9 research outputs found
Antisymmetric exchange and pseudo Jahn-Teller instability in spin-frustrated metal clusters
Tsukerblat BS, Tarantul A, MĆ¼ller A. Antisymmetric exchange and pseudo Jahn-Teller instability in spin-frustrated metal clusters. In: Journal of Molecular Structure. JOURNAL OF MOLECULAR STRUCTURE. Vol 838. ELSEVIER SCIENCE BV; 2007: 124-132.In this article, we analyze the interplay between the antisymmetric (AS) exchange interaction and Jahn-Teller (JT) vibronic coupling in spin-frustrated systems with triangular units. AS exchange in these systems creates a strong first order magnetic anisotropy related to the orbitally degenerate frustrated ground state that is analyzed for the unique structure of the cluster anion present in K-6[(V15As6O42)-As-IV(H2O)] center dot 8H(2)O (V-15 cluster) exhibiting layers of different magnetization. Spin-frustration inherently related to the orbital degeneracy creates a structural instability that is shown to be competitive to the AS exchange. The vibronic pseudo JT coupling arising from the modulation of the exchange interaction by the double degenerate molecular vibrations is shown to reduce AS exchange giving rise to a restoration of magnetization quenched by AS exchange. This leads to an essential reduction of the magnetic anisotropy in spin-frustrated triangular clusters so that in the limit of strong JT coupling the isotropic exchange model becomes adequate. (c) 2007 Elsevier B.V. All rights reserved
About the nature of luminescent bands in undoped and Eu2+ doped SrAl2O4 phosphors
In this paper we report the luminescence properties of nanosized undoped and Eu2+ (1%) doped SrAl2O4 phosphors. The samples were prepared by combustion method at 600 ā¦C followed by annealing of the resultant combustion ash at 1000 ā¦C in a reductive (Ar + H2) atmosphere. Photo luminescence (PL) and cathodoluminescence (CL) analyses were applied to characterize the phosphors. The qualitative energy level scheme of doped crystal SrAl2O4:Eu2+ and nature of defects in undoped phosphor is proposed and discussed. Within a simplified model of a single vibration and linear vibronic coupling the shape-function of the vibrationally assisted band (Pekarian) is analyzed with the emphasis on the vibrational structure. We propose an approximate approach to pass from the discrete Pekarian distribution to the structureless crystal field spectra taking into account phonon dispersion. This approach is expected to be useful for the description of the shape of the bands when the electronic levels are close to the conduction band of the host crystal
Theoretical Modeling of the Magnetic Behavior of Thiacalix[4]arene Tetranuclear Mn<sup>II</sup><sub>2</sub>Gd<sup>III</sup><sub>2</sub> and Co<sup>II</sup><sub>2</sub>Eu<sup>III</sup><sub>2</sub> Complexes
In
view of a wide perspective of 3dā4f complexes in single-molecule
magnetism, here we propose an explanation of the magnetic behavior
of the two thiacalix[4]Āarene tetranuclear heterometallic complexes
Mn<sup>II</sup><sub>2</sub>Gd<sup>III</sup><sub>2</sub> and Co<sup>II</sup><sub>2</sub>Eu<sup>III</sup><sub>2</sub>. The energy pattern
of the Mn<sup>II</sup><sub>2</sub>Gd<sup>III</sup><sub>2</sub> complex
evaluated in the framework of the isotropic exchange model exhibits
a rotational band of the low-lying spin excitations within which the
LandeĢ intervals are affected by the biquadratic spināspin
interactions. The nonmonotonic temperature dependence of the Ļ<i>T</i> product observed for the Mn<sup>II</sup><sub>2</sub>Gd<sup>III</sup><sub>2</sub> complex is attributed to the competitive influence
of the ferromagnetic MnāGd and antiferromagnetic MnāMn
exchange interactions, the latter being stronger (<i>J</i>(Mn, Mn) = ā1.6 cm<sup>ā1</sup>, <i>J</i><sub>s</sub>(Mn, Gd) = 0.8 cm<sup>ā1</sup>, <i>g</i> = 1.97). The model for the Co<sup>II</sup><sub>2</sub>Eu<sup>III</sup><sub>2</sub> complex includes uniaxial anisotropy of the seven-coordinate
Co<sup>II</sup> ions and an isotropic exchange interaction in the
Co<sup>II</sup><sub>2</sub> pair, while the Eu<sup>III</sup> ions
are diamagnetic in their ground states. Best-fit analysis of Ļ<i>T</i> versus <i>T</i> showed that the anisotropic
contribution (arising from a large zero-field splitting in Co<sup>II</sup> ions) dominates (weak-exchange limit) in the Co<sup>II</sup><sub>2</sub>Eu<sup>III</sup><sub>2</sub> complex (<i>D</i> = 20.5 cm<sup>ā1</sup>, <i>J</i> = ā0.4
cm<sup>ā1</sup>, <i>g</i><sub>Co</sub> = 2.22). This
complex is concluded to exhibit an easy plane of magnetization (arising
from the Co<sup>II</sup> pair). It is shown that the low-lying part
of the spectrum can be described by a highly anisotropic effective
spin-<sup>1</sup>/<sub>2</sub> Hamiltonian that is deduced for the
Co<sup>II</sup><sub>2</sub> pair in the weak-exchange limit
Mixed-Valence Molecular Unit for Quantum Cellular Automata: Beyond the BornāOppenheimer Paradigm through the Symmetry-Assisted Vibronic Approach
Unusual Magnetic Behavior of Six-Coordinate, Mixed-Ligand Re(II) Complexes:Ā Origin of a Strong Temperature-Independent Paramagnetism ā
Single-Ion Magnet Et<sub>4</sub>N[Co<sup>II</sup>(hfac)<sub>3</sub>] with Nonuniaxial Anisotropy: Synthesis, Experimental Characterization, and Theoretical Modeling
In
this article we report the synthesis and structure of the new
CoĀ(II) complex Et<sub>4</sub>NĀ[Co<sup>II</sup>Ā(hfac)<sub>3</sub>] (<b>I</b>) (hfac = hexafluoroacetylacetonate) exhibiting
single-ion magnet (SIM) behavior. The performed analysis of the magnetic
characteristics based on the complementary experimental techniques
such as static and dynamic magnetic measurements, electron paramagnetic
resonance spectroscopy in conjunction with the theoretical modeling
(parametric Hamiltonian and ab initio calculations) demonstrates that
the SIM properties of <b>I</b> arise from the nonuniaxial magnetic
anisotropy with strong positive axial and significant rhombic contributions