Showing papers on "Magnetoresistance published in 2008"

Tunnel magnetoresistance of 604% at 300K by suppression of Ta diffusion in CoFeB∕MgO∕CoFeB pseudo-spin-valves annealed at high temperature

Abstract: The authors observed tunnel magnetoresistance (TMR) ratio of 604% at 300K in Ta∕Co20Fe60B20∕MgO∕Co20Fe60B20∕Ta pseudo-spin-valve magnetic tunnel junction annealed at 525°C. To obtain high TMR ratio, it was found critical to anneal the structure at high temperature above 500°C, while suppressing the Ta diffusion into CoFeB electrodes and in particular to the CoFeB∕MgO interface. X-ray diffraction measurement of MgO on SiO2 or Co20Fe60B20 shows that an improvement of MgO barrier quality, in terms of the degree of the (001) orientation and stress relaxation, takes place at annealing temperatures above 450°C. The highest TMR ratio observed at 5K was 1144%.

Prediction of very large values of magnetoresistance in a graphene nanoribbon device.

Abstract: On the basis of first-principles computer simulations, theorists have predicted that zigzag graphene nanoribbons should display magnetoresistance values that are thousands of times higher than previously reported experimental values, and also should be able to generate highly spin-polarized currents.

Measurement of the spin-transfer-torque vector in magnetic tunnel junctions

Abstract: The transfer of spin angular momentum from a spin-polarized current to a ferromagnet can generate sufficient torque to reorient the magnet’s moment. This torque could enable the development of efficient electrically actuated magnetic memories and nanoscale microwave oscillators. Yet difficulties in making quantitative measurements of the spin-torque vector have hampered understanding. Here we present direct measurements of both the magnitude and direction of the spin torque in magnetic tunnel junctions, the type of device of primary interest for applications. At low bias V, the differential torque dτ/dV lies in the plane defined by the electrode magnetizations, and its magnitude is in excellent agreement with recent predictions for near-perfect spin-polarized tunnelling. We find that the strength of the in-plane differential torque remains almost constant with increasing bias, despite a substantial decrease in the device magnetoresistance, and that with bias the torque vector also rotates out of the plane.

Classical and quantum routes to linear magnetoresistance.

Abstract: The hallmark of materials science is the ability to tailor the microstructure of a given material to provide a desired response. Carbon mixed with iron provides the steel of buildings and bridges; impurities sprinkled in silicon single crystals form the raw materials of the electronics revolution; pinning centres in superconductors let them become powerful magnets. Here, we show that either adding a few parts per million of the proper chemical impurities to indium antimonide, a well-known semiconductor, or redesigning the material's structure on the micrometre scale, can transform its response to an applied magnetic field. The former approach is purely quantum mechanical; the latter a classical outgrowth of disorder, turned to advantage. In both cases, the magnetoresistive response-at the heart of magnetic sensor technology-can be converted to a simple, large and linear function of field that does not saturate. Harnessing the effects of disorder has the further advantage of extending the useful applications range of such a magnetic sensor to very high temperatures by circumventing the usual limitations imposed by phonon scattering.

Low field magnetotransport in manganites

Abstract: The perovskite manganites with generic formula RE1?xAExMnO3 (RE = rare earth, AE = Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is strong correlation between structure, transport and magnetic properties. They exhibit extraordinary large magnetoresistance named CMR in the vicinity of the insulator?metal/paramagnetic?ferromagnetic transition at relatively large applied magnetic fields. However, for applied aspects, occurrence of significant CMR at low applied magnetic fields would be required. This review consists of two sections: in the first section we have extensively reviewed the salient features, e.g.?structure, phase diagram, double-exchange mechanism, Jahn?Teller effect, different types of ordering and phase separation of CMR manganites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for various doped manganites having natural grain boundaries such as polycrystalline, nanocrystalline bulk and films, manganite-based composites and intrinsically layered manganites, and artificial grain boundaries such as bicrystal, step-edge and laser-patterned junctions. Some other potential magnetoresistive materials, e.g.?pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc, are also briefly dealt with. The review concludes with an overview of grain-boundary-induced low field magnetotransport behavior and prospects for possible applications.

