42 research outputs found
Magnetodielectric Response of the Spin-Ice Dy2Ti2O7
We report the magneto-dielectric response of single crystals of the spin-ice
compound Dy2Ti2O7 down to 0.26 K. The dielectric constant under zero magnetic
field exhibits a clear decrease reflecting the development of the local
two-spins-in, two-spins-out structure below about 1.2 K. Both the real and
imaginary parts of the dielectric constant under magnetic fields sensitively
respond to various changes in the spin structures. We found that the real part
can be described in terms of local spin correlations among the moments of
tetrahedra, rather than among individual Dy3+ moments. Using the peaks in the
imaginary part, we have constructed a precise field-temperature phase diagram
in the [111] field direction. We thus demonstrate that the magneto-dielectric
response can be a high-sensitivity local probe of the spin state of
geometrically frustrated systems.Comment: 6 pages, 6 figure
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
Magnetoelastics of a spin liquid: X-ray diffraction studies of Tb2Ti2O7 in pulsed magnetic fields
We report high resolution single crystal x-ray diffraction measurements of
the frustrated pyrochlore magnet Tb2Ti2O7, collected using a novel low
temperature pulsed magnet system. This instrument allows characterization of
structural degrees of freedom to temperatures as low as 4.4 K, and in applied
magnetic fields as large as 30 Tesla. We show that Tb2Ti2O7 manifests
intriguing structural effects under the application of magnetic fields,
including strongly anisotropic giant magnetostriction, a restoration of perfect
pyrochlore symmetry in low magnetic fields, and ultimately a structural phase
transition in high magnetic fields. It is suggested that the magnetoelastic
coupling thus revealed plays a significant role in the spin liquid physics of
Tb2Ti2O7 at low temperatures.Comment: 4 pages, 4 figures, submitted for publicatio
Structural and magnetic properties of two branches of the tripod-kagome-lattice family A
Lack of Evidence for a Singlet Crystal Field Ground State in the Tb2Ti2O7 Magnetic Pyrochlore
We present new high resolution inelastic neutron scattering data on the
candidate spin liquid Tb2Ti2O7. We find that there is no evidence for a zero
field splitting of the ground state doublet within the 0.2 K resolution of the
instrument. This result contrasts with a pair of recent works on Tb2Ti2O7
claiming that the spin liquid behavior can be attributed to a 2 K split
singlet-singlet single-ion spectrum at low energies. We also reconsider the
entropy argument presented in Chapuis {\it et al.} as further evidence of a
singlet-singlet crystal field spectrum. We arrive at the conclusion that
estimates of the low temperature residual entropy drawn from heat capacity
measurements are a poor guide to the single ion spectrum without understanding
the nature of the correlations.Comment: 4 pages, 3 figure. Submitted to Physical Review
Structural Fluctuations in the Spin Liquid State of Tb2Ti2O7
High resolution X-ray scattering measurements on single crystal Tb2Ti2O7
reveal finite structural correlations at low temperatures. This geometrically
frustrated pyrochlore is known to exhibit a spin liquid, or cooperative
paramagnetic state, at temperatures below ~ 20 K. Parametric studies of
structural Bragg peaks appropriate to the Fdm space group of Tb2Ti2O7
reveal substantial broadening and peak intensity reduction in the temperature
regime 20 K to 300 mK. We also observe a small, anomalous lattice expansion on
cooling below a density maximum at ~ 18 K. These measurements are consistent
with the development of fluctuations above a cooperative Jahn-Teller,
cubic-tetragonal phase transition at very low temperatures.Comment: 5 pages, 4 figures, submitted for publicatio
Revisiting Static and Dynamic Spin Ice Correlations in Ho2Ti2O7
Elastic and inelastic neutron scattering studies have been carried out on the
pyrochlore magnet Ho2Ti2O7. Measurements in zero applied magnetic field show
that the disordered spin ice ground state of Ho2Ti2O7 is characterized by a
pattern of rectangular diffuse elastic scattering within the [HHL] plane of
reciprocal space, which closely resembles the zone boundary scattering seen in
its sister compound Dy2Ti2O7. Well-defined peaks in the zone boundary
scattering develop only within the spin ice ground state below ~ 2 K. In
contrast, the overall diffuse scattering pattern evolves on a much higher
temperature scale of ~ 17 K. The diffuse scattering at small wavevectors below
[001] is found to vanish on going to Q=0, an explicit signature of expectations
for dipolar spin ice. Very high energy-resolution inelastic measurements reveal
that the spin ice ground state below ~ 2 K is also characterized by a
transition from dynamic to static spin correlations on the time scale of
10^{-9} seconds. Measurements in a magnetic field applied along the
[10] direction in zero-field cooled conditions show that the system
can be broken up into orthogonal sets of polarized alpha chains along
[10] and quasi-one-dimensional beta chains along [110]. Three
dimensional correlations between beta chains are shown to be very sensitive to
the precise alignment of the [10] externally applied magnetic field.Comment: 11 pages, 10 figures. Submitted for publicatio
Vibrating-coil magnetometry of the spin liquid properties of Tb2Ti2O7
We have explored the spin liquid state in Tb2Ti2O7 with vibrating coil
magnetometry down to 0.04 K under magnetic fields up to 5 T. We observe
magnetic history dependence below K reminiscent of the classical spin
ice systems Ho2Ti2O7 and Dy2Ti2O7. The magnetic phase diagram inferred from the
magnetization is essentially isotropic, without evidence of magnetization
plateaux as anticipated for so-called quantum spin ice, predicted theoretically
for [111] when quantum fluctuations renormalize the interactions. Instead, the
magnetization for agrees semi-quantitatively with the predictions of
"all-in/all-out" (AIAO) antiferromagnetism. Taken together this suggests that
the spin liquid state in Tb2Ti2O7 is akin to an incipient AIAO-antiferromagnet.Comment: accepted in Physical Review Letter