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\textbf{Cryptoferromagnetic State in Superconductor-Ferromagnet Multilayers}

\bigskip {Konstantin Efetov}

\bigskip {\small \emph{Ruhr-Universitaet Bochum, D-44780 Bochum,Germany}} \\[%
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\texttt{efetov@tp3.ruhr-uni-bochum.de}
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In recent experiments on Fe-Nb multilayers influence of
superconducting Nb and ferromagnetic Fe on each other has been studied. It
is clear that the ferromagnetic layers should suppress the superconductivity
but in some samples an opposite effect of the superconductivity on the
ferromagnetism was observed: below the superconducting temperature one could
see a decrease of the average magnetic moment in the ferromagnetic layers.

A possibility of formation of the cryptoferromagnetic state in the magnetic
layers is discussed. Such a state is characterized by a rotating or
oscillating magnetic moment in the magnetic layers. Although the energy of the state with
the non-homogeneous magnetic moment is higher than that of the ferromagnet,
the superconducting energy is reduced if the period of the oscillations does
not exceed the superconducting coherence length. If the ferromagnetic layers
are sufficiently thin, the cryptoferromagnetic state can become more
favorable and this might explain the experiments. Basic equations describing
the ferromagnet-superconductor multilayers and a calculational scheme are
discussed.

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