Basal plane anisotropy contribution

Following the /-dependence of the appropriate Stevens factor, Dy exhibits a crystal field contribution to the basal plane anisotropy considerably larger than Tb and, as already noted (section 2.2.2), the a-axis is the easy direction so that If is negative. Although Feron et al. estimated kI from 1.7 to 105 K, below 50 K the available magnetic field was insufficient to pull the magnetization completely into the hard direction. Thus the extrapolations used to calculate K6(0) = -1.1 0.1 X 10 Jm should be treated with caution. 

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