85 research outputs found
Triplet spin resonance of the Haldane compound with interchain coupling
Spin resonance absorption of the triplet excitations is studied
experimentally in the Haldane magnet PbNi2V2O8. The spectrum has features of
spin S=1 resonance in a crystal field, with all three components, corresponding
to transitions between spin sublevels, being observable. The resonance field is
temperature dependent, indicating the renormalization of excitation spectrum in
interaction between the triplets. Magnetic resonance frequencies and critical
fields of the magnetization curve are consistent with a boson version of the
macroscopic field theory [Affleck 1992, Farutin & Marchenko 2007], implying the
field induced ordering at the critical field, while contradict the previously
used approach of noninteracting spin chains.Comment: 7 pages, 9 figure
High-Field Low-Frequency Spin Dynamics
The theory of exchange symmetry of spin ordered states is extended to the
case of high magnetic field. Low frequency spin dynamics equation for
quasi-goldstone mode is derived for two cases of collinear and noncollinear
antiferromagnets.Comment: 2 page
Magnetic phase diagram of the frustrated S=1/2 chain magnet LiCu_2O_2
We present the results of the magnetization and dielectric constant
measurements on untwinned single crystal samples of the frustrated S=1/2 chain
cuprate LiCu_2O_2. Novel magnetic phase transitions were observed. A spin flop
transition of the spiral spin plane was observed for the field orientations
H||a,b. The second magnetic transition was observed at H~15 T for all three
principal field directions. This high field magnetic phase is discussed as a
collinear spin-modulated phase which is expected for an S=1/2 nearest-neighbor
ferromagnetic and next-nearest-neighbor antiferromagnetic chain system
Magnetic excitations in dipolar pyrochlore antiferromagnet GdSnO
The spin dynamics in the geometrically frustrated pyrochlore antiferromagnet
is studied by means of the electron spin resonance. In the
ordered phase ( K), we have detected three gapped resonance modes.
Their values agree well with the developed spin-wave theory which takes into
account the Heisenberg nearest-neighbor exchange, the single-ion anisotropy and
the long-range dipolar interactions. The theory also predicts a fourth
lowest-frequency gap, which lies beyond the experimental range of frequencies,
but determines the exponential decrease of the specific heat at low
temperature.Comment: 8 pages, 5 figure
Magnetic excitations in dipolar pyrochlore antiferromagnet GdSnO
The spin dynamics in the geometrically frustrated pyrochlore antiferromagnet
is studied by means of the electron spin resonance. In the
ordered phase ( K), we have detected three gapped resonance modes.
Their values agree well with the developed spin-wave theory which takes into
account the Heisenberg nearest-neighbor exchange, the single-ion anisotropy and
the long-range dipolar interactions. The theory also predicts a fourth
lowest-frequency gap, which lies beyond the experimental range of frequencies,
but determines the exponential decrease of the specific heat at low
temperature.Comment: 8 pages, 5 figure
Quasi two-dimensional antiferromagnet on a triangular lattice RbFe(MoO4)2
RbFe(MoO4)2 is a rare example of a nearly two-dimensional Heisenberg
antiferromagnet on a triangular lattice. Magnetic resonance spectra and
magnetization curves reveal that the system has a layered spin structure with
six magnetic sublattices. The sublattices within a layer are arranged in a
triangular manner with the magnetization vectors 120 degree apart. The H-T
phase diagram, containing at least five different magnetic phases is
constructed. In zero field, RbFe(MoO4)2 undergoes a phase transition at T_N=3.8
K into a non-collinear triangular spin structure with all the spins confined in
the basal plane. The application of an in-plane magnetic field induces a
collinear spin state between the fields H_c1=47 kOe and H_c2=71 kOe and
produces a magnetization plateau at one-third of the saturation moment. Both
the ESR and the magnetization measurements also clearly indicate an additional
first-order phase transition in a field of 35 kOe. The exact nature of this
phase transition is uncertain.Comment: 9 pages incl 11 figure
Magnetocaloric effect in pyrochlore antiferromagnet Gd2Ti2O7
An adiabatic demagnetization process is studied in Gd2Ti2O7, a geometrically
frustrated antiferromagnet on a pyrochlore lattice. In contrast to conventional
paramagnetic salts, this compound can exhibit a temperature decrease by a
factor of ten in the temperature range below the Curie-Weiss constant. The most
efficient cooling is observed in the field interval between 120 and 60 kOe
corresponding to a crossover between saturated and spin-liquid phases. This
phenomenon indicates that a considerable part of the magnetic entropy survives
in the strongly correlated state. According to the theoretical model, this
entropy is associated with a macroscopic number of local modes remaining
gapless till the saturation field. Monte Carlo simulations on a classical spin
model demonstrate good agreement with the experiment. The cooling power of the
process is experimentally estimated with a view to possible technical
applications. The results for Gd2Ti2O7 are compared to those for Gd3Ga5O12, a
well-known material for low temperature magnetic refrigeration.Comment: 6 pages, 4 figures, accepted versio
Exact solution of the geometrically frustrated spin-1/2 Ising-Heisenberg model on the triangulated Kagome (triangles-in-triangles) lattice
The geometric frustration of the spin-1/2 Ising-Heisenberg model on the
triangulated Kagome (triangles-in-triangles) lattice is investigated within the
framework of an exact analytical method based on the generalized star-triangle
mapping transformation. Ground-state and finite-temperature phase diagrams are
obtained along with other exact results for the partition function, Helmholtz
free energy, internal energy, entropy, and specific heat, by establishing a
precise mapping relationship to the corresponding spin-1/2 Ising model on the
Kagome lattice. It is shown that the residual entropy of the disordered spin
liquid phase is for the quantum Ising-Heisenberg model significantly lower than
for its semi-classical Ising limit (S_0/N_T k_B = 0.2806 and 0.4752,
respectively), which implies that quantum fluctuations partially lift a
macroscopic degeneracy of the ground-state manifold in the frustrated regime.
The investigated model system has an obvious relevance to a series of polymeric
coordination compounds Cu_9X_2(cpa)_6 (X=F, Cl, Br and cpa=carboxypentonic
acid) for which we made a theoretical prediction about the temperature
dependence of zero-field specific heat.Comment: 13 pages, 7 figures, submitted to Phys. Rev.
BIOLOGICAL EFFECTS OF THE NEW SILVER-CONTAINING POLYMER NANOCOMPOSITE
In the materials there are presented, results of the synthesis and biological testing of the new nanocomposite which, contains silver nanoparticles incapsulated. in the synthetic polymer poly-1-vinyl-1,2,4-triazole. The synthesis method, of silvercontaining polymere nanocomposite is shown. With the use of modern equipment evidence of the nanocomposite with the size of 2-10 nm silver production, are substinated. It was revealed, that the new nanocomposite causes slight changes in the tissue structure of experimental animals internal organs and biochemical shift that is characteristic for the compensatory-adaptive reactions development. There are presented, the results of the silver nanoparticles distribution, mainly in the kidney and. liver tissue
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