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@PHDTHESIS{Ovsianikov:952911,
author = {Ovsianikov, Aleksandr},
othercontributors = {Roth, Georg and Brückel, Thomas},
title = {{T}he role of exchange interactions in the formation of the
magnetic structure in rare-earth orthoferrites
{RF}e{O}$_{3}$ ({R}={H}o, {T}b, {Y}b)},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2023-02316},
pages = {1 Online-Ressource : Illustrationen, Diagramme},
year = {2023},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2023, Kumulative Dissertation},
abstract = {The rare earth orthoferrite family RFeO3, where R is a rare
earth element, demonstrates a remarkable variety of magnetic
properties. Its compounds crystallize in an orthorhombic
perovskite structure with the space group Pnma. Different
combinations of Dzyaloshinsky-Moriya interactions (DMI) and
rare-earth ions with different ionic radii and filling of
outer shells lead to a variety of magnetic effects. Rare
earth orthoferrites are nowadays well known as multiferroics
- materials with typically large magnetoelectric (ME)
coupling and show the magnetocaloric effect (MCE). This
cumulative dissertation investigates various orthoferrites
RFeO3 (R=Ho, Tb, Yb) using neutron scattering methods. The
orthoferrite HoFeO3 was studied by single crystal inelastic
neutron scattering. It was shown that the spin dynamics of
the Fe subsystem does not change through the spin
reorientation transitions. The observed spectrum of magnetic
excitations was analyzed in the framework of linear
spin-wave theory. Within this approach the antiferromagnetic
exchange interactions of nearest neighbors and next nearest
neighbors were obtained for the Fe subsystem. Parameters of
DMI at the Fe subsystem were refined. The temperature
dependence of the gap in the Fe spin-wave spectrum indicates
the temperature evolution of the anisotropy parameters.
Estimations for the values of the Fe-Ho and Ho-Ho exchange
interactions were made as well. Using the new polarized
neutron diffraction (PND) setup of the instrument POLI at
MLZ the spin reorientation transition in the HoFeO3 was
studied at different wavelengths. The various experiments
provided reproducible results demonstrating high reliability
of the used setup. It was shown that during the phase
transition at TSR=53 K in an external magnetic field applied
along the crystal c-axis, the ordered magnetic moment of the
Fe sublattice rotates from the crystallographic direction b
to a not just in the ab plane, but through the z axis. This
means that the applied field breaks the orthorhombic
symmetry allowing some magnetization parallel to z within a
small temperature region. Interestingly, this is the same
temperature region where the large magnetocaloric effect for
HoFeO3 was previously reported. A general model of the
magnetic structure of HoFeO3, unconstrained by the
orthorhombic symmetry, would allow the magnitudes and
directions of the moments on each of the 8 magnetic
sublattices in the unit cell to be independent of
one-another, leading to 24 independent magnetic parameters.
PND measurements were used to determine the absolute sign of
the DMI in the ab plane for the Fe magnetic sublattice at 65
K. DMI plays an important role in the energy balance of the
system. Neutron diffraction studies of HoFeO3 single
crystals were performed under external magnetic fields. The
interplay between the external magnetic fields,
Dzyaloshinsky-Moria antisymmetric exchange, isotropic
exchange interactions between Fe and Ho sublattices and
within the Fe sublattice provides a rich magnetic phase
diagram. As result of the balance of exchange interactions
inside the crystal and external magnetic fields, eight
different magnetic phases were found, which are induced or
suppressed dependent on the external field. Investigations
of the orthoferrites TbFeO3 and YbFeO3 were performed by
neutron inelastic scattering and neutron single crystal
diffraction in magnetic fields. The low temperature
evolution of energy gaps was explored for both compounds and
considered from the point of view of changes of rare earth
ion anisotropy. Exchange parameters between nearest
neighbors for Fe3+ in TbFeO3 were obtained. The magnetic
phase diagram for YbFeO3 was obtained and discussed as a
result of the energy balance between Heisenberg exchange
interactions, Dzyaloshinsky-Moriya interaction, anisotropy
and external magnetic field.},
cin = {542220 / 530000},
ddc = {550},
cid = {$I:(DE-82)542220_20140620$ / $I:(DE-82)530000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2023-02316},
url = {https://publications.rwth-aachen.de/record/952911},
}
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