Large Positive Magnetoresistance 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.

Unconventional room-temperature negative magnetoresistance effect in Au/n-Ge:Sb/Au devices

Non-magnetic semiconductor materials and their devices have attracted wide attention since they are usually prone to exhibit large positive magnetoresistance (MR) effect in a low static magnetic field environment at room temperature. However, how to obtain a large room-temperature negative MR effect in them remains to be studied. In this paper, by designing an Au/n-Ge:Sb/Au device with metal electrodes located on identical side, we observe an obvious room-temperature negative MR effect in a specific 50 T pulsed high magnetic field direction environment, but not in a static low magnetic field environment. Through the analysis of the experimental measurement of the Hall effect results and bipolar transport theory, we propose that this unconventional negative MR effect is mainly related to the charge accumulation on the surface of the device under the modulation of the stronger Lorentz force provided by the pulsed high magnetic field. This theoretical analytical model is further confirmed by regulating the geometry size of the device. Our work sheds light on the development of novel magnetic sensing, magnetic logic and other devices based on non-magnetic semiconductors operating in pulsed high magnetic field environment.

Role of morphology and composition in the transport properties of highly conducting CVD grown PtTex

Platinum telluride (PtTex), a metallic noble transition-metal dichalcogenide, has emerged as a central candidate for magnetic and optoelectronic applications. Recently, PtTex has drawn great attention because of its large positive magnetoresistance and broadband photodetection owing to its astounding electron transport properties. Here, we report the role of morphology, texture, and composition in the transport properties of CVD-grown highly conducting PtTex. Two different compositions of Te atoms, named tellurium stoichiometric (TS) and tellurium rich (TR), with different morphology have been obtained in the PtTex films. The highest longitudinal conductivity was found to be ∼ 3.57 × 107 and 3.83 × 106 S m−1 at 200 K in the TR and TS-PtTex samples, respectively. The maximum carrier density in TR and TS-PtTex samples was found to be of the order of 1022 and 1021 cm−3, respectively. Further, the negative magnetoresistance has also been found to be ∼ −2.2% at 300 K in the presence of an in-plane magnetic field of 9 T. Our results, reporting extremely high conductivity along with negative magnetoresistance, promise exciting applications of PtTex for nanoelectronic devices.

Large positive magnetoresistance and high mobility in topological insulator candidate LaP

We reported herein the single crystal growth and the comprehensive study of basic physical properties including electronic transport, magnetic, specific heat of topological insulator candidate LaP. Single crystal LaP of rock salt type structure was synthesized by Sn flux method. Under low temperature and high magnetic field of T= 2 K and B= 9 T, large positive magnetoresistance (LMR) of 500% was discovered. The Hall effect measurements show that the conduction carriers are dominated by holes among the temperature range from 300 K to 2 K, the carrier concentration n_{h} =4.94times 10^{19} cm−3 and n_{e} =5.02times 10^{16} cm−3 and the mobility of LaP reached as high as mu _{h}=1.57times 10^{4} cm2 V−1 S−1 and mu _{e} = 1.55times 10^{3} cm2 V−1 S−1 obtained at 2 K, which can be explained by multiband model physics like other topological quantum material systems with large MR. LaP shows diamagnetism over a wide temperature range from 2 K to 300 K without any magnetic phase transition by susceptibility measurements. No evidence of phase transitions from 2 K to 300 K was observed in the specific heat measurement. The electronic specific heat coefficient is obtained 0.538 m J mol−1 K−2 for LaP single crystal, which responds to a small electron density state near the Fermi level. Our results would be helpful in renewing interest in studying emergent phenomena arisen from topological semimetals. LaP offers a platform for understanding the interactions between large magnetoresistance, high mobility and topological band structure.

