Negative Magnetoresistance Effect Research Articles

Anisotropy and size effects in Bi1-xSbx semiconductor wires in a magnetic field

The electron transport and longitudinal and transverse magnetoresistance (MR) of glass-insulated Bi0.92Sb0.08 single-crystal wires with diameters of 180 nm to 2.2 μm and the (1011) orientation along the wire axis have been studied. The wires have been prepared by liquid-phase casting. It has been first found that the energy gap ΔE increases by a factor of 4 with a decrease in wire diameter d due to the manifestation of the quantum size effect, which can occur under conditions of a linear energy–momentum dispersion law characteristic of both the gapless state and the surface states in topological insulators (TIs). It has been revealed that, in strong magnetic fields at low temperatures, a semiconductor–semimetal transition occurs, which is evident as an anomalous decrease in the transverse MR anisotropy and the appearance of a metallic temperature dependence of resistance at T < 100 K. It has been found that the effect of negative MR, the appearance of an anomalous maximum in the longitudinal MR, and the dependence of Hmax ~ d-1 at 4.2 K is a manifestation of the classical MacDonald–Chambers size effect. The calculated value of the component of the Fermi momentum perpendicular to the magnetic induction vector H is 2 times higher than the value for pure bismuth wires. The features of the manifestation of the quantum size effect in Bi0.92Sb0.08 wires, semiconductor–semimetal electronic transitions induced by a magnetic field, and a decrease in the transverse MR anisotropy indicate the occurrence of new effects in low-dimensional structures based on semiconductor wire TIs, which require new scientific approaches and applications.

The Effect of Negative Magnetoresistance in Silicon to Create Multifunctional Sensors

The Effect of Negative Magnetoresistance in Silicon to Create Multifunctional Sensors

A large negative magnetoresistance effect in semiconducting crystals composed of an octahedrally ligated phthalocyanine complex with high-spin manganese(iii).

A design for an octahedrally ligated phthalocyanine complex with high-spin manganese(iii) (S = 2) and MnIII(Pc)Cl2 (Pc = phthalocyanine) is presented. The presence of high-spin state MnIII in the fabricated Ph4P[MnIII(Pc)Cl2]2 (Ph4P = tetraphenylphosphonium) semiconducting molecular crystal is indicated by the Mn–Cl distance, which suggests an electronic configuration of (dyz, dzx)2(dxy)1(dz2)1. This was confirmed by the Curie constant (C = 5.69 emu K mol−1), which was found to be significantly larger than that of the isostructural Ph4P[MnIII(Pc)(CN)2]2, where MnIII adopts a low-spin state (S = 1). The magnetoresistance (MR) effects of Ph4P[MnIII(Pc)Cl2]2 at 26.5 K under 9 T static magnetic fields perpendicular and parallel to the c-axis were determined to be −30% and −20%, respectively, which are significantly larger values than those of Ph4P[MnIII(Pc)(CN)2]2. Furthermore, the negative MR effect is comparable to that of Ph4P[FeIII(Pc)(CN)2]2 (S = 1/2), which exhibits the largest negative MR effect reported for [MIII(Mc)L2]-based systems (Mc = macrocyclic ligand, L = axial ligand). This suggests that the spin state of the metal ion is the key to tuning the MR effect.

Electrical and galvanomagnetic properties of black phosphorus single crystals

Black phosphorus (b-P) single crystals having the n-type electrical conductivity produced in a high pressure set-up (~1 GPa) with six diamond anvils at 800 °C for 12 h have been studied. The electrical conductivity σ(Т,В) and the Hall constant Rh(Т,В) have been analyzed within one-band and two-band models as functions of temperature in the 2 < Т < 300 K range and magnetic field in the 0 < В < 8 T range. Fitting of the experimental σ(Т,В) and Rh(Т,В) curves suggests the following key properties of the crystals: (1) intrinsic conductivity type, (2) approximately equal electron and hole concentrations and mobilities, (3) anisotropic behavior of electron and hole conductivities, concentrations and mobilities and (4) combination of negative and positive contributions to magnetoresistance (magnetoresistive effect, MR). In a zero magnetic field the anisotropy coefficient α = [σа (Т) – σс (Т)]/σс (Т) below 50–70 K is positive whereas above 220 K its sign changes to negative due to a specific combination of the temperature dependences of carrier concentration and mobility. It has been shown that the negative sign of relative MR (negative magnetoresistive effect) dominates at T < 25 K and B < 6 T and is presumably caused by the effects of strong localization resulting from structural disorder. The positive MR sign (positive magnetoresistive effect) is associated with the Lorentz mechanism of carrier movement and exhibits itself above 25 K in 6–8 T magnetic fields.

