Among the molecular magnets, multi-decker sandwiches of early transition metals and benzenes are a large, promising class. In particular, quasi-one-dimensional vanadium-benzene complexes recently attracted much experimental and theoretical interest. They reveal large spin moments and ferromagnetic coupling with increasing number of vanadium ions, making them very promising candidates for technology.
In our work we analyzed the stability of magnetism in these molecules by evaluating a quantity crucial for practical applications, the magnetic anisotropy energy. Magnetic anisotropy energy links the direction of the magnetization to the lattice and gives an estimate for an energy required to change this direction in space. Using first-principles electronic structure calculations we discovered that the anisotropy values of V-based sandwiches are rather small. Moreover, we proposed a way to effectively increase this energy barrier by one to two orders of magnitude, paving a way to stable magnetism in this type of compounds at higher temperatures. This can be achieved by substituting vanadium with heavier transition metal ions, in particular niobium and tantalum, which have a significantly stronger influence of spin-orbit coupling on the electronic and magnetic structure. The latter sandwich compounds are well known experimentally and, according to our predictions, reveal ferromagnetic order with large spin moments as well.
Relativistic effects in these molecules don’t only result in large anisotropy energies with consequent possibility for high temperature applications. The crucial influence of the enhanced spin-orbit coupling on the electronic structure of the heavy transition-metal-benzene sandwiches could also reveal itself in transport measurements via large values of anisotropic magnetoresistance, an effect currently being explored in magnetic break junctions.