Colossal anomalous Hall conductivity and topological Hall effect in ferromagnetic kagome metal Nd3Al

We report a polycrystalline kagome metallic ferromagnet Nd3Al with a large unconventional positive magnetoresistance (∼80%) and a colossal anomalous Hall conductivity of 3 × 104 S/cm. We find that, though it is predominantly ferromagnetic, the low temperature phase is rather complex. The reduction in the effective moment, thermomagnetic irreversibility, anomalous temperature dependence of magnetization, large and non-saturating positive magnetoresistance, and existence of the finite topological Hall effect make this compound quite interesting. Various experimental proofs point toward topological band structure and topological spin texture in the frustrated kagome lattice. Ab initio calculations broadly confirm the presence of flatbands and Weyl points originating from the itinerant Nd-moments. The non-trivial band structure, enhanced skew scattering, and topological spin texture in a frustrated kagome lattice are found to be responsible for the colossal Hall conductivity and the topological Hall effect.

Tunable magnetoresistance driven by electronic structure in Kagome semimetal Co1−xFexSn

Topological quantum phenomena due to the presence of both Dirac fermions and flat bands in Kagome semimetal CoSn promise novel transport properties. In Kagome materials, the transport properties can be enhanced by moving the position of the flat band and Dirac fermions with doping. Here, we have investigated the magnetotransport and electronic properties by Fe doping in CoSn. A large positive magnetoresistance (MR) of ≈ 105% at 3 K and 8 T magnetic field has been observed in CoSn. Fe doping in Co1−xFexSn gives rise to negative MR with a large negative value for x = 0.2 (−8.4%) but decreases for x = 0.3 (−3.7%) and x = 0.4 (−2.7%). Tuning of MR from positive to negative values with Fe doping in CoSn is associated with the changes in energy position of the localized flat band and emergence of quasi-localized states near the Fermi level. The systematic variation of MR with increasing x depends both on the structural changes due to increase in the lattice parameters and on the near neighbor interactions of the Sn atoms with the Co atoms in the Kagome plane. The origin of quasi-localized states is associated with the electron hopping and the variation in the strength of localization with doping that leads to tunable MR properties in Co1−xFexSn.

Magnetotransport in the double-atomic-layer rect-7×3-In phase on Si(111)

Magnetoresistance of the double-atomic-layer $\text{rect-}\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{-In}/\mathrm{Si}(111)$ phase was measured in situ using the four-point-probe technique in the temperature range from 2.5 to 33 K and magnetic fields from $\ensuremath{-}8$ to $+8$ T. The $\text{rect-}\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{-In}/\mathrm{Si}(111)$ was found to demonstrate a classical quadratic behavior of magnetoresistance at fields lower than $\ensuremath{\sim}0.2$ T, and a large positive linear magnetoresistance at fields up to 8 T in the low-temperature range of up to $\ensuremath{\sim}10$ K. In contrast, the results obtained on the parent single- and double-atomic In layers formed on the ${\mathrm{NiSi}}_{2}/\text{Si}(111)$ substrate show much lower values of magnetoresistance. In view of the density functional theory calculation results, which reveal the presence of tiny Fermi pockets in the $\text{rect-}\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{-In/Si}(111)$ electronic band structure, we attribute the observed large linear magnetoresistance at the low-temperature range to a quantum mechanical origin.

Magnetism and Transport Properties of EuCdBi2 with Bi Square Net

We report a possible coexistence of nontrivial topology and antiferromagnetism in the newly discovered compounds EuCdBi2, with magnetic Eu layer locating above and below Bi square net. The X-ray diffraction on single crystals and powder indicats that this 112-type material crystalizes in space group of I4/mmm, the same as SrMnBi2 and EuMnBi2. Our combined measurements of magnetization, electrical transport and specific heat consistently reveal antiferromagnetic (AFM) transition of Eu2+ moments at TN = 20 K. The Eu moments are not saturated under a field of 7 T at 1.8 K. The anisotropic susceptibility suggests the Eu moments lie in the ab plane, and a metamagnetic (MM) transition is observed near 1 T below TN. Large positive magnetoresistance (MR) present for both H ‖ ab and H ‖ c, which are considered to contain part contributions from Dirac bands. Hall measurements show the electron-hole compensation effect is prominent above 100 K, with a crossover of Hall resistance from negative to positive values at ∼150 K. The fitted mobility of electrons is as high as 3250 cm2 V−1 S−1 at 1.8 K. Interestingly, the rapid increase of carrier density and suppression of mobility appear at around TN, indicating non-negligible interaction between Eu moments and electron/hole bands. EuCdBi2 may provide a new platform to investigate the interplay of topological bands and antiferromagnetic order.

