Low-temperature Magnetoresistance Research Articles

Microstructure dependent magnetotransport properties in polycrystalline La0.7Sr0.3MnO3 ultrathin films on thermally oxidized Si substrates

The magnetotransport properties of polycrystalline La0.7Sr0.3MnO3 (LSMO) ultrathin films depend strongly on their thickness and microstructure. In particular, the resistivity and magnetoresistance can be tuned by varying the film thickness and grain size. Here, we have deposited LSMO films with thicknesses ranging from 100 nm down to 10 nm by pulsed laser deposition on thermally oxidized Si substrates, retaining the films’ metallic nature. To avoid reaction of the films with SiO2 at high temperatures, we have introduced a two-step deposition method (half the film thickness grown at 400 °C and the remaining half at 800 °C). X-ray diffraction (XRD) revealed the polycrystalline nature, while atomic force microscopy (AFM) revealed the granular structure of the films from which, surface roughness and grain size are found to decrease with decreasing film thickness. However, low temperature resistivity upturn increases with decreasing film thickness and grain size. The variations of low temperature resistivity minima and magnetoresistance (MR) with film thickness and grain size have been interpreted in the regime of quantum interference effects (viz. weak localization and e-e interaction), intergranular spin polarized transport (SPT) phenomena and their dominance. It is found that SPT was the dominating phenomena. The signature of SPT was also evidenced clearly from low field MR at low temperatures. These kinds of ultrathin films are suitable as ferromagnetic electrode for spintronic device applications.

Linear magnetoresistance with a universal energy scale in the strong-coupling superconductor Mo8Ga41 without quantum criticality

The recent discovery of a nonsaturating linear magnetoresistance in several correlated electron systems near a quantum critical point has revealed an interesting interplay between the linear magnetoresistance and the zero-field linear-in-temperature resistivity. These studies suggest a possible role of quantum criticality on the observed linear magnetoresistance. Here we report our discovery of a nonsaturating, linear magnetoresistance in ${\mathrm{Mo}}_{8}{\mathrm{Ga}}_{41}$, a nearly isotropic strong electron-phonon coupling superconductor with a linear-in-temperature resistivity from the transition temperature to $\ensuremath{\sim}55$ K. The growth of the resistivity in field is comparable to that in temperature, provided that both quantities are measured in the energy unit. Our data sets are remarkably similar to magnetoresistance data of the optimally doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$, despite the clearly different crystal and electronic structures, and the apparent absence of quantum critical physics in ${\mathrm{Mo}}_{8}{\mathrm{Ga}}_{41}$. A new empirical scaling formula is developed, which is able to capture the key features of the low-temperature magnetoresistance data of ${\mathrm{Mo}}_{8}{\mathrm{Ga}}_{41}$, as well as the data of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$.

Weak localization and weak antilocalization in doped Ge1-y Sn y layers with up to 8% Sn

Low-temperature magnetoresistance measurements of n- and p-doped germanium–tin (Ge1-y Sn y ) layers with Sn concentrations up to 8% show contributions arising from effects of weak localization for n-type and weak antilocalization for p-type doped samples independent of the Sn concentration. Calculations of the magnetoresistance using the Hikami–Larkin–Nagaoka model for two-dimensional transport allow us to extract the phase-coherence length for all samples as well as the spin–orbit length for the p-type doped samples. For pure Ge, we find phase-coherence lengths as long as (349.0 ± 1.4) nm and (614.0 ± 0.9) nm for n-type and p-type doped samples, respectively. The phase-coherence length decreases with increasing Sn concentration. From the spin–orbit scattering length, we determine the spin-diffusion scattering length in the range of 20–30 nm for all highly degenerate p-type doped samples irrespective of Sn concentration. These results show that Ge1-y Sn y is a promising material for future spintronic applications.

