Dc Magnetotransport Research Articles

DC Transport and Magnetotransport Properties of the 2D Isotropic Metallic System with the Fermi Surface Reconstructed by the Charge Density Wave

We report the ground state stabilization and corresponding electrical transport and magnetotransport properties of a 2D metallic system with an isotropic Fermi surface reconstructed by a charge density wave. The onset of the charge density wave is a spontaneous process, stabilized by the condensation energy gain due to the self-consistent mechanism of topological reconstruction of the Fermi surface and opening of the pseudo-gap around it. We address the signature of the uni-axial reconstruction in terms of the measurable quantities, such as the intra-band transport properties, including the one-particle density of states, the total and effective concentration of electrons, and the Hall coefficient. Additionally, we analyze the magnetotransport properties of the system reconstructed by the bi-axial, checkerboard-like charge density wave, under conditions of magnetic breakdown. It manifests huge quantum oscillations in diagonal components of magnetoconductivity, while the Hall conductivity changes sign, varying the external magnetic field with a finite region of vanishing Hall coefficient in between.

Giant ac magnetoconductivity in rGO-Fe3O4 composites

In this work we report detailed investigations of structural, ac transport and magnetotransport properties of composites of reduced graphene oxide (rGO) and magnetite nanoparticles measured in the frequency range 10 Hz–2 MHz and in static magnetic fields upto 500 mT. AC conductivity of composite samples was enhanced by a factor of ~3 over that of the pure rGO even though the conductivity of magnetite is five orders of magnitude lower than that of rGO. We attribute this to charge transfer effects between the magnetite nanoparticles and the rGO matrix. Evidence of this is also observed in detailed Raman analysis of all samples. Further, we find a giant magnetoconductivity in the ac transport behavior of the composite samples. The rGO sample showed a negative magnetoconductivity of ~−15% at room temperature, while all composite samples showed positive magnetoconductivity with weak frequency dependence between 10 Hz and 254 kHz. We report a room temperature ac magnetoconductivity of 57% (at f = 254 kHz) and 40% (at f = 10 Hz) in a magnetic field B = 500 mT for the sample with 40 wt% magnetite nanoparticles. This is much larger than that obtained in dc magnetotransport, where room temperature magnetoconductivities in similar composites are typically 10% or less in comparable magnetic fields.

Soliton defects and topological -periodic superconductivity from an orbital magnetic field effect in edge Josephson junctions

Recently, much research has been dedicated to understanding topological superconductivity and Majorana zero modes induced by a magnetic field in hybrid proximity structures. This paper proposes a realization of topological superconductivity in a short Josephson junction at an edge of a 2D topological insulator subject to a perpendicular magnetic field. The magnetic field effect is entirely orbital, coming from a gradient of the order parameter phase at the edge, which results in a soliton defect at the junction with a pair of gapless Andreev bound states. The latter are reducible to Majorana zero modes by a unitary rotation and protected by a chiral symmetry. Furthermore, both ground state and excitations are quasiperiodic in the magnetic flux enclosed in the junction, with the period equal to the double flux quantum . This behaviour follows from the gauge invariance of the —phase periodicity of the Majorana states and manifests itself as —spaced magnetic oscillations of the critical current. Another proposed observable is a persistent current occurring in the absence of an external phase bias. Beside the oscillations, it shows a sign reversal prompted by the neutral Majorana zero modes. These findings offer the possibility to access topological superconductivity through low-field dc magnetotransport measurements.

Interesting magnetic properties observed in the 4f ferromagnet NdRu2

We have investigated dc magnetization, magneto-transport and magnetostriction of C15 Laves phase 4f ferromagnetic compound NdRu2 as a function of temperature and applied magnetic field. We find that the magnetic properties of this compound can not be explained within a simple model based on localized magnetic moment even though Nd based compounds and alloys are usually considered as localized magnetic moment systems. Our experimental results on temperature dependence of magnetization, electrical resistivity and magnetoresistance suggest the existence of spin fluctuations in the paramagnetic state of NdRu2. This compound shows unusually high and non-linear forced magnetostriction. We conjecture the hybridization of Nd 4f states with valence electrons as one of the possibilities for explaining the experimental results.

