L10 FeNi Research Articles

Structural, Vibrational and Electronic properties of FeNi

The Fe-Ni alloys attract the great consciousness of researchers due to their anomalously low thermal expansion coefficient for Fe-rich compositions and numerous technological and practical applications. The structural, vibrational and electronic properties of tetrataenite L10 FeNi studied using the first-principles plane wave self-consistent method with the Ultrasoft pseudopotential scheme under the framework of density functional theory. Calculated c/a ratio and isothermal bulk modulus for tetrataenite L10 FeNi presented with the other available results. Conclusions based on the phonon dispersion curves, phonon density of states and electronic band structure along with the total and projected density of states are outlined.

Thermodynamic and kinetic parameters of the chemical order–disorder transformation in L10 FeNi (tetrataenite)

The thermodynamic and kinetic parameters describing the chemical disordering process of L10 FeNi (tetrataenite) are quantified through analysis of calorimetric data obtained from material (Fe57Ni43) extracted from the NWA 6259 meteorite. The material presents an endotherm with peak temperatures of 800–860 K depending on heating rate and a transformation enthalpy of 4.0 ± 0.2 kJ/mol of atoms that corresponds to the L10 → A1 chemical order–disorder phase transformation. Using reported values of the enthalpy of mixing of the A1 phase and the enthalpy difference between the A1(γ) and the A2(α) phases of Fe, the enthalpy of formation of the L10 phase at the composition 43 at.% Ni constituted from Fe and Ni in their standard state is calculated as −4.4 ± 0.6 kJ/mol of atoms. These results indicate that the L10 FeNi phase is at the edge of enthalpic stability relative to a mixture of α-Fe and L12 FeNi3. The measured kinetic chemical-disordering transformation temperature is well above the reported order–disorder temperature of 593 K (320 °C) confirming that the disordering process is highly kinetically limited. The activation energy of chemical disordering of L10 FeNi (43 at% Ni) is 3.08 ± 0.07 eV/atom. Results obtained in this study are consistent with a model of one-dimensional growth of the disordered phase wherein the transformation rate is limited by the creation and migration of vacancies. Establishment of the enthalpy difference between the chemically-ordered and -disordered phases as well as determination of the activation energy for transformation provide guidance for laboratory synthesis of the L10 FeNi phase, which is of significant applied interest as a new advanced permanent magnet material.

L10 phase formation in ternary FePdNi alloys

Metallurgical routes to highly metastable phases are required to access new materials with new functionalities. To this end, the stability of the tetragonal chemically ordered L10 phase in the ternary Fe–Pd–Ni system is quantified to provide enabling information concerning synthesis of L10-type FeNi, a highly attractive yet highly elusive advanced permanent magnet candidate. Fe50Pd50−xNix (x = 0–7 at%) samples were arc-melted and annealed at 773 K (500 °C) for 100 h to induce formation of the chemically ordered L10 phase. Coupled calorimetry, structural and magnetic investigations allow determination of an isothermal section of the ternary Fe–Pd–Ni phase diagram featuring a single phase L10 region near the FePd boundary for x < 6 at%. It is demonstrated that increased Ni content in Fe50Pd50−xNix alloys systematically decreases the order-disorder transition temperature, resulting in a lower thermodynamic driving force for the ordering phase transformation. The Fe50Pd50−xNix L10 → fcc disordering transformation is determined to occur via a two-step process, with compositionally-dependent enthalpies and transition temperatures. These results highlight the need to investigate ternary alloys with higher Ni content to determine the stability range of the L10 phase near the FeNi boundary, thereby facilitating kinetic access to the important L10 FeNi ferromagnetic phase.

