Significant Magnetoresistance Research Articles

The structure, magnetic properties, magnetotransport and temperature coefficient of resistivity of hexagonal 4H-SrMnO3 manganite

The perovskite hexagonal SrMnO3 manganite with four layers (4H-SrMnO3) have been one of the hot topics in materials science due to the physical characteristics of antiferromagnetic, ferroelectric, and thermoelectric effects. The transport properties and magnetoresistance (MR) effect of 4H-SrMnO3 have seldom been reported. The structure, temperature coefficient of resistivity (TCR), magnetic and magnetotransport properties of the 4H-SrMnO3 SrMnO3 manganite obtained by solid state reaction have been investigated in this work. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) measurements at room temperature reveal that the SrMnO3 sample exhibits a pure hexagonal symmetric structure, with uniform distribution of particles and elements. Under applied magnetic fields of 2000 Oe, the SrMnO3 sample exhibits a transition from antiferromagnetic (AFM) to paramagnetic (PM) behavior at approximately 280 K, known as the Néel temperature (TN). The resistance is influenced by the magnetic field, but overall exhibits an insulating state. Remarkably, the SrMnO3 sample shows significant MR effect near room temperature. The negative MR near room temperature varies from 5.8 % with 1 T field to 16.8 % with 6 T field, indicating an existence of obvious MR in the hexagonal AFM SrMnO3 manganite. Furthermore, the maxmium value of TCR reaches 4.34 % K−1 near room temperature. These findings of room-temperature TCR and room-temperature MR in the AFM 4H-SrMnO3 hold immense value for the research and potential application of the AFM spintronics and devices.

Structure, impedance spectroscopy, and magnetic properties of nanostructured composites (1-x)CoFe2O4–xPbTiO3

The article presents a comprehensive investigation of the crystal structure, impedance spectra, magnetic, magnetodielectric, and magnetoresistive properties of (1-x)CoFe2O4–xPbTiO3 composites with varying concentrations (х = 0, 0.2, 0.4, and 0.6). The study also examines the effects of applying a uniaxial pressure of 1 GPa to the synthesized powders using Bridgman anvils. Our findings reveal that the synthesis of these composites results in the formation of an additional phase, lead hexaferrite PbFe12O19, which exhibits multiferroic properties. Additionally, the coherent scattering region of the components is significantly reduced after mechanical activation. Notably, the real part of the resistivity ρ′(ω) of nanostructured CoFe2O4 ceramics increases eightfold at T = 240 °C. The composites demonstrate significant magnetoresistance at room temperature, reaching up to 250 %. The study also reveals that the signs of the magnetodielectric MD(B) and magnetoresistive coefficient MR(B) vary with the frequency of the measuring field for certain concentrations. Using the first-order reversal curve (FORC) method, it was observed that after nanostructuring CoFe2O4 through mechanical activation, the interaction field Hu shifts from ± 0.6 kOe to ± 0.8 kOe, while the coercive field Hc increases from 1.05 kOe to 5 kOe. Moreover, the two-dimensional FORC maps of the composites show increased complexity, due to the formation of additional magnetic phases.

One-step ultrafast Joule heating synthesis of EuB6 ceramics and their electronic transport and magnetic properties

The synthesis of high-performance rare-earth borides (e.g., RB2, RB4, R2B5, RB6, RB12, and RB66) typically necessitates sintering at high temperatures and high pressures for prolonged durations. Traditional methods, such as hot-pressing sintering and spark plasma sintering, are marked by high production costs and low efficiency. Here, using EuB6 as an example, we demonstrate the successful synthesis of rare-earth boride ceramics utilizing the one-step ultrafast joule heating technology. Using this method, we optimized the sintering temperature and duration and synthesized EuB6 ceramics with superior electronic and magnetic properties at 1600 °C in just 5 min. The EuB6 ceramics prepared under optimized conditions show distinct paramagnetic to ferromagnetic phase transition, significant negative magnetoresistance, and enhanced magnetization. The combination of the one-step ultrafast Joule heating technology and boron thermal reduction method offers a simple and fast synthesis approach for fabricating polycrystalline EuB6 and may be extended to prepare other rare-earth and transition-metal borides.

