Overcoming Super-Paramagnetism at Elevated Temperatures in HAMR

The idea behind heat assisted magnetic recording (HAMR) is to be able to reverse high anisotropy grains by heating them and consequently reducing their anisotropy. The high anisotropy grains are necessitated because of increased demands on density and consequent reduction in grain sizes. Elevated temperature does reduce the anisotropy of the grain but also increases the super-paramagnetic fluctuations. This puts a limit on how high the temperature can be raised and as a result on how low the anisotropy can be reduced. Here I show that by increasing the length of the grain and by allowing the grain to have multiple domains at elevated temperatures the un-reversing effect of super-paramagnetic fluctuations can be minimized. The domains do undergo toggling between their moments aligning and opposing the field. But due to the presence of multiple domains at-least one domain in the direction of the field exists. As the temperature reduces the field acts on the domain-wall which propagates and the grain undergoes reversal by wall propagation.