Showing papers on "Magnetoresistance published in 2012"

Exchange bias in LaNiO3-LaMnO3 superlattices.

Abstract: Interfaces between insulating oxides have revealed exotic electronic and magnetic properties. It is now shown that a complex magnetic structure can emerge in an oxide superlattice, and that specific interfaces can unexpectedly exhibit exchange bias. The observations reveal the induction of antiferromagnetism in a material that is usually paramagnetic.

The Physics of Ferromagnetism

Abstract: Part I: Foundation of magnetism.- Basis of magnetism.- Magnetism of atoms.- Magnetism of solids.- Exchange interaction.- Magnetic anisotropy.- Magnetostrictive effects.- Magnetic domain.- Micromagnetism.- Part II: Magnetic materials.- Soft magnetism.- Hard magnetism.- Part III: Spintronics.- Magnetoresistance effect.- Tunnel magnetoresistance effect.- Magnetoresistive random access memory (MRAM).- Technologies that accompany the development of spintronics devices.

Room temperature giant and linear magnetoresistance in topological insulator Bi2Te3 nanosheets

Abstract: Topological insulators, a new class of condensed matter having bulk insulating states and gapless metallic surface states, have demonstrated fascinating quantum effects. However, the potential practical applications of the topological insulators are still under exploration worldwide. We demonstrate that nanosheets of a ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ topological insulator several quintuple layers thick display giant and linear magnetoresistance. The giant and linear magnetoresistance achieved is as high as over 600% at room temperature, with a trend towards further increase at higher temperatures, as well as being weakly temperature-dependent and linear with the field, without any sign of saturation at measured fields up to 13 T. Furthermore, we observed a magnetic field induced gap below 10 K. The observation of giant and linear magnetoresistance paves the way for 3D topological insulators to be useful for practical applications in magnetoelectronic sensors such as disk reading heads, mechatronics, and other multifunctional electromagnetic applications.

Giant tunneling magnetoresistance in epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions by half-metallicity of Co2MnSi and coherent tunneling

Abstract: Giant tunnel magnetoresistance (TMR) ratios of up to 1995% at 4.2 K and up to 354% at 290 K were obtained for epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions (MTJs) featuring a reduced lattice mismatch in the MTJ trilayer by introducing a thin Co2MnSi lower electrode deposited on a Co50Fe50 buffer layer. The obtained giant TMR ratios can be explained by the enhanced contribution of coherent tunneling originating from the increased misfit dislocation spacing at the lower and upper interfaces with a MgO barrier along with the half-metallicity of Co2MnSi electrodes.

Quantized Hall effect and Shubnikov-de Haas oscillations in highly doped Bi2Se3: evidence for layered transport of bulk carriers.

Abstract: Bi2Se3 is an important semiconductor thermoelectric material and a prototype topological insulator. Here we report observation of Shubnikov-de Hass oscillations accompanied by quantized Hall resistances (R(xy)) in highly doped n-type Bi2Se3 with bulk carrier concentrations of few 10(19) cm(-3). Measurements under tilted magnetic fields show that the magnetotransport is 2D-like, where only the c-axis component of the magnetic field controls the Landau level formation. The quantized step size in 1/R(xy) is found to scale with the sample thickness, and average ~e(2)/h per quintuple layer. We show that the observed magnetotransport features do not come from the sample surface, but arise from the bulk of the sample acting as many parallel 2D electron systems to give a multilayered quantum Hall effect. In addition to revealing a new electronic property of Bi2Se3, our finding also has important implications for electronic transport studies of topological insulator materials.

Room-temperature perpendicular exchange coupling and tunneling anisotropic magnetoresistance in an antiferromagnet-based tunnel junction.

Abstract: We investigate the exchange coupling between perpendicular anisotropy (PMA) Co/Pt and IrMn in-plane antiferromagnets (AFMs), as well as tunneling anisotropic magnetoresistance (TAMR) in [Pt/Co]/IrMn/AlO_{x}/Pt tunnel junctions, where Co/Pt magnetization drives rotation of AFM moments with the formation of exchange-spring twisting. When coupled with a PMA ferromagnet, the AFM moments partially rotate with out-of-plane magnetic fields, in contrast with being pinned along the easy direction of IrMn for in-plane fields. Because of the superior thermal tolerance of perpendicular exchange coupling and the stability of moments in ~6 nm-thick IrMn, TAMR gets significantly enhanced up to room temperature. Their use would advance the process towards practical AFM spintronics.