Predictable Gate-Field Control of Spin in Altermagnets with Spin-Layer Coupling.

Spintronics, a technology harnessing electron spin for information transmission, offers a promising avenue to surpass the limitations of conventional electronic devices. While the spin directly interacts with the magnetic field, its control through the electric field is generally more practical, and has become a focal point in the field. Here, we propose a mechanism to realize static and almost uniform effective magnetic field by gate-electric field. Our method employs two-dimensional altermagnets with valley-mediated spin-layer coupling (SLC), in which electronic states display valley-contrasted spin and layer polarization. For the low-energy valley electrons, a uniform gate field is approximately identical to a uniform magnetic field, leading to predictable control of spin. Through symmetry analysis and abinitio calculations, we predict altermagnetic monolayer Ca(CoN)_{2} and its family materials as potential candidates hosting SLC. We show that an almost uniform magnetic field (B_{z}) indeed is generated by gate field (E_{z}) in Ca(CoN)_{2} with B_{z}∝E_{z} in a wide range, and B_{z} reaches as high as about 10^{3} T when E_{z}=0.2 eV/Å. Furthermore, owing to the clean band structure and SLC, one can achieve perfect and switchable spin and valley currents and significant tunneling magnetoresistance in Ca(CoN)_{2} solely using the gate field. Our work provides new opportunities to generate predictable control of spin and design spintronic devices that can be controlled by purely electric means.

Emergence of flat bands and ferromagnetic fluctuations via orbital-selective electron correlations in Mn-based kagome metal

Kagome lattice has been actively studied for the possible realization of frustration-induced two-dimensional flat bands and a number of correlation-induced phases. Currently, the search for kagome systems with a nearly dispersionless flat band close to the Fermi level is ongoing. Here, by combining theoretical and experimental tools, we present Sc3Mn3Al7Si5 as a novel realization of correlation-induced almost-flat bands in the kagome lattice in the vicinity of the Fermi level. Our magnetic susceptibility, 27Al nuclear magnetic resonance, transport, and optical conductivity measurements provide signatures of a correlated metallic phase with tantalizing ferromagnetic instability. Our dynamical mean-field calculations suggest that such ferromagnetic instability observed originates from the formation of nearly flat dispersions close to the Fermi level, where electron correlations induce strong orbital-selective renormalization and manifestation of the kagome-frustrated bands. In addition, a significant negative magnetoresistance signal is observed, which can be attributed to the suppression of flat-band-induced ferromagnetic fluctuation, which further supports the formation of flat bands in this compound. These findings broaden a new prospect to harness correlated topological phases via multiorbital correlations in 3d-based kagome systems.

Voltage tunable sign inversion of magnetoresistance in van der Waals Fe3GeTe2/MoSe2/Fe3GeTe2 tunnel junctions

The magnetic tunnel junctions (MTJs) based on van der Waals (vdW) materials possess atomically smooth interfaces with minimal element intermixing. This characteristic ensures that spin polarization is well maintained during transport, leading to the emergence of richer magnetoresistance behaviors. Here, using all 2D vdW MTJs based on magnetic metal Fe3GeTe2 and non-magnetic semiconductor MoSe2, we demonstrate that the magnitude and even sign of the magnetoresistance can be tuned by the applied voltage. The sign inversion of the magnetoresistance is observed in a wide temperature range below the Curie temperature. This tunable magnetoresistance sign may be attributed to the spin polarizations of the tunneling carriers and the band structure of the two ferromagnetic electrodes. Such robust electrical tunability of magnetoresistance extends the functionalities of low-dimensional spintronics and makes it more appealing for next-generation spintronics with all-vdW MTJs.

