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@PHDTHESIS{Reim:200944,
author = {Reim, Johannes},
title = {{M}agnetic order and spin dynamics in the extended kagome
system {C}a{B}a{C}o$_{2}${F}e$_{2}${O}$_{7}$},
volume = {115},
school = {RWTH Aachen},
type = {Dr.},
address = {Jülich},
publisher = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
reportid = {FZJ-2015-03285},
isbn = {978-3-95806-097-5},
series = {Schriften des Forschungszentrums Jülich. Reihe
Schlüsseltechnologien / Key Technologies},
pages = {VIII, 144 S.},
year = {2015},
note = {RWTH Aachen, Diss., 2015},
abstract = {The extended kagome system in the hexagonal Swedenborgite
structure displays similarly to the pyrochlores a highly
frustrated network of tetrahedrally coordinated magnetic
ions. These ions form stacked kagome and triangular layers
resulting in double tetrahedra columns perpendicular to said
layers. However, its broken inversion symmetry raises
further the complexity of ordering due to non-vanishing
Dzyaloshinskii-Moriya interactions. Recently investigated
compounds of this family show various signs for unusual
geometric frustration and disordered ground states despite
of the typically strong antiferromagnetic exchange. In the
present thesis, the magnetic order and the spin dynamics
observed on the antiferromagnetically coupled material
CaBaCo$_{2}$Fe$_{2}$O$_{7}$ a Swedenborgite are studied in
detail. As a primary tool for the experimental
investigation, different types of neutron scattering
instruments have been used. The experiments are complemented
from the theoretical side with Monte Carlo simulations for
the modelling of the magnetic order and spin dynamics
simulations for the determination of the excitations. Both
methods are based on the same Heisenberg nearest neighbour
model, in which the in-plane J$_{in}$ and out-of-plane
J$_{out}$ exchange interactions are distinguished. In the
phase diagram determined using this model an
antiferromagnetically ordered phase was found for ratios
$\tau$ = J$_{out}/J_{in} \geq$ 1.5 and a 3D spin liquid at
lower ratios. The double tetrahedron formed by a triangle of
the kagome and two sites of different triangular lattices
was found to be the essential building block for the
magnetic structure. From the so-called sum rule for the
building block derived from the energy minimization follows,
that the triangular spins align parallel and the sum of the
kagome spins compensates the triangular spins weighted with
ratio $\tau$. Thus, in the spin liquid phase the kagome
spins tilt with regard to the antiferromagnetically ordered
spin structure depending on the ratio of the interactions.
In most materials of this compound family, the deviation is
so high that only short range order is present. So far,
CaBaCo$_{2}$Fe$_{2}$O$_{7}$ is the only known compound of
this family, which exhibits a long-range three dimensional
order, without a structural transition to an orthorhombic
symmetry. This alone already makes the present material
unique and interesting as a model system for understanding
the interactions and the influence of geometric frustration.
A commensurate magnetic order with a $\sqrt{3}$ x $\sqrt{3}$
larger unit cell was reported to arise below 160 K. Using
high-resolution powder as well as single crystal
diffractometers, a long-periodic modulation was observed in
addition to the commensurate magnetic order. The long
periodic modulation is only present within the kagome and
triangular layers, with the order remaining commensurate
perpendicular to the layers. The period of the modulation
found to be temperature-independent corresponds to a
periodicity of about 370Å or 58 crystallographic unit cells
and a propagation vector $\textbf{k} \approx$ (0.342, 0.342,
0). The scattering pattern observed in neutron single
crystal measurements is related to a multi-$\textit{q}$
antiferromagnetic structure with $\textbf{k}$ and its
symmetrically equivalents. In order to investigate the spin
structure related to the long periodic order, neutron
scattering experiments with polarization analysis were
performed. From the polarization channels, the scattering
contributions from moments parallel and perpendicular to the
scattering plane as well as the chiral magnetic scattering,
a scattering contribution sensitive to vectorial chirality,
have been separated. These reveal that the long periodic
spin structure (prominent in intensity below 20 K) is canted
more strongly in comparison to the more coplanar spin
structure in the case of the commensurate order (prominent
in intensity above 80 K). A continuous reorientation between
the two spin structures is visible in neutron scattering as
well as magnetisation data. With the broken inversion
symmetry the Dzyaloshinskii-Moriya (DM) interactions can be
present. A suitable choice of DM vectors was shown to create
energy minima in the energy-vs-reciprocal [...]},
cin = {PGI-4 / JCNS-2 / JARA-FIT / JCNS (München) ; Jülich
Centre for Neutron Science JCNS (München) ; JCNS-FRM-II},
cid = {I:(DE-Juel1)PGI-4-20110106 / I:(DE-Juel1)JCNS-2-20110106 /
$I:(DE-82)080009_20140620$ /
I:(DE-Juel1)JCNS-FRM-II-20110218},
pnm = {144 - Controlling Collective States (POF3-144) / 524 -
Controlling Collective States (POF3-524) / 6213 - Materials
and Processes for Energy and Transport Technologies
(POF3-621) / 6G4 - Jülich Centre for Neutron Research
(JCNS) (POF3-623)},
pid = {G:(DE-HGF)POF3-144 / G:(DE-HGF)POF3-524 /
G:(DE-HGF)POF3-6213 / G:(DE-HGF)POF3-6G4},
experiment = {EXP:(DE-MLZ)DNS-20140101},
typ = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
url = {https://juser.fz-juelich.de/record/200944},
}
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