our findings, together with a through structural characterization, substantiate the model assigning spontaneous magnetization of (Zn,Co)O to uncompensated spins at the surface of antiferromagnetic nanocrystal of Co-rich wurtzite (Zn,Co)O. The model explains a large anisotropy observed in both magnetization and magnetoresistance in terms of spin Hamiltonian of Co ions in the crystal field of the wurtzite lattice.

Anisotropic magnetoresistance components in (Ga,Mn)As.

Abstract: We explore the basic physical origins of the noncrystalline and crystalline components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. The sign of the noncrystalline AMR is found to be determined by the form of spin-orbit coupling in the host band and by the relative strengths of the nonmagnetic and magnetic contributions to the Mn impurity potential. We develop experimental methods yielding directly the noncrystalline and crystalline AMR components which are then analyzed independently. We report the observation of an AMR dominated by a large uniaxial crystalline component and show that AMR can be modified by local strain relaxation. Generic implications of our findings for other dilute moment systems are discussed.