Magnetoresistance Characteristics Research Articles

Effect of electrolyte components on the magnetic and magnetoresistive characteristics of Fe-containing plasma electrolytic oxide coatings on titanium

The effect replacing Na3PO4 with Na2HPO4 in aqueous phosphate–borate–tungstate electrolyte that additionally contains Fe2(C2O4)3 on the magnetic and magnetoresistive characteristics of oxide coating/ titanium composites formed by means of plasma electrolytic oxidation (PEO) is investigated. It is established that PEO coatings with ferromagnetic characteristics form on titanium in an electrolyte containing Na3PO4 (pH ~ 11) upon adding iron(III) oxalate, while replacing Na3PO4 with Na2HPO4 and the respective drop in the pH of the base electrolyte down to 9.8 results in the formation of coatings with different magnetic characteristics. The correlation between changes in the values of the charge carriers’ activation energy and the magnetic susceptibility is demonstrated for the latter. An increase in the electric resistance of coatings in a magnetic field is observed, and the type of the magnetic resistance temperature dependence is established.

Effect of temperature and magnetic field on disorder in semiconductor structures

We present the results of consistent theoretical analysis of various factors that may lead to influence of temperature and external magnetic field on disorder in semiconductor structures. Main attention is paid to quantum well (QW) structures in which only QWs or both QW and barriers are doped (the doping level is assumed to be close to the value corresponding to the metal–insulator transition). The above factors include (i) ionization of localized states to the region of delocalized states above the mobility edge, which is presumed to exist in the impurity band; (ii) the coexistence in the upper and lower Hubbard bands (upon doping of QWs as well as barriers); in this case, in particular, the external magnetic field determines the relative contribution of the upper Hubbard band due to spin correlations at doubly filled sites; and (iii) the contribution of the exchange interaction at pairs of sites, in which the external magnetic field can affect the relation between ferromagnetic and antiferromagnetic configurations. All these factors, which affect the structure and degree of disorder, lead to specific features in the temperature dependence of resistivity and determine specific features of the magnetoresistance. Our conclusions are compared with available experimental data.

Room-Temperature Spin Filtering in Metallic Ferromagnet-Multilayer Graphene-Ferromagnet Junctions.

We report room-temperature negative magnetoresistance in ferromagnet-graphene-ferromagnet (FM|Gr|FM) junctions with minority spin polarization exceeding 80%, consistent with predictions of strong minority spin filtering. We fabricated arrays of such junctions via chemical vapor deposition of multilayer graphene on lattice-matched single-crystal NiFe(111) films and standard photolithographic patterning and etching techniques. The junctions exhibit metallic transport behavior, low resistance, and the negative magnetoresistance characteristic of a minority spin filter interface throughout the temperature range 10 to 300 K. We develop a device model to incorporate the predicted spin filtering by explicitly treating a metallic minority spin channel with spin current conversion and a tunnel barrier majority spin channel and extract spin polarization of at least 80% in the graphene layer in our structures. The junctions also show antiferromagnetic coupling, consistent with several recent predictions. The methods and findings are relevant to fast-readout low-power magnetic random access memory technology, spin logic devices, and low-power magnetic field sensors.

Rectifying behavior and photovoltage effect in La1.3Sr1.7Mn2O7/SrTiO3-Nb heterostructure

