Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 11 de 11
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
Nat Commun ; 10(1): 2463, 2019 06 05.
Artigo em Inglês | MEDLINE | ID: mdl-31165737

RESUMO

A method to measure the superconducting (SC) stiffness tensor [Formula: see text], without subjecting the sample to external magnetic field, is applied to La1.875Sr0.125CuO4. The method is based on the London equation [Formula: see text], where J is the current density and A is the vector potential which is applied in the SC state. Using rotor free A and measuring J via the magnetic moment of superconducting rings, [Formula: see text] at T → Tc is extracted. The technique is sensitive to very small stiffnesses (penetration depths on the order of a few millimeters). The method is applied to two different rings: one with the current running only in the CuO2 planes, and another where the current must cross planes. We find different transition temperatures for the two rings, namely, there is a temperature range with two-dimensional stiffness. Additional low energy muon spin rotation measurements on the same sample determine the stiffness anisotropy at T < Tc.

2.
Nat Commun ; 9(1): 2850, 2018 07 20.
Artigo em Inglês | MEDLINE | ID: mdl-30030427

RESUMO

Two-dimensional magnetic systems with continuous spin degrees of freedom exhibit a rich spectrum of thermal behaviour due to the strong competition between fluctuations and correlations. When such systems incorporate coupling via the anisotropic dipolar interaction, a discrete symmetry emerges, which can be spontaneously broken leading to a low-temperature ordered phase. However, the experimental realisation of such two-dimensional spin systems in crystalline materials is difficult since the dipolar coupling is usually much weaker than the exchange interaction. Here we realise two-dimensional magnetostatically coupled XY spin systems with nanoscale thermally active magnetic discs placed on square lattices. Using low-energy muon-spin relaxation and soft X-ray scattering, we observe correlated dynamics at the critical temperature and the emergence of static long-range order at low temperatures, which is compatible with theoretical predictions for dipolar-coupled XY spin systems. Furthermore, by modifying the sample design, we demonstrate the possibility to tune the collective magnetic behaviour in thermally active artificial spin systems with continuous degrees of freedom.

3.
Chem Commun (Camb) ; 54(56): 7826-7829, 2018 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-29947367

RESUMO

Muon spin relaxation (µSR) experiments on a single-molecule magnet enriched in different Dy isotopes detect unambiguously the slowing down of the zero field spin dynamics for the non-magnetic isotope. This occurs in the low temperature regime dominated by quantum tunnelling, in agreement with previous ac susceptibility investigations. In contrast to the latter, however, µSR is sensitive to all fluctuation modes affecting the lifetime of the spin levels.

5.
Nat Commun ; 7: 12195, 2016 07 20.
Artigo em Inglês | MEDLINE | ID: mdl-27435800

RESUMO

New exotic phenomena have recently been discovered in oxides of paramagnetic Ir(4+) ions, widely known as 'iridates'. Their remarkable properties originate from concerted effects of the crystal field, magnetic interactions and strong spin-orbit coupling, characteristic of 5d metal ions. Despite numerous experimental reports, the electronic structure of these materials is still challenging to elucidate, and not attainable in the isolated, but chemically inaccessible, [IrO6](8-) species (the simplest molecular analogue of the elementary {IrO6}(8-) fragment present in all iridates). Here, we introduce an alternative approach to circumvent this problem by substituting the oxide ions in [IrO6](8-) by isoelectronic fluorides to form the fluorido-iridate: [IrF6](2-). This molecular species has the same electronic ground state as the {IrO6}(8-) fragment, and thus emerges as an ideal model for iridates. These results may open perspectives for using fluorido-iridates as building-blocks for electronic and magnetic quantum materials synthesized by soft chemistry routes.

6.
ACS Nano ; 10(6): 5663-9, 2016 06 28.
Artigo em Inglês | MEDLINE | ID: mdl-27139335

RESUMO

The organization of single-molecule magnets (SMMs) on surfaces via thermal sublimation is a prerequisite for the development of future devices for spintronics exploiting the richness of properties offered by these magnetic molecules. However, a change in the SMM properties due to the interaction with specific surfaces is usually observed. Here we present a rare example of an SMM system that can be thermally sublimated on gold surfaces while maintaining its intact chemical structure and magnetic properties. Muon spin relaxation and ac susceptibility measurements are used to demonstrate that, unlike other SMMs, the magnetic properties of this system in thin films are very similar to those in the bulk, throughout the full volume of the film, including regions near the metal and vacuum interfaces. These results exhibit the robustness of chemical and magnetic properties of this complex and provide important clues for the development of nanostructures based on SMMs.