Characterization of High Mobility InAs/InGaAs/InAlAs Composite Channels by THz Magneto-Photoresponse Spectroscopy

Inserted narrow InAs quantum wells in InAs/InGaAs/InAlAs heterostructures have been used to achieve higher mobility for high-electron-mobility transistors (HEMTs) with ultra-low-power and low-noise amplification characteristics and for spin-based devices. Due to the large nonparabolicity of the conduction band of InAs and the penetration of the confined electronic envelope function into the adjacent layer(s), accurate calculations of effective mass and g-factor of charge carriers can be problematic. Methods of making precise determinations of the mass and other electronic parameters are thus of interest. We have applied magneto-photoresponse and -transmissions measurements at several THz laser frequencies in concert with dc magnetotransport measurements at low temperature (T = 1.6 K) to determine various electronic parameters (effective mass, carrier density, g-factor, mobility and the quantum scattering time) of the 2DEG in an InAs/In0.75Ga0.25As/In0.75Al0.25As inserted channel structure. This characterization method can also be used to probe the effect of strain, Rashba field, etc on the properties of charge carriers in such structures.

Bias-voltage-controlled ac and dc magnetotransport phenomena in hybrid structures

We report some ac and dc magnetotransport phenomena in silicon-based hybrid structures. The giant impedance change under an applied magnetic field has been experimentally found in the metal/insulator/semiconductor (MIS) diode with the Schottky barrier based on the Fe/SiO2/p-Si and Fe/SiO2/n-Si structures. The maximum effect is found to observe at temperatures of 10–30K in the frequency range 10Hz–1MHz. Below 1kHz the magnetoresistance can be controlled in a wide range by applying a bias to the device. A photoinduced dc magnetoresistance of over 104% has been found in the Fe/SiO2/p-Si back-to-back Schottky diode. The observed magnetic-field-dependent effects are caused by the interface states localized in the insula-tor/semiconductor interface.

Far-infrared and dc magnetotransport of CaMnO3-CaRuO3superlattices

We report temperature- and magnetic-field-dependent measurements of the dc resistivity and the far-infrared reflectivity (FIR) (photon energies $\ensuremath{\hbar}\ensuremath{\omega}=50--700$ cm${}^{\ensuremath{-}1}$) of superlattices comprising ten consecutive unit cells of the antiferromagnetic insulator CaMnO${}_{3}$, and four to ten unit cells of the correlated paramagnetic metal CaRuO${}_{3}$. Below the N\'eel temperature of CaMnO${}_{3}$, the dc resistivity exhibits a logarithmic divergence upon cooling, which is associated with a large negative, isotropic magnetoresistance. The $\ensuremath{\omega}\ensuremath{\rightarrow}0$ extrapolation of the resistivity extracted from the FIR reflectivity, on the other hand, shows a much weaker temperature and field dependence. We attribute this behavior to scattering of itinerant charge carriers in CaRuO${}_{3}$ from sparse, spatially isolated magnetic defects at the CaMnO${}_{3}$-CaRuO${}_{3}$ interfaces. This field-tunable ``transport bottleneck'' effect may prove useful for functional metal-oxide devices.

Single spin transport spectroscopy: Current blockade and spin decay

We present a theory of a single-electron transistor exchange-coupled to a localized spin. We show how to gain detailed quantitative knowledge about the attached spin such as spin size, exchange coupling strength, Landé g-factor, and spin decay time T1 by utilizing a robust blockade phenomenon of dc magnetotransport with accompanying noise enhancement. Our studies are of particular relevance to spin-resolved scanning single-electron transistor microscopy, electronic transport through nanomagnets, and the effect of hyperfine interaction on transport electrons by surrounding nuclear spins.

