Magnetoresistance Phenomena Research Articles

Weak localization and magnetoresistance phenomena of hydrogen-annealed WO3 films

We explore the electronic transport properties of hydrogenated epitaxial WO3 films. Ionized hydrogen not only generates an electron carrier but also acts as an impurity center, providing a useful approach to investigate a macroscopic quantum transport phenomenon of weak localization. Temperature dependent-resistivity and anomalous magnetoresistance results are analyzed based on Mott's variable range hopping and weak localization. From the model studies, the phase coherence length is estimated to have a temperature dependence of T −1.6 and is determined to vary from ∼22 nm at 80 K to ∼180 nm at 20 K. The length at 20 K is of a similar order of magnitude of film thickness, which requires a dimensional crossover between three-dimensional and two-dimensional localization.

Magnetic proximity sensor based on colossal magnetoresistance effect

Magnetic proximity sensors are widely used to determine the distance to ferromagnetic or highly conductive materials and to investigate their shape or to detect their motion. In this study, a magnetic proximity sensor incorporating a cylindrical permanent magnet and a scalar magnetic field sensor (measuring the magnitude of the field independently from its orientation) based on the phenomenon of colossal magnetoresistance in manganite films is presented. Two versions of the sensor, one with a solid cylindrical magnet and one with a hollow cylindrical magnet, were constructed and tested. A steel plate and a superconducting YBaCuO disk were used to investigate the static response. The measurements of the dynamic behavior were carried out with rotating steel and duralumin plates. The results of the static experiments and modelling showed that the output response (change of the magnetic field) of the sensor had a hyperbolic relationship with the distance between the sensor and the measured object. This change was positive for solid cylindrical magnet design and negative for hollow cylindrical magnet design when the target was steel. In contrast, the solid magnet-based sensor produced a negative signal for the superconducting material, while the hollow cylindrical magnet sensor showed a positive signal. It was found that the main features of the dynamic response of the proximity sensors can be explained by the magnetization of steel and the induction of eddy currents in duralumin. The present study demonstrates that the proposed magnetic proximity sensor can be successfully used to search for ferromagnetic objects, investigate the properties of superconductors and detect the motion of non-ferromagnetic conductive materials and can have advantages over conventional magnetic proximity sensors.

A Review of Synthesis and Novel Transport Properties of Multivalent Manganate Perovskite: Progress, Opportunities, and Challenges

AbstractUnderstanding the microstructure of materials and their impact on performance is crucial for new electronic devices design. The strong interaction between multiple degrees of freedom (spin, orbit, charge, and lattice) within perovskite type oxides can generate rich electron phase diagrams. The microscopic modulation of manganese ions electron states in rare earth manganate perovskites is the root of many novel macroscopic quantum behaviors, such as giant magnetoresistance phenomenon. For ABO3 type perovskites, the differences of radii and valence states of A‐site ions are used to regulate the coexistence of B site manganese multivalent states (Mn3+, Mn4+, Mn5+), and its ordered structure in the metastable state will create a new combination of electronic states. The ordered arrangement of the multi‐dimensional space of electronic states can induce unique transport properties. Therefore, the preparation of manganate perovskites will provide strong chemical support to construct new generation quantum devices. This review summarizes the challenges and difficulties on constructing trivalent coexisting metastable phase in manganate perovskites through disproportionation reactions, and discusses the status and potential of designing ideal rectified p‐n junctions taking advantage of this unique structure. Finally, the foregrounds of new quantum states driven by this chemical reaction are prospected.

The synthesis and tunable topological properties of Sm-doped (Bi, Sb)2Te2S crystal

Topological insulators exhibit Dirac points in their topological surface states, but often such Dirac points are far from the Fermi level and sometimes even reside outside the bulk band gap. Here, we report the synthesis of a new topological insulator material Sm-doped (Bi, Sb)2Te2S, which has its surface tunable Dirac point exposed within the bulk gap and close to the Fermi level at a minimal distance of 73 meV with a large tunability of 157 meV, as evidenced by angle-resolved photoemission spectroscopy. The compensation doping by Sm creates a nearly insulating bulk and triggers an interesting phenomenon of all-configuration negative magnetoresistance. This signals a nearly compensated state where the bulk transport is mainly via percolation through charge puddles. Our work offers a new material platform suitable for regulating the position of surface Dirac points and exploring unique physical properties of topological insulators.

