4 research outputs found
Appearance of room temperature ferromagnetism in Cu-doped TiO films
In recent years there has been an intense search for room temperature
ferromagnetism in doped dilute semiconductors, which have many potentially
applications in spintronics and optoelectronics. We report here the unexpected
observation of significant room temperature ferromagnetism in a semiconductor
doped with nonmagnetic impurities, Cu-doped TiO thin films grown by Pulsed
Laser Deposition. The magnetic moment, calculated from the magnetization
curves, resulted surprisingly large, about 1.5 per Cu atom. A large
magnetic moment was also obtained from ab initio calculations using the
supercell method for TiO with Cu impurities, but only if an oxygen vacancy
in the nearest-neighbour shell of Cu was present. This result suggests that the
role of oxygen vacancies is crucial for the appearance of ferromagnetism. The
calculations also predict that Cu doping favours the formation of oxygen
vacancies.Comment: 4 pages, 3 figures, published in Phys. Rev. B (Rapid Comm.
Vortex pinning vs superconducting wire network: origin of periodic oscillations induced by applied magnetic fields in superconducting films with arrays of nanomagnets
Hybrid magnetic arrays embedded in superconducting films are ideal systems to
study the competition between different physical (such as the coherence length)
and structural length scales such as available in artificially produced
structures. This interplay leads to oscillation in many magnetically dependent
superconducting properties such as the critical currents, resistivity and
magnetization. These effects are generally analyzed using two distinct models
based on vortex pinning or wire network. In this work, we show that for
magnetic dot arrays, as opposed to antidot (i.e holes) arrays, vortex pinning
is the main mechanism for field induced oscillations in resistance R(H),
critical current Ic(H), magnetization M(H) and ac-susceptibility Xac(H) in a
broad temperature range. Due to the coherence length divergence at Tc, a
crossover to wire network behavior is experimentally found. While pinning
occurs in a wide temperature range up to Tc, wire network behavior is only
present in a very narrow temperature window close to Tc. In this temperature
interval, contributions from both mechanisms are operational but can be
experimentally distinguished
Anomalies in vortex lattice dynamics driven by induced ac currents in superconducting films with magnetic arrays of two-fold symmetry
We study the dynamics of the vortex lattice driven by ac induced currents in the critical state regime, for T>0.70 T C . The samples are superconducting films grown on top of two-fold symmetry array of magnetic dots. In these heterostructures, the induced ac currents flow parallel to the short and to the long side of the pinning array in different areas of the samples simultaneously. This behavior produces remarkable effects in the vortex lattice dynamics. First of all, periodic features are observed in the ac susceptibility versus applied magnetic field measurements which are related to matching effects between the vortex lattices and the magnetic array. However, the vortex lattice reconfiguration observed in magnetotransport experiments is absent. Some of these features are revealed as maxima instead of being minima, indicating higher mobility at certain matching fields. Competing unstable vortex configurations could lead to increase vortex mobility precluding the reconfiguration transition. At high temperatures, where the matching effects show up, the magnetic permeability of the dots is the mechanism that governs the J C (T) behavior. Moreover, the temperature dependence of the pinning force F P (T) shows a temperature crossover related to an unexpected enhancement in vortex mobility. Vortex-vortex interaction and the interplay between trapped and interstitial vortices are a hint to explain these phenomena.Fil: Moreno Jiménez, Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Chiliotte, Claudio Ezequiel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Pasquini, Gabriela. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Bekeris, Victoria Isabel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: J. del Valle. Universidad Complutense de Madrid; EspañaFil: E. M. González. Universidad Complutense de Madrid; EspañaFil: J. L. Prieto. Universidad Politécnica de Madrid; EspañaFil: J. L. Vincent. Universidad Complutense de Madrid; Españ
Commensurability effects in magnetic properties of superconducting Nb thin films with periodic submicrometric pores
Pinning properties in 100 nm thick continuous and porous superconducting Nb films are examined by ac susceptibility and dc magnetization measurements. The Nb film was deposited on a smooth Si substrate, while the porous film, NbP, was deposited on an anodized Al oxide substrate. Pores or "antidots" 40 nm in diameter, 100 nm apart, form a triangular array. The porous film presents commensurate or matching field effects for applied magnetic fields where the magnetic flux threading each unit cell is an integer number of the flux quantum, where ac shielding capability and dc diamagnetic magnetization show an abrupt increase. The response to ac fields as a function of temperature and dc field provided a way to determine that NbP sample has higher pinning than the continuous one, and that TC suppression due to fluxoid quantization is not relevant for the investigated temperature range. © 2009 Elsevier B.V. All rights reserved.Fil: Chiliotte, Claudio Ezequiel. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Perez Daroca, Diego Raul. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Pasquini, Gabriela. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Bekeris, Victoria Isabel. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Li, C. P.. University of California at San Diego; Estados UnidosFil: Casanova, F.. University of California at San Diego; Estados UnidosFil: Villegas, J. E.. University of California at San Diego; Estados UnidosFil: Schuller, I.K.. University of California at San Diego; Estados Unido