Anisotropic magnetoresistance in spin\u2013orbit semimetal {hbox {SrIrO}}_{3}

{hbox {SrIrO}}_{3}, the three-dimensional member of the Ruddlesden–Popper iridates, is a paramagnetic semimetal characterised by a the delicate interplay between spin–orbit coupling and Coulomb repulsion. In this work, we study the anisotropic magnetoresistance (AMR) of {hbox {SrIrO}}_{3} thin films, which is closely linked to spin–orbit coupling and probes correlations between electronic transport, magnetic order and orbital states. We show that the low-temperature negative magnetoresistance is anisotropic with respect to the magnetic field orientation, and its angular dependence reveals the appearance of a fourfold symmetric component above a critical magnetic field. We show that this AMR component is of magnetocrystalline origin, and attribute the observed transition to a field-induced magnetic state in {hbox {SrIrO}}_{3}.

Symmetry breaking and skyrmionic transport in twisted bilayer graphene

Motivated by recent low-temperature magnetoresistance measurements in twisted bilayer graphene aligned with hexagonal Boron Nitride substrate, we perform a systematic study of possible symmetry breaking orders in this device at a filling of two electrons per Moir\'e unit cell. We find that the surprising non-monotonic dependence of the resistance on an out-of-plane magnetic field is difficult to reconcile with particle-hole charge carriers from the low-energy bands in symmetry broken phases. We invoke the non-zero Chern numbers of the twisted bilayer graphene flat bands to argue that skyrmion textures provide an alternative for the dominant charge carriers. Via an effective field-theory for the spin degrees of freedom, we show that the effect of spin Zeeman splitting on the skyrmion excitations provides a possible explanation for the non-monotonic magnetoresistance. We suggest several experimental tests, including the functional dependence of the activation gap on the magnetic field, for our proposed correlated insulating states at different integer fillings. We also discuss possible exotic phases and quantum phase transitions that can arise via skyrmion-pairing on doping such an insulator.

Multiband Ballistic Transport and Anisotropic Commensurability Magnetoresistance in Antidot Lattices of AB-stacked Trilayer Graphene

Ballistic transport was studied in a multiple-band system consisting of an antidot lattice of AB-stacked trilayer graphene. The low temperature magnetoresistance showed commensurability peaks arising from matching of the antidot lattice period and radius of cyclotron orbits for each mono- and bilayer-like band in AB stacked trilayer graphene. The commensurability peak of the monolayer-like band appeared at a lower magnetic field than that of the bilayer-like band, which reflects the fact that the Fermi surface of the bilayer-like band is larger than that of monolayer-like band. Rotation of the antidot lattice relative to the crystallographic axes of graphene resulted in anisotropic magnetoresistance, which reflects the trigonally warped Fermi surface of the bilayer-like band. Numerical simulations of magnetoresistance that assumed ballistic transport in the mono- and bilayer-like bands approximately reproduced the observed magnetoresistance features. It was found that the monolayer-like band significantly contributes to the conductivity even though its carrier density is an order smaller than that of the bilayer-like band. These results indicate that ballistic transport experiments could be used for studying the anisotropic band structure of multiple-band systems.

Modification of Shubnikov–de Haas oscillation and quantum Hall effect for Bi2Se3 crystals by Fe

Shubnikov–de Haas (SdH) oscillations and the quantum Hall effect (QHE) can give insights into quantum oscillations of charge carriers. We report the SdH oscillations and QHE of magnetically doped topological insulator FexBi2−xSe3 crystals, in which the lattice constant c decreases with the increasing dopant concentration but increases when x is greater than 0.08. From the fitting of low-temperature magnetoresistance data to the Lifshitz–Kosevich formula, the effective mass of the carriers and the Dingle temperature of the samples were extracted and show a similar trend of change as the lattice constant. A constant step size of 1/Rxy observed during Hall measurements at low temperatures implies two-dimensional-like transport behavior associated with the layered structure of the doped Bi2Se3 samples. Two-dimensional and three-dimensional charge carrier densities were calculated, and the latter had a similar trend of change as the lattice constant. A systematic study of the changes in all these properties with increasing dopant concentration suggests that they are highly correlated.

