Single Crystalline La0 Research Articles

Emergence of superconductivity in single-crystalline LaFeAsO under simultaneous Sm and P substitution

We report on the high-pressure growth, structural characterization, and investigation of the electronic properties of single-crystalline LaFeAsO co-substituted by Sm and P, in both its normal- and superconducting states. Here, the appearance of superconductivity is attributed to the inner chemical pressure induced by the smaller-size isovalent substituents. X-ray structural refinements show that the partial substitution of La by Sm and As by P in the parent LaFeAsO compound leads to a contraction in both the conducting Fe2(As,P)2 layers and the interlayer spacing. The main parameters of the superconducting state, including the critical temperature, the lower- and upper critical fields, as well as the coherence length, the penetration depth, and their anisotropy, were determined from magnetometry measurements on a single-crystalline La0.87Sm0.13FeAs0.91P0.09O sample. The critical current density (jc), as resulting from loops of magnetization hysteresis in the self-generated magnetic field, is 2 × 106 A/cm2 at 2 K. Overall, our findings illustrate a rare and interesting case of how superconductivity can be induced by co-substitution in the 1111 family. Such approach delineates new possibilities in the creation of superconductors by design, thus stimulating the exploration of related systems under multi-chemical pressure conditions.

Epitaxial growth of metastable La0.67Sr0.33MnO3−δ films by using Sr3Al2O6 as multifunctional buffer layers

In doped manganites, a substantial tuning of the magnetic and electrical transport properties can be realized by engineering the concentration of oxygen vacancies. To date, most oxygen-deficient La1−xSrxMnO3−δ (0 ≤ x ≤ 1) films are synthesized by after-growth treatments. However, the direct growth of La1−xSrxMnO3−δ films remains challenging due to the metastability of this material. Here, we report the epitaxial growth of high quality single crystalline La0.67Sr0.33MnO3−δ films with an extremely large out-of-plane lattice parameter of 4.26 Å by reactive oxide molecular beam epitaxy. To stabilize this metastable phase, Sr3Al2O6 buffer layers are used to block the oxygen diffusion from the SrTiO3 substrate to the film during the growth process. This work provides an efficient way to obtain metastable La0.67Sr0.33MnO3−δ films.

Spectroscopic characterization of electronic structures of ultra-thin single crystal La0.7Sr0.3MnO3

We have successfully fabricated high quality single crystalline La0.7Sr0.3MnO3 (LSMO) film in the freestanding form that can be transferred onto silicon wafer and copper mesh support. Using soft x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) spectroscopy in transmission and reflection geometries, we demonstrate that the x-ray emission from Mn 3s-2p core-to-core transition (3sPFY) seen in the RIXS maps can represent the bulk-like absorption signal with minimal self-absorption effect around the Mn L3-edge. Similar measurements were also performed on a reference LSMO film grown on the SrTiO3 substrate and the agreement between measurements substantiates the claim that the bulk electronic structures can be preserved even after the freestanding treatment process. The 3sPFY spectrum obtained from analyzing the RIXS maps offers a powerful way to probe the bulk electronic structures in thin films and heterostructures when recording the XAS spectra in the transmission mode is not available.

Ferromagnetic Resonance of Single-Crystalline La0.67Sr0.33MnO3 Thin Film Integrated on Silicon

Integration of high-quality multifunctional oxide thin films in silicon technology promises a wide range of potential applications in low loss spintronic devices such as sensors, detectors, data storage media, and so on. However, the heteroepitaxial growth of functional complex oxides on silicon substrates has been proved challenging, which limits the development of semiconductor-based spintronic devices. In this work, epitaxial single-crystalline La0.67Sr0.33MnO3 (LSMO) thin films have been integrated on silicon substrates by epitaxy and transfer carried out at room temperature. The microwave magnetisms of the LSMO thin films transferred on silicon substrates have been investigated under multiple directions of magnetic field by ferromagnetic resonance. The transferred LSMO thin films on silicon substrates preserve the microwave magnetic characteristics of the primary as-grown LSMO thin films. Our results demonstrate that the epitaxy and transfer method has enormous potential in future spintronic applications of functional oxide devices compatible with semiconductor technology without thermal damage.

