In the present paper we report the effect of graphene oxide (GO) doping on the structural and superconducting properties of MgB 2 . Bulk polycrystalline samples have been synthesized via a solid state reaction route with compositions MgB 2 + x wt% of GO (x = 0, 1, 2, 3, 5, 7 and 10) by sintering at ∼850 • C in a reducing atmosphere of Ar/H 2 (9:1). The x-ray diffraction results confirm the formation of the MgB 2 phase in all samples, together with traces of a MgO impurity phase. The XRD data results also show substitution of carbon for boron, but in the present case the actual amount of carbon substituting for boron is very small as compared to other carbon sources. A substantial improvement in the critical current density, J c (H), has been observed in the entire magnetic field range (0-8 T) for samples x = 1, 2 and 3 as compared to the undoped sample. In addition to J c (H), marginal improvements in the upper critical field (H c2 ) and the irreversibility field (H irr ) have been observed for the doped samples x = 1, 2 and 3 with respect to pristine MgB 2 . Furthermore, a curious result of the present investigation is that there is no change in the superconducting transition temperature (T c ) up to a doping level of 10 wt%. The possible mechanisms of flux pinning and correlations between the observed superconducting properties and structural characteristics of the samples have been described and discussed in this paper.
Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa2Cu3O7 and ferromagnetic-metallic La0.67Ca0.33MnO3 or ferromagnetic-insulating LaMnO(3+δ). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La0.67Ca0.33MnO3 and almost absent for ferromagnetic-insulating LaMnO(3+δ). We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.
Flexoelectricity is an electromechanical coupling between electrical polarization and a strain gradient that enables mechanical manipulation of polarization without applying an electrical bias. Recently, flexoelectricity was directly demonstrated by mechanically switching the out-of-plane polarization of a uniaxial system with a scanning probe microscope tip. However, the successful application of flexoelectricity in low-symmetry multiaxial ferroelectrics and therefore active manipulation of multiple domains via flexoelectricity have not yet been achieved. Here, we demonstrate that the symmetry-breaking flexoelectricity offers a powerful route for the selective control of multiple domain switching pathways in multiaxial ferroelectric materials. Specifically, we use a trailing flexoelectric field that is created by the motion of a mechanically loaded scanning probe microscope tip. By controlling the SPM scan direction, we can deterministically select either stable 71° ferroelastic switching or 180° ferroelectric switching in a multiferroic magnetoelectric BiFeO thin film. Phase-field simulations reveal that the amplified in-plane trailing flexoelectric field is essential for this domain engineering. Moreover, we show that mechanically switched domains have a good retention property. This work opens a new avenue for the deterministic selection of nanoscale ferroelectric domains in low-symmetry materials for non-volatile magnetoelectric devices and multilevel data storage.
Boron nitride occurs in several polymorphs, among which cubic and hexagonal varieties have been extensively studied over many years and have found numerous practical applications. Although various forms of BN have been used as lubricant and abrasive material for many years, the unique electronic properties of BN have not been yet utilized. Due to their wide optical bandgaps (the widest among nitride semiconductors), cubic and hexagonal polymorphs of BN are promising candidates for optoelectronic devices operating in the deep‐ultraviolet range and for high‐power electronic devices for switching and radio‐frequency applications. The layered structure of hexagonal BN allows preparation of ultrathin nanosheets with very smooth surfaces and low defect density, which are proved to be useful as supporting substrates and gate dielectric layers in graphene‐based structures. The first device demonstrations are very encouraging. In this contribution, the recent progress in properties, fabrication technologies, and emerging applications of BN as a wide‐gap semiconductor material is reviewed.
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