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Register a new userIntraplanar Magnetic Excitations in Na0.5CoO2: An Inelastic Neutron Study
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Added by
Jennifer Wooldridge
Publication date
2006
fields
Physics
and research's language is
English
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Inelastic neutron scattering measurements mapping the in-plane magnetic interactions of Na0.5CoO2 reveal dispersive excitations at points above an energy gap Eg = 11.5(5) meV at the superstructural Bragg reflections. The excitations are highly damped, broadening with increasing energy, and disappear at hw ~ 35 meV, a strong indication that the magnetism is itinerant. Tilting into the ac plane reduces the value of Eg by 25%, suggesting that the dispersion along c is significant and the magnetic correlations are three-dimensional, as seen at the higher doping levels.
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We report polarized and unpolarized neutron scattering measurements of the magnetic order in single crystals of Na0.5CoO2. Our data indicate that below T_N=88 K the spins form a novel antiferromagnetic pattern within the CoO2 planes, consisting of alternating rows of ordered and non-ordered Co ions. The domains of magnetic order are closely coupled to the domains of Na ion order, consistent with such a two-fold symmetric spin arrangement. Magnetoresistance and anisotropic susceptibility measurements further support this model for the electronic ground state.
We have investigated the phonon and the magnetic excitations in LaCoO3 by inelastic neutron scattering measurements. The acoustic phonon dispersions show some characteristic features of the folded Brillouin zone (BZ) for the rhombohedrally distorted perovskite structure containing two chemical formula units of LaCoO3 in the unit cell. We observed two transverse optical (TO) phonon branches along (delta, delta, delta), consistent with previously reported Raman active Eg modes which show remarkable softening associated with the spin-state transition [Ishikawa et al., (Phys. Rev. Lett. 93 (2004) 136401.)]. We found that the softening takes place in the TO mode over the whole BZ. In contrast, the acoustic phonons show no anomalous softening associated with the spin-state transition. The low-energy paramagnetic scattering at 8 K is weak, increasing towards a maximum at E > 15 meV, consistent with excitation of the nonmagnetic low-spin to magnetic intermediate-spin state of Co 3+ ions.
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.
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.
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