Self-Powered Magnetic Sensor Based on a Triboelectric Nanogenerator

Abstract: Magnetic sensors are usually based on the Hall effect or a magnetoresistive sensing mechanism. Here we demonstrate that a nanogenerator can serve as a sensor for detecting the variation of the time-dependent magnetic field. The output voltage of the sensor was found to exponentially increase with increasing magnetic field. The detection sensitivities for the change and the changing rate of magnetic field are about 0.0363 ± 0.0004 ln(mV)/G and 0.0497 ± 0.0006 ln(mV)/(G/s), respectively. The response time and reset time of the sensor are about 0.13 and 0.34 s, respectively. The fabricated sensor has a detection resolution of about 3 G and can work under low frequencies (<0.4 Hz).

Linear magnetoresistance in topological insulators: Quantum phase coherence effects at high temperatures

Abstract: In addition to the weak antilocalization cusp observed in the magnetoresistance (MR) of topological insulators at low temperatures and low magnetic fields, we find that the high-field MR in Bi2Te2Se is linear in field. At fields up to B=14T the slope of this linear-like MR is nearly independent of temperature over the range T=7 to 150K. We find that the linear MR arises from the competition between a logarithmic phase coherence component and a quadratic component. The quantum phase coherence dominates up to high temperatures, where the coherence length remains longer than the mean free path of electrons.

Weak localization and antilocalization in topological insulator thin films with coherent bulk-surface coupling

Abstract: We evaluate quantum corrections to conductivity in an electrically gated thin film of a three-dimensional topological insulator. We derive approximate analytical expressions for the low-field magnetoresistance as a function of bulk doping and bulk-surface tunneling rate. Our results reveal parameter regimes for both weak localization and weak antilocalization, and include diffusive Weyl semimetals as a special case.

Extensive study of giant magnetoresistance properties in half-metallic Co2(Fe,Mn)Si-based devices

Abstract: Fully epitaxial Co2FexMn1−xSi(CFMS)/Ag/Co2FexMn1−xSi current-perpendicular-to-plane giant magnetoresistive devices with various Fe/Mn ratios x and top CFMS layer thicknesses tCFMS were prepared. The highest magnetoresistance (MR) ratios, 58% at room temperature and 184% at 30 K, were observed in the sample with x = 0.4 and tCFMS = 3 nm. Enhancement of interface spin-asymmetry was suggested for x = 0.4 compared with that at x = 0. A MR ratio of 58% was also observed even in a very thin trilayer structure, CFMS(4 nm)/Ag(3 nm)/CFMS(2 nm), which is promising for a next-generation magnetic read sensor for high-density hard disk drives.

Ferromagnetism in thin-film Cr-doped topological insulator Bi2Se3

Abstract: We report on the observation of ferromagnetism in epitaxial thin films of the topological insulator compound Bi2Se3 with chromium doping. The structural, magnetic, and magnetoelectrical properties of Bi2Se3 were investigated for Cr concentrations up to 10%. For a Cr content up to ∼5% the films are of good crystalline quality, with the lattice parameter a decreasing and the lattice parameter c increasing with increasing Cr concentration. The Curie temperature reached a maximum TC = 20 K for 5.2% Cr. Well-defined ferromagnetic hysteresis in the magnetization and in the magnetoresistance was also observed in these films.

