Magnetic Field Dependence Of Magnetoresistance Research Articles

Magneto-Transport Properties in LaMnO<sub>3</sub> Thin Films on a-SiO<sub>2</sub> Substrates Produced by Metal Organic Decomposition Method

We have studied the magneto-transport properties in LMO thin films on a-SiO2 substrates produced by the metal organic decomposition (MOD) method. LMO thin films have been prepared by the MOD method in the 100 % O2 gas atmosphere on different heat treatment conditions. Although LMO single crystal is an antiferromagnetic insulator, LMO thin films we have produced in the 100 % O2 gas atmosphere by use of the MOD method shows the ferromagnetic metal properties for suitable heat treatment conditions. We consider that the excess of O2- ions in LMO thin films produced in the 100 % O2 gas atmosphere induces the strong hole self-doping into those and changes to ferromagnetic metal. The quantity of excess O2- ions in LMO is sensitive to the heat treatment conditions of the LMO production, especially the temperature, time and atmosphere gas. We have obtained the coercive forces from the magnetic field dependence of magnetoresistance. Based on the temperature dependence of the coercive forces, we have estimated the Curie temperature of LMO thin films.

Linear magnetoresistance in the charge density wave state of quasi-two-dimensional rare-earth tritellurides

We report measurements of the magnetoresistance in the charge density wave (CDW) state of rare-earth tritellurides, namely TbTe$_3$ and HoTe$_3$. The magnetic field dependence of magnetoresistance exhibits a temperature dependent crossover between a conventional quadratic law at high $T$ and low $B$ and an unusual linear dependence at low $T$ and high $B$. We present a quite general model to explain the linear magnetoresistance taking into account the strong scattering of quasiparticles on CDW fluctuations in the vicinity of "hot spots" of the Fermi surface (FS) where the FS reconstruction is the strongest.

Hopping Transport and Spin-Polarized Tunneling Mechanism in Cr-Doped Gd0.7Ca0.3Mn1−x Cr x O3 (x = 0.0–0.5)

Temperature-dependent DC electrical resistivity of polycrystalline samples with orthorhombic perovskite structure Gd0.7Ca0.3Mn1−x Cr x O3 (x = 0.0–0.5) has been reported. The samples were prepared by the solid-state reaction method. The resistivity of all the samples indicates a semiconducting nature. In the high-temperature region, the electrical conduction process follows the small polaron hopping (SPH) mechanism showing exponential variation of resistivity with temperature. The activation energy increases with Cr concentration. The low-temperature resistivity data follows Mott’s ln(ρT −1/2)∼T −1/4 law of variable-range hopping (VRH) conduction mechanism. The estimated values of hopping distances (R) and hopping energies (W) of the samples start to increase with increasing Cr concentrations from and above x = 0.5 and satisfy essential conditions of the Mott VRH, i.e., αR>> 1 (α = inverse localization length) and W/ K B T>> 1. This signifies that evaluation of these parameters being logical and the effect can be attributed to high density of doping which introduces simultaneously potential disorder and localized states. The magnetic field-dependent magnetoresistance has been explained by a phenomenological model considering spin-polarized tunneling at grain boundaries in the samples.

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.

Investigation of magnetoresistance and localization behavior in nanobulk assembly of Bi2Te3

Magnetoresistance (MR) measurements are used to investigate the topological nature of nanobulk assembly of two dimensional (2D) Bi2Te3 nanoflakes prepared by microwave assisted solvothermal reactions. Average edge to edge length of individual hexagonal nanoflakes is found to be of ∼3-4 microns as demonstrated by SEM micrographs. Nanobulk assemblies prepared out of this 2D nanoflakes exhibit rhombohedral structure and have a high surface to volume ratio that is advantageous for probing the signatures of surface states through bulk measurements. At low temperatures, magnetic field dependent MR measurements demonstrate a clear positive MR with a prominent dip below ±1T. Significant changes happening in magnetic field dependent MR measurements are analyzed and discussed in the light of Weak anti localization (WAL) effect arising likely from surface states in nanobulk Bi2Te3 materials which on proper optimization can open up a new gateway for large scale application of these topological insulating materials in NEMS and MEMS technology.

