Enhanced effects of local moment–carrier couplings in the high-purity alloys Cu_xM_1-x(phthalocyanine)I (M = H₂ or Ni)₂

Utilizing techniques for synthesizing very pure M(pc), (pc=phthalocyanine), we prepared alloys of the type Cu_xM_1-x(pc)I, M=H₂ or Ni to examine the interaction between the conduction electrons and the local moments associated with the S=1/2Cu^II metal ions embedded in the macrocycle center. The effects of the local moments, which include large decreases in conductivity and large positive magnetoconductivities, are greatly enhanced in these purer materials. Introduction of 5% Cu(pc) reduces the conductivity by a factor of 3. This same alloy shows a positive magnetoconductivity with a relative increase in conductivity of tenfold; this effect is four times greater than that observed in Cu_0.05Ni_0.95(pc)I made from commercially available material. Magnetic studies including susceptibility and electron paramagnetic resonance confirm that the two spin systems are strongly coupled. A magnetic susceptibility fit to a Curie-Weiss expression yields a Weiss constant, ?, that increases linearly with x, the Cu content. The slope of ? is explained in the companion paper by Martin and Phillips [Phys. Rev. B 60, 530 (1999)].