436 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
The time dependence of muon spin relaxation spectra and spin correlation functions
The existing theory of the microscopic interpretation of the dynamical
contribution to zero-field muon depolarization spectra in a longitudinal
geometry is developed. The predicted relaxation of the muon depolarization is
calculated from two forms of the spin correlation function. First, when the
spin correlation function has an exponential form with a single wave vector
dependent relaxation rate is considered, it is shown that this form of the spin
correlation function reproduces the slow and fast fluctuation limits of
stochastic spin theory regardless of the choice of microscopic spin model.
Second, if the spin correlation function is a homogeneous scaling function
(such as a power-law decay with time), as suggested by the mode-coupling theory
of spin dynamics, this results in a stretched exponential relaxation of the
muon spectra. For simple spin diffusion, the muon spectra are shown to be relax
with a root-exponential form.Comment: 5 pages, 1 figur
Zero field muon spin lattice relaxation rate in a Heisenberg ferromagnet at low temperature
We provide a theoretical framework to compute the zero field muon spin
relaxation rate of a Heisenberg ferromagnet at low temperature. We use the
linear spin wave approximation. The rate, which is a measure of the spin
lattice relaxation induced by the magnetic fluctuations along the easy axis,
allows one to estimate the magnon stiffness constant.Comment: REVTeX 3.0 manuscript, 5 pages, no figure. Published in Phys. Rev. B
52, 9155 (1995
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
Low-temperature magnetization in geometrically frustrated Tb2Ti2O7
The nature of the low temperature ground state of the pyrochlore compound
Tb2Ti2O7 remains a puzzling issue. Dynamic fluctuations and short-range
correlations persist down to 50 mK, as evidenced by microscopic probes. In
parallel, magnetization measurements show irreversibilities and glassy behavior
below 200 mK. We have performed magnetization and AC susceptibility
measurements on four single crystals down to 57 mK. We did not observe a clear
plateau in the magnetization as a function of field along the [111] direction,
as suggested by the quantum spin ice model. In addition to a freezing around
200 mK, slow dynamics are observed in the AC susceptibility up to 4 K. The
overall frequency dependence cannot be described by a canonical spin-glass
behavior.Comment: 5 pages, 4 figures + Supp. Mat (3 pages, 5 figures
Evidence for an antiferromagnetic component in the magnetic structure of ZrZn2
Zero-field muon spin rotation experiments provide evidence for an
antiferromagnetic component in the magnetic structure of the intermetallics
ZrZn2.Comment: 5 pages, 2 figure
2D Kagome Ordering in the 3D Frustrated Spinel Li2Mn2O4
muSR experiments on the geometrically frustrated spinel oxide, Li2Mn2O4, show
the development of spin correlations over a range of length scales with
decreasing temperature. Increased relaxation below 150 K is consistent with the
onset of spin correlations. Below 50 K, spin order on a length scale, which is
long range for the muSR probe, appears abruptly in temperature, consistent with
prior neutron diffraction results. The oscillations in the zero field asymmetry
are analyzed using a three frequency model. By locating the muon site this is
shown to be consistent with the unexpected 2D q = root 3 x root 3 structure on
the Kagome planes proposed originally from neutron data. Longitudinal field
data demonstrate that some spin dynamics persist even at 2 K. Thus, a very
complex magnetic ground state, featuring the co-existence of long length scale
2D ordering and significant spin dynamics, is proposed. This is unusual
considering the 3D topology of the Mn3+ spins in this material.Comment: 9 pages, 9 figures, to be submitted to J. Phys. Cond. Mat
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