Magnetic heterostructures: examples and perspectives

Artificial magnetic heterostructures and superlattices have received much a ttention in recent years because of their scientific and technological rele vance. Heterostructures consist of different material layers promising join tly to display physical properties different from any of their single layer s. Paramagnetic layers sandwiched between ferromagnetic films are an excell ent example of a magnetic heterostructure, since together they display an o scillatory exchange coupling as a function of the paramagnetic spacer thick ness not present in any single layer. The strength of the exchange coupling and the oscillation period depend on the details of the Fermi surfaces inv olved, whereas the overall features of the exchange coupling appear univers al. More complex couplings are observed for magnetic superlattices with Cr spacer layers. This is due to the intrinsic spin-density wave state of Cr. Extensive experiments with synchrotron and neutron radiation have recently unravelled the Nt el state of thin Cr layers and proximity effects between Fe and Cr, elucidating the mutual interdependence of the Cr spin structure and the Fe exchange coupling. In Co/Cr superlattices the structural mismatc h between hcp Co and bcc Cr adds another complexity, which affects strongly the magnetic anisotropy. Utilizing the different coercivities of Co and Fe layers, spin valve systems can be constructed from Co/Cr/Fe heterostructur es. Both the current status and some future perspectives are briefly review ed here.