This thesis presents the experimental studies of the magnetic and hyperfine interaction
properties of four novel Fe-based superconductors (ThFeAsN, CsEuFe4As4, Rb1-δEuFe4As4, and
EuFe0.97Ni0.03As2) and one new non-superconducting compound (EuFeAs2). It is supplemented by
ab-initio calculations of the electronic structure and magnetism of the three superconductors. The
experimental studies are based on the results of x-ray diffraction, magnetic susceptibility,
magnetization, and 57Fe and 151Eu Mössbauer spectroscopy measurements.
The superconductor ThFeAsN crystallizes in the tetragonal space group P4/nmm with the
lattice parameters a = 4.0356(1) Å and c = 8.5286(1) Å. It is shown that there is no magnetic order
of the Fe magnetic moments down to 2.0 K. We suggest that nonappearance of Fe magnetism in
ThFeAsN may be because of an internal uniaxial chemical pressure whose presence is manifested
by the unusually small c/a ratio. We provide theoretical evidence for a mixture of ionic and
covalent chemical bonding and metallic characteristics. We present a detailed analysis of the
calculated energy band structure of ThFeAsN. A quadrupole doublet well describes the shape of
the Mössbauer spectra with a small quadrupole splitting that increases with lowering temperature.
Good agreement is found between the calculated and extrapolated 0 K quadrupole splitting values.
A fair agreement is noted between the experimental Debye temperature 332(2) K and 370 K of the
calculated one.
We show that the superconductor CsEuFe4As4 crystallizes in the tetragonal space group
P4/mmm with the lattice parameters a = 3.8956(1) Å and c = 13.6628(5) Å. We show that the Fe
atoms carry no magnetic moment down to 2.1 K and that the ferromagnetic order is related to the
Eu magnetic moments. We establish that the Curie temperature Tc = 15.97(8) K found from the
temperature dependence of the hyperfine magnetic field at 151Eu nuclei is well-matched with the
temperature dependence of the transferred hyperfine magnetic field at 57Fe nuclei that is produced
by the ferromagnetically ordered Eu sublattice. The magnetic moments of the Eu atoms are shown
to be perpendicular to the crystallographic c-axis. The T 3⁄2 power-law perfectly describes the
temperature dependence of the principal component of the electric field gradient tensor, both at Fe
and Eu sites. The calculated and the measured parameters of the hyperfine-interaction are in
excellent agreement with each other. We determine that the Debye temperature of CsEuFe4As4 is
295(3) K.
Ab-initio calculations suggest a mixture of ionic, covalent, and metallic bonding between
the constituent atoms in the CsEuFe4As4 superconductor. We confirm that the strongly localized
Eu f states are the origin of the magnetic moment of CsEuFe4As4, in agreement with the
experimental results. We show that an almost zero magnetic moment carried by the Fe atoms is
caused by the spin-up and spin-down states' apparent symmetry. We show that the Fermi surfaces
have hole-like and electron-like pockets located at the center and corners of the Brillouin zone,
respectively.
The superconductor Rb1-δEuFe4As4 crystallizes in the tetragonal space group P4/mmm with
the lattice parameters a = 3.8849(1) Å and c = 13.3370(3) Å. We show that the Fe atoms carry no
magnetic moment down to 2.1 K and that the ferromagnetic order is associated solely with the Eu
magnetic moments. The Curie temperature Tc = 16.54(8) K is found from the temperature
dependence of both the hyperfine magnetic field at 151Eu nuclei and the transferred hyperfine
magnetic field at 57Fe nuclei induced by the ferromagnetically ordered Eu sublattice. We find that
the Eu magnetic moments lie in the ab plane. It is observed that the temperature dependence of the
principal component of the electric field gradient tensor, at both Fe and Eu sites, is well described
by a T3⁄2 power-law relation. There is good agreement between the calculated and measured
parameters of the hyperfine-interaction. We determine that the Debye temperature of
Rb1-δEuFe4As4 is 391(8) K.
Ab-initio calculations indicate the presence of a mixture of ionic, covalent, and metallic
bonding between the constituent atoms in the RbEuFe4As4 superconductor. We show that the
magnetic moment of RbEuFe4As4 is mainly a result of the strongly localized Eu f states. It is shown
that an almost zero magnetic moment carried by the Fe atoms originates from an apparent
symmetry of the spin-up and spin-down states. We show that the electrical and chemical properties
of RbEuFe4As4 are closely associated with the presence of the Fe 3d states in the Fermi energy
region. The Fermi surfaces display hole-like and electron-like pockets, respectively, at the center
and corners of the Brillouin zone.
We find that in both the EuFeAs2 compound and 14 K superconductor EuFe0.97Ni0.03As2
the antiferromagnetic ordering of the Fe sublattice is of a spin-density-wave type with the Néel
temperatures and Fe saturation magnetic moments of 106.2(1.9) K, 0.78(1) μB and 56.6(2.2) K,
0.47(1) μB, respectively. We show that the Néel temperatures and the saturation hyperfine
magnetic fields in the two compounds with the antiferromagnetically ordered Eu sublattice are
44.4(5) K, 294.2(7) kOe and 43.5(1) K, 290.5(1) kOe respectively. The 3% substitution of Fe by
Ni in EuFeAs2, aside from producing superconductivity in EuFe0.97Ni0.03As2, radically reduces the
strength of magnetism of the Fe sublattice and has nearly no impact on the magnetism of the Eu
sublattice. The appearance of antiferromagnetically ordered Fe and Eu sublattices in
EuFe0.97Ni0.03As2 verifies that superconductivity and magnetism coexist in this compond. The
growth of the magnitude of the main component of the electric field gradient tensor, at both Fe
and Eu sites, with reducing temperature, is well described by a T3⁄2 power-law relation. We
determine the Debye temperatures of EuFeAs2, EuFe0.97Ni0.03As2, and the FeAs2 impurity phase
to be 355(18), 428(14), and 594(25) K, respectively.
In summary, for all of the studied compounds, there is no magnetic ordering associated
with iron sub-lattices in the ThFeAsN, CsEuFe4As4, and Rb1-δEuFe4As4 compounds. The iron
sublattice is magnetically ordered in the EuFeAs2 and the EuFe0.97Ni0.03As2 superconductor. There
is a coexistence of magnetism and superconductivity associated with europium in the CsEuFe4As4,
Rb1-δEuFe4As4, EuFe0.97Ni0.03As2 compounds. There is a good agreement between the calculated
and the measured hyperfine and magnetic parameters for most studied compounds.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/41636 |
| Date | 08 January 2021 |
| Creators | Albedah, Mohammed |
| Contributors | Stadnik, Zbigniew |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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