Resistivity anomaly in conducting europium chalcogenides

Abstract

Ferromagnetic and semiconducting Eu chalcogenides exhibit a negative magnetoresistance effect near the magnetic transition temperature (T C) which is much too large to be explained in terms of free electron spin disorder scattering. In order to define experimentally the parameters involved in this phenomenon we have measured the temperature dependence of resistance and magnetoresistance in the chalcogenide series using diamagnetic (e.g., La) or magnetic (e.g., Gd) trivalent RE ions. The trivalent ions donate an excess electron and change the character of the material from insulating to conducting. The resistivity increases for all dopants exponentially with decreasing temperature. This corresponds to an activation energy for carrier mobility which depends more on the carrier concentration than on the character of the anions and dopants. The resistivity rise is followed by a decrease below TC. The resulting peak is dependent on the magnetic order and decreases markedly with applied field strength and increasing electron concentration. Some possibilities for describing strong interactions between conduction electrons and localized spins will be discussed. © 1968 The American Institute of Physics.