Negative Magnetoresistance Behavior Research Articles

Magneto-transport and thermoelectric studies of antiperovskite semimetal: Mn3SnC

We explore the magnetotransport and thermoelectric (Seebeck and Nernst coefficients) properties of Mn3SnC, an antiperovskite magnetic Nodal line semimetal. Mn3SnC shows paramagnetic (PM) to concurrent antiferromagnetic (AFM)/ferromagnetic (FM) transition at T ∼ 286 K. The electrical resistivity and Seebeck coefficient indicate the importance of electron–magnon scattering in the concurrent AFM/FM regime. We observed a large positive magnetoresistance (MR) of ∼8.2 at 8 T field near magnetic transition, in the otherwise negative MR behaviour for low temperatures. The electrical resistivity and MR show a weak thermal hysteresis around the boundary of transition temperature and the width of hysteresis decreases as magnetic field increases. Interestingly the Hall and Seebeck coefficients change sign from positive to negative below the transition temperature, highlighting the different scattering for holes and electrons in this multi-band system. The Seebeck and Nernst signal exhibit two sharp anomalies; one at the transition temperature and another at ∼50 K. The anomaly at magnetic transition in the Nernst signal disappear at 8 T magnetic field, owing to the reduction of magnetic fluctuation. A pseudo-gap near the Fermi level produces an upturn with a broad minimum in the Seebeck signal.

Magneto-Transport and Enhanced Spin-Polarized Photo Response in Solution-Processed Vertically Aligned Zn0.9Ni0.1O Nanowires

In this study, we grew pristine and Ni-doped vertically aligned zinc oxide nanowires (NWs) on a glass substrate. Both the doped and pristine NWs displayed dominant 002 peaks, confirming their vertical alignment. The Ni-doped NWs exhibited a leftward shift compared to the pristine NWs. TEM measurements confirmed the high crystallinity of individual NWs, with a d-spacing of ~0.267 nm along the c-axis. Ni-doped NWs had a higher density, indicating increased nucleation sites due to nickel doping. Doped NW films on glass showed enhanced absorbance in the visible region, suggesting the creation of sub-gap defect levels from nickel doping. Magnetization vs. magnetic field measurements revealed a small hysteresis loop, indicative of soft ferromagnetic behavior. Current transient plots demonstrated an increase in current with an applied magnetic field. Two-terminal devices exhibited a photo response that intensified with magnetic field application. This increase was attributed to parallel grain alignment, resulting in enhanced carrier concentration and photo response. In the dark, transport properties displayed negative magnetoresistance behavior. This magneto-transport effect and enhanced photo response (under an LED at ~395 nm) were attributed to giant magnetoresistance (GMR) in the aligned NWs. The observed behavior arose from reduced carrier scattering, improved transport properties, and parallel spin alignment in the magnetic field.

Enhanced quantum oscillations and scattering effect in quaternary InAlGaN/GaN two-dimensional electron gas

Quantum transport properties of a large bandgap In0.15Al0.79Ga0.06N/GaN quaternary GaN high electron mobility transistor (HEMT) heterostructure are studied at low temperatures up to 2 K. Herein, we report the first evidence of weak localization in a quaternary GaN two-dimensional electron gas (2DEG) system. We observe negative magnetoresistance behavior and extracted dephasing time (τΦ) using a Hikami–Larkin–Nagaoka model at 2.2 K. Linear dependency of dephasing rate with temperature (τΦ−1∝T) is established below 20 K. Furthermore, Shubnikov–de Haas quantum oscillation induced by 2DEG is observed using perpendicular magnetic (B⊥) field strengths up to 14 T. From the temperature-dependent oscillation amplitude, we extracted an effective mass m*≈0.237me. The dominance of small-angle scattering in the 2DEG channel is identified from less than unit ratio (τq/τt≪1) of quantum lifetime (τq) to the Hall transport lifetime (τt). In our study, we have demonstrated that the In0.15Al0.79Ga0.06N/GaN quaternary heterostructure possesses high dephasing time (τΦ=5.4 ps) and larger quantum lifetime (τq=0.102 ps) indicating better suitability and a way forward to high-power–high-frequency GaN HEMT development.

