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Strong anisotropic magnetotransport is reported in high-quality TaTe4 single crystals synthesized by flux methods. Large positive magnetoresistance (MR) and field-induced metal-semiconductor-like transition are observed at low temperatures with B perpendicular to c axis. The MR value reaches 3200% in 9T at 2K with B parallel to a axis, contrast to 79% for B along c axis. Angle dependent magnetoresistance with B rotated within ab plane displays eightfold symmetry and pronounced Shubnikov-de Haas (SdH) oscillations at low temperatures. The analysis of angle dependent resistivity, Hall effect and observed SdH oscillations suggest the high mobile electron and anisotropic Fermi surface responsible for the large anisotropic MR in TaTe4. [ABSTRACT FROM AUTHOR]

Anisotropic magnetoresistance (AMR) and related planar Hall resistance (PHR) are ubiquitous phenomena of magnetic materials. Although the universal angular dependences of AMR and PHR in magnetic polycrystalline materials with one order parameter are well known, no similar universal relation for other class of magnetic materials are known to date. Here a general theory of galvanomagnetic effects in magnetic materials is presented with two vector order parameters, such as magnetic single crystals with a dominated crystalline axis or polycrystalline non-collinear ferrimagnetic materials. It is shown that AMR and PHR have a universal angular dependence. In general, both longitudinal and transverse resistivity are non-reciprocal in the absence of inversion symmetry: Resistivity takes different values when the current is reversed. Different from simple magnetic polycrystalline materials where AMR and PHR have the same magnitude, and π /4 out of phase, the magnitudes of AMR and PHR of materials with two vector order parameters are not the same in general, and the phase difference is not π /4. Instead of π periodicity of the usual AMR and PHR, the periodicities of materials with two order parameters are 2 π. [ABSTRACT FROM AUTHOR]

A non-saturating linear magnetoresistance (MR) is observed in Sb2SeTe2 topological insulator. The results show that the MR slope and the critical magnetic field of the linear MR are proportional to the carrier mobility and inverse mobility, respectively. These are consistent with the prediction of a model, which is constructed by Parish and Littlewood [Nature 426, 162 (2003)], in the weak mobility fluctuation condition. The Kohler plot of the magnetoresistance does not collapse onto a single curve that supports the multi-carriers scattering mechanisms. [ABSTRACT FROM AUTHOR]

The spin orbital torques in Ta/CoFeB/MgO structures are experimentally investigated utilizing the planar Hall effect and magnetoresistance measurement. By angular field characterization of the planar Hall resistance at ±current, the differential resistance which is directly related to the spin orbital torques is derived. Upon curve fitting of the analytical formulas over the experimental results, it is found that the anti-damping torque, also known as spin Hall effect, is sizable while a negligible fieldlike torque is observed. A spin Hall angle of about 18±0.6% is obtained for the Ta layer. Temperature dependent study of the spin orbital torques is also performed. It is found that temperature does not significantly modify the spin Hall angle. By cooling down the sample down to 100K, the obtained spin Hall angle has a maximum value of about 20.5±0.43%. [ABSTRACT FROM AUTHOR]

We synthesize an iron-doped PbTe single-crystal ingot and investigate the phase composition and distribution of the iron impurity along the ingot as well as galvanomagnetic properties in weak magnetic fields (4.2K ≤ T ≤ 300 K, B ≤ 0.07 T) of Pb1-yFeyTe alloys. We find microscopic inclusions enriched with iron and regions with a chemical composition close to FeTe in the heavily doped samples, while the iron impurity content in the main phase rises only slightly along the length of the ingot reaching the impurity solubility limit at approximately 0.6 mol. %. Samples from the initial and the middle parts of the ingot are characterized by p-type metal conductivity. An increase of the iron impurity content leads to a decrease in the free hole concentration and to a stabilization of galvanomagnetic parameters due to the pinning of the Fermi level by the iron resonant impurity level EFe lying under the bottom of the valence band (Ev - EFe ≈ 16 meV). In the samples from the end of the ingot, a p-n inversion of the conductivity type and an increase of the free electron concentration along the ingot are revealed despite the impurity solubility limit being reached. The kinetics of changes of charge carrier concentration and of the Fermi energy along the ingot is analyzed in the framework of the six-band Dimmock dispersion relation. A model is proposed for the electronic structure rearrangement of Pb1-yFeyTe with doping, which may also be used for PbTe doped with other transition metals. [ABSTRACT FROM AUTHOR]

Magnetoresistance effects observed in ferromagnet/superconductor (FM/SC) hybrids, FM/SC bilayers (BLs) and FM/SC/FM trilayers (TLs), have attracted much interest. Here, we focus on the stray-fields-based superconducting magnetoresistance effect (sMRE) observed in Co(d Co)/Nb(dNb)/Co(dCo) TLs with sufficiently thick Co outer layers so that out-of-plane magnetic domains (MDs) and MDs walls (MDWs) emerge all over their surface when subjected to a parallel external magnetic field, Hex, equal to the coercive field, Hc. To explore the conditions necessary for maximization of the sMRE, we focus on the different kinds of the stray dipolar fields, Hdip, that emerge at the interior of the out-of-plane MDs and at the boundaries of MDWs; these have a different inherent tendency to create straight and semi-loop vortices, respectively. In the recent literature, the creation of straight and semi-loop vortices has been addressed at some extent both theoretically [Laiho et al., Phys. Rev. B 67, 144522 (2003)] and experimentally [Bobba et al., Phys. Rev. B 89, 214502 (2014)] for the case of FM/SC BLs. Here, we address these issues in FM/SC/FM TLs in connection to the sMRE. Specifically, we focus on an experimental finding reported recently [D. Stamopoulos and E. Aristomenopoulou, J. Appl. Phys. 116, 233908 (2014)]; strong magnetostatic coupling of the FM outer layers is accompanied by an intense sMRE in TLs in which the thickness of the SC interlayer, dSC, matches the width of MDWs, DMDWs. To investigate this finding, we employ simulations-modeling and energy-considerations and propose two quantitative criteria that facilitate the creation of straight vortices over semi-loop ones. The first focuses on the maximization of the stray Hdip that occur at the interior of the out-of-plane MDs. The second enables the estimation of a crossover between the preferable creation of one kind of vortices over the other. Both criteria respond well, when tested against experimental results. These generic criteria on the interference between dSC and DMDWs can assist the design of cryogenic devices based on FM/SC/FM TLs. [ABSTRACT FROM AUTHOR]

