ZrZn2: geometrical enhancement of the local DOS and quantum design of magnetic instabilities

The recent discovery of coexisting ferromagnetism and superconductivity in ZrZn2, and the fact that they are simultaneously suppressed on applying pressure suggest the possibility of a pairing mechanism which is mediated by exchange interactions and connected with the proximity to a magnetic quantum critical point. On the basis of first principles, full potential calculations we study the conditions that, for ZrZn2, determine the proximity to this magnetic instability. More specifically, we discuss the role played by the geometrical arrangement of the lattice, the hybridization and the presence of disorder, in addition to the widely used method of applying external pressures. These circumstances influence the width of the relevant Zr d bands whose narrowing, due to the reduction of the effective number of neighbours or to an increase of the cell volume, causes an enhancement of the density of states at the Fermi level. Finally, we highlight some general features that may aid the design of other materials close to a magnetic instability.