Transport properties of Mn-doped/adsorbed zigzag boron nitride nanoribbon based nanodevices

We are conducting research into the spin transport characteristics of models that are built from zigzag Boron Nitride Nanoribbons (ZBNNRs) that have been doped and adsorbed with Mn. These investigations are made possible by using the non-equilibrium Green’s function (NEGF) formalism in conjunction with Density Functional Theory (DFT). We have scrutinized the electronic structures of six different models, and have chosen two for a deeper exploration of its’ transport characteristics. Our computational findings highlight an unusual occurrence where a Mn atom substitutes a B atom in ZBNNRs, resulting in ZB(Mn)NNRs that exhibit significant Giant Magnetoresistance (GMR) and Rectification ratio (RR) - peaking at 106 and 105 respectively, along with the presence of negative differential conductance (NDC) effects. The detected GMR, NDC, and rectification characteristics in ZB(Mn)NNRs offer an optimistic outlook for the conceptualization of future nanodevices.

An all phosphorene lattice nanometric spin valve

Phosphorene is a unique semiconducting two-dimensional platform for enabling spintronic devices integrated with phosphorene nanoelectronics. Here, we have designed an all phosphorene lattice lateral spin valve device, conceived via patterned magnetic substituted atoms of 3d-block elements at both ends of a phosphorene nanoribbon acting as ferromagnetic electrodes in the spin valve. Through First-principles based calculations, we have extensively studied the spin-dependent transport characteristics of the new spin valve structures. Systematic exploration of the magnetoresistance (MR) of the spin valve for various substitutional atoms and bias voltage resulted in a phase diagram offering a colossal MR for V and Cr-substitutional atoms. Such MR can be directly attributed to their specific electronic structure, which can be further tuned by a gate voltage, for electric field controlled spin valves. The spin-dependent transport characteristics here reveal new features such as negative conductance oscillation and switching of the sign of MR due to change in the majority spin carrier type. Our study creates possibilities for the design of nanometric spin valves, which could enable integration of memory and logic elements for all phosphorene 2D processors.

Angular Position Sensor Based on Anisotropic Magnetoresistive and Anomalous Nernst Effect

Magnetic position sensors have extensive applications in various industrial sectors and consumer products. However, measuring angles in the full range of 0–360° in a wide field range using a single magnetic sensor remains a challenge. Here, we propose a magnetic position sensor based on a single Wheatstone bridge structure made from a single ferromagnetic layer. By measuring the anisotropic magnetoresistance (AMR) signals from the bridge and two sets of anomalous Nernst effect (ANE) signals from the transverse ports on two perpendicular Wheatstone bridge arms concurrently, we show that it is possible to achieve 0–360° angle detection using a single bridge sensor. The combined use of AMR and ANE signals allows a mean angle error in the range of 0.51–1.05° within a field range of 100 Oe–10,000 Oe to be achieved.

Mechanisms of Electrical Switching of Ultrathin CoO/Pt Bilayers.

We study current-induced switching of the Néel vector in CoO/Pt bilayers to understand the underlying antiferromagnetic switching mechanism. Surprisingly, we find that for ultrathin CoO/Pt bilayers electrical pulses along the same path can lead to an increase or decrease of the spin Hall magnetoresistance signal, depending on the current density of the pulse. By comparing these results to XMLD-PEEM imaging of the antiferromagnetic domain structure before and after the application of current pulses, we reveal the details of the reorientation of the Néel vector in ultrathin CoO(4 nm). This allows us to understand how opposite resistance changes can result from a thermomagnetoelastic switching mechanism. Importantly, our spatially resolved imaging shows that regions where the current pulses are applied and regions further away exhibit different switched spin structures, which can be explained by a spin-orbit torque-based switching mechanism that can dominate in very thin films.

Interfacial random roughness effects on the magnetoresistance of Co–Cu metallic superlattices

The purpose of this study is to better understand the interfacial random geometric roughness contribution regarding the magnetoresistance (MR) effect in structure consisting “Ferromagnetic (FM) metal/Non-magnetic (NM) metal” metallic multilayer (ML) film by treating the interfaces in two different approaches. The calculation of the film in-plane conductivity is realised for two configurations: the FM layers aligned (i) parallel and (ii) antiparallel to each other. The results reveal in the first approach that the MR behaviour is strongly influenced by interfaces state by varying their chemical composition and also their magnitude of its geometric roughness. The scattering effects across non-rough interfaces on the magnetoresistive signal amplitude are observed in the second approach. The results show that a [Substrate//Co(1.1 nm)/Cu(1.1 nm)//Co(1.1 nm)//Capping layer] ML with smooth interfaces exhibit a giant magnetoresistive ratio of 97.5 % for the largest asymmetric spin-dependent scattering.