Magnetic field assisted α-Fe2O3/Zn1−xFexO heterojunctions for accelerating antiviral agents degradation under visible-light

Reducing the recombination efficiency of photo-induced carriers has been found as an effective means to improve the degradation of antiviral agents. Given that the Lorentz forces can cause the abnormal charge to move in the opposite direction, external magnetic field improved α-Fe2O3/Zn1−xFexO heterojunctions (FZHx) were developed to remove increasing antiviral agents that were attributed to the COVID-19 pandemic under visible light. The characterization of the mentioned FZHx in the external magnetic field indicated that FZHx had perfect photocatalytic activity for degrading antiviral agents. In the external magnetic field, the quantities of photo-generated carriers and free radicals (•OH and •O2-) derived from FZHx increased significantly, which improved antiviral agent removal by 30.0%. Though the band structure (α-Fe2O3) is unlikely to change due to some orders of magnitude weaker of Zeeman energy in magnetic fields, which insignificantly impacts photocatalytic performance. However, this study proposed a strategy of negative magnetoresistance effects and heterojunctions to facilitate the separation and transfer of photo-induced carriers in magnetic fields. Based on the proposed strategy, spin oriented electrons were selected and accumulated on the conduction band, which contributed to the degradation of antiviral agents. Overall, this study presented novel insights into the improved degradation performance of antiviral agents by applying Fe-based heterojunctions in an external magnetic field.

Unique Signatures of Rashba Effect in Angle Resolved Magnetoresistance

AbstractAn unusual dependence of electrical resistance on the direction of the magnetic field, relative to that of current, in a 2D electron gas with strong spin‐orbit coupling formed at the LaVO3–KTaO3 interface is reported. The observations are incompatible with any previously reported magneto‐transport measurements. Surprisingly, on the one hand the system exhibits signatures of chiral anomaly such as negative magnetoresistance and planar Hall effect, on the other hand, a number of features are even qualitatively beyond the existing theories. It is found that all the unusual features in transport are controlled by the quantum effects originating from strong spin‐orbit coupling induced spin‐momentum locking, and the traditional Lorentz mechanism plays a minimal role. The results not only open up a new avenue related to magneto‐transport in spin‐orbit coupled metals but also pave a path to engineer non‐magnetic materials as sensors for vector magnetic fields.

Nonnegative magnetoresistance in hydrodynamic regime of electron fluid transport in two-dimensional materials

Electron transport in ultraclean two-dimensional materials has received much attention. However, the sign of the magnetoresistance effect in various electron flow regimes remains controversial. In this work, the complete electron Boltzmann transport equation is numerically solved with the discrete ordinate method in the real space to clarify the condition of the negative magnetoresistance effect under a weak magnetic field. It turns out from the numerical results that this effect occurs only within the ballistic regime under a low electric field rather than the hydrodynamic regime. It is noteworthy that the existence of momentum-conserving scattering dramatically reverses the sign of magnetoresistance in the ballistic regime. When the electric field becomes strong enough compared to the magnetic field, its effect on the deflection of the electrons is not negligible and will lead to positive magnetoresistance in the whole parameter domain. The possible influence of boundary conditions and internal electric field models on the sign of magnetoresistance is also discussed. Our work provides insight into electron fluid transport under electromagnetic fields.

