Anomalous Positive Magnetoresistance Research Articles

Слабая антилокализация в сильно разупорядоченном двумерном полуметалле в квантовой яме HgTe

Weak localization in a highly disordered quantum well CdxHg1−xTe/HgTe/CdxHg1−xTe with a thickness of d = 20 nm is experimentally investigated. An analysis is made of the anomalous positive magnetoresistance (APM) caused by the suppression of the interference correction to the conductivity by a magnetic field on both sides of the charge neutrality point: for a two-dimensional semimetal and for a two-dimensional electronic metal. For the same values of resistivity, the APM peak in a 2D semimetal has a much wider width than in a 2D electron gas. A quantitative comparison of the obtained results with the theory allows, in particular, to conclude that the intensity of carrier transitions between subsystems in the 2D semimetal binary system is maximum near the charge neutrality point, where the concentrations of electrons and holes are close, and decreases as the difference in concentrations increases.

Magnetotransport properties of chiral helimagnet Cr1/3NbS2 near phase transition

Many interesting physical phenomena emerge in the vicinity of phase transition temperature (Tc) for magnetic materials. However, it remains a big challenge to elucidate the near-Tc behaviors of chiral helimagnets, owing to their diversified magnetic interactions and complicated phase transitions. Here, we have systematically studied the near-Tc properties of a chiral helimagnet Cr1/3NbS2 by magnetotransport measurements with fine tuning of temperature and magnetic field. An anomalous positive magnetoresistance (MR) appears around the critical field (Hc) in a narrow temperature range (~ 128–132 K), which can be attributed to the competing scattering effects on electrons between the chiral magnetic order and thermal fluctuation near critical point. Moreover, the negative MR continues to increase even above Hc, and this behavior becomes more pronounced near Tc. Based on the MR results, a near-Tc magnetic phase diagram of Cr1/3NbS2 is presented, with a particular emphasis on the determination of internal phase boundary of chiral soliton lattice (CSL). This work gains an insight on the near-Tc behaviors of chiral magnetic materials and sheds light on the related electronic and spintronic applications.

Tunable Berry curvature and transport crossover in topological Dirac semimetal KZnBi

Topological Dirac semimetals have emerged as a platform to engineer Berry curvature with time-reversal symmetry breaking, which allows to access diverse quantum states in a single material system. It is of interest to realize such diversity in Dirac semimetals that provides insight on correlation between Berry curvature and quantum transport phenomena. Here, we report the transition between anomalous Hall and chiral fermion states in three-dimensional topological Dirac semimetal KZnBi, which is demonstrated by tuning the direction and flux of Berry curvature. Angle-dependent magneto-transport measurements show that both anomalous Hall resistance and positive magnetoresistance are maximized at 0° between net Berry curvature and rotational axis. We find that the unexpected crossover of anomalous Hall resistance and negative magnetoresistance suddenly occurs when the angle reaches to ~70°, indicating that Berry curvature strongly correlates with quantum transports of Dirac and chiral fermions. It would be interesting to tune Berry curvature within other quantum phases such as topological superconductivity.

Evidence of weak-antilocalization phenomenon in Al-induced crystallization grown polycrystalline-SiGe thin film

We report the low-temperature magneto-transport properties of a polycrystalline-SiGe thin film realized on glass at 350 ℃ via Al-induced layer exchange crystallization. The film exhibits a degenerate p-type behavior, and we observed evidence of weak-antilocalization (WAL) effect that manifests as an anomalous positive magnetoresistance (MR) below 10 K and weak magnetic fields (B) between ± 1 Tesla. While the spin–orbit coupling length is nearly constant at ~ 40 nm, the temperature dependence of phase-coherence length, which has a maximum value at 4 K (~131 nm), indicates hole-hole scattering as the predominant de-phasing mechanism. This result shows the degenerate Al doping causes the WAL effect in the present system.

