Giant bipolar unidirectional photomagnetoresistance

Studies of negative magnetoresistance in novel materials have recently been in the forefront of spintronic research. Here, we report an experimental observation of the temperature dependent negative magnetoresistance in Bi2Te3 topological insulator (TI) nanowires at ultralow temperatures (20 mK). We find a crossover from negative to positive magnetoresistance while increasing temperature under longitudinal magnetic field. We observe a large negative magnetoresistance which reaches −22% at 8 T. The interplay between negative and positive magnetoresistance can be understood in terms of the competition between dephasing and spin-orbit scattering time scales. Based on the first-principles calculations within a density functional theory framework, we demonstrate that disorder (substitutional) by Ga+ ion milling process, which is used to fabricate nanowires, induces local magnetic moments in Bi2Te3 crystal that can lead to spin-dependent scattering of surface and bulk electrons. These experimental findings show a significant advance in the nanoscale spintronics applications based on longitudinal magnetoresistance in TIs. Our experimental results of large negative longitudinal magnetoresistance in 3D TIs further indicate that axial anomaly is a universal phenomenon in generic 3D metals.

A large negative magnetoresistance (NMR) is anticipated in topological semimetals in parallel magnetic fields, demonstrating the chiral anomaly, a long-sought high-energy-physics effect, in solid-state systems. Recent experiments reveal that the Dirac semimetal Cd3As2 has the record-high mobility and positive linear magnetoresistance in perpendicular magnetic fields. However, the NMR has not yet been unveiled. Here we report the observation of NMR in Cd3As2 microribbons in parallel magnetic fields up to 66% at 50 K and visible at room temperatures. The NMR is sensitive to the angle between magnetic and electrical fields, robust against temperature and dependent on the carrier density. The large NMR results from low carrier densities in our Cd3As2 samples, ranging from 3.0 × 1017 cm−3 at 300 K to 2.2 × 1016 cm−3 below 50 K. We therefore attribute the observed NMR to the chiral anomaly. In perpendicular magnetic fields, a positive linear magnetoresistance up to 1,670% at 14 T and 2 K is also observed.

The magnetoresistance (MR) effect in Co-doped ZnO films prepared by pulsed laser deposition on $a$-plane sapphire substrates with electron concentration at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ ranging from $8.3\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}9.9\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ has been studied experimentally and theoretically. A large positive MR of 124% has been observed in the film with the lowest electron concentration of $8.3\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, while only a negative MR of $\ensuremath{-}1.9%$ was observed in the film with an electron concentration of $9.9\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The positive MR is attributed to the quantum correction on the conductivity due to the $s\text{\ensuremath{-}}d$ exchange interaction induced spin splitting of the conduction band. The negative MR is attributed to the magnetic field suppressed weak localization. The presented modeling of superimposed positive and negative MR well agrees with the experimentally observed MR and hints at the physical origin of MR in Co-doped ZnO.

The effect of Ga and Co doping on the hopping transport of electrons in ZnO films was studied. All investigated films were deposited on monocrystalline substrates by water assisted metal organic chemical vapour deposition method. Temperature dependencies of the resistivity of all films obeyed the Mott's law at low temperatures. Both positive and negative magnetoresistance were observed in the temperature range of the Mott's law. The values of the localization length obtained from magneto transport data for ZnO and ZnO:Ga films are larger than the effective Bohr radii of a shallow donor state and are independent on the Ga content. These observations point to the localization of electrons in the random potential originating from the structural disorder. In Co doped ZnO films large positive magnetoresistance was observed in low magnetic fields at low temperatures. The value of positive magnetoresistance increases with an increase of Co content and decrease of temperature. Observed magneto transport properties of Co-doped films were interpreted by the decrease of electron hopping probability due to decrease of the density of localized states in magnetic field.

