W0035

Role of the Fe Sub-lattice on Magnetostriction in R2Fe14B Compounds, (R = Y, Nd, Gd, Tb). Matthew Kramer, Ning Yang, Peter Lee, Yuegang Zhang., Ames Laboratory, Iowa State University, 37 Wilhlem Hall, Ames, IA 50011.

The R₂TM₁₄M (R = rare earth, TM = transition metals and M = B,C,N) compounds, are all isostructural with space group P4₂/mnm and only form stoichiometric compounds of 2:14:1 of rare earths, transition metals and metaloids. This allows for various substitutions of magnetic and non-magnetic atoms to probe the magnetic coupling between the R and TM sites. For instance, the magnetic moments of rare earth and iron in these compounds are coupled ferromagnetically with light rare earths (Pr, Nd, Sm), antiferromagnetically with heavy rare earths (Gd, Tb, Dy, Ho, Er, Tm, Yb) or non-magnetic for La, Ce, Lu, and Y. Thermal expansion anomalies of R₂Fe₁₄B (R = Y, Nd, Gd, Tb) stoichiometric compounds have been studied by X-ray diffraction with high energy synchrotron radiation using a Debye-Scherrer geometry from room temperature to 1000 K. R₂Fe₁₄B compounds all have similar temperature dependence of lattice parameters and bond distances up to their Curie temperatures (T_c). The volumetric spontaneous magnetostriction defined by following equation

Eq.1

where _ is the linear coefficient of thermal expansion for the paramagnetic component of the lattice fitted to the high temperature data. The term a_o refers to the extrapolated values at 0 K and the subscripts a, c and v will refer to the a-axis, c-axis and volume components to _ and _ at room temperature for Nd₂Fe₁₄B and Y₂Fe₁₄B were determined to be 1.49% and 1.45% respectively, but was larger for Gd₂Fe₁₄B and Tb₂Fe₁₄B, 1.82%, due to relatively larger thermal expansion coefficients. Above the T_c, both samples have an anisotropic thermal expansion. The iron sub-lattice was shown to dominate the spontaneous volumetric magnetostriction. Among the Fe-Fe bonds, those bonds containing Fe(j2), whose projections are dominantly in the basal plane, have the highest contribution to the invar effect. These bonds have magnetostictive strain ranging from 0.4% to 0.8% at room temperature. The iron sub-lattices dominate the spontaneous volumetric magnetostriction of these compounds regardless if the R atoms are non-magnetic, ferromagnetically or anti-ferromagentically coupled to the Fe sub-lattice.