Influence of the NiFe/Cu/NiFe Structure Dimensions and Position in External Magnetic Field on Resistance Changes under the Magnetoresistance Effect.

The subject of this work is NiFe/Cu/NiFe thin-film structures made by magnetron sputtering and showing the phenomenon of magnetoresistance. Three series of samples differing in spatial dimensions and thickness of the Cu spacer were produced. During the sputtering process, an external magnetic field of approx. 10 mT was applied to the substrate. Measurements of the resistance of the structures were carried out in the field of neodymium magnets in three different positions of the sample in relation to the direction of the field. The measurements allowed us to indicate in which position the structures of different series achieved the greatest changes in resistance. For each of the three series of layer systems, the nature of changes in the determined coefficient of giant magnetoresistance ΔR/R remained similar, while for the series with the smallest copper thickness (2.5 nm), the coefficient reached the highest value of about 2.7‱. In addition, impedance measurements were made for the structures of each series in the frequency range from 100 Hz to 100 kHz. For series with a thinner copper layer, a decrease in impedance values was observed in the 10-100 kHz range.

MXene-Based Sc2CHO/Sc2NHO/Sc2CHO Magnetic Tunnel Junctions for Multi-Value Logic Computing Devices: A First Principles Study

The rapid development of technologies such as machine learning and artificial intelligence has further accelerated the growth of multi-value logic (MVL) computing nanodevices. However, current MVL nanodevices based on conventional materials face the challenges of low peak–valley ratio (PVR) and unstable spin filtering effects. Therefore, the intrinsic half-metal nanoscale MXene has become the answer to the development bottleneck of MVL nanodevices for its outstanding properties such as high Curie temperature and an appropriate band structure. The present work attempts to design spintronic magnetic tunnel junctions (MTJs) based on new predicted Sc2CHO as half-metallic electrodes and Sc2NHO as semiconductor-based scattering regions. Furthermore, we simulate its transport properties from density functional theory combined with nonequilibrium Green’s function. And we have considered the effect of the length of the scattering region on MTJ performance. According to our calculations, the spintronic transport of the MTJs demonstrates up to perfect 100% polarizability spin currents stably due to the single-channel conduction capability of half-metal Sc2CHO. The MTJs exhibit a significant negative differential resistance (NDR) phenomenon, with the PVR of the type 2N reaching 1.28 × 106, the highest value among the two-dimensional spintronic nanodevices currently being theoretically explored. MTJs with a high PVR will be beneficial for the application and development of MVL nanodevices. The spin filtering effect and NDR phenomenon can be well-maintained as the scattering region length increases, and the PVR usually almost always remains 104. This is well reflected in the transmission spectra and molecular projection self-consistent Hamiltonian. The tunneling magnetoresistance phenomenon is also observed.

Large Negative Magnetoresistance in All‐2D‐Materials‐Based Spin Valves

Two‐dimensional (2D) magnetic materials have attracted enormous interest and opened up a new direction to novel spintronic applications. Herein, a large negative magnetoresistance (MR) in all‐2D‐materials‐based vertical magnetic spin valves with nano/micro bubbles existing at the interface between the spacer layer (MoS2) and the top ferromagnetic electrode layer (Cr0.85Te or Fe3GeTe2) is demonstrated. The large negative MR, with a magnitude of ≈21.5% attained in Cr0.85Te/MoS2/Fe3GeTe2, could be fully suppressed by the elimination of the bubbles after a short argon‐protected annealing, indicating the determining role of the bubbles in modulating the spin‐related transport behavior. First‐principles calculations based on density function theory (DFT) illustrate that the compressive strain can significantly modify the spin‐resolved electronic band structures of the 2D ferromagnetic materials, and hence alter the spin polarization near the Fermi level, accounting for the negative MR phenomena with the conventional two‐current model. Herein, a new route to all‐2D‐materials‐based spintronic devices via strain engineering is provided, and also offers a new insight into the role of bubbles formed during fabricating 2D heterostructures in developing exotic properties and novel functionalities.

