Magnetic Anisotropy of Rare Earth Metals

Torque measurements have been made to determine K1 on single crystals of Tb, Dy, and Ho. This constant has been measured as a function of field and temperature in the antiferro- and paramagnetic regions. For Dy and Ho, κ1 decreases with decreasing T between TN and TC, changing sign in Ho and becoming large and negative again when ferromagnetism appears. The results are used to estimate critical fields which agree well with those derived from magnetization measurements. In the paramagnetic region K1=βH2 as predicted by Callen and Callen and β∝ (T−TC)−2 where TC is the paramagnetic Curie temperature. The basal plane constant, K4, has also been obtained for Gd, Tb, and Ho. There is no detectable anisotropy above TC. For Gd the value of K4 varies with T as (σ/σ0)21 and for Tb a good fit is obtained with a function of the form Î13/12[£−1(m)] as suggested by Callen and Callen. The magnitude of K4 extrapolated to 0°K for Gd is −800 erg·g−1; the value for Tb is about 500 times larger. A twelvefold symmetry is obtained with Ho and a further constant, K5, is required to describe the results.