Magnetic-field-induced avalanches in magnetization and magnetoresistance of La0.9Ce0.1Fe12B6 compound

Competition between paramagnetic (PM), antiferromagnetic (AFM) and ferromagnetic (FM) phases in La0.9Ce0.1Fe12B6 itinerant-electron metamagnetic system is studied by a combination of magnetic, electrical resistivity and magnetoresistance measurements. La0.9Ce0.1Fe12B6 is an antiferromagnet below the Néel temperature (35 K) and presents two consecutive magnetic transitions AFM–FM and FM–PM during warming under certain applied magnetic field values. We further demonstrate that both AFM and PM phases may be converted into the FM phase irreversibly and reversibly via a first-order metamagnetic transition accompanied by a large magnetic field hysteresis. At very low temperatures, the magnetic-field-driven AFM–FM metamagnetic transformation is discontinuous and proceeds by multiple abrupt steps in magnetization and magnetoresistance. In addition, an extraordinarily large negative magnetoresistance (MR) effect of −77% is observed. The consistency found in the magnetotransport and magnetization data suggests strong correlations between charge and spin degrees of freedom in La0.9Ce0.1Fe12B6 system.

Colossal magneto-resistive relaxation effects in La0.9Ce0.1Fe12B6

The study of the magnetic, electronic transport, and magnetotransport properties of La0.9Ce0.1Fe12B6 itinerant-electron system has been performed by combining magnetization, electrical resistivity, and magnetoresistance experiments. Along with the antiferromagnetic (AFM) ordering at TN = 35 K, two consecutive magnetic transformations, antiferromagnetic–ferromagnetic (AFM–FM) and ferromagnetic–paramagnetic (FM–PM), occur upon heating under certain magnetic field values. At fixed temperatures, it is revealed that both AFM and PM phases can be converted into the FM phase irreversibly and reversibly via a first-order metamagnetic transition associated with a large hysteresis. Below 8 K, the metamagnetic transition is discontinuous, manifesting itself by multiple sudden jumps in magnetoresistance and magnetization. A giant negative magnetoresistance effect of about −78% is found. We further demonstrate that the time dependencies of the electrical resistivity and the magnetization exhibit colossal spontaneous steps after an incubation time in conditions where both the applied magnetic field and temperature are constant. Another intriguing observation in the phase diagram is the presence of a critical point at the crossover of the three distinct PM, FM, and AFM magnetic states.

Site mixing induced ferrimagnetism and anomalous transport properties of the Weyl semimetal candidate MnSb2Te4

$\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ has been proposed to have magnetic topological states as a potential Weyl semimetal. We synthesized single crystals of $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ and systematically investigated their structural and physical properties. $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ has an isostructural septuple-layered structure that is similar to the magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ but possesses transpositional Mn and Sb atoms between the sublayers. Magnetic and specific-heat measurements revealed a ferrimagnetic phase transition with a Curie temperature ${T}_{\mathrm{C}}$ of \ensuremath{\sim}31 K in $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$, which originates from the interaction of the interexchanged ${\mathrm{Mn}}^{2+}$ ions. As the temperature decreases below ${T}_{\mathrm{C}}$, negative longitudinal magnetoresistance and anomalous Hall effect are observed, implying a non-negligible connection between the magnetism and expected transport carriers that may be driven by topological bands. Our results indicate that ferrimagnetic $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ provides insights for further studies on magnetic topological materials.

Current-dependent positive magnetoresistance in La0.8Ba0.2MnO3 ultrathin films* *Project supported by the National Natural Science Foundation of China (Grant No. 11674298), the National Key Research and Development Program of China (Grant No. 2017YFA0403502), and the Users with Excellence Project of Hefei Science Center CAS (Grant No. 2018HSC-UE013).

We report an investigation into the magnetoresistance (MR) of La0.8Ba0.2MnO3 ultrathin films with various thicknesses. While the 13 nm-thick film shows the commonly reported negative magnetoresistive effect, the 6 nm- and 4 nm-thick films display unconventional positive magnetoresistive (PMR) behavior under certain conditions. As well as the dependence on the film’s thickness, it has been found that the electrical resistivity and the PMR effect of the thinner films are very dependent on the test current. For example, the magnetoresistive ratio of the 4 nm-thick film changes from +46% to –37% when the current is increased from 10 nA to 100 nA under 15 kOe at 40 K. In addition, the two thinner films present opposite changes in electrical resistivity with respect to the test current, i.e., the electroresistive (ER) effect, at low temperatures. We discuss the complex magnetoresistive and ER behaviors by taking account of the weak contacts at grain boundaries between ferromagnetic metallic (FMM) grains. The PMR effect can be attributed to the breaking of the weak contacts due to the giant magnetostriction of the FMM grains under a magnetic field. Considering the competing effects of the conductive filament and local Joule self-heating at grain boundaries on the transport properties, the dissimilar ER effects in the two thinner films are also understandable. These experimental findings provide an additional approach for tuning the magnetoresistive effect in manganite films.

