Influence of thermal fluctuations on the Nernst signal in superconducting (K,Ba)BiO3 single crystals

International audienceWe report on the Nernst effect, specific heat and transportmeasurements performed in high quality (K,Ba)BiO3 single crystals close to optimal doping (Tc ∼ 31 K). We show that a nonzero Nernst effect remains visible well above the upper critical field unambiguously deduced from the onset of the specific heat anomaly. This finite Nernst signal is attributed to fluctuations of the amplitude of the order parameter in a region where the free energy is smaller than kBT . Despite the absence of any vortex liquid phase (and hence of any significant phase fluctuations), the field and temperature dependence of the Nernst coefficient is very similar to the one obtained in electron-doped cuprates

Low-temperature transport in highly boron-doped nanocrystalline diamond

International audienceWe studied the transport properties of highly boron-doped nanocrystalline diamond thin films at temperatures down to 50 mK. The system undergoes a doping-induced metal-insulator transition with an interplay between intergranular conductance g and intragranular conductance g0, as expected for a granular system. The conduction mechanism in the case of the low-conductivity films close to the metal-insulator transition has a temperature dependence similar to Efros-Shklovskii type of hopping. On the metallic side of the transition, in the normal state, a logarithmic temperature dependence of the conductivity is observed, as expected for a metallic granular system. Metallic samples far away from the transition show similarities to heavily borondoped single-crystal diamond. Close to the transition, the behavior is richer. Global phase coherence leads in both cases to superconductivity also checked by ac susceptibility , but a peak in the low-temperature magnetoresistance measurements occurs for samples close to the transition. Corrections to the conductance according to superconducting fluctuations account for this negative magnetoresistance

Coupling of single, double, and triple-decker metal-phthalocyanine complexes to ferromagnetic and antiferromagnetic substrates

We report a survey of the magnetic properties of metal-organic complexes coupled to ferromagnetic and antiferromagnetic surfaces. Using element-resolved X-ray magnetic circular dichroism, we investigate the magnetism of single, double, and triple-decker phthalocyanines focusing on MnPc, TbPc, and TbPc deposited on Ni, Mn, and CoO thin films. Depending on the number of Pc ligands, we find that the metal ions within the molecules couple either parallel or antiparallel to a ferromagnetic substrate. Whereas single-decker complexes such as MnPc form a unique magnetic entity with ferromagnetic films, the intrinsic single molecule magnet properties of TbPc and TbPc remain largely unaltered. TbPc deposited on perpendicularly magnetized Ni films exhibits enhanced magnetic stability compared to TbPc in molecular crystals, opposite to TbPc deposited on in-plane magnetized Ni. Depending on the competition between uniaxial anisotropy, superexchange, and Zeeman interaction, the magnetic moment of TbPc can be aligned parallel or antiparallel to that of the substrate by modulating the intensity of an external magnetic field. This occurs also for TbPc, but the substrate-induced exchange coupling in triple-decker molecules is found to be short-ranged, that is, limited to the Tb ion closer to the ferromagnetic surface. Finally, we discuss the conditions required to establish exchange bias between molecules and antiferromagnetic substrates. We show that TbPc deposited on antiferromagnetic Mn thin films exhibits both exchange bias and enhanced coercivity when field cooled parallel to the out-of-plane easy axis. However, exchange bias does not extend to all molecules on the surface. On oxide antiferromagnets such as CoO we find no evidence of exchange bias for either TbPc or MnPc

Retournement de l'aimantation assisté par un champ micro-onde d'une nanoparticule individuelle