Robust tunability of magnetoresistance in half-HeuslerRPtBi(R=Gd, Dy, Tm, and Lu) compounds

We present the magnetic field dependencies of transport properties for $R$PtBi ($R$ = Gd, Dy, Tm, and Lu) half-Heusler compounds. Temperature and field dependent resistivity measurements of high quality $R$PtBi single crystals reveal an unusually large, non-saturating magnetoresistance (MR) up to 300 K under a moderate magnetic field of $H$ = 140 kOe. At 300 K, the large MR effect decreases as the rare-earth is traversed from Gd to Lu and the magnetic field dependence of MR shows a deviation from the conventional $H^{2}$ behavior. The Hall coefficient ($R_{H}$) for $R$ = Gd indicates a sign change around 120 K, whereas $R_{H}$ curves for $R$ = Dy, Tm, and Lu remain positive for all measured temperatures. At 300 K, the Hall resistivity reveals a deviation from the linear field dependence for all compounds. Thermoelectric power measurements on this family show strong temperature and magnetic field dependencies which are consistent with resistivity measurements. A highly enhanced thermoelectric power under applied magnetic field is observed as high as $\sim$100 $\mu$V/K at 140 kOe. Analysis of the transport data in this series reveals that the rare-earth-based Half-Husler compounds provide opportunities to tune MR effect through lanthanide contraction and to elucidate the mechanism of non-trivial MR.

Spin transport and temperature-dependent giant positive junction magnetoresistance in CoFe2O4/SiO2/p-Si heterostructure

The observation of giant positive junction magnetoresistance (JMR) in our cobalt ferrite (CFO)/p-Si heterojunction has been reported here. The pulsed laser deposition technique has been used for fabrication of the heterojunction. The junction confirms a very good rectifying magnetic diode like behavior at low temperature, whereas at high temperatures the junction shows nonlinear I–V characteristics. The magnetic field-dependent magnetoresistance (MR) of both CFO film and across the heterojunction have been studied in detail. The CFO film shows negative MR behavior, whereas the junction shows large positive JMR behavior throughout the temperature range. The spin lifetime (142 ps) and spin diffusion length (331 nm) have been estimated of the heterostructure at 10 K. The highest JMR value (~1600 %) has been observed at 10 K, and it gradually decreases at higher temperature range. The origin of positive JMR in our heterojunction has been best explained by the standard spin injection theory.

Near room-temperature magnetoresistance effect in double perovskite La2NiMnO6

Near room-temperature magnetoresistance (MR) effect in ferromagnetic semiconductor La2NiMnO6 has been reported. The magnetic field dependent MR curve exhibits two distinct regions: the low-field MR which shows a rapid increase in an applied field within 200 Oe and the high-field MR which shows a relatively slow increase with the field increasing (more than 200 Oe). Based on the analyses of magnetic and electronic properties, we suggest that the low-field MR comes from the increased electron tunneling probability accompanied by the alignment of ferromagnetic domains under external field and the high-field MR is due to the suppression of scattering from the spin defects arising from the Ni/Mn antisite disorders.

Synthesis of La0.67Sr0.33MnO3 and polyaniline nanocomposite with its electrical and magneto-transport properties

We have synthesized composite of polyaniline nanotubes with lanthanum strontium manganite (La0.67Sr0.33MnO3) (LSMO) nanoparticles. Scanning electron microscopy, x-ray diffraction, Fourier transform infrared, and thermogravimetric analysis scans are done for morphological, structural, and thermal characterization of polyaniline-LSMO nanocomposite. Room temperature hysteresis loop shows ferromagnetic behavior of polyaniline-LSMO nanocomposite but with very low saturation magnetization. Overall pattern of temperature dependence of resistivity for polyaniline has been best-fitted with the form ln[ρ(T)]∝T−1/2. This may be attributed to quasi-one dimensional hopping and also tunneling between mesoscopic ferromagnetic metallic islands. Total resistivity due to grain and grain boundary reduces by application of magnetic field. Remarkable increase (∼73%) in magnetoresistance (MR) is obtained in these polymer coated LSMO compared to the without coated counterpart (16%). The MR% is found to increase with the increase in percentage of LSMO nanoparticles content in the composite. Temperature and magnetic field dependence of MR is observed. This nanocomposite can be used as a sensitive giant MR sensor.