A comparison of magnetoconductivities between type-I and type-II Weyl semimetals

It is well known that Weyl semimetals (WSMs) are classified into two types of type-I and type-II depending on whether or not they have electron and hole pockets. Also, these WSMs have peculiar transport properties such as negative longitudinal magnetoresistance and planar Hall effect because of a chiral anomaly. In this paper, however, we show that the chiral anomaly can cause positive longitudinal magnetoresistance in type-II WSMs. Here, we investigate longitudinal and transverse magnetoconductivities of time reversal symmetry broken type-I and type-II WSMs using a tight-binding model. The model allows us to describe both types of type-I and type-II WSMs by tuning parameters, and it has two Weyl points that are separated along the kx-direction. The numerical calculations of these conductivities are performed using the Boltzmann equation including the Berry curvature. It is found that longitudinal magnetoconductivities in the x-direction can have both positive and negative values depending on the magnitude of the inclination of a Weyl cone. This is because the zeroth Landau energy-level becomes either a hole-like one or an electron-like one depending on the magnitude of the inclination of the Weyl cone in type-II WSMs. These results imply that we can make a high MR-ratio device using type-II WSMs by tuning the inclination of their cones if it is possible to change their energy bands by the application of electric field and so on.

Successive destruction of charge density wave states by pressure in LaAgSb2

We comprehensively studied the magnetotransport properties of LaAgSb$_2$ under high pressure up to 4 GPa, which showed unique successive charge density wave (CDW) transitions at $T_{CDW1}\sim 210$ K and $T_{CDW2}\sim 190$ K at ambient pressure. With the application of pressure, both $T_{CDW1}$ and $T_{CDW2}$ were suppressed and disappeared at the critical pressures of $P_{CDW1}=3.0$--3.4 GPa and $P_{CDW2}=1.5$--1.9 GPa, respectively. At $P_{CDW1}$, the Hall conductivity showed a step-like increase, which is consistently understood by the emergence of two-dimensional hollow Fermi surface at $P_{CDW1}$. We also observed a significant negative magnetoresistance effect when the magnetic field and current were applied parallel to the $c$ axis. Shubnikov--de Haas (SdH) oscillation measurements under pressure directly showed the changes in the Fermi surface across the CDW phase boundaries. In $P<P_{CDW2}$, three major oscillation components, $\alpha$, $\beta$, and $\gamma$, were identified, whose frequencies were increased by application of pressure. The increment rate of these frequencies was considerably larger than that expected from the shrinkage of lattice constant, indicating the unignorable band modification under pressure. In the normal metallic phase above $P>P_{CDW1}$, we observed a single frequency of $\sim 48$ T with a cyclotron effective mass of 0.066 $m_0$, whose cross section in the reciprocal space corresponded to only 0.22\% of the first Brillouin zone. Besides, we observed another oscillation component with frequency of $\sim 9.2$ T, which is significantly enhanced in the limited pressure range of $P_{CDW2}<P<P_{CDW1}$. The amplitude of this oscillation was anomalously suppressed in the high-field and low-temperature region, which cannot be explained by the conventional Lifshitz--Kosevich formula.

