184 research outputs found
Magnetization Process of Kagome-Lattice Heisenberg Antiferromagnet
The magnetization process of the isotropic Heisenberg antiferromagnet on the
kagome lattice is studied. Data obtained from the numerical-diagonalization
method are reexamined from the viewpoint of the derivative of the magnetization
with respect to the magnetic field. We find that the behavior of the derivative
at approximately one-third of the height of the magnetization saturation is
markedly different from that for the cases of typical magnetization plateaux.
The magnetization process of the kagome-lattice antiferromagnet reveals a new
phenomenon, which we call the "magnetization ramp".Comment: 4 pages, 5figures, accepted in J. Phys. Soc. Jpn
Distribution of exchange energy in a bond-alternating S=1 quantum spin chain
The quasi-one-dimensional bond-alternating S=1 quantum antiferromagnet NTENP
is studied by single crystal inelastic neutron scattering. Parameters of the
measured dispersion relation for magnetic excitations are compared to existing
numerical results and used to determine the magnitude of bond-strength
alternation. The measured neutron scattering intensities are also analyzed
using the 1st-moment sum rules for the magnetic dynamic structure factor, to
directly determine the modulation of ground state exchange energies. These
independently determined modulation parameters characterize the level of spin
dimerization in NTENP. First-principle DMRG calculations are used to study the
relation between these two quantities.Comment: 10 pages, 10 figure
High field level crossing studies on spin dimers in the low dimensional quantum spin system NaT(CO)(HO) with T=Ni,Co,Fe,Mn
In this paper we demonstrate the application of high magnetic fields to study
the magnetic properties of low dimensional spin systems. We present a case
study on the series of 2-leg spin-ladder compounds
NaT(CO)(HO) with T = Ni, Co, Fe and Mn. In all
compounds the transition metal is in the high spin configuation. The
localized spin varies from S=1 to 3/2, 2 and 5/2 within this series. The
magnetic properties were examined experimentally by magnetic susceptibility,
pulsed high field magnetization and specific heat measurements. The data are
analysed using a spin hamiltonian description. Although the transition metal
ions form structurally a 2-leg ladder, an isolated dimer model consistently
describes the observations very well. This behaviour can be understood in terms
of the different coordination and superexchange angles of the oxalate ligands
along the rungs and legs of the 2-leg spin ladder. All compounds exhibit
magnetic field driven ground state changes which at very low temperatures lead
to a multistep behaviour in the magnetization curves. In the Co and Fe
compounds a strong axial anisotropy induced by the orbital magnetism leads to a
nearly degenerate ground state and a strongly reduced critical field. We find a
monotonous decrease of the intradimer magnetic exchange if the spin quantum
number is increased
Spin-lattice instability to a fractional magnetization state in the spinel HgCr2O4
Magnetic systems are fertile ground for the emergence of exotic states when
the magnetic interactions cannot be satisfied simultaneously due to the
topology of the lattice - a situation known as geometrical frustration.
Spinels, AB2O4, can realize the most highly frustrated network of
corner-sharing tetrahedra. Several novel states have been discovered in
spinels, such as composite spin clusters and novel charge-ordered states. Here
we use neutron and synchrotron X-ray scattering to characterize the fractional
magnetization state of HgCr2O4 under an external magnetic field, H. When the
field is applied in its Neel ground state, a phase transition occurs at H ~ 10
Tesla at which each tetrahedron changes from a canted Neel state to a
fractional spin state with the total spin, Stet, of S/2 and the lattice
undergoes orthorhombic to cubic symmetry change. Our results provide the
microscopic one-to-one correspondence between the spin state and the lattice
distortion
Measuring every particle's size from three-dimensional imaging experiments
Often experimentalists study colloidal suspensions that are nominally
monodisperse. In reality these samples have a polydispersity of 4-10%. At the
level of an individual particle, the consequences of this polydispersity are
unknown as it is difficult to measure an individual particle size from
microscopy. We propose a general method to estimate individual particle radii
within a moderately concentrated colloidal suspension observed with confocal
microscopy. We confirm the validity of our method by numerical simulations of
four major systems: random close packing, colloidal gels, nominally
monodisperse dense samples, and nominally binary dense samples. We then apply
our method to experimental data, and demonstrate the utility of this method
with results from four case studies. In the first, we demonstrate that we can
recover the full particle size distribution {\it in situ}. In the second, we
show that accounting for particle size leads to more accurate structural
information in a random close packed sample. In the third, we show that crystal
nucleation occurs in locally monodisperse regions. In the fourth, we show that
particle mobility in a dense sample is correlated to the local volume fraction.Comment: 7 pages, 5 figure
Calculating the energy spectra of magnetic molecules: application of real- and spin-space symmetries
The determination of the energy spectra of small spin systems as for instance
given by magnetic molecules is a demanding numerical problem. In this work we
review numerical approaches to diagonalize the Heisenberg Hamiltonian that
employ symmetries; in particular we focus on the spin-rotational symmetry SU(2)
in combination with point-group symmetries. With these methods one is able to
block-diagonalize the Hamiltonian and thus to treat spin systems of
unprecedented size. In addition it provides a spectroscopic labeling by
irreducible representations that is helpful when interpreting transitions
induced by Electron Paramagnetic Resonance (EPR), Nuclear Magnetic Resonance
(NMR) or Inelastic Neutron Scattering (INS). It is our aim to provide the
reader with detailed knowledge on how to set up such a diagonalization scheme.Comment: 29 pages, many figure
Critical Behavior of Anisotropic Heisenberg Mixed-Spin Chains in a Field
We numerically investigate the critical behavior of the spin-(1,1/2)
Heisenberg ferrimagnet with anisotropic exchange coupling in a magnetic field.
A quantized magnetization plateau as a function of the field, appearing at a
third of the saturated magnetization, is stable over whole the
antiferromagnetic coupling region. The plateau vanishes in the ferromagnetic
coupling region via the Kosterlitz-Thouless transition. Comparing the quantum
and classical magnetization curves, we elucidate what are essential quantum
effects.Comment: 5 pages, Revtex, with 7 eps figures, to appear in Phys. Rev. B (An
extra ps figure (fig7.ps) is included for printing.
Field-Induced Magnetic Ordering in the Quantum Spin System KCuCl
KCuCl is a three-dimensional coupled spin-dimer system and has a singlet
ground state with an excitation gap K. High-field
magnetization measurements for KCuCl have been performed in static magnetic
fields of up to 30 T and in pulsed magnetic fields of up to 60 T. The entire
magnetization curve including the saturation region was obtained at K.
From the analysis of the magnetization curve, it was found that the exchange
parameters determined from the dispersion relations of the magnetic excitations
should be reduced, which suggests the importance of the renormalization effect
in the magnetic excitations. The field-induced magnetic ordering accompanied by
the cusplike minimum of the magnetization was observed as in the isomorphous
compound TlCuCl. The phase boundary was almost independent of the field
direction, and is represented by the power law. These results are consistent
with the magnon Bose-Einstein condensation picture for field-induced magnetic
ordering.Comment: 9 pages, 7 figures, 9 eps files, revtex styl
Multi-plateau magnetization curves of one-dimensional Heisenberg ferrimagnets
Ground-state magnetization curves of ferrimagnetic Heisenberg chains of
alternating spins and are numerically investigated. Calculating several
cases of , we conclude that the spin- chain generally exhibits
magnetization plateaux even at the most symmetric point. In the double- or
more-plateau structure, the initial plateau is generated on a classical basis,
whereas the higher ones are based on a quantum mechanism.Comment: 6 pages, 6 figures embedded, to appear in Phys. Rev. B 01 August 200
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