Perovskite oxide heterostructure possesses attractive magnetic, optical and electric properties, such as superconducting interface between two insulators, two-dimensional electron gas, positive giant magnetoresistance, photoelectric response characteristic, magnetocaloric effect, and coexistent different magnetic structures. Especially for the photoelectric response behaviors of A1-xAxMnO3 (A=La, Pr etc.; A = Sr, Ca etc.) perovskite manganese oxide heterostructure, one has made a systematic study on the photoelectric conversion efficiency, the photovoltaic response speed, and the in-plane lateral photovoltage. Besides A1-xAxMnO3 structure, manganese oxides can also exhibit the double layered perovskite structure A2-2xA1+2xMn2O7. Double layered perovskite structure can be regarded as the layers of perovskite and rock salt which are alternately stacked. This double layered perovskite manganese oxide (such as La2-2xSr1+2xMn2O7) is a natural structure of the tunnel structure: ferromagnetic metal layer-insulating layer-ferromagnetic metal layer. Double layered perovskite manganese oxide has not only the characteristics of giant magnetoresistance, but also the novel physical properties, such as persistent photoconductivity, etc. However, there are few reports on the physical properties of the double layered perovskite manganite oxides, heterostructures, especially the photovoltaic properties. In this work, the La1.3Sr1.7Mn2O7 (LSMO) film is deposited on an n-type SrTiO3-Nb (NSTO) single crystal substrate by a pulsed laser deposition method. Additionally, we study the transporting properties of LSMO/NSTO heterostructure and its photovoltaic effect. The heterostructure exhibits benign rectifying and palpable photovoltaic effect. Under the 532 nm laser irradiation, the photovoltage first increases and then decreases with temperature rising. The maximal photovoltage reaches 400 mV at 150 K which is consistent with the metal-insulator transition temperature of LSMO film. It is indicated that the photovoltaic effect of the heterostructure is regulated by the inner transporting characteristics of LSMO film. The dynamical process of the heterostructure, photovoltaic response, is analyzed. Meanwhile, by analyzing the relationship between the photovoltage and time, it is found that the rising edge fits to the first order exponential function, which is related to the migration of carriers. While the falling edge of second-order exponential function indicates that the compound of carriers has two different physical processes: 1 corresponds to the neutralization process of the carriers aggregated on both junction sides through the external circuit, and 2 corresponds to the annihilation process of non-equilibrium carriers. The carrier lifetime of our p-n junction is longer, on the order of ms, than those of other manganese oxides p-n junctions. Remarkably, the time constants of both the rising edge and falling edge first increase and then decrease as temperature increases, and the maximum values occur at the metal-insulator transition temperature of LSMO film.

Influence of Pb Additive to the Spacer Layer on the Structure and Giant Magnetoresistance of Electrodeposited Co/Cu Multilayers

In an effort to see the possible surfactant effect of Pb on the formation of electrodeposited multilayers, Co/Cu(Pb) multilayers were prepared by this technique and their structure and giant magnetoresistance (GMR) were investigated. The multilayers were deposited from a perchlorate bath with various amounts of Pb2+ ions in the solution. The composition analysis by energy dispersive X-ray spectroscopy revealed that the Pb mole fraction in the deposit varies in a non-monotonous manner with Pb2+ ion concentration. By fitting the measured X-ray diffraction patterns, superlattice satellites could be identified in some of these multilayers. A ferromagnetic-type GMR behavior was observed for Co/Cu(Pb) deposits prepared from baths with small Pb2+ ion concentration, corresponding to the formation of a layered structure. The GMR magnitude decreased from 8 to 10% with increasing Pb concentration and, also, changed to a superparamagnetic-type GMR; finally, for high Pb2+ ion concentrations, the magnetoresistance behavior turned over to anisotropic magnetoresistance characteristic of bulk materials.

Numerical Investigation of the Magnetodynamics of Self-Organizing Nanoparticle Ensembles: A Hybrid Molecular and Spin Dynamics Approach

Ensembles of magnetic nanoparticles which are dispersed in a conductive gel matrix, show promising magnetoresistive characteristics [1-3]. By structuring the nanoparticles in the liquid gel by applying external magnetic fields, the magnetoresistance of such an arrangement can be adjusted. Furthermore, if the gel exhibits a liquid-solid transition the system structure and magnetoresistance characteristics can be preserved without the need of an external magnetic field. In order to predict the magnetoresistive properties of combinations of different magnetic nanoparticles and conductive gels, one has to simulate the magneto-dynamics of magnetic nanoparticles immersed in a gel. However, this requires to perform molecular dynamics and spin dynamics simulations simultaneously. Here, we present a hybrid approach, which combines two highly developed and well-established software packages into a new software tool that allows us to study this new class of problems.

