Proximity Effects in Superconductor-ferromagnet Heterostructures

The very special characteristic of the proximity effect in superconductor-ferromagnet systems is the damped oscillatory behavior of the Cooper pair wave function in a ferromagnet. In some sense, this is analogous to the inhomogeneous superconductivity, predicted long time ago by Larkin and Ovchinnikov (1964), and Fulde and Ferrell (1964), and constantly searched since that. After the qualitative analysis of the peculiarities of the proximity effect in the presence of the exchange field, the author provides a unified description of the properties of the superconductor-ferromagnet het-erostructures. Special attention is paid to the striking non-monotonous dependance of the critical temperature of the multilayers and bilayers on the ferromagnetic layer thickness and conditions of the realization of the " π "-Josephson junctions. The recent progress in the preparation of the high quality hybrid systems permitted to observe on experiments many interesting effects, which are also discussed in the article. Finally, the author analyzes the phenomenon of the domain-wall superconductivity and the influence of superconductivity on the magnetic structure in superconductor-ferromagnet bilayers. CONTENTS I. Introduction 2 II. Paramagnetic limit and qualitative explanation of the non-uniform phase formation 3 A. The (H, T) phase diagram 3 B. Exchange field in the ferromagnet 4 C. Why does the Fulde-Ferrell-Larkin-Ovchinnikov state appear? 4 D. Generalized Ginzburg-Landau functional 5 III. Proximity effect in ferromagnets 6 A. Some generalities about superconducting proximity effect 6 B. Damped oscillatory dependence of the Cooper pair wave function in the ferromagnets 7 C. Density of states oscillations 9 D. Andreev reflection at the S/F interface 10 IV. Oscillatory superconducting transition temperature in S/F multilayers and bilayers 12 A. First experimental evidences of the anomalous proximity effect in S/F systems 12 B. Theoretical description of the S/F multilayers 12 C. " 0 "-and " π "-phases 14 D. Oscillating critical temperature 15 V. Superconductor-ferromagnet-superconductor " π "-junction 17 A. General characteristics of the " π "-junction 17 B. Theory of " π "-junction 17 C. Experiments with " π "-junctions 21 VI. Complex S/F structures 22 A. F/S/F spin-valve sandwiches 22 B. S-F-I-F'-S heterostructures and triplet proximity effect 24 VII. Atomic thickness S/F multilayers 26 A. Layered ferromagnetic superconductors 26 B. Exactly solvable model of the " π "-phase 27 VIII. Superconductivity near the domain wall 28 IX. Modification of ferromagnetic order by superconductivity 30 A. Effective exchange field in thin S/F bilayers 30 B. Domain structure 31 C. Negative domain wall energy 32 …