Magnetotransport Irreversibility in Single Crystalline La0.18Pr0.40Ca0.42MnO3 Thin Films

Magnetotransport irreversibility in single crystalline La0.18Pr0.40Ca0.42MnO3 thin films is probed with respect to intrinsic electronic phase separation (IEPS). Temperature‐dependent magnetization and resistivity measurements show that (i) the high temperature non‐hysteretic regime is dominated by the antiferromagnetic insulator (AFMI) and the charge ordered (CO) phases; (ii) at intermediate temperatures hysteretic regime is akin to a spin liquid; and (iii) the glass transition occurs at temperature Tg below which the spin liquid freezes. The suppression of the ferromagnetic and insulator–metal transitions (TC and TIM) during cooling confirms supercooled magnetic liquid. Magnetic field‐dependent resistivity (ρ–H) measured during cooling and warming highlights the differences in the spin‐ordered structures through (i) reversible behavior at T < Tg; (ii) colossal irreversibility in the isothermal cooling and warming ρ–H at Tg < T < TC/TIM; and (iii) reversible ρ–H at low/moderate fields but irreversible behavior at high fields at T > TIM (warming). The present study demonstrates that the scaling of area between the isothermal cooling and warming cycle ρ–H curves with temperature mimics the ρ–T behavior and hence also reflects the insulator–metal transition. The observed irreversibility and the area scaling in the different spin regimes have been explained in terms of the intrinsic electronic phase separation.

Quantifying phase separation in terms of magnetoresistive hysteresis loops in strongly phase-separated manganite thin films

The present work reports the scaling behaviour of thermomagnetic hysteresis in temperature and magnetic field-dependent resistivity [(ρ–T) and (ρ–H)] measured during cooling/warming and H increasing/decreasing cycles in single crystalline La0.21Pr0.42Ca0.37MnO3 thin films. The zero-magnetic field (H = 0) insulator–metal transition temperature (IMT) measured in warming cycle ( $$T_{{{\text{IM}}}}^{{\text{W}}}$$ ~ 163 K) is higher than that in the cooling cycle ( $$T_{{{\text{IM}}}}^{{\text{C}}}$$ ~ 116 K) and the difference between the two IMTs shrinks as H is increased. The magnetic field dependence of the two IMTs is well fitted by the equation $${T_{{\text{IM}}}}=T_{{{\text{IM}}}}^{0}+\beta {H^\alpha }$$ . Here, $$T_{{{\text{IM}}}}^{0}$$ is the H-independent contribution and the constants, pre-factor $$\beta$$ and exponent $$\alpha$$ determine the H-dependent part of the IMT. The ρ–T loop area diminishes with the increasing H and its scaling with H in the regime which is akin to a magnetic liquid and unstable towards external H (H < 30 kOe) is nicely described by $${A_T}={A_{T0}}{e^{ - {\Gamma}H}}$$ . Here, $${A_{T0}}$$ is the zero-field normalized area and Γ is a constant related to the degree of phase separation. The analysis of the isothermal ρ–H loop area, which increases with H shows a scaling behaviour of the type $$A(H)=a{(H - {H_{{\text{IM}}}})^\eta }$$ . Here, the constant HIM corresponds to the induced AFMI–FMM phase transition and decreases with temperature, while the exponent ‘η’ measures the degree of phase separation.

Understanding thermomagnetic hysteresis in La1−x−yPryCaxMnO3 thin films

The present work reports the scaling behaviour of thermomagnetic hysteresis in temperature and magnetic field dependent resistivity [(ρ–T) and (ρ–H)] measured during cooling/warming and H increasing/decreasing cycles in single crystalline La0.21Pr0.42Ca0.37MnO3 thin films. The zero-magnetic field (H = 0) insulator–metal transition temperature (IMT) measured in warming cycle ~ 166 K is higher than that in the cooling cycle = 128 K and the difference between them shrinks as H is increased. The two IMTs scale with H as . Here is the H-independent contribution, and the constants, pre-factor and exponent determine the magnetic field dependent part. The ρ–T loop area (AT) diminishes with the increasing H as the magnetic liquid is extremely unstable with respect to external H (H < 30 kOe) and consequently AT shows an exponential decay given by . Here, is the zero-field normalized area and Γ is a constant related to the degree of phase separation. The analysis of the isothermal ρ–H loop area, which increases with H shows scaling behaviour of the type . Here, the constant HIM corresponds to the magnetic field that induces AFMI to FMM phase transition and decreases with temperature, while the exponent ‘η’ measures the degree of phase separation. The value of η is found to be temperature dependent and hence related to the relative fraction of the two coexisting phases.