Admittance spectroscopy of dopants implanted in silicon and impurity state-induced AC magnetoresistance effect

A silicon structure doped with Ga using ion implantation has been investigated by admittance spectroscopy. It has been established that the presence of the Ga impurity, along with the B one, in the silicon structure leads to the appearance of the second peak in the temperature dependence of the real part of the impedance (admittance). Moreover, switching-on a magnetic field parallel to the sample plane shifts the singularities in the temperature curve to the high-temperature region. This results in the manifestation of both the positive and negative magnetoresistance effect upon temperature and magnetic field variation. It has been found by the standard admittance spectroscopy analysis of the impedance data that the energy structure of the investigated sample includes two interfacial energy levels ES1(0) = 42 meV and ES2(0) = 69.4 meV. As expected, these energies are consistent with the energies of B and Ga dopants. In a magnetic field, these levels increase by 3 meV for B and 2 meV for Ga, which induces the magnetoresistance effect. It has been demonstrated that the interfacial state-induced magnetoresistance effect can be tuned by ion implantation and dopant selection.

An electrically conducting molecular crystal composed of a magnetic iron(iii) complex (S = 1/2) with a large aromatic ligand, 1,2-naphthlalocyanine (C4h isomer): towards the development of molecular spintronics.

The field of molecular spintronics has gained significant attention for the development of second-generation spintronic devices. Therefore, an electrically conducting molecular crystal, Ph4P[FeIII(1,2-Nc)(CN)2]2 (Ph4P = tetraphenylphosphonium and 1,2-Nc = C4h isomer of 1,2-naphthalocyanine), was fabricated as a new coordination compound with a strong π-d interaction. Furthermore, it is a mixed-valence compound with a local spin of S = 1/2 at the center of the conduction path. Crystal structure analysis revealed that Ph4P[FeIII(1,2-Nc)(CN)2]2 was isostructural to its non-magnetic analogue Ph4P[CoIII(1,2-Nc)(CN)2]2 but possessed higher electrical resistivity, indicating that the strong intramolecular π-d interaction is present in the [FeIII(1,2-Nc)(CN)2] unit. Although the magnetic interaction between π-conduction electrons and FeIII-d spins (π-d interaction) is crucial for the emergence of a negative magnetoresistance effect, the negative magnetoresistance effect of Ph4P[FeIII(1,2-Nc)(CN)2]2 was significantly smaller (-6% at 30 K under a static 9 T magnetic field) than those of Ph4P[FeIII(Pc)(CN)2]2 (-32%) and Ph4P[FeIII(tbp)(CN)2]2 (-13%) analogues (Pc = phthalocyanine and tbp = tetrabenzoporphyrin). This small negative magnetoresistance effect of Ph4P[FeIII(Pc)(CN)2]2 could be ascribed to the weak intermolecular antiferromagnetic interaction between its d spins. Hence, this study showed that constructing a molecular design for strengthening the intermolecular antiferromagnetic interaction is key to enhancing the negative magnetoresistance effect.

Отрицательное магнитосопротивление в структуре n-InSb/ЖИГ

It is shown that for the n-InSb/YIG/GGG (YIG - yttrium iron garnet, GGG - gallium-gadolinium garnet) structure in the tangent to the substrate plane geometry of magnetization (H &lt;10 kOe) at a temperature T≈300 K the magnetoresistance of about 1% is negative, while for n-InSb/GGG structure, in the same magnetization geometry, the magnetoresistance is positive (an increase in electrical resistance in a magnetic field). The negative magnetoresistance effect in the InSb/YIG/GGG structure is due to the influence of the YIG magnetization on the conduction electrons of InSb (proximity effect) and the magnitude of the effect is determined by the value of YIG magnetization and parameters of InSb films.

Temperature Dependent In-Plane Anisotropic Magnetoresistance in HfTe5 Thin LayersSupported by the National Key R&D Program (Grant Nos. 2017YFB0405703, 2017YFA0205004, and 2018YFA0306600), and the National Natural Science Foundation of China (Grant Nos. 11974327, 11474265, 11674295, 11674024, and 11874193), the Fundamental Research Funds for the Central Universities (Grant Nos. WK2030020032 and WK2340000082), Anhui Initiative in Quantum Information Technologies, and the Shenzhen Fundamental