Interplay between ferromagnetism, surface states, and quantum corrections in a magnetically doped topological insulator

Abstract: The breaking of time-reversal symmetry by ferromagnetism is predicted to yield profound changes to the electronic surface states of a topological insulator. Here, we report on a concerted set of structural, magnetic, electrical, and spectroscopic measurements of Mn-Bi${}_{2}$Se${}_{3}$ thin films wherein photoemission and x-ray magnetic circular dichroism studies have recently shown surface ferromagnetism in the temperature range $15\phantom{\rule{4.pt}{0ex}}\text{K}\ensuremath{\le}T\ensuremath{\le}100$ K, accompanied by a suppressed density of surface states at the Dirac point. Secondary-ion mass spectroscopy and scanning tunneling microscopy reveal an inhomogeneous distribution of Mn atoms, with a tendency to segregate towards the sample surface. Magnetometry and anisotropic magnetoresistance measurements are insensitive to the high-temperature ferromagnetism seen in surface studies, revealing instead a low-temperature ferromagnetic phase at $T\ensuremath{\lesssim}5$ K. The absence of both a magneto-optical Kerr effect and an anomalous Hall effect suggests that this low-temperature ferromagnetism is unlikely to be a homogeneous bulk phase but likely originates in nanoscale near-surface regions of the bulk where magnetic atoms segregate during sample growth. Although the samples are not ideal, with both bulk and surface contributions to electron transport, we measure a magnetoconductance whose behavior is qualitatively consistent with predictions that the opening of a gap in the Dirac spectrum drives quantum corrections to the conductance in topological insulators from the symplectic to the orthogonal class.

Spontaneously gapped ground state in suspended bilayer graphene.

Abstract: Bilayer graphene bears an eightfold degeneracy due to spin, valley, and layer symmetry, allowing for a wealth of broken symmetry states induced by magnetic or electric fields, by strain, or even spontaneously by interaction. We study the electrical transport in clean current annealed suspended bilayer graphene. We find two kinds of devices. In bilayers of type $B1$ the eightfold zero-energy Landau level is partially lifted above a threshold field revealing an insulating $\ensuremath{ u}=0$ quantum-Hall state at the charge neutrality point. In bilayers of type $B2$ the Landau level lifting is full and a gap appears in the differential conductance even at zero magnetic field, suggesting an insulating spontaneously broken symmetry state. Unlike $B1$, the minimum conductance in $B2$ is not exponentially suppressed, but remains finite with a value $G\ensuremath{\lesssim}{e}^{2}/h$ even in a large magnetic field. We suggest that this phase of $B2$ is insulating in the bulk and bound by compressible edge states.

High-field linear magneto-resistance in topological insulator Bi2Se3 thin films

Abstract: Linear magneto-resistance is observed in high magnetic field in topological insulator Bi2Se3 films. As revealed by tilted magnetic field measurement, this linear magneto-resistance is associated with the gapless topological surface states and of quantum origin. In the ultra-thin limit, the inter-surface tunneling induced surface state gap opening quenches the linear magneto-resistance. Instead, weak negative magneto-resistance is observed in high magnetic fields in ultra-thin films.

Giant Resistance Change across the Phase Transition in Spin-Crossover Molecules

Abstract: The electronic origin of a large resistance change in nanoscale junctions incorporating spin-crossover molecules is demonstrated theoretically by using a combination of density functional theory and the nonequilibrium Green's function method for quantum transport. At the spin-crossover phase transition, there is a drastic change in the electronic gap between the frontier molecular orbitals. As a consequence, when the molecule is incorporated in a two-terminal device, the current increases by up to 4 orders of magnitude in response to the spin change. This is equivalent to a magnetoresistance effect in excess of 3000%. Since the typical phase transition critical temperature for spin-crossover compounds can be extended to well above room temperature, spin-crossover molecules appear as the ideal candidate for implementing spin devices at the molecular level.

Design of a Voltage‐Controlled Magnetic Random Access Memory Based on Anisotropic Magnetoresistance in a Single Magnetic Layer

Abstract: A simple and fully gate-voltage-controlled magnetic random access memory is designed based on anisotropic magnetoresistance. This multiferroic memory device consists of just a single magnetic film grown on a ferroelectric layer with bistable in-plane anisotropic ferroelastic or piezo strains induced by out-of-plane voltages. It can simultaneously achieve ultrahigh storage density, ultralow energy consumption, and GHz high-speed operation at room temperature.