Exchange bias controlled antisymmetric-symmetric magnetoresistances in Fe3GeTe2/graphite/Fe3GeTe2 trilayer

The magnetoresistance (MR) of spin values usually displays a symmetric dependence on the magnetic field. An antisymmetric MR phenomenon has been discovered recently that breaks the field symmetry and has the potential to realize multi-bit memory. In this work, we report a controllable switch between the antisymmetric and symmetric MRs and propose a multi-bit memory performance in Fe3GeTe2 (FGT)/graphite/FGT trilayer with modified vertical geometry. Via investigating the evolution of the antisymmetric MR depending on the spatial distribution, current direction, and magnetization configuration, we demonstrate that the antisymmetric MR results from the local nonequilibrium current through the trilayer. Furthermore, an exchange bias effect is induced which modifies the antisymmetric MR. A stable multi-bit memory is presented in the heterostructure. Such architecture for multi-state memory provides insights into other spin-valve structures to improve storage density.

Magnetic field dependent metal insulator transition by monovalent doping (Na+) in PrMnO3: Investigation through structural, magnetic and transport properties

A detailed study on the structural, magnetic, and transport properties of a monovalent (Na+) doped Pr0.75Na0·25MnO3 (PNMO) compound has been carried out. Temperature (T) and magnetic field (H) dependent magnetization (M), and resistivity (ρ) measurements have been carried out which reveal a second-order magnetic phase transition around TC = 60 K accompanied by a metal-insulator transition around TMI = 138 K. Magnetization measurements also confirm the coexistence of antiferromagnetic (AFM) and ferromagnetic (FM) components in the low-temperature regime. M − H has been used to calculate the magnetocaloric behavior and the system is found to show a maximum magnetic entropy change i.e. –ΔSmax as 2.54 J kg −1.K −1 for the field change of 5 T. Temperature dependence resistivity has been measured under the applied magnetic field and the system exhibits a colossal magnetoresistance (≅ 81%). Spin-polarized tunneling (SPT) model is applied to explain magnetoresistance (MR) phenomenon under applied magnetic field. Analysis of zero-field resistivity data signifies that a transition from Mott-VRH to Efros-Shklovskii-VRH (ES-VRH) conduction occurs in the low-temperature region in this present PNMO system.

Inhibitivni efekat magnetnog polja na koroziju mekog čelika u kiselom mediju

The inhibitive effect of applied magnetic fields on the corrosion of mild steel in 1.0M HCl and 0.5M H2SO4 acid solutions has been investigated at room temperature by using the gravimetric technique. A locally made electromagnet with variable outputs and powered by a direct current source voltage provided the magnetic fields. In a typical experiment, the applied magnetic field was applied perpendicularly onto the surface of a mild steel coupon immersed completely in a chosen acid solution for an exposure time of two hours. It was observed that the corrosion rates of mild steel determined for each case of 1.0M HCl and 0.5M H2SO4 test solutions decrease as the applied magnetic field increases. This indicates that the presence of the applied magnetic field inhibits the corrosion of the mild steel in the test solutions. Results also show that the protection efficiency of the magnetic field increases as the applied magnetic field increases. More specifically, as the magnetic field was increased from 118.2 - 764.3mG [or 0.01182 - 0.07643 mT], the protection efficiency increased from 36.34 - 62.67% for mild steel in 1.0M HCl acid solution, while for mild steel in 0.5M H2SO4 acid solution, the protection efficiency increased from 12.68 - 46.88%. It was demonstrated that the observed inhibitive effect of the magnetic fields can be attributed partly to the phenomenon of transverse magnetoresistance and partly to the effect of the magnetic fields on the mass transport properties of the constituent ions in the electrochemical system

Bilinear Magnetoresistance in HgTe Topological Insulator: Opposite Signs at Opposite Surfaces Demonstrated by Gate Control.

