No abstract

It is generally believed that the electrical transport in magnetoresistive manganites above T C is governed by small polarons associated with large lattice distortions. We present evidence which confirms the existence of magnetic clusters ͑polarons͒ associated with minimal lattice distortions above the Curie temperature. We take advantage of the fact that the Mössbauer spectra of the relatively large and slowly fluctuating magnetic polarons consist of incompletely collapsed magnetic component with extensive wings.

Whiskers of iron carbide encased in carbon shells have been prepared from iron (I1) phthalocyanine. Solid FePc is oxygenated and then thermally treated under H2 flow. With proper control of conditions, Fe3C free from metallic iron can be synthesized. The product was characterized with the help of Mossbauer effect spectroscopy, X-ray diffraction, X-ray absorption, and transmission electron microscopy. The iron carbide whiskers range in length from 300 to 500 nm, and their widths are approximately 100 nm. The carbon coating is uniform and about 2.7 nm wide.

No abstract

We follow the step-by-step progression of events while approaching the Curie temperature TC from below using Co57 substituent as a microprobe in an emission Mössbauer study combined with resistivity measurements. In the temperature range 0.33<T/TC⩽1, the material consists of a mixture of ferromagnetic and paramagnetic regions within the same matrix. An increase of the amount of paramagnetic fraction is accompanied by a decrease in electron delocalization in the ferromagnetic regions. At T⩾TC, the electrons are localized to neighboring Mn4+/Mn3+ pairs only, in about 46% of the paramagnetic species. The strength with which Mn atoms are bound to the neighbors also decreases progressively and rather steeply in the range 0.65⩽T/TC=1. Zero field resistivity, ρ0, follows linearly with the amount of the paramagnetic phase in the range 0.65⩽T/TC⩽1 and still shows metal-like behavior up to T/TC=1.03.