Magnetic anisotropy observed at surface of amorphous silicate and its implications for the mechanism of dust alignment

Amorphous silicate is considered to be one of the major dust-forming material, however its possibility of magnetic alignment by magnetic anisotropy has not been considered as yet. This is because the anisotropy of an amorphous material is generally considered to be negligibly small. In the present study, depth profile of paramagnetic anisotropy Δχpara was experimentally obtained for the first time on an amorphous silicate sample, namely moldavite, with a spatial resolution of 0.5 mm. For the above purpose, Δχpara of square plate (2 mm×2 mm×0.5 mm), separated from a quadrangular prism (2 mm×2 mm×10 mm), were measured one by one; the prism was cut from the amorphous sample with its long axis directed normal to the surface. In order to realize the above measurement, a new experimental device was adopted which could detect Δχpara of a sub-mm sized sample by measuring its field-induced rotational oscillation in an μG area. The observed magnetic unstable axis of the plate was always normal to surface plane. The magnitude of Δχpara at the surface was at a level of 10−6 emu/g, which was comparable to the value previously obtained for of olivine and pyroxene, which are major dust components in the proto-planetary disk region. Accordingly, most of the dust forming materials in the above region is expected to possess finite amount of Δχ to cause partial dust alignment.