We have investigated dc voltage generation induced by ferromagnetic resonance in a Co75Fe25/Pt film. In order to reduce rectification effects of anisotropic magnetoresistance and the planar Hall effect, which may be observed simultaneously with the inverse spin Hall effect, we selected Co75Fe25 with extremely small anisotropic magnetoresistance as a spin injector. Using the difference in the spectral shape of voltage and in the angle dependence of in-plane magnetization among the effects, we demonstrated that the generated dc voltage is governed by the inverse spin Hall effect induced by spin pumping. [ABSTRACT FROM AUTHOR]

A recent experiment in CuBi alloys obtained a large spin Hall angle (SHA) of -0.24 (Niimi et al., Phys. Rev. Lett. 109, 156602 (2012)). We find that the SHA can be dramatically enhanced by Bi impurities close to the Cu surface. The mechanisms of this enhancement are two-fold. One is that the localized impurity state on surface has a decreased hybridization and combined with Coulomb correlation effect. The other comes from the low-dimensional state of conduction electrons on surface, which results in a further enhancement of skew scattering by impurities. Furthermore, we note that a discrepancy in sign of SHA between the experiment and previous theories is simply caused by different definitions of SHA. This re-establishes skew scattering as the essential mechanism underlying the spin Hall effect in CuBi alloys. [ABSTRACT FROM AUTHOR]

The close coupling between the spin and momentum degrees of freedom in topological insulators (TIs) presents the opportunity for the control of one to manipulate the other. The momentum can, for example, be confined on a curved surface and the spin influenced by applying a magnetic field. In this work, we study the surface states of a cylindrical TI magnetized in the x direction perpendicular to the cylindrical axis lying along the z direction. We show that a large magnetization leads to an upwards bending of the energy bands at small ∣kz∣. The bending leads to an anomalous magnetoresistance where the transmission between two cylinders magnetized in opposite directions is higher than when the cylinders are magnetized at intermediate angles with respect to each other. [ABSTRACT FROM AUTHOR]

We study the evolution of quantum anomalous Hall (QAH) effect for a Z2 topological insulator (TI) thin films in a proximity induced magnetic phase by a realistic layered k.p model with interlayer coupling. We examine three different magnetic configurations in which ferromagnetic (FM) layer(s) is added either from one side (FM-TI), from both sides (FM-TI-FM), or homogeneously distributed (magnetically doped) in a TI slab. We map out the thickness-dependent topological phase diagram under various experimental conditions. The critical magnetic exchange energy for the emergence of QAH effect in the latter two cases decreases monotonically with increasing number of quintuple layers (QLs), while it becomes surprisingly independent of the film thickness in the former case. The gap size of the emergent QAH insulator depends on the non-magnetic "parent" gap of the TI thin film and is tuned by the FM exchange energy, opening a versatile possibility to achieve roomtemperature QAH insulator in various topological nanomaterials. Finally, we find that the emergent spin-texture in the QAH effect is very unconventional, non-"hedgehog" type; and it exhibits a chiral out-of-plane spin-flip texture within the same valence band which is reminiscent of dynamical "skyrmion" pattern, except our results are in the momentum space. [ABSTRACT FROM AUTHOR]

We have experimentally investigated the crystal orientation dependence of band matching in current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) pseudo-spin-valves using Co2Fe(Ge0.5Ga0.5) (CFGG) Heusler alloy ferromagnetic layer and NiAl spacer. The high quality epitaxial CFGG/NiAl/CFGG all-B2-trilayers structure devices were fabricated on both MgO(001) and sapphire (1120) single crystal substrates to create (001) and (110) crystal orientations. Same magneto-transport properties were observed from these two differently orientated devices indicating that there is no or little orientation dependence of band matching on MR output. We also found that all-B2-trilayer structure was free of lattice matching influence depending on the crystal orientation, which made it a good candidate for CPP-GMR device. [ABSTRACT FROM AUTHOR]

This paper deals with the modeling of sensitivity of epitaxial graphene Hall bars, from submicrometer to micrometer size, to the stray field generated by a magnetic microbead. To demonstrate experiment feasibility, the model is first validated by comparison to measurement results, considering an ac-dc detection scheme. Then, a comprehensive numerical analysis is performed to investigate signal detriment caused by sensor material heterogeneities, saturation of bead magnetization at high fields, increment of bead distance from sensor surface, and device width increase. [ABSTRACT FROM AUTHOR]

