The development of Fe73.5Si13.5B9Nb3Cu1 (FINEMET) by Yoshizawa etal and Fe88Zr7B4Cu1 (NANOPERM) by Inoue etal have shown that nanocrystalline microstructures can play an important role in the production of materials with outstanding soft magnetic properties. The FINEMET and NANOPERM materials rely on nanocrystalline α-Fe3Si and α-Fe, respectively, for their soft magnetic properties. The magnetic properties of a new class of nanocrystalline magnets are described herein. These alloys with a composition of (Fe,Co)–M–B–Cu (where M=Zr and Hf) are based on the α- and α′-FeCo phases, have been named HITPERM magnets, and offer large magnetic inductions to elevated temperatures. This report focuses on thermomagnetic properties, alternating current (ac) magnetic response, and unambiguous evidence of α′-FeCo as the nanocrystalline ferromagnetic phase, as supported by synchrotron x-ray diffraction. Synchrotron data have distinguished between the HITPERM alloy, with nanocrystallites having a B2 structure from the FINEMET alloys, with the D03 structure, and NANOPERM alloys, with the A2 structure. Thermomagnetic data shows high magnetization to persist to the α→γ phase transformation at 980 °C. The room temperature ac permeability has been found to maintain a high value of 1800 up to a frequency of ∼2 kHz. The room temperature core loss has also been shown to be competitive with that of commercial high temperature alloys with a value of 1 W/g - at BS=10 kG and f=1 kHz. © 1998 American Institute of Physics.