Jaime A. Fernandez-Baca

Short-range polaron correlations in the ferromagnetic La1-xCaxMnO3

We use neutron scattering to demonstrate the presence of lattice polarons and their short-range correlations for several samples of La1-xCaxMnO3 in the Ca doping range 0.15</=x</=0.3. We establish the doping dependence of the orientation, commensuration, and coherence length of the polaron correlations and show that the populations of correlated and uncorrelated polarons are intimately related to the transport properties of the materials.

Evolution of the low-frequency spin dynamics in ferromagnetic manganites

Neutron scattering was used to study the ferromagnetic manganites Nd0.7Sr0.3MnO3 (T-C = 197.9 K) and PT0.63Sr0.37MnO3 (T-C = 300.9 K). The spin dynamical behavior of these two systems is similar at low temperatures but drastically different at temperatures around T-C. While the formation of spin clusters of size (similar to 20 Angstrom) dominates the spin dynamics of the 197.9 K sample close to T-C, the paramagnetic to ferromagnetic transition for the 300.9 K sample is mon conventional. These results, combined with seemingly inconsistent earlier reports, reveal clear systematics in the spin dynamics of the manganites.

Magnon damping by magnon-phonon coupling in manganese perovskites

Inelastic neutron scattering was used to systematically investigate the spin-wave excitations (magnons) in ferromagnetic manganese perovskites. In spite of the large differences in the Curie temperatures (Tcs) of different manganites, their low-temperature spin waves have very similar dispersions with the zone-boundary magnon softening and broadening that cannot be explained by the canonical double exchange mechanism. From the wave-vector dependence of the magnon lifetime effects and its correlation with the dispersions of the optical-phonon modes, we argue that a strong magnetoelastic (magnon-phonon) coupling is responsible for the observed low-temperature anomalous spin dynamical behavior of the manganites.

Existence of Ferroelectric Ice in the Universe

The question as to whether or not ferroelectric ice, named ice XI, exists in a stable low-temperature phase attracts much interest. This question arose as a condensed-matter issue and became of interest in astronomy (e.g., does ice XI exist on Pluto?) because astronomical observations identified the existence of crystalline ice in our solar system. From neutron diffraction experiments, we found the temperature conditions for the transformation of the largest fraction of ice into ice XI using the lowest level of impurity dopant. The finding of bulk crystal of ordered structure firmly supports that ice XI is stable. This suggests the existence of naturally occurring ice XI at a narrow temperature range (57-66 K) in our solar system.

Magnetic dispersion of the diagonal incommensurate phase in lightly doped La2-xSrxCuO4.

We present inelastic neutron scattering experiments on a single-domain crystal of lightly doped La1.96Sr0.04CuO4. We find that the magnetic excitation spectrum in this insulating phase with a diagonal incommensurate spin modulation is remarkably similar to that in the superconducting regime, where the spin modulation is bond parallel. In particular, we find that the dispersion slope at low energy is essentially independent of doping and temperature over a significant range. The energy at which the excitations cross the commensurate antiferromagnetic wave vector increases roughly linearly with doping through the underdoped regime.

Neutron diffraction study of the magnetic structures of CeMn2Ge2 and CeMn2Si2

We have performed high‐resolution neutron powder diffraction measurements on CeMn2Si2 and CeMn2Ge2 at temperatures between 12 and 550 K. Our measurements indicate that at high temperatures both compounds are paramagnetic. Below TN=380 K CeMn2Si2 becomes a collinear AF, with a structure similar to that reported by Siek et al. in which the magnetic propagation vector is τ=(0 0 1). CeMn2Ge2, on the other hand, exhibits two different magnetic transitions. At TN≊415 K there is a transition to a collinear AF phase characterized by the commensurate propagation wave vector τ=(1 0 1). At TC≊318 K there is a transition to a conical structure with a ferromagnetic component along the c axis and a helical component in the ab plane. The helical component is characterized by the incommensurate propagation vector τ=(1 0 1−qz), where qz is temperature dependent.

Robust ferroelectric state in multiferroic Mn 1 − x Zn x WO 4

We report on the remarkably robust ferroelectric state in the multiferroic compound

Long-Range Magnetic Interactions in the Multiferroic Antiferromagnet MnWO4

{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{WO}}_{4}

Magnetic interactions in the geometrically frustrated triangular lattice antiferromagnet CuFeO2.

. Substitution of the magnetic

Incommensurate magnetic structure in the orthorhombic perovskite ErMnO3

{\mathrm{Mn}}^{2+}