Lower-current and fast switching of a perpendicular TMR for high speed and high density spin-transfer-torque MRAM

Abstract: We investigate extremely low programming current and fast switching time of a perpendicular tunnel-magnetoresistance (P-TMR) for spin-transfer torque using a P-TMR cell of 50 nm-diameter. A L10-crystalline ordered alloy is used as a free layer that has excellent thermal stability and a damping constant of about 0.03. The programming current of 49 uA and the switching time of 4 nsec are also demonstrated.

Spin transfer induced coherent microwave emission with large power from nanoscale MgO tunnel junctions

Abstract: In low resistance-area product MgO magnetic tunnel junction nanopillars, we observe high integrated power (up to 43nW), narrow linewidth (down to 10MHz), spin transfer induced microwave emission at frequencies up to 14GHz due to precession of the free layer magnetization at room temperature. Although all devices were fabricated on the same wafer, they present bimodal transport and precessional characteristics. The devices in which the narrowest linewidths were observed exhibited low resistance and tunneling magnetoresistance (30%), while maintaining large integrated power.

Inversion of Magnetoresistance in Organic Semiconductors

Abstract: We report that organic semiconductors such as alpha-sexithiophene (alpha-6T) have magnetoresistance (MR) with unexpected sign changes; depending on applied voltage, temperature, and layer thickness, the resistance may either increase or decrease upon application of a small magnetic field (<100 mT). We propose that MR and the inversion of MR are due to the role of hyperfine interaction in a magnetic field, as illustrated by the recombination-limited regime.

Tunneling magnetoresistance of magnetic tunnel junctions using perpendicular magnetization L10-CoPt electrodes

Abstract: Magnetic tunnel junctions (MTJs) using L10-ordered CoPt electrodes with perpendicular magnetic anisotropy were fabricated. Full-epitaxial CoPt∕MgO∕CoPt-MTJs were prepared onto single crystal MgO-(001) substrate by sputtering method. X-ray diffraction analyses revealed that both bottom and top CoPt electrodes were epitaxially grown with (001)-orientation. The L10-chemical order parameter of 0.82 was obtained for the bottom CoPt electrode deposited at substrate temperature of 600°C. The transport measurements with applying magnetic field perpendicular to the film plane showed a tunnel magnetoresistance ratio of 6% at room temperature and 13% at 10K.

High-pressure synthesis of orthorhombic SrIrO3 perovskite and its positive magnetoresistance

Abstract: The orthorhombic SrIrO3 perovskite was synthesized under 5GPa and 1000°C. It is paramagnetic below about 170K and transfers to an unknown magnetism under higher temperature. A band type metal to insulator transition caused by a pseudogap was observed at about 44K. Interestingly a positive magnetoresistance, i.e., resistance increased with applying magnetic field, was observed in the orthorhombic SrIrO3 perovskite below about 170K.

Method and system for testing P2 stiffness of a magnetoresistance transducer at the wafer level

Abstract: A method of testing P 2 stiffness of a magnetoresistance (MR) sensor stack including a P 2 pinned layer is provided. The method comprises the step of applying an external magnetic field to the MR sensor stack. The external magnetic field is oriented substantially perpendicular to a magnetic field of the P 2 pinned layer. The method further comprises varying an amplitude of the external magnetic field, measuring a change in a resistance of the MR sensor stack in response to the varying amplitude of the external magnetic field, and calculating the P 2 stiffness based on the measured change in resistance.

