Spin excitations used to probe the nature of the exchange coupling in the magnetically ordered ground state of Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$

We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$, in its magnetic and orbitally ordered phase. The data, which cover multiple Brillouin zones and the entire energy range of the excitations, are compared with several different models that are all consistent with the CE-type magnetic order, but arise through different exchange coupling schemes. The Goodenough model, i.e. an ordered state comprising strong nearest neighbor ferromagnetic interactions along zig-zag chains with antiferromagnetic inter-chain coupling, provides the best description of the data, provided that further neighbor interactions along the chains are included. We are able to rule out a coupling scheme involving formation of strongly bound ferromagnetic dimers, i.e. Zener polarons, on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e. recovery of the Goodenough model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping, and provide a recipe for how to interpret future measurements away from half-doping, where degenerate double exchange models predict more complex ground states.

The Magnetic Structure of DyMn2O5 Determined by Resonant X-ray Scattering

Resonant magnetic x-ray scattering has been used to investigate the magnetic structure of the magnetoelectric multiferroic DyMn2O5. We have studied the magnetic structure in the ferroelectric phase of this material, which displays the strongest ferroelectric polarisation and magnetodielectric effect of the RMn2O5 (where R is a rare earth ion, Y or Bi) family. The magnetic structure observed is similar to that of the other members of the series, but differs in the direction of the ordered moments. In DyMn2O5 both the Dy and Mn moments lie close to the b-axis, whereas in other RMn2O5 they lie close to the a-axis.

Itinerant Spin Excitations in SrFe2As2 Measured by Inelastic Neutron Scattering

We report inelastic neutron scattering measurements of the magnetic excitations in SrFe2As2, the parent of a family of iron-based superconductors. The data extend throughout the Brillouin zone and up to energies of ~260meV. An analysis with the local-moment J_1-J2 model implies very different in-plane nearest-neighbor exchange parameters along the $a$ and $b$ directions, both in the orthorhombic and tetragonal phases. However, the spectrum calculated from the J1-J2 model deviates significantly from our data. We show that the qualitative features that cannot be described by the J1-J2 model are readily explained by calculations from a 5-band itinerant mean-field model.

Ferromagnetic Excitations in La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ Observed Using Neutron Inelastic Scattering

Polarized neutron inelastic scattering has been used to measure spin excitations in ferromagnetic La$_{0.82}$Sr$_{0.18}$CoO$_{3}$. The magnon spectrum of these spin excitations is well defined at low energies but becomes heavily damped at higher energies, and can be modeled using a quadratic dispersion. We determined a spin wave stiffness constant of $D=94pm 3$,meV,AA$^{2}$. Assuming a nearest-neighbor Heisenberg model we find reasonable agreement between the exchange determined from D and the bulk Curie temperature. Several possible mechanisms to account for the observed spin-wave damping are discussed.

An x-ray resonant diffraction study of multiferroic DyMn2O5

X-ray resonant scattering has been used to measure the magnetic order of the Dy ions below 40K in multiferroic DyMn$_{2}$O$_{5}$. The magnetic order has a complex behaviour. There are several different ordering wavevectors, both incommensurate and commensurate, as the temperature is varied. In addition a non-magnetic signal at twice the wavevector of one of the commensurate signals is observed, the maximum intensity of which occurs at the same temperature as a local maximum in the ferroelectric polarisation. Some of the results, which bear resemblence to the behaviour of other members of the RMn$_{2}$O$_{5}$ family of multiferroic materials, may be explained by a theory based on so-called acentric spin-density waves.