Complex ferromagnetic state transformation in single crystal [formula omitted

The magnetic properties of Nd 0.5 Pb 0.5 MnO 3 single crystal have been studied by dc magnetization and magnetotransport data. The ferromagnetic (FM) to paramagnetic (PM) transition temperature T C and the metal–insulator transition temperature are 177 and 194 K, respectively. The deviation of the inverse susceptibility from the Curie–Weiss law indicates the presence of a Griffiths-like phase in this system, viz., an inverse susceptibility characterized by χ - 1 ∝ ( T - T C Rand ) 1 - λ with λ = 0.81 and T C Rand = 183 K close to the T C 177 K. The slopes of H / M vs. M 2 isotherms are negative between 300 and 194 K and positive above 300 K and below 194 K. The transition cannot be regarded as first or second order on the basis of the Banerjee criterion. The above interesting and complex magnetic transition is a manifestation of competition between the double exchange mechanism and correlations arising from coupled spin and lattice degrees of freedom present in studied system. To analyze the magnetism around T C , the critical behavior of Nd 0.5 Pb 0.5 MnO 3 is investigated based on the data of static magnetization measurements. It is seen that the critical behavior of Nd 0.5 Pb 0.5 MnO 3 belongs to the mean-field model and shows a second-order like magnetic transition at high fields, however, the poor scaling is found toward low fields, which suggest that the critical behavior is related to the observed complicated magnetic transition. The possible origin of observed complex FM state has been discussed based on the competition between the charge/orbital order clusters and the A site disorder.

Phase separation versus spin glass behavior in La0.85Sr0.15CoO3

We present comprehensive studies of dc magnetization, ac susceptibility, and magnetotransport of two sets of La0.85Sr0.15CoO3 samples, one exhibits phase separation and the other exhibits spin glass behavior. Our study reveals that the phase separation in La0.85Sr0.15CoO3 is neither inherent nor ubiquitous; rather, it is a consequence of preparation condition. It is realized that the low temperature annealed sample exhibits phase separation while the high temperature annealed one shows the characteristic of spin glass behavior. This study shows that the most probable magnetic state of La0.85Sr0.15CoO3 is spin glass.

Novel Magnetoresistive Structures Using Self-Assembly and Nanowires on Si

AbstractAnodic Aluminum Oxide (AAO) was grown both as free-standing membranes and as integrated layers on Si as templates for arrays of magnetoresistive nanowires. These structures will be useful for applications such as current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) sensors, magnetic random access memory (MRAM) and microwave oscillator arrays. As the AAO was formed, using a two-step anodization process, columnar nanopores self-assembled inside the oxide to form a close-packed array. The pore diameters could be varied from 10-60nm by changing the anodization conditions. As the diameter of the AAO nanopores decreased, the distance between the nanopores also decreased. The free-standing membranes had pores with lengths of 17um. The pores that were grown directly onto Si with an adhesion layer of Ti were 600nm in length. In addition to growing these latter pores directly onto Si, they were also grown onto Co/Cu/Co thin films that were evaporated onto the Si. Au nanocontacts were electroplated into these nanopores to study point-contact magnetoresistance and microwave response. For the magnetoresistive nanostructures, multilayered Co/Cu nanowires were fabricated via electrochemical deposition. The samples were measured with vibrating sample magnetometry (VSM) and also using ac and dc magnetotransport systems. The highest magnetoresistance was found in nanowires that had hysteresis loops that were identical as measured in plane and perpendicular to the plane. The highest measured MR (Delta R/R = 11%) of the multilayers was calculated as 33% by subtracting the resistance of the Cu leads on either side of the multilayers from the denominator. Shorter wires are currently under construction to avoid this effect. Spin transfer torque (STT) was also measured in the samples. For 10-60nm diameter nanowires, the change in resistance due to STT was around 6% which represents the full magnetoresistance of the larger wires, but only half that of the smaller nanowires. It is therefore concluded that the 10-nm Co layers do not align antiparallel to parallel as fully at the switching current density of JAP-P = 2.7 × 108 A/cm2 compared to the larger wires which switch at JAP-P = 3.2 × 107 A/cm2. With diameters in the 10-60 nm range and integration with Si, these nanostructures have great potential for future nanosensors, MRAM and microwave oscillator arrays.

