748 research outputs found
Intracluster interactions in butterfly {Fe3 LnO2} molecules with the non-Kramers ions Tb(III) and Ho(III)
The intracluster exchange interactions within the "butterfly" Fe3Ln(µ3-O)2(CCl3COO)8(H2O)(THF)3] molecules, where Ln(III) represents a lanthanide cation, have been determined by a combination of x-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometry (VSM) along with an interaction model. We have studied the compounds with Ln=Tb and Ho, both non-Kramers lanthanides and with high uniaxial anisotropy, and Ln=Lu(III) and Y(III) as pseudolanthanides, which supply nonmagnetic Ln reference cases. At low temperature, the three Fe atoms can be considered as a self-unit with total spin SFe3=5/2. Using the element selectivity of the XMCD magnetometry, measured at the Ln L2,3 edges, together with the VSM measurements, the local magnetization of the Ln ion and the Fe3 subcluster, as a function of the field and low temperature (T˜2.5K), has been determined separately. These results are described quantitatively in the framework of a theoretical model based on an effective spin Hamiltonian, which considers the competing effects of intracluster interactions and the external applied magnetic field. The Ln-Fe3 exchange interaction within the {Fe3LnO2} cluster has been determined to be antiferromagnetic, in both Tb and Ho compounds, with JFeTb/kB=-0.13(1)K and JFeHo/kB=-0.18(1)K, respectively. In both cases, a field-induced reorientation of the Fe3 and Ln spins from antiparallel to parallel orientation takes place at a threshold field µ0H=1.1 and 2 T, for the {Fe3TbO2} and {Fe3HoO2} compounds, respectively. By comparison with other compounds of the series with uniaxial anisotropy, it is concluded that the polarizability of the Fe3 subcluster magnetic moment decreases in the trend {Fe3YO2}¿{Fe3TbO2}¿{Fe3HoO2}¿{Fe3DyO2}, because of the increasing opposition of the exchange antiferromagnetic field caused by the Ln ion. In the Ln=Tb, Ho, and Dy, the magnetization of the whole molecule is dominated by the anisotropy of the Ln ion. The intracluster Fe3-Ln exchange interactions are very weak compared to the Ln ligand field and Fe-Fe exchange interactions
Two-dimensional approach to relativistic positioning systems
A relativistic positioning system is a physical realization of a coordinate
system consisting in four clocks in arbitrary motion broadcasting their proper
times. The basic elements of the relativistic positioning systems are presented
in the two-dimensional case. This simplified approach allows to explain and to
analyze the properties and interest of these new systems. The positioning
system defined by geodesic emitters in flat metric is developed in detail. The
information that the data generated by a relativistic positioning system give
on the space-time metric interval is analyzed, and the interest of these
results in gravimetry is pointed out.Comment: 11 pages, 5 figures. v2: a brief description of the principal
bibliography has been adde
Spin glass behavior in an interacting gamma-Fe2O3 nanoparticle system
In this paper we investigate the superspin glass behavior of a concentrated
assembly of interacting maghemite nanoparticles and compare it to that of
canonical atomic spin glass systems. ac versus temperature and frequency
measurements show evidence of a superspin glass transition taking place at low
temperature. In order to fully characterize the superspin glass phase, the
aging behavior of both the thermo-remanent magnetization (TRM) and ac
susceptibility has been investigated. It is shown that the scaling laws obeyed
by superspin glasses and atomic spin glasses are essentially the same, after
subtraction of a superparamagnetic contribution from the superspin glass
response functions. Finally, we discuss a possible origin of this
superparamagnetic contribution in terms of dilute spin glass models
Heteronuclear {TbxEu1-x} furoate 1D polymers presenting luminescent properties and SMM behavior