Fluctuation conductivity and vortex state in a superconductor with strong paramagnetic pair breaking

The fluctuation conductivity of a moderately clean type II superconductor with strong Pauli paramagnetic pair-breaking (PPB) is studied by focusing on the quantum regime at low temperatures and in high magnetic fields. First, it is pointed out that, as the PPB effect becomes stronger, the quantum superconducting fluctuation is generally enhanced so that the Aslamasov-Larkin (AL) fluctuation conductivity tends to vanish upon cooling. Further, by examining other (the DOS and the Maki-Thompson (MT)) terms of the fluctuation conductivity, the field dependence of the resulting total conductivity is found to depend significantly on the type of the vortex lattice (or, glass) ordered state at low temperatures where the strong PPB plays important roles. By comparing the present theoretical results with the fluctuation-induced negative magnetoresistance behavior upon entering a PPB-induced novel SC phase of Iron selenide (FeSe), it is argued that the vortex matter states of the superconducting order parameter in the second lowest ($n=1$) Landau level are realized in FeSe in the parallel field configuration in high fields and at low temperatures

Negative Magnetoresistance in the GeSn Strip.

Discovery of topological materials associated with an exotic phenomenon has attracted increasing attention in modern condensed matter physics. A typical example is the chiral anomaly proposed in the Dirac or Weyl semimetals. In addition to the well-known topological semimetals, such as TaAs and Na3Bi, recently, group IV GeSn alloys were also proposed to be Dirac semimetals in theory, demonstrating potential applications compatible with current Si-based technology. Here, we report the observation of large negative magnetoresistance (MR) that is sensitive to the orientation of the magnetic and electric field in the GeSn strip. This negative MR emerges only when the applied magnetic field is parallel to the electric field, which is consistent with the chiral anomaly in topological semimetals. This work paves a new way toward exploring the negative MR behavior and underlying mechanism in a new class of Dirac semimetals.

Electrical Properties and Thermal Annealing Effects of Polycrystalline MoS2-MoSX Nanowalls Grown by Sputtering Deposition Method

Straightforward growth of nanostructured low-bandgap materials is a key issue in mass production for electronic device applications. We report here facile nanowall growth of MoS2-MoSX using sputter deposition and investigate the electronic properties of the nanowalls. MoS2-MoSX nanowalls become gradually thicker and taller, with primarily (100)-plane growth directions, with increasing deposition time. Nanowalls combine with nearby walls when a rapid thermal annealing (RTA, 200 °C–500 °C) process is applied. All samples have conventional low-bandgap semiconductor behavior with exponential resistance increase as measurement temperature decreases. The 750 nm-thick MoS2-MoSX nanowalls have a sheet carrier mobility of up to 2 cm2·V−1·s−1 and bulk carrier concentration of ~1017–1019 cm−3 range depending on RTA temperature. Furthermore, perpendicular field-dependent magnetoresistance at 300 K shows negative magnetoresistance behavior, which displays resistance decay by applying a magnetic field (MR ratio in the −1 % range at 5 T). Interestingly, 400 °C RTA treated samples show a resistance upturn when applying an external magnetic field of more than 3 T. Our research suggests tuneability of MoS2 nanowall size and mesoscopic electronic transport properties.

Charge localization crossover from Mott to Efros-Shklovskii type variable range hopping mechanism in In1−xPbxTe compounds

We have investigated the electrical transport properties of the In1−xPbxTe compounds. Small Pb-doping is not incorporated in the In-site but randomly distributed in the matrix, found from the X-ray diffraction and elemental mapping by energy-dispersive X-ray spectroscopy measurements. The random distribution of Pb elements induces robust charge localization at low temperatures causing the variable range hopping (VRH) transport. The electrical resistivity ρ(T) of pristine InTe exhibits semiconducting to metal transition near 175 K. From the analysis of temperature exponent, we found that the VRH transport is changed from Mott to Efros-Shklovskii (ES) type with decreasing temperature, where the crossover temperatures are found as 14.4 K (x=0.01) and 13.36 (x=0.02), respectively. The magnetoresistance (MR) of the InTe shows that the weak antilocalization at low temperature (T≤3 K) and low magnetic field (H≤1 T) competes with weak localization with increasing temperature (T≥5 K). It is noteworthy that small Pb-doping exhibits unconventional negative MR (NMR) behavior because it is not a magnetic or topological material. The unconventional NMR behavior of Pb-doped compounds is attributed to the quantum mechanical interference under the magnetic field in ES type VRH transport. The charge localization crossover from Mott to ES type VRH transport mechanism suggests the strong electron-electron Coulomb interaction in the compounds, leading to the significant change of density of states and inducing the Coulomb gap at low temperatures.