Synergistic effect of sequential solvent treatment on the structural and low temperature charge transport of PEDOT:PSS films

A detailed study on the low temperature charge transport and magnetoresistance (MR) measurements of spin-coated and solvent treated poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films has been presented. The samples were prepared by sequential treatment of PEDOT:PSS film using dimethyl sulfoxide and sulphuric acid. X-ray diffraction and Raman spectroscopy elucidate the effect of solvent treatment on the structural modifications of the film, which correlates to the enhanced conductivity values. The nature of solvent and the sequence of successive treatments using different solvents has been effectively utilized to tune the conductivity of the film over a wide range. Further, the low temperature (300–2 K) charge transport study indicates that the resistance of the film rises sharply below 50 K, which implies the samples lie in the category of disordered materials. A linear fit of ln R vs. T −1/4 for all the samples indicates that the temperature dependent resistance of solvent treated PEDOT:PSS films follow 3D variable range hopping model. The observed large positive MR behavior of the films has been discussed in terms of the wave function shrinkage of the charge carriers. The MR follows ∼B 2 and ∼B 1/3 dependence at low and high magnetic field regimes, respectively. The sequential treatment of PEDOT:PSS films shown in the present study is an effective method to enhance the electrical conductivity significantly.

Enhancement of the Magnetoresistance in the Mobility‐Engineered Compensated Metal Pt5P2

AbstractThe magnetoresistance (MR) in nonmagnetic materials continues to be a fertile research area in materials science. The search for giant, positive MR has been limited to a rather small window of materials such as high‐mobility semimetals in single‐crystalline form. Here, the observation of a very large positive MR in metallic Pt5P2 in polycrystalline form is reported. The observations reveal that improvement of the crystallinity results in a significant enhancement of the positive MR, exceeding 10 000% at 9 T, comparable to high‐mobility semimetals. Based on first‐principles calculations combined with magnetotransport and thermoelectric measurements, the Fermi surface of Pt5P2 is found to consist of a collection of multiple electron and hole pockets compensating one another, along with a characteristic pocket continuously connected to the adjacent Brillouin zone, together with possible topologically protected band crossings. This work extends the landscape of high MR candidate materials to polycrystalline metals, which demonstrates the importance of crystallinity and purity of the samples for the optimization of the MR.

High-Mobility Magnetic Two-Dimensional Electron Gas in Engineered Oxide Interfaces

The engineered interfaces of complex oxides have abundant physical properties and provide a powerful platform for the exploration of fundamental physics and emergent phenomena. In particular, research on the two-dimensional magnetic systems with high mobility remains a long-standing challenge for the discovery of quantum phase and spintronic applications. Here, we introduce a few atomic layers of the delta doping layer at LaAlO3/SrTiO3 interfaces through elaborately controllable epitaxial growth of SrRuO3. After inserting a SrRuO3 buffer layer, the interfaces exhibit a well-defined anomalous Hall effect up to 100 K and their mobility is enhanced by 3 orders of magnitude at low temperatures. More intriguingly, a large unsaturated positive magnetoresistance is created at interfaces. Combining with the density functional theory calculation, we attribute our findings to the electron transfer at interfaces and the magnetic moment of Ru4+ 4d bands. The results pave a way for further research of two-dimensional ferromagnetism and quantum transport in all-oxide systems.

