Contribution To Magnetoresistance Research Articles

Structural characterization, electrical and magnetic properties of (In0.97-xFexMg0.03)2O3 films

Structural, electrical, and magnetic properties of (In0.97-xFexMg0.03)2O3 films with different Fe concentration are investigated. The doped Fe ions replace the In1 sites of In2O3 lattice with the mixed-valence of +2 and +3 states and form FeIn1+2VO complex based on the real-space multiple-scattering simulation. The room temperature ferromagnetic hysteresis loop of Fe/Mg co-doped In2O3 films shows ferromagnetic ordering with coercivity Hc of about 150-300 Oe. Obvious increase of ferromagnetic hysteresis loop and saturation magnetic moment with a maximum value of 8.45 emu/cm3 (x = 0.08) are observed due to the more incorporation of Fe ions. All the films exhibit n-type conductive behavior, which are mainly dominated by Mott variable range hopping with strong localization of carriers. Interestingly, the contribution of positive and negative magnetoresistance (MR) strongly correlates with Fe dopant and temperature. It can conclude that the FeIn1+2VO complex based on bound magnetic polaron model plays a key role for achieving the intrinsic ferromagnetism.

Spin mixing conductance and spin magnetoresistance of the iridate/manganite interface

We present results on experimental studies of spin current, measured under spin pumping at ferromag-netic resonance in wide frequency band 2-20 GHz for SrIrO3/La0.7Sr0.3MnO3 heterostructures fabricated by RF magnetron sputtering at high temperature. The epitaxial growth of the thin film in heterostructure by a cube-on-cube mechanism was confirmed by XRD and TEM analysis. Taking into account the con-tribution of anisotropic magnetoresistance the spin current was estimated as 1/3 of the total response. We show that both real and imaginary parts of spin mixing conductance are valuable for heterostructures with strong spin-orbit interaction in SrIrO3. Imaginary part of spin mixing conductance was estimated by means of shift of ferromagnetic resonance field of La0.7Sr0.3MnO3 layer in heterostructure. The spin mag-netoresistance was evaluated from angular dependencies of magnetoresistance measured in planar Hall configuration. In order to extract the influence of anisotropic magnetoresistance a La0.7Sr0.3MnO3 film was measured as well. The spin Hall angle for heterostructure was found higher than for interface Pt/ La0.7Sr0.3MnO3.

Magnetic and magnetotransport properties of MnSb polycrystals near equatomic composition

In this work, we studied properties of the MnSb polycrystalline samples synthesized from a melt of high-purity elements. We observed the presence of a ferromagnetic state with Curie temperature significantly higher than room temperature (Tc≈568 K). The results of structural and magnetic studies suggest the presence of a slight deviation from the equatomic composition towards the small excess of Mn (∼ 1 at.%). It was found that the studied sample have high resistivity (up to 425 μΩ⋅cm) at room temperature, however, it significantly decreases upon cooling, breaking the Mooij correlation. The magnetoresistance of the MnSb polycrystal is positive at low temperatures and does not get saturated in studied magnetic field range (≤40 kOe), which suggests that this contribution stems from the classical Lorenz mechanism. Negative magnetoresistance contribution, associated with the suppression of thermal spin fluctuations by external magnetic field, becomes dominant at higher temperatures (T≥74 K). The normal Hall effect coefficient is positive and significantly increases upon heating, which indicates a substantial variation of delocalized holes density with temperature. The anomalous Hall effect coefficient is also positive, and its temperature dependence suggests that the skew scattering mechanism provides a considerable contribution at low temperatures.

Magnetoresistance effect in vertical NiFe/graphene/NiFe junctions

For convenient and efficient verification of the magnetoresistance effect in graphene spintronic devices, vertical magnetic junctions with monolayer graphene sandwiched between two NiFe electrodes are fabricated by a relatively simple way of transferring CVD graphene onto the bottom ferromagnetic stripes. The anisotropic magnetoresistance contribution is excluded by the experimental result of magnetoresistance (MR) ratio dependence on the magnetic field direction. The spin-dependent transport measurement reveals two distinct resistance states switching under an in-plane sweeping magnetic field. A magnetoresistance ratio of about 0.17 % is obtained at room temperature and it shows a typical monotonic downward trend with the bias current increasing. This bias dependence of MR further verifies that the spin transport signal in our device is not from the anisotropic magnetoresistance. Meanwhile, the I–V curve is found to manifest a linear behavior, which demonstrates the Ohmic contacts at the interface and the metallic transport characteristic of vertical graphene junction.

