Doon Gibbs

Interplay between Charge, Orbital, and Magnetic Order in Pr{sub 1-x}Ca{sub x}MnO{sub 3}

We report resonant x-ray scattering studies of charge and orbital order in Pr{sub 1-x}Ca {sub x}MnO{sub 3} with x=0.4 and 0.5. Below the ordering temperature, T{sub O}=245 K , the charge and orbital order intensities follow the same temperature dependence, including an increase at the antiferromagnetic ordering temperature, T{sub N} . High resolution measurements reveal, however, that long range orbital order is never achieved. Rather, an orbital domain state is formed. Above T{sub O} , the charge order fluctuations are more highly correlated than the orbital fluctuations. We conclude that the charge order drives the orbital order at the transition. (c) 1999 The American Physical Society.

Magnetic rare earth superlattices

Abstract Advances in molecular beam epitaxy deposition techniques have recently made it possible to grow, an atomic plane at a time, single crystalline superlattices composed of alternating layers of a magnetic rare earth, such as Gd, Dy, Ho, or Er, and metallic Y, which has an identical chemical structure. The primary goal of this article is to review the new and interesting magnetic structures which have been discovered in these novel superlattice systems and to consider what implications the observed phases have on our understanding of the underlying microscopic magnetic interactions. In particular, the effects of the artificial periodicity or compositional modulation, finite layer thickness, and epitaxial strain on the resulting long range magnetic order of Gd-Y, Dy-Y, Ho-Y, Er-Y, and Gd-Dy superlattices are described.

Spin dynamics and orbital state in LaTiO3

A neutron scattering study of the Mott-Hubbard insulator LaTiO3 ( T(N) = 132 K) reveals a spin wave spectrum that is well described by a nearest-neighbor superexchange constant J = 15.5 meV and a small Dzyaloshinskii-Moriya interaction ( D = 1.1 meV). The nearly isotropic spin wave spectrum is surprising in view of the absence of a static Jahn-Teller distortion that could quench the orbital angular momentum, and it may indicate strong orbital fluctuations. A resonant x-ray scattering study has uncovered no evidence of orbital order in LaTiO3.

Spin slips and lattice modulations in holmium: A magnetic x-ray scattering study

The analysis of data obtained in magnetic X-ray scattering experiments on rare earth metals, particularly holmium, has led to a phenomenological model for one-dimensional spatially propagating magnetic structures. Based on the concept of spin slips or discommensurations, this model explains the observed lock-in transitions in the magnetic spirals of the rare earths in terms of simple commensurate structures. Further, the anomalous intensities of previously observed higher harmonic magnetic satellites, as well as the qualitative behavior of the magnetic wave vector in the presence of a magnetic field, are understood directly within the spin slip description. We discuss how the presence of spin slips in the magnetic structure can lead to modulations of the crystal lattice as demonstrated by a recent magnetic X-ray study of holmium.

DIFFRACTION STUDIES OF RARE EARTH METALS AND SUPERLATTICES

We review the results of X-ray scattering studies of the rare earth metals and present related new results for superlattices and thin slabs. In rare earth crystals we have observed weak structural modulations which accompany the magnetic ordering. The wave length of this modulation can be derived from a spin-slip model in accordance with symmetry considerations. X-ray scattering of both the charge and magnetization density modulations allow for highly accurate determination of the magnetic wave vector. The physical basis of our discussion is given in the context of lattice modulations. The implications of these results for the understanding of magnetic structure of rare earth superlattices are also discussed in the light of recent neutron diffraction studies of holmium-yttrium superlattices. The effect of the finite size of the magnetic block in a superlattice is considered and it is shown that significantly different behavior than in bulk is expected. In particular it is found that for thin slabs the ferromagnetic phase has the lowest energy.

X-ray resonant scattering studies of orbital and charge ordering in Pr 1 − x Ca x MnO 3

We present the results of a systematic x-ray scattering study of the charge and orbital ordering in the manganite series

X-Ray anomalous scattering study of a charge-ordered state in NaV2O5

{\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}

Magnetic rare-earth superlattices (invited)

with

Reconstruction of the (111) and (001) surfaces of Au and Pt: thermal behavior

x=0.25,

RESONANT X-RAY DIFFRACTION OF THE MAGNETORESISTANT PEROVSKITE PR0.6CA0.4MNO3

0.4, and 0.5. The temperature dependence of the scattering at the charge and orbital wavevectors, and of the lattice constants, was characterized throughout the ordered phase of each sample. It was found that the charge- and orbital-order wavevectors are commensurate with the lattice, in striking contrast to the results of electron and neutron diffraction studies of samples with