21 research outputs found
Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCoF and NaSrCoF with magnetic pair distribution function analysis
We present time-of-flight neutron total scattering and polarized neutron
scattering measurements of the magnetically frustrated compounds
NaCaCoF and NaSrCoF, which belong to a class of recently
discovered pyrochlore compounds based on transition metals and fluorine. The
magnetic pair distribution function (mPDF) technique is used to analyze and
model the total scattering data in real space. We find that a
previously-proposed model of short-range XY-like correlations with a length
scale of 10-15 \AA, combined with nearest-neighbor collinear antiferromagnetic
correlations, accurately describes the mPDF data at low temperature, confirming
the magnetic ground state in these materials. This model is further verified by
the polarized neutron scattering data. From an analysis of the temperature
dependence of the mPDF and polarized neutron scattering data, we find that
short-range correlations persist on the nearest-neighbor length scale up to 200
K, approximately two orders of magnitude higher than the spin freezing
temperatures of these compounds. These results highlight the opportunity
presented by these new pyrochlore compounds to study the effects of geometric
frustration at relatively high temperatures, while also advancing the mPDF
technique and providing a novel opportunity to investigate a genuinely
short-range-ordered magnetic ground state directly in real space
Breathing Pyrochlore Lattice Realized in <em>A</em>-Site Ordered Spinel Oxides LiGaCr<sub>4</sub>O<sub>8</sub> and LiInCr<sub>4</sub>O<sub>8</sub>
A unique type of frustrated lattice is found in two A-site ordered spinel
oxides, LiGaCr4O8 and LiInCr4O8. Because of the large size mismatch between Li+
and Ga3+/In3+ ions at the A site, the pyrochlore lattice, made up of Cr3+ ions
carrying spin 3/2, becomes an alternating array of small and large tetrahedra,
i.e., a "breathing" pyrochlore lattice. We introduce a parameter, the breathing
factor Bf, which quantifies the degree of frustration in the pyrochlore
lattice: Bf is defined as J'/J, where J' and J are nearest-neighbor magnetic
interactions in the large and small tetrahedra, respectively. LiGaCr4O8 with Bf
~ 0.6 shows magnetic susceptibility similar to that of conventional Cr spinel
oxides such as ZnCr2O4. In contrast, LiInCr4O8 with a small Bf ~ 0.1 exhibits a
spin-gap behavior in its magnetic susceptibility, suggesting a proximity to an
exotic singlet ground state. Magnetic long-range order occurs at 13.8 and 15.9
K for LiGaCr4O8 and LiInCr4O8, respectively, in both cases likely owing to the
coupling to structural distortions.Comment: 5 pages, 5 figures, accepted for publication in Phys. Rev. Let
Phase diagram of multiferroic KCuAsO(OD)
The layered compound KCuAsO(OD), comprising distorted kagome
planes of Cu ions, is a recent addition to the family of type-II
multiferroics. Previous zero field neutron diffraction work has found two
helically ordered regimes in \kns, each showing a distinct coupling between the
magnetic and ferroelectric order parameters. Here, we extend this work to
magnetic fields up to ~T using neutron powder diffraction, capacitance,
polarization, and high-field magnetization measurements, hence determining the
phase diagram. We find metamagnetic transitions in both low temperatures
phases around ~T, which neutron powder diffraction reveals
to correspond to a rotation of the helix plane away from the easy plane, as
well as a small change in the propagation vector. Furthermore, we show that the
sign of the ferroelectric polarization is reversible in a magnetic field,
although no change is observed (or expected on the basis of the magnetic
structure) due to the transition at ~T. We finally justify the temperature
dependence of the polarization in both zero-field ordered phases by a symmetry
analysis of the free energy expansion
Magnetic, electronic, and structural investigation of the strongly correlated YSmCo system
SmCo and YCo are isostructural compounds both showing large
magnetocrystalline anisotropy, where the former originates mainly from the
crystal-electric field and magnetic interactions. We investigate the
contribution of both interactions by partially substituting Y by Sm in as-cast
polycrystalline YSmCo (with =0, 0.1, 0.2, 0.3, and 0.4)
and measuring their structural, magnetic, and electrical properties through
X-ray diffraction, magnetization, and electrical transport measurements. Our
results suggest an interplay between microstructure strain in as-cast samples
and the electronic and magnetic interactions
Realizing square and diamond lattice S =1/2 Heisenberg antiferromagnet models in the α and β phases of the coordination framework, KTi(C2O4)2⋅xH2O
Provision of a PhD studentship to A.H.A. by the University of Liverpool and the Science and Technology Facilities Council (STFC) is gratefully acknowledged. The work of T.L. was funded by the University of St Andrews and China Scholarship Council (CSC) joint scholarship (201606280032). A.T. was funded by the Federal Ministry of Education and Research through the Sofja Kovalevskaya Award of Alexander von Humboldt Foundation. Work at St Andrews was supported by the Leverhulme Trust (RPG-2013-343).We report the crystal structures and magnetic properties of two pseudopolymorphs of the S=1/2 Ti3+ coordination framework, KTi(C2O4)2⋅xH2O. Single-crystal x-ray and powder neutron diffraction measurements on α−KTi(C2O4)2⋅xH2O confirm its structure in the tetragonal I4/mcm space group with a square planar arrangement of Ti3+ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with J1≈7 K and J2/J1=0.11 indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below 1.8 K, α−KTi(C2O4)2⋅xH2O undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the c axis of the tetragonal structure. The estimated ordered moment of Ti3+ in α−KTi(C2O4)2⋅xH2O is suppressed from its spin-only value to 0.62(3) μB, thus verifying the two-dimensional nature of the magnetic interactions within the system. β−KTi(C2O4)2⋅2H2O, on the other hand, realizes a three-dimensional diamondlike magnetic network of Ti3+ moments within a hexagonal P6222 structure. An antiferromagnetic exchange coupling of J≈54 K—an order of magnitude larger than in α−KTi(C2O4)2⋅xH2O—is extracted from magnetometry and specific heat data. β−KTi(C2O4)2⋅2H2O undergoes Néel ordering at TN=28 K, with the magnetic moments aligned within the ab plane and a slightly reduced ordered moment of 0.79 μB per Ti3+. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the α and β pseudopolymorphs. Given their observed magnetic behaviors, we propose α−KTi(C2O4)2⋅xH2O and β−KTi(C2O4)2⋅2H2O as close to ideal model S =1/2 Heisenberg square and diamond lattice antiferromagnets, respectively.PostprintPeer reviewe
One dimensional magnetism in synthetic Pauflerite, -VOSO
We have synthesized single-crystal samples of -VOSO and fully
characterized their magnetic properties. Our magnetic susceptibility, high
field magnetization and powder inelastic neutron scattering results are in
excellent agreement with theoretical expressions for a one-dimensional spin-1/2
Heisenberg chain with an exchange parameter of \,meV. Ab-initio
calculations identify the superexchange pathway, revealing that the spin-chain
does not run along the expected crystallographic chain -direction but
instead between VO octahedra that are linked via SO
tetrahedra along the -axis. We do not detect any phase transition to a
long-range magnetic order within our experimental conditions, indicating
-VOSO is very close to an ideal one-dimensional magnetic system