Scaling of Berry-curvature monopole dominated large linear positive magnetoresistance

The linear positive magnetoresistance (LPMR) is a widely observed phenomenon in topological materials, which is promising for potential applications on topological spintronics. However, its mechanism remains ambiguous yet, and the effect is thus uncontrollable. Here, we report a quantitative scaling model that correlates the LPMR with the Berry curvature, based on a ferromagnetic Weyl semimetal CoS2 that bears the largest LPMR of over 500% at 2 K and 9 T, among known magnetic topological semimetals. In this system, masses of Weyl nodes existing near the Fermi level, revealed by theoretical calculations, serve as Berry-curvature monopoles and low-effective-mass carriers. Based on the Weyl picture, we propose a relation [Formula: see text], with B being the applied magnetic field and [Formula: see text] the average Berry curvature near the Fermi surface, and further introduce temperature factor to both MR/B slope (MR per unit field) and anomalous Hall conductivity, which establishes the connection between the model and experimental measurements. A clear picture of the linearly slowing down of carriers, i.e., the LPMR effect, is demonstrated under the cooperation of the k-space Berry curvature and real-space magnetic field. Our study not only provides experimental evidence of Berry curvature-induced LPMR but also promotes the common understanding and functional designing of the large Berry-curvature MR in topological Dirac/Weyl systems for magnetic sensing or information storage.

Quasi-linear and Plateaus-like Tunneling Magnetoresistance in Graphene-based Tunable Magnetic Barrier Nanostructures

The effect of electrostatic and magnetic vector potentials pattern on electrical properties and the tunneling magnetoresistance in graphene junction with periodic magnetic vector potentials are theoretically investigated using the transfer matrix method. The magnetic structure on graphene can control the direction of the magnetizations which correspond to the parallel and anti-parallel (AP) configurations. In AP magnetic structures with the applied gate voltage pattern, USUBA/SUB (U₁ = U₂), the shift of the conductance-peak position as a function of Fermi energy. The peak corresponds to resonant tunneling, where the incidence energy of the tunneling electron equals the confinement energy, and the conductance peak decrease approximately linearly with increasing electrostatic potential. The peak position occurs at ESUBF/SUB = 0.5U where it is shifted to higher Fermi energy with higher gate potential, but for the case of USUBB/SUB (U₁ = -U₂), the peak height is reduced rapidly, and the width increase as gate potential increases. Because of the periodic magnetic field with zero spatial average in the antiparallel structure, we found that the minimum conductivity decreases with increasing magnetic energy. They show suppression of Klein tunneling which occurs in the zero-conductance plateaus and leads to the robust magnetoresistance plateau and large positive magnetoresistance appears below magnetic energy. For the case of USUBA/SUB and ESUBF/SUB more than magnetic energy, we found that the quasi-linear magnetoresistance feature is applied by an electric field instead of the usual magnetically driven magnetoresistance.

Large positive and negative magnetoresistance in the magnetic EuCu4As2 crystal

In this article, we studied the magnetic and transport properties by magnetization, resistivity, and specific heat measurements for EuCu4As2 crystals with a rhombohedral structure. Strongly anisotropic magnetism and rich magnetic phases were observed under magnetic field in this compound. Interestingly, we observed two types of magnetoresistance: a large negative magnetoresistance (−55%, at 35 K, 9 T) near the magnetic transition temperature, due to reduction of magnetic scattering under magnetic field; a large positive magnetoresistance (60% at 2 K, 9 T) at lower temperatures when Eu2+ moments ordering ferromagnetically, similar to that observed in the non-magnetic isostructural analogue SrAg4As2 with possible nontrivial topological bands, which may be related to its special band structure.

Large unsaturated magnetoresistance of 2D magnetic semiconductor Fe-SnS2 homojunction

A magnetic semiconductor whose electronic charge and spin can be regulated together will be an important component of future spintronic devices. Here, we construct a two-dimensional (2D) Fe doped SnS2 (Fe-SnS2) homogeneous junction and investigate its electromagnetic transport feature. The Fe-SnS2 homojunction device showed large positive and unsaturated magnetoresistance (MR) of 1800% in the parallel magnetic field and 600% in the vertical magnetic field, indicating an obvious anisotropic MR feature. In contrast, The MR of Fe-SnS2 homojunction is much larger than the pure diamagnetic SnS2 and most 2D materials. The application of a gate voltage can regulate the MR effect of Fe-SnS2 homojunction devices. Moreover, the stability of Fe-SnS2 in air has great application potential. Our Fe-SnS2 homojunction has a significant potential in future magnetic memory applications.