Quantum criticality tuned by magnetic field in optimally electron-doped cuprate thin films

Antiferromagnetic (AF) spin fluctuations are commonly believed to play a key role in electron pairing of cuprate superconductors. In electron-doped cuprates, it is still in paradox about the interplay among different electronic states in quantum perturbations, especially between superconducting and magnetic states. Here, we report a systematic transport study on cation-optimized La2-xCexCuO4 (x = 0.10) thin films in high magnetic fields. We find an AF quantum phase transition near 60 T, where the Hall number jumps from nH =-x to nH = 1-x, resembling the change of nH at the AF boundary (xAF = 0.14) tuned by Ce doping. In the AF region a spin dependent state manifesting anomalous positive magnetoresistance is observed, which is closely related to superconductivity. Once the AF state is suppressed by magnetic field, a polarized ferromagnetic state is predicted, reminiscent of the recently reported ferromagnetic state at the quantum endpoint of the superconducting dome by Ce doping. The magnetic field that drives phase transitions in a similar but distinct manner to doping thereby provides a unique perspective to understand the quantum criticality of electron-doped cuprates.

Magnetoresistance of Ge-Si Whiskers in the Vicinity to Metal– Insulator Transition

The magnetoresistance of GeхSi1-х (x = 0.002¸ 0.11) whiskers with an acceptor concentration (Na = 3x1018¸ 2.1019 cm-3) near the metal-insulator transition (MIT) was studied at low temperatures (4.2 - 77)K in magnetic fields up to 14 T. It is shown that at 4.2 K the magnetoresistance of the Ge-Si whiskers on thedielectric side of the MIT is quadratic, while the magnetoresistance of the crystals on the metal side of the MIThas an exponential dependence on the magnetic field. In the samples in the immediate vicinity to the MIT on thedielectric side, negative magnetoresistance was detected, whereas in metal samples with a high germaniumcontent (x = 11 at.%) an anomalous positive magnetoresistance occurs. The resulting anomalous dependences arerespectively explained by the conductivity with respect to the delocalized A+ states of the upper Hubbard bandand the increase in the electron-electron interaction in Ge-Si whiskers at increasing germanium content.

Tunnel Magnetoresistive Effect in Nanostructured Composite Systems

The results of the investigation of tunnel magnetoresistive effect in a large number of metal-dielectric nanocomposites obtained by the same technology and under identical conditions are presented. The composites with multi-elemental amorphous metal phase were considered. It has been shown that the large values of the tunnel magnetoresistive effect in these composites are ensured when the value of the density of electronic states on the Fermi surface is large while other conditions are being equal. The conditions for the appearance of an anomalous positive tunneling magnetoresistance are formulated based on the study of composites with a mono-elemental ferromagnetic phase from cobalt.

Heterojunction metal-oxide-metal Au-Fe3O4-Au single nanowire device for spintronics

In this report, we present the synthesis of heterojunction magnetite nanowires in alumina template and describe magnetic and electrical properties from a single nanowire device for spintronics applications. Heterojunction Au-Fe-Au nanowire arrays were electrodeposited in porous aluminum oxide templates, and an extensive and controlled heat treatment process converted Fe segment to nanocrystalline cubic magnetite phase with well-defined Au-Fe3O4 interfaces as confirmed by the transmission electron microscopy. Magnetic measurements revealed Verwey transition shoulder around 120 K and a room temperature coercive field of 90 Oe. Current–voltage (I-V) characteristics of a single Au-Fe3O4-Au nanowire have exhibited Ohmic behavior. Anomalous positive magnetoresistance of about 0.5% is observed on a single nanowire, which is attributed to the high spin polarization in nanowire device with pure Fe3O4 phase and nanocontact barrier. This work demonstrates the ability to preserve the pristine Fe3O4 and well defined electrode contact metal (Au)–magnetite interface, which helps in attaining high spin polarized current.

Magnetotransport properties of undoped amorphous carbon films

Anomalous positive magnetoresistance (MR) up to 36% was observed at 2K and 12T in the undoped amorphous carbon (a-C) film deposited on glass substrate by pulsed laser deposition at 500°C. There is no tendency of saturation of MR with increase of the magnetic field. The MR decreases as the measurement temperature increases from 2 to 80K, and could hardly be observed above 80K. As the deposition temperature grows from 300 to 600°C, the disorder degree of the a-C film decreases, and the value of MR also decreases, indicating that the lower disorder degree results in a smaller MR. The mechanism of this MR could be ascribed to the wave function shrinkage. It could help to understand the MR phenomenon in amorphous material systems.

Magnetoresistance of two-dimensional ballistic microcontacts with electron-electron scattering

In the semiclassical approximation we calculate corrections to the Sharvin conductance of two-dimensional ballistic microcontact. They are caused by electron-electron interactions outside the contact, the main contribution to the corrections are made by the collisions of electrons with almost opposite momenta. The corrections are positive, increase with temperature and are strongly suppressed by a magnetic field. We assume that the anomalous positive magnetoresistance observed in a recent experiment may be a consequence of this suppression.