A low temperature microSQUID magnetometer coupled to a microwave antenna was used to probe the assisted switching of the magnetization of a ferromagnetic nanoparticle. Using an original measurement protocol based on the control of the microwave amplitude and phase, we studied the basins of attraction of the precessional modes of the magnetization dynamics. The switching of the magnetization can thus be controlled by choosing an amplitude and phase in one particular basin. In particular, we evidenced the counter intuitive fact that the magnetization switching can be prevented by choosing a “wrong” phase, even though the amplitude is high enough to switch the system with another phase. Moreover, we used the basin's sensitivity to the parameters governing the magnetization dynamics to determine a value for the Gilbert's damping constant α. This is the first measurement of the damping constant on an individual nanoparticle.Un magnétomètre microSQUID basse température couplé à une antenne micro-onde a été utilisé pour sonder la dynamique du retournement assisté de l'aimantation d'une nanoparticule ferromagnétique. Grâce au développement d'une technique de mesure originale, basée notamment sur le contrôle de l'amplitude et de la phase du champ micro-onde, nous avons pu mettre en évidence les bassins d'attraction liés aux modes de précession présents dans la dynamique de l'aimantation. Il devient possible de contrôler le retournement de l'aimantation selon que l'amplitude et la phase du champ AC sont judicieusement choisis dans l'un ou l'autre des bassins d'attraction. Nous avons pu mettre en évidence un fait contre-intuitif où le retournement est bloqué par une phase "mal" choisie alors que l'amplitude est largement suffisante pour retourner le système avec une autre phase. De plus, nous avons pu utiliser la sensibilité des bassins d'attraction aux paramètres gouvernant la dynamique de l'aimantation pour déterminer expérimentalement une valeur de la constante d'amortissement de Gilbert α. C'est d'ailleurs la première mesure de la constante d'amortissement sur une nanoparticule unique

Microwave Assisted Switching of Magnetisation of a Single Nanoparticle

Un magnétomètre microSQUID basse température couplé à une antenne micro-onde a été utilisé pour sonder la dynamique du retournement assisté de l'aimantation d'une nanoparticule ferromagnétique. Grâce au développement d'une technique de mesure originale, basée notamment sur le contrôle de l'amplitude et de la phase du champ micro-onde, nous avons pu mettre en évidence les bassins d'attraction liés aux modes de précession présents dans la dynamique de l'aimantation. Il devient possible de contrôler le retournement de l'aimantation selon que l'amplitude et la phase du champ AC sont judicieusement choisis dans l'un ou l'autre des bassins d'attraction. Nous avons pu mettre en évidence un fait contre-intuitif où le retournement est bloqué par une phase "mal" choisie alors que l'amplitude est largement suffisante pour retourner le système avec une autre phase. De plus, nous avons pu utiliser la sensibilité des bassins d'attraction aux paramètres gouvernant la dynamique de l'aimantation pour déterminer expérimentalement une valeur de la constante d'amortissement de Gilbert α. C'est d'ailleurs la première mesure de la constante d'amortissement sur une nanoparticule unique.A low temperature microSQUID magnetometer coupled to a microwave antenna was used to probe the assisted switching of the magnetization of a ferromagnetic nanoparticle. Using an original measurement protocol based on the control of the microwave amplitude and phase, we studied the basins of attraction of the precessional modes of the magnetization dynamics. The switching of the magnetization can thus be controlled by choosing an amplitude and phase in one particular basin. In particular, we evidenced the counter intuitive fact that the magnetization switching can be prevented by choosing a “wrong” phase, even though the amplitude is high enough to switch the system with another phase. Moreover, we used the basin's sensitivity to the parameters governing the magnetization dynamics to determine a value for the Gilbert's damping constant α. This is the first measurement of the damping constant on an individual nanoparticle

Phase Dependence of Microwave-Assisted Switching of a Single Magnetic Nanoparticle

International audienc

Coupling of single, double, and triple-decker metal-phthalocyanine complexes to ferromagnetic and antiferromagnetic substrates

We report a survey of the magnetic properties of metal-organic complexes coupled to ferromagnetic and antiferromagnetic surfaces. Using element-resolved X-ray magnetic circular dichroism, we investigate the magnetism of single, double, and triple-decker phthalocyanines focusing on MnPc, TbPc2, and Tb2Pc3 deposited on Ni, Mn, and CoO thin films. Depending on the number of Pc ligands, we find that the metal ions within the molecules couple either parallel or antiparallel to a ferromagnetic substrate. Whereas single-decker complexes such as MnPc form a unique magnetic entity with ferromagnetic films, the intrinsic single molecule magnet properties of TbPc2 and Tb2Pc3 remain largely unaltered. TbPc2 deposited on perpendicularly magnetized Ni films exhibits enhanced magnetic stability compared to TbPc2 in molecular crystals, opposite to TbPc2 deposited on in-plane magnetized Ni. Depending on the competition between uniaxial anisotropy, superexchange, and Zeeman interaction, the magnetic moment of TbPc2 can be aligned parallel or antiparallel to that of the substrate by modulating the intensity of an external magnetic field. This occurs also for Tb2Pc3, but the substrate-induced exchange coupling in triple-decker molecules is found to be short-ranged, that is, limited to the Tb ion closer to the ferromagnetic surface. Finally, we discuss the conditions required to establish exchange bias between molecules and antiferromagnetic substrates. We show that TbPc2 deposited on antiferromagnetic Mn thin films exhibits both exchange bias and enhanced coercivity when field cooled parallel to the out-of-plane easy axis. However, exchange bias does not extend to all molecules on the surface. On oxide antiferromagnets such as CoO we find no evidence of exchange bias for either TbPc2 or MnPc.We received financial support from the European Research Council (StG 203239 NOMAD), the Ministerio de Ciencia e Innovación (MAT2010-15659), the Agència de Gestió d'Ajuts Universitaris i de Recerca (2009 SGR 695), and the Swiss Competence Centre for Materials Science and Technology (CCMX).Peer Reviewe

