Magnetoresistance Research Articles

Anomalous magnetothermoelectric behavior in massive Dirac materials

Extensive studies of electron transport in Dirac materials have shown positive magneto-resistance (MR) and positive magneto-thermopower (MTP) in a magnetic field perpendicular to the excitation current or thermal gradient. In contrast, measurements of electron transport often show a negative longitudinal MR and negative MTP for a magnetic field oriented along the excitation current or thermal gradient; this is attributed to the chiral anomaly in Dirac materials. Here, we report a very different magneto-thermoelectric transport behavior in the massive Dirac material ZrTe5. Although thin flakes show a commonly observed positive MR in a perpendicular magnetic field, distinct from other Dirac materials, we observe a sharp negative MTP. In a parallel magnetic field, we still observe a negative longitudinal MR, however, a remarkable positive MTP is observed for the fields parallel to the thermal gradients. Our theoretical calculations suggest that this anomalous magneto-thermoelectric behavior can be attributed to the screened Coulomb scattering. This work demonstrates the significance of impurity scattering in the electron transport of topological materials and provides deep insight into the novel magneto-transport phenomena in Dirac materials.

Field direction dependent quantum-limit magnetoresistance of correlated Dirac electrons in perovskite CaIrO3

Correlated-electron perovskite ${\mathrm{CaIrO}}_{3}$ shows the topologically protected line node near the Fermi level, wherein the high-mobility Dirac electrons reach the quantum limit (QL) with the one-dimensionally (1D) dispersive $n=0$ Landau level (LL) state at a moderate magnetic field. In the QL, the longitudinal magnetoresistance (MR) shows an extreme anisotropy against the field $(B)$ direction in spite of nearly isotropic Dirac band dispersions. The resistivity for $B\ensuremath{\parallel}a$ shows an insulating behavior with the MR ratio exceeding $2000%$ around 18 T, whereas the $B\ensuremath{\parallel}c$ MR always remains metallic. This is accounted for in terms of the large difference of Fermi velocity of the $n=0$ LL between $B\ensuremath{\parallel}a$ and $B\ensuremath{\parallel}c$ due to the $B$ direction dependent $5d$ orbital electron hopping, which triggers the different instability toward the charge density wave formation for the 1D $(\ensuremath{\parallel}B)$ dispersive $n=0$ LL.

Electronic structure, magnetic properties, spin orientation, and doping effect in Mn3Si2Te6

The layered material Mn$_3$Si$_2$Te$_6$, with alternating stacking honeycomb and triangular layers, is attracting considerable attention due to its rich physical properties. Here, using density functional theory and classical Monte Carlo (MC) methods, we systematically study this system. Near the Fermi level, the states are mainly contributed by Te $5p$ orbitals hybridized with Mn $3d$ orbitals, resembling a charge transfer system. Furthermore, the spin orientations of the ferrimagnetic (FiM) ground state display different conductive behaviors when along the $ab$ plane or out-of-plane directions: insulating vs. metallic states. The energy difference between the FiM [110] insulating and FiM [001] metallic phases is very small($ \sim 0.71$ meV/Mn). Changing the angle $\theta$ of spin orientation from in-plane to out-of-plane directions, the band gaps of this system are gradually reduced, leading to an insulator-metal transition, resulting in an enhanced electrical conductivity, related to the colossal angular magnetoresistance (MR) effect. In addition, we also constructed the magnetic phase diagram using the classical $XY$ spin model studied with the MC method. Three magnetic phases were obtained including antiferromagnetic order, noncollinear spin patterns, and FiM order. Moreover, we also investigated the Se- and Ge- doping into the Mn$_3$Si$_2$Te$_6$ system: the FiM state has the lowest energy among the magnetic candidates for both Se- or Ge- doped cases. The magnetic anisotropy energy (MAE) decreases in the Se-doped case because the Mn orbital moment is reduced as the doping $x$ increases. Due to the small spin-orbital coupling effect of Se, the insulator-metal transition caused by the spin orientation disappears in the Se-doped case, resulting in an insulating phase in the FiM [001] phase. This causes a reduced colossal angular MR.