This paper presents a novel microfluidic chip for in-vitro detection of biomolecules tagged by magnetic microparticles (MAPs) suspended in a static fluid. The system consists of two microfluidic channels: a reference channel in which bare MAPs are suspended and a detection channel in which magnetically tagged biomolecules are suspended (LMAPs). The LMAPs are functionalized MAPs (of the same magnetic volume as the ones in the reference channel) with attached biomolecules. The overall, non-magnetic volume of the LMAPs is greater than that of the bare MAPs. Current carrying microconductors are positioned underneath the channels in order to impose a magnetic field gradient to the MAPs and LMAPs and move them from the inlet to the outlet of the channels without flow. The innovative aspect of the proposed method is that the induced velocity on the MAPs and LMAPs, while imposed to the same magnetic field gradient, is inversely proportional to their overall, non-magnetic volume. This is due to the enhanced Stokes drag force exerted on the LMAPs, resulting from the greater volume and altered hydrodynamic shape. This induced velocity is measured by utilizing Giant Magnetoresistance (GMR) sensor pairs fabricated underneath the first and the last microconductors. Detected differences in velocity between the LMAPs and the reference MAPs indicate the presence of biomolecules in the static liquid sample. We also present a novel method for signal acquisition and demodulation: expensive function generators, data acquisition devices, and lock-in amplifiers were substituted by a generic PC sound card and an algorithm combining the Fast Fourier Transform of the signal with a peak detection routine. Experiments with functionalized MAPs and magnetically tagged Escherichia coli (representing the LMAPs) were carried out as a proof of concept. In order to identify the detection limit of the GMR sensor, single MAP (2.8 μm diameter) detection was performed. [ABSTRACT FROM AUTHOR]

An effective detection structure for a dynamical spin injection in a ferromagnetic/nonmagnetic metal bilayer has been developed. We demonstrate a clear detection of a highly symmetric signal, indicating a significant suppression of spurious signals due to the galvanomagnetic effects from the ferromagnetic layer. The angular dependence of the signal amplitude assures that the observed signal is caused by the inverse spin Hall effect due to the dynamical spin injection. We also find the importance of the eddy current that induces spin-rectified signals from a ferromagnetic layer. [ABSTRACT FROM AUTHOR]

The phase and elemental compositions and galvanomagnetic properties (4.2 K ≤ T ≤ 300 K, B ≤ 0.07 T) of samples from a single-crystal Pb1−x−ySnxNiyTe ingot (x = 0.08, y = 0.01) synthesized by the Bridgman–Stockbarger method were studied. Microscopic inclusions enriched in nickel were found. It is shown that in the main phase, the tin concentration increases exponentially along the ingot (x = 0.06–0.165), while the concentration of nickel impurity does not exceed 0.4 mol %. A significant increase in the concentration of holes along the ingot and an abnormal increase in the Hall coefficient with increasing temperature were found; both are due to the pinning of the Fermi level by the resonant nickel level located in the valence band. The dependences of the hole concentration and of the Fermi energy at T = 4.2 K on the tin concentration in alloys are calculated using the two-band Kane dispersion law. A qualitative model of electronic structure rearrangement is proposed. The model takes into account the movement of the nickel level into the depth of the valence band with an increase in tin concentration and the redistribution of electrons between the valence band and the level. The energy position of the nickel level and the speed of its movement relative to the top of the valence band with an increase in the tin content in Pb1−xSnxTe alloys are estimated. [ABSTRACT FROM AUTHOR]

Magnetotransport measurements were performed on a series of nanostructured Bi1-xSbx alloy samples with an Sb content in the range between 0% and 60%. The samples were prepared by cold pressing and annealing of crystalline Bi1-xSbx nanoparticles, which were synthesized by mechanical alloying. The incorporation of Sb changes the band structure of these nanotextured alloys as well as their transport behavior. With increasing Sb content the band gap increases and reaches a maximum band gap of 42 meV at an Sb concentration of about 14% determined from temperature dependent resistivity measurements. For even higher Sb content, the band gap decreases again. The bands and thus the band gaps are shifted with respect to bulk material due to quantum confinement effects in the nanostructures. The change of the band structure with varying Sb content strongly affects the magnetoresistance behavior as well as the magnetic field dependence of the Hall-coefficient. Using a three band model in order to describe both properties, it was possible to determine the main band parameters of the nanostructured material as a function of the Sb content. [ABSTRACT FROM AUTHOR]

Magnetotransport properties of spin-glass-like Ge1-x-ySnxMnyTe mixed crystals with chemical composition changing in the range of 0.083≤x≤0.142 and 0.012≤y≤0.119 are presented. The observed negative magnetoresistance we attribute to two mechanisms, i.e., weak localization occurring at low fields and spin disorder scattering giving contribution mainly at higher magnetic fields. A pronounced hysteretic anomalous Hall effect (AHE) was observed. The estimated AHE coefficient shows a small temperature dependence and is dependent on Mn-content, with changes in the range of 10-7

The extraordinary magnetoresistance (EMR) in metal-semiconductor hybrid structures was first demonstrated using a van der Pauw configuration for a circular semiconductor wafer with a concentric metallic inclusion in it. This effect depends on the orbital motion of carriers in an external magnetic field, and the remarkably high magnetoresistance response observed suggests that the geometry of the metallic inclusion can be optimized to further significantly enhance the EMR. Here, we consider the theory and simulations to achieve this goal by comparing both two-dimensional (2D) and three-dimensional (3D) structures in an external magnetic field to evaluate the EMR in them. New results for 3D structures are presented to show the feasibility of such modeling. Examples of structures that are compatible with present day technological capabilities are given together with their expected responses in terms of EMR. For a 10 μm 2D square structure with a square metallic inclusion, we find an MR up to 107 percent for an applied magnetic field of 1 T. In 3D, for a 10 μm cube with a 5 μm centered metallic inclusion, we obtain an MR of ∼104 percent, which is comparable with the 2D structure of equivalent dimensions. The results presented here for specific geometries are scalable to smaller dimensions down to the onset of ballistic effects in the transport. The present calculations open up the possibility of 3D magnetic field sensors capable of determining the magnitude and also direction of the magnetic field once a full characterization of the sensor response is performed. [ABSTRACT FROM AUTHOR]