Coupling of single, double, and triple-decker metal-phthalocyanine complexes to ferromagnetic and antiferromagnetic substrates

We report a survey of the magnetic properties of metal-organic complexes coupled to ferromagnetic and antiferromagnetic surfaces. Using element-resolved X-ray magnetic circular dichroism, we investigate the magnetism of single, double, and triple-decker phthalocyanines focusing on MnPc, TbPc, and TbPc deposited on Ni, Mn, and CoO thin films. Depending on the number of Pc ligands, we find that the metal ions within the molecules couple either parallel or antiparallel to a ferromagnetic substrate. Whereas single-decker complexes such as MnPc form a unique magnetic entity with ferromagnetic films, the intrinsic single molecule magnet properties of TbPc and TbPc remain largely unaltered. TbPc deposited on perpendicularly magnetized Ni films exhibits enhanced magnetic stability compared to TbPc in molecular crystals, opposite to TbPc deposited on in-plane magnetized Ni. Depending on the competition between uniaxial anisotropy, superexchange, and Zeeman interaction, the magnetic moment of TbPc can be aligned parallel or antiparallel to that of the substrate by modulating the intensity of an external magnetic field. This occurs also for TbPc, but the substrate-induced exchange coupling in triple-decker molecules is found to be short-ranged, that is, limited to the Tb ion closer to the ferromagnetic surface. Finally, we discuss the conditions required to establish exchange bias between molecules and antiferromagnetic substrates. We show that TbPc deposited on antiferromagnetic Mn thin films exhibits both exchange bias and enhanced coercivity when field cooled parallel to the out-of-plane easy axis. However, exchange bias does not extend to all molecules on the surface. On oxide antiferromagnets such as CoO we find no evidence of exchange bias for either TbPc or MnPc

Coupling of single, double, and triple-decker metal-phthalocyanine complexes to ferromagnetic and antiferromagnetic substrates

We report a survey of the magnetic properties of metal-organic complexes coupled to ferromagnetic and antiferromagnetic surfaces. Using element-resolved X-ray magnetic circular dichroism, we investigate the magnetism of single, double, and triple-decker phthalocyanines focusing on MnPc, TbPc, and TbPc deposited on Ni, Mn, and CoO thin films. Depending on the number of Pc ligands, we find that the metal ions within the molecules couple either parallel or antiparallel to a ferromagnetic substrate. Whereas single-decker complexes such as MnPc form a unique magnetic entity with ferromagnetic films, the intrinsic single molecule magnet properties of TbPc and TbPc remain largely unaltered. TbPc deposited on perpendicularly magnetized Ni films exhibits enhanced magnetic stability compared to TbPc in molecular crystals, opposite to TbPc deposited on in-plane magnetized Ni. Depending on the competition between uniaxial anisotropy, superexchange, and Zeeman interaction, the magnetic moment of TbPc can be aligned parallel or antiparallel to that of the substrate by modulating the intensity of an external magnetic field. This occurs also for TbPc, but the substrate-induced exchange coupling in triple-decker molecules is found to be short-ranged, that is, limited to the Tb ion closer to the ferromagnetic surface. Finally, we discuss the conditions required to establish exchange bias between molecules and antiferromagnetic substrates. We show that TbPc deposited on antiferromagnetic Mn thin films exhibits both exchange bias and enhanced coercivity when field cooled parallel to the out-of-plane easy axis. However, exchange bias does not extend to all molecules on the surface. On oxide antiferromagnets such as CoO we find no evidence of exchange bias for either TbPc or MnPc