451 research outputs found
A Spin-1/2 Model for CsCuCl_3 in an External Magnetic Field
CsCuCl_3 is a ferromagnetically stacked triangular spin-1/2 antiferromagnet.
We discuss models for its zero-temperature magnetization process. The models
range from three antiferromagnetically coupled ferromagnetic chains to the full
three-dimensional situation. The situation with spin-1/2 is treated by
expansions around the Ising limit and exact diagonalization. Further,
weak-coupling perturbation theory is used mainly for three coupled chains which
are also investigated numerically using the density-matrix renormalization
group technique. We find that already the three-chain model gives rise to the
plateau-like feature at one third of the saturation magnetization which is
observed in magnetization experiments on CsCuCl_3 for a magnetic field
perpendicular to the crystal axis. For a magnetic field parallel to the crystal
axis, a jump is observed in the experimental magnetization curve in the region
of again about one third of the saturation magnetization. In contrast to
earlier spinwave computations, we do not find any evidence for such a jump with
the model in the appropriate parameter region.Comment: 13 pages LaTeX2e with EPJ macro package (included), 8 (e)ps figures
included using psfig.sty; this is the final version to appear in Eur. Phys. J
B; a few further explanations and one reference adde
Comment on "Anomalous Thermal Conductivity of Frustrated Heisenberg Spin Chains and Ladders"
In a recent letter [Phys. Rev. Lett. 89, 156603 (2002); cond-mat/0201300],
Alvarez and Gros have numerically analyzed the Drude weight for thermal
transport in spin ladders and frustrated chains of up to 14 sites and have
proposed that it remains finite in the thermodynamic limit. In this comment, we
argue that this conclusion cannot be sustained if the finite-size analysis is
taken to larger system sizes.Comment: One page REVTeX4, 1 figure. Published version (minor changes
Finite-temperature ordering in a two-dimensional highly frustrated spin model
We investigate the classical counterpart of an effective Hamiltonian for a
strongly trimerized kagome lattice. Although the Hamiltonian only has a
discrete symmetry, the classical groundstate manifold has a continuous global
rotational symmetry. Two cases should be distinguished for the sign of the
exchange constant. In one case, the groundstate has a 120^\circ spin structure.
To determine the transition temperature, we perform Monte-Carlo simulations and
measure specific heat, the order parameter as well as the associated Binder
cumulant. In the other case, the classical groundstates are macroscopically
degenerate. A thermal order-by-disorder mechanism is predicted to select
another 120^\circ spin-structure. A finite but very small transition
temperature is detected by Monte-Carlo simulations using the exchange method.Comment: 11 pages including 9 figures, uses IOP style files; to appear in J.
Phys.: Condensed Matter (proceedings of HFM2006
Magnetic structure factor of correlated moments in small-angle neutron scattering
The interplay between structural and magnetic properties of nanostructured magnetic materials allows one to
realize unconventional magnetic effects, which results in a demand for experimental techniques to determine
the magnetization profile with nanoscale resolution. Magnetic small-angle neutron scattering (SANS) probes
both the chemical and magnetic nanostructure and is thus a powerful technique, e.g., for the characterization of
magnetic nanoparticles. Here, we show that the conventionally used particle-matrix approach to describe SANS
of magnetic particle assemblies, however, leads to a flawed interpretation. As a remedy, we provide general
expressions for the field-dependent two-dimensional magnetic SANS cross section of correlated moments. It
is shown that for structurally disordered ensembles the magnetic structure factor is in general, and contrary to
common assumptions, (i) anisotropic also in zero field and (ii) that even in saturation the magnetic structure factor
deviates from the nuclear one. These theoretical predictions explain qualitatively the intriguing experimental,
polarized SANS data of an ensemble of dipolar-coupled iron oxide nanoparticles.This project has received funding from the European Commission Framework Programme 7 under Grant Agreement No. 604448 (NanoMag)
Supraferromagnetic correlations in clusters of magnetic nanoflowers
Magnetic nanoflowers are densely packed aggregates of superferromagnetically coupled iron oxide nanocrystallites, which excel during magnetic hyperthermia experiments. Here, we investigate the nature of the moment coupling within a powder of such nanoflowers using spin-resolved small-angle neutron scattering. Within the powder, the nanoparticles are agglomerated to clusters, and we can show that the moments of neighboring nanoflowers tend to align parallel to each other. Thus, the whole system resembles a hierarchical magnetic nanostructure consisting of three distinct levels, i.e., (i) the ferrimagnetic nanocrystallites as building blocks, (ii) the superferromagnetic nanoflowers, and (iii) the supraferromagnetic clusters of nanoflowers. We surmise that such a supraferromagnetic coupling explains the enhanced magnetic hyperthermia performance in the case of interacting nanoflowers.This project received funding from the European Commission Framework Programme 7 under Grant Agreement No. 604448 (NanoMag), the National Research Fund of Luxembourg (No. CORE SANS4NCC Grant), and the Spanish Government (No. MAT2017-83631-C3-R)
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