Dependence Of Magnetoresistance Research Articles

Variable range hopping and positive magnetoresistance in n type semiconductor CuIn3Se5

Variable range hopping mechanism of Mott type is observed for the first time in two different temperature ranges between 10K and 65K in the ordered defect compound n-CuIn3Se5. Analysis of magnetic field dependence of positive magnetoresistance up to 27T that has been made at several temperatures up to 55K, shows B2 and B1/3 behavior in low and high field regions. This is in good agreement with the theory of Shklovskii-Efros for hopping conduction in lightly doped semiconductors. The field dependence of the localization temperature To and localization length ξ is discussed.

Spin-Flop in Synthetic Antiferromagnet and Anhysteretic Magnetic Reversal in FeMn-Based Spin Valves

The deviation of the pinning direction from the easy axis was created in spin valves by thermomagnetic treatment. The angle between the uniaxial and the unidirectional magnetic anisotropy axes is shown to depend on the value of the field applied during the thermomagnetic treatment. Anhysteretic dependence of magnetoresistance on the magnetic field is obtained for the spin valves with crossed anisotropy configuration. The interval within which the magnetic field dependence of magnetoresistance is linear depends on the spacer thickness.

Magnetotransport properties of Fe0.8Ga0.2 films with stripe domains

Magnetotransport properties of ${\mathrm{Fe}}_{0.8}{\mathrm{Ga}}_{0.2}$ films with stripe domains are studied. The anisotropic magnetoresistance dominates the low field behavior, which is extremely dependent on the magnetic domains configuration. The magnetoresistance measured at different temperatures displays qualitatively different behaviors depending on the measurement configuration. When the stripes are oriented along the electric current, the low-field magnetoresistance changes sign with temperature, while when the stripes are perpendicular to the electric current the magnetoresistance curves are nearly temperature independent. A simple model considering parallel (series) conduction along (across) the stripes, plus the temperature dependence of anisotropic magnetoresistance and domains configuration, accounts for these experimental results.

Magnetic scattering and spin-orbit coupling induced magnetoresistance in nonmagnetic heavy metal and magnetic insulator bilayer systems

We report on the experimental study of the angular dependent magnetoresistance (MR) of heavy metal/ferromagnetic insulator bilayer structures. Through altering the relative composition in heavy metal $\mathrm{P}{\mathrm{t}}_{\ensuremath{\delta}}\mathrm{T}{\mathrm{a}}_{1--\ensuremath{\delta}}$ alloy, we continuously tune its spin Hall angle from positive, crossing zero, and to negative and study its impact on the MR. Most notably, both spin Hall effect and MR disappear simultaneously in $\mathrm{P}{\mathrm{t}}_{0.32}\mathrm{T}{\mathrm{a}}_{0.68}$ (3 nm)/YIG when the effective spin Hall angle vanishes, evidencing the essential role of spin-orbit coupling in heavy metal for the MR. By introducing Fe impurities, we further identify that magnetic scattering is also essential to induce the MR in Pt/Fe-doped $\mathrm{Si}{\mathrm{O}}_{2}$ at large magnetic field, where the MR ratio increases monotonically with doping level.

Robust quantum oscillations with non-zero Berry phase in Bi 2 Te 3

Abstract We performed angle dependent magnetoresistance study of a metallic single crystal sample of Bi2Te3. We find that the magnetoresistance is highly asymmetric in positive and negative magnetic fields for small angles between the magnetic field and the direction perpendicular to the plane of the sample. The magnetoresistance becomes symmetric as the angle approaches 90°. The quantum Shubnikov de-Haas oscillations are symmetric and show signatures of topological surface states with Dirac dispersion in the form of non-zero Berry phase. However, the angular dependence of these oscillations suggests a complex three dimensional Fermi surface as the source of these oscillations, which does not exactly conform with the six ellipsoidal model of the Fermi surface of Bi2Te3. We attribute the asymmetry in the magnetoresistance to a mixing of the Hall voltage in the longitudinal resistance due to the comparable magnitude of the Hall and longitudinal resistance in our samples. This provides a clue to understanding the asymmetric magnetoresistance often seen in this and similar materials. Moreover, the asymmetric nature evolves with exposure to atmosphere and thermal cycling, which we believe is either due to exposure to atmosphere or thermal cycling, or both affecting the carrier concentration and hence the Hall signal in these samples. However, the quantum oscillations seem to be robust against these factors which suggests that the two have different origins.

Large Unidirectional Magnetoresistance in a Magnetic Topological Insulator.

