59 research outputs found
Candidate Quantum Spin Liquid in the Ce\textsuperscript{3+} Pyrochlore Stannate CeSnO
We report the low temperature magnetic properties of CeSnO, a
rare-earth pyrochlore. Our susceptibility and magnetization measurements show
that due to the thermal isolation of a Kramers doublet ground state,
CeSnO has Ising-like magnetic moments of
. The magnetic moments are confined to the local trigonal axes,
as in a spin ice, but the exchange interactions are antiferromagnetic. Below 1
K the system enters a regime with antiferromagnetic correlations. In contrast
to predictions for classical -Ising spins on the
pyrochlore lattice, there is no sign of long-range ordering down to 0.02 K. Our
results suggest that CeSnO features an antiferromagnetic liquid
ground state with strong quantum fluctuations.Comment: 6 pages, 4 figure
Palmer-Chalker correlations in the XY pyrochlore antiferromagnet Er2Sn2O7
\ersn\, is considered, together with \erti, as a realization of the XY
antiferromagnet on the pyrochlore lattice. We present magnetization
measurements confirming that \ersn\, does not order down to 100 mK but exhibits
a freezing below 200 mK. Our neutron scattering experiments evidence the strong
XY character of the \er moment and point out the existence of short range
correlations in which the magnetic moments are in peculiar configurations, the
Palmer-Chalker states, predicted theoretically for an XY pyrochlore
antiferromagnet with dipolar interactions. Our estimation of the \ersn\,
parameters confirm the role of the latter interactions on top of relatively
weak and isotropic exchange couplings
Magnetic structure and dynamics of a strongly one-dimensional cobalt metal-organic framework
We investigate the magnetism of the
Co(OH)(CHO) metal-organic framework which
displays complex inorganic chains separated from each other by distances of 1
to 2 nm, and which orders at ~5.4 K. The zero-field magnetic structure is
determined using neutron powder diffraction: it is mainly antiferromagnetic but
posseses a ferromagnetic component along the -axis. This magnetic
structure persists in presence of a magnetic field. Ac susceptibility
measurements confirm the existence of a single thermally activated regime over
7 decades in frequency () whereas time-dependent relaxation
of the magnetization after saturation in an external field leads to a two times
smaller energy barrier. These experiments probe the slow dynamics of domain
walls within the chains: we propose that the ac measurements are sensitive to
the motion of existing domain walls within the chains, while the magnetization
measurements are governed by the creation of domain walls.Comment: 12 pages, 14 figure
Candidate quantum spin ice in the pyrochlore PrHfO
We report the low temperature magnetic properties of the pyrochlore
PrHfO. Polycrystalline and single-crystal samples are investigated
using time-of-flight neutron spectroscopy and macroscopic measurements,
respectively. The crystal-field splitting produces a non-Kramers doublet ground
state for Pr, with Ising-like anisotropy. Below 0.5 K ferromagnetic
correlations develop, which suggests that the system enters a spin ice-like
state associated with the metamagnetic behavior observed at
~T. In this regime, the development of a discrete inelastic
excitation in the neutron spectra indicates the appearance of spin dynamics
which are likely related to cooperative quantum fluctuations.Comment: 7 pages, 5 figures, 1 tabl
Magnetisation process in Er2Ti2O7 and Tb2Ti2O7 at very low temperature
We present a model which accounts for the high field magnetisation at very
low temperature in two pyrochlore frustrated systems, Er2Ti2O7 and Tb2Ti2O7.
The two compounds present very different ground states: Er2Ti2O7, which has a
planar crystal field anisotropy, is an antiferromagnet with T_N=1.2K, whereas
Tb2Ti2O7 is expected to have Ising character and shows no magnetic ordering
down to 0.05K, being thus labelled a ``spin liquid''. Our model is a mean field
self-consistent calculation involving the 4 rare earth sites of a tetrahedron,
the building unit of the pyrochlore lattice. It includes the full crystal field
hamiltonian, the infinite range dipolar interaction and anisotropic nearest
neighbour exchange described by a 3-component tensor. For Er2Ti2O7, we discuss
the equivalence of our treatment of the exchange tensor, taken to be diagonal
in a frame linked to a rare earth - rare earth bond, with the pseudo-spin
hamiltonian recently developped for Kramers doublets in a pyrochlore lattice.