We have investigated the sensitivity of a planar Hall resistance sensor as a function of the ring radius in the trilayer structure Ta(3)/IrMn(10)/Cu(0.2)/NiFe(10)/Ta(3) (nm). The diagonal components of magnetoresistivity tensor in rectangular prism corresponding to anisotropic magnetoresistance are few ten times larger than that of off-diagonal component corresponding to planar Hall resistance. However, it is noteworthy that the resultant contribution is governed by the off-diagonal components due to the cancellation of diagonal components in the self-balanced bridge configuration. Both the experimental and theoretical results show that the sensitivity varies linearly with the ring radius. In multi-ring architecture, the circumference can be increased to a limit, which consequently enhances sensitivity. We found the sensitivity of the investigated 7-rings planar Hall to be more than 600 μV/Oe. [ABSTRACT FROM AUTHOR]

The phase and elemental composition and galvanomagnetic properties of the Pb1 –yNiyTe alloys at temperatures of 4.2 K < T < 300 K in magnetic fields of B < 7 T are investigated upon a variation in the nickel-impurity concentration along a single-crystal ingot synthesized by the Bridgman–Stockbarger technique. The solubility of nickel is shown to be no more than 0.35 mol %. The anomalous temperature dependences of the Hall coefficient and temperature and field dependences of the magnetoresistance are established. The results obtained are explained under the assumption of the formation of an inversion layer with n-type conductivity on the sample surface and the existence of several competing conductivity mechanisms in the samples. A model of the Pb1 –yNiyTe electronic structure is proposed that suggests Fermi-level pinning within the Ni impurity band located at the edge of the valence band and moving deeper into it with increasing temperature. [ABSTRACT FROM AUTHOR]

The article offers information on the anomalous Hall effect (AHE) as one of the most fundamental phenomena in physics. It mentions about an electronic anomalous Hall effect (AHE), where charge carriers acquire a velocity component orthogonal to an applied electric field without an external magnetic field; also highlights the understanding of different mechanisms that can give rise to skew scattering AHEs.

A brief review is given of the physical properties of antiferromagnets (AFMs) that can be used as active elements of terahertz and subterahertz range nanooscillators based on the excitation of spin oscillations by spin transfer torque. Possible schemes of such devices are considered. The analysis is carried out from a unified point of view on the basis of the nonlinear sigma model for the antiferromagnetic vector with regard to the magnetic symmetry of real AFMs. Specific properties of AFMs, first of all, the possibility of faster (compared to ferromagnets) spin dynamics, as well as the manifestations of antiferromagnetic ordering in galvanomagnetic and optical effects, are described. The history of the development of AFM physics is briefly discussed, first of all, those aspects of it that may be important for the practical application of AFMs, in particular, in ultrafast spintronics. [ABSTRACT FROM AUTHOR]

Sheet resistance, carrier mobility, and sheet carrier density are important parameters in semiconductor production, and it is therefore important to be able to rapidly and accurately measure these parameters even on small samples or pads. The interpretation of four-point probe measurements on small pads is non-trivial. In this paper we discuss how conformal mapping can be used to evaluate theoretically expected measurement values on small pads. Theoretical values calculated from analytical mappings of simple geometries are compared to the values found from the numerical conformal mapping of a square onto the infinite half-plane, where well-established solutions are known. Hall effect measurements are performed to show, experimentally, that it is possible to measure Hall mobility in less than one minute on squares as small as 70 × 70 μm2 with a deviation of ±6.5% on a 1σ level from accurate reference measurements, when only small sheet resistance variations are present. For samples with a sheet resistance variation of more than 5%, a more time consuming method must be used. Here, the theoretically expected measurement values are computed based on the sheet resistance variation using a finite element method. This more accurate method has an experimental repeatability of ±1.6% even on inhomogeneous samples. [ABSTRACT FROM AUTHOR]

Interest in the magnetic properties of the ordered equiatomic alloy FeRh has been revived in recent years due to potential applications in heat assisted magnetic recording. This is based on the existence of a first order hysteretic phase transition from strongly anisotropic antiferromagnet (AF) to relatively isotropic ferromagnet (F) upon warming to ∼370 K. Here we investigate this transition, which shows significant coupling between structural, magnetic, and electronic degrees of freedom, via wide temperature range magnetotransport and magnetometry measurements on ordered epitaxial MgO(001)/FeRh(1000 Å) films. Consistent with bulk measurements, a large decrease in resistivity (∼30%) occurs on warming through the transition. The transition temperature shifts with applied magnetic field at -0.75 K/kOe due to the field-induced stabilization of the F phase, leading to large negative magnetoresistance (∼40% in 90 kOe) in the transition region (340-400 K). Isothermal field cycles reveal consistent behavior where the resistivity is controlled by the magnetization changes as the AF/F phase boundary is crossed. In the F state at high temperature anisotropic magnetoresistance (AMR) is observed (maximum amplitude ∼0.45%), in addition to high field negative MR due to field-induced suppression of electron-magnon scattering. The temperature dependence of the AMR reveals clear two-phase coexistence in the hysteresis region, providing a sensitive probe of remnant F regions upon cooling through the transition. This behavior, and the accompanying coercivity enhancement, is discussed in terms of possible two-phase microstructures across the magnetic phase transformation. [ABSTRACT FROM AUTHOR]