Spin-Circuit Representation of Spin-Torque Ferromagnetic Resonance

Spin-torque ferromagnetic resonance (ST-FMR), particularly using magnetic insulators and heavy metals possessing a giant spin Hall effect (SHE), has received a lot of attention for the development of spintronic devices. To devise complex functional devices, it is necessary to construct the equivalent spin-circuit representations of different phenomena. Such representations are useful for translating physical equations into circuit elements, benchmarking experiments, and then developing creative and efficient designs. We utilize the superposition principle in circuit theory to separate the spin Hall magnetoresistance and spin-pumping contributions in ST-FMR experiments. We show that the proposed spin-circuit representation reproduces the standard results found in the literature. We further consider multilayers like a spin-valve structure with a SHE layer sandwiched by two magnetic layers and show how the corresponding spin-circuit representation can be constructed by simply writing a vector netlist and then solving using circuit theory.

Optimization of spin Hall magnetoresistance in heavy-metal/ferromagnetic-metal bilayers

We present experimental data and their theoretical description on spin Hall magnetoresistance (SMR) in bilayers consisting of a heavy metal (H) coupled to in-plane magnetized ferromagnetic metal (F), and determine contributions to the magnetoresistance due to SMR and anisotropic magnetoresistance (AMR) in five different bilayer systems: hbox {W}/text {Co}_{20}text {Fe}_{60}text {B}_{20}, text {Co}_{20}text {Fe}_{60}text {B}_{20}/hbox {Pt}, hbox {Au}/text {Co}_{20}text {Fe}_{60}text {B}_{20}, W/Co, and Co/Pt. The devices used for experiments have different interfacial properties due to either amorphous or crystalline structures of constitutent layers. To determine magnetoresistance contributions and to allow for optimization, the AMR is explicitly included in the diffusion transport equations in the ferromagnets. The results allow determination of different contributions to the magnetoresistance, which can play an important role in optimizing prospective magnetic stray field sensors. They also may be useful in the determination of spin transport properties of metallic magnetic heterostructures in other experiments based on magnetoresistance measurements.

Scaling analysis of anomalous Hall resistivity in the Co2TiAl Heusler alloy

A comprehensive magnetotransport study including resistivity (ρxx), isothermal magnetoresistance, Hall resistivity (ρxy) and magnetization have been carried out at different temperatures on the Co2TiAl Heusler alloy. Co2TiAl alloy shows a paramagnetic to ferromagnetic (FM) transition below the Curie temperature (TC) ∼ 125 K. In the FM region, resistivity and magnetoresistance reveal a spin flip electron–magnon scattering and the Hall resistivity unveils the anomalous Hall resistivity. Scaling of anomalous Hall resistivity with resistivity establishes the extrinsic scattering process responsible for the anomalous Hall resistivity; however skew scattering is the dominant mechanism compared to the side-jump contribution. A one to one correspondence between magnetoresistance and side-jump contribution to anomalous Hall resistivity verifies the electron–magnon scattering being the source of side-jump contribution to the anomalous Hall resistivity.

Effect of current-induced electron heating on irradiated magnetoresistance in high-mobility 2D electron systems

We present a theoretical work to study the influence of an external DC-current on the irradiated magnetoresistance through a very high-mobility 2D electron system. Recent experiments report that an increasing DC-current through the system under radiation, dramatically reduces magnetoresistance, including background and irradiated contributions. On the one hand, the former ends up progressively negative (negative giant magnetoresistance) and on the other hand, the effect of radiation, i.e., the radiation-induced resistance oscillations, tends to vanish. In our theoretical model, the DC-current–induced hot electrons when scattered by charged impurities end up, to an increasing extent, between Landau levels with a very low density of states to get to. This makes the scattering process much less effective, decreasing both the magnetoresistance itself and the radiation-induced resistance oscillations. We conclude that both effects are not separable, being tightly linked by the scattering process.