Anisotropic three-dimensional weak localization in ultrananocrystalline diamond films with nitrogen inclusions

We present a study of the structural and electronic properties of ultrananocrystalline diamond films that were modified by adding nitrogen to the gas mixture during chemical vapor deposition growth. Hall bar devices were fabricated from the resulting films to investigate their electrical conduction as a function of both temperature and magnetic field. Through low-temperature magnetoresistance measurements, we present strong evidence that the dominant conduction mechanism in these films can be explained by a combination of three-dimensional weak localization (3DWL) and thermally activated hopping at higher temperatures. An anisotropic 3DWL model is then applied to extract the phase-coherence time as a function of temperature, which shows evidence of a power-law dependence in good agreement with theory.

A comparative study in different rare earth ions in multiferroic nanocomposites: Low temperature resistivity minima and low field magnetoresistance

Low temperature magnetoresistance and resistivity properties are systematically studied in rare earth based multiferroic nanocomposites La0.7Sr0.3MnO3-TMnO3 (T = Er,Ho) under magnetic fields up to 5 Tesla. Resistivity with temperature exhibits metal-insulator transition. At very low temperature, the sharp upturn of resistivity is observed, which is found to be suppressed by field. The upturn of resistivity is characterized with spin fluctuation of the system. The reduction of the upturn of resistivity with magnetic field ensures the progress of the electrons tunneling. Effective reduction of charging energy is obtained upon an applied magnetic field. Temperature dependent resistivity shows minima at low temperature under different applied magnetic fields. The resistivity fit at low temperature is made with consideration of equally electron-electron interactions concerning (temperature)1/2 term and the Kondo-identical spin dependent scattering for ln(temperature) dependence. Lessening of the spin fluctuation by an applied magnetic field plays a dynamical role in defining the magnetoresistive behavior in samples. Experimental data are analyzed and demonstrated through a simple proposed phenomenological model to provide an insight on the physical mechanism behind the low temperature transport phenomena. The derived surface spin susceptibility is accordance with the temperature dependence of magnetoresistance involving an intergranular surface magnetization. Temperature dependent field cooled and zero field cooled dc magnetization curve having a large bifurcation between them, is a suggestion of a glassy like phase that may be originated from the challenging magnetic exchange interactions. The low temperature glassy magnetic behavior is endorsed to a spin-glass identical frozen state for the surface of the nanoparticles. Magnetic field dependent magnetization study shows an increase in magnetization without saturation in decreasing temperature. The multiferroic nanocomposites exhibit the magnetoelectric coupling, which arises due to the strain mediated magnetostriction behavior of the piezomagnetic phase (La0.7Sr0.3MnO3) in the nanocomposites.

Inkjet-printed graphene Hall mobility measurements and low-frequency noise characterization

We report room temperature Hall mobility measurements, low temperature magnetoresistance analysis and low-frequency noise characterization of inkjet-printed graphene films on fused quartz and SiO2/Si substrates. We found that thermal annealing in vacuum at 450 °C is a necessary step in order to stabilize the Hall voltage across the devices, allowing their electrical characterization. The printed films present a minimum sheet resistance of 23.3 Ω sq-1 after annealing, and are n-type doped, with carrier concentrations in the low 1020 cm-3 range. The charge carrier mobility is found to increase with increasing film thickness, reaching a maximum value of 33 cm2 V-1 s-1 for a 480 nm-thick film printed on SiO2/Si. Low-frequency noise characterization shows a 1/f noise behavior and a Hooge parameter in the range of 0.1-1. These results represent the first in-depth electrical and noise characterization of transport in inkjet-printed graphene films, able to provide physical insights on the mechanisms at play.

