[Show abstract][Hide abstract] ABSTRACT: We report neutron inelastic scattering measurements and analysis of the
spectrum of magnons propagating within the Fe2O4 bilayers of LuFe2O4. The
observed spectrum is consistent with six magnetic modes and a single prominent
gap, which is compatible with a single bilayer magnetic unit cell containing
six spins. We model the magnon dispersion by linear spin-wave theory and find
very good agreement with the domain-averaged spectrum of a spin-charge bilayer
superstructure comprising one Fe3+ -rich monolayer and one Fe2+ -rich
monolayer. These findings indicate the existence of polar bilayers in LuFe2O4,
contrary to recent studies that advocate a charge-segregated non-polar bilayer
model. Weak scattering observed below the magnon gap suggests that a fraction
of the bilayers contain other combinations of charged monolayers not included
in the model. Refined values for the dominant exchange interactions are
Full-text · Article · Jun 2014 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Information on the anisotropy of the atomic magnetic moment density can be obtained from neutron scattering. Within the dipole approximation the effect of the anisotropy is usually neglected. We show how the anisotropy of the charge-, spin-, orbital moment and current densities of an unfilled shell can be calculated. The theoretical formulae have been implemented in the McPhase software suite. We illustrate the theory for some representative examples of different magnetic ions in an idealized crystal field. Moreover, we also evaluate the various densities for some real systems.
No preview · Article · Oct 2011 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: We present data on the magnetic and magneto-elastic coupling in the hexagonal multiferroic manganite LuMnO3 from inelastic neutron scattering, magnetization and thermal expansion measurements. We measured the magnon dispersion along the main symmetry directions and used this data to determine the principal exchange parameters from a spin-wave model. An analysis of the magnetic anisotropy in terms of the crystal field acting on the Mn is presented. We compare the results for LuMnO3 with data on other hexagonal RMnO3 compounds. Comment: 7 pages, 8 figures, typo corrected
[Show abstract][Hide abstract] ABSTRACT: We present the results of muon-spin relaxation and heat capacity measurements on LiFeSi2O6 and NaFeSi2O6. In synthetic samples of both compounds we see a single muon oscillation frequency consistent with commensurate magnetic structures below TN. In contrast, for a specimen of naturally occurring NaFeSi2O6, in which multiferroicity has been observed, a rapid Gaussian depolarization of the muon polarization is observed instead, showing that the magnetic structure in this case is more complex. Heat capacity measurements reproduce the phase diagrams previously derived from other techniques and demonstrate that the main contribution to the magnetic entropy is associated with the buildup of correlations in the quasi-one-dimensional Fe3+ chains.
[Show abstract][Hide abstract] ABSTRACT: We present the results of muon-spin relaxation (muSR) measurements on the
hexagonal manganite HoMnO3. Features in the temperature-dependent relaxation
rate, lambda, correlate with the magnetic transitions at 76 K, 38 K and 34 K.
The highest temperature transition, associated with the ordering of Mn3+
moments has the largest effect on lambda. The application of a static electric
field of E=10^4 Vm^-1 below T=50 K causes a small reduction in lambda which is
suggestive of coupling between ferroelectric and magnetic domain walls in the
ordered state of the material.
[Show abstract][Hide abstract] ABSTRACT: We present a muon-spin relaxation investigation of the Ising chain magnet Ca3Co2-xMnxO6 (x≈0.95) . We find dynamic spin fluctuations persisting down to the lowest measured temperature of 1.6 K. The previously observed transition at around 18K is interpreted as a subtle change in dynamics for a minority of the spins coupling to the muon that we interpret as spins locking into clusters. The dynamics of this spin fraction freeze below a temperature TSF≈8K , while a majority of spins continue to fluctuate. An explanation of the low-temperature behavior is suggested in terms of the predictions of the anisotropic next-nearest-neighbor Ising model.
[Show abstract][Hide abstract] ABSTRACT: Transverse-field muon-spin rotation measurements performed on two samples of LiFeAs demonstrate that the superfluid stiffness of the superconducting condensate in relation to its superconducting transition temperature is enhanced compared to other pnictide superconductors. Evidence is seen for a field-induced magnetic state in a sample with a significantly suppressed superconducting transition temperature. The results in this system highlight the role of direct Fe-Fe interactions in frustrating pairing mediated by antiferromagnetic fluctuations and indicate that, in common with other pnictide superconductors, the system is close to a magnetic instability.
[Show abstract][Hide abstract] ABSTRACT: We report muon-spin relaxation measurements of highly homogeneous samples of FeAs and FeAs2, both previously found as impurity phases in some samples of recently synthesized pnictide superconductors. We observe well defined muon precession in the FeAs sample with two precession frequencies of 38.2(3) and 22.7(9) MHz at 7.5 K, with the majority of the amplitude corresponding to the lower frequency component. In FeAs2 we confirm previous measurements showing that no long-ranged magnetic order occurs above 2 K and measure the muon spin relaxation rate, which increases on cooling. Our results exclude the possibility that previous muon-spin relaxation measurements of pnictide superconductors have been measuring the effect of these possible impurities. Comment: 4 pages, 3 figures, corrected Figure 3