Intrinsic magnetic properties of L1 FeNi obtained from meteorite NWA 6259

FeNi having the tetragonal L10 crystal structure is a promising new rare-earth-free permanent magnet material. Laboratory synthesis is challenging, however, tetragonal L10 FeNi—the mineral “tetrataenite”—has been characterized using specimens found in nickel-iron meteorites. Most notably, the meteorite NWA 6259 recovered from Northwest Africa is 95 vol. % tetrataenite with a composition of 43 at. % Ni. Hysteresis loops were measured as a function of sample orientation on a specimen cut from NWA 6259 in order to rigorously deduce the intrinsic hard magnetic properties of its L10 phase. Electron backscatter diffraction showed that NWA 6259 is strongly textured, containing L10 grains oriented along any one of the three equivalent cubic directions of the parent fcc structure. The magnetic structure was modeled as a superposition of the three orthonormal uniaxial variants. By simultaneously fitting first-quadrant magnetization data for 13 different orientations of the sample with respect to the applied field direction, the intrinsic magnetic properties were estimated to be saturation magnetization 4πMs = 14.7 kG and anisotropy field Ha = 14.4 kOe. The anisotropy constant K = 0.84 MJ/m3 is somewhat smaller than the value K = 1.3 MJ/m3 obtained by earlier researchers from nominally equiatomic FeNi prepared by neutron irradiation accompanied by annealing in a magnetic field, suggesting that higher Ni content (fewer Fe antisite defects) may improve the anisotropy. The fit also indicated that NWA 6259 contains one dominant variant (62% by volume), the remainder of the sample being a second variant, and the third variant being absent altogether.

Addition of Co to L10-ordered FeNi films: influences on magnetic properties and ordered structures

L10-type ordered FeNi alloy is a good candidate material for rare-earth-free permanent magnets due to its large saturation magnetization, large uniaxial magnetic anisotropy (Ku) and high Curie temperature. Previously, we investigated the fundamental properties of L10–FeNi by preparing single crystal films by alternate deposition of Fe and Ni monatomic layers. In the present study, we investigated the influences of the addition of Co on the magnetic and structural properties of L10-type ordered FeNi films. Three types of Co addition, with the substitution of Fe layers, Ni layers and both layers, were performed. X-ray diffraction using synchrotron radiation indicated that Co atoms were randomly distributed in films in all addition processes. A long-range order parameter of the L10–FeNi phase and Ku showed a tendency to decrease with the amount of Co for additions substituting Fe layers and both layers. However, Ku showed no decrease with Co addition substituting ⩽10% of Ni layers, even though the order parameter notably decreased. This indicates that an achievement of the Co addition substituting Ni atoms without any reduction of the order parameter enhances Ku in L10–FeNi.

Magnetic Anisotropy in Ordered L10 type NiFe Alloy

Magnetic anisotropy energy (MAE) is obtained in the frame of a first principles calculation for L10 type FeNi alloy. The L10 type FeNi is only found in the iron meteorite in the nature that has an extraterrestrial texture called a Widmanstatten structure. The presence of L10 FeNi influences the unusual domain structure referred as “head-on”, in which the magnetization in alternate domains runs with each other. This magnetic domain is provided due to the large anisotropy in the L10 lamella at the interface between the α-and γ-phase in the FeNi alloy. In the present study, the MAE of L10 FeNi is calculated as a function of the c/a ratio for the lattice distortion by using the LMTO-ASA formula. The result indicates that the distortion enlarges the MAE when c/a

Fe–Ni many-body potential for metallurgical applications

A many-body interatomic potential for the Fe–Ni system is fitted, capable of describing both the ferritic and austenitic phase. The Fe–Ni system exhibits two stable ordered intermetallic phases, namely, L10 FeNi and L12 FeNi3, that are key issues to be tackled when creating a Fe–Ni potential consistent with thermodynamics. A procedure, based on a rigid lattice Ising model and the theory of correlation functions space, is developed to address all the intermetallics that are possible ground states of the system. While controlling the ground states of the system, the mixing enthalpy and defect properties were fitted. Both bcc and fcc defect properties are compared with density functional theory calculations and other potentials found in the literature. Finally, the potential is thermodynamically validated by constructing the alloy phase diagram. It is shown that the experimental phase diagram is reproduced reasonably well and that our potential gives a globally improved description of the Fe–Ni system in the whole concentration range with respect to the potentials found in the literature.