35 research outputs found
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
Spin dynamics in the ordered spin ice TbSnO
Geometrical frustration is a central challenge in contemporary condensed
matter physics, a crucible favourable to the emergence of novel physics. The
pyrochlore magnets, with rare earth magnetic moments localized at the vertices
of corner-sharing tetrahedra, play a prominent role in this field, with a rich
variety of exotic ground states ranging from the "spin ices" \hoti\ and \dyti\
to the "spin liquid" and "ordered spin ice" ground states in \tbti\ and \tbsn.
Inelastic neutron scattering provides valuable information for understanding
the nature of these ground states, shedding light on the crystal electric field
(CEF) level scheme and on the interactions between magnetic moments. We have
performed such measurements with unprecedented neutron flux and energy
resolution, in the "ordered spin ice" \tbsn. We argue that a new interaction,
which involves the spin lattice coupling through a low temperature distortion
of the trigonal crystal field, is necessary to account for the data
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
Origin of the large dispersion of magnetic properties in nanostructured oxides: FexO/Fe3O4 nanoparticles as a case study
The intimate relationship in transition-metal oxides between stoichiometry and physiochemical properties makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxide
Origin of the large dispersion of magnetic properties in nanostructured oxides: FexO/Fe3O4 nanoparticles as a case study
The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between the magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides