Low TC Fe-Cr-Nb-B glassy submicron powders for hyperthermia applications

Fe79.7−xNb0.3CrxB20 (x = 11.5–13.5 at. %) submicron powders with glassy structure and sizes between a few tens of nm and 1 μm have been prepared by high-energy ball milling from glassy melt-spun ribbons (MSRs) precursors. The glassy structure existing in the MSRs, and consisting of Fe-Cr clusters embedded within the (Fe,Nb)3B residual amorphous matrix, is preserved by milling the annealed MSRs in oleic acid and n-heptane. The glassy structure of the submicron powders confers the specific magnetic properties and allows tuning the Curie temperatures in the range of 15–50 °C. The precipitation of α-Fe in excess shifts the Curie temperature to 780 °C. The use of surfactants (oleic acid or heptane) avoids partially the agglomeration of powders, and their individual size goes down to 25–40 nm. The heating efficiency of the submicron powders in a.c. fields (H = 350 mT and f = 153 kHz) evidences a working regime in the range of 33–45 °C, depending on Cr content, indicating the suitability of using the Fe-Cr-Nb-B powders for hyperthermia applications.