We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within density-functional theory using two complementary techniques, one based on the exact muffin-tin orbital theory within the coherent potential approximation and another one based on the projector augmented-wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.

A systematic ab initio study of static ionic displacements in a face-centered-cubic Fe 65 Ni 35 alloy has been carried out. Theoretical results for magnitudes of average Fe-Fe, Fe-Ni, and Ni-Ni ͗110͘ bond vectors agree well with experimental measurements. In addition, we have observed that in collinear ferrimagnetic states, iron-iron nearest-neighbor pairs with antiparallel local magnetic moments are shorter on average than those with parallel moments. Furthermore, having considered different states ͑ferromagnetic, nonmagnetic, and collinear ferrimagnetic states͒ for the same lattice spacing, we have shown that the magnetic structure strongly influences local geometrical properties of the alloy. For example, a transition from a ferromagnetic state to a collinear ferrimagnetic state induces a significant contraction of the volume associated with an iron site where the moment flips. A model system based on a Hamiltonian written as the sum of Lennard-Jones energies and a classical Heisenberg Hamiltonian has been introduced. It yields structural properties which are qualitatively similar to those obtained ab initio. We have found that some of the phenomena can be classified as magnetovolume effects.

The Invar effect in ferromagnetic Fe-Ni, Fe-Pt, and Fe-Pd alloys is investigated theoretically by means of a computationally efficient scheme. The procedure can be divided into two stages: study of magnetism and calculations of structural properties. In the first stage, an Ising model is considered and fractions of Fe moments which point up as a function of temperature are determined. In the second stage, density-functional theory calculations are performed to evaluate free energies of alloys in partially disordered local moment states as a function of lattice constant for various temperatures. Extensive tests of the scheme are carried out by comparing simulation results for thermal expansion coefficients of Fe1−xNix with x = 0.35, 0.4, . . . , 0.8, Fe0.72Pt0.28, and Fe0.68Pd0.32 with measurements. The scheme is found to perform well, at least qualitatively, throughout the whole spectrum of test compounds. For example, the significant reduction of the thermal expansion coefficient of Fe1−xNix as x decreases from 0.55 to 0.35 near room temperature, which was discovered by Guillaume, is reliably reproduced. As a result of the overall qualitative agreement between theory and experiment, it appears that the Invar effect in Fe-Ni alloys can be investigated within the same computational framework as Fe-Pt and Fe-Pd.

Static local displacements of ions in disordered face-centered cubic Fe50Ni50 alloy are studied from first principles in the framework of the density functional theory. The disordered alloy is modeled using a 64 atom supercell constructed as a special quasirandom structure. Fully relaxed atomic positions inside the supercell are calculated by means of projected augmented wave method as implemented in Vienna ab initio simulation package. According to our calculation, the relative changes of mean nearest neighbor interatomic distances due to local lattice relaxations are relatively small (⩽0.6%), in agreement with experiment. At the same time, we predict that for all types of pairs, Fe–Fe, Fe–Ni, and Ni–Ni, the dispersion of the nearest neighbor interatomic distances is rather large, and the individual changes of distances between certain pairs of atoms due to local lattice relaxations can be one order of magnitude larger than the mean values for the corresponding pair of atoms.

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Magnetopneumography is the study of the remanent magnetism of foreign intrathoracic ferromagnetic particles after magnetization by an external magnetic field. Given knowledge of the magnetic characteristics of the inhaled particles, this highly sensitive and non-invasive technique allows the measurement of lung dust loads. Many groups of workers have been examined in this way, e.g. welders, coalminers, asbestos, foundry and steel workers. Magnetopneumography also allows analysis of the distribution of aerocontaminants in the different anatomical structures and, when repeated, the study of clearance speeds and migration from site to site of such particles. Emphasis has been laid on the importance of study of the fading of the remanent magnetic signal as time elapses. This short-term phenomenon, called relaxation, seems highly significant for the study of the dynamic properties of the immediate environment of extra pulmonary particles and especially for the study of macrophage activity.

A method is proposed for investigating the spontaneous magnetization, the spontaneous volume magnetostriction, and their relationship in disordered facecentered-cubic Fe 0.72 Pt 0.28 and Fe 0.65 Ni 0.35 in the temperature interval 0 ≤ T /T C < 1. It relies on the disordered local moment formalism and the observation that the reduced magnetization in each of the investigated materials is accurately described by an equation of the formThe present approach yields interesting results. The alloys at zero Kelvin share several physical properties: the volume in a partially disordered local moment state shrinks as the fraction of Fe moments which point down increases in the interval 0 < x Fe↓ < 1/2, following closely V (0) − 4[V (0) − V (1/2)]x Fe↓ (1 − x Fe↓ ), while the magnetization collapses, following closely M (0) − 2M (0)x Fe↓ ; the volume in the homogeneous ferromagnetic state greatly exceeds that in the disordered local moment state; x Fe↓ (0) is close to zero. These common properties can account for a variety of intriguing phenomena displayed by both alloys, including the anomaly in the magnetostriction at zero Kelvin and, more surprisingly perhaps, the scaling between the reduced magnetostriction and the reduced magnetization squared below the Curie temperature. However, the thermal evolution of the fraction of Fe moments which point down depends strongly on the alloy under consideration. This, in turn, can explain the observed marked difference in the temperature dependence of the reduced magnetization between the two alloys.