Electron transport in manganite bicrystal junctions

The transport and magnetic properties of junctions created in La_0.67Sr_0.33MnO₃ thin films epitaxially grown on substrates with a bicrystal boundary have been investigated. In tilted neodymium gallate bicrystal substrates, the NdGaO₃(110) planes are inclined at angles of 12° and 38°. The temperature dependences of the electrical resistance, magnetoresistance, and differential conductance of the junctions at different voltages have been measured and analyzed. It has been found that the magnetoresistance and electrical resistance of the junction significantly increase with an increase in the misorientation angle, even though the misorientation of the easy magnetization axes remains nearly unchanged. The ratio of the spin-dependent and spin-independent contributions to the conductance of the bicrystal junction increases by almost an order of magnitude with an increase in the misorientation angle from 12° to 38°. The magnetoresistance of the junction increases with decreasing temperature, which is most likely associated with an increase of the magnetic polarization of the electrons. It has been shown that, at low (liquid-helium) temperatures, the conductance depends on the voltage V according to the law V ^1/2, which indicates the dominant contribution from the electron-electron interaction to the electrical resistance of the junction. An increase in the temperature leads to a decrease in this contribution and an increase in the contribution proportional to V ^3/2, which is characteristic of the mechanism involving inelastic spin scattering by surface antiferromagnetic magnons.     

Charge transport studies on chemically grown manganite based heterostructures

In this communication, we have successfully fabricated mixed valent La_0.7Ca_0.3MnO₃ (LCMO) manganite based (i) ZnO/LCMO/LAO and (ii) LMO/LCMO/LAO (LMO: LaMnO_3–d thin layer; LAO: LaAlO₃ substrate) thin film heterostructures using chemical solution deposition (CSD) method. 100 nm LCMO layer was initially grown on single crystalline (100) LAO substrate followed by the growth of 50 nm ZnO and LMO layers separately on the two different heterostructures. In the present study, upper layers of ZnO and LMO were intentionally prepared at 700 °C for 12 h under air environment, thereby some naturally created oxygen vacancies are expected to be present in their lattices. Presence of oxygen vacancies makes ZnO and LMO layers as n–type oxides in the heterostructures. Temperature dependent current–voltage (I–V) characteristics and interface resistivity (under different applied electric fields across interface only) were carried out to understand their charge transport behavior. A strong effect of electric field on the resistivity behavior has been observed due to a reasonable electrically polarizable (active) nature of ZnO and LMO thin layers. Zener double exchange (ZDE) polynomial law has been employed to understand various scattering processes as source of resistivity across, both, ZnO/LCMO and LMO/LCMO interfaces. Transport properties and charge conduction mechanisms have been discussed and compared for both the interfaces in the context of interface state and barrier between electrically active layer and LCMO film. Also, power consumption criteria have been discussed in detail for the presently studied heterostructures for their practical device applications such as field effect devices, memory devices, read–write head devices or any other spintronic devices.     

Cuprate/manganite heterostructures

Advances in thin film deposition techniques have made it possible to produce heterostructures of high-temperature superconducting compounds and manganite perovskites. The latter exhibit the phenomenon of colossal magnetoresistance (CMR). The half-metallic character of the CMR compounds results in their carriers being spin polarized. Experiments with these heterostructures have demonstrated that the injection of spin-polarized carriers into high-temperature superconductors results in a reduction of parameters such as the critical current and critical temperature. The experimental situation is reviewed, and open questions are identified.     

Electron transport in interacting hybrid mesoscopic systems

A unified theory for the current through a mesoscopic region of interacting electrons connected to two leads which can be either ferromagnet or superconductor is presented, yielding Meir-Wingreen-type formulas when applied to specific circumstances. In such a formulation, the requirement of gauge invariance is satisfied automatically. Moreover, one can judge unambiguously what quantities can be measured in the transport experiment. Received 22 August 2002 / Received in final form 14 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: phyzengz@nus.edu.sg     

Theory of half-metal/superconductor heterostructures

We investigate the Josephson coupling between two singlet superconductors separated by a half-metallic magnet. The mechanism behind the coupling is provided by the rotation of the quasiparticle spin in the superconductor during reflection events at the interface with the half metal. Spin rotation induces triplet correlations in the superconductor which, in the presence of surface spin-flip scattering, results in an indirect Josephson effect between the superconductors. We present a theory appropriate for studying this phenomenon and calculate physical properties for a superconductor/half-metal/superconductor heterostructure.     

Thermal valve of superconductor heterostructures with different pairing types

Electron heat transport across the interface between a two-band superconductor with the inter- and intra- band pairing types and a normal metal, and a ferromagnet formed by domains with different magnetization directions is analyzed. The possibility of controlling the heat flow by varying the magnetization direction of one of the ferromagnetic domains is considered. The microrefrigerating applications of these structures are discussed.     