Low-field magnetoresistance of La 0.7Ca 0.3MnO 3 perovskite synthesized by reactive milling method

Nanocrystalline, granular samples of La 0.7Ca 0.3MnO 3 (LCMO) were synthesized by reactive milling method and annealed at various temperatures. Effect of grain size on the low-field magnetoresistance (LFMR) has been investigated. Based upon a phenomenological model taking into account the spin-polarized transport across grain boundaries, we explained magnetic field dependence of magnetoresistance in LCMO samples. The contribution to the magnetoresistance coming from spin-polarized tunneling was separated out from the intergranular contribution. The fitted results showed that the temperature dependence of the LFMR displays a Curie–Weiss law-like behavior.

Low-field positive magnetoresistance near room temperature in three-component perovskite-type artificial superlattices

Perovskite-type superlattices consisting of La0.9Sr0.1MnO3, LaAlO3, and SrTiO3 were fabricated on (001) SrNb0.01Ti0.99O3 substrates by laser molecular beam epitaxy. The three-component superlattice exhibited desirable low-field magnetoresistance effects. At a given current bias of 10μA, positive magnetoresistance ratio was enhanced from 2% to 15% and 22% with increasing temperatures from 15 to 300 and 325K at 300Oe, at which a maximum value occurred from the external magnetic field dependence of magnetoresistance. The nature of the enhanced magnetoresistance is discussed in connection to the interfacial effect.

Magnetoresistance in thin wires with granular structure

We report on studies of magnetotransport properties and structure at 5–300 K temperature region in glass-coated microwires prepared from immiscible (Co 10Cu 90 and Co 29Ni 25Mn 1Cu 45) elements. The crystalline structure consists of nanocrystals of Co (Ni) and Cu with average grain size below 30 nm. Considerable magnetoresistance (MR) (up to about 18%) is found in Co 10Cu 90 microwires depending on fabrication parameters. Magnetic field dependence of MR of Co 10Cu 90 microwires is completely un-hysteretic showing monotonic decay with magnetic field. On the other hand, MR (up to about 4%) is also found in as-prepared and annealed Co 29Ni 25Mn 1Cu 45 microwires. Annealing of Co 29Ni 25Mn 1Cu 45 microwires (973 K) results in increasing of the coercivity ( H c) from 65 up to 750 Oe. Annealed Co 29Ni 25Mn 1Cu 45 sample exhibits considerable hysteresis on magnetic field dependence of MR. Neutron and X-ray diffraction allows to attribute changes in magnetic properties and MR after annealing of Co 29Ni 25Mn 1Cu 45 microwires to the decomposition of the metastable phase.

Magnetic field dependence of conductance and conduction noise of an artificial grain boundary junction in a La 0.67Ba 0.33MnO 3 epitaxial thin film

A single artificial grain boundary in La 0.67Ba 0.33MnO 3 (LBMO) thin film has been prepared by depositing the film on a bicrystal substrate using laser ablation technique. We investigated the magnetic field dependence of magnetoresistance and conductance–voltage characteristics of the grain boundary at 77 K. A decrease of nonlinearity of current–voltage characteristics was observed upon application of magnetic field. The results are explained by assuming the presence of two different types of parallel conducting channels (metallic and highly resistive) across the grain boundary. The analysis of the results reveals that the application of magnetic field suppresses magnetic disorders at the grain boundary region and increases metallic conduction channels across the grain boundary. The temperature dependence of the conduction noise of the bicrystal grain boundary was measured at 0 and 1.5 kG magnetic field and compared with a microbridge on the LBMO film having no grain boundary. The presence of the grain boundary was found to enhance noise by one order of magnitude. The noise of a bicrystal grain boundary showed a decrease in the presence of 1.5 kG magnetic field for T<210 K. This decrease of noise confirms that the application of a magnetic field induces more metallic channels across the grain boundary.

