Temperature Dependence Of Magnetoresistance Research Articles

Influence of the electron density on the giant negative magnetoresistance in two-dimensional electron gases

variation of the electron density via a metallic gate can control the disorder potentials in two-dimensional electron gases (2DEGs). This also influences the negative magnetoresistance at low magnetic fields, which is commonly observed in ultrahigh mobility 2DEGs. We investigate the temperature-dependent giant negative magnetoresistance (GNMR) as a function of the electron density for several temperatures and currents. Thereby, we find that the GNMR behavior depends decisively on the electron density. This observation is attributed to a changed disorder potential with electron density. In the case of higher electron densities, a nonlinear current dependency of the GNMR is observed, which could be described within the hydrodynamic regime. Published by the American Physical Society 2024

Magnetoresistance properties in nickel-catalyzed, air-stable, uniform, and transfer-free graphene

A transfer-free graphene with high magnetoresistance (MR) and air stability has been synthesized using nickel-catalyzed atmospheric pressure chemical vapor deposition. The Raman spectrum and Raman mapping reveal the monolayer structure of the transfer-free graphene, which has low defect density, high uniformity, and high coverage (>90%). The temperature-dependent (from 5 to 300 K) current–voltage (I–V) and resistance measurements are performed, showing the semiconductor properties of the transfer-free graphene. Moreover, the MR of the transfer-free graphene has been measured over a wide temperature range (5–300 K) under a magnetic field of 0 to 1 T. As a result of the Lorentz force dominating above 30 K, the transfer-free graphene exhibits positive MR values, reaching ∼8.7% at 300 K under a magnetic field (1 Tesla). On the other hand, MR values are negative below 30 K due to the predominance of the weak localization effect. Furthermore, the temperature-dependent MR values of transfer-free graphene are almost identical with and without a vacuum annealing process, indicating that there are low density of defects and impurities after graphene fabrication processes so as to apply in air-stable sensor applications. This study opens avenues to develop 2D nanomaterial-based sensors for commercial applications in future devices.

Unconventional superconductivity in PdxBi2Se3 whiskers

Studies of temperature dependence of magnetoresistance for PdxBi2Se3 whiskers in the temperature range 1.6-77K in magnetic field up to 10 T were carried out. Crystals were grown by chemical transport reaction method in closed bromide system. The source and crystallization zone temperatures were 1100 K and 780 K, respectively. Doping of the crystals was carried out during the growth process with palladium impurity to concentrations of (1 – 2) × 1019 cm−3. In the low-temperature region beginning at a temperature of 5 K and reaching a temperature 3.5 K, a sharp decrease in resistance was observed, which is associated with the transition to superconducting state. Based on the analysis of the temperature dependence of the resistance at fixed magnetic fields, the Curie temperature Tc1=5.3 K and Tc2=3.5 K as well as the upper critical magnetic field Bc2=1.45 T and 0.25 T were determined. The established parameters indicates in II type supercondor. This is indicated by the ratio Δ0/kBTc = 2.0, which exceeds BCS limit of 1.76 and indicates a relatively large value of the superconducting gap Δ0=0.8 meV. The determined ratio A/γ2, which establishes the relationship between the electron-electron and electron-phonon interaction, is about of 2ao, which indicates a strong fermionic interaction with phonons in the PdxBi2Se3 superconductor. The estimated value of the ratio of the Curie temperature to the effective Fermi temperature equal to 0.04 also falls within the range of 0.01 ⩽ Tc/TF ⩽ 0.1, which confirms the unconventional superconductivity in the investigated whiskers.

Magnetoresistance behavior across the critical region in ferrimagnet FeCr2Te4 single crystal

In this paper, we carried out a systematic study on the magnetic and electrical transport properties of weak-itinerant ferrimagnet FeCr2Te4 single crystal. We observe a substantial suppression of electron-magnon scattering and spin-fluctuation scattering coefficients with increasing magnetic field below the magnetic ordering temperature ≈123 K. Temperature dependent magnetoresistance exhibits a maximum in the close vicinity of the Curie temperature (Tc) associated with paramagnetic to ferrimagnetic transition. Our Hall resistivity measurements suggest that the obtained carrier concentration is directly related to the magnetoresistance behavior near the transition temperature. Angle dependent measurements show that the magnetoresistance is anisotropic near Tc.

