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The substitutional effects of cobalt in (FeCo)PB have been studied with respect to crystalline structure and chemical order with X-ray diffraction and Mössbauer spectroscopy. The magnetic properties have been determined from magnetic measurements, and density functional theory calculations have been performed for the magnetic properties of both the end compounds, as well as the chemically disordered intermediate compounds. The crystal structure of (FeCo)PB is tetragonal (space group I4/mcm) with two different metal sites, with a preference for cobalt atoms in the M(2) position (4c) at higher cobalt contents. The substitution also affects the magnetic properties with a decrease of the Curie temperature (T) with increasing cobalt content, from 622 to 152 K for FePB and (FeCo)PB, respectively. Thus, the Curie temperature is dependent on composition, and it is possible to tune T to a temperature near room temperature, which is one prerequisite for magnetic cooling materials.

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The Fe 5 PB 2 compound offers tunable magnetic properties via the possibility of various combinations of substitutions on the Fe and P-sites. Here, we present a combined computational and experimental study of the magnetic properties of (Fe 1−x Cox) 5 PB 2 . Computationally, we are able to explore the full concentration range, while the real samples were only obtained for 0 ≤ x ≤ 0.7. The calculated magnetic moments, Curie temperatures, and magnetocrystalline anisotropy energies (MAEs) are found to decrease with increasing Co concentration. Co substitution allows for tuning the Curie temperature in a wide range of values, from about six hundred to zero kelvins. As the MAE depends on the electronic structure in the vicinity of Fermi energy, the geometry of the Fermi surface of Fe 5 PB 2 and the k-resolved contributions to the MAE are discussed. Low temperature measurements of an effective anisotropy constant for a series of (Fe 1−x Cox) 5 PB 2 samples determined the highest value of 0.94 MJ m −3 for the terminal Fe 5 PB 2 composition, which then decreases with increasing Co concentration, thus confirming the computational result that Co alloying of Fe 5 PB 2 is not a good strategy to increase the MAE of the system. However, the relativistic version of the fixed spin moment method reveals that a reduction in the magnetic moment of Fe 5 PB 2 , by about 25%, produces a fourfold increase of the MAE. Furthermore, calculations for (Fe 0.95 X 0.05 ) 5 PB 2 (X = 5d element) indicate that 5% doping of Fe 5 PB 2 with W or Re should double the MAE. These are results of high interest for, e.g., permanent magnet applications, where a large MAE is crucial.