This project seeks to elucidate materials factors that control the thermodynamic and kinetic stability of the equiatomic compound FeNi with the chemically-ordered tetragonal L10 structure that holds very high potential for rare-earth-free advanced permanent magnet applications. Fundamental exploratory research on magnetic alloys in the near-equiatomic compositional region of the Fe-Ni phase diagram will take place in materials with and without the ternary alloying additions of Ti, V, and Al. These materials will be synthesized in both bulk and thick-film form, with insight gained from film analogs of the focus alloys used to guide rational design of bulk alloys. Correlations between chemical ordering and magnetic properties such as magnetization, anisotropy and Curie temperature will be determined and quantified to facilitate trend prediction.
Astronomers have identified the L10 FeNi phase in selected meteorites and attributed its presence to an extraordinarily slow cooling rate that fosters long-range chemical ordering over a period of 4.6 billion years. Confirmation of the L10 structure in the Fe-Ni system is extremely significant because the tetragonal distortion that accompanies the chemical ordering in this structure gives rise to appreciable anisotropy. The development of rare-earth-free permanent magnet materials is essential to offset supply limitations and ensure U.S. competitiveness. Attainment of a rare-earth-free magnetic material with very high magnetocrystalline anisotropy would carry tremendous impact that ranges from the basic science realm all the way to advanced applications of great societal importance. Students supported by this grant will obtain an interdisciplinary research experience via educational exchange at three geographically diverse, strong science and engineering schools.
Astronomers have identified the chemically ordered L10 FeNi phase in selected meteorites and attributed its presence to an extraordinarily slow cooling rate that fosters long-range chemical ordering over a period of 4.6 billion years. The (equiatomic) L10 ordered phase is a natural superlattice in which one elemental atomic layer (Fe) alternates with the other (Ni) in the crystallographic c-axis direction. Confirmation of the L10 structure in the Fe-Ni system is extremely significant because the tetragonal distortion that accompanies the chemical ordering in this structure gives rise to appreciable magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant of L10 FeNi is in the range 1.0-1.3 MJ/m3, which, together with its high saturation magnetization of 1.42 T, implies the prospect for realization of technologically-useful coercivity in a rare-earth free permanent magnet. The development of rare-earth-free permanent magnet materials is essential to offset supply limitations and ensure U.S. competitiveness. Attainment of a rare-earth-free magnetic material with very high magnetocrystalline anisotropy would carry tremendous impact that ranges from the basic science realm all the way to advanced applications of great societal importance. Intellectual Merit: This project sought to elucidate materials factors that control the thermodynamic and kinetic stability of the equiatomic compound FeNi with the chemically-ordered tetragonal L10 structure. Fundamental exploratory research on magnetic alloys in the near-equiatomic (50-50) and in the Ni-rich region of the Fe-Ni phase diagram took place. For the near equiatomic alloys, samples with and without the ternary alloying additions of Ti, V and Al were studied. These materials were synthesized in both bulk and thick-film form. However, unequivocal evidence for the formation of the ordered phase proved difficult. This difficulty partly stemmed from the small difference in atomic scattering factor between Fe and Ni, which gives rise to very low intensity superlattice peaks even in well-ordered, bulk sample (intensities of 0.3-0.5% for the 100 and 110 superlattice peaks relative to the 111 fundamental peak). The difficulty was increased in thin films where the nanoscale grain size resulted in broadening of diffraction peaks and a lowering of the peak intensity relative to background. Given the difficulty in verifying the presence of the L10 FeNi phase, the project sought to answer the question: Is atom mobility (diffusivity) in films with nanoscale grains large enough to allow L10 formation? Interdiffusion of Fe and Ni was studied in sub-10 nm period Fe/Ni multilayers using X-ray reflectivity (XRR) and in sub-100 nm period Fe/Ni multilayers using a calorimetric method developed during the course of the project. For the sub-10 nm period films, the activation energy and the pre-exponential term for the effective interdiffusion coefficient were determined as 1.06 eV ± 0.07 and 5×10-10 cm2/s, respectively. For the sub-100 nm period multilayers, the activation energy and pre-exponential term were measured as 1.6±0.1 eV and 4×10-3 cm2/s, respectively. In order to produce L10 FeNi from the disordered matrix through isothermal annealing at a laboratory time scale, the diffusivity below the reported order-disorder temperature of 320oC must be high enough to allow the phase transformation to proceed at a reasonable rate. The measured diffusivities in Fe/Ni multilayer films are more than five orders of magnitude higher than lattice diffusivity below 320oC, and are comparable to interdiffusivities in Fe/Pt multilayers measured with XRR. Previous reports show that in FePt films the A1 to L10 disorder-to-order transformation is completed within hours at an annealing temperature of 300°C. Therefore, the measured diffusivity in nanocrystalline FeNi is sufficiently high so as to not be the limiting factor for the formation of L10 FeNi. Thus, it is concluded that the laboratory scale formation of L10 FeNi is instead limited by a low thermodynamic driving force. Thus, it is the thermodynamic driving force that must be increased through the use of alloying elements that increase the thermodynamic stability of L10 ordered FeNi, without degrading its magnetic properties. Broader Impact: This CMMI project featured three specific Broader Impacts accomplishments that support desired societal goals: (1) students were introduced to the collaborative and scientific working environments in different cultures (mixing gender and national origin) thereby developing the knowledge, skills and networks needed to become productive and successful participants in the global scientific community; (2) the project featured strong participation from women at all levels, contributing to increased participation of women in all aspects of science and technology; (3) students involved in the project were provided the opportunity to carry out experiments at large scientific user facilities, pushing the boundaries of scientific literacy. The project provided partial support for one doctoral student at Columbia University, in addition to supporting the research experience of three masters students, and three undergraduate students, two of whom were female students who participated in the research experience for undergraduates (REU) summer program.