Negative Magnetoresistance Behavior in Polymer Spin Valves Based on Donor−Acceptor Conjugated Molecules

AbstractOrganic spin valves (OSVs) have become an essential building block of next‐generation memory devices which focus on spin degree of transporting carriers. Meanwhile, negative magnetoresistance (MR) effect in the OSV devices is increasingly observed which deserves further exploration for the rich spin physics behind. In this work, the negative MR response in ferromagnetic (FM) metal‐based OSVs using donor−acceptor (D−A) conjugated polymer based on the naphthalenediimide units as a spacer material is observed. The negative MR effect does not result from negative polarization at spin injection and detection interface as well as tunneling anisotropic magnetoresistance effect, but from the spin transport inside the D−A polymer spacer. Even the stacking sequence of the spin injection and detection electrodes is reversed or changed the polymer coating solvents, the D−A polymer contributed negative MR response still can be observed. For further identifying negative MR origin, the bottom FM metal polarizer is replaced into high‐polarization La0.7Sr0.3MnO3 thin film, negative MR feature is well reproduced. Based on these points, it is deduced that the spin‐orientation reversal inside D−A polymer spacer is the ultimate reason for such negative MR response. The filtering‐like spin reversal activity is also in positive proportion to intermolecular interaction.

Evidence of tunable magnetic coupling in hydrogenated graphene

Many efforts have been devoted to understanding the origin and effects of magnetic moments induced in graphene with carbon atom vacancy, and light adatoms like hydrogen or fluorine. In the meantime, the large negative magnetoresistance (MR) widely observed in these systems is not well understood, nor has it been associated with the presence of magnetic moments. In this paper, we study the systematic evolution of the large negative MR of in-situ hydrogenated graphene in ultrahigh-vacuum (UHV) environment. We find for most combinations of electron density (${n}_{e}$) and hydrogen density (${n}_{\mathrm{H}}$), MR at different temperature can be scaled to $\ensuremath{\alpha}={\ensuremath{\mu}}_{B}B/{k}_{B}(T\ensuremath{-}{T}^{*})$, where ${T}^{*}$ is the Curie-Weiss temperature. The sign of ${T}^{*}$ indicates the existence of tunable ferromagneticlike (${T}^{*}>0$) and antiferromagneticlike (${T}^{*}<0$) coupling in hydrogenated graphene. However, the lack of hysteresis of MR or anomalous Hall effect below $|{T}^{*}|$ points to the fact that long-range magnetic order did not emerge, which we attribute to the competition of different magnetic orders and disordered arrangement of magnetic moments on graphene. We also find that localized impurity states introduced by H adatoms could modify the capacitance of hydrogenated graphene. This work provides a way to extract information from large negative MR behavior and can be a key to understanding interactions of magnetic moments in graphene.

Synthesis, structure, and physical properties of bilayer molybdate Sr3Mo2O7 with flat-band

ABSTRACT Bilayer molybdate SrMoO has been successfully synthesized by a modified solid-state reaction method. Magnetization measurements show that the compound is a paramagnetic material with a weak spin-glass state below about 4.5 K. Fitting to the magnetic susceptibility under a high magnetic field based on the Currie–Weiss law yields a magnetic moment of about 1.61 per Mo atom. The temperature dependence of resistivity exhibits a metallic behaviour in wide temperature region from 20 K to 300 K, but a slight upturn of resistivity with the behaviour of is observed at temperatures below about 20 K. Negative magneto-resistance behaviour has also been observed in the temperature region with resistivity upturning. Both effects indicate that a Kondo-like scattering occurs in the material. Resistivity measurements under high pressures up to 33.7 GPa are carried out, no superconductivity above 2 K has been observed, which is in contradiction to the theoretical prediction. We argue that the absence of superconductivity maybe because the flat-band is still too far away from the Fermi energy.

Coupling of magnetoresistance switching and glassy magnetic state at the metal–insulator transition in Ruddlesden-Popper manganite Ca4Mn3O10