Magnetoresistance and Kohler rule in the topological chiral semimetal CoSi

The transverse and longitudinal magnetoresistance (MR) of two samples of the topological chiral semimetal CoSi with different RRR was studied. It is shown that the Kohler rule works for the transverse MR. The Kohler rule is also fulfilled in the case of longitudinal MR at a low reduced magnetic field. A sharp deviation of longitudinal MR curve for sample with low RRR from the Kohler prediction at high fields reveals its tendency to a sign change at higher magnetic fields. The Shubnikov de Haas quantum oscillations were observed and analyzed in both perpendicular and parallel configurations of the current and magnetic field in sample CoSi 1 with RRR 9.33 at low temperatures.

Magnetotransport induced by anomalous Hall effect

In a magnetic metal, the Hall resistance is generally taken to be the sum of the ordinary Hall resistance and the anomalous Hall resistance. Here it is shown that this empirical relation is no longer valid when either the ordinary Hall angle or the anomalous Hall angle is not small. Using the proper conductivity relation, we reveal an unexpected magnetoresistance (MR) induced by the anomalous Hall effect (AHE). A $B$-linear MR arises and the sign of the slope depends on the sign of the anomalous Hall angle, giving rise to a characteristic bowtie shape. The Hall resistance in a single-band system can exhibit a nonlinearity which is usually considered as a characteristic of a two-band system. A $B$-symmetric component appears in the Hall. These effects reflect the fundamental difference between the ordinary Hall effect and the AHE. Furthermore, we experimentally reproduce the unusual MR and Hall reported before in Co$_3$Sn$_2$S$_2$ and show that these observations can be well explained by the proposed mechanism. MR often observed in quantum anomalous Hall insulators provides further confirmation of the picture. The effect may also account for the large MR observed in non-magnetic three-dimensional topological Dirac semimetals.

Quantum Oscillation and Electronic Structure of Sn4As3 and Sn4P3

We report the magnetotransport properties and electronic structures of the single crystals of Sn4As3 and Sn4P3. The magnetization curves display strong de Haas–van Alphen oscillations at low temperatures and high magnetic fields. The oscillation frequencies and corresponding Fermi surfaces (FSs) are identified from the analysis of the oscillations. The in-plane magnetization of Sn4As3 shows the split of FSs induced by nonsymmetric spin–orbit interaction. The extracted cyclotron effective masses are very small and in the range 0.03–0.11 me. First-principles calculations reveal the electronic structures and topological properties. Large transverse magnetoresistance (MR) has also been observed in Sn4As3 and Sn4P3. The Hall measurements eliminate the possibility of electron–hole compensation. It is suggested that the large MR mainly originates from the open FSs.

Effects of Partial Manganese Substitution by Cobalt on the Physical Properties of Pr0.7Sr0.3Mn(1-x)CoxO3 (0 ≤ x ≤ 0.15) Manganites.

We have investigated the structural, magnetic, and electrical transport properties of Pr0.7 Sr0.3 Mn(1-x)Cox O3 nanopowders (x = 0, 0.05, 0.10 and 0.15). The Pechini Sol-gel method was used to synthesize these nanopowders. X-ray diffraction at room temperature shows that all the nano powders have an orthorhombic structure of Pnma space group crystallography. The average crystallite size of samples x = 0, 0.05, 0.10, and 0.15 are 33.78 nm, 29 nm, 33.61 nm, and 24.27 nm, respectively. Semi-quantitative chemical analysis by energy dispersive spectroscopy (EDS) confirms the expected stoichiometry of the sample. Magnetic measurements indicate that all samples show a ferromagnetic (FM) to paramagnetic (PM) transition with increasing temperature. The Curie temperature TC gradually decreases (300 K, 270 K, 250 K, and 235 K for x = 0, 0.05, 0.10, and 0.15, respectively) with increasing Co concentrations. The M-H curves for all compounds reveal the PM behavior at 300 K, while the FM behavior characterizes the magnetic hysteresis at low temperature (5 K). The electrical resistivity measurements show that all compounds exhibit metallic behavior at low temperature (T < Tρ) well fitted by the relation ρ = ρ0 + ρ2T2 + ρ4.5T4.5 and semiconductor behavior above Tρ (T > Tρ), for which the electronic transport can be explained by the variable range hopping model and the adiabatic small polaron hopping model. All samples have significant magnetoresistance (MR) values, even at room temperature. This presented research provides an innovative and practical approach to develop materials in several technological areas, such as ultra-high density magnetic recording and magneto resistive sensors.

