The magnetic properties of binary rare-earth compounds are reviewed. The influence of crystal field and exchange interaction on the magnetic properties is shown to vary systematically in the group of LnX compounds [Ln=Lanthanide atom, X=N, P, As, Sb]. Crystal-field parameters are calculated.
The europium-chalcogenides are of particular interest. From measurements of the magnetic susceptibility at normal and under high pressure it is concluded that EuSe is antiferromagnetic above 3°K approximately with a Néel temperature of 4.6°K, but it assumes a ferro- or ferrimagnetic structure at lower temperatures.
The optical absorption of EuO, EuS, EuSe, and EuTe was measured as a function of temperature. Energy gaps and their variation with temperature were determined. A giant decrease of the gaps was observed below the Curie and Néel temperatures, respectively. The change of the gaps is shown to be proportional to the magnetization of the specimen. This effect yields a new method to measure the spontaneous magnetization without a magnetic field. A long tail of the spontaneous magnetization above the Curie temperature is found.
Gray (1731), Desagullier (1742) and Volta (1782) discovered and investigated electric conduction in solids. Davy (1821) found a decrease of the conductivity sigma in metals and Faraday (1833) observed a strong increase with temperature in a number of binary chemical compounds. Hittorf's (1851) measurements on Ag2S and Cu2S led to a linear relation of log sigma against 1/T. The controversial case of Ag2S is described. Hall (1879) and Rowland (1880) discovered a transverse voltage of a current carrying metal film in a magnetic field. Riecke (1901) and Drude (1900) developed the first electron theory of metals and Koenigsberger (1914) tried to explain the temperature dependence of the electrical conductivity by a dissociation theory. Baedeker (1908) was the first to observe semiconducting properties of CuI depending on the stoichiometric composition. Wagner (1933) proved that the conductivity of Ag2S is essentially electronic and not ionic. Gudden (1930) suggested that semiconduction is the result of impurities and imperfections in solids and Wagner and Schottky (1931) developed their theories of lattice defects (Fehlordnungs-Erscheinungen). Wilson (1931) presented the first band theory of intrinsic and extrinsic semiconductors. The existence of intrinsic conduction has been questioned by experimentalists and is verified only by the preparation and investigation of high-purity semiconducting elements.
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