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The development of Fe73.5Si13.5B9Nb3Cu1 (FINEMET) by Yoshizawa et al. and Fe88Zr7B4Cu1 (NANOPERM) by Inoue et al. 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.

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Magnetic nanoparticles (MNPs) offer promise for local hyperthermia or thermoablative cancer therapy. Magnetic hyperthermia uses MNPs to heat cancerous regions in an rf field. Metallic MNPs have larger magnetic moments than iron oxides, allowing similar heating at lower concentrations. By tuning the magnetic anisotropy in alloys, the heating rate at a particular particle size can be optimized. Fe–Co core-shell MNPs have protective CoFe2O4 shell which prevents oxidation. The oxide coating also aids in functionalization and improves biocompatibility of the MNPs. We predict the specific loss power (SLP) for FeCo (SLP ∼450W∕g) at biocompatible fields to be significantly larger in comparision to oxide materials. The anisotropy of Fe-Co MNPs may be tuned by composition and/or shape variation to achieve the maximum SLP at a desired particle size.

Fe–Co alloys in the α phase are soft magnetic materials which have high saturation inductions over a wide range of compositions. However, above about 1250 K, an α to γ phase transition occurs. The fcc-based, γ, high-temperature phase is paramagnetic at this temperature. In this work the low-temperature ordered B2, or α′, phase, as well as the disordered bcc phase of FeCo alloys, have been studied with first-principles electronic-structure calculations using the layer Korringa–Kohn–Rostoker method. The variation of moment with composition (Slater–Pauling curve) is discussed. For equiatomic FeCo, interatomic exchange couplings are derived from first principles. These exchange interactions are compared to those obtained for pure Fe and Co, and are used within a mean-field theory to estimate the hypothetical Curie temperature of the α phase.