Transport and localisation in the presence of strong structural and spin disorder

We study a tight binding model including both on site disorder and coupling of the electrons to randomly oriented magnetic moments. The transport properties are calculated via the Kubo-Greenwood scheme, using the exact eigenstates of the disordered system and large system size extrapolation of the low frequency optical conductivity. We first benchmark our method in the model with only structural disorder and then use it to map out the transport regimes and metal-insulator transitions in problems involving (i) scattering from random magnetic moments, and (ii) the combined effect of structural disorder and magnetic scattering. We completely map out the dependence of the d.c conductivity on electron density (n) the structural disorder (Delta) and the magnetic coupling (J'), and locate the insulator-metal phase boundary in the space of n -Delta - J'. These results serve as a reference for understanding transport in systems ranging from magnetic semiconductors to double exchange 'colossal magnetoresistance' systems. A brief version of this study appears in our earlier paper Europhys. Lett. 65, 75 ( 2004).