Ultrahigh Density Magnetic Recording Media: Quantitative Kinetic Experiments and Models of the A1 to L1₀ Phase Transformation in FePt and Related Ternary Alloy Films

L1₀ ordered FePt continues to be of interest for ultrahigh density magnetic recording media, in particular, heat assisted magnetic recording (HAMR) media. However, when deposited at room temperature, the alloy film forms 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.

Binary and ternary alloy films were sputter deposited from elemental targets at nominally room temperature at two different thicknesses of 1 micron and 500 nm. The latter films were used for composition analysis using energy dispersive X-ray spectrometry (EDS). The transformation from the A1 phase to the L1₀ phase was studied by differential scanning calorimetry (DSC) using freestanding micron-thick films. The kinetic ordering temperature (KOT), defined as the peak temperature of the DSC trace at a heating rate of 40 °C/min, was used to evaluate the impact of alloy composition and alloying additions on the ordering transformation

The nine ternary alloying elements and composition ranges are: 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. Compared with binary FePt, Cu additions have no impact on the (KOT), whereas all the other additions except C result in an increase of KOT. Additional experiments are necessary to better evaluate the impact of C additions. Elevated temperature deposition experiments showed that higher deposition temperatures result in faster transformation only for binary films with > 46 at.% Pt.

The time-temperature-transformation (TTT) and isothermal transformation curves for binary FePt and ternary Fe_46.7Cu_2.4Pt_50.9 films were calculated using the Michaelsen- Dahms (MD), k₂(T), k₂(T)N(T) and a new continuous nucleation model, k₂(T)N(T,t). The model that most closely agrees with all available experimental data is the k₂(T) model.