We report current-direction dependent or unidirectional magnetoresistance (UMR) in magnetic or nonmagnetic topological insulator (TI) heterostructures, Cr_{x}(Bi_{1-y}Sb_{y})_{2-x}Te_{3}/(Bi_{1-y}Sb_{y})_{2}Te_{3}, that is several orders of magnitude larger than in other reported systems. From the magnetic field and temperature dependence, the UMR is identified to originate from the asymmetric scattering of electrons by magnons. In particular, the large magnitude of UMR is an outcome of spin-momentum locking and a small Fermi wave number at the surface of TI. In fact, the UMR is maximized around the Dirac point with the minimal Fermi wave number.

Low-field magnetoresponse of strongly disordered two-dimensional superconductors

We show that inhomogeneities in the spatial distribution of Cooper pairs and in the phase of the local superconducting order parameter in the vicinity of a superconductor-normal state transition (SNT) in two dimensions can be highly sensitive to a perpendicular magnetic field. We focus on the role of orbital effects in the field-dependence of local superfluid stiffness and superconducting phase disorder in homogeneously-disordered two-dimensional superconductor thin films. The relative importance of these orbital effects is analyzed in different physical regimes dominated by Coulomb blockade, thermal phase fluctuations and Aharanov-Bohm phase disorder respectively. Following this approach, we obtain explicit expressions for the field dependence of magnetoresistance and superfluid stiffness near the SNT, and attempt an understanding of some recent experimental findings.

Magnetic, transport, and magnetotransport properties of the textured Fe3O4 thin films reactively deposited onto SiO2/Si

The structural, magnetic, transport, and magnetotransport properties of Fe3O4 thin films with thicknesses from 38 nm to 95 nm are systematically investigated. The occurrence of the Verwey transition in these films at a temperature of about 120 K is established. It is found that the temperature dependences of the magnetic moment have a feature near 40 K, which can be attributed to the multiferroic phase. According to the X-ray diffraction data, the film structure represents a (001) texture. As was established using transmission electron microscopy, the height and width of texture crystallites increase with film thickness. Analysis of the temperature dependences of the resistivity showed that the dominant carrier transport mechanism in the films is thermoactivated tunneling. The thermoactivation energy, along with the room-temperature resistivity, decreases with increasing film thicknesses, which is most likely related to the variation in the crystallite size, especially in the crystallite width. The field dependence of magnetoresistance behaves similarly over the entire temperature range and has a positive MR peak in weak fields, which is related to spin-dependent tunneling through Fe3O4 grains and antiferromagnetically coupled antiphase boundaries.

Improvement of Thermal Tolerance of CoFeB–MgO Perpendicular-Anisotropy Magnetic Tunnel Junctions by Controlling Boron Composition

We investigated annealing temperature $T_{a}$ dependence of tunnel magnetoresistance (TMR) ratio and magnetic properties for perpendicular-anisotropy (CoFe)100– X B X /MgO magnetic tunnel junctions (MTJs) with single (CoFe)100– X B X /MgO interface (s-MTJ) and double CoFeB–MgO interface (d-MTJ) structures with various boron compositions X. High TMR ratio over 100% was observed in the s-MTJ with $X= 35$ at.% after annealing at 360 °C–400 °C, whereas the s-MTJ with $X = 30$ at.% showed the degradation of TMR ratio with the increase of $T_{a}$ above 360 °C, resulting from the decrease of perpendicular anisotropy. The d-MTJ with $X = 25$ at.% maintained high TMR ratio up to $T_{a} = 400~^{\circ }\text{C}$ owing to its higher perpendicular anisotropy compared with the s-MTJ. The difference of perpendicular anisotropy between the s-MTJ and the d-MTJ can be attributed to higher interfacial anisotropy together with lower saturation magnetization of the d-MTJs. The lower saturation magnetization is attributable to two MgO layers that suppress boron diffusion from CoFeB layers, which was verified by cross-sectional line analysis using electron energy-loss spectroscopy.

In-plane anisotropy in two-dimensional electron gas at LaAlO3/SrTiO3(110) interface**Project supported by the Ministry of Science and Technology of China (Grant Nos. 2013CB921701, 2013CBA01603, and 2014CB920903), the National Natural Science Foundation of China (Grant Nos. 10974019, 51172029, 91121012, 11422430, 11374035, 11474022, and 11474024), the Program for New Century Excellent Talents in the University of the Ministry of Education of China (Grant

A systematic study of the two-dimensional electron gas at LaAlO3/SrTiO3(110) interface reveals an anisotropy along two specific directions, [001] and . The anisotropy becomes distinct for the interface prepared under high oxygen pressure with low carrier density. Angular dependence of magnetoresistance shows that the electron confinement is stronger along the direction. Gate-tunable magnetoresistance reveals a clear in-plane anisotropy of the spin–orbit coupling, and the spin relaxation mechanism along both directions belongs to D’yakonov–Perel’ (DP) scenario. Moreover, in-plane anisotropic superconductivity is observed for the sample with high carrier density, the superconducting transition temperature is lower but the upper critical field is higher along the direction. This in-plane anisotropy could be ascribed to the anisotropic band structure along the two crystallographic directions.

