As vacancies as magnetic defects in iron pnictide superconductors

Unusual magnetic response of As-deficient iron pnictide LaOFeAs is  
discussed. It is shown that As vacancies being nominally non-magnetic  
defects induce strong enhancement of paramagnetic response of  
nonstoichiometric 1111 iron pnictides. This strong magnetic response  
however is not detrimental for superconductivity. The microscopic  
theory explaining unconventional properties
of defects in Fe-As planes is based on a generalized Anderson - Wolff  
impurity model adapted for multivalley semimetals.