In Tb2Ti2O7, an essential ingredient of our model is a symmetry breaking
developping at very low temperature. We compare its prediction for the
isothermal magnetisation with that of ``the quantum spin ice'' model
Antiferro-quadrupolar correlations in the quantum spin ice candidate Pr2Zr2O7
We present an experimental study of the quantum spin ice candidate pyrochlore
coumpound \przr\ by means of magnetization measurements, specific heat and
neutron scattering up to 12 T and down to 60 mK. When the field is applied
along the and directions, field induced
structures settle in. We find that the ordered moment rises slowly, even at
very low temperature, in agreement with macroscopic magnetization.
Interestingly, for , the ordered moment appears on the
so called chains only. The spin excitation spectrum is essentially
{\it inelastic} and consists in a broad flat mode centered at about 0.4 meV
with a magnetic structure factor which resembles the spin ice pattern. For (at least up to 2.5 T), we find that a well defined mode
forms from this broad response, whose energy increases with , in the same
way as the temperature of the specific heat anomaly. We finally discuss these
results in the light of mean field calculations and propose a new
interpretation where quadrupolar interactions play a major role, overcoming the
magnetic exchange. In this picture, the spin ice pattern appears shifted up to
finite energy because of those new interactions. We then propose a range of
acceptable parameters for \przr\, that allow to reproduce several experimental
features observed under field. With these parameters, the actual ground state
of this material would be an antiferroquadrupolar liquid with spin-ice like
excitations
Strong quantum fluctuations due to competition between magnetic phases in a pyrochlore iridate
We report neutron diffraction measurements of the magnetic structures in two
pyrochlore iridates, Yb2Ir2O7 and Lu2Ir2O7. Both samples exhibit the
all-in-all-out magnetic structure on the Ir4+ sites below TN~ 150,K, with a low
temperature moment of around 0.45 muB/Ir. Below 2\,K, the Yb moments in
Yb2Ir2O7 begin to order ferromagnetically. However, even at 40 mK the ordered
moment is only 0.57(3)muB/Yb, well below the saturated moment of the ground
state doublet of Yb3+ (1.9 muB/Yb), deduced from magnetization measurements and
from a refined model of the crystal field environment, and also significantly
smaller than the ordered moment of Yb in Yb2Ti2O7 (0.9 muB/Yb). A mean-field
analysis shows that the reduced moment on Yb is a consequence of enhanced phase
competition caused by coupling to the all-in-all-out magnetic order on the Ir
sublattice.Comment: 14 pages, 10 figures, resubmitted to PR
Vibronic collapse of ordered quadrupolar ice in the pyrochore magnet TbTiO
While the spin liquid state in the frustrated pyrochlore
TbTiO has been studied both experimentally and
theoretically for more than two decades, no definite description of this
unconventional state has been achieved. Using synchrotron based THz
spectroscopy in combination with quantum numerical simulations, we highlight a
significant link between two previously unrelated features: the existence of a
quadrupolar order following an ice rule and the presence of strong
magneto-elastic coupling in the form of hybridized Tb crystal-field and
phonon modes. The magnitude of this so-called vibronic process, which involves
quadrupolar degrees of freedom, is significantly dependent on small
off-stoichiometry and favors all-in all-out like correlations between
quadrupoles. This mechanism competes with the long range ordered quadrupolar
ice, and for slightly different stoichiometry, is able to destabilize it.Comment: Main text: 7 pages, 3 figures ; Supplemental Material: 6 pages, 2
figure
Geometric Frustration on the Trillium Lattice in a Magnetic Metal-Organic Framework
In the dense metal-organic framework Na[Mn(HCOO)3], Mn2+ ions (S = ) occupy the nodes of a ‘trillium’ net. We show that the system is strongly magnetically frustrated: the Neel transition is suppressed well ´ below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Neel ´ temperature; and the magnetic susceptibility exhibits a pseudo-plateau at -saturation magnetisation. A simple model of nearest-neighbour Heisenberg antiferromagnetic and dipolar interactions accounts quantitatively for all observations, including an unusual 2-k magnetic ground-state. We show that the relative strength of dipolar interactions is crucial to selecting this particular ground-state. Geometric frustration within the classical spin liquid regime gives rise to a large magnetocaloric response at low applied fields that is degraded in powder samples as a consequence of the anisotropy of dipolar interactions
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