We report the synthesis, crystal structure and photo-magnetic properties of novel Tb/Eu polymeric complexes of general formula {TbxEu1-x(a-fur)3(H2O)3}n, supported by 2-furancarboxilic acid: the homonuclear Tb(iii) complex {Tb} (1), four heterodinuclear complexes, {Tb0.8Eu0.2} (2), {Tb0.7Eu0.3} (3), {Tb0.3Eu0.7} (4), and {Tb0.1Eu0.9} (5), and the homonuclear Eu(iii)-only complex {Eu} (6). X-ray diffraction experiments show that the a-furoate ligands, acting in bridging mode, consolidate the 1D polymeric chains along the c-axis. Luminescence studies show the sensitization capability of the furoic acid ligand. Color tuning from green to red can be successfully achieved through the heterodinuclear strategy. We have measured Eu emission by direct excitation at the resonant 7F0 ¿ 5L6 (395 nm), and indirectly, by excitation of the non-resonant wavelength (280 nm) which provokes ligand ¿ Tb ¿ Eu energy transfer. Besides, ac susceptibility measurements under varying frequencies and temperatures reveal that mixed {TbxEu1-x} complexes exhibit field-induced slow relaxation dynamics, with extremely slow relaxation times, owing to direct processes affected by the bottleneck effect. Thus, the {TbxEu1-x} complexes represent interesting low-dimensional multifunctional materials combining both luminescent and SMM magnetic properties
Heteronuclear {TbxEu1-x} furoate 1D polymers presenting luminiscent properties and SMM behavior
We report the synthesis, crystal structure and photo-magnetic properties of novel Tb/Eu polymeric complexes of general formula {TbxEu1−x(α-fur)3(H2O)3}n, supported by 2-furancarboxilic acid: the homonuclear Tb(III) complex {Tb} (1), four heterodinuclear complexes, {Tb0.8Eu0.2} (2), {Tb0.7Eu0.3} (3), {Tb0.3Eu0.7} (4), and {Tb0.1Eu0.9} (5), and the homonuclear Eu(III)-only complex {Eu} (6). X-ray diffraction experiments show that the α-furoate ligands, acting in bridging mode, consolidate the 1D polymeric chains along the c-axis. Luminescence studies show the sensitization capability of the furoic acid ligand. Color tuning from green to red can be successfully achieved through the heterodinuclear strategy. We have measured Eu emission by direct excitation at the resonant 7F0 → 5L6 (395 nm), and indirectly, by excitation of the non-resonant wavelength (280 nm) which provokes ligand → Tb → Eu energy transfer. Besides, ac susceptibility measurements under varying frequencies and temperatures reveal that mixed {TbxEu1−x} complexes exhibit field-induced slow relaxation dynamics, with extremely slow relaxation times, owing to direct processes affected by the bottleneck effect. Thus, the {TbxEu1−x} complexes represent interesting low-dimensional multifunctional materials combining both luminescent and SMM magnetic properties
Heteronuclear {TbxEu1−x} furoate 1D polymers presenting luminescent properties and SMM behavior
We report the synthesis, crystal structure and photo-magnetic properties of novel Tb/Eu polymeric complexes of general formula {TbxEu1−x(α-fur)3(H2O)3}n, supported by 2-furancarboxilic acid: the homonuclear Tb(III) complex {Tb} (1), four heterodinuclear complexes, {Tb0.8Eu0.2} (2), {Tb0.7Eu0.3} (3), {Tb0.3Eu0.7} (4), and {Tb0.1Eu0.9} (5), and the homonuclear Eu(III)-only complex {Eu} (6). X-ray diffraction experiments show that the α-furoate ligands, acting in bridging mode, consolidate the 1D polymeric chains along the c-axis. Luminescence studies show the sensitization capability of the furoic acid ligand. Color tuning from green to red can be successfully achieved through the heterodinuclear strategy. We have measured Eu emission by direct excitation at the resonant 7F0 → 5L6 (395 nm), and indirectly, by excitation of the non-resonant wavelength (280 nm) which provokes ligand → Tb → Eu energy transfer. Besides, ac susceptibility measurements under varying frequencies and temperatures reveal that mixed {TbxEu1−x} complexes exhibit field-induced slow relaxation dynamics, with extremely slow relaxation times, owing to direct processes affected by the bottleneck effect. Thus, the {TbxEu1−x} complexes represent interesting low-dimensional multifunctional materials combining both luminescent and SMM magnetic properties
