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Register a new userFerromagnetic Excitations in La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ Observed Using Neutron Inelastic Scattering
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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.
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We have investigated the effect of pressure on the electronic, magnetic, and structural properties on a single crystal of conducting, ferromagnet (T$_{C}$=157K) La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ located near the boundary of the metal-insulator transition. Contrary to the results reported on related systems, we find a transition from the conducting state to an insulating state and a decrease of T$_{C}$ with increasing pressure while the lattice structure remains unchanged. We show that this unusual behavior is driven by a gradual change of the spin state of Co$^{3+}$ ions from magnetic intermediate-spin (t$_{2g}^5$e$_{g}^{1}$; S=1) to a nonmagnetic low-spin (t$_{2g}^6$e$_{g}^{0}$; S=0) state.
We present neutron scattering spectra taken from a single crystal of Na0.75CoO2, the precursor to a novel cobalt-oxide superconductor. The data contain a prominent inelastic signal at low energies (~10 meV), which is localized in wavevector about the origin of two-dimensional reciprocal space. The signal is highly dispersive, and decreases in intensity with increasing temperature. We interpret these observations as direct evidence for the existence of ferromagnetic spin fluctuations within the cobalt-oxygen layers.
High-energy, local multiplet excitations of the d-electrons are revealed in our inelastic neutron scattering measurements on the prototype magnetic insulator NiO. These become allowed by the presence of both non-zero crystal field and spin-orbit coupling. The observed excitations are consistent with optical, x-ray, and EELS measurements of d-d excitations. This experiment serves as a proof of principle that high-energy neutron spectroscopy is a reliable and useful technique for probing electronic excitations in systems with significant crystal field and spin-orbit interactions.
Among colossal magnetoresistive manganites the prototypical ferromagnetic manganite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ has a relatively small magnetoresistance, and has been long assumed to have only weak electron-lattice coupling. Here we report that La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ has strong electron-phonon coupling: Our neutron and x-ray scattering experiments show strong softening and broadening of transverse acoustic phonons on heating through the Curie temperature T$_C$ = 350 K. Simultaneously, we observe two phases where metallic resistivity and polarons coexist. The ferromagnetic polaronic metal phase between 200 K and T$_C$ is characterized by quasielastic scattering from dynamic CE-type polarons with the relatively short lifetime of $mathbf{tau}approx 1,rm{ps}$. This scattering is greatly enhanced above T$_C$ in the paramagnetic polaronic metal phase. Our results suggest that the strength of magnetoresistance in manganites scales with the inverse of polaron lifetime, not the strength of electron-phonon coupling.
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.
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