Colossal negative magnetoresistance in spin glass Na(Zn,Mn)Sb

We report the study of magnetic and transport properties of polycrystalline and single crystal Na(Zn,Mn)Sb,a new member of“111”type of diluted magnetic materials.The material crystallizes into Cu2Sb-type structure which is isostructural to“111”type Fe-based superconductors.With suitable carrier and spin doping,the Na(Zn,Mn)Sb establishes spin-glass ordering with freezing temperature(Tf)below 15 K.Despite lack of long-range ferromagnetic ordering,Na(Zn,Mn)Sb single crystal still shows sizeable anomalous Hall effect below Tf.Carrier concentration determined by Hall effect measurements is over 1019 cm-3.More significantly,we observe colossal negative magnetoresistance(MR≡[ρ(H)−ρ(0)]/ρ(0))of-94%in the single crystal sample.

Negative magnetoresistance in Dirac semimetal Cd_(3)As_(2)with in-plane magnetic field perpendicular to current

Topological insulators and semimetals have exotic surface and bulk states with massless Dirac or Weyl fermions,demonstrating microscopic transport phenomenon based on relativistic theory.Chiral anomaly induced negative magnetoresistance(negative MR)under parallel magnetic field and current has been used as a probable evidence ofWeyl fermions in recent years.Here we report a novel negative MR result with mutually perpendicular in-plane magnetic field and current in Cd_(3)As_(2)nanowires.The negative MR has a considerable value of-16%around 1.5 K and could persist to room temperature of 300 K with value of-1%.The gate tuning and angle dependence of the negative MR demonstrate the mechanism of the observed negative MR is different from the chiral anomaly.Percolating current paths induced by charge puddles and disorder might be involved to produce such considerable negative MR.Our results indicate the negative MR effect in topological semimetals involves synergistic effects of many mechanisms besides chiral anomaly.

Colossal negative magnetoresistance from hopping in insulating ferromagnetic semiconductors

Ferromagnetic semiconductor Ga_(1–x)Mn_(x)As_(1–y)P_(y) thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers.In this regime,we report a colossal negative magnetoresistance(CNMR)coexisting with a saturated magnetic moment,unlike in the traditional magnetic semiconductor Ga_(1–x)Mn_(x)As.By analyzing the temperature dependence of the resistivity at fixed magnetic field,we demonstrate that the CNMR can be consistently described by the field dependence of the localization length,which relates to a field dependent mobility edge.This dependence is likely due to the random environment of Mn atoms in Ga_(1-x)Mn_(x)As_(1-y)P_(y) which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field.

Tunneling Negative Magnetoresistance via δ Doping in a Graphene-Based Magnetic Tunnel Junction

We investigate the tunneling magnetoresistance via δ doping in a graphei2e-based magnetic tunnel junction in detail. It is found that the transmission probability and the conductance oscillates with the position and the aptitude of the 8 doping. Also, both the transmission probability and the conductance at the paxallel configuration are suppressed by the magnetic field more obviously than that at the antiparallel configuration, which implies a large negative magnetoresistance for this device. The results show that the negative magnetoresistance of over 300% at B = 1.0 T is observed by choosing suitable doped parameters, and the temperature plays an important role in the magnetoresistance. Thus it is possible to open a way to effectively manipulate the magnetoresistance devices, and to make a type of magnetoresistance device by controlling the structural parameter of the δ doping.

Negative magnetoresistance in Weyl semimetals NbAs and NbP： Intrinsic chiral anomaly and extrinsic effects

Chiral anomaly-induced negative magnetoresistance （NMR） has been widely used as critical transport evidence for the existence of Weyl fermions in topological semimetals. In this mini-review, we discuss the general observation of NMR phenomena in non-centrosymmetric NbP and NbAs. We show that NMR can arise from the intrinsic chiral anomaly of Weyl fermions and/or extrinsic effects, such as the superimposition of Hall signals; field-dependent inhomogeneous current flow in the bulk, i.e., current jetting; and weak localization （WL） of coexistent trivial carriers. The WLmcontrolled NMR is heavily dependent on sample quality and is characterized by a pronounced crossover from positive to negative MR growth at elevated temperatures, resulting from the competition between the phase coherence time and the spin-orbital scattering constant of the bulk trivial pockets. Thus, the correlation between the NMR and the chiral anomaly need to be scrutinized without the support of complimentary techniques. Because of the lifting of spin degeneracy, the spin orientations of Weyl fermions are either parallel or antiparallel to the momentum, which is a unique physical property known as helicity. The conservation of helicity provides strong protection for the transport of Weyl fermions, which can only be effectively scattered by magnetic impurities. Chemical doping with magnetic and non-magnetic impurities is thus more convincing than the NMR method for detecting the existence of Weyl fermions.

Large unsaturated positive and negative magnetoresistance in Weyl semimetal TaP

After successfully growing single-crystal TaP, we measured its longitudinal resistivity （Pxx） and Hall resistivity （Pyx） at magnetic fields up to 9 T in the temperature range of 2-300 K. At 8 T, the magnetoresistance （MR） reached 3.28 ×10^5% at 2 K, 176% at 300 K. Neither value appeared saturated. We confirmed that TaP is a hole-electron compensated semimetal with a low carrier concentration and high hole mobility ofμh=3.71 × 105 cm2/V s, and found that a magnetic-field-induced metal-insulator transition occurs at room temperature. Remarkably, because a magnetic field （H） was applied in parallel to the electric field （E）, a negative MR due to a chiral anomaly was observed and reached -3000% at 9 T without any sign of saturation, either, which is in contrast to other Weyl semimetals （WSMs）. The analysis of the Shubnikov-de Haas （SdH） oscillations superimposed on the MR revealed that a nontrivial Berry＇s phase with a strong offset of 0.3958, which is the characteristic feature of charge carriers enclosing a Weyl node. These results indicate that TaP is a promising candidate not only for revealing fundamental physics of the WSM state but also for some novel applications.