Parametric resonance of a two-dimensional electron gas under bichromatic irradiation

In an ultrahigh mobility 2D electron gas, even a weak nonparabolicity of the electron dispersion, by violating Kohn's theorem, can have a drastic effect on dc magnetotransport under ac drive. We study theoretically the manifestation of this effect in the dc response to the combined action of two driving ac-fields (bichromatic irradiation). Compared to the case of monochromatic irradiation, which is currently intensively studied both experimentally and theoretically, the presence of a second microwave source provides additional insight into the properties of an ac-driven 2D electron gas. In particular, we find that nonparabolicity, being the simplest cause for a violation of Kohn's theorem, gives rise to new qualitative effects specific to bichromatic irradiation. Namely, when the frequencies $\omega_1$ and $\omega_2$ are well away from the cyclotron frequency, $\omega_c$, our simple classical considerations demonstrate that the system becomes parametrically unstable with respect to fluctuations with frequency $(\omega_1+\omega_2)/2$. As an additional effect of nonparabolicity, this parametric instability can manifest itself in the dc properties of the system. This happens when $\omega_1$, $\omega_2$ and $\omega_c$ are related as 3:1:2, respectively. Even for weak detuning between $\omega_1$ and $\omega_2$, the effect of the bichromatic irradiation on the dc response in the presence of nonparabolicity can differ dramatically from the monochromatic case. In particular, the equations of motion can acquire multistable solutions. As a result, the diagonal dc-conductivity can assume several stable negative values at the same magnetic field.

Magnetic and Electronic Properties of Eu0.8Sr0.2CoO3

Results of dc magnetization, ac susceptibility and magnetotransport measurements on Eu0.8Sr0.2CoO3 are presented. The compound shows glassy ferromagnetic character. The compound is semiconducting-metallic with negative magneto-resistance.

THEORY OF THE DC MAGNETOTRANSPORT IN LATERALLY MODULATED QUANTUM HALL SYSTEMS NEAR FILLING ν=½

A quasiclassical theory for DC magnetotransport in a modulated quantum Hall system near filling factor ν=½ is presented. A weak one-dimensional electrostatic potential acts on the two-dimensional electron gas. Closed form analytic expressions are obtained for the resistivity ρ⊥ corresponding to a current at right angles to the direction of the modulation lines as well as a smaller component ρ‖ for a current along the direction of the modulation lines. It is shown that both resistivity components are affected by the presence of the modulation. Numerical results are presented for ρ⊥ and ρ‖ and show reasonable agreement with the results of recent experiments.

Dynamics of disordered quantum Hall crystals

Charge density waves are thought to be common in two-dimensional electron systems in quantizing magnetic fields. Such phases are formed by the quasiparticles of the topmost occupied Landau level when it is partially filled. One class of charge density wave phases can be described as electron solids. In weak magnetic fields (at high Landau levels) solids with many particles per unit cell—bubble phases—predominate. In strong magnetic fields (at the lowest Landau level) only crystals with one particle per unit cell—Wigner crystals—can form. Experimental identification of these phases is facilitated by the fact that even a weak disorder influences their DC and AC magnetotransport in a very specific way. In the AC domain, a range of frequencies appears where the electromagnetic response is dominated by magnetophonon collective modes. The effect of disorder is to localize the collective modes and to create an inhomogeneously broadened absorption line, the pinning mode. In recent microwave experiments pinning modes have been discovered both at the lowest and at high Landau levels. We present the theory of the pinning mode for a classical two-dimensional electron crystal collectively pinned by weak impurities. We show that long-range Coulomb interaction causes a dramatic line narrowing, in qualitative agreement with the experiments.

Magnetotransport in a modulated two-dimensional electron gas

We propose a semiclassical theory of dc magnetotransport in a two-dimensional electron gas modulated along one direction with weak electrostatic modulations. We show that oscillations of the magnetoresistivity ρ∥ corresponding to the current driven along the modulation lines observed at moderately low magnetic fields can be explained as commensurability oscillations.