Some features of experimental determination of localization length of charges in carbon nanomaterials for positive and negative magnetoresistance

In recent years, antiferromagnetic kagome materials have attracted considerable attention in condensed matter physics owing to their distinctive lattice geometry. In this work, high-quality single crystals of D019-structured Mn2.16Ga were grown using the flux method, and their magnetotransport properties were systematically studied. Measurements of magnetization versus field (M–H), temperature-dependent magnetization (M–T), and the anomalous Hall effect confirm that the crystal undergoes a magnetic-structural transition driven by both temperature and the magnetic field. Remarkably, a coexistence of positive and negative longitudinal magnetoresistance (MR) is observed in Mn2.16Ga. The MR shows a field-induced sign change from negative to positive. The negative MR is attributed to field-modified magnetic ordering, whereas the positive MR originates mainly from interlayer electron conduction in the kagome lattice and distortion of the in-plane triangular arrangement of Mn magnetic moments. These results offer valuable insights into the electronic and magnetic transport behavior of Mn-based antiferromagnetic single crystals.

Recently discovered Dirac and Weyl semimetals display unusual magnetoresistance phenomena, including a large, non-saturating, linear transverse magnetoresistance and a negative longitudinal magnetoresistance. The latter is often considered as evidence of fermions having a defined chirality. Classical mechanisms, due to disorder or non-uniform current injection, can however, also produce negative longitudinal magnetoresistance. Here, we report on magnetotransport measurements performed on epitaxial thin films of Cd3As2, a three-dimensional Dirac semimetal. Quasi-linear positive transverse magnetoresistance and negative longitudinal magnetoresistance are observed. By evaluating films of different thickness and by correlating the temperature dependence of the carrier density and mobility with the magnetoresistance characteristics, we demonstrate that both the quasi-linear positive and the negative magnetoresistance are caused by conductivity fluctuations. Chiral anomaly is not needed to explain the observed features.

The magnetic, transport and magnetotransport properties of antiferromagnetic epsilon-(Mn(1-x)Fe(x))(3.25)Ge (x=0.17 and 0.2) compounds are investigated systematically. For both compounds, a large positive magnetoresistance (MR) is observed in the temperature regime below the temperature (T(t)) for a transition from a triangular to a collinear antiferromagnetic spin configuration, while a negative magnetoresistance appears between T(t) and the Neel temperature T(N), and then the MR ratio turns to be positive again above T(N). Below T(t), the positive MR reaches a maximum ratio of 6.1% at 5 K at an applied magnetic field of 5 T for the epsilon-(Mn(0.83)Fe(0.17))(3.25)Ge compound. For the epsilon-(Mn(0.83)Fe(0.2))(3.25)Ge compound, the positive MR ratio reaches its maximum at 120 K and nearly vanishes at 5 K. The origin of the anomalous positive and negative MR effects is discussed in terms of the shrinkage of the orbits, the spin fluctuations, the spin configurations, and the magnetic transitions.

Polyaniline microfibers were synthesized by using a self-assembly method. The electron hopping transport in this partially dedoped polyaniline single fiber showed typical bias-dependent characteristics. The magnetoresistance properties of the fiber were systematically investigated. In this microfiber, both positive and negative magnetoresistances were observed. An apparent positive magnetoresistance (> 1%) was found at 50 K. This temperature is much higher than previous reports of single or few nanofiber/tubes bundles (< 10 K). With the increase in temperature, the positive magnetoresistance was converted to the negative magnetoresistance where the transition happened at ca. 90 K. Moreover, the magnetoresistance was also sensitive to the applied bias. These observations could be explained by using the combination or competition of electron wave function shrinkage effect, quantum decoherence effect, as well as localized states coupling effect.

Ni-substituted to perovskites La0.67Ba0.33Mn1-xNixO3 with x = 0; 0.01; 0.02; 0.03; 0.04; and 0.05, have been done and investigated by x-ray diffraction spectrometer, permagraph spectrometer, and four-point probe (FPP) measurements. X-ray diffraction spectrometer result refined by high score software. All samples show an orthorhombic structure at room temperature. At room temperature the cell parameter a, b and c of the samples seem to be constant although the doping of Ni was increased. The permagraph spectrometer results showed that all the samples were paramagnet at room temperature since magnetic remanence is zero for all samples. The four-point probe (FPP) results negative magnetoresistance and positive magnetoresistance. The negative magnetoresistance appears at x = 0 with ratio 15.625% and the positive magnetoresistance appears at x = 0.01 until 0.05 with maximum ratio 18.341% for x = 0.01. Theseevidences showed that highest sensitivity’s respond for resistivity change before and after applying external magnetic fields to perform magnetoresistance phenomena is for x = 0.01.