8.
Sci Rep ; 5: 8871, 2015 Mar 09.
Artigo em Inglês | MEDLINE | ID: mdl-25747456

RESUMO

Diamagnetic oxides can, under certain conditions, become ferromagnetic at room temperature and therefore are promising candidates for future material in spintronic devices. Contrary to early predictions, doping ZnO with uniformly distributed magnetic ions is not essential to obtain ferromagnetic samples. Instead, the nanostructure seems to play the key role, as room temperature ferromagnetism was also found in nanograined, undoped ZnO. However, the origin of room temperature ferromagnetism in primarily non-magnetic oxides like ZnO is still unexplained and a controversial subject within the scientific community. Using low energy muon spin relaxation in combination with SQUID and TEM techniques, we demonstrate that the magnetic volume fraction is strongly related to the sample volume fraction occupied by grain boundaries. With molecular dynamics and density functional theory we find ferromagnetic coupled electron states in ZnO grain boundaries. Our results provide evidence and a microscopic model for room temperature ferromagnetism in oxides.

9.
Artigo em Inglês | MEDLINE | ID: mdl-25353500

RESUMO

The results of many experiments on polymers such as polystyrene indicate that the polymer chains near a free surface exhibit enhanced dynamics when compared with the bulk. We have investigated whether this is the case for poly(tetrafluoroethylene) (PTFE) by using zero-field muon-spin-relaxation spectroscopy to characterize a local probe, the F-Mu(+)-F state, which forms when spin-polarized positive muons are implanted in PTFE. Low-energy muons (implantation energies from 2.0 to 23.0 keV) were used to study the F-Mu(+)-F state between ∼ 23 and 191 nm from the free surface of PTFE. Measurements were also made with surface muons (4.1 MeV) where the mean implantation depth is on the order of ∼ 0.6 mm. The relaxation rate of the F-Mu(+)-F state up to ∼ 150 K was found to be significantly higher for muons implanted at 2.0 keV than for higher implantation energies, which suggests that the polymer chains in a region on the order of a few tens of nanometers from the free surface are more mobile than those in the bulk.


Assuntos
Espectroscopia Dielétrica/métodos , Mésons , Modelos Químicos , Politetrafluoretileno/química , Simulação por Computador , Teste de Materiais , Polímeros/química , Marcadores de Spin , Propriedades de Superfície
10.
Phys Rev Lett ; 110(21): 217208, 2013 May 24.
Artigo em Inglês | MEDLINE | ID: mdl-23745924

RESUMO

We have used low-energy implanted muons as a volume sensitive probe of the magnetic properties of EuO(1-x) thin films. We find that static and homogeneous magnetic order persists up to the elevated T(C) in the doped samples, and the muon signal displays the double dome feature also observed in the sample magnetization. Our results appear incompatible with either the magnetic phase separation or bound magnetic polaron descriptions previously suggested to explain the elevated T(C), but are compatible with an RKKY-like interaction mediating magnetic interactions above 69 K.

11.
ACS Nano ; 6(9): 8390-6, 2012 Sep 25.
Artigo em Inglês | MEDLINE | ID: mdl-22917162

RESUMO

We present measurements of the magnetic properties of thin film TbPc(2) single-molecule magnets evaporated on a gold substrate and compare them to those in bulk. Zero-field muon spin relaxation measurements were used to determine the molecular spin fluctuation rate of TbPc(2) as a function of temperature. At low temperature, we find that the fluctuations in films are much faster than in bulk and depend strongly on the distance between the molecules and the Au substrate. We measure a molecular spin correlation time that varies between 1.4 µs near the substrate and 6.6 µs far away from it. We attribute this behavior to differences in the packing of the magnetic cores, which change gradually on the scale of ~10-20 nm away from the TbPc(2)/Au interface.


Assuntos
Imãs , Membranas Artificiais , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Nanotecnologia/instrumentação , Térbio/química , Teste de Materiais , Mésons , Marcadores de Spin
SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA