A density functional characterisation of the exchange properties in transition metal phthalocyanines

Due to their chemical and thermal stability as well as the variance observed in their cooperative magnetic ordering, the family of transition metal phthalocyanines (TMPcs) has been examined extensively within the context of both spintronics and quantum information processing. The self-assembly of individual TMPc molecules into stacked chain structures, facilitates the opportunity to study low-dimensional nanomagnetic materials. However, a pre-condition for successful commercialisation of a device is magnetic ordering at operable temperatures. Whilst this has not been observed in any known TMPc system, alpha-phase cobalt (II) phthalocyanine (α-CoPc) has exhibited distinctively strong antiferromagnetic coupling, with a Neel transition temperature, T_N ~ 100 K. However, the mechanism underpinning this magnetic behaviour is not fully understood. Using a density functional-based approach, the underlying exchange interactions in TMPcs may be elucidated and understood in terms of the electronic structure of each respective molecule. This has enabled a predictive approach to the relationship between the magnetic properties, molecular electronic configuration and intrachain geometry. Using this model, specific modifications to the TMPc systems have been predicted to significantly raise the coupling interaction. Using broken symmetry density functional theory, an evaluation of the exchange strength in these modified systems has been obtained and is shown to explicitly support the predictive capabilities of the model.