Electronic phase separation is a frequent ingredient of the physics of manganites for which the colossal magnetoresistance (CMR) phenomenon is at debate. We investigate the switchable magnetoresistance (MR) effect of the Ruddelsden-Popper (RP) oxide Ca4Mn3O10 (CMO), which shows a metal-to-insulator transition (MIT) at 70 K (TMIT). The static (DC) and dynamic (AC) magnetic susceptibilities of CMO indicate an unusual magnetic phase coexistence below the Néel temperature (TN) i.e., at TMIT, which we describe as a glassy magnetic behavior. Furthermore, by means of synchrotron X-ray diffraction, we observed a structural anomaly directly associated with the MIT. Hence, the onset of glassiness correlates with a slight structural distortion. Interestingly, CMO displays a large positive magneto-resistance (PMR) effect (up to 550% at 4 K) in the metallic state below TMIT, while a small negative magneto-resistance (NMR) effect (-3.5% at 112 K) in the insulating region above TMIT. At TMIT, the PMR switches to NMR behavior. PMR is probably induced via enhancing the exchange interaction by localization of itinerant moment resulting in formation of magnetic polarons, whereas NMR is possibly due to reduced electron-spin scattering. This appears to be a special type of magnetoresistance, different from regular observations, and may open a new avenue in searching for exotic phenomena in manganese-based CMR materials.

Investigation of the electrical charge transport mechanism and magnetoresistance response in chloride-doped polyaniline–Fe composite nanofibers

One-dimensional composite nanostructures of chloride-doped polyaniline (PANI-Cl) and Fe nanoparticles (NPs) were fabricated via deposition of Fe NPs onto the PANI nanofiber matrix at room temperature. Morphological and structural characterization performed using microscopic techniques, e.g. scanning electron microscopy and transmission electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy confirmed effective deposition of crystalline Fe NPs onto the amorphous PANI-Cl nanofiber matrix. The temperature-dependent magnetic property measurement results revealed the ferromagnetic nature of the prepared PANI-Cl–Fe composite nanofibers (CNFs) structure. Depending on the Fe NP loading, the PANI-Cl–Fe CNFs showed both metallic and semiconducting behaviour. The temperature-dependent resistivity of semiconducting PANI-Cl–Fe CNFs was best described by Efros–Shklovskii variable range hopping (ES-VRH) and Mott three-dimensional variable range hopping (Mott-3D-VRH) mechanisms at low and high temperature regimes. Magnetoresistance (MR) investigation was executed at different temperatures and magnetic fields for the PANI-Cl–Fe CNF pellets in semiconducting form. Room temperature (300 K) negative MR values were detected in both the low and high magnetic field regions. A significant increase in MR values was noted with a decrease in temperature from 300 K to 5 K. Meanwhile, the observed low-field negative MR behaviour of PANI-Cl–Fe CNFs was explained by the forward interference model. Therefore, the implications of these findings might be significant for the application of PANI-Cl–Fe CNFs as sensing materials for magnetic field sensor devices.

Spin-Triplet p-Wave Superconductivity Revealed under High Pressure in UBe_{13}.

To unravel the nature of the superconducting symmetry of the enigmatic 5f heavy-fermion UBe_{13}, the pressure dependence of the upper critical field and of the normal state are studied up to 10GPa. Remarkably, the pressure evolution of the anomalous H_{c2}(T,P) over the entire pressure range up to 5.9GPa can be successfully explained by the gradual admixture of a field-pressure-induced E_{u} component in an A_{1u} spin-triplet ground state. This result provides strong evidence for parallel-spin pairing in UBe_{13}, which is also supported by the recently observed fully gapped excitation spectrum at ambient pressure. Moreover, we have also found a novel non-Fermi-liquid behavior of the resistivity, ρ(T)∼T^{n} (n≲1), which disappears with the collapse of the negative magnetoresistance behavior and the existence of a superconducting ground state around P=6 GPa, suggesting a close interplay between Kondo scattering and superconductivity.

Magnetotransport in Ultrathin 2-D Superconducting Mo2C Crystals

Ultrathin transition metal carbides are a class of emerging 2-D materials with many intriguing properties and applications. Here, we report the transport measurements on high-quality ultrathin Mo2C crystals, which are synthesized by a chemical vapor deposition method. We demonstrate 2-D nature of the observed superconductivity in ultrathin Mo2C crystals, being consistent with a Berezinskii–Kosterlitz–Thouless transition. As the sample thickness decreases, the Mo2C crystals exhibit negative magnetoresistance behavior at low magnetic fields deep in the superconducting state. We attribute these results to strong phase fluctuations of the superconducting order parameters near the superconductor–insulator transition. Our results demonstrate that these high-quality ultrathin Mo2C crystals provide an appealing platform to gain insights into 2-D superconductivity in a clean system.

