Magnetic correlation in itinerant magnetic materials above T C (invited)

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Abstract
Although the properties of magnetic insulators are relatively well understood, the same cannot be said of materials in which the unpaired electrons participate in the Fermi surface. A number of conflicting theories exist which all give a more or less satisfactory account of the ground state properties of magnetic metals but give widely different descriptions of magnetic fluctuations above the ordering temperature (TC). The paramagnetic state of a material can be characterized by the spin‐density spin‐density correlation function (SDSDCF) which is accessible to measurement through the magnetic scattering cross section. We have made measurements of the paramagnetic scattering from the transition metals Cr, Mn, Fe, Ni, and a number of intermetallic compounds over a wide range of temperature. Polarization analysis was used to separate the magnetic part of the scattering cleanly. In all cases where atoms with open d shells are in contact we have found significant structure in the SDSDCF over a range of several interatomic distances. This has loosely been called ‘‘short range order above TC.’’ The resolution of the spectrometer for the majority of the measurements was some 12 THz FWHM, so that although the true equal time SDSDCF is not obtained, essentially all contributions with a lifetime greater than 5×10−10 s are included and in particular most of the thermal fluctuations up to kTC. The paramagnetic scattering results are coherent with inelastic neutron scattering data both above and below the transition temperature. It has been found difficult to give a consistent account of all these results using theories based on extensions of the Heisenberg model. The fluctuating band models, in which correlations in the paramagnetic phase appear naturally, seem more appropriate.