E. O. Wollan

Bio: E. O. Wollan is an academic researcher from Oak Ridge National Laboratory. The author has contributed to research in topics: Neutron diffraction & Antiferromagnetism. The author has an hindex of 34, co-authored 69 publications receiving 5673 citations. Previous affiliations of E. O. Wollan include University of Chicago.

Neutron Diffraction Study of the Magnetic Properties of the Series of Perovskite-Type Compounds [ ( 1 − x ) La , x Ca ] Mn O 3

Abstract: A study has been made of the magnetic properties of the series of perovskite-type compounds $[(1\ensuremath{-}x)\mathrm{La}, x\mathrm{Ca}]\mathrm{Mn}{\mathrm{O}}_{3}$ The investigations have been made primarily by neutron diffraction methods, but x-ray diffraction measurements of lattice distortions and ferromagnetic saturation data are also included This series of compounds exhibits ferromagnetic and antiferromagnetic properties which depend upon the relative trivalent and tetravalent manganese ion content The samples are purely ferromagnetic over a relatively narrow range of composition ($x\ensuremath{\sim}035$) and show simultaneous occurrence of ferromagnetic and antiferromagnetic phases in the ranges ($0lxl025$) and ($040lxl05$) Several types of antiferromagnetic structures at $x=0$ and $xg05$ have also been determined The growth and mixing of the various phases have been followed over the whole composition range, the ferromagnetic and antiferromagnetic moment contributions to the coherent reflections have been determined, and Curie and N\'eel temperatures have been measured The results have been organized into a scheme of structures and structure transitions which is in remarkable accord with Goodenough's predictions based on a theory of semicovalent exchange

Neutron Diffraction by Paramagnetic and Antiferromagnetic Substances

Abstract: Neutron scattering and diffraction studies on a series of paramagnetic and antiferromagnetic substances are reported in the present paper. The paramagnetic diffuse scattering predicted by Halpern and Johnson has been studied, resulting in the determination of the magnetic form factor for ${\mathrm{Mn}}^{++}$ ions. From the form factor, the radial distribution of the electrons in the $3d$-shell of ${\mathrm{Mn}}^{++}$ has been determined, and this is compared with a theoretical distribution of Dancoff. Antiferromagnetic substances are shown to produce strong, coherent scattering effects in the diffraction pattern. The antiferromagnetic reflections have been used to determine the magnetic structure of the material below the antiferromagnetic Curie temperature. For some substances the magnetic unit cell is found to be larger than the chemical unit cell. The temperature dependence of the antiferromagnetic intensities has been studied, and the directional effects which characterize neutron scattering by aligned atomic moments have been used to determine the moment alignment with respect to crystallographic axes. From studies with magnetic ions possessing both orbital and spin moments, it is found that the antiferromagnetic intensities contain partial orbital moment components along with the spin moment component. The degree of orbital moment contribution agrees satisfactorily with that predicted by models of lattice quenching.

Neutron Diffraction Investigations of the Magnetic Ordering in Fe Br 2 , Co Br 2 , Fe Cl 2 , and Co Cl 2

Abstract: Neutron diffraction experiments have been performed on anhydrous Fe${\mathrm{Br}}_{2}$, Co${\mathrm{Br}}_{2}$, Fe${\mathrm{Cl}}_{2}$, and Co${\mathrm{Cl}}_{2}$ at temperatures from 295\ifmmode^\circ\else\textdegree\fi{}K to 4.2\ifmmode^\circ\else\textdegree\fi{}K to investigate the existence of magnetic ordering in these hexagonal layer-type structures. All four compounds have an antiferromagnetic transition at low temperatures to structures in which the atomic magnetic moments within a metal layer form ferromagnetic sheets and the moments in adjacent layers are antiparallel. In the iron compounds the moments are oriented parallel to the hexagonal $c$ axis and in the cobalt compounds the moment orientation is perpendicular to that axis. Values of the atomic magnetic moments are close to those expected for the divalent metallic ions if the orbital contribution is quenched. Small-angle scattering experiments on Fe${\mathrm{Cl}}_{2}$ and Co${\mathrm{Cl}}_{2}$ have shown that the ferromagnetic coupling between moments within a layer is much stronger than the antiferromagnetic coupling between atoms in adjacent layers, and single-crystal investigations on these two compounds have determined the method by which large net magnetization values are obtained at temperatures below ${T}_{N}$ in moderate magnetic fields.