A Heavy Fermion Zn-Deficient CaBe2Ge2-Type Phase with Rare Ce-Based Ferromagnetism and Large Magnetoresistance

We report on the hitherto unknown compound CeZn2-dGe2 (d approx 0.41). We find this compound to crystallize in a defect version of the well-known CaBe2Ge2 structure type. The phase forms in a Zn/In flux and with Zn-deficiency on one of its crystallographic sites. We find the compound to display uncommon localized Ce-based (4f1) ferromagnetism with a Tc = 6.6 K, a large positive magnetoresistance reaching an MR of approximately 32 % below $T$ = 10 K, and strongly correlated electrons, as evidenced by a Kadowaki-Woods ratio A/gamma^2 close to known heavy fermion compounds. The here discovered material is therefore a promising model platform for the investigation of these entangled interacting and potentially competing electronic states paired with complex crystal chemistry.

Spin transport in fully ferromagnetic p–n junctions

We systematically investigate the spin-dependent transport properties of fully ferromagnetic (Ga,Fe)Sb/GaSb/(In,Fe)Sb and (Ga,Fe)Sb/(In,Fe)As p–n junctions grown by low-temperature molecular beam epitaxy. The (Ga,Fe)Sb/GaSb/(In,Fe)Sb p–n junctions show high Curie temperature (170–310 K) and rectifying characteristics. The polarity and magnitude of magnetoresistance of the (Ga,Fe)Sb/GaSb/(In,Fe)Sb junctions strongly depend on the GaSb spacer thickness (t). Large positive magnetoresistance (MR, 58%) and negative MR (−1.6%) were observed at 3.7 K for the samples with t = 2 and 4 nm, respectively. When the n-type (In,Fe)Sb layer was replaced by the n-type (In,Fe)As, giant MR over 500% was observed, which can be explained by spin-valve and spin-splitting effects. Our results shed light on rich spin-transport physics observed in fully ferromagnetic p–n junctions.

High-Pressure Synthesis of a Massive and Non-Symmorphic Dirac Semimetal Candidate MoP4

Single crystal and polycrystalline samples of MoP$_4$ with a black-phosphorus-derived structure have been successfully synthesized by a high-pressure technique. The polycrystalline samples show a large positive magnetoresistance and a small negative Seebeck coefficient at low temperatures, reflecting a semi-metallic nature with high-mobility electrons. Consistent with the transport properties, the band structure calculation reveals a semi-metallic state with the presence of two types of Dirac nodes slightly below the Fermi level. The Dirac node along the $\Gamma$-X direction normal to the phosphorus layers is gapped out in the presence of spin-orbit coupling (SOC), whereas the band crossing at the Z-point is immune to SOC because of the non-symmorphic symmetry. This work demonstrates a great potential of phosphorus-based layered Zintl compounds for topological semimetal candidates allowing chemical band engineering.

Intrinsic V vacancy and large magnetoresistance in V1 — δ Sb2 single crystal

The binary pnictide semimetals have attracted considerable attention due to their fantastic physical properties that include topological effects, negative magnetoresistance, Weyl fermions, and large non-saturation magnetoresistance. In this paper, we have successfully grown the high-quality V1 – δ Sb2 single crystals by Sb flux method and investigated their electronic transport properties. A large positive magnetoresistance that reaches 477% under a magnetic field of 12 T at T = 1.8 K was observed. Notably, the magnetoresistance showed a cusp-like feature at the low magnetic fields and such feature weakened gradually as the temperature increased, which indicated the presence of a weak antilocalization effect (WAL). In addition, based upon the experimental and theoretical band structure calculations, V1 – δ Sb2 is a research candidate for a flat band.

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.