Anomalous Transport Properties in Acceptor-Type Magnetic GICs

Abstract

Magnetic phase transitions have been studied in both donor and acceptor-type magnetic GICs using AC magnetic susceptibility χ and magnetization M measurements.¹ However, C₆Eu is the only GIC for which a magnetic phase diagram has been successfully determined using magnetoresistance (transport) measurements.² The strong coupling between graphite layers and Eu layers in C₆Eu results in strong scattering effects on the graphite conduction π-electrons by the magnetic excitations of the Eu ions. Therefore dramatic changes in the magnetoresistance of C₆Eu are observed as the system undergoes magnetic phase transitions. In contrast, acceptor-type magnetic GICs have a much weaker magnetic coupling between intercalate and graphite layers because of the relatively larger spacing between the graphite and the magnetic species.^3,4,5,6 Therefore the change in magnetoresistance due to changes in magnetic ordering is expected to be smaller than for the case of C₆Eu. In this work we show that transport-property measurements (magnetoresistance and temperature-dependent zero-field resistivity) can yield measurable effects associated with magnetic phase transitions for the acceptor-type magnetic stage-1 and stage-2 CoC1₂-GICs. Thus studies on the transport properties of acceptor magnetic GICs provide a complementary way to investigate the magnetic phase diagrams.¹ In addition, such measurements provide information on the π-d electron coupling constants (J _π−d) and the interplane exchange coupling constants (J′).⁷