Two-dimensional electron gases with very high mobility show a huge or giant negative magnetoresistance at low temperatures and low magnetic fields. We present an experimental and theoretical work on the influence of the applied current on the negative huge magnetoresistance of these systems. We obtain an unexpected and strong nonlinear behavior consisting in an increase of the negative huge magnetoresistance with increasing current, in other words, for increasing current the magnetoresistance collapses at small magnetic fields. This nonlinearity is explained by the subtle interplay of elastic scattering within Landau levels and between Landau levels.

We report a systematic study on magnetotransport properties of the single crystal of cadmium (Cd). When the applied magnetic field B is perpendicular to the current I, the resistivities for both directions (I || a, I || c) show field induced metal-to-insulator-like transitions. The isothermal magnetoresistance (MR) at low temperatures increases approximately as the square of the magnetic field without any sign of saturation, and reaches up to 1140000% and 58000% at T = 2 K and B = 9 T for I || a and I || c, respectively. As the magnetic field rotates to parallel to the current, no sign of negative MR is observed for I || a, while an obvious negative MR appears up to −70% at 2 K and 9 T for the current flowing along the c-axis, and the negative longitudinal MR shows a strong dependence of the electrode position on the single crystal. These results suggest that the negative longitudinal MR is caused by the dislocations formed in the process of crystal growing along the c-axis. Further studies are needed to clarify this point.

Herein, we report the transport properties of Ge films. The variable-range hopping transport at low temperatures (T≲50K) and thermal activation transport at high temperatures (T≳50K) are observed in our Ge films. In the different temperature regimes, the anomalous magnetotransport properties are observed. In the low-temperature regime (T≲15K), the negative magnetoresistance (MR) at low field and positive MR at high field can be seen. In the moderate-temperature regime (15K≲T≲100K), the positive MR curve gradually evolves from a linear curve to a parabolic curve with increasing temperature, and the MR magnitude appears to be insensitive to temperature. In the high-temperature regime (T≳100K), the positive MR value increases with increasing temperature. By considering the angular-dependent MR, we can determine that the negative MR comes from the spin-related mechanism, and the positive MR is caused by the orbital-related mechanism. However, further study is required to determine the exact mechanisms behind the anomalous magnetotransport properties.

Large positive and negative magnetoresistance in the magnetic EuCu4As2 crystal

Magnetoresistance measurements have been performed in high magnetic fields up to 15 teslas in fluorine-intercalated vapor-deposited graphite fibers with concentrations ${\mathrm{C}}_{4.5}$F and ${\mathrm{C}}_{3.6}$F. Previous studies had shown that the intercalation of fluorine, an acceptor in graphite, induces a two-dimensional hole gas in the graphene layers. The fluorine-intercalation process also introduces disorder into the highly ordered pristine fiber, as exhibited by the transport properties. At high magnetic fields, a positive magnetoresistance is observed and is found to vary strongly with temperature. This phenomenon is explained by the effect of the magnetic field on the hole-hole interaction when the spin subbands are split by the magnetic field. A negative magnetoresistance is observed at low fields, and is explained in terms of weak-localization theory. The magnetoresistance data are fitted with the weak-localization contribution in the weak-field regime, then extrapolated to high fields and this contribution is finally subtracted from the measured values to obtain the hole-hole interaction contribution to the magnetoresistance. From a fit to this latter contribution, a value for the screening parameter is found. The relative importance of the weak localization and hole-hole interaction contributions to the logarithmic increase in \ensuremath{\rho}(T) at low temperature is discussed.