Anomalous magnetotransport behavior in single-crystal VSb2

We report the anomalous magnetotransport properties in high-quality single-crystal ${\mathrm{VSb}}_{2}$ without V atom vacancies. ${\mathrm{VSb}}_{2}$ shows typical metallic behavior with convex-upward characteristics, which can be explained using a parallel-resistor model. The three-dimensional weak antilocalization effect leads to a large saturation magnetoresistance (MR) up to $595%$, which disappears when the magnetic field is along the $ab$ plane. Instead, a clear Shubnikov--de Haas oscillation at low temperatures is observed, based on which the fast Fourier transformation spectrum analyses reveal three fundamental frequencies. The two-carrier fitting of Hall resistivity suggests that the saturation MR originates from the giant magnitude difference of the two carrier concentrations. Moreover, the angle-resolved magnetoresistance is measured by rotating the sample around its $c$ axis and $ab$ plane. A transition from twofold symmetry to isotropy is observed.

Defect assisted magneto-tunable photoresponse in ZnO-rGO/La0.7Sr0.3MnO3/ITO heterojunctions

We have investigated light and magnetic field dependent electrical transport response of (100−x)%ZnO-x%rGO/La0.7Sr0.3MnO3 (LSMO)/ITO (x= 0, 100, 0.6, 0.8) heterostructure devices at room temperature. Negative and positive magnetoresistance(MR) response, respectively for bare (x= 0, 100) and composite devices (x= 0.6, 0.8) is associated with spin dependent scattering at eg2↑ and t2g↓ band of LSMO space charge region. Positive bias current, for bare devices, is supposed to have a defect induced suppression with light leading to an overall decrease of device current at low field. However, the bias current may have started increasing with light at high fields thereby reversing photosensitivity sign from negative (NPS) to positive (PPS). Such reversal from NPS to PPS has been attributed to high defect induced carrier transport through depinning of electrons from weak pinning centers with magnetic field. (100−x)%ZnO-x%rGO/LSMO barrier widths that are directly proportional with defect state population seem to support field induced change of photosensitivity behavior with light illumination.

Modulation of magnetic anisotropy by bending in Ni and Ni78Fe22 thin films on polycarbonate organic substrates

We investigate surface morphologies and magnetic/electrical properties, in which the bending dependence of the anisotropic magnetoresistance (AMR) effect and magnetic anisotropic field are focused on, of Ni and Ni78Fe22 thin films on polycarbonate (PC) substrates. The magnetic easy axis of the Ni (Ni78Fe22) thin films is rotated by bending the films in a convex (concave) shape. Bending the Ni/PC films in a concave shape enhances the magnetoresistance (MR) ratio by a factor of 1.4 and increases the magnetic anisotropic field from 13 to 240 mT. Whereas, bending Ni78Fe22/PC films in a convex shape enhances the MR ratio by a factor of 1.9 and increases the magnetic anisotropic field from 3.4 to 31 mT. These results are attributed to the inverse magnetostrictive effect. The results also reveal that the bending direction, which enhances the AMR effect, is determined by the sign of the magnetostriction constant. Bending-induced magnetic tuning can be a powerful tool for developing high-performance magnetic materials and spintronic devices.

Role of cationic size mismatch and effect of disorder in mixed valent manganites

Comparative studies of structure, magnetism, and magnetoresistance (MR) have been carried out in A-site ordered NdBaMn2O6 (O-NB), A-site disordered NdBaMn2O6 (D-NB) and A-site disordered NdCaMn2O6 (D-NC). O-NB, where A-site cations, Nd3+ and Ba2+ (of different ionic sizes) are arranged periodically, undergoes structural transition with temperature, while no structural change is present in D-NB where A-site cations are arranged randomly. However, structural transitions are observed in D-NC where Nd3+ and Ca2+ have similar ionic sizes. Magnetization (M) data shows O-NB has AFM ground state associated with a lower structurally symmetric phase and an FM ground state is observed for D-NB with higher structural symmetry. However, AFM ground state is observed in D-NC similar to that of O-NB. Both the disorder systems exhibit semiconductive transport characteristics over the entire temperature range. The resistivity data of disorder compounds have been fitted with different theoretical models to elucidate the conduction process in these systems. Further, MR studies depict a three times higher value of MR in both disorder compounds compared to that of order one. However, the behavior of MR with H is different for D-NB and D-NC, implying a different origin of this large MR in these compounds. We believe that the different magnetic ground state of D-NB and D-NC is the possible origin of their distinct MR behavior to the magnetic field.