Enhancement of perpendicular magnetic anisotropy in FeB free layers using a thin MgO cap layer

Abstract: We prepared magnetic tunnel junction films with PtMn/CoFe/Ru/CoFeB/MgO tunnel barrier/FeB free layer/MgO cap layer/Ta multilayers using sputtering and measured magnetic and magnetoresistive properties of the films at room temperature. The magnetization curves of the FeB plane film measured under perpendicular-to-plane magnetic fields showed much smaller saturation fields (Hs) than those expected from the demagnetizing field. Hs decreased from 4 to 0.4 kOe with increasing MgO cap layer thickness. The small Hs is due to the perpendicular magnetic anisotropy (PMA) induced at both MgO barrier–FeB and FeB–MgO cap interfaces. After microfabrication, the small free layer cells having a 1.6 nm thick MgO cap layer showed a magnetization easy axis in the perpendicular-to-plane direction. By inducing PMA from both upper and lower interfaces, we can stabilize the magnetization of the relatively thick (2 nm) FeB free layer in the perpendicular-to-plane direction.

Magneto-resistance up to 60 Tesla in topological insulator Bi2Te3 thin films

Abstract: We report magneto-transport studies of topological insulator Bi2Te3 thin films grown by pulsed laser deposition. A non-saturating linear-like magneto-resistance (MR) is observed at low temperatures in the magnetic field range from a few Tesla up to 60 Tesla. We demonstrate that the strong linear-like MR at high field can be well understood as the weak antilocalization phenomena described by Hikami-Larkin-Nagaoka theory. Our analysis suggests that in our system, a topological insulator, the elastic scattering time can be longer than the spin-orbit scattering time. We briefly discuss our results in the context of Dirac Fermion physics and “quantum linear magnetoresistance.”

Weak antilocalization and conductance fluctuation in a submicrometer-sized wire of epitaxial Bi 2 Se 3

Abstract: In this paper, we address the phase-coherent transport in a submicrometer-sized Hall bar made of epitaxial Bi${}_{2}$Se${}_{3}$ thin film by probing the weak antilocalization (WAL) and the magnetoresistance fluctuation below 22 K. The WAL effect is well described by the Hikami-Larkin-Nagaoka model, where the temperature dependence of the coherence length indicates that electron conduction occurs quasi-one-dimensionally in the narrow Hall bar. The temperature-dependent magnetoresistance fluctuation is analyzed in terms of the universal conductance fluctuation, which gives a coherence length consistent with that derived from the WAL effect.

The extrinsic origin of the magnetodielectric effect in the double perovskite La2NiMnO6.

Abstract: A La2NiMnO6 polycrystalline sample prepared by the sol–gel method showed monoclinic crystal structure with the P21/n space group and a saturation magnetization of 4.63 μB/f.u. at 5 K. Impedance spectroscopy results in the temperature range of 10 K < T < 300 K have revealed a distinct conduction process at grains and grain boundaries, where the grains followed the variable range hopping mechanism and the grain boundaries obeyed Arrhenius thermal activation. A negative magnetoresistance of 2.5% was observed at the paramagnetic to ferromagnetic transition, and this became temperature independent below the magnetic ordering. A marginal positive magnetodielectric (MD) effect that followed the dielectric relaxation was observed and its magnitude was found to decrease with increase of the frequency. A systematic study on the magnetic field induced dielectric properties, dc transport and dc bias effect on the dielectric permittivity has revealed the extrinsic origin of the MD effect in the bulk sample of La2NiMnO6.

Gate-controlled linear magnetoresistance in thin Bi2Se3 sheets

Abstract: We explore the emergence of linear magnetoresistance in thin Bi2Se3 sheets upon tuning the carrier density using a back gate. With increasingly negative gate voltage, a pronounced magnetoresistance of ∼100% is observed, while the associated B-field dependence changes from quadratic to linear. Concomitantly, the resistance-versus-temperature curves evolve from metallic to semiconductor-like, and increasingly strong weak anti-localization behavior is manifested. Analysis of the magnetoresistance data reveals two contributions, namely from the bulk conduction band and from a state inside the bulk gap. The latter is responsible for the linear magnetoresistance and likely represents the topologically protected surface state.

