J. B. MacChesney

Bio: J. B. MacChesney is an academic researcher from Bell Labs. The author has contributed to research in topics: Néel temperature & Antiferromagnetism. The author has an hindex of 9, co-authored 10 publications receiving 846 citations.

Electric and Magnetic Properties of the Strontium Ferrates

Abstract: A series of strontium ferrate compositions ranging from SrFeO2.7 to SrFeO3.0 have been produced by equilibration at oxygen pressures ranging from 0.2 to 855 atm. The stoichiometric composition SrFeO3.0 exhibits a simple cubic perovskite structure (a=3.850 A). Ceramic disks of this material are conductive (ρ≈10−3 Ω·cm) and their temperature dependence is characteristic of metallic conduction. Magnetic measurements indicate that SrFeO3.0 is antiferromagnetic below 130°K. With increasing oxygen deficiency, specimens show increased cell constants and eventually tetragonal distortion. Decrease in the Neel temperature and increased resistivity accompany these changes.

Magnetic Study of the Manganate Phases: CaMn O 3 , Ca 4 Mn 3 O 10 , Ca 3 Mn 2 O 7 , Ca 2 Mn O 4

Abstract: Two series of manganate phases have been prepared and their magnetic properties measured. The first series consists, in addition to CaMn${\mathrm{O}}_{3}$, of three phases having layered structures. In these, tetravalent magnanese ions are contained in single or multiple perovskite layers which are separated from each other by nonmagnetic calcium-oxygen layers. Magnetic exchange of the type ${\mathrm{Mn}}^{4+}$-O-${\mathrm{Mn}}^{4+}$ is disrupted between layers, and decreased N\'eel temperatures were expected to result. In a second series of phases, the size of of the CaMn${\mathrm{O}}_{3}$ unit cell was increased by partial substitution of strontium for calcium. Increased separation of manganese ions produced by this means was expected to reduce N\'eel temperatures. However, contrary to these expectations, N\'eel temperatures remained approximately constant for the first series of phases and increased for the second.

Preparation and Low‐Temperature Magnetic Properties of the Terbium Oxides

Abstract: The magnetic properties of terbium oxide phases (Tb2O3 cubic and monoclinic polymorphs, TbO1.715, TbO1.809, TbO1.823, and TbO2) have been measured between 1.4° and 300°K. With the exception of TbO1.809, each of these phases is observed to order magnetically at low temperatures. The ordering is predominantly antiferromagnetic, several of the lower oxide phases show metamagnetic behavior, wherein the observed moment has a nonlinear dependence upon the applied field at fields above a critical value, approximately 2000 Oe. The phase TbO1.023 exhibits a tendency toward ferrimagnetic alignment as evidenced by remanent magnetization at 1.4°K. TbO2 is antiferromagnetic with a Neel temperature of 3°K. At higher temperatures, in the paramagnetic region all of the oxide phases give effective Bohr magneton numbers close to the expected values for the free ions.

Oxygen Stoichiometry in the Barium Ferrates; Its Effect on Magnetization and Resistivity

Abstract: The barium ferrates comprise a series of compositions exhibiting a wide range of oxygen deficiency. Members of this series are isostructural with the high‐temperature polymorph of barium titanate which is hexagonal with space group C63/mmc. Polycrystalline specimens with varying oxygen contents, i.e., Fe4+ concentrations, were prepared by equilibration at oxygen pressures ranging from 0.2 to 2400 atm. The highest oxygen content achieved was BeFeO2.95 which had lattice parameters: a=5.674 A, c=13.645 A. This specimen exhibited unusual magnetic properties marked by an abrupt change in magnetization at 164°K. Above this temperature the material appeared to be ferrimagnetic. The peak value of the magnetization per gram, σg, is 13.8 emu at 164°K. At lower temperatures the compound is antiferromagnetic but exhibits a small residual moment. The transition appears to be first order with a thermal hysteresis of approximately 4°C between heating and cooling curves, and is accompanied by a discontinuity in the plot ...

Experiments on simple magnetic model systems

Abstract: “…. For the truth of the conclusions of physical science, observation is the supreme Court of Appeal….” (Sir Arthur Eddington, The Philosophy of Physical Science.) In this paper we shall review the theoretical and experimental results obtained on simple magnetic model systems. We shall consider the Heisenberg, XY and Ising type of interaction (ferro and antiferromagnetic), on magnetic lattices of dimensionality 1, 2 and 3. Particular attention will be paid to the approximation of these model systems in real crystals, viz. how they can be realized or be expected to exist in nature. A large number of magnetic compounds which, according to the available experimental information, meet the requirements set by one or the other of the various models are considered and their properties discussed. Many examples will be given that demonstrate to what extent experiments on simple magnetic systems support theoretical descriptions of magnetic ordering phenomena and contribute to their understanding. It will a...

Growth and applications of Group III-nitrides

Abstract: Recent research results pertaining to InN, GaN and AlN are reviewed, focusing on the different growth techniques of Group III-nitride crystals and epitaxial films, heterostructures and devices. The chemical and thermal stability of epitaxial nitride films is discussed in relation to the problems of deposition processes and the advantages for applications in high-power and high-temperature devices. The development of growth methods like metalorganic chemical vapour deposition and plasma-induced molecular beam epitaxy has resulted in remarkable improvements in the structural, optical and electrical properties. New developments in precursor chemistry, plasma-based nitrogen sources, substrates, the growth of nucleation layers and selective growth are covered. Deposition conditions and methods used to grow alloys for optical bandgap and lattice engineering are introduced. The review is concluded with a description of recent Group III-nitride semiconductor devices such as bright blue and white light-emitting diodes, the first blue-emitting laser, high-power transistors, and a discussion of further applications in surface acoustic wave devices and sensors.

Spintronics: a challenge for materials science and solid-state chemistry.

Abstract: Spintronics is a multidisciplinary field involving physics, chemistry, and engineering, and is a new research area for solid-state scientists. A variety of new materials must be found to satisfy different demands. The search for ferromagnetic semiconductors and stable half-metallic ferromagnets with Curie temperatures higher than room temperature remains a priority for solid-state chemistry. A general understanding of structure-property relationships is a necessary prerequisite for the design of new materials. In this Review, the most important developments in the field of spintronics are described from the point of view of materials science.