We analyze spin-dependent transport through a quantum ring coupled to two ferromagnetic leads, whose magnetic moments lie in a common plane and form an arbitrary angle with respect to each other. The Rashba spin-orbit (RSO) interaction existed in the ring arms is taken into consideration. We calculate the linear conductance in terms of the Green's functions method based on the equation of motion technique. It is found that due to the quantum interference effect arising from the RSO-induced spin precession phase factor, the conductance is greatly suppressed when the Fermi energy is aligned to the on-site energies of the ring, where the spin polarization and the tunnel magnetoresistance (TMR) have their maximums. The conductance, spin polarization, and the TMR are monotonously tuned by the relative angle of the leads' magnetization directions, which shows the typical spin-valve effect. We pay special attention on the situation when one magnetic lead is polarized along z axis while the other one is pointing at x direction. The peak value of the TMR is suppressed now and can become either positive or negative when the on-site energies of the two ring arms are different from each other. This device is realizable with current technology and may practical applications in spintronics. [ABSTRACT FROM AUTHOR]

We present systematic studies of magnetic and transport properties of Zn1-xMnxGeAs2 semimagnetic semiconductor with the chemical composition varying between 0.053≤x≤0.182. The transport characterization showed that all investigated samples had p-type conductivity strongly depending on the chemical composition of the alloy. The Hall effect measurements revealed carrier concentrations p≥1019 cm-3 and relatively low mobilities, μ≤15 cm2/(V s), also chemical composition dependent. The magnetic investigations showed the presence of paramagnet-ferromagnet phase transitions with transition temperatures greater than 300 K for the samples with x≥0.078. We prove by means of x-ray diffraction, nuclear magnetic resonance, and scanning electron microscopy techniques that the observed room temperature ferromagnetism is due to the presence of MnAs inclusions. The high field magnetoresistance showed the presence of giant magnetoresistance effect with maximum amplitudes around 50% due to the presence of nanosize ferromagnetic grains. [ABSTRACT FROM AUTHOR]

The electronic properties of armchair graphene nanoribbons (AGNRs) can be significantly modified from semiconducting to metallic states by applying a uniform perpendicular magnetic field (B-field). Here, we theoretically study the band gap modulation induced by a perpendicular B-field. The applied B-field causes the lowest conduction subband and the topmost valence subband to move closer to one another to form the n=0 Landau level. We exploit this effect to realize a device relevant magnetoresistive (MR) modulation. Unlike in conventional spin-valves, this intrinsic MR effect is realized without the use of any ferromagnetic leads. The AGNRs with number of dimers, Na=3p+1[p=1,2,3,...] show the most promising behavior for MR applications with large conductance modulation, and hence, high MR ratio at the optimal source-drain bias. However, the MR is suppressed at higher temperature due to the spread of the Fermi function distribution. We also investigate the importance of the source-drain bias in optimizing the MR. Lastly, we show that edge roughness of AGNRs has the unexpected effect of improving the magnetic sensitivity of the device and thus increasing the MR ratio. [ABSTRACT FROM AUTHOR]

It was numerically demonstrated that current-in-plane (CIP) and current-perpendicular-to-plane (CPP) Ir20Mn80 based giant magnetoresistance spin valve read sensors operating at an extremely high current density (J≥1×108 A/cm2) show completely different electrical and magnetic failure mechanisms: (1) CIP read sensors, electromigration-induced Cu spacer diffusion and correspondingly degraded interlayer coupling were primarily responsible for the failures; while, (2) CPP read sensors, the deterioration of exchange bias due to thermomigration-induced Mn interdiffusion at the Co80Fe20/Ir20Mn80 interface was found to be dominant. The different temperature and current distribution resulting in different mass-transport mechanisms are the main physical reasons for the failure. [ABSTRACT FROM AUTHOR]

We present the microscopic treatment of edge magnetoplasmons (EMPs) for the regime of not-too-low temperatures defined by the condition hωc>kBT>hvg/2ℓ0, where vg is the group velocity of the edge states, ℓ0=h/m*ωc is the magnetic length and ωc is the cyclotron frequency. We find a weakly damped symmetric mode, named helical EMP, which is localized at the edge states region for filling factors ν=1,2 and very strong dissipation ηT=ξ/kxℓT>=ln(1/kxℓT)>1, where the characteristic length ℓT=kBTℓ02/hvg>ℓ0/2 with ξ being the ratio of the local transverse conductivity to the local Hall conductivity at the edge states and kx is the wave vector along the edge; here other EMP modes are strongly damped. The spatial structure of the helical EMP, transverse to the edge, is strongly modified as the wave propagates along the edge. In the regime of weak dissipation, ηT<1, we obtain exactly the damping of the fundamental mode as a function of kx. For ν=4 and weak dissipation we find that the fundamental modes of n=0 and 1 Landau levels are strongly renormalized due to the Coulomb coupling. Renormalization of all these EMPs coming from a metal gate and air half-space is studied. [ABSTRACT FROM AUTHOR]