We report the observation of in-plane anisotropic magnetoresistance and planar Hall effect in non-magnetic HfTe5 thin layers. The observed anisotropic magnetoresistance as well as its sign is strongly dependent on the critical resistivity anomaly temperature T p. Below T p, the anisotropic magnetoresistance is negative with large negative magnetoresistance. When the in-plane magnetic field is perpendicular to the current, the negative longitudinal magnetoresistance reaches its maximum. The negative longitudinal magnetoresistance effect in HfTe5 thin layers is dramatically different from that induced by the chiral anomaly as observed in Weyl and Dirac semimetals. One potential underlying origin may be attributed to the reduced spin scattering, which arises from the in-plane magnetic field driven coupling between the top and bottom surface states. Our findings provide valuable insights for the anisotropic magnetoresistance effect in topological electronic systems and the device potential of HfTe5 in spintronics and quantum sensing.

Диодные гетероструктуры с ферромагнитными узкозонными полупроводниками A-=SUP=-3-=/SUP=-FeB-=SUP=-5-=/SUP=- разного типа проводимости

Diode structures with ferromagnetic narrow-gap semiconductors A3FeB5 as only p-region (p-GaFeSb/n-InGaAs), only n-region (n-InFeSb/p-InGaAs), p- and n-regions (p-GaFeSb/n-InFeSb, p-GaFeSb/n-InFeAs) for p-n junction were fabricated by pulsed laser deposition in vacuum. The composition of ferromagnetic semiconductor layers and their thicknesses, determined by X-ray photoelectron spectroscopy, generally correspond to the technological data for diode structures. In particular, the thickness of the GaFeSb layer is 25–30 nm, and the thickness of the InFeAs and InFeSb layers is 35–40 nm. The iron content in InFeSb ranges from 25 to 35 at.%. The GaFeSb layer contains from 15 to 41 iron at. %, and the InFeAs layer - 35 iron at. %. The chemical analysis of the structures revealed the presence of chemical bonds Fe-As (Sb), In-Fe and Fe-Ga. Therefore, it can be assumed that Fe atoms in the fabricated structures can substitute for elements of groups III and V simultaneously. All structures exhibit the effect of negative magnetoresistance at sufficiently low observation voltages of the effect (up to 50 mV), in low magnetic fields (up to 3600 Oe), and at high measurement temperatures. For GaFeSb/InFeSb, GaFeSb/InFeAs diodes, negative magnetoresistance was first observed at room temperature. The hysteresis form of the dependences of the resistance on the magnetic field suggests the effect of the ferromagnetic properties of the layers of narrow-gap semiconductors on the transport of carriers in the structures.

Magnetoresistance of a Ferromagnet/Superconductor/Ferromagnet Trilayer Microbridge Based on Diluted PdFe Alloy

A negative magnetoresistive effect has been observed for ferromagnet/superconductor/ferromagnet (FSF) microbridges based on diluted ferromagnetic PdFe alloy containing as small as 1% of magnetic atoms. The effect is represented by sharp negative peaks in magnetoresistance at magnetic fields opposite in sign to the initial saturated magnetizations. Microstructuring of the FSF trilayers does not suppress the effect: the most pronounced dips were obtained for the smallest bridges 6–8 µm wide and 10–15 µm long. The negative magnetoresistance peak was observed at temperatures within the superconducting transition and reaches a noticeable value of up to 1.3% of the normal state resistance.

Tensor monopoles and the negative magnetoresistance effect in optical lattices

We propose that a kind of four-dimensional (4D) Hamiltonians, which host tensor monopoles related to quantum metric tensor in even dimensions, can be simulated by ultracold atoms in the optical lattices. The topological properties and bulk-boundary correspondence of tensor monopoles are investigated in detail. By fixing the momentum along one of the dimensions, it can be reduced to an effective three-dimensional model manifesting with a nontrivial chiral insulator phase. Using the semiclassical Boltzmann equation, we calculate the longitudinal resistance against the magnetic field $B$ and find the negative relative magnetoresistance effect of approximately $ -B^{2} $ dependence when a hyperplane cuts through the tensor monopoles in the parameter space. We also propose an experimental scheme to realize this 4D Hamiltonian by introducing an external cyclical parameter in a 3D optical lattice. Moreover, we show that the quantum metric tensor and Berry curvature can be detected by applying an external drive in the optical lattices.