Spin-orbit effects appearing in topological insulators (TI) and at Rashba interfaces are currently revolutionizing how we can manipulate spins and have led to several newly discovered effects, from spin-charge interconversion and spin-orbit torques to novel magnetoresistance phenomena. In particular, a puzzling magnetoresistance has been evidenced as bilinear in electric and magnetic fields. Here, we report the observation of bilinear magnetoresistance (BMR) in strained HgTe, a prototypical TI. We show that both the amplitude and sign of this BMR can be tuned by controlling with an electric gate the relative proportions of the opposite contributions of opposite surfaces. At magnetic fields of 1 T, the magnetoresistance is of the order of 1% and has a larger figure of merit than previously measured TIs. We propose a theoretical model giving a quantitative account of our experimental data. This phenomenon, unique to TI, offers novel opportunities to tune their electrical response for spintronics.

Double perovskite Sr2FeReO6 oxides: Structural, dielectric, magnetic, electrical, and optical properties

AbstractDouble perovskite Sr2FeReO6 (SFRO) powders were synthesized by so‐gel process and annealed in argon atmosphere. Their structural, dielectric, magnetic, electrical, and optical properties were comprehensively investigated. It was found that the SFRO powders possessed a tetragonal crystal structure with I4/m space group and exhibited spherical shapes with some agglomeration due to the magnetic interactions between particles of the powders. Quantitative energy dispersive X‐ray spectrometer data revealed the atomic ratio of Sr, Fe, Re, and O elements close to the nominal values of 2:1:1:6. X‐ray photoemission spectroscopy spectra reveal two species of Re5+ and Re6‐7+ coexist in the SFRO powders. Sr, Fe, and O elements are present as Sr2+, Fe3+, and lattice oxygen, respectively.Dielectric property measurements revealed a Maxwell–Wagner type dielectric dispersion in the SFRO ceramics. Ferromagnetic behavior was verified by the observed magnetic hysteresis loops in the SFRO powders at 2 K and 300 K. The remanent magnetization and coercive field at 2 K were 8.23 emu/g and 3152 Oe, respectively, and the saturated magnetization was estimated to be 21.8 emu/g (or 2.0 μB/f.u.), smaller than the theoretical value of 3.0 μB/f.u. owing to the presence of the anti‐site defects. Magnetic Curie temperature (TC) was estimated to be 432.3 K. Intergranular tunneling magnetoresistance and hysteresis phenomena were observed in the SFRO powders at low temperatures, and the MR (2 K, 6 T) was measured to be −15% and −10% for MR (100 K, 6 T). Electrical transport and optical absorption measurements demonstrate the semiconducting nature of the SFRO with optical band gap of 1.39 eV. The electrical transport process follows the small polaron variable range hopping theory. The unique combination of high TC ferromagnetism with the semiconductivity enables the SFRO to be a promising candidate for spintronic devices.

Technology and measurements of three-layer NiFeCuMo/Ti/NiFeCuMo structures exhibiting the giant magnetoresistance phenomenon

Magnetoresistive phenomena are widely used in the construction of magnetic field sensors for biological, space, automotive, etc., applications. For this purpose, a number of methods of obtaining and testing this type of sensor are used. Therefore, the motivation to take up this topic is to obtain a structure showing magnetoresistance and carry out electrical measurements in a magnetic field to confirm the assumptions about the presence of magnetoresistance. This paper focuses on the technology and measurements of thin-film three-layer resistors. Structures NiFeCuMo/Ti/NiFeCuMo with 100/10/100 nm thicknesses were developed in situ using a DC magnetron sputtering method. A scanning electron microscopy or energy dispersive spectroscopy analysis was conducted on a sample structure and the target alloy to establish their chemical composition. The occurrence of the giant magnetoresistance exhibited by the developed three-layers was successfully confirmed in the presence of an applied magnetic field and the maximum value of resistance changes ΔR was approximately 2.75 Ω. Additionally, the thickness of the resistors was determined with a confocal microscope to verify the accuracy of the applied deposition method.

