Interdiffusion in the Fe-Pt System
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sed alloys have drawn special interest toward research studies because of their use as magnetic storage media and permanent magnets. The L1o FePt, which is an ordered 50:50 stoichiometric intermetallic compound, is one of the most promising materials for these applications because of its high magnetocrystalline anisotropy energy density (Ku = 107 ergs/cm3).[1] This large Ku arises from the Fe and Pt interactions and its lattice structure. It is distorted along the c axis, which leads to lower lattice symmetry; hence, it possesses higher crystal field anisotropy. The Fe-Pt phase diagram is reviewed by Okamoto as shown in Figure 1[2] based on the diagrams published by Okamoto,[2] Osaka et al.,[3] and Nose et al.[4] They have shown different composition ranges of the phases and confusion still exists. This system has three ordered phases; Fe3Pt and FePt3 have L12 and FePt has L1o crystal structure. In applications, the phases are formed by solid-state interdiffusion from Fe/Pt multilayer in thin film. So, understanding the diffusion process is important. However, study in this condition may not be straightforward. The stress generated during deposition and nonequilibrium concentration of vacancies might complicate the diffusion process. Moreover, nonequilibrium phases also may grow in the interdiffusion zone. Thus, it is important to study diffusion characteristics in bulk condition first to understand the growth mechanism of the phases. Even diffusion couple is one of the indirect techniques to find phase boundary compositions at different temperatures.[5] SANGEETA SANTRA, Ph.D. Student, AVIK MONDAL, M.E. Student, and ALOKE PAUL, Associate Professor, are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Contact e-mails: [email protected]. in; [email protected] Manuscript submitted November 18, 2011. Article published online January 19, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
Experiments were conducted following the diffusion couple technique. Pt foils (99.99 wt pct pure) were cut into pieces of dimension 5 9 5 9 0.5 mm3. Fe granules supplied by Alfa Aesar (99.99 pct wt pct pure; Alfa Aesar, Ward Hill, MA) were melted into button-shaped ingots in an arc melting unit in argon atmosphere and were cut into small pieces with dimensions of 5 9 5 9 1 mm3. The starting materials were polished using the standard metallographic technique to a 1-lm finish followed by ultrasonic cleaning for 10 minutes. Y2O3 particles were used as inert markers and were dispersed on one of the polished sample. The couple halves were clamped in a special fixture with minimum required pressure to ensure good bonding. Tungsten foil was used as the diffusion barrier between the fixture and the end members. The couples were annealed in the temperature range of 1323 K to 1423 K (1050 C to 1150 C) for 25 hours in a calibrated tube furnace in a vacuum of ~104 Pa. The annealing temperature was controlled within ±5 K (5 C). After the annealing, the diffusion couples were quenched in water and prepared
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