Neutron Diffraction Study of the Magnetic Properties of Rare-Earth-Iron Perovskites

Abstract: A neutron diffraction study has been made of the magnetic properties of the rare-earth-iron perovskites, NdFe${\mathrm{O}}_{3}$, HoFe${\mathrm{O}}_{3}$, and ErFe${\mathrm{O}}_{3}$, at temperatures ranging from 955\ifmmode^\circ\else\textdegree\fi{} to 1.25\ifmmode^\circ\else\textdegree\fi{}K. The iron ions in each of these compounds undergo a transition to an antiferromagnetic configuration in which each moment has six oppositely directed moments at nearest neighbor distances. The N\'eel temperatures are 760\ifmmode^\circ\else\textdegree\fi{}K, 700\ifmmode^\circ\else\textdegree\fi{}K, and 620\ifmmode^\circ\else\textdegree\fi{}K, respectively, for the compounds of Nd, Ho, and Er. The moment directions in HoFe${\mathrm{O}}_{3}$ and ErFe${\mathrm{O}}_{3}$ are parallel and antiparallel to the orthorhombic [100] direction at room temperature: at 43\ifmmode^\circ\else\textdegree\fi{}K the moments are found to be in a ($1\overline{1}0$) plane. In HoFe${\mathrm{O}}_{3}$ the iron-ion moments at 1.25\ifmmode^\circ\else\textdegree\fi{}K are parallel to [001]; in Er${\mathrm{Feo}}_{3}$ at the same temperature they are parallel to [110]. The magnitudes of the ordered iron moments at temperature saturation are ${4.5}_{7}$, ${4.6}_{0}$, and ${4.6}_{2}$ Bohr magnetons in NdFe${\mathrm{O}}_{3}$, HoFe${\mathrm{O}}_{3}$, and ErFe${\mathrm{O}}_{3}$, respectively. In the liquid helium temperature range, magnetic ordering transitions of the rare-earth ions in HoFe${\mathrm{O}}_{3}$ (${T}_{N}=6.5\ifmmode^\circ\else\textdegree\fi{}$K) and ErFe${\mathrm{O}}_{3}$ (${T}_{N}=4.3\ifmmode^\circ\else\textdegree\fi{}$K) are observed. The ${\mathrm{Er}}^{+3}$ ion moments form a nearly ideal antiferromagnetic configuration in which a chain of parallel moments is surrounded by four chains of oppositely directed moments at nearest neighbor distances. In this compound the ${\mathrm{Er}}^{+3}$ ion moments are parallel and antiparallel to [001] and at 1.25\ifmmode^\circ\else\textdegree\fi{}K have a magnitude of 5.8 Bohr magnetons. In HoFe${\mathrm{O}}_{3}$ the ions are ordered in a distorted antiferromagnetic configuration in which, at 1.25\ifmmode^\circ\else\textdegree\fi{}K, each ${\mathrm{Ho}}^{+3}$ moment with magnitude of 7.5 Bohr magnetons, makes an angle, in the (001) plane, of about 27\ifmmode^\circ\else\textdegree\fi{} with the [010] direction so as to produce a net ferromagnetic moment of 3.4 Bohr magnetons per HoFe${\mathrm{O}}_{3}$ molecule parallel to [100].

Magnetic Structures of Holmium. I. The Virgin State

Abstract: Neutron-diffraction measurements have been made on single-crystal specimens of holmium at temperatures ranging from room temperature to 4.2\ifmmode^\circ\else\textdegree\fi{}K. Below the N\'eel temperature of 133\ifmmode^\circ\else\textdegree\fi{}K, the moments order in a helical structure in which the $c$ axis is the screw axis. The interlayer angle varies from about 50\ifmmode^\circ\else\textdegree\fi{} per layer at ${T}_{N}$ to 30.0\ifmmode^\circ\else\textdegree\fi{} per layer at 4.2\ifmmode^\circ\else\textdegree\fi{}K. Below about 20\ifmmode^\circ\else\textdegree\fi{}K the structure is a conical configuration in which there is a net moment of $1.7{\ensuremath{\mu}}_{B}$ parallel to the $c$ axis. The configuration of moments in the basal plane at 4.2\ifmmode^\circ\else\textdegree\fi{}K is a distorted helical one in which moments of $9.5{\ensuremath{\mu}}_{B}$ are bunched around the easy $b$ directions in the plane.

Thousandfold Change in Resistivity in Magnetoresistive La-Ca-Mn-O Films

Abstract: A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77 kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are discussed.

Why Are There so Few Magnetic Ferroelectrics

Abstract: Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result, they have a spontaneous magnetization that can be switched by an applied magnetic field, a spontaneous polarization that can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature or have been synthesized in the laboratory. In this paper, we explore the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. In addition, we examine the properties of some known magnetically ordered ferroelectric materials. We find that, in general, the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Consequently, an additional electronic or structural driving force must be present for ferromagnetism and ferroelectricity to occur simultaneously.

Mixed Valence Chemistry-A Survey and Classification

Abstract: Publisher Summary This review is concerned with the neglected class of inorganic compounds, which contain ions of the same element in two different formal states of oxidation. Although the number of references cited in our review show that many individual examples of this class have been studied, yet they have very rarely been treated as a class, and there has never before, to our knowledge, been a systematic attempt to classify their properties in terms of their electronic and molecular structures. In the past, systems containing an element in two different states of oxidation have gone by various names, the terms “mixed valence,” nonintegral valence,” “mixed oxidation,” “oscillating valency,” and “controlled valency” being used interchangeably. Actually, none of these is completely accurate or all-embracing, but in our hope to avoid the introduction of yet another definition, we have somewhat arbitrarily adopted the phrase “mixed valence” for the description of these systems. The concept of resonance among various valence bond structures is one of the cornerstones of modern organic chemistry.