17 research outputs found
Spin correlations in the extended kagome system YBaCo3FeO7
The transition metal based oxide YBaCo3FeO7 is structurally related to the
mineral Swedenborgite SbNaBe4O7, a polar non-centrosymmetric crystal system.
The magnetic Co3Fe sublattice consists of a tetrahedral network containing
kagome-like layers with trigonal interlayer sites. This geometry causes
frustration effects for magnetic ordering, which were investigated by
magnetization measurements, M\"ossbauer spectroscopy, polarized neutron
diffraction, and neutron spectroscopy. Magnetization measurement and neutron
diffraction do not show long range ordering even at low temperature (1 K)
although a strong antiferromagnetic coupling (~2000 K) is deduced from the
magnetic susceptibility. Below 590 K, we observe two features, a spontaneous
weak anisotropic magnetization hysteresis along the polar crystallographic axis
and a hyperfine field on the Fe kagome sites, whereas the Fe spins on the
interlayer sites remain idle. Below ~50 K, the onset of a hyperfine field shows
the development of moments static on the M\"ossbauer time scale also for the Fe
interlayer sites. Simultaneously, an increase of spin correlations is found by
polarized neutron diffraction. The relaxation part of the dynamic response has
been further investigated by high-resolution neutron spectroscopy, which
reveals that the spin correlations start to freeze in below ~50 K. Monte Carlo
simulations show that the neutron scattering results at lower temperatures are
compatible with a recent proposal that the particular geometric frustration in
the Swedenborgite structure promotes quasi one dimensional partial order.Comment: 13 pages, 7 figure
Magnetic structure of the swedenborgite compound CaBaMn2 Fe 2 O7 derived by powder neutron diffraction and Mössbauer spectroscopy
We present a study combining neutron diffraction and 57 Fe Mössbauer spectroscopy on a powder sample ofCaBaMn 2 Fe 2 O7 belonging to the large family of swedenborgite compounds. The undistorted hexagonal crystal structure (space group P63mc) is preserved down to low temperatures, and all employed techniques reveal a transition into a magnetically long-range ordered phase at TN = 205 K. The magnetic Bragg peak intensities from the powder diffraction patterns together with a symmetry analysis of the employed models unambiguously reveal the classical √3 × √3 magnetic structure on a hexagonal lattice with propagation vector q = ( 1/3 1/3 0). The nuclear Bragg peak intensities allowed the statistical distribution of Fe and Mn ions on both trigonal and kagome sites of the complex swedenborgite structure to be analyzed which was considered to explain the complex shape of the Mössbauer spectra
Magnetoelectric effect and magnetic phase diagram of a polar ferrimagnet CaBaFe4O7
The magnetic phase diagram of a polar ferrimagnet
CaBaFe4O7 with a magnetic easy axis has been investigated by measurements of magnetization, specific heat, and magnetoelectricity. A ferrimagnetic transition takes place at TC1=275 K within the orthorhombic phase followed by a second magnetic transition at TC2=211 K. Below
TC2, successive metamagnetic transitions occur for magnetic fields applied perpendicular to the easy axis, implying a sequential emergence of magnetic states which are neither collinear nor coplanar. The observation of the static magnetoelectric effect was limited to temperatures below 120 K due to the conducting nature of the crystals at higher temperatures. The magnitude of the ferroelectric polarization shows large changes between the different field-induced magnetic phases. The low-field state is characterized by a large linear magnetoelectric coefficient of αcc=39 ps/m, while a gigantic polarization change of ΔP=850μC/m2 is observed for μoH=14 T applied along the easy axis
Orbital occupation and magnetic moments of tetrahedrally coordinated iron in CaBaFe4O7
CaBaFe4O7 is a mixed-valent transition metal oxide having both Fe2+ and Fe3+
ions in tetrahedral coordination. Here we characterize its magnetic properties
by magnetization measurements and investigate its local electronic structure
using soft x-ray absorption spectroscopy at the Fe L2,3 edges, in combination
with multiplet cluster and spin-resolved band structure calculations. We found
that the Fe2+ ion in the unusual tetrahedral coordination is Jahn-Teller active
with the high-spin e^2 (up) t2^3 (up) e^1 (down) configuration having a
x^2-y^2-like electron for the minority spin. We deduce that there is an
appreciable orbital moment of about L_z=0.36 caused by multiplet interactions,
thereby explaining the observed magnetic anisotropy. CaBaFe4O7, a member of the
'114' oxide family, offers new opportunities to explore charge, orbital and
spin physics in transition metal oxides
Neutron diffraction study and theoretical analysis of the antiferromagnetic order and diffuse scattering in the layered Kagome system CaBaCoFeO
The hexagonal swedenborgite, CaBaCoFeO, is a chiral frustrated
antiferromagnet, in which magnetic ions form alternating Kagome and triangular
layers. We observe a long range antiferromagnetic
order setting in below K by neutron diffraction on single crystals
of CaBaCoFeO. Both magnetization and polarized neutron single
crystal diffraction measurements show that close to spins lie
predominantly in the -plane, while upon cooling the spin structure becomes