Magneto-resistance up to 60 Tesla in Topological Insulator Bi2Te3 Thin Films

Abstract: We report magneto-transport studies of topological insulator Bi_{2}Te_{3} thin films grown by pulsed laser deposition. A non-saturating linear-like magneto-resistance (MR) is observed at low temperatures in the magnetic field range from a few Tesla up to 60 Tesla. We demonstrate that the strong linear-like MR at high field can be well understood as the weak antilocalization phenomena described by Hikami-Larkin-Nagaoka theory. Our analysis suggests that in our system, a topological insulator, the elastic scattering time can be longer than the spin-orbit scattering time. We briefly discuss our results in the context of Dirac Fermion physics and 'quantum linear magnetoresistance'.

Large magnetoresistance through a single molecule due to a spin-split hybridized orbital.

Abstract: Using organic materials in spintronic devices raises a lot of expectation for future applications due to their flexibility, low cost, long spin lifetime, and easy functionalization. However, the interfacial hybridization and spin polarization between the organic layer and the ferromagnetic electrodes still has to be understood at the molecular scale. Coupling state-of-the-art spin-polarized scanning tunneling spectroscopy and spin-resolved ab initio calculations, we give the first experimental evidence of the spin splitting of a molecular orbital on a single non magnetic C(60) molecule in contact with a magnetic material, namely, the Cr(001) surface. This hybridized molecular state is responsible for an inversion of sign of the tunneling magnetoresistance depending on energy. This result opens the way to spin filtering through molecular orbitals.

Highly tunable electron transport in epitaxial topological insulator (Bi1-xSbx)2Te3 thin films

Abstract: Atomically smooth, single crystalline (Bi1−xSbx)2Te3 films have been grown on SrTiO3(111) substrates by molecular beam epitaxy. A full range of Sb-Bi compositions have been studied in order to obtain the lowest possible bulk conductivity. For the samples with optimized Sb compositions (x=0.5±0.1), the carrier type can be tuned from n-type to p-type across the whole thickness with the help of a back-gate. Linear magnetoresistance has been observed at gate voltages close to the maximum in the longitudinal resistance of a (Bi0.5Sb0.5)2Te3 sample. These highly tunable (Bi1−xSbx)2Te3 thin films provide an excellent platform to explore the intrinsic transport properties of the three-dimensional topological insulators.

Weak antilocalization and electron-electron interaction effects in Cu-doped Bi 2 Se 3 films

Abstract: We investigate the low-temperature transport properties in Cu${}_{x}$Bi${}_{2\ensuremath{-}x}$Se${}_{3}$ films prepared by a hot-wall-epitaxy growth of Bi${}_{2}$Se${}_{3}$ layers on Cu-deposited substrates. We observe a positive magnetoresistance due to the weak antilocalization effect and a classical magnetoresistance that exhibits a power-law dependence on the magnetic field. The resistance increases logarithmically with lowering temperature regardless of the strength of the magnetic field. The electron-electron interaction effect is thus evidenced to be strong. While the magnitude of the weak antilocalization effect is in reasonable agreement with theory, the correction to the conductivity due to the electron-electron interaction effect is unaccountably larger than the theoretical prediction. The discrepancy may indicate that the contribution from the bulk state is as large as that from the surface states, at least, for the interaction effect.

The multi-step tunneling analogue of conductivity mismatch in organic spin valves

Abstract: Carbon-based, molecular semiconductors offer several attractive attributes for spintronics, such as exceptionally weak spin-orbit coupling and compatibility with bottom-up nanofabrication. In spite of the promising properties of organic spin valves, however, the physical mechanisms governing spin-polarized conduction remain poorly understood. An experimental study of C60-based spin valves is presented and their behavior is modeled with spin-polarized tunneling via multiple intermediate states with a Gaussian energy distribution. It is shown that, analogous to conductivity mismatch in the diffusive regime, the magnetoresistance decreases with the number of intermediate tunnel steps, regardless of the value of the spin lifetime. This mechanism has been largely overlooked in previous studies of organic spin valves. In addition, using measurements of the temperature and bias dependence of the magnetoresistance, inhomogeneous magnetostatic fields resulting from interfacial roughness are identified as a source for spin relaxation and dephasing. These findings constitute a comprehensive understanding of the processes underlying spin-polarized transport in these structures and shed new light on previous studies of organic spin valves.

