Ferromagnetic inclusions in silicate thin films: insights into the magnetic properties of cosmic grains

Abstract

Context. We recently reported the formation of metallic inclusions in an amorphous and/or crystalline silicate matrix by thermal annealing of thin films in reducing atmospheres. Experimentally, the obtained microstructures closely resemble those of the glass with embedded metal and sulphides (GEMS) found in chondritic porous interplanetary dust particles (CP IDPs). We present here the magnetic properties of these synthetic samples. Aims. In this paper we report the detection and measurements of single domain and super paramagnetic ferromagnetic inclusions (SD/SP) in annealed silicate thin films of composition analogous to interstellar silicates and discuss the implications for the alignment of cosmic grains in astrophysical environments, in the presence of weak magnetic fields. Methods. We investigate the magnetic properties of synthesized laboratory silicate samples by measuring their magnetization when subjected to a given magnetic field. The measurements were performed at different temperatures including those compatible with interstellar dust. Results. The high values of remanent magnetization at saturation obtained in this work suggest the ability of our samples to indefinitely maintain a significant magnetization which may contribute to their alignment in weak magnetic fields. Conclusions. From our laboratory experimental simulation we propose that interstellar grains contain iron in form of nm-sized metallic beads. This can explain the non-detection of iron in interstellar grains. These inclusions could play a role in the alignment of grains. We propose a possible scenario for the magnetization of the cosmic grains and give a minimum value for the magnetic susceptibilty for GEMS.