Electronic structure, ferromagnetism, and EELS spectra of crystalline alloys ${\mathrm{Fe}}_{\mathit{n}}$B and ${\mathrm{Ni}}_{\mathit{n}}$B (n=1,2,3): Aspects of universal behavior

We have performed an extensive study of the electronic structure, optical properties, and electron-energy-loss spectra (EELS) for the series of the ${\mathit{M}}_3$B, ${\mathit{M}}_2$B, and MB (M=Fe, Ni) crystalline alloys. The electron density of states (DOS) of iron and nickel borides of the same composition have almost the same shape in spite of some minor differences in actual atomic structures. The Fermi level in nickel borides is shifted upwards in comparison with its position in iron borides, away from the main DOS peak formed by the nonbonding M d states. This behavior provides insight into the ‘‘marginal’’ stability of the nickel magnetic moment upon dilution by nonmagnetic atoms. As a result of competitive interaction between d-d metallic bonding and d-p M-metalloid covalent bonding the magnetic moment on the Fe atom gradually decreases with increasing boron content. This tendency is in accordance with the results of a simple generalized Stoner theory, which is capable of describing the ferromagnetic behavior in detail with good accuracy for the estimated magnetic moments. In spite of some differences in actual crystal structure and a high degree of crystalline anisotropy, the calculated EELS spectra are practically identical for all iron compounds studied. The spectra are dominated by a giant peak at about 20 eV, with some fine structure at lower energies (at about 10 eV) relevant to B p–M d transitions. The d-d transitions appear to be very strong in the low-energy region (0–10 eV) leaving the usual Drude term effective only at energies below 1.5 eV. These transitions suppress the low-energy plasmons both in para- and ferromagnetic phases and make just small differences in the calculated EELS spectra, in accordance with the available experimental data. The implications of the present results for the amorphous systems are discussed.