Anomalous Magnetoresistance Research Articles

Giant anisotropic magnetoresistance in bilayered La1.2Sr1.8Mn2O7 single crystals

We report an observation of anomalous anisotropic magnetoresistance (AMR) in bilayered La1.2Sr1.8Mn2O7 single crystals. A giant AMR is found to be 80% under a magnetic field of 1 T near the metal-insulator transition temperature, where AMR is defined as AMR=[ρ(H⊥c)−ρ(H∥c)]/ρ(H⊥c)×100%, and ρ(H⊥c) and ρ(H∥c) are the resistivity with the magnetic field perpendicular and parallel to c-axis, respectively. The AMR effect shows strong temperature and magnetic field dependences, and indicates a close interrelation with the anisotropic field-tuned metal-insulator transition.

LINEAR CONDUCTANCE AND TUNNEL MAGNETORESISTANCE IN A RASHBA QUANTUM DOT RING COUPLED TO FERROMAGNETIC LEADS

Spin-dependent electronic transport through a quantum ring with a quantum dot (QD) inserted in one of its arms is investigated within the Keldysh Green's function theory. We consider that the ring is connected to external ferromagnetic electrodes and there is Rashba spin-orbit interaction (SOI) in the QD. It is found that the anomalous tunnel magnetoresistance (TMR) peak originated from the electron correlations is split into two by the direct coupling between the two leads, with the two normal Coulomb resonances unchanged. The peak position and the magnitude of the conductance and the TMR can be controlled in terms of the SOI-induced phase factor, which can be adjusted by the gate voltage or the structure parameters. The present device should be realizable within present technology and may have practical usage in spintronics.

Electro- and magneto-transport properties of amorphous carbon films doped with iron

Electro- and magneto-transport properties of amorphous carbon films doped with iron element have been systematically studied. The electro-transport mechanism of the films is dominated by thermal activation at T>200K, Mott-type variable range hopping (VRH) at 200K>T>60K and Efros–Shklovskii type (ES-) VRH at T<60K. An anomalous giant positive magnetoresistance (MR) 6.40% is found at the ES–VRH range, which is attributed to the spin blockage effect. At high temperatures, an anomalous Hall effect (AHE) is also found with a large AHE coefficiency 49.6μΩcm/T. Electron energy loss spectroscopy (EELS) reveals that iron atoms chemically bond with carbon matrix. These iron carbides exist as amorphous nanoparticles with a diameter of 6–12nm, which is regarded as the origin of the MR and AHE. Besides, the films are p-type conductive at high temperature, which might be related with the iron doping. These properties make iron doped amorphous carbon films applicable in carbon-based solar cells, magnetic sensors or some other multifunctional devices.

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.

Anomalous butterfly-shaped magnetoresistance loops in the alloyTb4LuSi3

Magnetic-field (H) induced first-order magnetic transition and the assiciated electronic phase-separation phenomena are active topics of research in magnetism. Magnetoresistance (MR) is a key property to probe these phenomena and, in literature, a butterfly-shaped MR loop has been noted while cycling the field, with the envelope curve lying below the virgin curve in MR versus H plots of such materials. Here, we report an opposite behavior of MR loop for an alloy, Tb4LuSi3, at low temperatures (<<20 K) in the magnetically ordered state. Such an anomalous curve reveals unexpected domination of higher resistive high-field phase in electronic conduction, unlike in other materials where conducion is naturally by low-resistive high-field phase that follows first-order transition. The observed features reveal an unusual electronic phase separation, namely involving high-resistive high-field phase and low-resistive virgin phase.

Spin-orbit coupling and anomalous angular-dependent magnetoresistance in the quantum transport regime of PbS

We measured magnetotransport properties of PbS single crystals which exhibit the quantum linear magnetoresistance (MR) as well as the static skin effect that creates a surface layer of additional conductivity. The Shubnikov-de Haas oscillations in the longitudinal MR signify the peculiar role of spin-orbit coupling. In the angular-dependent MR, sharp peaks are observed when the magnetic field is slightly inclined from the longitudinal configuration, which is totally unexpected for a system with nearly spherical Fermi surface and points to an intricate interplay between the spin-orbit coupling and the conducting surface layer in the quantum transport regime.

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.

Anomalous magnetoresistance of a two-dimensional ferromagnet/ferromagnet junction on the surface of a topological insulator

We investigate charge transport in two-dimensional ferromagnet/ferromagnet junction on a topological insulator. The conductance across the interface depends sensitively on the directions of the magnetizations of the two ferromagnets, showing anomalous behaviors compared with the conventional spin valve. This stems from the way how the wave functions connect between both sides. It is found that the conductance depends strongly on the in-plane direction of the magnetization. Moreover, in sharp contrast to the conventional magnetoresistance effect, the conductance at the parallel configuration can be much smaller than that at the antiparallel configuration.