Temperature dependence of magnetoresistance in GdFeCo/Pt heterostructure

The temperature dependence of magnetoresistance is investigated in ferrimagnetic GdFeCo/Pt heterostructures. An anomalous Hall effect (AHE) shows a monotonic behavior in temperature even across the magnetization compensation temperature TM, implying that the FeCo moment is responsible for the magnetotransport properties. An anisotropic magnetoresistance (AMR) exhibits a steep increase at low temperatures, which we ascribe to the contribution of a weak antilocalization in an amorphous GdFeCo layer. A spin Hall magnetoresistance (SMR) is found to exist in ferrimagnet/Pt systems and shows a moderate temperature dependence, in contrast to the SMR in YIG/Pt where a significant temperature dependence was observed. These results provide a basic understanding of the magnetotransport in amorphous ferrimagnets/heavy metal heterostructures.

Electrophysical Properties of Onion-Like Carbon

The paper examines electrophysical properties of onion-like carbon (OLC) samples, where particles have the average size of 4–8 nm and are formed by 5–10 nested fullerene-like spheres connected by 1-3 common curved graphene shells into aggregates with a size of 50–300 nm. We measured the temperature dependence of electrical resistance from 4.2 to 300 K and dependence of magnetoresistance in magnetic fields up to 6 T at the temperature of 4.2 K. Temperature dependences of electrical resistance of samples can be described within the framework of the Mott law with variable hop length for the one-dimensional case or within the framework of the Efros–Shklovskii Coulomb gap. We observed the quadratically increasing positive magnetoresistance up to 6 T associated with compression of wave functions of conduction electrons. Negative magnetoresistance was observed in the range of magnetic fields up to 1–2 T in the case of some samples. This is due to the fact that magnetic field suppresses the contributions to magnetoresistance made by interference effects in the area of hopping conductivity. The measurements were used to estimate the localization radius that is comparable to the diameter of OLC particles (nano-onions).

The annealing effect on structure, magnetoresistance and magnetic properties of Co/Bi/Co thin films

Co/Bi/Co thin films were prepared by alternately sputtering at substrate temperature 460K. Their structure, magnetoresistance and magnetic properties were studied using transmission electron microscopy (TEM), software-hardware complex with current-in-plane geometries, and vibrating sample magnetometer (VSM). The trilayer systems have been investigated as functions of the Bi layer thickness and temperature. The film structure in both as-deposited and annealed at 680K states is a granulated structure which consists of a Co magnetic matrix with embedded Bi granules. The film structure after annealing at 900K depends on Bi concentrations and changes from the fcc-Co+amorphous Bi to a fcc-Co+tetragonal phase of Bi2O3 of around 17 at.%. Magnetoresistance has demonstrated independence from annealing processes in the temperature range from 300 to 680K. Besides, the oscillation dependence of magnetoresistance with Bi thickness has been observed. The saturation and remanent magnetization are reduced with the increase of Bi thickness and increase with increasing the annealing temperature; the coercivity slightly depends on Bi thickness in as-deposited state and increases more than 10 times at \( t_{Bi}=30\) nm after annealing at 680K.

Low-temperature spin relaxation in compacted powders of CrO2 solid solutions with iron impurities

The effect iron impurities have on the low-temperature behavior of the magnetoresistance of compacted powders of CrO2 ferromagnetic half-metal with anisotropic particle shape is investigated. The strong dependence of magnetoresistance on the spin relaxation rate during specimen magnetization reversal is revealed. It is shown that the addition of Fe impurities increases the powders’ coercive force and reduces tunnel magnetoresistance. A resonance mechanism of carrier tunneling in powders of CrO2 solid solutions with iron impurities is proposed.

Unusual angular dependent magnetoresistance in single-crystalline Co/Pt bilayers

The magnetoresistance (MR) of the single-crystalline Co/Pt(111) bilayers was studied systematically as a function of Co or Pt thickness at room temperature. For Co thickness dCo < 6 nm, the resistances with in-plane magnetization M parallel (R∥) and transverse (R⊥) to current and with a perpendicular M normal (RP) to film show the unconventional behavior of RP>R∥>R⊥. This MR behavior can be attributed to the combination effect of spin Hall MR with RP≈R∥>R⊥ and the geometrical size effect. In addition, the angular dependent magnetoresistance behavior of the (111)-oriented Co/Pt bilayer shows striking difference with that of the (001)-oriented Co/Pt bilayer.

