Neutron Diffraction Studies of Magnetic Materials
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NEUTRON DIFFRACTION STUDIES OF MAGNETIC MATERIALS WILLIAM J. JAMES Department of Chemistry and Graduate Center for Materials Research, sity of Missouri-Rolla, Rolla, Missouri 65 401
Univer-
ABSTRACT The ability of neutron diffraction in determining the nature and extent of magnetic ordering is illustrated for the intermetallic compounds, Substitution with other 3d transition metals influY,(Fe,Mn) 2, and ErFe,. ences the Fe-Fe exchange forces such as to alter, sometimes considerably, local site magnetic anisotropies in the magnetic properties, e.g., Er(Fe,Ni) 3 and thermal expansion anomalies in the R2 (FeCo),4B compounds. When the 3d atoms are near neighbors in the periodic chart, their nuclear scattering lengths for neutrons are sufficiently different to permit the detection of preferential occupation of the several nonequivalent crystallographic 3d metal sites, i.e., atomic ordering, present in the RM2, and R2 Fe1 4B structures. INTRODUCTION Determination of magnetic structures is not an easy task. Many aspects of the magnetic structure can only be inferred or guessed from conventional A reliable picture of the complete structure is magnetization studies. often elucidated only by combining these studies with data from neutron Although neutron diffraction provides a direct diffraction experiments. probe of magnetic structure and changes therein, it may at times be Inadequate as ambiguities can arise in interpretation of the data, particularly for polycrystalline samples. Usually magnetization data, Mossbauer data, or sometimes specific heat data can remove these ambiguities. It is best if single crystals can be obtained for neutron diffraction experiments as these can allow one to determine absolute directions of However, for many materials, it Is impossible to obtain magnetic moments. Typically, crystal sufficiently large single crystals for neutron work. dimensions need to be around 0.3 to 0.5 cm. Thus, for these materials, one must be content to obtain data on a polycrystalline sample where the moment directions can be expressed only in terms of degrees from a unique axis. There now exists a refinement method which is quite powerful in extracting the maximum amount of information from neutron diffraction patterns. This method was developed by H. M. Rietveld Ell in the late 60's and is designed to allow analysis of entire powder diffraction patterns since it The older, integrated, accounts for reflection overlap and background. intensity method does not do this. The Rietveld approach has made powder diffraction studies nearly as valuable as single crystal work in structure determination. The nuclear component of scattered neutrons can provide useful information. Elements having nearly equal atomic numbers are practically indistinguishable using x-ray diffraction techniques, but can have quite different Because of this, neutrons are superior to scattering lengths for neutrons. x-rays when studying intermetallic alloys containing metals with nearly equal atomic numbers, e.g., Fe and Co. Also, light atoms are nea
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