Magnetoresistance in Granular Co–Cu Glass-Coated Microwires

In this paper, we report on a considerable (up to about 18%) magnetoresistance (MR) effect in glass-coated Co-Cu microwires prepared from immiscible elements. Samples of Co/sub 10/Cu/sub 90/ microwires with a metallic nucleus diameter of 3-17 /spl mu/m and total diameter up to 25 /spl mu/m were prepared by the Taylor-Ulitovsky technique. The Co/sub 10/Cu/sub 90/ microwires generally consist of nanocrystals of Co and Cu with average grain size of about 40 nm, although the Co-phase content depends on the microwire geometry (metallic nucleus radius and glass-coating thickness). The magnetic field dependence of MR is completely anhysteretic, showing monotonous decay with the magnetic field.

Magnetoresistance in Co–Ni–Cu glass coated microwires

Abstract Considerable magneto-resistance (MR) is found in Co 29 Ni 25 Mn 1 Cu 45 microwires. Magnetization curves, magnetic field dependence of MR and coercivity, H c , in as-prepared and annealed Co 29 Ni 25 Mn 1 Cu 45 microwires are quite different. Roughly monotonic decay of MR with magnetic field above 1000 Oe is observed in as-prepared microwires. Annealed sample exhibits a maximum and considerable hysteresis on magnetic field dependence of MR. The position of this maximum correlates well with the values of coercivity, H c , obtained from magnetization curves.

Phenomenological theory of the giant magnetoresistance of superparamagnetic particles

A phenomenological theory is presented for the magnetic field dependence of the magnetoresistance (MR) of a granular sample containing very small superparamagnetic particles embedded in a nonmagnetic host. It is found that if the particles are present in a range of sizes, then MR varies almost linearly with the magnetization. The calculated magnetic field dependence of MR is in excellent agreement with recent data for a melt-spun granular sample of CuCo.

Scaling of Negative Magnetoresistance and Extraordinary Hall Effect in CaVO3-δ

We have investigated the detailed temperature- and magnetic field dependence of magnetoresistance and Hall resistance in single-crystal CaVO 3-δ (δ∼0.2). We have found that the low-temperature magnetoresistance and extraordinary contribution to the Hall resistance are both expressed well in terms of a Brillouin function B J [ g J µ B H / k B ( T -Θ)], Θ=-2 K. From the magnetization vs magnetic field data, it may be deduced that 4% of vanadium ions have a localized magnetic moment of S =3/2 or that 6% have S =1. However a resistance minimum (the Kondo effect) was not observed in this system and its possible reasons are discussed.

Ferromagnetic-to-antiferromagnetic transition in UCu2Ge2: Magnetoresistance study.

We report a detailed study of magnetoresistance in the intermetallic compounds ${\mathrm{UCu}}_{2}$${\mathrm{Ge}}_{2}$ and (${\mathrm{U}}_{0.95}$${\mathrm{Ce}}_{0.05}$)${\mathrm{Cu}}_{2}$${\mathrm{Ge}}_{2}$ in the temperature range of 4.5 to 140 K and in magnetic fields up to 50 kOe. The present measurements support the earlier conjecture drawn from the magnetic studies that the ferromagnetic to antiferromagnetic transition takes place gradually over a wide temperature range. The magnetic-field dependence of magnetoresistance shows that the low-temperature antiferromagnetic state is more fragile in ${\mathrm{UCu}}_{2}$${\mathrm{Ge}}_{2}$ than in (${\mathrm{U}}_{0.95}$${\mathrm{Ce}}_{0.05}$)${\mathrm{Cu}}_{2}$${\mathrm{Ge}}_{2}$.