On the Effects of Magnetic Bonding in Rare Earth Transition Metal Intermetallics
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ON THE EFFECTS OF MAGNETIC BONDING IN RARE EARTH TRANSITION METAL INTERXMETALLICS R. Kumar and J. Bentley, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6376 and W. B. Yelon, University of Missouri Research Reactor, Columbia, MO 65211.
A.BSTRACT Neutron diffraction experiments on rare-earth transition metal magnetic alloys Er 2 Fe 14 B and Er 2 Fe 17 have been carried out at temperatures above and below the ordering temperature (T.). An anomalously large magnetic moment is observed at the crystallographic j 2 site in Er 2 Fe1 5 B which is the intersection point of the major ligand lines in the crystal structure. The interatomic Fe-Fe distances are in the range of strong ferromagnetic bonds'(Ž2.66 A). The analogous f site in Er 2 Fe,7 does not develop as large a magnetic moment. In addition, the same sites show strong preference for Fe atoms in the respective substituted compounds. Due to poor phase stability of Er 2 (Co.Fe-,.) 1 ,B compounds, iron substitution has been studied in detail in Er 2 (CoFe 1 _-) 17 alloys for site specific order and lattice distortion effects. However, a nonlinear change in the c lattice parameter observed in the neutron diffraction results cannot be explained on the basis of site preference alone. The neutron refinement results indicate iron rich compositions in Er 2 (Co2 Fe 1 -. )17 materials, which is related to random substitution of Fe dumbbell pairs on the rare earth sites in the lattice. However, extensive electron microscopy (selected area electron diffraction and high resolution imaging) of Er 2 Fe 17 and Er 2 (Co • 0 Fe.6 0 ) 17 failed to reveal any microscopic inhomogeneity. At this stage, the concept of direct negative exchange interaction between dumbbell Fe atom pairs at very short distances of -2.38 A is invoked and its effect is correlated with phase stability. We further suggest that the addition of boron in Er 2 Fe 1 4 B suppresses the substitution of dumbbell Fe pairs as a result of which T. is raised signifi..ý..tly. I.
INTRODUCTION
Rare earths are prolific compound formers with most of the elements in the periodic table. Interest in rare earth-transition metal intermetallics stems from the fact that they constitute excellent starting materials for permanent magnetics technology. Outstanding magnetic properties result from the combined magnetic features of the rare earth (4f magnetism) and the transition metal (3d magnetism). Simultaneously, one can benefit from the intrinsic properties of both partners, rare earths providing a very high saturation moment per atom and strong single-ion magnetic crystalline anisotropy, and a high T. arising from the high magnetic coupling strengths of the moments of the 3d transition metals. This unique combination is exploited in fabricating high coercivity, high T¢ materials for high temperature applications. Samarium-cobalt was invariably the material of choice until a new hard magnetic phase in the Neodymium-Iron-Boron 1 2 system was discovered. , The structure consists of layers
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