Observation of a Griffiths-like phase in the paramagnetic regime of ErCo_2
A systematic x-ray magnetic circular dichroism study of the paramagnetic
phase of ErCo2 has recently allowed to identify the inversion of the net
magnetization of the Co net moment with respect to the applied field well above
the ferrimagnetic ordering temperature, Tc. The study of small angle neutron
scattering measurements has also shown the presence of short range order
correlations in the same temperature region. This phenomenon, which we have
denoted parimagnetism, may be related with the onset of a Griffiths-like phase
in paramagnetic ErCo2. We have measured ac susceptibility on ErCo2 as a
function of temperature, applied field, and excitation frequency. Several
characteristics shared by systems showing a Griffiths phase are present in
ErCo2, namely the formation of ferromagnetic clusters in the disordered phase,
the loss of analyticity of the magnetic susceptibility and its extreme
sensitivity to an applied magnetic field. The paramagnetic susceptibility
allows to establish that the magnetic clusters are only formed by Co moments as
well as the intrinsic nature of those Co moments
Magnetic circular dichroism of x-ray absorption spectroscopy at rare-earth L2,3 edges in RE2Fe14B compounds (RE = La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu)
Magnetic circular dichroism (MCD) in the x-ray absorption spectroscopy (XAS)
at the L2,3 edges for almost entire series of rare-earth (RE) elements in
RE2Fe14B, is studied experimentally and theoretically. By a quantitative
comparison of the complicated MCD spectral shapes, we find that (i) the 4f-5d
intra-atomic exchange interaction not only induces the spin and orbital
polarization of the 5d states, which is vital for the MCD spectra of the
electric dipole transition from the 2p core states to the empty 5d conduction
band, but also it accompanies a contraction of the radial part of the 5d wave
function depending on its spin and orbital state, which results in the
enhancement of the 2p-5d dipole matrix element, (ii) there are cases where the
spin polarization of the 5d states due to the hybridization with the spin
polarized 3d states of surrounding irons plays important roles, and (iii) the
electric quadrupole transition from the 2p core states to the magnetic vale!
nce 4f states is appreciable at the pre-edge region of the dipole spectrum.
Especially, our results evidence that it is important to include the
enhancement effect of the dipole matrix element in the correct interpretation
of the MCD spectra at the RE L2,3 edges.Comment: 9 pages, 5 figures, 1 table, REVTe
Magnetic and thermal properties of 4f-3d ladder-type molecular compounds
We report on the low-temperature magnetic susceptibilities and specific heats
of the isostructural spin-ladder molecular complexes L[M(opba)]_{3\cdot
xDMSOHO, hereafter abbreviated with LM (where L =
La, Gd, Tb, Dy, Ho and M = Cu, Zn). The results show that the Cu containing
complexes (with the exception of LaCu) undergo long range magnetic
order at temperatures below 2 K, and that for GdCu this ordering is
ferromagnetic, whereas for TbCu and DyCu it is probably
antiferromagnetic. The susceptibilities and specific heats of TbCu
and DyCu above have been explained by means of a model
taking into account nearest as well as next-nearest neighbor magnetic
interactions. We show that the intraladder L--Cu interaction is the predominant
one and that it is ferromagnetic for L = Gd, Tb and Dy. For the cases of Tb, Dy
and Ho containing complexes, strong crystal field effects on the magnetic and
thermal properties have to be taken into account. The magnetic coupling between
the (ferromagnetic) ladders is found to be very weak and is probably of dipolar
origin.Comment: 13 pages, 15 figures, submitted to Phys. Rev.
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