Growth and characterization of the sputtered type-II topological semimetal PdTe2 thin films and PdTe2/Co60Fe20B20 heterostructures

The topological phases such as Dirac/Weyl semimetals are recently observed in Layered transition metal dichalcogenides (TMDs). The family of materials XTe2 (with X = Pd, Pt and Ni) possesses a type-II topological semimetal (TSM) phase. Here, we report the synthesis of the layered XTe2 thin films using the industry-friendly sputtering technique. The structural and morphological qualities of the sputtered-grown XTe2 thin films are analysed using X-ray diffraction and Raman measurements. The crystallites are highly oriented along the c-axis of the sapphire substrate, which suggests that growth is driven by van der Waals epitaxy. The magneto-transport measurements namely temperature-dependent resistivity and magnetoresistance measurement in out-of-plane field configuration are performed on PdTe2 thin films. The semi-metallic trend and superconducting transition are observed in the temperature-dependent resistivity curve. The weak antilocalization behavior observed in the magnetoconductance curves at a low magnetic field signifies the possibility of a non-zero Berry phase, possibly arising from the Dirac fermionic nature of electronic states. The spin dynamics measurements are performed on the PdTe2/Co60Fe20B20 bilayers. The effective spin mixing conductance and spin current density are found to be ∼2×1020m-2and 2.5MAm-2, respectively in PdTe2/Co60Fe20B20 bilayer which are quite larger than those observed in heavy metal-ferromagnets bilayers, and comparable to MBE-grown topological insulator-ferromagnets heterostructures. This work paves the way to synthesize Te-based layered TMDs using the industrially viable sputtering technique, and demonstrates the prospects for exploring the topological superconductive phase in PdTe2 films and large spin-charge conversion efficiency in PdTe2/Co60Fe20B20 heterostructures.

The colossal magnetoresistance in wide temperature region of Pr0.7Sr0.3MnO3 film on Pb(Mg1/3Nb2/3)0.7Ti0.3O3 substrate

The magnetotransport and colossal magnetoresistance (MR) in wide temperature region of Pr0.7Sr0.3MnO3 film grown on Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMNPT) substrate have been studied in detail. The X-ray diffraction (XRD) measurement shows that the Pr0.7Sr0.3MnO3/PMNPT film reveal the out-of-plane single orientation and the atomic force microscopy (AFM) measurement indicates that the surface of the sample is very smooth. Moreover, some obvious interference peaks of the film and the full width at half maximum (FWHM) of 0.28° for (002) diffraction peak are observed in the XRD pattern. These phenomena suggest the high quality of the thin film. The magnetotransport and the MR effects associated to both the double exchange (DE) interaction and the spin polarized tunneling have been investigated using the temperature dependence of resistivity (ρ-T), the temperature dependence of MR (MR-T) and the magnetic filed dependence of resistivity (ρ-H). The ρ is increased gradually with decreasing the temperature, but a metal-insulator (MI) transition appears in the ρ-T curves. The MI temperature (TMI) is increased with increasing the applied field. The MR values (MR = [ρ-ρ0/ρ0]×100%) reach 25.2%, 47.8%, 69.4% and 77.8% at the applied fields of 1 , 2 , 4 and 6 T, respectively. At around TMI, the ρ changes significantly with the applied field. This indicates a colossal MR at around TMI, which can be explained by the DE mechanism. Compared with other works on Pr0.7Sr0.3MnO3, the Pr0.7Sr0.3MnO3 film on PMNPT exhibits the colossal MR in wide temperature region. Remarkably, there is two types of MR mechanisms in this work, i.e., the MR effect at around TMI caused by the DE interaction and the MR effect at low temperature (below TMI) resulted from the spin polarized tunneling, which lead to the wide temperature region of the MR.

Anomalous Hall effect in ferromagnetic LaCo2As2 and ferrimagnetic NdCo2As2

Abstract We conducted a comparative study of the magnetic and transport properties of single-crystalline LaCo2As2 and NdCo2As2. LaCo2As2 is a soft metallic ferromagnet which exhibits purely intrinsic anomalous Hall effect (AHE) due to Co-3d electrons. With Nd-4f electronic magnetism, ferrimagnetic NdCo2As2 manifests pronounced sign reversal and multiple hysteresis loops in temperature and field dependent magnetization, Hall resistivity and magnetoresistance, due to complicated magnetic structural changes. We reveal that the AHE for NdCo2As2 is stemming from the Co sub-lattice and deduce its phase diagram which includes magnetic compensation and two meta-magnetic phase transitions. The sensitivity of the Hall effect on the details of the magnetic structures in ferrimagnetic NdCo2As2 provides a unique opportunity to explore the magnetic interaction between 4f and 3d electrons and its impact on the electronic structure.

Interfacial quantum interference effect and dual magnetoresistance in La0.7Sr0.3MnO3 thin films grown on (001) Si

Transmission electron microscope image and electronic transport of La0.7Sr0.3MnO3 (LSMO) films grown on (001) oriented Si using the sputtered pulsed plasma method confirmed the presence of around 8 nm thick, less dense, and highly resistive LSMO at the interface below the conducting phase. Thicker LSMO films, in addition to metal-insulator transition, show an anomaly around the Curie temperature in temperature-dependent resistivity and magnetoresistance (MR), which is a unique observation. The conduction in these LSMO films at temperatures below low-temperature resistivity minimum is dominated by Kondo-like scattering over electron–electron scattering, established using the phenomenological model. At 20 K, the maximum positive MR is ∼ 12% for the in-plane field, while it is ∼ 7.2% for the out-of-plane field. The maximum negative in-plane MR is found to be ∼ 42.5%, which becomes ∼ 30% when the orientation of the field changes to the out-of-plane direction. The two-dimensional field-dependent change in the magneto-conductance model evidenced the quantum interference effect (QIE). The existence of QIE is associated with magnetic scattering and scattering due to spin–orbit coupling. These results may be used to modulate the electrical properties of future electronic devices and can encourage scientists to explore the multi-functionalities of complex oxides grown on bare Si substrates.

Strain-induced massless Dirac fermion state of the molecular conductor α-(BEDT-TTF)2I3

Uniaxial pressure can dramatically change the properties of low-dimensional systems and induce electronic phase transitions. The first-discovered molecular massless Dirac electron system α-(BEDT-TTF)2I3 has intensively been studied because of its unique features, such as the Fermi energy being very close to the Dirac point, strong electron correlation, and the quantum phase transition from the charge-ordered insulating state. However, the Dirac state is realized only under high pressure of 15 kbar, which limits the measurement of physical properties (e.g., experimental determination of band dispersion and density of states). Here, we demonstrate that the Dirac state of α-(BEDT-TTF)2I3 can be realized by applying uniaxial bending strain without confining it in a pressure cell. Uniaxial strain below 1% completely suppresses the metal–insulator transition observed at ambient pressure. Under strain, a characteristic temperature dependence of magnetoresistance associated with the formation of the n = 0 Landau level is observed, indicating the realization of the massless Dirac state.

Field effect two-dimensional electron gases in modulation-doped InSb surface quantum wells

We report on transport characteristics of field effect two-dimensional electron gases (2DEGs) in surface indium antimonide quantum wells. The topmost 5 nm of the 30 nm wide quantum well is doped and shown to promote the formation of reliable, low resistance Ohmic contacts to surface InSb 2DEGs. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus is observed to filling factor ν = 1 in magnetic fields of up to B = 18 T. We show that the electron density is gate-tunable, reproducible, and stable from pinch-off to 4 ×1011 cm−2, and peak mobilities exceed 24 000 cm2/V s. Large Rashba spin–orbit coefficients up to 110 meV ·Å are obtained through weak anti-localization measurements. An effective mass of 0.019me is determined from temperature-dependent magnetoresistance measurements, and a g-factor of 41 at a density of 3.6 ×1011 cm−2 is obtained from coincidence measurements in tilted magnetic fields. By comparing two heterostructures with and without a delta-doped layer beneath the quantum well, we find that the carrier density is stable with time when doping in the ternary Al0.1In0.9Sb barrier is not present. Finally, the effect of modulation doping on structural asymmetry between the two heterostructures is characterized.

Temperature-driven changes in the Fermi surface of graphite

We report on temperature-dependent size and anisotropy of the Fermi pockets in graphite revealed by magnetotransport measurements. The magnetoresistances obtained in fields along the c-axis obey an extended Kohler's rule, with the carrier density following prediction of a temperature-dependent Fermi energy, indicating a change in the Fermi pocket size with temperature. The angle-dependent magnetoresistivities at a given temperature exhibit a scaling behavior. The scaling factor that reflects the anisotropy of the Fermi surface is also found to vary with temperature. Our results demonstrate that temperature-driven changes in Fermi surface can be ubiquitous and need to be considered in understanding the temperature-dependent carrier density and magnetoresistance anisotropy in semimetals.

Study of metal–insulator like transition in Co[formula omitted]Fe[formula omitted]S[formula omitted

The metal–insulator like transition of the ferromagnetic (FM) state in Co1−xFexS2 (0 ≤x≤ 0.7) with varying x is investigated using X-ray diffraction, Mössbauer spectroscopy, and magnetoresistance measurements. The linear decrease of unit cell parameter with x confirmed Fe substitution at the Co site. The room temperature Mössbauer study showed the absence of magnetic moment on Fe and both isomer shift and quadrupole splitting were found to vary linearly with x. The temperature-coefficient-of-resistivity (TCR) in the FM state became negative for x≥ 0.15. For intermediate compositions i.e., x= 0.05 and 0.1, TCR changed from positive to negative (T ≤ TC) to positive with an increase in temperature. By studying the temperature-dependent and isothermal magnetoresistance data around the transition region, it was shown that resultant behavior could be arising due to changes in spin polarization over a broad temperature range.

The effect of heat treatment on temperature-dependent transport and magnetoresistance in polyacrylonitrile-based carbon fibers

The effects of heat treatment temperatures from 1000 to 2700 °C on the structural, chemical, and mechanical properties of polyacrylonitrile-based carbon fibers (CFs) were investigated. Structural changes affected mechanical properties, and in particular, a higher sp3 content induced a higher tensile strength below 1600 °C. For a comprehensive study on the heat treatment effect and the mechanism for developing high-strength CFs, electrical transport properties were measured for the CFs. The transport behaviors in temperature-dependent conductivity and magnetoresistance were also significantly modified, which led to crossover from strong localization to weak localization due to sp2 carbon ordering. Interestingly, in samples that were subjected to intermediate heat treatment temperatures ranging from 1100 to 1600 °C, anomalous metallic temperature-dependent behavior was observed at low temperatures of below 40 K. This unusual charge transport was analyzed further with a pertinent heterogeneous model employing three-dimensional variable range hopping, weak localization, and linear metallic transport. The results suggest that mixed noncrystalline and crystalline structures with sp2/sp3 carbons, oxygen, and quaternary N groups in CFs enabled the transition behavior at such low temperatures.

Temperature Dependence of Two-Terminal Local Magnetoresistance in Co-Based Heusler Alloy/Ge Lateral Spin-Valve Devices

We investigate temperature dependence of two-terminal local magnetoresistance (MR) effect for germanium (Ge)-based lateral spin-valve (LSV) devices with three different ferromagnet (FM)/Ge Schottky tunnel contacts. When we insert a 0.7-nm-thick Fe layer between Co-based Heusler alloy and Ge interfaces in LSV devices, the magnitude of the local MR signals is significantly improved from low temperatures to room temperature. However, the temperature dependence of the MR ratio is still large even for LSV devices with Fe insertion. From the analysis based on the standard theory, the temperature-dependent MR ratio can be interpreted in terms of thermally excited spin waves ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T^{3/{2}}$ </tex-math></inline-formula> law) in FM contacts. The physical origin of the decay of the two-terminal MR ratio in Ge-based LSV devices is the same as that of the four-terminal nonlocal spin signals because small temperature dependence of contact resistance is demonstrated in FM/Ge Schottky tunnel contacts which show almost linear <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|J|$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> characteristics and low interface resistance.

Escalating Ferromagnetic Order via Se-Vacancies Near Vanadium in WSe2 Monolayers.

