Magnetism in ZnO nanowire with Fe/Co codoping: First-principles density functional calculations

Using first-principles density functional calculations, at both generalized gradient approximation (GGA) and GGA+U levels we have investigated the electronic structure and magnetic properties of Fe/Co codoped ZnO nanowire. Here we have addressed some of the key issues such as, the preferable sites that Fe/Co can occupy, the coupling mechanism, and role of defects in coupling. We found that the spin alignment between the transition-metal atoms depends on their location. When Fe and Co atoms are nearest neighbors on the outer surface of the nanowire along [0001] direction is the lowest energy configuration with ferrimagnetic (FiM) ground state. At GGA level of description ferromagnetic ordering is observed when impurity atoms sit at surface and subsurface interface forming Fe-O-Co magnetic path, however at GGA+Ulevel of description antiferromagnetic superexchange interaction dominates and all configuration leads to FiM ground state. GGA+U are found to give more realistic description of electronics structure of Fe/Co codoped ZnO nanowire. Interestingly Fe-VO-Co defect configurations formed by removing the O atom from Fe-O-Co magnetic path are ferromagnetic when Fe-Co separation is less than 2.596 at GGA and 2.801 Å at GGA+U irrespective of the location of transition ions. We have also found that Co atom has a tendency to form clusters around Fe atom leading to inhomogeneous doping concentrations. O vacancy is found to be crucial in case of promoting ferromagnetism in this system. Two competing factors are the Ruderman-Kittel-Kasuya-Yosida (RKKY) type of exchange interaction in bulk environment due to O vacancy and direct exchange interaction of carriers due to Fe-VO-Codefect configuration on the surface