High temperature molecular magnetism caused by π-electrons: copper phthalocyanine doped with alkaline metals

Abstract

Electron spin resonance spectra of copper phthalocyanine doped with alkaline metals ( A x CuPc ) have been investigated. The temperature dependence of ESR spectra indicates ferromagnetic behavior. The Curie-Weiss temperature varies from 30 to 115 K depending on the stoichiometry x of the samples. Some particles of polycrystalline samples were attracted to a weak magnet at temperatures slightly higher than 77 K. The observed magnetism is caused by unpaired π-electrons of phthalocyanine anions on the eg doubly degenerated molecular orbital. The observed ferromagnetism can be understood within the framework of the McConnell-2 model proposed for organic ferromagnetic charge-transfer complexes. The high-temperature magnetism in A x CuPc is considered to be a result of the Zener mechanism of double exchange between phthalocyanine molecular anions of different valence.