Magnetism In Thin Films And Multilayers

G.P. Felcher, S.Te Velthuis; in collaboration with S. D. Bader, A. Berger and J. S. Jiang, Magnetic Thin Films Group

We are using a polarized neutron reflectometer (POSY 1) in order to study the magnetism of thin films and multilayers. The research is conducted in close collaboration with the thin film group in MSD and with leading universities. The main object is to map out the configuration of the magnetic moments in thin films and multilayers and to link them with expectations due to micromagnetic calculations and transport phenomena such as magnetization, magnetic anisotropy and magnetoresistance.

Superconducting films.Films composed of multilayers of Nb and Si, Cu, Al are superconducting with a transition temperature dependent on the Nb and the spacer layer thicknesses. When a magnetic field is applied parallel to the layer planes the magnetization curve exhibits, above Hc1, one or more cusps which have been interpreted as due to transitions between different configurations of arrays of vortices parallel to the surface. Polarized neutron scattering at  grazing incidence is the only technique that can in principle observe arrays of vortices parallel to the surface.  Results on Nb/Si multilayers indicate the presence of Josephson-type vortices, with a periodicity due to the chemical layering. Results on  Nb/al multilayers indicate instead the presence of layers of vortices whose number is defined by the overall thickness of the film and the applied magnetic field. The experiments are in collaboration with the Northwestern University and the University of Missouri.

Imprinting magnetic structures. Magnetic spiral structures were created by depositing in sequence layers of lanthanum and iron (each 30 � thick) on a substrate rotating in a fixed field of a few Oersteds. The magnetic field was sufficient to define the magnetization of each layer during growth, without disturbing the magnetization of the layers already formed.  As a result ,the imprinted helical magnetic structure had a chirality given by the direction of rotation of the substrate: the pitch was given by the speed of rotation and the rate of deposition. This finding, in which neutron reflectivity has been used as the primary diagnostic tool, shows that some of the magnetic structures found in superlattices can be, in effect ,artificial. The finding also indicates that it is possible to engineer mesoscopic magnetic structures. The first application proposed has been to use such structures as polarizers of low energy neutrons. The research is done in collaboration with the University of G�ttingen, Germany.

Exchange anisotropy.Exchange bias is a long-known phenomenon but not yet understood. Its most conspicuous manifestation is a hysteresis B-H loop that asymmetrical with respect to the H axis. The effect takes place in the following conditions: a ferromagnet (FM) is in immediate contact with an antiferromagnet (AF);  TC>TN; and the pair is cooled through TN in a magnetic field HC. The bias in the measuring field (HM) is in the direction of HC. Several models have been proposed, yet none of them is capable of describing unambiguously the effect. In spite of an extensive research effort, conducted over an arch of several years with the universities of California at San Diego and the University of Minnesota, it is extremely difficult to verify satisfactorily the detailed magnetic structure at the interface of naturally occurring AF/FM couples. Recently we proposed to substitute the "natural" exchange bias  system with  an artificial system that should have the same phenomenology. The system consists of a superlattice with AF interlayer interaction in contact with a superlattice with FM interlayer interaction. The new system is much easier to study because is truly one-dimensional and made of layers 20 � (rather than 2 �) thickness. This research, conducted jointly with the thin film group of MSD, has been fully successful. The samples produced behaved exactly as predicted, opening the possibility of using them as a template for studying exchange anisotropy in a controllable way.