Resistance Switching Phenomenon Associated with Anisotropic Magnetoresistance of the ReRAM Device with Ferromagnetic Electrodes

A resistive random access memory (ReRAM) is expected to be a next-generation nonvolatile memory substituting NAND Flash because of good scalability against downsizing, fast switching speed and low power consumption. ReRAM operation relies on the ability of the metal oxide to reversely change its resistance from high resistance to low resistance and vice versa by SET or RESET operation. The device structure is composed of a metal oxide layer sandwiched between two metal electrodes. In general, it is considered that resistive switching is caused by the formation and rupture of conductive filament (CF) such as metallic filaments or an oxygen vacancy filaments formed in the oxide layer. It is widely accepted that the type of filaments formed in the ReRAM devices depends on the combination of materials used as the electrodes and the oxide layer. The metallic filament is considered to be formed by metal ions migrated from the oxidizable electrode when a positive bias is applied to the oxidizable electrode. In the previous study, we found that ferromagnetic filament was formed at the on-state of the Ni/TiO2/Pt device [1]. Actually we observed a weak anisotropic magnetoresistance (MR) at room temperature. This study suggested that the conductive filament was formed with migration of Ni atoms from Ni electrode. In this study, we fabricated ReRAM device having two ferromagnetic electrodes; Ni and Fe-Ni. Both electrodes may be active under the voltage stress. Figure 1 shows the MR characteristics measured for the Ni/HfOx/Fe-Ni device at the room temperature. MR ratio is defined as α = [R(H)-R(H0)/R(H0)]×100. The ferromagnetism of filament was confirmed because the resistance of filament changed against magnetic field. It is most likely that this behavior is categorized as anistropic magnetoresistance. From the results, it is suggested that the conductive filament of Ni/HfOx/Fe-Ni device is formed by ferromagnetic materials. Further discussions and analysis will be presented in the symposium.

Synthesis and characterization of nanostructured magnetoresistive Ni doped ZnO films

We report a method to produce magnetic nanostructured semiconductor films based in ZnO doped with Nickel to control their magnetic properties. The method is based on a combined diffusion–oxidation process within a controlled atmosphere chamber to produce a uniform distribution of Ni ions in the ZnO films (ZnO:Ni). The synthesis of ZnO:Ni films is reported as well as the magnetoresistive characteristics, the used method yields films with reproducible and homogeneous properties. The films were also characterized structurally by X-Ray Diffraction (XRD) and Raman spectroscopy, and by Hall–van der Pauw measurements. The XRD measurements confirm the nanocrystalline films character. The films resulted of n-type conductivity with electron concentrations of ~1020cm−3 in average and carrier mobilities of 5cm2/Vs. The Magnetoresistance (MR) behavior of the films at 300K shows negative changes of ΔR~0.5% in accordance with the usual literature reports on samples produced by other methods.

Investigation of interfaces of multilayer Co/Cu structures using the method of nuclear magnetic resonance

The method of nuclear magnetic resonance has been used to investigate the structural features of the interfaces in [Co/Cu]n sublattices prepared using the method of magnetron sputtering. Changes in the structural characteristics of the interfaces and magnetoresistive properties of the sublattices have been analyzed depending on the number n of Co/Cu bilayers. Correlations have been found between the magnitude of the magnetoresistance and structural characteristics of the interfaces.

