  The design and fabrication of materials that exhibit both semiconducting and
magnetic properties for spintronics and quantum computing has proven difficult.
Important starting points are high-purity thin films as well as fundamental
theoretical understanding of the magnetism. Here we show that small molecules
have great potential in this area, due to ease of insertion of localised spins
in organic frameworks and both chemical and structural purity. In particular,
we demonstrate that archetypal molecular semiconductors, namely the metal
phthalocyanines (Pc), can be readily fabricated as thin film quantum
antiferromagnets, important precursors to a solid state quantum computer. Their
magnetic state can be switched via fabrication steps which modify the film
structure, offering practical routes into information processing. Theoretical
calculations show that a new mechanism, which is the molecular analogue of the
interactions between magnetic ions in metals, is responsible for the magnetic
states. Our combination of theory and experiments opens the field of organic
thin film magnetic engineering.
