  Ferromagnetic semiconductors (FMSs), which have the properties and
functionalities of both semiconductors and ferromagnets, provide fascinating
opportunities for basic research in condensed matter physics and device
applications. Over the past two decades, however, intensive studies on various
FMS materials, inspired by the influential mean-field Zener (MFZ) model have
failed to realise reliable FMSs that have a high Curie temperature (Tc > 300
K), good compatibility with semiconductor electronics, and characteristics
superior to those of their non-magnetic host semiconductors. Here, we
demonstrate a new n type Fe-doped narrow-gap III-V FMS, (In,Fe)Sb, in which
ferromagnetic order is induced by electron carriers, and its Tc is unexpectedly
high, reaching ~335 K at a modest Fe concentration of 16%. Furthermore, we show
that by utilizing the large anomalous Hall effect of (In,Fe)Sb at room
temperature, it is possible to obtain a Hall sensor with a very high
sensitivity that surpasses that of the best commercially available InSb Hall
sensor devices. Our results reveal a new design rule of FMSs that is not
expected from the conventional MFZ model. (This work was presented at the JSAP
Spring meeting, presentation No. E15a-501-2:
https://confit.atlas.jp/guide/event/jsap2017s/subject/15a-501-2/advanced)