Glassy ferromagnetism and magnetic phase separation inLa1−xSrxCoO3

We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and magnetotransport properties of the glassy ferromagnet ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}.$ The compositions studied span the range from the end-member ${\mathrm{LaCoO}}_{3}$ $(x=0.0)$ through to $x=0.7.$ These materials have attracted attention recently, primarily due to the spin-state transition phenomena in ${\mathrm{LaCoO}}_{3}$ and the unusual nature of the magnetic ground state for finite x. In this paper we present a consistent picture of the magnetic behavior of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ in terms of short-range ferromagnetic ordering and intrinsic phase separation. At high Sr doping $(xg0.2)$ the system exhibits unconventional ferromagnetism (with a Curie temperature up to 250 K), which is interpreted in terms of the coalescence of short-range-ordered ferromagnetic clusters. Brillouin function fits to the temperature dependence of the magnetization as well as high-temperature Curie-Weiss behavior suggest that the ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions are both in the intermediate spin state. At lower Sr doping $(xl0.18)$ the system enters a mixed phase that displays the characteristics of both a spin glass and a ferromagnet. A cusp in the zero-field-cooled dc magnetization, a frequency-dependent peak in the ac susceptibility and time-dependent effects in both dc and ac magnetic properties all point towards glassy behavior. On the other hand, field cooling results in a relatively large ferromagneticlike moment, with zero-field-cooled and field-cooled magnetizations bifurcating at an irreversibility point. Even in the region above $x=0.2$ the out-of-phase component of the ac susceptibility shows frequency-dependent peaks below the Curie temperature (indicative of glassy behavior) which have previously been interpreted in terms of the freezing of clusters. All of the results are consistent with the existence of a strong tendency towards magnetic phase separation in this material, a conclusion which is further reinforced by consideration of the electronic properties. The metal-insulator transition is observed to be coincident with the onset of ferromagnetic ordering $(x=0.18)$ and has a behavior in the doping dependence of the low-temperature conductivity which is strongly suggestive of percolation. This can be interpreted as a percolation transition within the simple ferromagnetic cluster model. On the metallic side of the transition the system exhibits colossal magnetoresistance-type behavior with a peak in the negative magnetoresistance (\ensuremath{\sim}10% in 90 kOe) in the vicinity of the Curie temperature. As the transition is approached from the metallic side we observe the onset of a negative magnetoresistance that increases in magnitude with decreasing temperature, reaching values as large as 90% in a 90-kOe field. This magnetoresistance is enhanced at the metal-insulator transition, where it persists even to room temperature.

The effect of the cross-gap alignment on magneto-transport in short period InAs/GaSb superlattices

We investigate dc and pulsed magneto-transport in InAs/GaSb superlattices, at low temperatures. High electric and magnetic fields up to 14.5T are applied parallel to the superlattice growth direction. In the low electric and magnetic field regimes we observe a temperature dependent threshold to conduction. At higher biases the superlattice exhibits a large magneto-resistance and in the quantum limit we observe conduction peaks, which satisfy the Stark-cyclotron-resonance condition. We have performed eight band k·p calculations, which show that the miniband width in this system is strongly k dependent.

On the theory of a new maximum observed in dc transport in modulated quantum hall systems near ν=1/2

We propose a theory for the new maximum recently observed by Willett et al. [Phys. Rev. Lett., 83 (1999) 2624] in the longitudinal magnetoresistance of a weakly modulated two-dimensional electron gas (2DEG) near filling factor ν=1/2 for the current driven along the modulation lines. The maximum is superimposed upon a new resonance structure. This occurs due to the geometric resonance of composite fermion cyclotron orbits with the period of modulation of the effective magnetic field. We propose here a semiquantitative theory of the dc magnetotransport in a modulated 2DEG near one half filling. Our analysis is based on the Boltzmann transport equation and it enables us to obtain this maximum which was neither observed nor predicted previously.

Transport properties of molecular conductor (DMe-DCNQI) 2Cu and its deuterated crystals

Pulsed-laser induced transient thermoelectric effect and dc magnetotransport have been measured for molecular conductors (DMe-DCNQI) 2Cu( h salt) and its deuterated crystals ( d 7 salt) over the temperature range 4.2–300 K. These dynamic and static data are reasonably explained by a two-carrier model of the three-dimensional hole bands located around the Λ and P points in the Brillouin zone at low temperatures, together with a contribution from the one-dimensional electron band at high temperatures (dimensional crossover at around 150 K). Discussions are given for the effect of deuteration on the band structure and metal-insulator transition.