Study of the magneto transport properties at room temperature in lacunary ceramics La0,8-x□xNa0,2-x□xMnO3 in site A

In this paper, we present the effect of lanthanum (La) and sodium (Na) vacancies in site A on the morphological, structural, magnetic, and electrical properties of La0,8-x□xNa0,2-x□xMnO3 compounds with x = 0.00, x = 0.05, and x = 0.075 prepared with the sol–gel method. The results of the X-ray powder diffraction diagrams show that these samples crystallize in a rhombohedral structure deformed with the space group R$$ \overline{3} $$c, while electron microscopy was carried out for the surface morphology. From the magnetic measurements, the transition from a ferromagnetic (FM) state to a paramagnetic (PM) state in the vicinity of Curie temperature TC is detailed. Moreover, in the electrical part, we study the gap effect on the transition temperature TMI; thus, the nature of the charge transport for the resistance behavior in the different regions (low temperature, metallic, and insulating) has been explained by different models. This work is completed by explaining the two types of magnetoresistance (MR) observed as a function of temperature and applied magnetic field: low-temperature extrinsic magnetoresistance (EMR) and high-temperature intrinsic magnetoresistance (IMR) have been found to be conserved (≈ 30 ℅) with decreasing precursor percentage at room temperature.

Crystal Growth and Basic Transport and Magnetic Properties of MnBi2Te4

We report successful growth of magnetic topological insulator (MTI) MnBi2Te4. The heating schedule basically deals with growth of the crystal from melt at 9000C and very slow cooling (10C/hr) to around 6000C with 24 hours hold time, followed by cooling to room temperature. Our detailed, PXRD Reitveld analysis showed that the resultant crystal is dominated mainly by MnBi2Te4 and minor phases of Bi2Te3 and MnTe. The transport measurements showed a step like behavior at around 150 K followed by cusp like structure in resistivity at around 25 K (TP) due reported anti-ferromagnetic ordering of Mn. Both the resistivity transitions are seen clearly in dρ/dT measurements at 150 K and 20 K, respectively. The 25 K transition of the compound is also seen in magnetic susceptibility. Low temperature (5 K) magneto-resistance (MR) in applied field of up to 6 Tesla exhibited –ve MR below 3 Tesla and + ve for higher fields. Also, seen are steps in MR below 1 Tesla. The studied MnBi2Te4 MTI crystal could be a possible candidate for quantum anomalous Hall (QAH) effect.

The Kondo effect in 2D electron gas of magnetically undoped AlGaN/GaN high-electron-mobility transistor heterostructures

Unusual observation of the Kondo effect in two-dimensional electron gas (2DEG) of magnetically undoped AlGaN/GaN high-electron-mobility transistor heterostructures is reported. The temperature-dependent zero-field resistivity data exhibited an upturn below 120 K, while the standard low-temperature weak-localization behaviour was revealed at T → 0. Magnetotransport characterization was carried out in the magnetic fields up to 80 kOe, applied perpendicular to the 2DEG plane, in the temperature range 1.8 ÷ 300 K. Negative low-temperature magnetoresistance with a magnitude of order of 1 % was detected. The data set is analysed in the frame of the multichannel Kondo model for d0-magnetic materials.

Transport properties and electroresistance of manganite based heterostructure

In the present communication, ZnO/La0.7Sr0.3MnO3/Al2O3 (ZnO/LSMO/Al2O3) heterostructure was grown using chemical solution deposition (CSD) technique. X–ray diffraction (XRD) measurement confirms the (100) crystallographic oriented growth of LSMO manganite and polycrystalline hexagonal growth of ZnO layer. Temperature dependent resistivity behavior under different applied magnetic fields, performed for two different geometries [current in plane (CIP) and current perpendicular to plane (CPP)], suggests the modifications in metal to insulator transition temperature TP and alterations in resistivity with applied magnetic field and measurement geometry. This also implies the better charge conduction across the ZnO/LSMO interface (CPP) as compared to LSMO film layer (CIP). Various charge conduction mechanisms have been employed to understand the charge transport for observed low temperature resistivity minimum, metallic behavior, insulating state and magnetoresistance (MR) of LSMO film and ZnO/LSMO interface. Electroresistance (ER) and field effect configuration (FEC) have been investigated across the ZnO/LSMO interface by recording the LSMO channel resistance, at room temperature, that suggest the tuning of signature and value of ER by considering forward and reverse bias modes across the interface. FEC studies show the large ER ∼ +750% and −85% across the LSMO channel with its tunability under different interface biases.

