CHALLENGES IN THE FIELD OF LARGE-M SUPERMIRRORS

Polarising and non-polarising supermirrors with angles of reflection t heta(c) of m similar or equal to 2 times the critical angle of Ni are now produced in large quantities mostly by using sputtering techniques . The coatings can be deposited with such perfection that the surface morphology of the substrates becomes as important for achieving high r eflectivities as the perfection of the layer structure itself. As a co nsequence, m can be increased to similar or equal to 4 simply by incre asing the number of layers to similar or equal to 1000. However, this requires that the individual thicknesses of the layers decrease too, a pproaching the thickness of the interdiffusion zones thus diminishing the reflectivity at large reflection angles. In addition, the total th ickness of the coatings ends up to be of the order of typically 5 mu m . Therefore, the forces on the substrate increase due to the stress wi thin the layers leading finally to the destruction of the substrates. Clearly, such supermirrors an of no practical use. The stress within t he layers can be significantly reduced by properly tuning the partial pressures of the reactive gases during sputtering. Moreover, by using anisotropic sputtering it is possible to induce an anisotropic stress distribution within the plane of the layers. As a consequence, it is p ossible to fabricate magnetic multilayers that exhibit a well-defined easy axis of magnetisation due to magnetostriction. This effect is par ticularly pronounced for polarising FeCoV/TiNx supermirrors, such that remanent mirrors can be produced that allow the polarisation of neutr ons in zero magnetic field. (C) 1998 Elsevier Science B.V. All rights reserved.