Optimization of Fe content in Electrodeposited FeCoCu/Cu magnetic multilayer

The structural, magnetic and magnetoresistance properties of FeCoCu/Cu multilayers prepared by electrochemical deposition from a single bath containing 0.025, 0.05 and 0.1 M Fe under potentiostatic control are investigated. The deposition potentials were selected −1.5 V for magnetic layers and −0.4 V for non-magnetic layers concerning saturated calomel electrode. The multilayers exhibited face-centered-cubic structure in the X-ray diffraction. In the multilayer produced from the electrolyte containing 0.05 M Fe, the high saturation magnetization value (572.3 kA/m) was observed and it value drastically decreased 140.6 kA/m in the electrolyte containing 0.1 M Fe. The magnetoresistance measurements were carried out in magnetic fields in the range of ±955 kA/m. The high giant magnetoresistance value (18.7%) were obtained in the multilayer with 0.05 M Fe. In the magnetic sublayers of the multilayers, the more than Cu atoms deposited and due to this, the magnetoresistance curves exhibited the 2.5, 4.7, 2.4% anisotropic magnetoresistance contributions between longitudinal and transverse magnetoresistance.

Low-temperature magnetoresistance of (111) ( La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3

The two dimensional conducting interfaces in SrTiO$_3$-based systems are known to show a variety of coexisting and competing phenomena in a complex phase space. Magnetoresistance measurements, which are typically used to extract information about the various interactions in these systems, must be interpreted with care, since multiple interactions can contribute to the resistivity in a given range of magnetic field and temperature. Here we review all the phenomena that can contribute to transport in SrTiO$_3$-based conducting interfaces at low temperatures, and discuss possible ways to distinguish between various phenomena. We apply this analysis to the magnetoresistance data of (111) oriented (La$_{0.3}$Sr$_{0.7}$)(Al$_{0.65}$Ta$_{0.35}$)/STO (LSAT/STO) heterostructures in perpendicular field, and find an excess negative magnetoresistance contribution which cannot be explained by weak localization alone. We argue that contributions from magnetic scattering as well as electron-electron interactions can provide a possible explanation for the observed magnetoresistance.

Surface termination dependent quasiparticle scattering interference and magneto-transport study on ZrSiS

Dirac nodal line semimetals represent a new state of quantum matters in which the electronic bands touch to form a closed loop with linear dispersion. Here, we report a combined study on ZrSiS by density functional theory calculation, scanning tunnelling microscope (STM) and magneto-transport measurements. Our STM measurements reveal the spectroscopic signatures of a diamond-shaped Dirac bulk band and a surface band on two types of cleaved surfaces as well as a spin-polarized surface band at at E ∼ 0.6 eV on S-surface, consistent with our band calculation. Furthermore, we find the surface termination does not affect the surface spectral weight from the Dirac bulk bands but greatly affect the surface bands due to the change in the surface orbital composition. From our magneto-transport measurements, the primary Shubnikov–de-Haas frequency is identified to stem from the hole-type quasi-two-dimensional Fermi surface between Γ and X. The extracted non-orbital magnetoresistance (MR) contribution D(θ, H) yields a nearly H-linear dependence, which is attributed to the intrinsic MR in ZrSiS. Our results demonstrate the unique Dirac line nodes phase and the dominating role of Zr-d orbital on the electronic structure in ZrSiS and the related compounds.

Full angular dependence of the spin Hall and ordinary magnetoresistance in epitaxial antiferromagnetic NiO(001)/Pt thin films

We report the observation of the three-dimensional angular dependence of the spin Hall magnetoresistance (SMR) in a bilayer of the epitaxial antiferromagnetic insulator NiO(001) and the heavy metal Pt, without any ferromagnetic element. The detected angular-dependent longitudinal and transverse magnetoresistances are measured by rotating the sample in magnetic fields up to 11 T, along three orthogonal planes (xy-, yz- and xz-rotation planes, where the z-axis is orthogonal to the sample plane). The total magnetoresistance has contributions arising from both the SMR and ordinary magnetoresistance. The onset of the SMR signal occurs between 1 and 3 T and no saturation is visible up to 11 T. The three-dimensional angular dependence of the SMR can be explained by a model considering the reversible field-induced redistribution of magnetostrictive antiferromagnetic S- and T-domains in the NiO(001), stemming from the competition between the Zeeman energy and the elastic clamping effect of the non-magnetic MgO substrate. From the observed SMR ratio, we estimate the spin mixing conductance at the NiO/Pt interface to be greater than $2\times10^{14}$ ${\Omega}^{-1}$ $m^{-2}$. Our results demonstrate the possibility to electrically detect the N\'eel vector direction in stable NiO(001) thin films, for rotations in the xy- and xz- planes. Moreover, we show that a careful subtraction of the ordinary magnetoresistance contribution is crucial to correctly estimate the amplitude of the SMR.