We present results from scanning tunneling microscopy based point-contact measurements of the local resistance in octagon shaped, Co(20 nm)/Cu(5 nm)/Fe19Ni81(2.5 nm) spin-valve rings. Through this technique one can detect the magnetoresistance with spatial resolution, and link it to magnetic domain wall motion within the ring. Measurements with varying currents indicate current-induced effects leading to offsets in the magnetic fields required for magnetic switching. The offsets can be attributed to current-induced spin-transfer torque effects for the thin Fe19Ni81 layer and to Oersted field effects for the thick Co layer. [ABSTRACT FROM AUTHOR]

The hole spin accumulation and spin-polarized transport in ferromagnetic semiconductor (FS)/d-wave superconductor (SC)/FS tunnel junctions with {110} interfaces are studied by using four-component Bogoliubov-de Gennes equations, in which the Andreev reflection and four-subband model for the FS are taken into account. It is found that due to the interplay of the d-wave SC and FS, the nonequilibrium hole spin accumulation, differential conductance, and tunneling magnetoresistance exhibit a rich dependence on the Andreev reflection, strengths of potential scattering at the interfaces, mismatches in the effective mass and band between the FS and SC, and types of incident holes, which is much different from that in FS/s-wave SC/FS tunneling junctions. Particularly, it is demonstrated that the differential conductance can be negative in both ferromagnetic and antiferromagnetic alignments for not only incident heavy holes but also incident light holes, and the variations in both the energy gap with temperature and hole spin accumulation with bias voltage can display a twofold behavior due to the Andreev reflection. [ABSTRACT FROM AUTHOR]

Anisotropic magnetoresistance effect has been studied systematically in (311)A-oriented as-grown and annealed GaMnAs films at different temperatures. Besides a uniaxial term, the measured angular dependence of longitudinal and transverse magnetoresistivities contains higher order terms arising from the crystal symmetry of GaMnAs. A third order crystalline term accounts for the observed antisymmetric planar Hall effect which is intrinsic to (311)A-oriented GaMnAs films. The uniaxial term persists above the Curie temperature, while the higher order terms can only be observed below the Curie temperature, with their magnitudes increasing with decreasing temperatures. It is revealed that the higher order crystalline terms originate from the long range ferromagnetic (FM) phase and the uniaxial one from isolated magnetic polarons (MPs) as well as the FM phase. With decreasing temperatures, the dominant contribution to the uniaxial term changes from isolated MPs to the FM phase. In addition, low temperature annealing of GaMnAs films is found to increase the magnitudes of all these terms. [ABSTRACT FROM AUTHOR]

Huge values of high-field magnetoresistance have been recently reported in large arrays of ferromagnetic CoFe nanoparticles embedded in an organic insulating matrix in the Coulomb blockade regime. The magnetoresistance displays two characteristic behaviors: a scaling with the magnetic field/temperature ratio and an unusual exponential decrease with increasing voltage. To describe all these features, we propose a model where the electronic charges tunnel from one nanoparticle to another through a paramagnetic impurity. It is assumed that the noncollinearity between the magnetic moment of the ferromagnetic nanoparticles and the paramagnetic moment induces an effective tunnel barrier, the height of which depends on the relative angle of the paramagnetic moment with respect to the ferromagnetic one. A systematic study of the magnetoresistance behavior as a function of the effective tunnel barrier parameters and applied bias voltage is carried out. Finally, we show that by using Fowler–Nordheim current expressions, i.e., in the hypothesis of small energy barriers, the main features of the magnetoresistance are well reproduced with realistic parameters. [ABSTRACT FROM AUTHOR]

Galvanomagnetic and thermomagnetic data of single crystal indium rich Pb1-xSnxTe are analyzed, and electronic band parameters are calculated using nonparabolic band model. Transport properties at 80 K are presented as a function of Sn (x=0–0.3) and In concentrations. Our results indicate pinning of Fermi level by indium impurity levels in these alloys. Effects of interaction of band with impurity level are investigated in terms of change in effective mass and energy scattering exponent. Indium doping slightly improves the thermoelectric properties of PbSnTe alloys. [ABSTRACT FROM AUTHOR]

This work is devoted to the study of the extraordinary Hall effect (EHE) in ferromagnet/normal-metal bilayers with a particular focus on surface and interface scattering. Systems selected are Co monolayers and series of Co-Pd bilayers, differing in the order of the materials deposition and the total and relative thicknesses. The extraordinary Hall coefficient was calculated using two models which we named “black box” and “two channels,” and compared them with Co monolayers. The EHE coefficient of bilayers with uniform Co layers was found to be independent of Pd thickness. Furthermore, contribution of Co-Pd interfaces to the effect is significantly weaker than Co-vacuum interface. In bilayers, as in monolayers, we find a linear correlation between the surface scattering contributions to resistivity and the EHE, which indicates that the skew scattering mechanism is the dominant source of the effect in these films. [ABSTRACT FROM AUTHOR]

The current perpendicular-to-plane (CPP) giant magnetoresistance (GMR) was investigated as a function of temperature in 500 nm diameter spin-valve pillars of the form PtMn 7 nm/AP2/Ru 0.8 nm/AP1/Cu 3.2 nm/F. AP2/Ru/AP1 represents a synthetic ferrimagnetic pinned layer and F is either a simple Co50Fe50 5 nm free layer or a laminated (Co50Fe50 1 nm/Cu 0.3 nm)4/Co50Fe50 1 nm free layer. The purpose of the lamination is to increase the CPP resistance by taking advantage of the large CoFe/Cu interfacial resistance. As expected, we observed that the CPP resistance and absolute magnetoresistance (AΔR) are increased in the laminated structure. However, the CPP magnetoresistance ratio (ΔR/R) does not increase as much as expected if considering the increase in the relative resistance weight of the active part of the spin valve due to lamination. To understand this observation, the semiclassical theory of CPP GMR including bulk and interfacial spin relaxation effects was used to quantitatively interpret the data between 2 and 300 K. The quantitative analysis indicates that a quite significant spin-memory loss takes place at CoFe/Cu interfaces which reduces the expected GMR of the laminated multilayer. The spin-memory loss at Co50Fe50/Cu interfaces is found to be 52% at 4 K and 56% at 300 K, to be compared with 25% at 300 K for Cu/Co interfaces [Eid et al., J. Appl. Phys. 93, 3445 (2003) and Bozec et al., J. Appl. Phys. 87, 4334 (2000)]. [ABSTRACT FROM AUTHOR]