Quantum corrections to resistance of microblock tellurium at ultralow temperatures under phonon freezing conditions

The low-temperature magnetoresistance of bulk tellurium samples with a microcrystalline structure is investigated. At ultralow temperatures T ≤ 1 K, an anomalous positive magnetoresistance (APMR), viz. the antilocalization effect, is observed. It is shown that this effect can be explained using the weak localization theory. The characteristic parameters of the theory are determined. It is concluded that charge carriers produce a predominant effect on the phase breakdown time in the APMR mode of elastic scattering from structure defects, which leads to intervalley transitions without spin flip.

Electron-electron scattering and magnetoresistance of ballistic microcontacts

Using a semiclassical Boltzmann equation, we calculate corrections to the Sharvin conductance of a wide 2DEG ballistic contact that result from an electron--electron scattering in the leads. These corrections are dominated by collisions of electrons with nearly opposite momenta that come from different reservoirs. They are positive, increase with temperature, and are strongly suppressed by a magnetic field. We argue that this suppression may be responsible for an anomalous positive magnetoresistance observed in a recent experiment.

Antilocalization in 3D tellurium and the role of intervalley scattering in the ‘frozen’ phonon mode

We report on an experimental study of magnetoresistance in single crystal Te at the hole concentration p 77 K = 1.2 × 1 0 17 cm − 3 in the temperature range of 75–300 mK, when inelastic scattering by photons no longer contributes significantly. It is found that negative magnetoresistance (NMR) in Te, known since 1948, changes its sign at these temperatures to transform to anomalous positive magnetoresistance (APMR). We interpret this effect in terms of quantum corrections to the conductivity of semiconductors with a strong spin-orbit interaction. The expression for the quantum correction to the conductivity in a weak localization of non-interacting particles, which was derived for 2D holes on Te surface, is extended to 3D conductivity in anisotropic Te crystals. A quantitative comparison with theory has shown that the anomalous behavior of Te magnetoresistance in classically low magnetic fields at low and superlow temperatures agrees with the predictions of quantum correction theory for this material.

Pressure effect on thermopower of [formula omitted] alloy system

Thermopower of Y 1 - x Gd x Co 2 pseudobinary compounds has been measured at temperatures from 1.5 to 300 K under hydrostatic pressure up to 2 GPa and in magnetic field 0–15 T. In the inhomogeneous and paramagnetic regions of the phase diagram the main contribution to the electronic transport is related to the strong static magnetic fluctuations, which arise due to interplay of structural disorder within Gd-sublattice and Co-3d itinerant electron metamagnetism. This complex magnetic disorder brings about novel transport phenomena, such as anomalous positive magnetoresistance found in ferrimagnetic state of the alloys. The low-temperature thermopower is almost independent of alloy composition in the ferrimagnetic range of the phase diagram ( x > 0.3 ) indicating that the alloying does not change electronic structure of the compounds in a close vicinity of Fermi energy. However, the thermopower shows substantial variation with the composition in the inhomogeneous and in the paramagnetic regions of the phase diagram reflecting evolution of the magnetic structure with the composition.

Anomalous positive magnetoresistance inFe0.75Mn1.35As

With increasing temperature, tetragonal Fe0.75Mn1.35As compound exhibits a first-order phase transition at T-s=165 K from a ferrimagnetic (FI) phase with a large unit-cell volume to an antiferromagnetic (AF) phase with a small unit-cell volume. An external magnetic field induces a metamagnetic transition from the AF to the FI state above T-s, stabilizing the phase with the larger cell volume. With increasing the number of warming and cooling cycles, the resistivity curve, rho(T), shifts to higher values, and this shift is ascribed to a pronounced reduction of the mean-free path of electron scattering. After several thermocycles, a large, irreversible lattice expansion of c axis of 0.18% occurs. The positive magnetoresistance ratios as large as 12% and 6% for the first magnetic-field cycles are observed at 170 and 167 K, respectively, at 5 T. The overall magnetotransport behaviors on the subsequently second and third magnetic-field cycles at 170 K are similar to that in the first magnetic-field cycle, but with MR ratios of only 1.6% and 0.6%, respectively, at 5 T.