Inverse magnetocaloric and exchange bias effects in single crystalline La0.5Sr0.5MnO3 nanowires

We report the first observation of inverse magnetocaloric effect (IMCE) in hydrothermally synthesized single crystalline La0.5Sr0.5MnO3 nanowires. The core of the nanowires is phase separated with the development of double exchange driven ferromagnetism (FM) in the antiferromagnetic (AFM) matrix, whereas the surface is found to be composed of disordered magnetic spins. The FM phase scales with the effective magnetic anisotropy, which is directly probed by transverse susceptibility experiments. The surface exhibits a glassy behavior and undergoes spin freezing, which manifests as a positive peak (TL ∼ 42 K) in the magnetic entropy change (−ΔSM) curves, thereby stabilizing the re-entrance of the conventional magnetocaloric effect. Precisely at TL, the nanowires develop the exchange bias (EB) effect. Our results conclusively demonstrate that the mere coexistence of FM and AFM phases along with a disordered surface below their Néel temperature (TN ∼ 210 K) does not trigger EB, but this develops only below the surface spin freezing temperature.

Fabrication and magnetic properties of single-crystalline La0.33Pr0.34Ca0.33MnO3/MgO nanowires

Single crystalline La0.33Pr0.34Ca0.33MnO3/MgO core-shell nanowires with diameters about tens of nanometers are synthesized by a two-step process. Structure and morphology characterizations confirm the epitaxial growth of La0.33Pr0.34Ca0.33MnO3 shell layers on MgO core layers. Clear interfaces are observed between the core and shell layers. Magnetic measurements suggest the existence of electronic phase separation in the one dimensional nanowires similar to the bulk. However, the nanowires exhibit significantly increased amount of magnetically frozen phase and increased coercivity, which are attributed to the strongly modulated magnetic structure in the one dimensional structure.

On the estimation of the magnetocaloric effect by means of microwave technique

The method based on low-field microwave absorption measurements is presented to estimate the relative change of entropy with magnetic field. This method is illustrated on both the polycrystalline Gd5Si2Ge2 alloy and the single crystalline La0.7Ca0.3MnO3 manganite. It is shown that there is the simple functional relation between magnetization and non-resonant absorption over a narrow temperature range near the magnetic phase transition. The magnetoresistance is assumed to be the dominating mechanism underlying this relation.

Single crystalline La0.5Sr0.5MnO3microcubes as cathode of solid oxidefuel cell

The efficiency of solid oxide fuel cells (SOFCs) is heavily dependent on the electrocatalytic activity of the cathode toward the oxygen reduction reaction (ORR). In order to achieve better cathode performance, single crystalline La0.5Sr0.5MnO3 (LSM) microcubes with the {200} facets have been synthesized by the hydrothermal method. It is found that the LSM microcubes exhibit lower polarization resistance than the conventional polycrystalline La0.8Sr0.2MnO3 powder in air from 700 °C to 900 °C. The ORR activation energy of the LSM microcubes is lower than that of the conventional powder. The ORR kinetics for the microcubes is limited by the charge transfer step while that for the conventional powder is dominated by the oxygen adsorption and dissociation on the cathode surface.

Oxygen in-diffusion at room temperature in epitaxial La0.7Ca0.3MnO3−δ thin films

Single-crystalline La0.7Ca0.3MnO3−δ thin-films, 20 nm thick, were grown coherently on lattice well-matched (LaAlO3)0.3(Sr2AlTaO6)0.7(0 0 1) substrates by pulsed-laser deposition, and the control, diffusion and stability of oxygen in the films were carefully investigated. Using x-ray reciprocal space maps and magnetic measurements, we show clearly that the out-of-plane lattice parameter and Curie temperature of the films can change simultaneously with the oxygen content, while the in-plane lattice coherency is maintained consistently at the same time. Remarkably, contrary to those with oxygen deficiencies created during the in situ deposition at lower oxygen pressures, the ex situ vacuum-annealed films show fast oxygen in-diffusion even at room temperature and in ambient atmosphere. The results strongly suggest that the control of oxygen content via the in situ deposition process is more favourable for manganite films.

A Study on the Relationship Between Low-Temperature Reducibility and Catalytic Performance of Single-Crystalline La0.6Sr0.4MnO3+δ Microcubes for Toluene Combustion

Single-crystalline La0.6Sr0.4MnO3+δ microcubes of cubic perovskite phase were prepared hydrothermally. The adopted temperature and time of hydrothermal treatment and the amount of KOH used had great effects on the low-temperature reducibility of La0.6Sr0.4MnO3+δ . The initial H2 consumption rate of La0.6Sr0.4MnO3+δ played a vital role in determining its catalytic activity for toluene combustion. It is concluded that the perovskite-type oxide catalyst with a higher initial H2 consumption rate displayed a higher catalytic activity for the addressed reaction.