Review of photoinduced effect in manganite films and their heterostructures

Light–matter interaction plays an important role in the non-equilibrium physics, especially in strongly correlated electron systems with complex phases. Photoinduced effect can cause the variation in the physical properties and produce some emergent phases. As a classical archetype, manganites have received much attention due to their colossal magnetoresistance(CMR) effect and the strong interaction of charge, spin, orbital, and lattice degrees of freedom. In this paper, we give an overview of photoinduced effect in manganites and their heterostructures. In particular, some materials, including ZnO, Si,BiFeO_3(BFO), titanate-based oxides, and 0.7 Pb(Mg_(1/3) Nb_(2/3))O_3-0.3 PbTiO_3(PMN-PT) have been integrated with manganites. Heterostructures composed of these materials display some exciting and intriguing properties. We do hope that this review offers a guiding idea and more meaningful physical phenomena will be discovered in active areas of solid state physics and materials science.     

Current transport along the [001] axis of YBCO in low-temperature superconductor-normal metal-high-temperature superconductor heterostructures

The electrophysical properties of heterojunctions several microns in size, obtained by successive deposition of the metal-oxide high-temperature superconductor YBa₂Cu₃O_x, a normal metal Au, and the low-temperature superconductor Nb, were studied experimentally. Current flows in the [001] direction of the epitaxial YBa₂Cu₃O_x film. It is shown, by comparing the experimental data with existing theoretical calculations, that for the experimentally realizable transmittances (D̄=10⁻⁵–10⁻⁶) of the YBa₂Cu₃O_x—normal metal boundary the critical current of the entire heterostructure is low (of the order of the fluctuation current) because of a sharp change in the amplitude of the potential of the superconducting carriers at this boundary. The current-voltage characteristics of the heterostructure studied correspond to tunnel junctions consisting of a superconductor with type symmetry of the superconducting wave function and a normal metal. Zh. éksp. Teor. Fiz. 116, 2140–2149 (December 1999)     

Odd triplet pairing in clean superconductor/ferromagnet heterostructures

We study triplet pairing correlations in clean ferromagnet (F)/superconductor (S) nanojunctions, via fully self-consistent solution of the Bogoliubov-de Gennes equations. We consider FSF trilayers, with S being an s-wave superconductor, and an arbitrary angle alpha between the magnetizations of the two F layers. We find that contrary to some previous expectations, triplet correlations, odd in time, are induced in both the S and F layers in the clean limit. We investigate their behavior as a function of time, position, and alpha. The triplet amplitudes are largest at times on the order of the inverse Debye frequency, and at that time scale they are long-ranged in both S and F. The zero temperature condensation energy is found to be lowest when the magnetizations are antiparallel.     

Electron transport in a mesoscopic superconducting / ferromagnetic hybrid conductor

We present electrical transport experiments performed on submicron hybrid devices made of a ferromagnetic conductor (Co) and a superconducting (Al) electrode. The sample was patterned in order to separate the contributions of the Co conductor and of the Co-Al interface. We observed a strong influence of the Al electrode superconductivity on the resistance of the Co conductor. This effect is large only when the interface is highly transparent. We characterized the dependence of the observed resistance decrease on temperature, bias current and magnetic field. As the differential resistance of the ferromagnet exhibits a non-trivial asymmetry, we claim that the magnetic domain structure plays an important role in the electron transport properties of superconducting / ferromagnetic conductors. Received 9 July 2002 / Received in final form 22 October 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: herve.courtois@grenoble.cnrs.fr RID="b" ID="b"associated to Université Joseph Fourier     

Electron transport across the interface of a normal metal and a two-band superconductor with interband pairing

A consistent scheme taking into account both intra- and interband pairings has been proposed for diagonalizing the Hamiltonian of a superconductor. The temperature dependence of the order parameter of the superconductor with interband pairing has been calculated. The electron transport across the interface of a normal metal and the two-band superconductor with interband pairing has been analyzed.     

Spin-dependent electron transport in manganite bicrystal junctions

Magnetic bicrystal films and junctions of magnetic La_0.67Sr_0.33MnO₃ (LSMO) and La_0.67Ca_0.33MnO₃ (LCMO) films epitaxially grown on NdGaO₃ substrates with the (110) planes of their two parts misoriented (tilted) at angles of 12°, 22°, 28°, and 38° are investigated. For comparison, bicrystal boundaries with a 90° misorientation of the axes of the NdGaO₃ (110) planes were fabricated. The directions of the axes and the magnetic anisotropy constants of the films on both sides of the boundary are determined by two independent techniques of magnetic resonance spectroscopy. The magnetic misorientation of the axes in the substrate plane has been found to be much smaller than the crystallographic misorientation for tilted bicrystal boundaries, while the crystallographic and magnetic misorientation angles coincide for boundaries with rotation of the axes. An increase in the magnetoresistance and characteristic resistance of bicrystal junctions with increasing misorientation angle was observed experimentally. The magnetoresistance of bicrystal junctions has been calculated by taking into account the uniaxial anisotropy, which has allowed the contributions from the tunneling and anisotropic magnetoresistances to be separated. The largest tunneling magnetoresistance was observed on LCMO bicrystal junctions, in which the characteristic resistance of the boundary is higher than that in LSMO boundaries.     