increasingly canted due to Dzyaloshinskii-Moriya interactions. The ordered
structure can be described and refined within the magnetic space group
. Diffuse scattering between the magnetic peaks reveals that the
spin order is partial. Monte Carlo simulations based on a Heisenberg model with
two nearest-neighbor exchange interactions show a similar diffuse scattering
and coexistence of the order with disorder. The
coexistence can be explained by the freedom to vary spins without affecting the
long range order, which gives rise to ground-state degeneracy. Polarization
analysis of the magnetic peaks indicates the presence of long-period cycloidal
spin correlations resulting from the broken inversion symmetry of the lattice,
in agreement with our symmetry analysis.Comment: 12 pages, 13 figures, 2 table
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Chiral Spin Liquid Ground State in YBaCo3FeO7
A chiral spin liquid state is discovered in the highly frustrated, noncentrosymmetric swedenborgite compound YBaCo3FeO7, a layered kagome system of hexagonal symmetry, by advanced polarized neutron scattering from a single domain crystalline sample. The observed diffuse magnetic neutron scattering has an antisymmetric property that relates to its specific chirality, which consists of three cycloidal waves perpendicular to the c axis, forming an entity of cylindrical symmetry. Chirality and symmetry agree with relevant antisymmetric exchanges arising from broken spatial parity. Applying a Fourier analysis to the chiral interference pattern, with distinction between kagome sites and the connecting trigonal interlayer sites of threefold symmetry, the chiral spin correlation function is determined. Characteristic chiral waves originate from the trigonal sites and extend over several periods in the kagome planes. The chiral spin liquid is remarkably stable at low temperatures despite strong antiferromagnetic spin exchange. The observation raises a challenge, since the commonly accepted ground states in condensed matter either have crystalline long-range order or form a quantum liquid. We show that, within the classical theory of magnetic order, a disordered ground state may arise from chirality. The present scenario, with antisymmetric exchange acting as a frustrating gauge background that stabilizes local spin lumps, is similar to the avoided phase transition in coupled gauge and matter fields for subnuclear particles
Condensation of a tetrahedra rigid-body libration mode in HoBaCo4O7 : the origin of phase transition at 355 K
Rietveld profiles, Moessbauer spectra and x-ray absorption fine structure
(XAFS) were analyzed through the structural phase transition at Ts = 355 K in
HoBaCo4O7. Excess of the oxygen content over O7 was avoided via annealing the
samples in argon flow at 600 degree C. Space groups (S.G.) Pbn21c and P63mc
were used to refine the structure parameters in the low- and high-temperature
phases, respectively. Additionally, the Cmc21 symmetry was considered as a
concurrent model of structure of the low-temperature phase. In the
high-temperature phase, severe anisotropy of thermal motion of the major part
of the oxygen atoms was observed. This anisotropic motion turns to be quenched
as the sample is cooled below Ts. The variation of quadrupole splitting near Ts
is not similar to a steplike anomaly frequently seen at the charge-ordering
transition. We observe instead a dip-like anomaly of the average quadrupole
splitting near Ts. Narrow distribution of the electric field gradient (EFG)
over different cobalt sites is observed and explained on the basis of
point-charge model. XAFS spectra show no evidence of significant difference
between YBaCo4O7 (T > Ts) and HoBaCo4O7 (T < Ts). The origin of the transition
at Ts is ascribed to the condensation of the libration phonon mode associated
with the rigid-body rotational movements of the starlike tetrahedral units, the
building blocks of kagome network. It is shown that the condensation of the
libration mode is not compatible with translation symmetry for the hexagonal
S.G., but compatible for the orthorhombic S.G. The orthorhombic lattice
parameters and EFG components (Vxx, Vyy, Vzz) vary smoothly with temperature at
approaching Ts and closely follow each other.Comment: 13 figure
Magnetic particles and strings in iron langasite
Particle-like topological magnetic defects that can propagate in all spatial
directions open a new dimension for design of magnetic memory and data
processing devices. We show that three-dimensional magnetic skyrmions can be
stabilized in non-collinear antiferromagnets, such as the Fe-langasite,
BaTaFeSiO. Spins in the crystallographic unit cell of this
material form a 120-degree ordering transformed by competing exchange
interactions into a short-period spiral, which in turn forms a basis for
complex large-scale magnetic superstructures stabilized by
Dzyaloshinskii-Moriya interactions and applied magnetic fields. We derive an
effective continuum model describing modulated states of Fe-langasite at the
100 nm scale and explore its magnetic phases and topological defects. The order
parameter space of this model is similar to that of superfluid He-A and
the three-dimensional topological defect is closely related to the Shankar
monopole and hedgehog soliton in the Skyrme model of baryons