V. L. Moruzzi

Aspects of transition‐metal magnetism

The following questions are considered: How do itinerant d electrons of the Heusler alloys produce magnetic moments that are completely localized on the Mn atoms? What is the microscopic origin of Invar anomalies? Why is elemental antiferromagnetism confined to Cr, Mn, and Fe? Why do the Fe atoms in Fe3Si carry two distinctly different magnetic moments? Can we use our understanding of transiton‐metal magnetism to ’’design’’ new magnetic compounds composed of nonmagnetic elements? Our answers to these questions are based on the local‐spin‐density theory of electronic exchange and correlation, implemented using parameter‐free self‐consistent energy‐band calculations.

Covalent magnetism: An alternative to the Stoner model

We show that itinerant magnetism in a variety of transition‐metal systems cannot be described by the familiar Stoner model (rigid‐band theory). Magnetism as found in elemental anti‐ferromagnets, in certain impurity systems (transition metals in transiton metals), and in certain transition‐metal compounds is shown to arise from spin‐dependent changes in the covalent interactions between the d states of neighboring transition‐metal atoms.

Magnetism of metastable phases: Band theory and epitaxy (invited)

Total‐energy band calculations are used to analyze the magnetic phases of metallic elements as functions of volume. The calculations utilize a fixed‐spin‐moment procedure, which is described and justified as a natural generalization of density‐functional theory. This procedure finds the ground‐state energies of electronic systems under two constraints, and hence determines the system energy as a function of two variables—volume and magnetic moment. The energy function is used to find the ferromagnetic phases and their ground‐state properties, including bulk moduli and magnetic susceptibilities. The systems studied are fcc Fe, fcc Co, bcc Ni, fcc Pd, and bcc Mn, each of which undergoes a phase transition for small changes of the lattice constant from equilibrium (zero‐pressure) values.

Theory of Invar and Heusler alloys

Abstract The physical picture of Invar and Heusler alloys provided by parameter-free, self-consistent, spin-polarized energy-band calculations is described. In our view, Invar differs from other itinerant magnets in possessing a nonmagnetic state that can be reached by thermal excitations. Calculations for ordered Fe-Ni compounds containing 0, 25, 50, 75 and 100% Ni agree with zero-temperature measurements, and show the total-energy separation of the magnetic and nonmagnetic states to vanish in the Invar composition range, where both states are stable, but at different volumes. Heusler alloys (X 2 MnY) possess localized magnetic moments (on the Mn atoms) that are not in nearest-neighbor contact, and therefore provide a testing ground for theories of indirect magnetic coupling. Our calculations indicate that the majority-spin electrons are completely itinerant and that the localized moments result from the exclusion of the minority-spin electrons from the interior of the Mn atoms. Interatomic exchange coupling is studied by comparing calculations for ferromagnetic and antiferromagnetic moment alignments.

Critical Point of the Cubic Antiferromagnet RbMnF3

We have studied the critical region of RbMnF3 (TN=83°K) using specific heat, x‐ray, and strain‐gauge measurements. Because RbMnF3 so closely resembles an ideal Heisenberg antiferromagnet, experimental results may be especially relevant to theoretical predictions. The specific heat is found to diverge logarithmically over three decades for T<TN, but only over two decades for T<TN. No distortion from cubic symmetry is detectable in the range 20° to 300°K. Finally a remarkably small thermal expansion anomaly is found in a narrow region near TN.

Magnetic and Magneto‐Optical Properties of Fe‐Doped EuO Films

EuO films with ∼8‐wt% Fe have saturation magnetic moments of about 180 emu/g at 4.2°K and Curie temperatures (Tc) as high as 200°K (for pure EuO, σ=232 emu/g, Tc=69.5°K). The temperature dependence of the magnetization does not scale relative to pure EuO but deviates from the normal magnetization curve in that it exhibits a long tail extending from about 100°K up to Tc. Optical absorption data at 6°K in the range 0.3–2.5 μ indicate a characteristic peak at λ=0.6 μ (due to the 4f‐5d transition) with α=2.2×105/cm, about 50% higher than pure EuO. The wavelength dependence of the normal Faraday rotation at 4.2°K is similar in shape to pure EuO; both peak at 0.7 μ but the peak rotation of the Fe‐doped film (6×105 deg/cm) is about 30% lower than pure EuO. Mossbauer measurements of 151Eu show mainly Eu+ + with an isomer shift of −12.25 mm/s as in bulk EuO. At 4.2°K the 151Eu hyperfine field is 280.8±5.6 kOe (pure EuO≈300 kOe). The isomer shift of the doublet observed on the 57Fe resonance at 300°K is characteris...

Sublattice Magnetizations in Rare‐Earth Iron Garnets

The individual iron sublattice magnetizations in yttrium iron garnet and gadolinium iron garnet have been measured by NMR techniques as a function of temperature. The observed resonance frequencies are slightly higher for GdIG in both cases, the differences at low temperatures being about 0.67% for the octahedral sites and 0.70% for the tetrahedral sites. These differences are tentatively interpreted as due to the volume dependence of the hyperfine coupling constant A. The difference for the octahedral sites decreases slightly with increasing temperature, while that for the tetrahedral sites increases appreciably. A molecular field calculation shows that this behavior can be explained by assuming that the Gd3+ ions interact much more strongly with tetrahedral sites than with octahedral sites.

Calculations of the energy loss of 4He ions in solid elements

We have calculated the energy loss of 4He ions in a number of elemental and rare‐gas solids in the energy range from 0.4 to 4.0 MeV. The calculations are based on the Lindhard‐Winther energy‐loss formalism and use solid‐state charge densities for the target atoms. Stopping cross sections eα are presented as a function of projectile energy and are compared with the Hartreee‐Fock‐Slater isolated‐atom calculations and semiempirical cross sections. It is found that the solid‐state charge densities generally produce a somewhat better energy dependence than the atomic charge densities but that neither produces an adequate fit in the low‐energy region. In particular, solid‐state effects of Ne and Ar lead to a 4–10% increase over the atomic calculations.

Triangular Moment Arrangements in Manganese-Iron Spinels

The anomalously low magnetic moments for MnxFe3−xO4 spinels with 1<x≤3 are qualitatively explained by a Yafet-Kittel triangular moment arrangement. Values of β, n, and α are calculated from measurements of magnetic moment, high field susceptibility, and Curie temperature. The measurements on Mn3O4 yield a moment of 1.73 Bohr magnetons and a susceptibility of 3.5×10−4 gauss/oersted. An extrapolated anisotropy field of 150 kilo-oersted is found. Transitions from the triangular state to a Neel-type arrangement are found to occur for x = 1.25, x = 1.55, and x = 1.80. For Mn3O4, αβ values suggest that the vanishing of the spontaneous magnetization at 42°K represents a transition to a paramagnetic-antiferromagnetic state.

Magnetic moments and bonding in Co-Nb-B alloys

Abstract Several new CoNbB amorphous and microcrystalline alloys have been melt quenched and their magnetic moments measured. Comparison is made with cobalt moment suppression in CoB, CoNb, CoV and other CoNbB alloys. Two issues are addressed: 1) the different rates at which Nb and V (same number of valence electrons) suppress the cobalt moment and 2) the combined effect of boron and niobium together in suppressing the cobalt moment. It is suggested that the characteristic moment variations can be accounted for by d-d hybridization in the first case and by p-d and d-d hybridization in the second case.