this era of information explosion, tons of data are generated every day and the appetite for larger storage capacity devices is never satisfied. The hard disk drive is the market leader in storage devices due to its lowest cost per byte of data, compared to other forms of storage. Further increase in the areal density

of hard disk drives requires a higher anisotropy magnetic recording media to overcome the thermal instability of media due to the reduction in the size of the recording bit. L10 FePt is the most promising magnetic material to support recording media beyond the areal density of Tbits/in2. However, the preparation of L10 FePt film with (001) texture and high anisotropy at the lower temperature is one of the technical challenges for implementing L10 FePt as a recording medium. The room-temperature deposited FePt thin films were of face-centeredcubic (fcc) structure which is a magnetically soft phase. For magnetic recording applications, high temperature (above 600°C) process is required to obtain magnetically hard L10 phase FePt. Phase transformation is a thermodynamic process and in many circumstances, one only considers the effect of temperature, as heat is the main driving force in phase transformation. In the present work, we demonstrated that stress can also affect the phase transformation. By using CrRu and MgO as the underlayer with a proper lattice misfit of about 6%, tensile stress was induced in the FePt layer which was in favor of the formation of L10 phase.

Figure 1: Cross-sectional TEM images of FePt films on 1 and 4 nm MgO layers with CrRu underlayer.

The phase transformation temperature (from fcc phase to L1₀ phase) was greatly decreased from about 600°C to below 300°C.

Cross-sectional TEM images of the FePt films on 1 and 4 nm MgO layer with CrRu underlayer are shown in Figure 1. MgO(200) texture was confirmed in the TEM images regardless of MgO layer thickness. For 4-nm MgO layer thickness, the lattice spacing of MgO layer along [001] axis was 0.211 nm, which was consistent with bulk MgO. While for the sample with 1 nm MgO layer, the lattice spacing of MgO layer (d₂₀₀= 0.226 nm) along [001] axis expanded by 7.1 % with respect to bulk MgO (d₂₀₀= 0.211 nm). A compression along [100] axis was expected due to the strain from the CrRu underlayer (d₀₂₂= 0.206 nm) and the lattice constant of MgO along [100] followed the CrRu underlayer. The lattice mismatch of FePt overlayer grown on MgO (1 nm)/CrRu and MgO(4 nm)/CrRu were 6.1% and 9.7%, respectively.

The out-of-plane hysteresis loops of the samples with 4 nm FePt layer and various thickness of MgO layer measured by PMOKE are shown in Figure 2. The coercivity of FePt film grown on MgO(1nm)/ CrRu was as high as 12 kOe. It is evident that the hysteresis loop was not saturated at the maximum field of 20 kOe. The actual coercivity of the films should be higher than 12 kOe. The coercivity of FePt grown on MgO (4 nm)/CrRu was only 6.3 kOe. FePt films directly grown on glass substrate were still fcc phase and magnetically soft.

This work was done in collaboration with Prof GM Chow (Department of Materials Science and Engineering), and Dr JF Hu and Mr BC Lim (Data Storage Institute).