Further experimental investigations on some ferromagnetic oxidic compounds of manganese with perovskite structure

Polycrystalline substances of the type La ₁- δSr _δMnO ₃ have been investigated. These are ferromagnetic semiconductors which exhibit some remarkable second order effects related to the electrical conductivity. The specific heat has been found to be in agreement with Weiss's theory. New data on the direct current resistivity includoing that at liquid hydrogen temperature have been obtained. The direct current magneto-resistance appears to be independent of the mutual orientation of magnetic field and current; most of the effect seems to be due to domain rotations. The alternating current resistivity of some samples depends strongly upon frequency and high values of the dielectric constant have been observed at low frequencies. Samples exhibiting these properties also show a magneto-resistance dependent on the frequency of the alternating current, and a variation of both resistivity and magneto-resistance with applied voltage. These effects strongly support the hypothesis of barrier-layer resistance in ceramic semiconductors. A phenomenological analysis of these effects is given and may be of some importance for the general problem of magneto-resistance in ferromagnetics. The Seebeck effect of various samples is roughly in agreement with certain basic ideas on conductivity in oxidic semiconductors. From Hall effect measurements no conclusions could be drawn except that the apparent electron mobility is extremely small.