Singular field response and singular screening of vacancies in   antiferromagnets
  For isolated vacancies in ordered local-moment antiferromagnets we show that the magnetic-field linear-response limit is generically singular: The magnetic moment associated with a vacancy in zero field is different from that in a finite field h in the limit h->0. The origin is a universal and singular screening cloud, which moreover leads to perfect screening as h->0 for magnets which display spin-flop bulk states in the weak-field limit. 
Alexander Wollny, Eric C. Andrade, Matthias Vojta
cond-mat.str-el cond-mat.stat-mech