Co /Pt multilayer based magnetic tunnel junctions using perpendicular magnetic anisotropy

Abstract: Magnetic tunnel junctions that utilize perpendicular magnetic anisotropy have attracted growing attention due to their potential for higher storage densities in future high capacity magnetic memory applications. In this study, we present an experimental demonstration of magnetic tunnel junctions composed of perpendicularly magnetized Co∕Pt multilayer electrodes and an AlOx tunnel barrier. The emphasis has been on how to maximize the thickness of the Co layers adjacent to the tunnel barrier while still magnetized perpendicularly for possible spin torque utilization in future applications. It is found that the thickness ratio between the Co and Pt layers and the number of bilayers were significant parameters to customize the magnetic properties. The difference between the switching fields of the soft and the hard layers can be adjusted by the number of repeats of the Co∕Pt bilayers. The measured hysteresis shows virtually zero exchange coupling between the two layers through the tunnel barrier. Measured tunneling magnetoresistance ratio of the fabricated submicron-size tunnel junctions ranges from 10% to 15% at room temperature.

Theory of huge tunneling magnetoresistance in graphene

Abstract: We investigate theoretically the spin-independent tunneling magnetoresistance effect in a graphene monolayer modulated by two parallel ferromagnets deposited on a dielectric layer. For the parallel magnetization configuration, Klein tunneling can be observed in the transmission spectrum but at specific oblique incident angles. For the antiparallel magnetization configuration, the transmission can be blocked by the magnetic-electric barrier provided by the ferromagnets. Such a transmission discrepancy results in a tremendous magnetoresistance ratio and can be tuned by the inclusion of an electric barrier.

Low temperature tunneling magnetoresistance on (La,Sr)MnO3∕Co junctions with organic spacer layers

Abstract: This paper concerns with giant magnetoresistance (MR) effects in organic spin valves, which are realized as layered (La,Sr)MnO3 (LSMO)-based junctions with tris-(8, hydroxyquinoline) aluminum (Alq3)-spacer and ferromagnetic top layers. The experimental work was focused on the understanding of the transport behavior in this type of magnetic switching elements. The device preparation was carried out in an ultrahigh vacuum chamber equipped with a mask changer by evaporation and sputtering on SrTiO3 substrates with LSMO stripes deposited by pulsed laser technique. The field and temperature dependences of the MR of the prepared elements are studied. Spin-valve effects at 4.2K have been observed in a broad resistance interval from 50Ω to MΩ range, however, without systematic dependence on spacer layer thickness and device area. In some samples, the MR changes sign as a function of the bias voltage. The observed similarity in the bias voltages dependences of the MR in comparison with conventional magnetic tunnel junctions with oxide barriers suggests a description of the found effects within the classical tunneling concept. This assumption is also confirmed by a similar switching behavior observed on ferromagnetically contacted carbon nanotube devices. The proposed model implies the realization of the transport via local Co chains embedded in the Alq3 layer and spin dependent tunneling over barriers at the interface Co grains∕Alq3∕LSMO. The existence of conducting Co chains within the organics is supported by transmission electron microscopic∕electron energy loss spectroscopic studies on cross-sectional samples from analogous layer stacks.

Spin polarization of amorphous CoFeB determined by point-contact Andreev reflection

Abstract: Point-contact Andreev reflection measurements reveal that amorphous CoxFe80−xB20 (x=20, 40, and 60) alloys possess spin polarization of as much as 65%, much higher than the values of 43%–45% for Co and Fe. This accounts for the high magnetoresistance values in magnetic tunnel junctions incorporating amorphous CoFeB as the ferromagnetic electrodes. The crystallization of the amorphous alloys substantially reduces the spin polarization.

Enhanced annealing, high Curie temperature and low-voltage gating in (Ga,Mn)As: A surface oxide control study

Abstract: Our x-ray photoemission, magnetization, and transport studies on surface-etched and annealed (Ga,Mn)As epilayers elucidate the key role of the surface oxide in controlling the outdiffusion of self-compensating interstitial Mn impurities. We achieved a dramatic reduction in annealing times necessary to optimize the epilayers after growth and synthesized (Ga,Mn)As films with the Curie temperature reaching 180 K. A $p\text{\ensuremath{-}}n$ junction transistor is introduced, allowing for a large hole depletion in (Ga,Mn)As thin films at a few volts. The surface oxide etching procedure is applied to controllably reduce the thickness of the (Ga,Mn)As layer in the transistor and we observe a further strong enhancement of the field-effect on the channel resistance. The utility of our all-semiconductor ferromagnetic field-effect transistor in spintronic research is demonstrated on the measured large field effect on the anisotropic magnetoresistance.