From positive to negative magnetoresistance behavior at low applied magnetic fields for polyaniline:titania quantum dot nanocomposites

Here, we report the tuning from the positive to negative magnetoresistance response at room temperature and low applied magnetic fields (H ∼ 200 mT) for polyaniline nancomposites prepared via in situ growth of titanium oxide quantum dots. In addition, we showed experimental Raman evidence revealing that the positive magnetoresistance response in these polyaniline nanocomposites is mediated by the bipolaron mechanism. Confocal Raman spectroscopy under applied magnetic field analysis showed the decrease of the polaron population to form bipolarons of polyaniline when exposed to an applied magnetic field for the TiO2 quantum dot diluted regime. Negative magnetoresistance, observed for the TiO2 quantum dot higher concentration regime, was attributed to the suppression of polyaniline polarons probably associated with its partial chemical functionalization at the interface due to the increasing concentration of TiO2 quantum dots.

Spin mediated enhanced negative magnetoresistance in Ni80Fe20 and p-silicon bilayer

In this work, we present an experimental study of spin mediated enhanced negative magnetoresistance in Ni80Fe20 (50nm)/p-Si (350nm) bilayer. The resistance measurement shows a reduction of ~2.5% for the bilayer specimen as compared to 1.3% for Ni80Fe20 (50nm) on oxide specimen for an out-of-plane applied magnetic field of 3T. In the Ni80Fe20-only film, the negative magnetoresistance behavior is attributed to anisotropic magnetoresistance. We propose that spin polarization due to spin-Hall effect is the underlying cause of the enhanced negative magnetoresistance observed in the bilayer. Silicon has weak spin orbit coupling so spin Hall magnetoresistance measurement is not feasible. We use V2ω and V3ω measurement as a function of magnetic field and angular rotation of magnetic field in direction normal to electric current to elucidate the spin-Hall effect. The angular rotation of magnetic field shows a sinusoidal behavior for both V2ω and V3ω, which is attributed to the spin phonon interactions resulting from the spin-Hall effect mediated spin polarization. We propose that the spin polarization leads to a decrease in hole-phonon scattering resulting in enhanced negative magnetoresistance.

Strong Localization of Anionic Electrons at Interlayer for Electrical and Magnetic Anisotropy in Two-Dimensional Y2C Electride.

We have synthesized a single crystalline Y2C electride of centimeter-scale by floating-zone method and successfully characterized its anisotropic electrical and magnetic properties. In-plane resistivity upturn at low temperature together with anisotropic behavior of negative magnetoresistance is ascribed to the stronger suppression of spin fluctuation along in-plane than that along the c-axis, verifying the existence of magnetic moments preferred for the c-axis. A superior magnetic moment along the c-axis to that along the in-plane direction strongly demonstrates the anisotropic magnetism of Y2C electride containing a magnetically easy axis. It is clarified from the theoretical calculations that the anisotropic nature of the Y2C electride originates from strongly localized anionic electrons with an inherent magnetic anisotropy in the interlayer spaces.

Negative magnetoresistance in Mn-doped p-CdSb under pressure

We report on the characterization and magnetotransport measurements as a function of applied pressure for Mn-doped CdSb magnetic semiconductor. A pressure-induced negative magnetoresistance (MR) is observed in the polycrystalline ingots of p-Cd1-xMnxSb with x = 0.03 and х = 0.06 at hydrostatic pressure up to 7 GPa and room temperatures. We found that a magnitude of negative MR is strongly depends from application of pressure, magnetic field and chemical composition. Only in a low magnetic field of H∼1 kOe we observed a weak positive MR that changes its sign with increase in pressure or magnetic field. The behavior of negative MR is closely linked with the magnetic inclusions in the studied samples, as it evident from analysis of composition and distribution of elements on the surface.

Magnetotransport properties of nonstoichiometric Si–Mn alloys with an excess of manganese relative to silicides Mn4Si7 and MnSi

Comparative examination of magnetotransport properties of nonstoichiometric Si1–xMnx alloys with roughly the same (δх/x ≈ 7%) excess of Mn relative to silicides Mn4Si7 and MnSi has been performed. Films of Si1–xMnx (х ≈ 0.39 and 0.535) with a thickness of 60–70 nm have been fabricated by means of pulsed laser deposition on Al2O3 (0001) substrates at a temperature of 340°С. It has been found that the excess of Mn produces the strongest effect on the Curie temperature in the case of MnSi (ТС increases from 30 K to 300 K and higher), while no such effect is observed in Mn4Si7. This may be attributed to strong nonmagnetic disorder in Si1–xMnx with x ≈ 0.39 and the associated unusual behavior of negative magnetoresistance (NMR): The NMR varies linearly with magnetic field at В ≤ 1.3 T and exhibits almost no temperature dependence at T = 40–85 K.

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.