Origin of Superparamagnetism in Core-Mantle Grains; MNRAS

It has been frequently suggested that interstellar dust grains contain clusters of metallic iron, iron sulphides or magnetic iron oxides having superparamagnetic properties. This paper presents a simple model for the origin of superparamagnetism in H₂O ice mantle grains. According to this model, impurity Fe atoms in H₂O ice mantles are rapidly oxidized to magnetite (Fe₃O₄) precipitates during mantle heating by cosmic ray impacts. These precipitates undergo ferromagnetic coupling and aggregate into single-domain Fe₃O₄ particles uniformly dispersed throughout the mantle like raisins in a pudding. The single-domain particles behave like an assembly of superparamagnets, greatly enhancing Davis-Greenstein alignment of core-mantle grains in dense clouds. Superparamagnetism alone, however, cannot explain the polarized far-infrared emission observed from cold dust in Orion clouds.