Magnetocapacitance effects in MnZn ferrites

The magnetocapacitance effects of MnZn ferrites with different initial permeabilities have been studied systematically. Both intrinsic effect associated with magnetoelectric coupling and extrinsic effect, which means the combined contribution of magnetoresistance and the Maxwell-Wagner effect, have been observed simultaneously. Analysis shows that the relationship between the origins of both is in competitive equilibrium. Either of both mechanisms plays a dominant role in magnetocapacitance effects under different conditions, respectively, such as permeability and frequency of applied signals.

Defect-induced negative magnetoresistance and surface state robustness in the topological insulatorBiSbTeSe2

Absence of backscattering and occurrence of weak anti-localization are two characteristic features of topological insulators. We find that the introduction of defects results in the appearance of a negative contribution to magnetoresistance in the topological insulator BiSbTeSe2, at temperatures below 50 K. Our analysis shows that the negative magnetoresistance originates from an increase in the density of defect states created by introduction of disorder, which leaves the surface states unaffected. We find a decrease in the magnitude of the negative magnetoresistance contribution with increasing temperature and a robustness of the topological surface states to external disorder.

Magnetothermopower and magnetoresistance of single Co-Ni/Cu multilayered nanowires

The magnetothermopower and the magnetoresistance of single Co Ni/Cu multilayered nan-owires with various thicknesses of the Cu spacer are investigated. Both kinds of measurement have been performed as a function of temperature (50 K to 325 K) and under applied magnetic fields perpendicular to the nanowire axis, with magnitudes up to 15 % at room temperature. A linear relation between thermopower S and electrical conductivity {\sigma} of the nanowires is found, with the magnetic field as an implicit variable. Combining the linear behavior of the S vs. {\sigma} and the Mott formula, the energy derivative of the resistivity has been determined. In order to extract the true nanowire materials parameters from the measured thermopower, a simple model based on the Mott formula is employed to distinguish the individual thermopower contributions of the sample. By assuming that the non-diffusive thermopower contributions of the nanowire can be neglected, it was found that the magnetic field induced changes of thermopower and resistivity are equivalent. The emphasis in the present paper is on the comparison of the magnetoresistance and magnetothermopower results and it is found that the same correlation is valid between the two sets of data for all samples, irrespective of the relative importance of the giant magnetoresistance or anisotropic magnetoresistance contributions in the various individual nanowires.

Anisotropic magnetothermal transport and spin Seebeck effect

The anisotropic properties of thermal transport in insulating or conducting ferromagnets are derived on the basis of the Onsager reciprocity relations applied to a magnetic system. It is shown that the angular dependence of the temperature gradient takes the same form as that of the anisotropic magnetoresistance, including anomalous and planar Hall contributions. The measured thermocouple generated between the extremities of the nonmagnetic electrode in thermal contact to the ferromagnet follows this same angular dependence. The sign and amplitude of the magnetovoltaic signal is controlled by the difference of the Seebeck coefficients of the thermocouple.

Composition, morphology and electrical transport properties of Co–Pb electrodeposits

Bath compositions were elaborated for the codeposition of Co and Pb by taking into account the chemical compatibility of Co2+ and Pb2+ with the appropriate anions. Electrolytes containing either acetate, chloride or nitrate anions were tested, but only the acetate bath proved to be appropriate for the preparation of compact Co–Pb films. Deposits were obtained with constant current, with constant potential or by using various current and potential pulses in order to investigate the possibility of multilayer formation. The variation in deposit composition as a function of the deposition parameters was elucidated by using cyclic voltammetry, current transient measurements and gravimetry. X-ray diffraction (XRD) patterns recorded for two-pulse plated deposits revealed a nanocrystalline structure with grain sizes in the range 5–20nm. The XRD peaks could be well indexed to pure face-centered cubic Co and Pb, indicating that the Pb codeposited with Co is not dissolved in Co but is segregated. Both the d.c. – plated and the two-pulse plated deposits exhibited anisotropic magnetoresistance without an indication for a noticeable giant magnetoresistance contribution. This means that the observed magnetoresistance arises from spin-dependent electron scattering events dominantly within the sufficiently large Co regions and not along electron paths between two Co regions via the Pb regions. Low-temperature resistivity measurements revealed a superconducting transition slightly below that of pure Pb. This may be ascribed to a proximity effect: the ferromagnetic Co grains suppress somewhat the superconductivity of the Pb phase due to the nanoscale phase mixture of the two constituents.