We study the magnetotransport in a magnetic-particle chain by Monte Carlo simulation. The influence of the correlations between magnetic particles on the system’s conductance is studied and an unusual tunneling magnetoresistance (TMR) effect is shown in the magnetic-particle chain. A large negative TMR at room temperature is obtained due to the strong magnetic interaction between particles. A separated hysteresis loop of TMR is observed at low temperature, and a large positive TMR peak appears at the coercive field. Both the theoretical calculation and the Monte Carlo simulation show an enhancement of TMR as the magnetic energy increases. [ABSTRACT FROM AUTHOR]

We demonstrate that a high-resolution Ga focused ion beam can be used to introduce artificial domain structures in Pt(1.6 nm)/Co(0.5 nm)/Pt(3.5 nm) trilayer transport structures. We have used thin SiO2 overlayers to control the effective energy and dose of Ga ions at the Pt/Co interface. The extraordinary Hall effect (EHE) was used to characterize the magnetic properties of the patterned films. Using 30 keV Ga ions and SiO2 overlayer thicknesses in the range of 0–24 nm, we achieve complete control of the coercive field of our Pt/Co/Pt trilayer structures. The magnetization reversal mechanism for an artificial domain of size of 3×0.5 μm2 is investigated using EHE. [ABSTRACT FROM AUTHOR]

Nanocrystalline La0.7Sr0.3MnO3 films with thickness t=10-60 nm were grown on LaAlO3(100) substrates by radio-frequency magnetron sputtering. Their electrical resistivity and low-field magnetoresistance (MR) were measured. Metal-insulator transitions occur above 275 K for films with t=20-60 nm, but the electron localization prevails in the 10 nm thick film. Furthermore, only the 10 nm thick film has an MR that depends on the inverse of temperature, consistent with the model of spin-polarized tunneling. This relationship may reflect a critical aspect of the structure of grain/grain-boundaries. Accordingly, the tunneling MR in this film is 27% at 75 K. [ABSTRACT FROM AUTHOR]

From the low-temperature (down to 10 K) dc-magnetization data of the La0.7Sr0.3MnO3 (LSM), La0.7Ca0.3MnO3 (LCM), and Pr0.7Ca0.3MnO3 (PCM) systems, we estimated the critical exponents β, γ, and hence δ from the analysis of the modified Arrot plots. The exponent β estimated for the LCM system is less than that predicted by Heisenberg model and resides within the zone predicted by Ising model while for the LSM sample, β is higher than that predicted from the Heisenberg model which is considered to be due to the presence of dipole-dipole interaction arising from the large spin moment in the LSM system. The magnetization data of the PCM system cannot be fitted to the modified Arrot plots, which suggest highly inhomogeneous ground state even under 5 T magnetic field. Both LSM and LCM have almost equal values of γ. Seebeck coefficient data indicate a crossover from higher-temperature n-type to lower-temperature p-type conductivity behavior in both LSM and LCM systems, while the semiconducting PCM system shows p-type conductivity throughout the temperature range (300–80 K). It is noticed that for LSM system TC (Curie temperature) and TMI (metal-insulator transition temperature) are almost equal (∼360 K), whereas for the LCM system there exist a large difference between TC and TMI (TC∼245 K and TMI∼265 K), which may give some idea regarding the critical behavior of the respective samples. [ABSTRACT FROM AUTHOR]

Rapidly solidified, annealed ribbons (20 μm thick) of elemental Bi show a room-temperature ordinary magnetoresistive effect of 250% at 5 T with the field applied perpendicular to the ribbon surface. The effect increases to 10000% at 5 K and 5 T. These values are intermediate to those obtained for single-crystal Bi films and sputtered or evaporated polycrystalline Bi films of comparable thicknesses. The large magnetoresistance of the ribbons is attributed to a very good crystallinity and partial c-axis texture of the ribbon achieved during solidification. Rapid solidification by melt spinning is a promising technique for synthesis of Bi with potential application in magnetoelectric devices. [ABSTRACT FROM AUTHOR]

Magnetic and magnetoresistance (MR) properties of spin valves composed of Co/Cu/Co/iron oxide layer were studied using epitaxial Fe3O4 (110) grown on MgO (110) as the pinning layer. It was confirmed from the in-plane magnetization curves that the epitaxial Fe3O4 (110) films have the easy axes along the <110> direction and the hard axes along the <001> direction. The spin valves composed of Co/Cu/Co/Fe3O4 (110) showed quite different MR curves depending upon the direction of the magnetic field applied in the plane of Fe3O4 (110). Butterflylike MR curves, which imply the difference in the coercive fields between the top and bottom Co layers, were observed when the field was applied along the <001> direction of Fe3O4. On the other hand, flat MR curves, which implied the difference of the saturation fields between top and bottom Co, were observed when the field was applied along the <110> direction of Fe3O4. The magnetization easy axis of Co layers might be perpendicular to that of Fe3O4. These magnetic and MR properties could be explained by assuming the 90° coupling between the bottom Co and the Fe3O4 layers, and the ferromagnetic coupling (so-called Néel-type coupling) between the top and bottom Co layers. [ABSTRACT FROM AUTHOR]