Study of the electrophysical and magnetic properties of a Dirac 3D semimetal Cd3As2 with nanogranules of MnAs

We report on the main results of studying the electrical and magnetoresistance (MR) of a composite material consisting of 70 % mol. Dirac semi-metal Cd3As2 and 30 % mol. ferromagnet MnAs at pressures up to 50 GPa in a diamond anvil cell with a «rounded cone-flat» type anvils, as well as magnetization at hydrostatic pressures up to 6 GPa in a toroid-shaped high-pressure cell, both at room temperature and in the temperature range of 180 – 350 K at atmospheric pressure. A mixture of methanol and ethanol in a ratio of 4:1 was used as a pressure transmitting medium. Elemental analysis of Cd3As2 + 30 % mol MnAs composites showed that much of the volume is occupied by the Cd3As2 phase. The proportion of MnAs phase inclusions is less than 5 %. The feature of Cd3As2 + MnAs is the presence of a significant region of non-mixing of the Cd3As2 and MnAs phase melts. A negative MR was revealed with increasing pressure in the entire studied baric zone. The maximum negative MR is observed in the baric zone of 22 – 26 GPa. Further increase in the pressure up to the maximum level result in the appearance of several extrema on the ΔR/R0(P) curve, with negative MR not exceeding 4 %. Upon pressure release from 50 GPa, the baric dependence of ΔR/R0(P) is characterized by an inversion of the MR sign: at pressures around 40 GPa, a negative MR is replaced by a positive MR, and at around 20 GPa, the maximum value of positive MR of ~5.3 % is observed. Signs of the instability of the monoclinic structure of Cd3As2 resulted from its partial decomposition upon decompression were revealed. The results obtained can be used in spintronics when using appropriate composite materials.

Sr2Mn0.5Hf1.5O6-δ double perovskite oxides: Structural, dielectric, magnetic, optical and transport properties

Double perovskite (DP) oxides with a chemical formula of A2B′B”O6 exhibit intriguing physical properties and structural flexibilities, which are widely used as a host structure to explore the novel candidates of half-metallic ferro-/ferrimagnetic materials. In this work, we have synthesized ferromagnetic Sr2Mn0.5Hf1.5O6-δ (SMHO) DP powders via solid-state reaction method. The powders had a magnetic Curie temperature up to 600 K, saturation magnetization of 0.59 emu/g at 2 K, and magnetic coercive field of 450 Oe. The SMHO ceramics displayed a butterfly-like magnetoresistance (MR)-H curve at 2 K due to the intergranular tunneling effect, and the MR (2 K, 7 T) value was −2.8 %. Structural investigations demonstrated the SMHO powders crystallized in an orthogonal crystal structure with Pnma symmetry. SEM images displayed a spherical morphology of the SMHO powders with particle size distribution of 100–200 nm. Quantitative EDS (energy dispersive X-ray spectra) data give out the percentage of Sr, Mn, Hf and O elements presented in approximately stoichiometric proportion. XPS spectra results confirmed the existence of Sr2+, Mn4+, and Hf4+/Hfx+(x＜4) ions in the powders, and oxygen elements were presented as the lattice oxygen and adsorbed oxygen species, respectively. Resistivity of the SMHO ceramics decreased continuously as the temperature increased from 2 K to 800 K, exhibiting a semiconductor behavior. The electrical transport data were well fitted by thermal activation model, small polaron hopping model, and Mott’s variable range hopping model, respectively in different temperature regions. Optical measurements gave out an indirect bandgap of 2.12 eV for the SMHO powders. Our present work demonstrates that the SMHO oxides are attractive in high temperature spintronic devices and solar cells owing to the integration of high TC, significant negative MR and optical bandgap located in the visible light window.

