Ferromagnetism of Dy films containing embedded Fe atoms and nanoparticles

We have studied the magnetic behaviour of polycrystalline dysprosium films grown in their pure state and containing Fe impurity. The films were grown by MBE in ultra-high vacuum and Fe impurity was introduced either as Fe atoms from a conventional MBE source or as 1.8 nm diameter Fe nanoparticles from a gas aggregation source. We show that with a suitable magnetothermal history it is possible to produce a large thermal hysteresis up to room temperature in all samples. In the Dy ferromagnetic state both types of Fe impurity cause a reduction in the observed magnetisation but the reduction of average moment observed in the case of the Fe nanoparticles is much greater than that for atoms for a similar quantity of Fe. Despite the low solubility of Fe in Dy EXAFS data clearly indicates alloying in the case of embedded Fe nanoparticles. We attribute the low moment in the Dy films containing nanoparticles to alloying within the particles producing a significant proportion of high-Fe-concentration Dy–Fe alloy.