Magnetization response to in-plane applied fields has been studied, using Stoner-Wohlfarth model, on the bilayer systems of synthetic antiferromagnet having an anisotropy configuration in which the induced intrinsic uniaxial anisotropy is set either parallel or orthogonal to the shape anisotropy. It has been found that the toggle-mode magnetoresistance random access memory operation is possible even when the shape anisotropy exceeds the intrinsic anisotropy to some extent due to the anisotropic effect of the magnetostatic coupling and that the minimum operating field and maximum relative operating field margin are attained when the two anisotropy compensate each other. The optimal operating fields increase with the increase of the absolute value of the total anisotropy constant. However, the tolerance of total anisotropy constant in the positive direction is greater than in the negative direction if zero total anisotropy is chosen to be optimal. [ABSTRACT FROM AUTHOR]

We have investigated magnetotransport behaviors of a series of single-phase, nanocrystalline La0.67Sr0.33MnO3 (LSMO) samples having grain sizes in the nanometric regime (14, 22, and 26 nm), all synthesized through chemical route “pyrophoric reaction process.” The motivation behind the present investigation is to study the effects of nanometric grain size on magnetoresistance (MR), specially its temperature and magnetic-field dependences. Magnetoresistance measurements show that in all samples there is a large negative MR at very low fields (LFMR), followed by a slower varying negative MR at comparatively high fields (HFMR), in the ferromagnetic regime. Surprisingly, we observed that at both low- and high-field regimes, the magnitude of MR remains constant up to sufficiently high temperature and then drops sharply with temperature. This temperature-dependent MR behavior gets enhanced with the decrease in particle size. Most interestingly, we found a considerable low-field MR (14%) persisting even at 200 K, which is an appreciable improvement on the results of previous workers. In order to explore the basic physics behind this unusual temperature dependence of MR, we analyzed our data in the light of a phenomenological model [P. Raychaudhuri et al., J. Appl. Phys. 84, 2048 (1998)], based on spin-polarized transport of conduction electrons at the grain boundaries, with major attention being paid to the gradual slippage of domain walls across the grain-boundary pinning centers in an applied magnetic field. Finally, we have attributed this feature of LFMR to the surface magnetization of our nanosize granular LSMO samples, which is crucial for nanodimensional systems. [ABSTRACT FROM AUTHOR]

We have investigated the low-temperature magnetoresistance (MR) behavior of Co nanowires with varying widths between 40 nm and 5.2 μm prepared by high-resolution electron-beam lithography. The wires have a thickness of 20 nm and are protected from oxidation by a 10 nm carbon cap layer. The magnetoresistance—for magnetic fields applied transversal or perpendicular to the wire—is negative and can be attributed to anisotropic magnetoresistance. The MR curves exhibit a strong dependence on the wire width from which the shape anisotropy and the demagnetizing factors can be deduced. The experimental results are in perfect agreement with the theoretical values for the demagnetization tensor of the general ellipsoid. During the magnetization rotation process a minor contribution from domain-wall nucleation and annihilation occurs. [ABSTRACT FROM AUTHOR]

We have measured the low-temperature magnetoresistance (MR) of single cobalt nanowires of various lengths, thicknesses, and widths prepared by electron-beam lithography. The MR measurements exhibit peaks at the coercive fields Hc where Hc increases with decreasing wire width. From the shape of the peaks different switching mechanisms can be distinguished. While the resistance jumps for platinum-covered Co nanowires are sharp, pure Co nanowires exhibit broadened peaks due to domain-wall pinning at the Co/CoO interface. Covering the Co nanowires with insulating carbon yields a sharp switching behavior while simultaneously offering the possibility to investigate the resistance behavior of pure cobalt. By reducing the thickness of the wires the sharpness of the switching behavior continuously degenerates. [ABSTRACT FROM AUTHOR]

The current-perpendicular-to-plane (CPP) giant-magnetoresistance (GMR) of spin valves with Fe50Co50 alloy was investigated. It has been reported that the Cu inserted in Fe50Co50 is effective for enhancing CPP-GMR. In this paper, we investigated the number of Cu lamination effect on CPP-GMR and clarified that higher CPP-GMR is obtained by decreasing the number of Cu layers. In order to ascertain the reason for this tendency, the crystalline structure and magnetization were examined. As a result, it was found that the d spacing of Fe50Co50 increases when the number of Cu layers is increased, although saturation magnetization does not change. [ABSTRACT FROM AUTHOR]

The magnetoresistance (MR) in cold-pressed magnetite nanopowders has been studied using pulsed magnetic field up to 42 T and steady field up to 12 T. Ball milling in air produces pure and stoichiometric Fe3O4 grains of nanometric size coated by a thin layer of Fe2O3, which electrically isolates the magnetite and acts as a tunnel barrier. Therefore, the intergrain magnetoresistance of magnetite grain boundaries can be analyzed regardless of the bulk transport properties. At high fields and high temperature, the MR depends linearly on the field, whereas at lower fields a direct tunneling contribution governed by the surface magnetization appears. Below the Verwey transition (T<120 K) the linear high-field MR disappears. We interpret these results in terms of the grain-boundary properties. [ABSTRACT FROM AUTHOR]