Angle-dependent magnetoresistance oscillations and magnetic breakdown in [formula omitted

We present results of stereographic measurements of interlayer magnetoresistance in the quasi-two-dimensional (Q2D) organic conductor, α ‐ ( BEDT ‐ TTF ) 2 KHg ( SCN ) 4 , at around charge density wave (CDW) transition temperature T c ∼ 8 K . At 2 K in the CDW phase, there exists the anomalous Lebed resonance pattern, the amplitude of which is modulated by Danner–Chaikin oscillations in case of applied magnetic field close to the Q2D conducting plane. On the contrary, at 7 K just below T c , we have found that the Kajita oscillation of the cylindrical Fermi surfaces appears superposed on the anomalous Lebed resonance. A quantum model of interlayer tunneling demonstrates these behaviors qualitatively. These results suggest that magnetic breakdown plays a key role in anomalous angle-dependent magnetoresistance oscillations.

Two-dimensional electron gas in a lattice of antidots with a period of 80 nm

The magnetotransport in a two-dimensional electron gas with a lattice of antidots, which has a record-breaking small (80 nm) period and size (20–40 nm) of antidots comparable with the de Broglie wavelength of electrons, has been experimentally studied. A wide variety of new features of the magnetoresistance behavior has been observed both under semiclassical conditions and in the regime of quantizing magnetic fields. In particular, the anomalous semiclassical magnetoresistance peak induced by the nonmonotonic scattering effects has been revealed. The Shubnikov-de Haas oscillations have been revealed to exhibit an unusual transition from the anomalous period constant in the magnetic field to the normal constant in the inverse magnetic field. The effect of the generation and suppression of the oscillations has also been observed; this effect is induced by the transformation of the short and long-range scattering potentials in the lattice owing to the variation of the density of the two-dimensional electrons.

Study of Nanostructured Array of Antidots Using Pulsed Magnetic Fields

The absence of the superparamagnetic limit in nanostructured antidots makes them strong candidates for ultra-high density recording media. In this work, nanoporous alumina templates (NpATs), with average pore diameters ∼35 nm and separation ∼100 nm, were grown using a two-step anodization method. A Ni80Fe20 thin film of 6.5 nm was then sputtered on top of such NpATs, building an antidot network. A detailed study of the magnetoresistance (MR) (fields up to 25 T and temperatures down to 77 K) was performed. The antidot network sample revealed an anomalous MR and R(T) behaviour at TM∼250 K arising from a spin-flip transition occurring in a thin iron oxide layer.

Influence of pressure on the magnetic behavior and the anomalous magnetoresistance inTb5Si3

The compound, Tb5Si3, crystallizing in Mn5Si3-type hexagonal structure, was recently reported by us to exhibit a sudden and huge enhancement in electrical resistivity (rho) at a critical magnetic field (H_cr) in the magnetically ordered state (<70 K) tracking isothermal magnetization (M) behavior. We have investigated the influence of external pressure (<15 kbar) and negative chemical pressure induced by Ge substitution for Si on M and rho as a function of temperature (5-300 K) and magnetic field (<120 kOe), with the primary aim of understanding the field-induced anomalies. Focussing on isothermal M and magnetoresistance (MR) at two temperatures, 5 and 20K, we find that this rho anomaly persists under external as well as negative chemical pressures, however with a large change in the H_cr. The pressure-derivative of H_cr is negative and this trend and the MR behavior at the H_cr are comparable to that observed in some Laves phase itinerant magnetic systems. On the basis of this observation, we speculate that the magnetic fluctuations induced at this critical field could be responsible for the MR anomal.ies

THEORY OF MAGNETO RESISTANCE OF AMORPHOUS SEMICONDUCTORS

We propose a simple theory to explain the anomalous magneto resistance of amorphous semiconductors in the hopping regime. We derive an expression for the magneto resistance (Δρ/ρ0), in this regime by using a reinterpretation of the effective electronic mass, velocity, and lifetime based on the density matrix approach and a three-site jump probability in topologically disordered systems. Our theory accounts for the experimental observation of (Δρ/ρ0) for low magnetic fields at room temperature and below and supports Mott's T-1/4 for the variable range hopping of electrical conductivity.

Magnetic impurity behavior in the type II broken-gap p-GaInAsSb/p-InAs:Mn heterostructures with 2D-semimetal channel at the interface

Abstract Magnetic impurity behavior have been studied in type II p-GaInAsSb/p-InAs heterostructures grown by liquid-phase epitaxy on InAs substrate heavily doped with manganese ( p =(5−7)×10 18 cm −3 ). Ferromagnetic properties of these structures become apparent during observation of anomalous Hall effect and negative magnetoresistance (Δ ρ ⊥ /ρ ) in low magnetic fields H =3–20 kOe at T =77–200 K. High value of negative magnetoresistance up to 30% and its quadratic dependence on a magnetic field were found. Effective magnetic moment M *=200 μ B and Curie temperature T C =200 K were evaluated. Contribution of Mn δ-layer placed on GaInAsSb epilayer was manifested in vertical quantum magnetotransport in high magnetic fields ( B >8 T) and low temperatures (1.5–4.2 K). Presence of the narrow layer of Mn can control over spin-oriented tunneling of electrons from a semimetal channel across the type II broken-gap heterointerface.