Tuning magnetoresistance and electrical resistivity by enhancing localization length in polyaniline and carbon nanotube composites

We report low temperature electrical resistivity and magnetoresistance (MR) measurements of conducting polyaniline (PANI) and multiwalled-carbon nanotube (MWCNT) composites. We have used an in-situ oxidative polymerization method to synthesize hydrochloric acid-doped PANI composites with MWCNT weight percentages of 0, 5, 10 and 15. The temperature dependence of resistivity is studied from room temperature to 4.2 K and analysed by a Mott variable range hopping (VRH) model. The resistivity increases from \(1.1\times 10^{-3}\,\Omega \mathrm {m}\) at 300 K to \(65.75\,\Omega \mathrm {m}\) at 4.2 K, almost four orders of the magnitude change with temperature for pure PANI. Whereas the PANI composite with 15% MWCNTs shows less variation from \(4.6\times 10^{-4}\,\mathrm{}\) to \(3.5\times 10^{-2}\,\Omega \mathrm{m}\). The huge change in resistivity is due to the localization of charge carriers in the presence of disorder. At 4.2 K MR shows transition from positive to negative with higher MWCNT loading. Samples with 5 and 10% MWCNTs show positive MR, whereas the 15% MWCNT loaded sample shows negative MR. The positive and negative MR are discussed in terms of the wave function shrinkage effect and quantum interference effect on VRH conduction.

Electric-field-induced two-dimensional hole gas in undoped GaSb quantum wells

We have measured hole transport in electrically induced two-dimensional hole gases in undoped GaSb/AlSb quantum wells. In order to access the electrically induced two-dimensional hole gas in GaSb quantum wells, recessed ohmic contacts were formed and the low-temperature magnetoresistance was measured for a gate-defined Hall bar geometry. The mobility of the sample increases with increasing hole density and reaches 20 000 cm2/V s at a hole density of 5.3 × 1011 cm−2 for an 8-nm-thick GaSb quantum well. The longitudinal and Hall resistivities show Shubnikov–de Haas oscillations and integer quantum Hall plateaus, respectively. These results establish a platform for realizing spin-based electronics using the strong spin–orbit interaction of this material and are also useful for understanding the transport properties of the two-dimensional topological insulator realized in InAs/GaSb double quantum well structures.

Low-Temperature Properties of CePt3P Tuned by Magnetic Field**Supported by the Zhejiang Provincial Natural Science Foundation of China under Grant No LQ19A040006, and the Scientific Research Fund of Zhejiang Provincial Education Department under Grant No Y201840160.

We present low-temperature magnetization, magnetoresistance and specific heat measurements on the Kondo lattice compound CePt3P under applied magnetic fields up to 9.0 T. At zero field, CePt3P exhibits a moderately enhanced Sommerfeld coefficient of electronic specific heat γCe = 86 mJ/mol·K2 as well as two successive magnetic transitions of Ce 4f moments: an antiferromagnetic ordering at TN1 = 3.0K and a spin reorientation at TN2 = 1.9 K. The value of TN1 shifts to lower temperature as magnetic field increases, and it is ultimately suppressed around Bc ∼ 3.0T at 1.5 K. No evidence of non-Fermi liquid behavior is observed around Bc down to the lowest temperature measured. Moreover, γ decreases monotonously with increasing the magnetic field. On the other hand, the electrical resistivity shows an anomalous temperature dependence ρ ∝ Tn with the exponent n decreasing monotonously from ∼2.6 around Bc down to ∼1.7 for B = 9.0 T. The T–B phase diagram constructed from the present experimental results of CePt3P does not match the quantum criticality scenario of heavy fermion systems.

