Structural origin of the anomalous temperature dependence of the local magnetic moments in the CaFe2As2 family of materials

We report a combination of Fe K β x-ray emission spectroscopy and density functional reduced Stoner theory calculations to investigate the correlation between structural and magnetic degrees of freedom in CaFe2 (As1−xPx)2. The puzzling temperature behavior of the local moment found in rare earth-doped CaFe2As2 [H. Gretarsson et al., Phys. Rev. Lett. 110, 047003 (2013)] is also observed in CaFe2(As1−xPx)2. We explain this phenomenon based on first-principles calculations with scaled magnetic interaction. One scaling parameter is sufficient to describe quantitatively the magnetic moments in both CaFe2 (As1−xPx) 2 (x=0.055) and Ca 0.78 La 0.22 Fe2 As2 at all temperatures. The anomalous growth of the local moments with increasing temperature can be understood from the observed large thermal expansion of the c-axis lattice parameter combined with strong magnetoelastic coupling. These effects originate from the strong tendency to form As-As dimers across the Ca layer in the CaFe2 As2 family of materials. Our results emphasize the dual local-itinerant character of magnetism in Fe pnictides.