Conductivity and magnetoresistance of La0.7Ce0.3MnO3−δ thin films under photoexcitation

La0.7Ce0.3MnO3 thin films of different thicknesses, degrees of CeO2-phase segregation and oxygen deficiency, grown on SrTiO3 single crystal substrates, were comparatively investigated with respect to both their spectral and temperature-dependent photoconductivity (PC) and their magnetoresistance (MR) behaviour under photoexcitation. While as-grown films were insensitive to optical excitation, oxygen reduction appeared to be an effective way to decrease the film resistance, but the film thickness was found to play a minor role. However, from the evaluation of the spectral behaviour of the PC and the comparison of the MR of the LCeMO/substrate-samples with a bare substrate under illumination we find that the photoconductivity data reflects not only contributions from (i) photogenerated charge carriers in the film and (ii) carriers injected from the photoconductive substrate (as concluded from earlier works), but also (iii) a decisive parallel photoconduction in the SrTiO3 substrate. Furthermore—also by analyzing the MR characteristics—the unexpected occurence of a strong electroresistive effect in the sample with the highest degree of CeO2 segregation and oxygen deficiency could be attributed to the electroresistance of the SrTiO3 substrate as well. The results suggest a critical reconsideration and possibly a reinterpretation of several previous photoconductivity and electroresistance investigations of manganite thin films on SrTiO3.

Enhancement of Spin-transfer torque switching via resonant tunneling

We propose the use of resonant tunneling as a route to enhance the spin-transfer torque switching characteristics of magnetic tunnel junctions. The proposed device structure is a resonant tunneling magnetic tunnel junction based on a MgO-semiconductor heterostructure sandwiched between a fixed magnet and a free magnet. Using the non-equilibrium Green's function formalism coupled self consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation, we demonstrate enhanced tunnel magneto-resistance characteristics as well as lower switching voltages in comparison with traditional trilayer devices. Two device designs based on MgO based heterostructures are presented, where the physics of resonant tunneling leads to an enhanced spin transfer torque thereby reducing the critical switching voltage by up to 44%. It is envisioned that the proof-of-concept presented here may lead to practical device designs via rigorous materials and interface studies.

Influence of the Fe Concentration on the Superconducting Properties of Fe $$_{1-y}$$ 1 - y Se

We present a comparative study of electrical transport properties in the normal state and in the dissipative superconducting state between pure \(\beta \)-FeSe phase and Fe deficient Fe\(_{1-y}\)Se crystals. We discuss the influence of the intergrowth of the magnetic hexagonal phase (Fe\(_7\)Se\(_8\)) in Fe deficient samples when compared to pure \(\beta \)-FeSe samples. In the superconducting state, we measured the \(ab\)-plane electrical resistivity with magnetic field up to 16 T and the electrical resistivity as a function of the angle between the \(c\) axis and the applied field. The angular dependence at fixed temperature below the superconducting critical temperature, \(T_c(H=0)\), is very different for both sets of crystals. The Fe deficient samples display a vortex pinning-related feature at \(\sim \)57\(^\circ \) off the plane while the pure \(\beta \)-FeSe phase samples show the persistence of a strong angular-dependent magnetoresistance characteristic of the normal state electronic structure.

Magnetic and magnetoresistive characteristics of neutron-irradiated Si0.97Ge0.03 whiskers

Magnetic and magnetoresistive characteristics of neutron-irradiated Si0.97Ge0.03 whiskers

Impurity-band transport in organic spin valves.

The central phenomenon in the field of organic spintronics is the large magnetoresistance in thick organic spin valves. A prerequisite for understanding the magnetoresistance is a reliable description of the device resistance, or the I-V characteristics. Here I show that the observed I-V characteristics in the organic spin valves is incompatible with charge injection into the organic's lowest unoccupied molecular orbital or highest occupied molecular orbital but can be explained by electrons tunnelling into a broad impurity band located in the gap between these molecular orbitals. Voltage drop takes place mainly across depletion layers at the two electrode/organic interfaces, giving rise to electrode-limited charge transport. Spin-dependent electron tunnelling into the impurity band from the ferromagnetic electrodes results in spin accumulations inside the organic, which rapidly diffuses through the organic primarily via the exchange between impurity-band electrons. This picture explains the major magnetoresistance features and predicts enhanced capacitance in these devices.

