88 research outputs found
New insights for the description of magnetic correlations inferred from muSR
Whenever a compound exhibits a spontaneous muSR oscillation, long-range
magnetic ordering is usually inferred. Here we show that some caution is
required. The coherence length needs not to be large for a spontaneous muon
spin precession to be observed. Establishing the incommensurate nature of a
magnetic structure, solely based on muSR measurements, may not be reliable. The
absence of a spontaneous muon precession at low temperature does not mean that
the system under investigation does not display long-range magnetic ordering.
The relaxation measured in zero and longitudinal field in the quasi-static
limit is usually analyzed in the framework of the strong-collision model, the
static polarization function being taken to be the famous Kubo-Toyabe function.
This might not be satisfactory if short-range correlation effects are strong.
Here we propose a method based on the maximum entropy concept and reverse Monte
Carlo technique which gives results consistent with those obtained in 2013 by
analytical means for the considered example.Comment: To appear in the proceedings of the 13th International Conference on
Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland, 1-6
June 201
Anomalously slow spin dynamics and short-range correlations in the quantum spin ice systems Yb2Ti2O7 and Yb2Sn2O7
We report a positive muon spin relaxation and rotation (\muSR) study of the
quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field
response. In agreement with earlier reports, data recorded in small
longitudinal fields evidence anomalously slow spin dynamics in the microsecond
range below the temperature T_c at which the specific heat displays an intense
peak, namely T_c = 0.24 K and 0.15 K, respectively, for the two systems. We
found that slow dynamics extends above T_c up to at least 0.7 K for both
compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the
\muSR spectra recorded above T_c. At lower temperatures a published analytical
extension of the Gaussian Kubo-Toyabe model provides a good description,
consistent with the existence of short-range magnetic correlations. While the
physical response of the two systems is qualitatively the same, Yb2Ti2O7
exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T_c.
Considering previously reported ac susceptibility, neutron scattering and \muSR
results, we suggest the existence of anomalously slow spin dynamics to be a
common physical property of pyrochlore magnetic materials. The possibility of
molecular spin substructures to be associated to the slow dynamics and
therefore the short-range correlations is mentioned. The slow spin dynamics
observed under field does not exclude the presence of much faster dynamics
detected in extremely low or zero field.Comment: 11 pages, 10 figure
Evidence for unidimensional low-energy excitations as the origin of persistent spin dynamics in geometrically frustrated magnets
We report specific heat, magnetic, and muon spin relaxation measurements
performed on a polycrystalline sample of the normal spinel CdHo2S4. The
rare-earth ions sit on a lattice of corner-sharing regular tetrahedra as in
pyrochlore compounds. Magnetic ordering is detected at Tc ~ 0.87 K. From
spin-lattice relaxation rate measurements on both sides of Tc we uncover
similar magnetic excitation modes driving the so-called persistent spin
dynamics at T < Tc. Unidimensional excitations are argued to be at its origin.
Often observed spin loop structures are suggested to support these excitations.
The possibility of a generic mechanism for their existence is discussed.Comment: 7 pages, 8 figure
Determination of the zero-field magnetic structure of the helimagnet MnSi at low temperature
Below a temperature of approximately 29 K the manganese magnetic moments of
the cubic binary compound MnSi order to a long-range incommensurate helical
magnetic structure. Here, we quantitatively analyze a high-statistic zero-field
muon spin rotation spectrum recorded in the magnetically ordered phase of MnSi
by exploiting the result of representation theory as applied to the
determination of magnetic structures. Instead of a gradual rotation of the
magnetic moments when moving along a axis, we find that the angle of
rotation between the moments of certain subsequent planes is essentially
quenched. It is the magnetization of pairs of planes which rotates when moving
along a axis, thus preserving the overall helical structure.Comment: 10 pages, 4 figure
Low temperature crystal structure and local magnetometry for the geometrically frustrated pyrochlore Tb2Ti2O7
We report synchrotron radiation diffraction and muon spin rotation (muSR)
measurements on the frustrated pyrochlore magnet Tb2Ti2O7. The powder
diffraction study of a crushed crystal fragment does not reveal any structural
change down to 4 K. The muSR measurements performed at 20 mK on a mosaic of
single crystals with an external magnetic field applied along a three-fold axis
are consistent with published a.c. magnetic-susceptibility measurements at 16
mK. While an inflection point could be present around an internal field
intensity slightly above 0.3 T, the data barely support the presence of a
magnetization plateau.Comment: To appear in the proceedings of the 13th International Conference on
Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland, 1-6
June 201
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