Magnetotransport Measurements Research Articles

Tunable even- and odd-denominator fractional quantum Hall states in trilayer graphene

Fractional quantum Hall (FQH) states are exotic quantum many-body phases whose elementary charged excitations are anyons obeying fractional braiding statistics. While most FQH states are believed to have Abelian anyons, the Moore–Read type states with even denominators – appearing at half filling of a Landau level (LL) – are predicted to possess non-Abelian excitations with appealing potential in topological quantum computation. These states, however, depend sensitively on the orbital contents of the single-particle LL wavefunctions and the LL mixing. Here we report magnetotransport measurements on Bernal-stacked trilayer graphene, whose multiband structure facilitates interlaced LL mixing, which can be controlled by external magnetic and displacement fields. We observe robust FQH states including even-denominator ones at filling factors ν = − 9/2, − 3/2, 3/2 and 9/2. In addition, we fine-tune the LL mixing and crossings to drive quantum phase transitions of these half-filling states and neighbouring odd-denominator ones, exhibiting related emerging and waning behaviour.

Room-temperature ferromagnetism in semimetallic Co-intercalated MoTe2

Fe-intercalated TaS2 prepared via the chemical intercalation of iron into van der Waals (vdW) layered 2H-TaS2 crystals enables hard ferromagnetic behavior, making it attractive for spintronic applications. However, the Curie transition at room temperature and no gate controllability in metallic TaS2 are still unfavorable. Here, we demonstrate the feasibility of realizing a ferromagnetic semimetal through the wet chemical intercalation of cobalt into semiconducting 2H-MoTe2, thereby leveraging its polymorphism. Transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy reveal the insertion of pure Co atoms within the vdW gap following the intercalation process. Co-intercalated MoTe2 (Co-MoTe2) demonstrates a conductivity of 1.5 orders of magnitude higher than that of 2H-MoTe2. This significant increase in the conductance facilitates the direct investigation of ferromagnetism through magnetotransport measurements. Co-MoTe2 exhibits a positive magnetoresistance of approximately 4%, with hysteresis observed even at room temperature. Furthermore, density functional theory calculations corroborate the enhancement of the magnetoresistance ratio and Curie temperature, which is attributed to the ferromagnetism induced by local magnetic moments resulting from the incorporation of cobalt. This study paves the way for enhanced practical applications in future spintronics by demonstrating the effectiveness of integrating foreign magnetic atoms into the vdW gap, thereby increasing the operational temperature.

Development and Comprehensive Evaluation of TMR Sensor-Based Magnetrodes.

Due to their compact size and exceptional sensitivity at room temperature, magnetoresistance (MR) sensors have garnered considerable interest in numerous fields, particularly in the detection of weak magnetic signals in biological systems. The "magnetrodes", integrating MR sensors with needle-shaped Si-based substrates, are designed to be inserted into the brain for local magnetic field detection. Although recent research has predominantly focused on giant magnetoresistance (GMR) sensors, tunnel magnetoresistance (TMR) sensors exhibit a significantly higher sensitivity. In this study, we introduce TMR-based magnetrodes featuring TMR sensors at both the tip and midsection of the probe, enabling detection of local magnetic fields at varied spatial positions. To enhance detectivity, we designed and fabricated magnetrodes with varied aspect ratios of the free layer, incorporating diverse junction shapes, quantities, and serial arrangements. Utilizing a custom-built magnetotransport and noise measurement system for characterization, our TMR-based magnetrode demonstrates a limit of detection (LOD) of 300pT/ at 1 kHz. This implies that neuronal spikes can be distinguished with minimal averaging, thereby facilitating the elucidation of their magnetic properties.

Concurrence of directional Kondo transport and incommensurate magnetic order in the layered material AgCrSe2

In this work, we report on the concurrent emergence of the directional Kondo behavior and incommensurate magnetic ordering in a layered material. We employ temperature- and magnetic field-dependent resistivity measurements, susceptibility measurements, and high resolution wavelength X-ray diffraction spectroscopy to study the electronic properties of AgCrSe2. Impurity Kondo behavior with a characteristic temperature of TK = 32 K is identified through quantitative analysis of the in-plane resistivity, substantiated by magneto-transport measurements. The excellent agreement between our experimental data and the Schlottmann’s scaling theory allows us to determine the impurity spin as S = 3/2. Furthermore, we discuss the origin of the Kondo behavior and its relation to the material’s antiferromagnetic transition. Our study uncovers a rare phenomenon—the equivalence of the Néel temperature and the Kondo temperature—paving the way for further investigations into the intricate interplay between impurity physics and magnetic phenomena in quantum materials, with potential applications in advanced electronic and magnetic devices.