Polarization dependent asymmetric magneto-resistance features in nanocrystalline diamond films

Polar angle-dependence of magneto-resistance (AMR) in heavily nitrogen-incorporated ultra-nanocrystalline diamond (UNCD) films is recorded by applying high magnetic fields, which shows strong anisotropic features at low temperatures. The temperature-dependence of MR and AMR can reveal transport in the weak-localization regime, which is explained by using a superlattice model for arbitrary values of disorder and angles. While a propagative Fermi surface model explains the negative MR features for low degree of disorder the azimuthal angle-dependent MR shows field dependent anisotropy due to the aligned conducting channels on the layers normal to film growth direction. The analysis of MR and AMR can extract the temperature dependence of dephasing time with respect to the elastic scattering time which not only establishes quasi-two dimensional features in this system but also suggests a potential application in monitoring the performance of UNCD based quantum devices.

Specific features of magnetoresistance in overcompensated manganese-doped silicon

It is shown experimentally that a rather high negative magnetoresistance in silicon is observed not only in compensated p-(Si:B):Mn, but also in overcompensated n-(Si:B):Mn with a Fermi level of F = EC − 0.35 eV ÷ EC − 0.55 eV. The magnitude and the temperature range of the negative magnetoresistance in materials of this kind are determined by the position of the Fermi level.

Interface Structure and Magnetoresistance Studies of [Co/C]<sub><i>n </i></sub>Superlattices by Means of NMR and TEM

Structural features of interfaces in [Co/Cu]n superlattices obtained by magnetron sputtering have been studied by nuclear magnetic resonance (NMR). Modification of interface structural characteristics and magnetoresistive properties of the superlattices with the increase of the number of [Co/C] bilayers is analyzed. Correlation between magnetoresistance and interface structural characteristics has been revealed.

Magnetoresistance Characteristics of Magnetorheological Gel under a Magnetic Field

A kind of magnetoresistance material was prepared, named as magnetorheological gel (MRG). The MRG samples were fabricated by dispersing carbonyl iron particles (CIP) into the polyurethane gel, and several experimental devices were prepared to investigate the magnetoresistance characteristics of MRG. The magnetoresistance characteristic was systematically tested and the influence of the magnetic field was analyzed. It is found that the resistance value of MRG with the CIP content of 70 wt % can be decreased from 7.56 to 2.44 MΩ with increasing of magnetic field from 0.1 T to 1 T. The experimental results have also proved that CIP content and the composition of matrix have a greater impact on the magnetoresistance characteristics. Because of the interaction between the magnetic force of CIP and motion resistance under a magnetic field, the obvious hysteresis phenomenon was observed and recorded. Lastly, it was observed that the magnetoresistance can be changed reversibly by controlling the magnetic field.

Weak localization effects as evidence for bulk quantization in Bi2Se3thin films

Strong spin-orbit coupling in topological insulators results in the ubiquitously observed weak antilocalization feature in their magnetoresistance. Here we present magnetoresistance measurements in ultra thin films of the topological insulator Bi_2Se_3, and show that in the 2D quantum limit, in which the topological insulator bulk becomes quantized, an additional negative magnetoresistance feature appears. Detailed analysis associates this feature with weak localization of the quantized bulk channels, providing thus evidence for this quantization. Examination of the dephasing fields at different temperatures indicates different scattering mechanism in the bulk vs the surface states.

ELECTRICAL TRANSPORT AND LOW-FIELD MAGNETORESISTANCE IN La0.8Sr0.2MnO3/Zn COMPOSITES

La 0.8 Sr 0.2 MnO 3 (LSMO)/ Zn composites were prepared by bonded method. The phase structure, electrical transport and low-field magnetoresistance (LFMR) characteristics have been investigated. The binder segregated mostly at the grain boundaries. The bonded composites exhibited enhanced LFMR, which was attributed to the enhanced intergrain spin-polarized tunneling behavior.