Anomalously large anisotropic magnetoresistance in a perovskite manganite.

The signature of correlated electron materials (CEMs) is the coupling between spin, charge, orbital and lattice resulting in exotic functionality. This complexity is directly responsible for their tunability. We demonstrate here that the broken symmetry, through cubic to orthorhombic distortion in the lattice structure in a prototype manganite single crystal, La(0.69)Ca(0.31)MnO(3), leads to an anisotropic magneto-elastic response to an external field, and consequently to remarkable magneto-transport behavior. An anomalous anisotropic magnetoresistance (AMR) effect occurs close to the metal-insulator transition (MIT) in the system, showing a direct correlation with the anisotropic field-tuned MIT in the system and can be understood by means of a simple phenomenological model. A small crystalline anisotropy stimulates a "colossal" AMR near the MIT phase boundary of the system, thus revealing the intimate interplay between magneto- and electronic-crystalline couplings.

Magnetoresistance studies on RPd2Si (R = Tb, Dy, Lu) compounds

Magnetoresistance studies on RPd2Si (R = Tb, Dy and Lu) compounds show large negative magnetoresistance (MR) inTbPd2Si andDyPd2Si near the magnetic ordering temperature. Positive MR at low temperature in theferromagnetic Dy compound is shown to arise from the orbital contribution (theLorentz force effect). As a consequence, a deviation from the linear relation betweenMR and isothermal entropy change (i.e. the magnetocaloric effect) is observed.In the case of the Tb compound, anomalous magnetoresistance behavior isobserved at 3 K, where the resistivity is found to be different before and aftermagnetic field cycling. These results suggest complex magnetic behavior inTbPd2Si.

Spin–orbit interaction and weak localization in heterostructures

Theory of weak localization in two-dimensional high-mobility semiconductor systems is developed with allowance for the spin–orbit interaction. The obtained expressions for anomalous magnetoresistance are valid in the whole range of classically weak magnetic fields and for arbitrary strengths of bulk and structural inversion asymmetry contributions to the spin splitting. The theory serves for both diffusive and ballistic regimes of electron propagation taking into account coherent backscattering and nonbackscattering processes. The transition between weak localization and antilocalization regimes is analyzed. The manifestation of the mutual compensation of Rashba and Dresselhaus spin splittings in magnetoresistance is discussed. A perfect description of experimental data on anomalous magnetoresistance in high-mobility heterostructures is demonstrated. The in-plane magnetic-field dependence of the conductivity caused by an interplay of the spin–orbit splittings and Zeeman effect is described theoretically.

Organic spin valves: effect of magnetic impurities on the spin transport properties of polymer spacers

We report the effect of magnetic impurities in the spacer layer of polymeric spin valves (PSV) with the sandwich configuration of La0.67Sr0.33MnO3(LSMO)/π-conjugated polymer regio-random poly(3-hexyl thiophene)/cobalt (Co), showing giant magnetoresistance (GMR) response. Different deposition rates of Co at the top electrode resulted in two types of devices: one with lower device resistance and linear current–voltage (I–V) characteristics and the other with very low inclusion of Co and exhibiting higher device resistance and nonlinear I–V characteristics. We observed an asymmetric dc bias dependence of magnetoresistance (MR) in devices with more Co inclusion, while for the other type of device, bias dependence was more symmetric. At higher bias, %MR of both types of device showed no significant difference (5–10%), but at low dc bias it ranged between 50 and 160% MR. This can be attributed to the higher tunneling probability of spin-polarized carriers from one ferromagnetic electrode to the other. Magnetic tunnel junction-like features are observed in the devices with greater Co inclusions. Anomalous MR peaks were also observed in these devices and their origin was explained in terms of presence of additional scattering centers around the included metal ions and increased spin relaxation due to high magnetic anisotropy in the system. Both types of PSVs showed a monotonic decrease in MR with temperature at high bias currents.

Exotic properties of zerogap state in α-(BEDT-TTF)2I3

We investigate effects of a non-magnetic impurity on zerogap state of molecular conductor α-(BEDT-TTF)2I3 and predict theoretically incommensurate oscillations of charge density. These oscillations reflect unique properties of the massless Dirac fermions, which are characterized by the incommensurate momenta ±κo of the contact points and the singular momentum distributions of the charge density. We examine an effective Hamiltonian in the presence of stripe charge ordering, neighboring the zerogap state. The energy bands are divided into two parts, having one-dimensional fermions and two-dimensional fermions. The one-dimensional-like bands exhibit a unique anisotropic dispersion which is linear along b-axis (parallel to the stripe) and parabolic along the a-axis (the stacking axis). Thus we expect anomalous magnetoresistance in stripe charge ordering state, since such a anisotropic dispersion indicates the unconventional Landau quantization.