150 research outputs found
Frustrated square lattice with spatial anisotropy: crystal structure and magnetic properties of PbZnVO(PO4)2
Crystal structure and magnetic properties of the layered vanadium phosphate
PbZnVO(PO4)2 are studied using x-ray powder diffraction, magnetization and
specific heat measurements, as well as band structure calculations. The
compound resembles AA'VO(PO4)2 vanadium phosphates and fits to the extended
frustrated square lattice model with the couplings J(1), J(1)' between
nearest-neighbors and J(2), J(2)' between next-nearest-neighbors. The
temperature dependence of the magnetization yields estimates of averaged
nearest-neighbor and next-nearest-neighbor couplings, J(1) ~ -5.2 K and J(2) ~
10.0 K, respectively. The effective frustration ratio alpha=J(2)/J(1) amounts
to -1.9 and suggests columnar antiferromagnetic ordering in PbZnVO(PO4)2.
Specific heat data support the estimates of J(1) and J(2) and indicate a likely
magnetic ordering transition at 3.9 K. However, the averaged couplings
underestimate the saturation field, thus pointing to the spatial anisotropy of
the nearest-neighbor interactions. Band structure calculations confirm the
identification of ferromagnetic J(1), J(1)' and antiferromagnetic J(2), J(2)'
in PbZnVO(PO4)2 and yield J(1)'-J(1) ~ 1.1 K in excellent agreement with the
experimental value of 1.1 K, deduced from the difference between the expected
and experimentally measured saturation fields. Based on the comparison of
layered vanadium phosphates with different metal cations, we show that a
moderate spatial anisotropy of the frustrated square lattice has minor
influence on the thermodynamic properties of the model. We discuss relevant
geometrical parameters, controlling the exchange interactions in these
compounds, and propose a new route towards strongly frustrated square lattice
materials.Comment: 14 pages, 9 figures, 5 table
Spin ladder compound Pb(0.55)Cd(0.45)V(2)O(5): synthesis and investigation
The complex oxide Pb(0.55)Cd(0.45)V(2)O(5) was synthesized and investigated
by means of X-ray powder diffraction, electron diffraction, magnetic
susceptibility measurements and band structure calculations. Its structure is
similar to that of MV(2)O(5) compounds (M = Na, Ca) giving rise to a spin
system of coupled S=1/2 two-leg ladders. Magnetic susceptibility measurements
reveal a spin gap-like behavior with \Delta ~ 270 K and a spin singlet ground
state. Band structure calculations suggest Pb(0.55)Cd(0.45)V(2)O(5) to be a
system of weakly coupled dimers in perfect agreement with the experimental
data. Pb(0.55)Cd(0.45)V(2)O(5) provides an example of the modification of the
spin system in layered vanadium oxides by cation substitution. Simple
correlations between the cation size, geometrical parameters and exchange
integrals for the MV(2)O(5)-type oxides are established and discussed.Comment: 8 pages, 7 figure
Magnetic properties of Ag(2)VOP(2)O(7): an unexpected spin dimer system
Magnetic properties of the silver vanadium phosphate Ag(2)VOP(2)O(7) are
studied by means of magnetic susceptibility measurements and electronic
structure calculations. In spite of the layered crystal structure suggesting 1D
or 2D magnetic behavior, this compound can be understood as a spin dimer
system. The fit of the magnetic susceptibility indicates an intradimer
interaction of about 30 K in perfect agreement with the computational results.
Our study emphasizes the possible pitfalls in interpreting experimental data on
structural basis only and points out the importance of microscopic models for
the understanding of the magnetic properties of vanadium phosphates.Comment: 8 pages, 5 figures; to appear in PR
Frustrated spin-1/2 square lattice in the layered perovskite PbVO(3)
We report on the magnetic properties of the layered perovskite PbVO(3). The
results of magnetic susceptibility and specific heat measurements as well as
band structure calculations consistently suggest that the S=1/2 square lattice
of vanadium atoms in PbVO(3) is strongly frustrated due to
next-nearest-neighbor antiferromagnetic interactions. The ratio of
next-nearest-neighbor (J(2)) to nearest-neighbor (J(1)) exchange integrals is
estimated to be J(2)/J(1)\approx 0.2-0.4. Thus, PbVO(3) is within or close to
the critical region of the J(1)-J(2) frustrated square lattice. Supporting
this, no sign of long-range magnetic ordering was found down to 1.8 K.Comment: 4 pages, 4 figures, 2 table
Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility
The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and
Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat
measurements. In both compounds, the results can be very well described by an
S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82
K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9,
antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange
of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was
found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain
coupling. In both compounds, we observe a pronounced Curie-like increase of the
susceptibility below 30 K, which we tentatively attribute to a staggered field
effect induced by the applied magnetic field. Results of LDA calculations
support the quasi one-dimensional character and indicate that in Sr2V3O9, the
magnetic chain is perpendicular to the structural one with the magnetic
exchange being transferred through VO4 tetrahedra.Comment: Submitted to Phy. Rev.
New methodology for the characterization of endoglucanase activity and its application on the Trichoderma longibrachiatum cellulolytic complex
The cellulolytic complex of Trichoderma longibrachiatum was separated in nine fractions using FPLC. The avicelase, cellobiase, carboxymethylcellulase, and endoglucanase activities of these fractions were characterized. The endoglucanase activity was measured by a method that allows the determination of the variation in the degree of polymerization of the insoluble celluloses. This method, which is based on the measurement of the reducing power of the insoluble fibers, is proposed as a tool for the identification and characterization of the endoglucanases. Using this technique and H3PO4-swollen cotton as substrate, the kinetic parameters of two proteins that showed high specific endoglucanase activity (pI = 5.25; Mr = 55 kDa; and pI = 4.70; Mr = 70 kDa) were determined. The measurement of the degree of polymerization variation during digestion of Sigmacell gives evidence that the endoglucanase activity is located at the beginning of the reaction.JNICT -Junta Nacional de Investigação CientÃfica e Tecnológicainfo:eu-repo/semantics/publishedVersio
Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry
In this paper we examine the role of crystal chemistry factors in creating
conditions for formation of magnetoelectric ordering in BiFeO3. It is generally
accepted that the main reason of the ferroelectric distortion in BiFeO3 is
concerned with a stereochemical activity of the Bi lone pair. However, the lone
pair is stereochemically active in the paraelectric orthorhombic beta-phase as
well. We demonstrate that a crucial role in emerging of phase transitions of
the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order
types belongs to the change of the degree of the lone pair stereochemical
activity - its consecutive increase with the temperature decrease. Using the
structural data, we calculated the sign and strength of magnetic couplings in
BiFeO3 in the range from 945 C down to 25 C and found the couplings, which
undergo the antiferromagnetic-ferromagnetic transition with the temperature
decrease and give rise to the antiferromagnetic ordering and its delay in
regard to temperature, as compared to the ferroelectric ordering. We discuss
the reasons of emerging of the spatially modulated spin structure and its
suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table
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