Anomalous magnetoresistance in a nonconjugated radical polymer glass.

Macromolecules bearing open-shell entities offer unique transport properties for both electronic and spintronic devices. This work demonstrates that, unlike their conjugated polymer counterparts, the charge carriers in radical polymers (i.e., macromolecules with nonconjugated backbones and with stable open-shell sites present at their pendant groups) are singlet cations, which opens significant avenues for manipulating macromolecular design for advanced solid-state transport in these highly transparent conductors. Despite this key point, magnetoresistive effects are present in radical polymer thin films under applied magnetic fields due to the presence of impurity sites in low (i.e., <1%) concentrations. Additionally, thermal annealing of poly(4-glycidyloxy-2,2,6,6- tetramethylpiperidine-1-oxyl) (PTEO), a nonconjugated polymer with stable open-shell pendant groups, facilitated better electron exchange and pairwise spin interactions resulting in an unexpected magnetoresistance signal at relatively low field strengths (i.e., <2 T). The addition of 4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxy (TEMPO-OH), a paramagnetic species, increased the magnitude of the MR effect when the small molecule was added to the radical polymer matrix. These macroscopic experimental observables are explained using computational approaches that detail the fundamental molecular principles. This intrinsic localized charge transport behavior differs from the current state of the art regarding closed-shell conjugated macromolecules, and it opens an avenue towards next-generation transport in organic electronic materials.

Excellent spin filtering and significant magnetoresistance of rhenium phthalocyanine molecular junctions on Au(100)

Spin transport properties of single rhenium phthalocyanine (RePc) and double-rhenium phthalocyanine (Re2Pc2) molecular devices with gold electrodes have been investigated by combining Density Functional theory and nonequilibrium Green’s functions. It was found that the RePc molecular devices exhibited good spin filtration efficiency in the small bias range even in the absence of magnetic electrodes. The Re2Pc2 molecular device exhibited a large magnetoresistance and excellent spin-filtering properties at a finite bias, which can only be achieved by altering the magnetization orientation of the rhenium atom at the center of the phthalocyanine molecule. These conclusions of the rhenium phthalocyanine molecule may have potential application in magnetoresistive devices and spin filters.

Significant thermoelectric power and large magnetoresistance associated with elastic coupling in ternary PbBi2Te4

We present a detailed analysis of the structural and transport properties of PbBi2Te4 crystals. Thermal variations of lattice constants, as well as unit cell volume, follow a linear dependence but deviate from linearity around ∼ 100 K. The resistivity exhibits a semi-metallic character in its temperature dependence with a reasonably high magnetoresistance (MR) up to ∼ 90% at 2 K. The MR results comply with Kohler's rule, being a semi-classical two-band approach, when measurements are carried out below ∼ 100 K along the crystallographic c-axis, suggesting possible structural correlation. However, the MR results do not confirm to this rule when measured perpendicular to the c-axis. The absolute value of S is noteworthy, as it leads to a systematic increase with temperature to a thermoelectric power of ∼ 5 μW/cm-K2 at 300 K. To understand the Hall conductivities, we employ a two-band model that considers the interplay between electron and hole mobilities, as well as carrier densities. This interplay, in addition to the structural correlation, plays crucial roles in fine-tuning the MR and thermoelectric properties.

Photo-switching of magnetoresistance in p-Co3O4/n-WS2 heterojunction

Tuning and controlling magnetic states by light is of great interest for applications such as magneto-optical recording and sensing devices. In this research, the change of sign and adjustability of magnetoresistance by light is shown experimentally. In a simple device, a p-n junction is created at the interface of a thin layer of n-type WS2 and p-type Co3O4. The device displays positive magnetoresistance in the dark, while under light illumination, it displays negative magnetoresistance. We suggest that this effect may be due to the suppression of photogenerated electron-hole recombination under the Zeeman effect. The findings of this research can lead to the development of practical fields such as light sensor systems or magnetoresistance memory devices of the new generation.