Structural, magnetic and transport properties of FeRuVZ (Z = Si, Ge) Heusler alloys: Experiment and theory

Heusler alloys based on 4d-elements are relatively less explored though they are known to host interesting electronic/magnetic properties useful for future spintronics applications. Here, we report a comparative study of the structural and magnetic properties of FeRuVSi (FRVS) and FeRuVGe (FRVG) using experimental and theoretical tools. Magneto-transport measurements have also been carried out to probe the role of magnetism on their transport behaviour. Room temperature (T) X-ray diffraction data reveals cubic crystal structure (space group No. 216) with B2 type antisite disorder between Fe and Ru sites in both the alloys. They show ferromagnetic behaviour below the Curie temperature (TC) 84 K for FeRuVSi and 57 K for FeRuVGe respectively. The field dependence of magnetization at low-T shows saturating (nearly saturating) behaviour for FRVS (FRVG). Transport measurements show metallic behaviour with a sharp reduction in the resistivity near TC. Ab-initio calculations predict nearly half metallic behaviour with ∼ 87 % and ∼ 89 % spin polarization for FRVS and FRVG in their ordered states. The inclusion of antisite disorder changes the magnetic configuration, leading to a visible decrease in the magnetic moment and spin polarization. The calculated net moment and the transport properties agree fairly well with those observed experimentally. These results confirm the importance of disorder in determining the intrinsic properties of these alloys. Such combined studies help in the identification of more novel quantum materials suitable for future spintronic applications.

Detection of Dirac fermions in capped SnTe film via magnetotransport measurements

In this work, we present the investigation of the magnetotransport properties of a capped SnTe film, grown by molecular beam epitaxy, using Shubnikov–de Haas oscillations for the detection of Dirac fermions. The cap layer used was a 10 nm thick Sn0.98Eu0.02Te film, which can also contribute to the transport such that it is mandatory to isolate its contribution from the electrical transport measured in the sample. To separate the contribution from both layers, photoconductivity measurements were performed. A detailed analysis of the Shubnikov–de Haas oscillations is carried out using theoretical expressions and building the Landau-level indexation. We found that Dirac fermions are detected in the SnTe layer, while the cap layer contributes with trivial fermions, protecting SnTe against deterioration due to exposure to the atmosphere.

Two-dimensional Sb net generated nontrivial topological states in SmAgSb2 probed by quantum oscillations

Abstract The REAgSb2 (RE = rare earth and Y) family has drawn considerable research interest because the two-dimensional Sb net in their crystal structure hosts topological fermions and hence rich topological properties. We report herein the magnetization and magnetotransport measurements of SmAgSb2 single crystal, which unveil very large magnetoresistance and high carrier mobility up to 6.2×103% and 5.58×103 cm2V-1s-1, respectively. The analysis of both Shubnikov-de Haas and de Haas-van Alphen quantum oscillations indicates nontrivial Berry phases in the paramagnetic state while trivial Berry curvature in the antiferromagnetic state, indicating a topological phase transition induced by the antiferromagnetic order. It is also supported by the first-principles calculations. The results not only provide a new interesting topological material but also offer valuable insights into the correlation between magnetism and nontrivial topological states.

Large magnetoresistance and nontrivial Fermi surface topology in quasi-one-dimensional Ta2PdSe6

We report the experimental investigation of quasi-one-dimensional (quasi-1D) Ta2PdSe6 single crystals by magnetotransport measurements under high magnetic fields up to 31 T. The crystals show large unsaturated magnetoresistance up to (1.8 × 105%) under 31 T at 1.6 K, accompanied by remarkable resistivity oscillations. The analysis of quantum oscillations reveals the existence of five Fermi pockets, consistent with theoretical calculations. Furthermore, our investigation suggested that band structure is modified by the electron correlations in Ta2PdSe6, which may provide a platform for exploring the interplay between electron effects and topological properties in quasi-1D materials.