Interfaces in superconducting hybrid heterostructures with an antiferromagnetic interlayer

The structural, X-ray diffraction, and electrophysical studies of hybrid superconducting hetero-structures with an interlayer of cuprate antiferromagnetic Ca_{1 ? x} Sr _x CuO₂ (CSCO) with the upper electrode Nb/Au and the lower electrode Y_Ba₂Cu₃O_{7 ? δ} (YBCO) have been carried out. It has been experimentally shown that the epitaxial growth of two cuprates, YBCO and CSCO, results in the formation of an interface on which the enrichment of the CSCO interlayer with charge carriers proceeds to a depth of about 20 nm. In this case, the conduction of the enriched CSCO region proves to be closer to metallic, whereas the CSCO film deposited onto the NdGaO₃ substrate is a Mott insulator with hopping conduction.     

Interband resonant tunneling in superconductor heterostructures in a quantizing magnetic field

The resonant tunneling of electrons through quasistationary levels in the valence band of a quantum well in double-barrier structures based on III–V materials with type-II heterojunctions is considered in a quantizing magnetic field directed perpendicularly to the interfaces. The transmission coefficients of the tunnel structure for transitions from states corresponding to different Landau levels are calculated using the Kane model. It is shown that transitions with a unit change in the Landau level index n as a result of mixing of the wave functions of states with opposite spin orientations are possible on the interfaces due to spin-orbit coupling. The probability of such transitions can be comparable to the probability of transitions without a change in the Landau level index for InAs/AlGaSb/GaSb resonant-tunneling structures. Fiz. Tverd. Tela (St. Petersburg) 40, 2121–2126 (November 1998)     

Substrate dependent transport and magnetotransport in manganite multilayer

We report magnetotransport properties of La_0.5Pr_0.2Sr_0.3MnO₃ (LPSMO) {5 layers}/La_0.5Pr_0.2Ba_0.3MnO₃ (LPBMO) {4 layers} manganite multilayers grown on single crystalline STO (h 0 0) and NGO (h 0 0) substrates using pulsed laser deposition (PLD) technique. An appreciable magnetoresistance (MR) ∼56% (80 kOe field) at room temperature (RT) is exhibited by the heterostructure deposited on STO substrate having field coefficient of resistance (FCR) ∼35% (100 Oe) while the multilayer deposited on NGO substrate exhibits MR∼61% (80 kOe field) and FCR∼18% (5 kOe) at RT. The observed values of temperature coefficient of resistance (TCR) are ∼3.15% and 2.77% at RT for the multilayers grown on STO and NGO substrates, respectively. A comparison of the field sensitivity of the multilayered structure studied with those reported for LPSMO/Al₂O₃/LPSMO (FCR∼20% at 220 K) and LPSMO/STO film (FCR∼13% at 250 K) shows that the multilayer exhibits higher field sensitivity, which can be attributed to the improved LPSMO-LPBMO interfaces. In this communication, the results of the structural, transport and magnetotransport studies on LPSMO/LPBMO multilayered systems have been discussed.     

Josephson effect in hybrid oxide heterostructures with an antiferromagnetic layer

Josephson coupling between an s- and d-wave superconductor through a 50 nm thick Ca1-xSrxCuO2 antiferromagnetic layer was observed for the hybrid Nb/Au/Ca1-xSrxCuO2/YBa2Cu3O7-delta heterostructures and investigated as a function of temperature, magnetic field, and applied millimeter-wave electromagnetic radiation. The magnetic field dependence of the supercurrent I(c)(H) exhibits anomalously rapid oscillations, which is the first experimental evidence of the theoretically predicted giant magneto-oscillations in Josephson junctions with antiferromagnetic interlayers.     

Probing charge transport in manganite film through switching parameters

We have investigated the bipolar resistive switching of Y_0.95Ca_0.05MnO₃ (YCMO) thin film on Si substrate using pulsed laser deposition. Simulation of Mn L_3,2 near-edge X-ray absorption fine structure has been executed by CTM4XAS to corroborate the presence of a mixed-valence state of Mn ions and oxygen vacancies. The charge transport in the film is described by the space charge limited mechanism. Murgatroyd and space charge limited mechanism relations are used to calculate the mobility and other switching parameters at high resistance state. With a decrease in the switching layer (near to positively biased electrode) thickness, better resistive switching was observed. This work indicates that the localized switching thickness and temperature strongly affect the resistive switching of the YCMO film.