Single‐Crystalline La0.7Sr0.3MnO3 Nanowires by Polymer‐Template‐Directed Chemical Solution Synthesis

A facile polymer-template-aided synthesis is adopted to prepare single crystalline La0.7Sr0.3MnO3 nanowires by chemical solution deposition. The La0.7Sr0.3MnO3 nanowires crystallize in a novel monoclinic phase and exhibit ferromagnetic ordering at room temperature.

Hydrothermal synthesis of single-crystalline La0.5Ca0.5MnO3 nanowires at low temperature

Single-crystalline La0.5Ca0.5MnO3 nanowires have been successfully prepared by a hydrothermal method at low temperature. X-Ray diffraction (XRD) results show that the La0.5Ca0.5MnO3 nanowires have an orthorhombic perovskite structure without any other impurity phase. Transmission electron microscopy (TEM) images demonstrate that the as-synthesized samples are made up of a large quantity of nanowires with lengths ranging from several to several tens of micrometers and uniform diameter (∼80 nm). High-resolution transmission electron microscopy (HRTEM) images reveal that the nanowires with very clean surfaces grow along [100]. The result of magnetic measurements indicates that the nanowires have an enhanced Tc due to the shrinkage of the unit cell volume.

Low-temperature resistivity minima in single-crystalline and ceramic La0.8Sr0.2MnO3: Mesoscopic transport and intergranular tunneling

A slight minimum in the zero-field resistivity of a single crystalline La0.8Sr0.2MnO3 and shallow one for ceramic La0.8Sr0.2MnO3 samples were observed at T∼4 K and at T∼25–30 K, respectively. The minimum for the ceramic shifts towards lower temperatures, flattens with increasing magnetic fields (H), and vanishes at some critical H. The above effects are accompanied by an appreciable negative magnetoresistance (MR). On the other hand, the minimum for the single-crystalline sample is almost field independent and the MR in the relevant temperature range is very small. Two different mechanisms were found to account for the results observed in the single crystal and the polycrystalline samples: (i) mesoscopic corrections to the bulk resistivity that include Coulomb interaction and weak localization and (ii) intergranular tunneling. The resistivity of the large-grain ceramic sample comprises both types of behavior. The minimum of the bulk contribution becomes clearly seen under H, which suppresses the ceramic-type minimum.

Anomalous magnetic field dependence of T AF in La 0.95 Sr 0.05 MnO 3

The magneto-elastic properties of single-crystalline La0.95Sr0.05MnO3 have been studied ultrasonically. Our investigations focussed on the temperature interval where magnetic ordering starts to evolve and results in a spin canted antiferromagnetic ground state. In detail the experiments revealed that the magnetic order parameter in low-doped manganite is only weakly coupled to lattice strains. Furthermore, the anomalous temperature dependence of the order parameter as found resembles highly that in stoichiometric LaMnO3. However, the main and most surprising finding is that external magnetic fields favor the spin canted phase in La0.95Sr0.05MnO3. It is unclear at present how the exchange interaction can be tuned by magnetic fields in the way observed and we are not aware of existing theoretical concepts which might give a plausible explanation for the unexpected field dependent behavior of the critical temperature. We believe, however, that this behavior primarily results from the fact that the exchange interaction depends sensitively on the orbital configuration of the manganese d electrons.

Microstructure of thin film La0.5Sr0.5CoO3 electrode for microelectronics applications

Increasing circuit densities in dynamic random access memories (DRAMs) is one of the major interests in microelectronics research. In a ferroelectric capacitor memory, conducting electrodes must make intimate and ohmic contact to both sides of the ferroelectric thin film. The bottom electrode serves as the substrate during growth and the top electrode is deposited on top of the ferroelectric film. Single crystalline La0.5Sr0.5CoO3 (LSCO) thin films, which are conductive and low electrical resistivity, have recently grown on both sides of Pb-Zr-Ti-O (PZT) ferroelectric film, which has shown a great potential for making fatigue-free small thickness storage capacitors. The present paper reports the microstructure studies of the LSCO thin films grown on MgO(00l) and LaAlO3(100) (LAO) substrates by the metalorganic chemical vapor deposition (MOCVD) technique.Cross-section specimens were made to examine the structure of the film and its relationship with the substrate. Select-area electron diffraction (SAD) and high-resolution lattice image show that the LSCO film has an epitaxial relationship with the MgO/LAO substrate.