The viability of dilute magnetic semiconductors in applications is linked to the strength of the magnetic couplings, and room temperature operation is still elusive in standard inorganic systems. Molecular semiconductors are emerging as an alternative due to their long spin-relaxation times and ease of processing, but, with the notable exception of vanadium-tetracyanoethylene, magnetic transition temperatures remain well below the boiling point of liquid nitrogen. Here we show that thin films and powders of the molecular semiconductor cobalt phthalocyanine exhibit strong antiferromagnetic coupling, with an exchange energy reaching 100 K. This interaction is up to two orders of magnitude larger than in related phthalocyanines and can be obtained on flexible plastic substrates, under conditions compatible with routine organic electronic device fabrication. Ab initio calculations show that coupling is achieved via superexchange between the singly occupied a1g () orbitals. By reaching the key milestone of magnetic coupling above 77 K, these results establish quantum spin chains as a potentially useable feature of molecular films.

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Iron phthalocyanine (FePc) is a molecular semiconductor whose building blocks are 1D ferromagnetic chains. It is shown that its optical and magnetic properties are controlled by the growth strategy, obtaining extremely high coercivities of over 1 T and modulating the exchange constant between 15 and 29 K through switching from thin films with controlled orientations, to ultralong nanowires. Magnetization measurements are analyzed using concepts and formulas with broad applicability to all 1D ferromagnetic chains. They show that FePc is best described by a xy model with moments preferentially lying in the molecular planes. The chain Hamiltonian is very similar to that for the classic inorganic magnet CsNiF 3 , but with ferromagnetic rather than antiferromagnetic interchain interactions. The dominant degrees of freedom are topological excitations called solitons, namely moving 1D magnetic domain walls, and at low temperatures and fields a "super-Curie-Weiss" law characteristic of nearly 1D xy and Heisenberg ferromagnets, where susceptibility scales as 1/(T 2 -θ 2 ), is observed. The ability to control the molecular orientation and ferromagnetism of FePc systems, and produce them on flexible substrates, together with excellent transistor characteristics reported previously for phthalocyanine analogues, makes them potentially useful for magneto-optical and spintronic devices.