Investigation of Structural, Magnetic and Magnetotransport Properties of Electrodeposited Co–TiO2 Nanocomposite Films

Nanocomposite Co-TiO2 thin films were pre- pared by simultaneous electrodeposition of Co and TiO2 on a Cu substrate from a solution based on Co sulfate in which TiO2 nanoparticles were suspended by stirring. We investigated the influence of the TiO2 nanoparticles concen- tration in the bath on the morphology, composition, mag- netic and magnetotransport properties of the films. The Co-TiO2 thin films were characterized by using scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction analyses, and their magnetic properties were evaluated by using an induction type device with data acquisition system and a torque magnetometer. The current in-plane transport properties of the films have been investi- gated. The results showed that the films were composed of a Co metal matrix containing embedded TiO2 nanoparticles and cobalt hydroxide which is formed simultaneously with cobalt metal deposition. The amount of TiO2 in the film in- creases with the rising concentration of TiO2 nanoparticles in the plating bath. This complex structure favored the in- crease of the magnetoresistance. The Co-TiO2 nanocom- posite films (containing about 1.3 at.% Ti) exhibit a gi- ant magnetoresistance contribution of 47.6 %. From the magnetic measurements, we have found that the satura- tion magnetization, the magnetic susceptibility, and the ef- fective magnetic anisotropy constant decrease with the in- creasing content of TiO2 in the thin layer. The easy mag- netization axis direction changes from in-plane to almost perpendicular-to-plane, with increasing TiO2 nanoparticles content in the film. The existence of a giant magnetoresis- tance effect in Co-TiO2 is very promising for potential ap- plications in spintronics.

Core contribution to magnetotransport of ferromagnetic dots in vortex state

We study the influence of the vortex core on magnetotransport of ferromagnetic dots in a vortex state. The extraordinary Hall effect generated in the core region has a different field symmetry compared to contributions of anisotropic magnetoresistance and the planar Hall effect, which can be used to detect chirality and polarity of the vortex. We propose a method for realization of two-bit per dot magnetic random access memory, in which two states are contributed by clockwise and counter-clockwise chirality and two by up and down core polarity. Dependence of the signal on vortex location, core diameter, and other parameters is discussed.

Electrical transport properties of nanostructured ferromagnetic perovskite oxides La0.67Ca0.33MnO3 and La0.5Sr0.5CoO3 at low temperatures (5 K ⩾ T ⩾ 0.3 K) and high magnetic field

We report a comprehensive study of the electrical and magneto-transport properties of nanocrystals of La0.67Ca0.33MnO3 (LCMO) (with size down to 15 nm) and La0.5Sr0.5CoO3 (LSCO) (with size down to 35 nm) in the temperature range 0.3–5 K and magnetic fields up to 14 T. The transport, magneto-transport and nonlinear conduction (I–V curves) were analysed using the concept of spin-polarized tunnelling in the presence of Coulomb blockade. The activation energy of transport, Δ, was used to estimate the tunnelling distances and the inverse decay length of the tunnelling wave function (χ) and the height of the tunnelling barrier (ΦB). The magneto-transport data were used to find the magnetic field dependences of these tunnelling parameters. The data taken over a large magnetic field range allowed us to separate out the magneto-resistance (MR) contributions at low temperatures arising from tunnelling into two distinct contributions. In LCMO, at low magnetic field, the transport and MR are dominated by the spin polarization, while at higher magnetic field the MR arises from the lowering of the tunnel barrier by the magnetic field, leading to an MR that does not saturate even at 14 T. In contrast, in LSCO, which does not have substantial spin polarization, the first contribution at low field is absent, while the second contribution related to barrier height persists. The idea of inter-grain tunnelling has been validated by direct measurements of the nonlinear I–V data in this temperature range, and the I–V data were found to be strongly dependent on magnetic field. We made the important observation that a gap-like feature (with magnitude ∼EC, the Coulomb charging energy) shows up in the conductance g(V ) at low bias for the systems with the smallest nanocrystal size at the lowest temperatures (T ⩽ 0.7 K). The gap closes when the magnetic field and temperature are increased.