Magnetic characterization of calcium-nickel-potassium oxide catalysts
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SQUID magnetometer characterization of Ca-Ni-K-O catalyst materials reveals complex magnetic behavior. The magnetic properties are generally determined by the antiferromagnetic material NiO, but samples with traces of elemental nickel show marked effects of ferromagnetism. Potassium doping enhances the formation of metallic nickel. Further deviations from bulk NiO properties can be attributed to NiO particle size effects (superparamagnetism) and to the presence of paramagnetic impurities, possibly Ni3+ ions.
I. INTRODUCTION
Mixed metal oxides in the Ca-Ni-K-O system are active in the catalyzed gasification of carbonaceous materials.1"6 In addition to having been synthesized as powders, they have also been fabricated as films.7 The accurate characterization of these materials is complex due to their sensitivity to details of the processing conditions used to make them. The magnetic properties of these materials may be expected to depend primarily on the spin magnetic moments of the electrons in the partially filled 3d orbital of the nickel atoms and ions. Isolated Ni2+ and Ni3+ ions are expected to have spins of 1 and 3/2, and moments of about 2.8 and 3.9 Bohr magnetons, respectively. In metallic nickel the 3d orbitals overlap to form a band that is not quite full. The electrons in this band experience an exchange interaction that tends to align spins parallel to one another. Since the band is not full, there is a net density of parallel spins, leading to ferromagnetism, with a saturation moment of 0.6 Bohr magnetons per atom. The material NiO is an antiferromagnetic semiconductor. In this case the magnetic Ni2+ ions order antiferromagnetically below the Neel temperature (525 K) due to electronic interactions mediated by the oxygen atoms. In the form of small particles, both nickel metal and nickel oxide exhibit the phenomenon of superparamagnetism. Consider the ferromagnetic case first. Small ferromagnetic particles below roughly 15 nm diameter contain only a single magnetic domain, due to the finite energy required to make a domain wall. The spontaneous moment can rotate freely if the temperature is high enough to overcome any energy anisotropy (due to stress, particle shape, crystal lattice, etc.). An assembly of such particles behaves thermally and mag-
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netically just as though it were an ordinary paramagnetic material, but with a moment that is much larger than an atomic moment. The resulting magnetic behavior was named superparamagnetism by Bean.8 Two important characteristics of superparamagnetism are the absence of magnetic hysteresis and, further, that the magnetic moment of the sample is a function of the ratio H/T only (field/temperature), after correcting for the temperature dependence of the moments of the individual particles. These superparamagnetic characteristics are seen only above the so-called blocking temperature (below which the effects of anisotropy are important). Provided that a sample is fully superparamagnetic, information on the s
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