Dramatically decreased magnetoresistance in non-stoichiometric WTe2 crystals

Recently, the layered semimetal WTe2 has attracted renewed interest owing to the observation of a non-saturating and giant positive magnetoresistance (~105%), which can be useful for magnetic memory and spintronic devices. However, the underlying mechanisms of the giant magnetoresistance are still under hot debate. Herein, we grew the stoichiometric and non-stoichiometric WTe2 crystals to test the robustness of giant magnetoresistance. The stoichiometric WTe2 crystals have magnetoresistance as large as 3100% at 2 K and 9-Tesla magnetic field. However, only 71% and 13% magnetoresistance in the most non-stoichiometry (WTe1.80) and the highest Mo isovalent substitution samples (W0.7Mo0.3Te2) are observed, respectively. Analysis of the magnetic-field dependent magnetoresistance of non-stoichiometric WTe2 crystals substantiates that both the large electron-hole concentration asymmetry and decreased carrier mobility, induced by non-stoichiometry, synergistically lead to the decreased magnetoresistance. This work sheds more light on the origin of giant magnetoresistance observed in WTe2.

Investigation of organic magnetoresistance dependence on spin-orbit coupling using 8-hydroxyquinolinate rare-earth based complexes

The recently discovered organic magnetoresistance effect (OMAR) reveals the spin-dependent behavior of the charge transport in organic semiconductors. So far, it is known that hyperfine interactions play an important role in this phenomenon and also that spin-orbit coupling is negligible for light-atom based compounds. However, in the presence of heavy atoms, spin-orbit interactions should play an important role in OMAR. It is known that these interactions are responsible for singlet and triplet states mixing via intersystem crossing and the change of spin-charge relaxation time in the charge mobility process. In this work, we report a dramatic change in the OMAR effect caused by the presence of strong intramolecular spin-orbit coupling in a series of rare-earth quinolate organic complex-based devices. Our data show a different OMAR lineshape compared with the OMAR lineshape of tris(8-hydroxyquinolinate) aluminum-based devices, which are well described in the literature. In addition, electronic structure calculations based on density functional theory help to establish the connection between this results and the presence of heavy central ions in the different complexes.

Magnetoelectric and magneto­resistive properties of the СexMn1-xS semiconductors

The effect of electron doping on the magnetoelectric and magnetoresistivity properties of cation-substituted antiferromagnetic СexMn1–xS (x ≤ 0.05) semiconductors at temperatures of 77–500 K in magnetic fields of up to 10 kOe was investigated. For all the compositions shifts of the temperature of the dielectric loss maximum toward higher temperature (80–500 K) with increasing magnetic field up to 10 kOe are found. The current dependence of magnetoresistance from the I–V characteristic measured in a magnetic field was established. The increase of the magnetoresistive effect with increase of the cerium ions concentration in the СexMn1–xS is found. The type of current carriers was determined on the basis of the Hall coefficient measurement. For explanation of experimental data the model of charge-orbital ordering, including orbital glass is used.

Large negative magnetoresistance induced by interplay between smooth disorder and antidots in AlGaN/GaN HEMT structures

Large low temperature negative magnetoresistance (NMR) experimentally observed in AlGaN/GaN high electron mobility transistors (HEMT) structures grown by metalorganic chemical vapour deposition on sapphire substrate has been reported. A linear B−1 ln B dependence of magnetoresistance observed in our samples indicates the presence of random antidot array together with smooth disorder. It is proposed that the antidots are linked with high bandgap AlN rich regions formed due to possible Al–Ga segregation at the interface during growth and the smooth random disorder is due to interface roughness. The antidot density is estimated to be of ∼7 to 8 × 1010 cm−2 in our samples. The magnitude of NMR is also correlated with the extent of interface roughness indicated by x-ray reflectivity. It is also proposed that the formation of antidots is related with the lattice mismatch between substrate and epitaxial heterostructures. The NMR in AlGaN/GaN HEMT structures grown on SiC substrates having relatively lower lattice mismatch has been shown to have a usual B2 and ln T dependences indicating only electron–electron interaction and absence of antidot-like scatterers.

Magneto-Seebeck effect in an ITO/PEDOT:PSS/Au thin-film device

This article reports giant magnetic field effects on the Seebeck coefficient by exerting a Lorentz force on charge diffusion based on vertical multi-layer ITO/PEDOT:PSS/Au thin-film devices. The Lorentz force, induced by an external magnetic field, changes the charge transport and consequently generates angular dependent magnetoresistance. The proposed mechanism of the magneto-Seebeck effect is proved by measuring the magnetoresistance at a parallel, 45o and perpendicular angle to the temperature gradient. The gradual change of the magnetoresistance from a parallel to perpendicular angle indicates that the Lorentz force is a key driving force to develop the magneto-Seebeck effect. Therefore, our experimental results demonstrate a magnetic approach to control the thermoelectric properties in organic materials.