Spin-Filter Effect in Magnetite Nanowire

Spin-dependent electron transport in individual magnetite (Fe3O4) nanowires contacted with normal metallic electrodes was investigated. Such a configured device demonstrated a spin-filter effect, that is, only the minority spin carriers can transport through the magnetite nanowire due to its negative spin polarization. An anomalous positive magnetoresistance approximately 7.5% is observed at room temperature. Moreover, the magnetoresistance can be controlled via bias voltage.

Grain Boundary Influence on the Electrical Properties of Tellurium Microstructure Ingots and Nanocluster Crystals

The high sensitivity of the low temperature electrical properties of p-type pure tellurium (Te) to impurities, structural boundaries, point defects and dislocations allows to investigate the structural imperfection profiles in crystals grown under different conditions. Our interest was focused on studying the influence of grain boundaries on the electrical properties of the samples that were remelted and directionally solidified in space (µg) without a seed (W-µg), in comparison with the sample grown under the normal earth conditions (1g0) and a nanocluster sample obtained by filling with melted Te of dielectric opal matrix voids (Opal<Te> sample). The W-µg ingot of Te was prepared in the "Crystallizator" furnace under microgravity conditions aboard the "Mir" space station [1]. The concentration variation of electrically active defects and neutral defects along the samples were studied by galvanomagnetic methods (Hall effect and electrical resistivity) in a wide temperature range from 0.4 to 300 K. In these measurements, the following effects caused by the micro- and nano- crystalline structure were found: low hole mobility, high concentration of neutral defects, and anomalous positive magnetoresistance in low magnetic fields at low temperatures. Besides, the specific resistivity of the space sample was found to oscillate (up to 20%) along the length which can be correlated with the presence of a few contact points of the melt with the ampoule wall. This ingot was formed as a result of rapid homogeneous spontaneous solidification, accompanied by forming a micro-block structure. The appearance of the anomalous positive magnetoresistance was observed in the micro-block W- sample and the nanocluster Opal<Te> sample. It is a consequence of intensive hole scattering at the grain boundaries which leads to an increase of the intervalley transition probability and to a change of the spin sign of holes in a low symmetry Te crystal. According to the weak localization theory [2], the spin variation during the scattering results in a positive magnetoresistance of the sample in low magnetic fields, in contrast to bulk Te crystals.

Quantum interference effect in single Pt(Ga)∕C nanowire

The electron transport behavior in single Pt(Ga)∕C nanowire fabricated using focused ion beam was studied. The Pt(Ga)∕C nanowire consists of <3nm Pt particles, which are sheathed with Ga+-doped amorphous carbon layers. The temperature dependence of the nanowire resistivity measured under four-probe method indicated the weak localization effect resulting from large disorder in the system. Moreover, low temperature resistivity of the nanowire increased with decrease of its temperature, following a −T law, which could be interpreted by electron-electron interaction in weak localization regime. An anomalous positive magnetoresistance was observed as well, which was ascribed to quantum interference effect arising from Coulomb interactions and weak localization in the strong spin-orbit scattering limit.

Anomalous positive magnetoresistance in Cox–C1−x granular films on Si substrates

We report the anomalous positive magnetoresistance (PMR) in Cox–C1−x granular films prepared on Si(100) substrates by pulsed laser deposition method. A much larger positive MR, up to 22% at magnetic field B=1 T and x=2 at. %, is observed at room temperature than at low temperature. Interestingly, the room-temperature response has a B2/3 dependence when B<1 T and a B1/2 dependence when B>1 T. Different from the low-temperature PMR, the room-temperature PMR are related with the Si(100) substrates. Furthermore, we show that the intrinsic positive MR at low temperature arise from shrinkage of the wave functions of localized electronic states, while the extrinsic MR effects of the Cox–C1−x/Si structure are obviously determined by another mechanism, which may exist in other similar structures. This in turn has important implications for the optimization of such materials.

Anomalous magnetoresistance in high-temperature organic-based magnetic semiconducting V(TCNE)x films

Anomalous positive magnetoresistance (MR) in high temperature organic-based magnet V(TCNE)x (TCNE=tetracynoethylene) thin films is reported. MR increases linearly with applied magnetic field and shows a maximum at the ferrimagnetic ordering temperature. The suggested roles of oppositely spin polarized π* electronic subbands and magnetization fluctuations due to the disordered nature of V(TCNE)x films are discussed.