Tunable superconductivity in parent cuprate Pr2CuO4±δ thin films**Project supported by the National Key Basic Research Program of China (Grant Nos. 2015CB921000, 2016YFA0300301, 2017YFA0303003, and 2018YFB0704100), the National Natural Science Foundation of China (Grant Nos. 11674374 and 11474338), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH008), the Strategic Priority Research Program of the

We studied the role of oxygen in Pr2CuO4±δ thin films fabricated by the polymer assisted deposition method. The magnetoresistance and Hall resistivity of Pr2CuO4±δ samples were systematically investigated. It was found that with decreasing oxygen content, the low-temperature Hall coefficient (RH) and magnetoresistance changed from negative to positive, similar to those with the increase of Ce-doped concentration in CexCuO4 (R=La, Nd, Pr, Sm, Eu). In addition, we observed that the dependence of the superconducting critical temperature Tc with RH for the Pr2−xCexCuO4 perfectly overlapped with that of Pr2CuO4±δ. These findings point to the fact that the doped electrons induced by the oxygen removal are responsible for the superconductivity of the -phase parent compounds.

Crystal Growth and Basic Transport and Magnetic Properties of MnBi <sub>2</sub>Te <sub>4</sub>

We report successful growth of magnetic topological insulator (MTI) MnBi2Te4. Depending, upon the heating schedule the phase formation in terms of various Bi2Te3 + MnTe structures viz. MnBi2Te4, MnBi4Te7, MnBi6Te10 and MnTe are seen in powder X-ray diffraction of crushed resultant crystals. The heating schedule basically deals with growth of the crystal from melt at 9000C and very slow cooling (10C/hr) to around 6000C with 24 hours hold time, followed by cooling to room temperature. Our detailed, PXRD Reitveld analysis showed that the resultant crystal is dominated by MnBi4Te7, MnBi6Te10 phases along with some MnTe content. The transport measurements showed a step like behavior at around 150K followed by cusp like structure in resistivity at around 25K (TP) due reported anti-ferromagnetic ordering of Mn. Both the resistivity transitions are seen clearly in dR/dT measurements at 150K and 20K respectively. The 25Ktransition of the compound is also seen in magnetic susceptibility. Low temperature (5K) magneto-resistance (MR) in applied field of up to 6 Tesla exhibited -ve MR below 3 Tesla and +ve for higher fields. Also, seen are steps in MR below one Tesla. The studied MnBi2Te4 MTI crystal could be a possible candidate for Quantum Anomalous Hall (QAH) effect.

Stoichiometry control and electronic and transport properties of pyrochlore Bi2Ir2O7 thin films

Synthesizing stoichiometric and epitaxial thin films of pyrochlore iridates is an essential step toward the experimental realization of unusual topological and magnetic states that are theoretically predicted in this unique spin-orbit coupled material system. Here, we report on the stoichiometry control and electronic and transport properties of pyrochlore iridate $\mathrm{B}{\mathrm{i}}_{2}\mathrm{I}{\mathrm{r}}_{2}{\mathrm{O}}_{7}$ thin films grown by pulsed laser deposition. The as-grown films form a bilayerlike structure, in which the top surface is highly Ir deficient while the bottom layer is mainly composed of iridium metal. By postannealing the as-deposited films in $\mathrm{Ir}{\mathrm{O}}_{2}+{\mathrm{O}}_{2}$ atmosphere, we improved the stoichiometry and homogeneity through the film thickness with the lattice constant close to the bulk value. Density functional theory calculation in the bulk limit shows a fourfold degenerate Dirac node slightly below the Fermi energy at the $X$ point, along with trivial bands around the \ensuremath{\Gamma} point. The projected partial density of states suggests that the states in the vicinity of the Fermi energy (\ensuremath{-}3 to 0 eV) mainly consist of highly hybridized Ir $5d$ and O $2p$ with minor contributions from Bi $6s$ and $6p$, while those far below the Fermi energy (\ensuremath{-}9 to --3 eV) are contributed primarily by the O bands. Transport measurements revealed a weakly metallic behavior at higher temperatures transitioning to a weakly insulating behavior below 150 K, and a low-temperature magnetoresistance qualitatively ascribed to multicarrier and band-structural effects. The transport features are influenced by a density of states sharply peaked at the Fermi energy, and by the coexistence of trivial bands with the Dirac node, as revealed by the density functional theory calculations.