The role of intra-atomic non-collinear magnetization density in weak   ferromagnetism
  We investigate the mechanism behind the breakdown of the compensation of large magnetic moments leading to weak ferromagnetism. For this we use first-principles calculations within density functional theory and we focus on the weak ferromagnetic compound Mn3Sn. Our new implementation allows for an exact treatment of the spin-density matrix and non-collinearity. In order to gain some insight, our results are compared to the ones obtained by using the atomic moment approximation (AMA) and its role is discussed. We find that the appearance of the weak ferromagnetic moment in this compound originates not so much as an effect of spin-orbit coupling as suggested previously from AMA calculations, as from the non-collinearity of the Sn atom magnetization density. This is confirmed by non-collinear calculations in which the SOC effects are neglected. 
Robert Laskowski, Gilles Santi