Exchange bias controlled antisymmetric-symmetric magnetoresistances in Fe3GeTe2/graphite/Fe3GeTe2 trilayer

The magnetoresistance (MR) of spin values usually displays a symmetric dependence on the magnetic field. An antisymmetric MR phenomenon has been discovered recently that breaks the field symmetry and has the potential to realize multi-bit memory. In this work, we report a controllable switch between the antisymmetric and symmetric MRs and propose a multi-bit memory performance in Fe3GeTe2 (FGT)/graphite/FGT trilayer with modified vertical geometry. Via investigating the evolution of the antisymmetric MR depending on the spatial distribution, current direction, and magnetization configuration, we demonstrate that the antisymmetric MR results from the local nonequilibrium current through the trilayer. Furthermore, an exchange bias effect is induced which modifies the antisymmetric MR. A stable multi-bit memory is presented in the heterostructure. Such architecture for multi-state memory provides insights into other spin-valve structures to improve storage density.

Measurement System for Short-Pulsed Magnetic Fields.

A measurement system based on the colossal magnetoresistance CMR-B-scalar sensor was developed for the measurement of short-duration high-amplitude magnetic fields. The system consists of a magnetic field sensor made from thin nanostructured manganite film with minimized memory effect, and a magnetic field recording module. The memory effect of the La1-xSrx(Mn1-yCoy)zO3 manganite films doped with different amounts of Co and Mn was investigated by measuring the magnetoresistance (MR) and resistance relaxation in pulsed magnetic fields up to 20 T in the temperature range of 80-365 K. It was found that for low-temperature applications, films doped with Co (LSMCO) are preferable due to the minimized magnetic memory effect at these temperatures, compared with LSMO films without Co. For applications at temperatures higher than room temperature, nanostructured manganite LSMO films with increased Mn content above the stoichiometric level have to be used. These films do not exhibit magnetic memory effects and have higher MR values. To avoid parasitic signal due to electromotive forces appearing in the transmission line of the sensor during measurement of short-pulsed magnetic fields, a bipolar-pulsed voltage supply for the sensor was used. For signal recording, a measurement module consisting of a pulsed voltage generator with a frequency up to 12.5 MHz, a 16-bit ADC with a sampling rate of 25 MHz, and a microprocessor was proposed. The circuit of the measurement module was shielded against low- and high-frequency electromagnetic noise, and the recorded signal was transmitted to a personal computer using a fiber optic link. The system was tested using magnetic field generators, generating magnetic fields with pulse durations ranging from 3 to 20 μs. The developed magnetic field measurement system can be used for the measurement of high-pulsed magnetic fields with pulse durations in the order of microseconds in different fields of science and industry.

Probing electron-electron interaction along with superconducting fluctuations in disordered TiN thin films

In two-dimensional (2D) disordered superconductors prior to superconducting (SC) transition, the appearance of a resistance peak in the temperature dependent resistance $R(T)$ measurements indicates the presence of weak localization (WL) and electron-electron interaction (EEI) in the diffusion channel and SC fluctuations in the Cooper channel. Here, we demonstrate an interplay between SC fluctuations and EEI by low-temperature magnetotransport measurements for a set of 2D disordered TiN thin films. While cooling the sample, a characteristic temperature ${T}^{*}$ is obtained from the $R(T)$ at which SC fluctuations start to appear. The upturn in $R(T)$ above ${T}^{*}$ corresponds to WL and/or EEI. By the temperature and field dependences of the observed resistance, we show that the upturn in $R(T)$ originates mainly from EEI with a negligible contribution from WL. Further, we have used the modified Larkin's electron-electron attraction strength $\ensuremath{\beta}(T/{T}_{\mathrm{c}})$, containing a field-induced pair-breaking parameter, in the Maki-Thompson SC fluctuation term. Here, the temperature dependence of $\ensuremath{\beta}(T/{T}_{\mathrm{c}})$ obtained from the magnetoresistance (MR) analysis shows a diverging behavior close to ${T}_{\mathrm{c}}$, and it remains almost constant at higher temperature within the limit of $\mathrm{ln}(T/{T}_{\mathrm{c}})&lt;1$. Interestingly, the variation of $\ensuremath{\beta}(T/{T}_{\mathrm{c}})$ on the reduced temperature $(T/{T}_{\mathrm{c}})$ offers a common trend which has been closely followed by all the concerned samples presented in this paper. Finally, the temperature dependence of inverse phase scattering time (${\ensuremath{\tau}}_{\ensuremath{\phi}}^{\ensuremath{-}1}$), as obtained from the MR analysis, clearly shows two different regimes; the first one close to ${T}_{\mathrm{c}}$ follows the Ginzburg-Landau relaxation rate $({\ensuremath{\tau}}_{\mathrm{GL}}^{\ensuremath{-}1})$, whereas the second one at high temperature varies almost linearly with temperature, indicating the dominance of inelastic electron-electron scattering for the dephasing mechanism. These two regimes are followed in a generic way by all the samples despite being grown under different growth conditions.