Zaštitno dejstvo magnetnog polja na koroziju aluminijumskih legura u rastvorima hlorovodonične kiseline

The effect of applied magnetic fields on the corrosion of aluminium alloys AA3003 and AA8011 in hydrochloric acid solutions at room temperature has been investigated by using the gravimetric technique. The magnetic fields were generated from a locally made electromagnet with variable outputs. They were applied at right angles to the surfaces of the aluminium alloy samples when completely immersed in the test solutions. The corrosion rates of the aluminium alloys AA3003 and AA8011 in 0.5M HCl acid solutions were monitored at different applied magnetic fields. Results show that the corrosion rates of the aluminium alloy samples studied decrease with increase in the applied magnetic field. This implies that the presence of magnetic field inhibits the corrosion of the aluminium alloy AA3003 and AA8011 samples. With applied magnetic fields in the range of 254.3 - 1040.6 mG [0.0254 - 0.1041 mT], the protection efficiency of the magnetic field observed for AA3003 in 0.5M HCl acid solutions ranges from 19.3 - 94.1% while for aluminium alloy AA8011 in identical test solutions, the protection efficiency ranges from 11.5 - 78.0%. The protection mechanism is attributed partly to the phenomenon of transverse magnetoresistance and partly to effect of the magnetic fields on the mass transport pattern of constituent ions in the electrochemical system.

Electron transport property of epitaixial bilayer graphene on SiC substrate

Graphene can find great potential applications in the future electronic devices. In bilayer graphene, the relative rotation angle between graphene layers can modulate the interlayer interaction and hence induces rich physical phenomena. We systematically study the temperature dependent magnetoresistance (MR) properties in the epitaxial bilayer graphene (BLG) grown on the SiC substrate. High quality BLG is synthesized by molecular beam epitaxy in ultra-high vacuum. We observe the negative MR under a small magnetic field applied perpendicularly at temperature < 80 K, which is attributed to a weak localization effect. The weak localization effect in our epitaxial BLG is stronger than previously reported ones in epitaxial monolayer and multilayer graphene system, which is possibly because of the enhanced interlayer electron transition and thus the enhanced valley scattering in the BLG. As the magnetic field increases, the MR exhibits a classical Lorentz MR behavior. Moreover, we observe a linear magnetoresistance behavior in a large field, which shows no saturation for the magnetic field of up to 9 T. In order to further investigate the negative and linear magnetoresistance, we conduct angle-dependent magnetoresistance measurements, which indicates the two-dimensional magnetotransport phenomenon. We also find that the negative MR phenomenon occurs under a parallel magnetic field, which may correspond to the moiré pattern induced local lattice fluctuation as demonstrated by scanning tunneling microscopy measurement on an atomic scale. Our work paves the way for investigating the intrinsic properties of epitaxial BLG under various conditions.

Superconducting triplet pairings and anisotropic magnetoresistance effects in ferromagnet/superconductor/ferromagnet double-barrier junctions

Ferromagnetic spin valves offer the key building blocks to integrate giant- and tunneling-magnetoresistance effects into spintronics devices. Starting from a generalized Blonder-Tinkham-Klapwijk approach, we theoretically investigate the impact of interfacial Rashba and Dresselhaus spin-orbit couplings on the tunneling conductance, and thereby the magnetoresistance characteristics, of ferromagnet/superconductor/ferromagnet spin-valve junctions embedding thin superconducting spacers between the either parallel or antiparallel magnetized ferromagnets. We focus on the unique interplay between usual electron tunnelings-that fully determine the magnetoresistance in the normal-conducting state-and the peculiar Andreev reflections in the superconducting state. In the presence of interfacial spin-orbit couplings, special attention needs to be paid to the spin-flip ("unconventional") Andreev-reflection process that is expected to induce superconducting triplet correlations in proximitized regions. As a transport signature of these triplet pairings, we detect conductance double peaks around the singlet-gap energy, reflecting the competition between the singlet and an additionally emerging triplet gap; the latter is an effective superconducting gap that can be ascribed to the formation of triplet Cooper pairs through interfacial spin-flip scatterings (i.e., to the generation of an effective triplet-pairing term in the order parameter). We thoroughly analyze the Andreev reflections' role in connection with superconducting magnetoresistance phenomena, and eventually unravel huge conductance and magnetoresistance magnetoanisotropies-easily exceeding their normal-state counterparts by several orders of magnitude-as another experimentally accessible fingerprint of unconventional Andreev reflections.