Magnetic order has been proposed to arise from a variety of defects, including vacancies, antisites, and grain boundaries, which are relevant in numerous electronics and spintronics applications. Nevertheless, its magnetism remains controversial due to the lack of structural analysis. The escalation of ferromagnetism in vanadium-doped WSe2 monolayer is herein demonstrated by tailoring complex configurations of Se vacancies (SeVac ) via post heat-treatment. Structural analysis of atomic defects is systematically performed using transmission electron microscopy (TEM), enabled by the monolayer nature. Temperature-dependent magnetoresistance hysteresis ensures enhanced magnetic order after high-temperature heat-treatment, consistent with magnetic domain analysis from magnetic force microscopy (MFM). The vanadium-Se vacancy pairing is a key to promoting ferromagnetism via spin-flip by electron transfer, predicted from density-functional-theory (DFT) calculations. The approach toward nanodefect engineering paves a way to overcome weak magnetic order in diluted magnetic semiconductors (DMSs) for renovating semiconductor spintronics.

Electron transport property of epitaixial bilayer graphene on SiC substrate

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

Thickness dependence of magnetotransport properties of tungsten ditelluride

We investigate the electronic structure of tungsten ditelluride (WTe$_2$) flakes with different thicknesses in magneto-transport studies. The temperature-dependent resistance and magnetoresistance (MR) measurements both confirm the breaking of carrier balance induced by thickness reduction, which suppresses the `turn-on' behavior and large positive MR. The Shubnikov-de-Haas oscillation studies further confirm the thickness-dependent change of electronic structure of WTe$_2$ and reveal a possible temperature-sensitive electronic structure change. Finally, we report the thickness-dependent anisotropy of Fermi surface, which reveals that multi-layer WTe$_2$ is an electronic 3D material and the anisotropy decreases as thickness decreases.

Modulation of Weyl semimetal state in half-Heusler GdPtBi enabled by hydrostatic pressure

The excitation of Weyl semimetals obeys the relativistic Weyl equation and attracted significant research attention due to its unique electronic state. In this paper, we present an emerging approach for modulating the electron state of half-Heusler GdPtBi by hydrostatic pressure. Through measurements of the temperature-dependent resistivity and magnetoresistance (MR), a phase transition from a Weyl semimetal to a semiconductor state was identified at about 2.0 GPa upon increasing the hydrostatic pressure. Electron transport in semiconductive GdPtBi is found to be well describable by Mott variable-range-hopping. The simulated electronic structures under different hydrostatic pressures further indicate that changes in the electronic states of atoms in the primary unit cell result in a phase transition in GdPtBi. This work presents an effective strategy for modulating the electronic state by tuning the lattice constant.

Magnetoresistance driven by the magnetic Berezinskii-Kosterlitz-Thouless transition

While the Berezinskii-Kosterlitz-Thouless transition (BKT) has been under intense scrutiny for decades, unambiguous experimental signatures in magnetic systems remain elusive. Here, we investigate the interplay between electronic and magnetic degrees of freedom near the BKT transition. Focusing on a metal with easy-plane ferromagnetic order, we establish a framework that accounts both for the coupling between the charge current and the flow of topological magnetic defects and for electron scattering on their inhomogeneous spin texture. We show that electron scattering is responsible for a temperature-dependent magnetoresistance effect scaling as the density of the topological defects, which is expected to increase dramatically above the BKT transition temperature. Our findings call for further experimental investigations.

Magnetoresistance and scaling laws in type-II Weyl semimetal WP2

Topological materials with extremely large magnetoresistance exhibit a prognostic feature of resistivity turn-on behaviour. This occurs when the temperature dependence of resistivity ρ(T) changes from metallic to semiconducting characteristics on application of external magnetic field above a threshold value. Here, we study the magneto-transport properties of type-II Weyl Semimetal WP2. The zero field electrical resistivity in the low temperature region indicates the dominant electron-phonon scattering. The saturation in ρ(T) curves under all applied magnetic fields are observed at low temperatures. A minimum in resistivity at ~40K is revealed in the temperature derivative of resistivity curves, which implies onset of field induced turn-on effect. Furthermore, a non-saturating linear magnetoresistance (MR) is observed above ~ 5 T which is generally assigned to linear energy dispersion near the Weyl nodes. However, Kohler's scaling fits the data well with resistivity ρ~(B/ρ0)m that implicitly explains the turn-on behaviour and the resistivity minimum. Thus, semi-classical theories of magnetoresistance are consistent with our data without the need to invoke topological surface states. Our findings in this work provides an alternative basis to understand the temperature dependence of magnetoresistance in topological materials.