Microwave heating-induced DC magnetic flux penetration in YBa2_{2}Cu3_{3}O7−δ_{7-\delta} superconducting thin films

The magneto-optical imaging technique is used to visualize the penetration of the magnetic induction in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin films during surface resistance measurements. The in-situ surface resistance measurements were performed at 7 GHz using the dielectric resonator method. When only the microwave magnetic field $H_{rf}$ is applied to the superconductor, no $H_{rf}$-induced vortex penetration is observed, even at high rf power. In contrast, in the presence of a constant magnetic field superimposed on $H_{rf}$ we observe a progression of the flux front as $H_{rf}$ is increased. A local thermometry method based on the measurement of the resonant frequency of the dielectric resonator placed on the YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin film shows that the $H_{rf}$--induced flux penetration is due to the increase of the film temperature.Comment: 6 pages, Journal of Applied Physic

Enhanced magnetic moment and conductive behavior in NiFe2O4 spinel ultrathin films

Bulk NiFe2O4 is an insulating ferrimagnet. Here, we report on the epitaxial growth of spinel NiFe2O4 ultrathin films onto SrTiO3 single-crystals. We will show that - under appropriate growth conditions - epitaxial stabilization leads to the formation of a spinel phase with magnetic and electrical properties that radically differ from those of the bulk material : an enhanced magnetic moment (Ms) - about 250% larger - and a metallic character. A systematic study of the thickness dependence of Ms allows to conclude that its enhanced value is due to an anomalous distribution of the Fe and Ni cations among the A and B sites of the spinel structure resulting from the off-equilibrium growth conditions and to interface effects. The relevance of these findings for spinel- and, more generally, oxide-based heterostructures is discussed. We will argue that this novel material could be an alternative ferromagetic-metallic electrode in magnetic tunnel junctions.Comment: accepted for publication in Phys. Rev.

Electric field control of exchange bias in multiferroic epitaxial heterostructures

The magnetic exchange bias between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO3 oxide and a soft ferromagnetic (FM) layer is used to couple the magnetic response of the ferromagnetic layer to the magnetic state of the antiferromagnetic one. We will show that biasing the ferroelectric YMnO3 layer by an appropriate electric field allows modifying and controlling the magnetic exchange bias and subsequently the magnetotransport properties of the FM layer. This finding may contribute to pave the way towards a new generation of electric-field controlled spintronics devices.Comment: 15 pages, 5 figures, submitte

Tuning the onset of ferromagnetism in heterogeneous bimetallic nanoparticles by gas phase doping

In the nanoregime, chemical species can reorganize in ways not predicted by their equilibrium bulk behavior. Here, we engineer Ni-Cr nanoalloys at the magnetic end of their compositional range (i.e., 0–15 at. % Cr), and we investigate the effect of Cr incorporation on their structural stability and resultant magnetic ordering. To ensure their stoichiometric compositions, the nanoalloys are grown by cluster beam deposition, a method that allows one-step, chemical-free fabrication of bimetallic nanoparticles. While full Cr segregation toward nanoparticle surfaces is thermodynamically expected for low Cr concentrations, metastability occurs as the Cr dopant level increases in the form of residual Cr in the core region, yielding desirable magnetic properties in a compensatory manner. Using nudged elastic band calculations, residual Cr in the core is explained based on modifications in the local environment of individual Cr atoms. The resultant competition between ferromagnetic and antiferromagnetic ordering gives rise to a wide assortment of interesting phenomena, such as a cluster-glass ground state at very low temperatures and an increase in Curie temperature values. We emphasize the importance of obtaining the commonly elusive magnetic nanophase diagram for M-Cr (M=Fe, Co, and Ni) nanoalloys, and we propose an efficient single-parameter method of tuning the Curie temperature for various technological applications.Peer reviewe

Isothermal tuning of exchange bias using pulsed fields

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Exchange bias,HE, and coercivity,HC, of antiferromagnetic (AFM)/ferromagnetic bilayers can be adjusted, after deposition, at temperatures below the Néel temperature of the AFM by subjecting the samples to large pulsed fields (in excess of HPulse=550 kOe). The efficiency of the process depends on the AFM system and the direction of the applied field with respect of the unidirectional anisotropy direction. Textured (111) Fe19Ni81/Fe50Mn50 bilayers show an HE reduction and a HC increase when the pulse field is applied antiparallel to the unidirectional anisotropy, while they only exhibit a reduction in HC when the pulse is applied parallel to their unidirectional anisotropy. On the other hand, textured (111) NiO/Co bilayers exhibit a change of the angular dependence of HE when the pulse is applied away from the unidirectional anisotropy. The effects could be caused by field induced changes in the domain structure of the AFM or transitions in the AFM (spin-flop or AFM-paramagnetic)

Exchange biasing and electric polarization with YMnO3

We report on the growth and functional characterization of epitaxial thin films of the multiferroic YMnO3. We show that using Pt as a seed layer on SrTiO3(111) substrates, epitaxial YMnO3 films (0001) textured are obtained. An atomic force microscope has been used to polarize electric domains revealing the ferroelectric nature of the film. When a Permalloy layer is grown on top of the YMnO3(0001) film, clear indications of exchange bias and enhanced coercivity are observed at low temperature. The observation of coexisting antiferromagnetism and electrical polarization suggests that the biferroic character of YMnO3 can be exploited in novel devices.Comment: 15 pages, 4 figures, Applied Physics Letters (in press

Observation et modélisation de nanostructures magnétiques excitées par microondes en vue d'une application capteur

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Caractérisation des champs électromagnétiques des systèmes rayonnants par thermo-fluorescence

La mesure du champ électromagnétique émis par diverses sources (antenne, radar) permet leur caractérisation et est utile en CEM (détection de fuites). Au-delà d'une mesure ponctuelle du champ proche, l'imagerie par thermographie permet une cartographie sur une surface, en un temps très court. La thermographie infrarouge a ainsi été développée par l'ONERA sous l'appellation EMIR : le rayonnement échauffe un film mince et cet échauffement, mesuré par une caméra infrarouge, donne l'amplitude du champ. Plus récemment, nous avons développé une technique de thermographie dérivée d'EMIR, appelée EMVI (ElectroMagnetic Visible Imaging). Là encore un film mince s'échauffe sous l'effet du champ. Mais ici, le film est recouvert par un fluorophore, dont l'émission dépend de la température, donc du champ. La cartographie du champ est ainsi obtenue avec une caméra optique. Nous rappelons ici des premiers résultats illustrant cette méthode et présentons des voies d'amélioration

Caractérisation des champs électromagnétiques des systèmes rayonnants par thermo-fluorescence

La mesure du champ électromagnétique émis par diverses sources (antenne, radar) permet leur caractérisation et est utile en CEM (détection de fuites). Au-delà d'une mesure ponctuelle du champ proche, l'imagerie par thermographie permet une cartographie sur une surface, en un temps très court. La thermographie infrarouge a ainsi été développée par l'ONERA sous l'appellation EMIR : le rayonnement échauffe un film mince et cet échauffement, mesuré par une caméra infrarouge, donne l'amplitude du champ. Plus récemment, nous avons développé une technique de thermographie dérivée d'EMIR, appelée EMVI (ElectroMagnetic Visible Imaging). Là encore un film mince s'échauffe sous l'effet du champ. Mais ici, le film est recouvert par un fluorophore, dont l'émission dépend de la température, donc du champ. La cartographie du champ est ainsi obtenue avec une caméra optique. Nous rappelons ici des premiers résultats illustrant cette méthode et présentons des voies d'amélioration