Maximum entropy mobility spectrum analysis for the type-I Weyl semimetal TaAs

Due to non-saturating magnetoresistance (MR) and the special compensation mechanism, the Weyl semimetal TaAs single crystal has attracted considerable attention in condensed matter physics. Herein, we use maximum entropy mobility spectrum analysis (MEMSA) to extract charge carrier information by fitting the experimentally measured longitudinal and transverse electric transport curves of TaAs. The carrier types and the number of bands are obtained without any hypothesis. Study of the temperature dependence shows details of carrier property evolution. Our quantitative results explain the non-saturated magnetoresistance and Hall sign change phenomena of TaAs.

Coexistence of large positive and negative magnetoresistance in Cr2Si2Te6 ferromagnetic semiconductor

Magnetoresistance (MR) phenomenon couples the electron transport with magnetic field, which has been at the forefront of condensed matter physics and materials science. Large-MR behaviors are of particularly importance for magnetic sensor and information memory applications, and their scarcity has aroused intensive research. Moreover, due to the different physical origins, combination of large positive and negative MR (pMR and nMR) in one single compound has rarely been reported. In present work, we achieved a coexistence of large pMR and nMR in Cr2Si2Te6 ferromagnetic semiconductor single crystal with different field configurations. Specifically, a large nMR of about −60% was obtained under the in-plane field, while a large pMR higher than 1000% took over in the out-of-plane direction. We attribute this field direction-sensitive dualistic large MR behavior to the competition and cooperation effect from the ferromagnetic interaction, orbital scattering and electronic correlation that coexist in Cr2Si2Te6, which contribute to nMR, pMR, and nMR, respectively, in dominated temperature and field ranges, and show different weights under different field directions. The elucidated multiple MR mechanism in this ferromagnetic semiconductor will shed light on the pursuit of coexistence of large pMR and nMR for field-sensitive device applications.

Non-reciprocal magnetoresistance, directional inhomogeneity and mixed symmetry Hall devices

Phenomenology similar to the non-reciprocal charge transport violating Onsager's reciprocity relations can develop in directionally inhomogeneous conducting films with non-uniform Hall coefficient along the current trajectory. The effect is demonstrated in ferromagnetic CoPd films and analyzed in comparison with the unidirectional magnetoresistance phenomena. We suggest to use an engineered inhomogeneity for spintronics applications and present the concept of mixed symmetry Hall devices in which transverse to current Hall signal is measured in a longitudinal contacts arrangement. Magnetization reversal and memory detection are demonstrated in the three-terminal and the partitioned normal metal–ferromagnet (NM-FM) device designs. Multi-bit memory is realized in the partitioned FM-NM-FM structure. The relative amplitude of the antisymmetric signal in the engineered ferromagnetic devices is few percent, which is 10–103 times higher than in their unidirectional magnetoresistance analogues.

Anisotropic magnetoresistance and memory effect in bulk systems with extended defects

To describe kinetic phenomena in disordered conductors, various acts of scattering of electrons can be often considered as independent, that is captured by the Boltzmann equation. However, in some regimes, especially, in a magnetic field, it becomes necessary to take into account the correlations between different scattering events of electrons on defects at different times in the past. Such memory effects can have a profound impact on the resistivity of 2D semiconductor systems, resulting in giant negative magnetoresistance and microwave-induced resistance oscillations phenomena. This work opens the discussion of the memory effects in 3D conducting systems featured by the presence of extended one-dimensional defects, such as screw dislocations or static charge stripes. We demonstrate that accounting for the memory effect, that is the capture of electrons on collisionless spiral trajectories winding around extended defects, leads to the strong negative magnetoresistance in case when the external magnetic field direction becomes parallel to the defects axis. This effect gives rise to a significant magnetoresistance anisotropy already for an isotropic Fermi surface and no spin–orbit effects. The proposed resistivity feature can be used to detect one-dimensional scattering defects in these systems.