Perpendicular magnetic tunnel junction with tunneling magnetoresistance ratio of 64% using MgO (100) barrier layer prepared at room temperature

Abstract: MgO (100) textured films can be prepared by reactive facing targets sputtering at room temperature without postdeposition annealing process when they were deposited on (100) oriented Fe buffer layers. This method allows fabrication of perpendicular magnetic tunnel junction (p-MTJ) with MgO (100) tunneling barrier layer and rare-earth transition metal (RE-TM) alloy thin films as perpendicularly magnetized free and pinned layers. The 3-nm-thick MgO tunneling barrier layer in p-MTJ multilayer prepared on glass substrate revealed (100) crystalline orientation. Extraordinary Hall effect measurement clarified that the perpendicular magnetic components of 3-nm-thick Fe buffer layers on the two ends of MgO tunneling barrier layer were increased by exchange coupling with RE-TM alloy layers. The RA of 35kΩμm2 and tunneling magnetoresistance ratio of 64% was observed in the multilayered p-MTJ element by current-in-plane-tunneling.

GaMnAs-based hybrid multiferroic memory device

Abstract: We report a nonvolatile hybrid multiferroic memory cell with electrostatic control of magnetization based on strain-coupled GaMnAs ferromagnetic semiconductor and a piezoelectric material. We use the crystalline anisotropy of GaMnAs to store information in the orientation of the magnetization along one of the two easy axes, which is monitored via transverse anisotropic magnetoresistance. The magnetization orientation is switched by applying voltage to the piezoelectric material and tuning magnetic anisotropy of GaMnAs via the resulting stress field.

Voltage control of magnetocrystalline anisotropy in ferromagnetic-semiconductor-piezoelectric hybrid structures

Abstract: We demonstrate voltage control of the magnetic anisotropy of a (Ga,Mn)As device bonded to a piezoelectric transducer. The application of a uniaxial strain leads to a large reorientation of the magnetic easy axis, which is detected by anisotropic magnetoresistance measurements. Calculations based on the mean-field kinetic-exchange model of (Ga,Mn)As provide a microscopic understanding of the measured effect. The reported smooth voltage control of the uniaxial in-plane anisotropy, electrically induced magnetization switching, and detection of unconventional crystalline components of the anisotropic magnetoresistance illustrate the generic utility of our multiferroic system in providing device functionalities and in the research of micromagnetic and magnetotransport phenomena in diluted magnetic semiconductors.

Magnetic field-effects in bipolar, almost hole-only and almost electron-only tris-(8-hydroxyquinoline) aluminum devices

Abstract: We present magnetoconductivity and magnetoluminescence measurements in sandwich devices made from the $\ensuremath{\pi}$-conjugated molecule tris-(8-hydroxyquinoline) aluminum $({\text{Alq}}_{3})$ and demonstrate effects of more than 25% and 50% magnitude, respectively. These effects are known to be caused by hyperfine coupling in pairs of paramagnetic species, and it is often assumed that these are electron-hole pairs. However, we show that the very large magnitude of the effect contradicts present knowledge of the electron-hole pair recombination processes in electroluminescent $\ensuremath{\pi}$-conjugated molecules and that the effect persists even in almost hole-only devices.