Robust absolute magnetometry with organic thin-film devices

Abstract: Magnetic field sensors based on organic thin-film materials have attracted considerable interest in recent years as they can be manufactured at very low cost and on flexible substrates. However, the technological relevance of such magnetoresistive sensors is limited owing to their narrow magnetic field ranges (∼30 mT) and the continuous calibration required to compensate temperature fluctuations and material degradation. Conversely, magnetic resonance (MR)-based sensors, which utilize fundamental physical relationships for extremely precise measurements of fields, are usually large and expensive. Here we demonstrate an organic magnetic resonance-based magnetometer, employing spin-dependent electronic transitions in an organic diode, which combines the low-cost thin-film fabrication and integration properties of organic electronics with the precision of a MR-based sensor. We show that the device never requires calibration, operates over large temperature and magnetic field ranges, is robust against materials degradation and allows for absolute sensitivities of <50 nT Hz(-1/2).

Magnetoimpedance spectroscopy of epitaxial multiferroic thin films

Abstract: The detection of true magnetocapacitance (MC) as a manifestation of magnetoelectric coupling (MEC) in multiferroic materials is a nontrivial task, because pure magnetoresistance (MR) of an extrinsic Maxwell-Wagner-type dielectric relaxation can lead to changes in capacitance [G. Catalan, Appl. Phys. Lett. 88, 102902 (2006)]. In order to clarify such difficulties involved with dielectric spectroscopy on multiferroic materials, we have simulated the dielectric permittivity e' of two dielectric relaxations in terms of a series of one intrinsic film-type and one extrinsic Maxwell-Wagner-type relaxation. Such a series of two relaxations was represented in the frequency- (f -) and temperature- (T -) dependent notations e' vs f and e' vs T by a circuit model consisting in a series of two ideal resistor-capacitor (RC) elements. Such simulations enabled rationalizing experimental f -, T-, and magnetic field- (H-) dependent dielectric spectroscopy data from multiferroic epitaxial thin films of BiMnO3 (BMO) and BiFeO3 (BFO) grown on Nb-doped SrTiO3. Concomitantly, the deconvolution of intrinsic film and extrinsic Maxwell-Wagner relaxations in BMO and BFO films was achieved by fitting f -dependent dielectric data to an adequate equivalent circuit model. Analysis of the H-dependent data in the form of determining the H-dependent values of the equivalent circuit resistors and capacitors then yielded the deconvoluted MC and MR values for the separated intrinsic dielectric relaxations in BMO and BFO thin films. Substantial intrinsic MR effects up to 65% in BMO films below the magnetic transition (TC ≈ 100 K) and perceptible intrinsic MEC up to −1.5% near TC were identified unambiguously.

Spin glass behaviour and extrinsic origin of magnetodielectric effect in non-multiferroic La 2 NiMnO 6 nanoparticles

Abstract: We report magnetization, dielectric and dc transport properties of La2NiMnO6 nanoparticles. Both dc and ac magnetization measurements indicated a metastable magnetic behaviour with random ferromagnetic and antiferromagnetic interactions below 110 K; critical slow-down, memory and rejuvenation properties signify the spin glass nature. The dc resistivity shows a semiconducting nature but the temperature dependent magnetoresistance (MR) shows a peak at the spin glass transition. The colossal dielectric property and its frequency dependence were interpreted using the Maxwell–Wagner (MW) interfacial polarization model. Impedance analysis along with magnetodielectric (MD) and magnetoresistance (MR) indicates that the observed MD originates from the combined effect of MR and MW interfacial polarization.

Magnetic states of an individual Ni nanotube probed by anisotropic magnetoresistance.

Abstract: Defined magnetization states in magnetic nanotubes could be the basic building blocks for future memory elements. To date, it has been extremely challenging to measure the magnetic states at the single-nanotube level. We investigate the magnetization states of an individual Ni nanotube by measuring the anisotropic magnetoresistance effect at cryogenic temperature. Depending on the magnitude and direction of the magnetic field, we program the nanotube to be in a vortex- or onion-like state near remanence.