TECHNICAL: The objective of these studies is to improve our understanding of the A1 to L10 transformation in FePt with ternary additions of V, Ti, Au, Ag or B, through the measurement of thermodynamic and kinetic parameters of the transformation (which occurs by nucleation and growth of the L10 phase in the A1 matrix). FePt is the leading candidate for the development of magnetic recording media in hard disk drives (HDDs). Since its introduction, thin film technology has allowed large increases in areal recording densities of HDDs by decreasing the thickness of the magnetic recording medium and the dimensions of a recorded bit. However, as recording densities increase, so does the approach to the fundamental limit posed by superparamagnetism. As a result, there has been a growing interest in FePt, CoPt and related ferromagnetic alloys with the tetragonal, L10 crystal structure. These chemically-ordered tetragonal alloys have the highest known Ku except for those containing rare-earth elements such as Sm, which have poor corrosion resistance. The anisotropies for FePt and CoPt are 20-40 times higher than today's Co-alloy based media. When deposited at room temperature FePt (and CoPt) alloy films form the chemically-disordered face-centered cubic (fcc, or A1) phase. The A1 phase has low magnetocrystalline anisotropy and is consequently unsuitable for use as a recording medium. The annealing times and temperatures needed to form the L10 phase, even for FePt with its faster kinetics than CoPt, are incompatible with current media manufacturing needs, and, at present, are major barriers to the implementation of these alloys in recording systems. To move beyond a trial-and-error effort of engineering the alloys for reduced (kinetic) ordering temperature, a deeper fundamental understanding of the A1 to L10 transformation has become necessary. Thus, as noted above, the objective of these studies is to improve our understanding of the A1 to L10 transformation in FePt films with ternary alloying additions. The particular emphasis of the work will be the impact of the choice and quantity of alloying additions on these parameters. The thermodynamic and kinetic parameters of the transformation (and the Curie temperature of the A1 and L10 phases) in alloy films will be measured by differential scanning calorimetry (DSC). In addition to DSC, x-ray and electron diffraction studies will be used for phase identification and determination of the long-range order parameter, transmission electron microscopy for microstructure characterization, and magnetometry for coercivity measurements. NON-TECHNICAL: The program will strongly support undergraduate research in addition to graduate research; will make an extensive effort to involve students from underrepresented groups; will support the operation and maintenance of the specialized experimental infrastructure within the PI?s laboratories, the Dept. of Materials Science and Eng. and the Data Storage Systems Center (DSSC) at Carnegie Mellon; will ensure student participation at meetings of professional societies, and in the semi-annual meetings of the DSSC; will help the development of commercial technology, namely, L10 media for HDDs. In addition to magnetic recording media, ferromagnetic L10 alloys are being considered for magnetic actuators and other elements in micro/nano-electromechanical systems (MEMS/NEMS). The formation of L10 alloys for these systems similarly involves transformation from a deposited A1 phase. Therefore, these studies are also expected to be of benefit in the development of these components.
Intellectual Merit- Since its introduction, thin film technology has allowed large increases in areal recording densities of hard disk drives by decreasing the thickness of the magnetic recording medium and the dimensions of a recorded bit. However, as recording densities increase, so does the approach to the fundamental limit posed by superparamagnetism. Superparamagnetic response refers to the probability of thermally activated switching of magnetic nanoparticle moments with consequent loss of recorded information. The switching frequency becomes larger for a smaller bit size, if not counterbalanced by an increased magnetocrystalline anisotropy energy density of the recording medium. Indeed, the critical (recording medium) limitation in reaching storage densities of 1 Terrabit/inch2 and beyond is that posed by superparamagnetism. As a result, there has been a growing interest in FePt (iron platinum), CoPt (cobalt platinum) and related ferromagnetic alloys with the tetragonal, L10 crystal structure. These chemically-ordered tetragonal alloys have the highest known magnetocrystalline anisotropy energy densities except for those containing rare-earth elements such as Sm (samarium), which have poor corrosion resistance. At 6.6×107 and 4.9×107 erg/cm3 for FePt and CoPt, respectively, the anisotropies are 20-40 times higher than today’s Co-alloy (cobalt-alloy) based media. The greater corrosion resistance of L10 alloys compared with rare-earth containing alloys makes them significantly more attractive for use as thin film media in hard disk drives. Over the past several years, FePt has emerged as the leading candidate for ultrahigh density magnetic recording media. Recording media are produced by sputter deposition. However, when sputter deposited at room temperature, FePt films form in the chemically disordered A1 state, requiring a post-deposition anneal at high temperatures or for long times, or deposition onto heated substrates at temperature ≥ 400 °C to form the ordered L10 phase. It has been of interest to identify ternary alloying additions that can reduce the post deposition annealing or the elevated deposition temperatures, and to examine the impact of deposition at elevated temperatures on the transformation kinetics. In this research project, the addition of nine ternary alloying elements to FePt was investigated. The compositions studied were: 0.0 - 2.6 at.% Mg, 0.7 – 12.2 at.% V, 2.2 – 16.3 at.% Mn, 1.6 – 21.5 at.% Ni, 1.3 – 17.3 at.% Cu, 0.0 – 16.7 at.% Ag, 1.9 – 13.8 at.% Au, 1.2 – 12.9 at.% B and 1.4 at.% C. No ternary alloying addition was found to result in faster transformation kinetics than in binary FePt alloys. The fastest transformation kinetics and the highest order parameter were found for binary FePt with compositions in the range of 47-49 at.% Pt. Elevated temperature deposition experiments showed that higher deposition temperatures resulted in faster transformation only for binary films with > 46 at.% Pt. Kinetic and thermodynamic parameters of the A1 to L10 transformation in the binary and ternary films were quantified. The heats of formation of binary Fe-Pt compounds were measured and were found to agree within 10% with values calculated using density functional theory. Broader Impact: This research program (i) strongly supported undergraduate research in addition to graduate research, (ii) supported the operation and maintenance of the specialized experimental infrastructure within the principal investigator’s laboratories, and the departments and centers within the university, (iv) ensured student participation at meetings of professional societies, (v) helped the development of commercial technology, namely, L10 media for hard disk drives.
