28 research outputs found
Magnetic frustration in BaCuSi2O6 released
Han Purple (BaCuSi2O6) is not only an ancient pigment, but also a valuable
model material for studying Bose-Einstein condensation (BEC) of magnons in high
magnetic fields. Using precise low-temperature structural data and extensive
density-functional calculations, we elucidate magnetic couplings in this
compound. The resulting magnetic model comprises two types of nonequivalent
spin dimers, in excellent agreement with the (63,65)Cu nuclear magnetic
resonance data. We further argue that leading interdimer couplings connect the
upper site of one dimer to the bottom site of the contiguous dimer, and not the
upper-to-upper and bottom-to-bottom sites, as assumed previously. This finding
is verified by inelastic neutron scattering data and implies the lack of
magnetic frustration in BaCuSi2O6, thus challenging existing theories of the
magnon BEC in this compound.Comment: 4.5 pages, 4 figures, 1 tabl
Cubic symmetry and magnetic frustration on the spin lattice in KIrCl
Cubic crystal structure and regular octahedral environment of Ir
render antifluorite-type KIrCl a model fcc antiferromagnet with a
combination of Heisenberg and Kitaev exchange interactions. High-resolution
synchrotron powder diffraction confirms cubic symmetry down to at least 20 K,
with a low-energy rotary mode gradually suppressed upon cooling. Using
thermodynamic and transport measurements, we estimate the activation energy of
eV for charge transport, the antiferromagnetic Curie-Weiss
temperature of K, and the extrapolated saturation
field of T. All these parameters are well reproduced \textit{ab
initio} using eV as the effective Coulomb repulsion
parameter. The antiferromagnetic Kitaev exchange term of K is about
one half of the Heisenberg term K. While this combination removes
a large part of the classical ground-state degeneracy, the selection of the
unique magnetic ground state additionally requires a weak second-neighbor
exchange coupling K. Our results suggest that KIrCl may
offer the best possible cubic conditions for Ir and demonstrates the
interplay of geometrical and exchange frustration in a high-symmetry setting.Comment: 9 page
Nanoskyrmion engineering with -electron materials: Sn monolayer on SiC(0001) surface
Materials with -magnetism demonstrate strongly nonlocal Coulomb
interactions, which opens a way to probe correlations in the regimes not
achievable in transition metal compounds. By the example of Sn monolayer on
SiC(0001) surface, we show that such systems exhibit unusual but intriguing
magnetic properties at the nanoscale. Physically, this is attributed to the
presence of a significant ferromagnetic coupling, the so-called direct
exchange, which fully compensates ubiquitous antiferromagnetic interactions of
the superexchange origin. Having a nonlocal nature, the direct exchange was
previously ignored because it cannot be captured within the conventional
density functional methods and significantly challenges ground state models
earlier proposed for Sn/SiC(0001). Furthermore, heavy adatoms induce strong
spin-orbit coupling, which leads to a highly anisotropic form of the spin
Hamiltonian, in which the Dzyaloshinskii-Moriya interaction is dominant. The
latter is suggested to be responsible for the formation of a nanoskyrmion state
at realistic magnetic fields and temperatures.Comment: 4 pages, supplemental materia
Magnetism of coupled spin tetrahedra in ilinskite-type KCuO(SeO)Cl
Synthesis, thermodynamic properties, and microscopic magnetic model of
ilinskite-type KCuO(SeO)Cl built by corner-sharing Cu
tetrahedra are reported, and relevant magnetostructural correlations are
discussed. Quasi-one-dimensional magnetic behavior with the short-range order
around 50\,K and the absence of long-range order down to at least 2\,K is
observed experimentally and explained in terms of weakly coupled spin ladders
(tubes) with a complex topology formed upon fragmentation of the tetrahedral
network. This fragmentation is rooted in the non-trivial effect of the SeO
groups that render the Cu--O--Cu superexchange strongly ferromagnetic.Comment: 9 pages, 7 figure
Toward cubic symmetry for Ir: structure and magnetism of antifluorite KIrBr
Crystal structure, electronic state of Ir, and magnetic properties of
the antifluorite compound KIrBr are studied using high-resolution
synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS),
thermodynamic and transport measurements, and ab initio calculations. The
crystal symmetry is reduced from cubic at room temperature to tetragonal below
170 K and eventually to monoclinic below 122 K. These changes are tracked by
the evolution of the non-cubic crystal-field splitting measured by
RIXS. Non-monotonic changes in are ascribed to the competing effects
of the tilt, rotation, and deformation of the IrBr octahedra as well as
tetragonal strain on the electronic levels of Ir. The N\'eel temperature
of K exceeds that of the isostructural KIrCl, and the
magnitude of frustration on the fcc spin lattice decreases. We argue that the
replacement of Cl by Br weakens electronic correlations and enhances magnetic
couplings.Comment: published version: 13 pages + Supplemental Materia
An effective spin model on the honeycomb lattice for the description of magnetic properties in two-dimensional FeGeTe
FeGeTe attracts significant attention due to technological
perspectives of realizing room temperature ferromagnetism in two-dimensional
materials. Here we show that due to structural peculiarities of the
FeGeTe monolayer, short distance between the neighboring iron atoms
induces a strong exchange coupling. This strong coupling allows us to consider
them as an effective cluster with a magnetic moment 5 , giving
rise to a simplified spin model on a bipartite honeycomb lattice with the
reduced number of long-range interactions. The simplified model perfectly
reproduces the results of the conventional spin model, but allows for a more
tractable description of the magnetic properties of FeGeTe, which is
important, e.g., for large-scale simulations. Also, we discuss the role of
biaxial strain in the stabilization of ferromagnetic ordering in
FeGeTe.Comment: 7 pages, 7 figure
Field evolution of the spin-liquid candidate YbMgGaO4
We report magnetization, heat capacity, thermal expansion, and
magnetostriction measurements down to mK temperatures on the triangular
antiferromagnet YbMgGaO. Our data exclude the formation of the distinct
-plateau phase observed in other triangular antiferromagnets, but
reveal plateau-like features in second derivatives of the free energy, magnetic
susceptibility and specific heat, at = 1.0 - 2.5 T for
and 2 - 5 T for . Using Monte-Carlo simulations of a realistic spin
Hamiltonian, we ascribe these features to non-monotonic changes in the
magnetization and the -plateau that is smeared out by the random
distribution of exchange couplings in YbMgGaO