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EPITAXIAL GROWTH OF RARE EARTHS ON RARE EARTH FLUORIDES AND RARE EARTH FLUORIDES ON RARE EARTHS : TWO NEW EPITAXIAL SYSTEMS ACCESSED BY MBE. R.F.C. Farrow%, S.S.P. Parkin%, M. Lang* , K.P. Roche*

" IBM

Almaden Research Center, 650 Harry Road, San Jose , Ca 95120-6099.

ABSTRACT We report two new epitaxial systems, prepared by MBE: basal plane epitaxy of the rare earth metal Dy onto LaF 3 films and vice versa. SQUID magnetometry studies indicate that buried epitaxial Dy films , of .,300A thick , order ferromagnetically at similar temperatures to bulk Dy crystals.These epitaxial systems are one member of a new family of epitaxial systems of basal plane epitaxy of rare earth metals on rare earth fluorides and vice versa. Such systems may be used to probe the effects of strain and dimensionality on magnetic ordering in rare earth metal films and multilayers. INTRODUCTION In this paper we report two new epitaxial systems, prepared by molecular beam epitaxy : basal plane epitaxy of the rare earth metal Dy onto LaF 3 films and vice versa. The significance of these systems is that they are representative of a new family of epitaxial metal-insulator systems which may be used to probe magnetic ordering in rare earth metal films and multilayers under controlled strain. The heavy rare earths ( Gd through Lu ) and several of their compounds with the transition metals have hexagonal close-packed crystal structures. On the other hand, the light rare earth trifluorides have hexagonal (tysonite) structures[1,2,] with basal plane lattice constants (a) close to a factor of two larger than the heavy rare earths.This suggests the possibility of basal plane epitaxy between these two classes of materials. Apart from geometry, another factor favoring epitaxy is the exceptionally large free energies of forma-

tion of the light rare earth trifluorides. There is , in fact , a negligible thermodynamic driving force for interfacial reactions between the heavy rare earths and the light rare earth trifluorides. Consider Figure 1. This illustrates the dependence of room temperature basal plane lattice constant on the atomic number of the rare earths. What is plotted is 2a since it is this quantity which has near coincidences with the basal plane lattice constant of appropriate rare earth trifluorides.The latter values are indicated by horizontal lines. For example in the case of Dy/LaF 3 the misfit defined as 2 ( aDy - aLaF 3 )/ aLaF3 = -7.4x10 -4

Other comparably matched combinations are Er/CeF 3 and Tm/PrF 3 for which the misfits are -1.9x10- 3 and -4.2x10-4 respectively. Furthermore, one can envisage , as in the case of group II fluoride-semiconductor systems[3], exactly matching the lattice constant of the metal to that of a solid solution of the fluorides. Inspection of Figure 1 shows that this should be possible for Ho on a solid solution of LaF 3-CeF 3 .

Mat. Res. Soc. Symp. Proc. Vol. 103. @1988 Materials Research Society

206

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