Resistivity minima in disordered Co2FeAl0.5Si0.5 Heusler alloy thin films

An exhaustive study of the effect of anti-site disorder on the ‘zero-field’, ρ(T, H = 0), and ‘in-field’, ρ(T, H = 80 kOe), electrical resistivity in 50 nm thick Co2FeAl0.5Si0.5 (CFAS) Heusler alloy thin films (deposited on the Si(100) or SiO2/Si(100) substrates at fixed substrate temperatures in the range 27°C≤TS≤550°C), has been carried out. Irrespective of the strength of disorder, resistivity goes through a minimum as a function of temperature at T = Tmin. With increasing substrate temperature, the crystalline order of the CFAS thin films improves so much so that the films deposited at 500 °C have the lowest anti-site disorder within the B2 structure and the least residual resistivity. A quantitative comparison of our results with the predictions of the existing theoretical models permits us to unambiguously identify the diffusive and ballistic transport mechanisms, responsible for ρ(T, H = 0) and ρ(T, H = 80 kOe) in different temperature ranges and accurately determine their relative magnitudes. The electron-diffuson (e – d) scattering and weak localization (WL) mechanisms, responsible for negative temperature coefficient of resistivity (TCR) for T < Tmin, compete with the positive TCR mechanisms, electron-magnon (e – m) and electron–phonon (e – p) scattering, to produce the resistivity minimum at Tmin. The e – d and WL contributions to ρ, ρe-d and ρwl, dominate over the e – m and e – p contributions, ρe-m and ρe-p, for T<Tmin whereas the reverse is true for T>Tmin. At any given temperature, ρe-d, ρwl and ρe-m decrease while ρe-p increases as the atomic order improves with increasing substrate temperature, TS. ρe-p and ρe-m originate from the phonon-induced non-spin-flip two-band (s↑↓- d↑↓) scattering and magnon - induced spin-flip s - d interband (s↑↓- d↓↑) transitions, respectively. In all the CFAS films, except for the one deposited at TS = 27°C, the thermal renormalization of the spin-wave stiffness, due to the electron - magnon interaction, contributes significantly to ρe-m(T). Furthermore, we demonstrate that the negative magnetoresistance (MR) is a consequence of a progressive suppression of the WL effect and e – m scattering by external magnetic field. However, the WL contribution to MR turns out to be negligibly small as the e – m contribution almost entirely accounts for MR over the temperature range 5K≤T≤300K.

Exploring the Dirac nature of RbBi2

Characteristics of topological semimetals such as a nonsaturating magnetoresistance (MR), a field-induced metal to semiconducting crossover and a robust resistivity plateau are observed under a magnetic field in type-I ${\mathrm{RbBi}}_{2}$ bulk superconductor with ${T}_{\mathrm{c}}=4.15$ K. The MR exhibits a notable 3500% increase at 2 K and 9 T and the resistivity follows a power law temperature dependence, while the MR $\ensuremath{\propto}\phantom{\rule{4pt}{0ex}}{H}^{1.26}$, indicating weak carrier compensation. First principles calculations provided insights into the dynamical stability of the cubic structure at 0 K. Both hole and electron pockets are observed at the Fermi surface. The electron-phonon interaction constant indicates weak coupling strength ($&lt;1$) that leads to a maximum predicted ${T}_{\mathrm{c}}$ of 2.852 K. Just below the Fermi level, ${\mathrm{E}}_{F}$, the electronic band structure consists of linear band crossings at the $X$ points in the Brillouin zone (BZ) corresponding to massless, symmetry-protected Dirac fermions.