Upper critical field, Hall effect and magnetoresistance in the iron-based layered superconductor LaFeAsO0.9F0.1−δ

Abstract: By using a two-step method, we successfully synthesized the new iron-based superconductor LaFeAsO0.9F0.1−δ. The resistive transition curves under different magnetic fields were measured, leading to the determination of the upper critical field Hc2(T) of this new superconductor. The value of Hc2 at zero temperature is estimated to be about 50 T roughly. In addition, the Hall effect and magnetoresistance were measured in a wide temperature region. A negative Hall coefficient RH has been found, implying a dominant conduction mainly by electron-like charge carriers in this material. The charge carrier density determined at 100 K is about 9.8 × 1020 cm−3, which is close to the cuprate superconductors. It is further found that the magnetoresistance does not follow Kohler's law. Meanwhile, the different temperature-dependent behaviors of resistivity, Hall coefficient and magnetoresistance have anomalous properties at about 230 K, which may be induced by some exotic scattering mechanism.

Tunneling anisotropic magnetoresistance in multilayer-(Co/Pt)/AlOx/Pt structures

Abstract: We report observations of tunneling anisotropic magnetoresitance (TAMR) in vertical tunnel devices with a ferromagnetic multilayer-(Co/Pt) electrode and a non-magnetic Pt counter-electrode separated by an AlOx barrier In stacks with the ferromagnetic electrode terminated by a Co film the TAMR magnitude saturates at 015% beyond which it shows only weak dependence on the magnetic field strength, bias voltage, and temperature For ferromagnetic electrodes terminated by two monolayers of Pt we observe order(s) of magnitude enhancement of the TAMR and a strong dependence on field, temperature and bias Discussion of experiments is based on relativistic ab initio calculations of magnetization orientation dependent densities of states of Co and Co/Pt model systems Anisotropic magnetoresistance (AMR) sensors replaced in the early 1990s classical magneto-inductive coils in hard-drive readheads launching the era of spintronics Their utility has, however, remained limited partly because the response of these ferromagnetic resistors to changes in magnetization orientation originates from generically subtle spin-orbit (SO) interaction effects [1] Currently widely used giant magnetoresistance [2] and tunneling magnetoresistance (TMR) [3] elements comprising (at least) two magnetically decoupled ferromagnetic layers provided a remarkably elegant way of tying the magnetoresistance response directly to the ferromagnetic exchange splitting of the carrier bands without involving SO-coupling Large magnetoresistances in these devices are, nevertheless, obtained at the expense of a significantly increased structure complexity, necessary to guarantee independent and different magnetization switching characteristics and spin-coherence of transport between the ferromagnetic layers Studies of AMR effects [4, 5, 6, 7] in ferromagnetic semiconductor tunneling devices showed that AMR response can in principle be huge and richer than TMR, with the magnitude and sign of the magnetoresistance dependent on the magnetic field orientation and electric fields Subsequent theoretical work predicted [8] that the tunneling AMR (TAMR) effect is generic in ferromagnets with SO-coupling, including the high Curie temperature transition metal systems A detailed investigation of the TAMR is therefore motivated both by its intricate relativistic quantum transport nature and by its potential in more versatile alternatives to current TMR devices which will not require two independently controlled ferromagnetic electrodes and spin-coherent tunneling Experimental demonstration of the TAMR in a tunnel junction with a ferromagnetic metal electrode has recently been reported [9] in an epitaxial Fe/GaAs/Au stack The observed TAMR in this structure is relatively small, bellow 05%, consistent with the weak SO-coupling in Fe In this paper we present a study of vertical tunnel devices in which the ferromagnetic electrode comprises alternating Co and Pt films We build upon the extensive literature [10, 11, 12, 13, 14, 15] on ferromagnetic/heavyelement transition metal multilayers in which large and tunable magnetic anisotropies are generated at the interfaces by combined effects of induced moments and strong SO-coupling We show that placing these interfaces adjacent to the tunnel barrier opens a viable route to highly sensitive metal TAMR structures

Tunable lateral displacement and spin beam splitter for ballistic electrons in two-dimensional magnetic-electric nanostructures

Abstract: We investigate the lateral displacements for ballistic electron beams in a two-dimensional electron gas modulated by metallic ferromagnetic (FM) stripes with parallel and antiparallel (AP) magnetization configurations. It is shown that the displacements are negative as well as positive, which can be controlled by adjusting the electric potential induced by the applied voltage and the magnetic field strength of FM stripes. Based on these phenomena, we propose an efficient way to realize a spin beam splitter, which can completely separate spin-up and spin-down electron beams in the AP configuration by their corresponding spatial positions.