Charge- 4e and Charge- 6e Flux Quantization and Higher Charge Superconductivity in Kagome Superconductor Ring Devices

The flux quantization is a key indication of electron pairing in superconductors. For example, the well-known h/2e flux quantization is considered strong evidence for the existence of charge-2e, two-electron Cooper pairs. Here we report evidence for multicharge flux quantization in mesoscopic ring devices fabricated using the transition-metal kagome superconductor CsV3Sb5. We perform systematic magnetotransport measurements and observe unprecedented quantization of magnetic flux in units of h/4e and h/6e in magnetoresistance oscillations. Specifically, at low temperatures, magnetoresistance oscillations with period h/2e are detected, as expected from the flux quantization for charge-2e superconductivity. We find that the h/2e oscillations are suppressed and replaced by resistance oscillations with h/4e periodicity when the temperature is increased. Increasing the temperature further suppresses the h/4e oscillations, and robust resistance oscillations with h/6e periodicity emerge as evidence for charge-6e flux quantization. Our observations provide the first experimental evidence for the existence of multicharge flux quanta and emergent quantum matter exhibiting higher-charge superconductivity in the strongly fluctuating region above the charge-2e Cooper pair condensate, revealing new insights into the intertwined and vestigial electronic order in kagome superconductors. Published by the American Physical Society 2024

In-plane gate induced transition asymmetry of spin-resolved Landau levels in InAs-based quantum wells

The crossover from quasi-two- to quasi-one-dimensional electron transport subject to transverse electric fields and perpendicular magnetic fields is studied in the diffusive to quasi-ballistic and zero-field to quantum Hall regime. In-plane gates and Hall-bars have been fabricated from an InGaAs/InAlAs/InAs quantum well hosting a 2DEG with a carrier density of about 6.8 × 1011 cm−2, a mobility of 1.8 × 105 cm2/Vs, and an effective mass of 0.042me after illumination. Magnetotransport measurements at temperatures down to 50 mK and fields up to 12 T yield a high effective Landé factor of g*=16, enabling the resolution of spin-split subbands at magnetic fields of 2.5 T. In the quantum Hall regime, electrostatic control of an effective constriction width enables steering of the reflection and transmission of edge channels, allowing a separation of fully spin-polarized edge channels at filling factors ν = 1 und ν = 2. A change in the orientation of a transverse in-plane electric field in the constriction shifts the transition between Zeeman-split quantum Hall plateaus by ΔB ≈ 0.1 T and is consistent with an effective magnetic field of Beff ≈ 0.13 T by spin-dependent backscattering, indicating a change in the spin-split density of states.

The role of magnetic anisotropy in the magnetoresistance of Cr2O3/Al2O3 thin film antiferromagnets

The magnetic states of antiferromagnetic insulating thin films are a promising medium for information storage, but characterization of these states has proven to be challenging. One approach is via magnetotransport measurements in an adjacent heavy metal layer. To this end, we synthesized and characterized a series of Cr2O3 films and bilayers on Al2O3 substrates with three different orientations: m-plane, a-plane, and c-plane. X-ray diffraction results demonstrated orientation control of the Cr2O3 thin film, with m-plane films displaying a higher degree of mosaic spread than the a- and c-plane films. Reciprocal space maps showed that the films are mostly relaxed, although there was a small and different degree of strain in each orientation. The m-plane films were under 2% compressive strain, the a-plane film was under 0.5% compressive strain, and the c-plane film was completely relaxed to bulk values. To probe the magnetic state of the films, we measured the angular dependent magnetoresistance of Cr2O3/Pt bilayers for each orientation. We found a nontrivial temperature dependence of the sign of the magnetoresistance, pointing to the complex interplay between the exchange and anisotropy energies that vary with orientation. We propose that strain and mosaic spread may contribute to a difference in magnetic anisotropies among the samples and the resulting temperature dependence of the magnetoresistance. This work demonstrates the importance of considering the competition between antiferromagnetic exchange and magnetic anisotropy when storing information in the spin state of an antiferromagnetic insulator.

Competition between chiral anomaly and weak antilocalization in Cd3As2 nanoplates

Negative longitudinal magnetoresistivity (nLMR) induced by the chiral anomaly together with the weak antilocalization (WAL) due to the quantum interference can be regarded as remarkable magnetotransport signatures for three-dimension (3D) topological semimetals. Here, we report the observation of high-temperature competition between the chiral anomaly and WAL by magnetotransport measurements on high-quality Cd3As2 nanoplates under parallel electromagnetic fields. We find that, the WAL dominates the magnetotransport in the weak magnetic fields, which decreases gradually and ultimately vanishes at a critical temperature Tc. In contrast, the chiral anomaly is robust against temperature and can survive up to room temperature. This competition between the chiral anomaly and WAL can be understood in terms of Berry phase, accompanying with the low carrier density in Cd3As2 nanoplates. Our work would offer a better understanding of magnetotransport properties governed by Berry phase and the nature of electronic states in topological semimetals.