Evaluation of MR ratio and reliability of MTJ device having SiN sidewall by modifying reference layer thickness

For high-performance magnetic tunnel junction devices, the ion beam etching (IBE) process is improved by including the fabrication of a SiN sidewall around the MgO tunnel barrier layer. This sidewall prevents the redeposition from lower metal layers on the edge of the MgO layer. The magnetoresistance (MR) ratio at an IBE angle of 20° is larger than that at 60°. Edge current by the redeposition is suppressed by the SiN sidewall, and etching damage is reduced by decreasing the IBE angle. The dependence of the MR ratio on the reference layer thickness is investigated using an FeB reference layer with thicknesses of 0.7, 0.9 and 1.05 nm. The MR ratio increases with the reference layer thickness, and an MR ratio of 180% is achieved at a layer thickness of 1.05 nm. A resistance variability of under 0.5% confirms the high resistance stability of the tested device.

Cross-plane transport in cyclo[18]carbon-based molecular devices

The isolated cyclo[18]carbon (C18) has a vertically discrete π-electron conjugate system that possesses unique electronic properties. However, the exploration on cross-plane transport properties of C18-based molecular devices is still in a rudimentary stage. In this work, we propose a type of spin-filtering device with AA-stacked C18 as the central molecule sandwiched between two zigzag-edged graphene nanoribbons (ZGNRs) and investigate the cross-plane transport behaviors in terms of a nonequilibrium Green's function method and a density functional theory. We find that it exhibits the integrated effect of negative differential resistance, spin-filtering, switching, and half-metallic properties. Moreover, it shows conspicuous magnetoresistance (MR) and rectification effects. The MR can be up to 12 480% when it is clamped vertically between two ZGNRs, and the rectification ratio can reach 2926 through inserting a transition metal atom V. Our results provide an avenue for the design of next-generation carbon-based molecular spintronic devices.

Selective Dual-Ion Modulation in Solid-State Magnetoelectric Heterojunctions for In-Memory Encryption.

Nanoionic technologies are identified as a promising approach to modulating the physical properties of solid-state dielectrics, which have resulted in various emergent nanodevices, such as nanoionic resistive switching devices and magnetoionic devices for memory and computing applications. Previous studies are limited to single-type ion manipulation, and the investigation of multiple-type ion modulation on the coupled magnetoelectric effects, for developing information devices with multiple integrated functionalities, remains elusive. Here, a dual-ion solid-state magnetoelectric heterojunction based on Pt/HfO2- x /NiOy /Ni with reconfigurable magnetoresistance (MR) characteristics is reported for in-memory encryption. It is shown that the oxygen anions and nickel cations can be selectively driven by voltages with controlled polarity and intensity, which concurrently change the overall electrical resistance and the interfacial magnetic coupling, thus significantly modulate the MR symmetry. Based on this device, a magnetoelectric memory prototype array with in-memory encryption functionality is designed for the secure storage of image and digit information. Along with the advantages including simple structure, multistate encryption, good reversibility, and nonvolatile modulation capability, this proof-of-concept device opens new avenues toward next-generation compact electronics with integrated information functionalities.

Doping-enhanced robustness of anomaly-related magnetoresistance in WTe2±α flakes

We study systematically the negative magnetoresistance (MR) effect in WTe2±α flakes with different thicknesses and doping concentrations. The negative MR is sensitive to the relative orientation between electrical-/magnetic-field and crystallographic orientation of WTe2±α . The analysis proves that the negative MR originates from chiral anomaly and is anisotropic. Maximum entropy mobility spectrum is used to analyze the electron and hole concentrations in the flake samples. It is found that the negative MR observed in WTe2±α flakes with low doping concentration is small, and the high doping concentration is large. The doping-induced disorder obviously inhibits the positive MR, so the negative MR can be more easily observed. In a word, we introduce disorder to suppress positive MR by doping, and successfully obtain the negative MR in WTe2±α flakes with different thicknesses and doping concentrations, which indicates that the chiral anomaly effect in WTe2 is robust.