Magnetoconductance oscillations in graphene antidot arrays

Abstract: Epitaxial graphene films have been formed on the C-face of semi-insulating 4H-SiC substrates by a high temperature sublimation process. Nanoscale square antidot arrays have been fabricated on these graphene films. At low temperatures, magnetoconductance in these films exhibits pronounced Aharonov–Bohm oscillations with the period corresponding to magnetic flux quanta added to the area of a single antidot. At low fields, weak localization is observed and its visibility is enhanced by intervalley scattering on antidot edges. At high fields, we observe two distinctive minima in magnetoconductance, which can be attributed to commensurability oscillations between classical cyclotron orbits and antidot array. All mesoscopic features, surviving up to 70K, reveal the unique electronic properties of graphene.

Sizable room-temperature magnetoresistance in cobalt based magnetic tunnel junctions with out-of-plane anisotropy

Abstract: Submicron alumina based magnetic tunnel junctions (MTJs) using electrodes with out-of-plane magnetic anisotropy were prepared and characterized. Both electrodes are industry-compatible Co∕Pt multilayers. The magnetic properties of the unpatterned samples have been investigated through superconducting quantum interference device (SQUID) magnetometry and extraordinary Hall effect: both electrodes have fully out-of-plane magnetic moments and nonoverlapping coercive fields. Transport measurements on the submicron MTJs showed a magnetoresistance (MR) ratio reaching 8% at room temperature. Nanopillars with diameters of 800, 400, and 200nm patterned from the same wafer show the expected out-of-plane magnetic properties and similar resistance×area products (RA) and MR ratios. The I(V) characteristics of pillars with diameters of 800 and 400nm could be accounted for with reasonable barrier heights and widths.

Cationic Ordering and Microstructural Effects in the Ferromagnetic Perovskite La0.5Ba0.5CoO3: Impact upon Magnetotransport Properties

Abstract: The synthesis and structural study of the stoichiometric perovskite La0.5Ba0.5CoO3 have allowed three forms to be isolated. Besides the disordered La0.5Ba0.5CoO3 and the perfectly ordered layered LaBaCo2O6, a third form called nanoscale-ordered LaBaCo2O6 is obtained. As evidenced by transmission electron microscopy investigations, the latter consists of 112-type 90° oriented domains fitted into each other at a nanometer scale which induce large strains and consequently local atomic scale lattice distortions. These three ferromagnetic perovskites exhibit practically the same TC ≅ 174–179 K, but differently from the other phases, the nanoscale-ordered LaBaCo2O6 is a hard ferromagnet, with HC ≈ 4.2 kOe, due to the strains which may pin domain walls, preventing the reversal of the spins in a magnetic field. The magnetotransport properties of these phases show that all of them exhibit a maximum intrinsic magnetoresistance, close to 6–7% around TC under ±70 kOe, but that the ordered phase exhibits a much higher...

All-Metal Current-Perpendicular-to-Plane Giant Magnetoresistance Sensors for Narrow-Track Magnetic Recording

Abstract: Read heads using current-perpendicular-to-plane (CPP) giant magnetoresistance sensors have been fabricated and tested under high-density recording conditions. A magnetoresistance of 5.5% and shield-to-shield spacing of 45 nm have been achieved by using an all-metal single-spin-valve with Heusler-alloy-based free and reference magnetic layers. Read heads with magnetic read widths ~45 nm were tested on perpendicular media, resulting in signals above 1 mV and signal-to-noise ratio ~30 dB. Linear densities in excess of 1050 kbpi were achieved with thermal fly-height control, compatible with recording areal densities of ~400 Gb/in2. Current-induced spin-torque effects in the recording head were observed to result in rapid performance degradation above a threshold bias voltage of about 75 mV, corresponding to current densities >108 A/cm2.