Crossover from clean to dirty superconducting limit in YBCO films with modulated disorder

Magnetotransport measurements have been carried out for pristine and irradiated Y Ba2Cu3O7−x films. The measurements reveal a crossover from the clean superconducting limit to the dirty one with an increase in the impurity scattering rate Γ produced by a certain dose of irradiation. For the clean limit, the critical temperature Tc0 decreases with Γ, while the initial slope K=|dHc2/dT|Tc0 increases. We find a nice agreement between these experimental data and a model of d-wave superconductors with anisotropic impurity scattering. Such dose-dependencies of Tc0 and K reveal the superconducting coherence length ξ0 to be quasi-invariant in the clean limit. For the dirty limit, ξ0 increases with Γ due to a decrease in both the quantities Tc0 and K. Though, this decrease appears slower than predicted by the textbook picture of d-wave superconductors. Such discrepancy can be produced by an irradiation-induced modulated disorder responsible for the formation of (d+s)-wave pairing.

Proximity effects induced exchange bias and perpendicular net moment in SrIrO3

The magnetic proximity effect enables the incorporation of magnetic properties into non-magnetic materials, thereby achieving high-performance electronic devices. The 5d perovskite-oxide iridates provide a versatile platform for investigating competing electronic states due to their strong spin–orbit coupling. By proximity to conventional 3d transition-metal oxide ferromagnets, originally paramagnetic SrIrO3 is expected to exhibit exotic magnetic phenomena. In this study, we report the emergence of perpendicular net moment and an unprecedented exchange bias effect in SrIrO3 sandwiched between two ferromagnetic LaMnO3 layers. The magnetization measurements clearly demonstrate antiferromagnetic coupling in the heterostructures, resulting in a shift of the magnetic hysteresis loops. Furthermore, magneto-transport measurements provide evidence for the induction of a net moment along the perpendicular direction of the SrIrO3 film plane due to proximity effects. Both the magnetic pinning effect and perpendicular magnetization can be controlled by an external magnetic field. These findings expand the potential applications of iridate-related heterostructures in modern information storage fields such as skyrmions, spin–orbit torques, and antiferromagnet-based spintronics.

Near room-temperature ferromagnetism in BixCr1−xTe2 epitaxial thin films grown on GaAs(111)B by molecular beam epitaxy

Two-dimensional (2D) ferromagnetic materials (FMs) are potentially the material foundation for future spintronics devices. However, at present, the Curie temperature (TC) of most 2D FM is relatively low and cannot meet the need for practical applications. Nowadays, CrTe2 thin films grown by molecular beam epitaxy (MBE) are reported to be room-temperature ferromagnetic only on graphene substrate instead of 3D substrates. In this work, we report high-quality Bi-doped CrTe2 (BixCr1−xTe2) thin films grown on conventional substrates of GaAs(111)B by MBE. Magnetotransport measurements reveal strong ferromagnetism of all the films, with out-of-plane magnetic anisotropy. More importantly, as more Bi atoms are doped into the film, the Curie temperature increases and reaches 305 K at x = 0.1. This improvement is a step forward for its application in spintronics and other fields.

Dual-component anomalous Hall effect in a helical spin-spiral metamagnet

We report a dual-component anomalous Hall effect (AHE) in polycrystalline Fe3Ga4 thin films grown on STO (001) and Al2O3 substrates. Systematic magnetic and magnetotransport measurements reveal an AHE consisting of positive and negative contributions that coexist across a wide range of temperatures and magnetic phases. We find that both magnitudes are nearly equal in the low-temperature ferromagnetic (FM) phase, but that their relative ratio is reduced upon heating through the antiferromagnetic helical spin-spiral state where they compete with metamagnetism and topological Hall effects, maintaining finite values at least up to the high-temperature FM phase.

Spin-Orbit Torques and Magnetization Switching in (Bi,Sb)2Te3/Fe3GeTe2 Heterostructures Grown by Molecular Beam Epitaxy.

Topological insulators (TIs) hold promise for manipulating the magnetization of a ferromagnet (FM) through the spin-orbit torque (SOT) mechanism. However, integrating TIs with conventional FMs often leads to significant device-to-device variations and a broad distribution of SOT magnitudes. In this work, we present a scalable approach to grow a full van der Waals FM/TI heterostructure by molecular beam epitaxy, combining the charge-compensated TI (Bi,Sb)2Te3 with 2D FM Fe3GeTe2 (FGT). Harmonic magnetotransport measurements reveal that the SOT efficiency exhibits a non-monotonic temperature dependence and experiences a substantial enhancement with a reduction of the FGT thickness to 2 monolayers. Our study further demonstrates that the magnetization of ultrathin FGT films can be switched with a current density of Jc ∼ 1010 A/m2, with minimal device-to-device variations compared to previous investigations involving traditional FMs.