Measurement Geometry and Hydrostatic Pressure‐Dependent Magnetoresistance in All‐Oxide‐Based Synthetic Antiferromagnets

AbstractThe electronic structure of constituent layers and the spin channel of propagating electrons are critical factors that affect the magnitude and sign of magnetoresistance (MR) in synthetic antiferromagnets (SAFMs), which are important for spintronic applications. However, for all‐oxide‐based SAFMs, where there is strong coupling between multiple degrees of freedom, spin transport becomes more complex and remains elusive. Here, using ultrathin half‐metallic manganite/doped ruthenate SAFMs as a model system, three sign reversals of MR are demonstrated accompanied by the crossover between underlying spin‐dependent transport mechanisms. Electron tunneling produces normal MR in the current‐perpendicular‐to‐plane (CPP) geometry at low temperatures, whereas carrier confinement causes inverse MR in the current‐in‐plane (CIP) geometry. Strikingly, CPP MR can undergo a temperature‐driven sign reversal due to resonant tunneling via localized states in the spacer. Moreover, hydrostatic pressure can modulate the interlayer exchange coupling and induce an asymmetric interfacial response to dramatically facilitate electron tunneling, driving a controllable sign reversal of CIP MR. These results provide new insights into understanding and optimization of MR in all‐oxide‐based SAFMs.

Large Magnetoresistance of Isolated Domain Walls in La2/3 Sr1/3 MnO3 Nanowires.

Generation, manipulation, and sensing of magnetic domain walls are cornerstones in the design of efficient spintronic devices. Half-metals are amenable for this purpose as large low field magnetoresistance signals can be expected from spin accumulation at spin textures. Among half metals, La1- x Srx MnO3 (LSMO) manganites are considered as promising candidates for their robust half-metallic ground state, Curie temperature above room temperature (Tc = 360 K, for x = 1/3), and chemical stability. Yet domain wall magnetoresistance is poorly understood, with large discrepancies in the reported values and conflicting interpretation of experimental data due to the entanglement of various source of magnetoresistance, namely, spin accumulation, anisotropic magnetoresistance, and colossal magnetoresistance. In this work, the domain wall magnetoresistance is measured in LSMO cross-shape nanowires with single-domain walls nucleated across the current path. Magnetoresistance values above 10% are found to be originating at the spin accumulation caused by the mistracking effect of the spin texture of the domain wall by the conduction electrons. Fundamentally, this result shows the importance on non-adiabatic processes at spin textures despite the strong Hund coupling to the localized t2g electrons of the manganite. These large magnetoresistance values are high enough for encoding and reading magnetic bits in future oxide spintronic sensors.

A reversal of positive to negative magnetoresistance in Fe3O4-based heterostructures at room temperature by annealing

In this paper, a transition of magnetoresistance (MR) from positive to negative was obtained at room temperature by vacuum annealing of Fe3O4 (128 nm) / Si (with natural SiO2) heterostructures at 780 °C. Considering the wire-like nanograins with varying length and the effect of interface, the sign of MR is decided by the contribution of two Fe3O4 layers. One is the layer at the Fe3O4/SiO2 interface where the scattering of conducting electrons in the natural SiO2 layer on the top of Si can lead to spin polarization reversal of Fe3O4 and the other is the Fe3O4 far away from the absorbing interface. The reversal sign of MR indicates the dominant contrition of Fe3O4 far away from the absorbing interface as the stabilized SiO2 phase on top of the Si substrate with fewer ion defects by annealing the Fe3O4 film on natural Si at 780 °C. For the Fe3O4 film prepared on the substrate of corroded Si (chemically etching the natural SiO2 layer), positive MR is maintained even vacuum annealed at 780 °C, while for the Fe3O4 film on SiO2, an additional α-Fe2O3 phase is obtained even negative MR for films both as deposited and annealed at 780 °C. It can be concluded that the SiO2 layer on top of the substrate plays an important role in the interface and then the magnetotransport properties of the Fe3O4 heterostructures. Well property and negative spin polarization of Fe3O4 film can be obtained by vacuum annealing the Fe3O4 film on Si substrate with a proper thickness of natural SiO2 layer at 780 °C.