Spin screening of magnetic moments in superconductors

– We consider ferromagnetic particles embedded into a superconductor and study the screening of their magnetic moments by the spins of the Cooper pairs in the superconductor. It is shown that a magnetic moment opposite to the one of the ferromagnetic particle is induced in the superconductor. In the case of a small itinerant ferromagnet grain and low temperatures the full screening of the magnetic moment takes place, i.e the absolute value of the total magnetic moment induced in the superconductor is equal to the one of the ferromagnetic particle. In type II superconductors the proposed screening by spins of the conduction electrons can be much stronger than the conventional screening by Meissner currents. The phenomenon of screening is very common in physics. The best known example is the screening of an electric charge in metals due to a redistribution of free electrons in space. This charge screening is very strong and the length characterizing an exponential decay of the electric field (the Thomas-Fermi length λ T F) is in most metals of the order of the Fermi wave length, i.e. of the order of the interatomic spacing. The length λ T F does not depend on whether the metal is in the normal or in the superconducting state. Another famous example is the screening of a magnetic field or a magnetic moment by superconducting currents in a superconductor (Meissner effect) [1]. Due to this effect the magnetic field decays over the length λ L (the penetration depth) and vanishes in a bulk superconductor. The same length characterizes the decay of the magnetic field created by a ferromagnetic (F) grain embedded in a superconductor. In contrast to the screening of the electric charge in normal metals the screening of the magnetic field and magnetic moments in superconductors is weaker and the penetration depth λ L can be of the order of hundreds interatomic distances or larger. The screening of the magnetic moment is a phenomenon specific for a superconductor and, in contrast to the charge screening, is very small in a normal metal. Although the stray field created by a ferromagnetic grain embedded in a nonmagnetic (N) metal may induce a negative local magnetization in some regions of the normal metal due to the Pauli paramagnetism, the susceptibility is rather small (µ 2 B ν ∼ 10 −6 , µ B is the Bohr magneton and ν the density of …