ELECTRONIC-STRUCTURE, FERROMAGNETISM, AND EELS SPECTRA OF CRYSTALLINEALLOYS FENB AND NINB (N=1,2,3) - ASPECTS OF UNIVERSAL BEHAVIOR

We have performed an extensive study of the electronic structure, opti cal properties, and electron-energy-loss spectra (EELS) for the series of the M3B, M2B and MB, (M=Fe, Ni) crystalline alloys. The electron d ensity of states (DOS) of iron and nickel borides of the same composit ion have almost the same shape in spite of some minor differences in a ctual 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 prov ides insight into the ''marginal'' stability of the nickel magnetic mo ment upon dilution by nonmagnetic atoms. As a result of competitive in teraction between d-d metallic bonding and d-p M-metalloid covalent bo nding the magnetic moment on the Fe atom gradually decreases with incr easing 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 estima ted magnetic moments. In spite of some differences in actual crystal s tructure and a high degree of crystalline anisotropy, the calculated E ELS spectra are practically identical for all iron compounds studied. The spectra are dominated by a giant peak at about 20 eV, with some fi ne structure at lower energies (at about 10 eV) relevant to B p-M d tr ansitions. The d-d transitions appear to be very strong in the low-ene rgy region (0-10 eV) leaving the usual Drude term effective only at en ergies below 1.5 eV. These transitions suppress the low-energy plasmon s both in para and ferromagnetic phases and make just small difference s in the calculated EELS spectra, in accordance with the available exp erimental data. The implications of the present results for the amorph ous systems are discussed.