Observation of spin-glass behavior in 1111-type magnetic semiconductor(La,Ba)(Zn,Mn)SbO

We report the successful fabrication of a new 1111-type bulk magnetic semiconductor(La,Ba)(Zn,Mn)SbO through the solid solution of(La,Ba)and(Zn,Mn)in the parent compound LaZnSbO.The polycrystalline samples(La,Ba)(Zn,Mn)SbO crystallize into ZrCuSiAs-type tetragonal structure,which has the same structure as iron-based superconductor LaFeAsO_(1-δ).The DC magnetization measurements indicate the existence of spin-glass ordering,and the coercive field is up to~11500 Oe(1 Oe=79.5775 A·m^(-1)).The AC magnetic susceptibility further determines that the samples evolve into a conventional spin-glass ordering state below the spin freezing temperature T_(f).In addition,the negative magnetoresistance(MR≡[ρ(H)-ρ(0)]/ρ(0))reaches-88%under 9 T.

Intergranular Tunnelling and Field-Induced Percolation Fluctuation of Granular Composites （La1-zAgzMnO3）/（MnO2/Mn2O3）

We investigate the giant magnetoresistance of composites （La1-xAgzMnO3）1-υ（MnO2/Mn2O3）υ. The magnetoresistive property of the composites shows the characteristics of intergranular tunnelling. A conductivity leap has been observed around y = 0.7. The composite （La0.926Ag0.074MnO3）0.698（MnO2/Mn2O3）0.302 has been found to show the greatest magnetoresistance in the samples. Its room-temperature MR ratio reaches 24% under a field of 1.8T. These phenomena suggest a percolation transformation and a kind of field-induced fluctuation in percolation in the samples investigated.

Electronic transport properties of Co cluster-decorated graphene

Interactions of magnetic elements with graphene may lead to various electronic states that have potential applications.We report an in-situ experiment in which the quantum transport properties of graphene are measured with increasing cobalt coverage in continuous ultra-high vacuum environment. The results show that e-beam deposited cobalt forms clusters on the surface of graphene, even at low sample temperatures. Scattering of charge carriers by the absorbed cobalt clusters results in the disappearance of the Shubnikov–de Haas（Sd H） oscillations and the appearance of negative magnetoresistance（MR）which shows no sign of saturation up to an applied magnetic field of 9 T. We propose that these observations could originate from quantum interference driven by cobalt disorder and can be explained by the weak localization theory.

Magneto-transport properties of thin flakes of Weyl semiconductor tellurium

As an elemental semiconductor,tellurium has recently attracted intense interest due to its non-trivial band topology,and the resulted intriguing topological transport phenomena.In this study we report systematic electronic transport studies on tellurium flakes grown via a simple vapor deposition process.The sample is self-hole-doped,and exhibits typical weak localization behavior at low temperatures.Substantial negative longitudinal magnetoresistance under parallel magnetic field is observed over a wide temperature region,which is considered to share the same origin with that in tellurium bulk crystals,i.e.,the Weyl points near the top of valence band.However,with lowering temperature the longitudinal magnetoconductivity experiences a transition from parabolic to linear field dependency,differing distinctly from the bulk counterparts.Further analysis reveals that such a modulation of Weyl behaviors in this low-dimensional tellurium structure can be attributed to the enhanced inter-valley scattering at low temperatures.Our results further extend Weyl physics into a low-dimensional semiconductor system,which may find its potential application in designing topological semiconductor devices.

Anomalous in-plane magnetoresistance of electron-doped cuprate La_(2.x)Ce_xCuO_(4±δ)

We report systematic in-plane magnetoresistance measurements on the electron-doped cuprate La2-x,.CexCuO4±δ thin films as a function of Ce doping and oxygen content in the magnetic field up to 14 T. A crossover from negative to positive magnetoresistance occurs between the doping level x = 0.07 and 0.08. Above x = 0.08, the positive magnetoresistance effect appears, and is almost indiscernible at x = 0.15. By tuning the oxygen content, the as-grown samples show negative magnetoresistance effect, whereas the optimally annealed ones display positive magnetoresistance effect at the doping level x = 0.15. Intriguingly, a linear-field dependence of in-plane magnetoresistanee is observed at the underdoping level x = 0.06, the optimal doping level x = 0. i and slightly overdoping level x = 0.11. These anomalies of in-plane magnetoresistance may be related to the intrinsic inhomogeneity in the cuprates, which is discussed in the framework of network model.

Giant magnetoresistance effect and temperature dependence of magnetoresistance in La_(2/3)Ca_(1/3)MnO_2 thin film

Magnetoresistanoe in the temperature region from 300 K to liquid helium temperature has been measured on good (100) oriented La2/3Ca1/3MnO2 thin films prepared by d.c. magnetron sputtering on ZrO2 (100) substrates. It is found that there appears a resistance peak at about TP = 72K at zero magnetic field, and this peak moves to higher temperature with increasing field H, and R decreases simultaneously. However, the magnetoresistance ratio MR = 5R /R (H) = (Ro~ RH) /RH has a different temperature dependence. There are also MR peaks, but the temperature of maximum MR is at about 25 K which does not change with changing external magnetic field. The heat treatment (annealed at 700C in 105’Pa O2 for 30min) will make R reduce dramatically and has an effect on the temperature dependence of R and its MR.