Role of local geometry in the spin and orbital structure of transition metal compounds

We analyze the role of local geometry in the spin and orbital interaction in transition metal compounds with orbital degeneracy. We stress that the tendency observed in the most studied case (transition metals in O₆ octahedra with one common oxygen—common corner of neighboring octahedra—and with ~180° metal–oxygen–metal bonds), that ferro-orbital ordering renders antiferro-spin coupling and, vice versa, antiferro-orbitals give ferro-spin ordering, is not valid in the general case, in particular, for octahedra with a common edge and with ~90° M–O–M bonds. Special attention is paid to the “third case,” that of neighboring octahedra with a common face (three common oxygens), which has largely been disregarded until now, although there are many real systems with this geometry. Interestingly enough, the spin-orbit exchange in this case turns out to be simpler and more symmetric than in the first two cases. We also consider, which form the effective exchange takes for different geometries in the case of strong spin–orbit coupling.     

New Half-Metallic States in Systems with Spin and Charge Density Waves (Brief Review)

JETP Letters - Studies of a recently suggested mechanism for the stabilization of half-metallic states in doped systems with the nesting of Fermi surface sheets are briefly reviewed. The...     

Isotope effect for transport and magnetic properties of La 0.35 Pr 0.35 Ca 0.3 MnO 3 thin films

The effect of ¹⁶ O → ¹⁸ O isotope substitution on electrical resistivity, magnetoresistance, and ac magnetic susceptibility was studied for La_0.35Pr_0.35Ca_0.3MnO₃ epitaxial thin films deposited onto LaAlO₃ and SrTiO₃ substrates. For the films on LaAlO₃, the isotope substitution resulted in the reversible transition from a metal-like to insulating state. The applied magnetic field ( H ≥ 2 T) transformed the sample with ¹⁸O back to the metallic state. The films on SrTiO₃ remained metallic at low temperatures for both ¹⁶O and ¹⁸O, but the shift of the resistivity peak corresponding to onset of metallic state exceeded 63 K after ¹⁶ O → ¹⁸ O substitution. The temperature dependence of both resistivity and magnetic susceptibility was characterized by hysteresis, especially pronounced in the case of the films on LaAlO₃. Such a behavior gives certain indications of the phase separation characteristic of interplay between ferromagnetism and charge ordering. Received 11 February 2000 and Received in final form 13 September 2000     

Metallindustrie

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Monitoring industrial wastewater by online GC-MS with direct aqueous injection

An online GC–MS-system for automated monitoring of crude wastewater at a complex chemical production site is presented. The modular system is, in principal, based on commercial equipment, but utilizes a special, two-stage injector, which consists of a splitless vaporization chamber on top of a PTV injector filled with Tenax. This set-up enables direct injection of wastewater. Almost 140 volatile and semi-volatile compounds are calibrated down to 1 mg L⁻¹, which is sufficient for analysis of the influent of the wastewater-treatment plant. Two instruments analyze alternately, every 20 min, and the instrument cycle time is 40 min. The quantitative results are transferred to a database which is connected to a process-control system. Depending on the nature and concentration of a compound, an alarm can be generated and the wastewater stream can be diverted into an “off spec tank” if necessary. The GC–MS-system operates quasi-continuously with a system availability >98%. Data quality is automatically controlled in each run and by daily analysis of a quality-control sample. The development of a novel stacked PTV–PTV injector design to expand the range of analytes to selected basic compounds is described.