Epitaxial thin films of binary Eu-compounds close to a valence transition

Intermetallic binary compounds of europium reveal a variety of interesting phenomena due to the interconnection between two different magnetic and 4f electronic (valence) states, which are particularly close in energy. The valence states, or magnetic properties are thus particularly sensitive to strain tuning in these materials. Consequently, we grew epitaxial EuPd2 (Eu2+, high magnetic moment) and EuPd3 (Eu3+, vanishing magnetic moment) thin films on MgO(001) substrates using molecular beam epitaxy. Ambient X-ray diffraction confirms an epitaxial relationship of cubic Laves-type (C15) EuPd2 with an [111]-out-of-plane orientation, whereby eight distinct in-plane crystallographic domains develop. For simple cubic EuPd3 two different out-of-plane orientations can be obtained by changing the substrate annealing temperature under ultra-high vacuum conditions from 873K to 1273K for one hour. A small resistance minimum evolves for EuPd3 thin films grown with low temperature substrate annealing, which was previously found even in single crystals of EuPd3 and might be attributed to a Kondo or weak localization effect. Absence of influence of applied magnetic fields and magnetotransport measurements suggest a purely trivalent Eu valence for EuPd3 thin films, as found in EuPd3 single crystals. For EuPd2 magnetic ordering below ∼72K is observed, quite similar to single crystal behavior. Field dependent measurements of the magnetoresistance and the Hall effect show hysteresis effects below ∼0.4T and an anomalous Hall effect below ∼70K, which saturates around 1.4T, thus proving a ferromagnetic ground state of the divalent Eu.

Field-induced compensation of magnetic exchange as the possible origin of reentrant superconductivity in UTe2

The potential spin-triplet heavy-fermion superconductor UTe2 exhibits signatures of multiple distinct superconducting phases. For field aligned along the b axis, a metamagnetic transition occurs at μ0Hm ≈ 35 T. It is associated with magnetic fluctuations that may be beneficial for the field-reinforced superconductivity surviving up to Hm. Once the field is tilted away from the b towards the c axis, a reentrant superconducting phase emerges just above Hm. In order to better understand this remarkably field-resistant superconducting phase, we conducted magnetic-torque and magnetotransport measurements in pulsed magnetic fields. We determine the record-breaking upper critical field of μ0Hc2 ≈ 73 T and its evolution with angle. Furthermore, the normal-state Hall effect experiences a drastic suppression indicative of a reduced band polarization above Hm in the angular range around 30° caused by a partial compensation between the applied field and an exchange field. This promotes the Jaccarino-Peter effect as a likely mechanism for the reentrant superconductivity above Hm.

Effect of an external/internal magnetic field on the photocurrent in Py-topological insulator heterojunction Ni80Fe20/TI (Bi2Te3/Bi2Se3/Bi2Te2Se)/p-Si devices.

This study demonstrates the fabrication and study of a permalloy (Py)/topological insulator heterojunction, i.e., the Ni80Fe20/TI(Bi2Te3/Bi2Se3/Bi2Te2Se)/p-Si heterojunction, for spintronic device applications at room temperature. In this work, the forward current values, under the absence of a magnetic field, for Ni80Fe20/Bi2Te2Se/p-Si, Ni80Fe20/Bi2Se3/p-Si, and Ni80Fe20/Bi2Te3/p-Si heterojunctions were 12.7 μA, 8.7 μA, and 6.85 μA, respectively; while in the presence of a magnetic field, the corresponding values were 10.8 μA, 7.6 μA, and 4.47 μA, respectively. Such reductions in current were attributed to an increase in the resistance of the p-n junction diode due to Lorentz force and a magnetoresistance effect, which was also verified using magneto-transport measurements. This resulted in a modification of the space charge shape and an increase in the potential barrier. Along with this, the magnetic field also affected the diffusion length, leading to a reduction in the current. Such a phenomenon enables the development of durable devices with improved reliability and longevity under harsh environments. The special features of topological edge states in the presence of a magnetic field can be used for sophisticated sensing applications. The future applications will likely lead to the emergence of other novel applications across disciplines such as computing, health, materials science, and energy harvesting.