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Nanostructured FePt films for magnetic media applications

Smaller, faster, denser and cheaper magnetic disk drives are demanded for information storage using magnetic media. Increase in high areal density in magnetic recording requires small bit cells with very small grain size.  The reduction of bit cell volume and grain size will eventually be constrained by the undesirable thermal instability of magnetization due to the superparamagnetic effect, thus ultimately limiting the achievable areal density. Very fine grained, thermally stable ferromagnetic media materials require high magnetocrystalline anisotropy constant. Nanostructured L1o FePt alloys, with very high anisotropy constant, large Ms, high corrosion resistance and small domain size, are potential high density magnetic recording media.

We investigated the effects of sputtering parameters on the texture, ordering temperature, reduction of grain size and exchange coupling of nanostructured longitudinal and perpendicular FePt thin films. The correlations of long range order (LRO) and short range order (SRO) with magnetic properties were studied.  Nanostructured CoCrPt films were also studied.

The grain size and grain size distribution of 30 nm L10 Fe50Pt50 magnetic thin films sputter-deposited on Cu (001) single crystal substrate were investigated using high resolution transmission electron microscopy (TEM). The planar TEM results showed that the grain size was about 56 Å in diameter with a standard deviation of ~10 % (Fig. 1). The cross-sectioned TEM images indicated the magnetic grains existed in columnar structures.  The L10 ordered FePt films with small grain size and narrow grain size distribution are potential longitudinal media candidate for the high-areal density magnetic recording.

Fig. 1. TME image of FePt film on Cu (001)

Improved coercivity was achieved in 10 nm, perpendicular FePt film (on glass substrate) through an inserted Ag layer. The out-of-plane coercivity increased with increasing Ag thickness, from 2.0 kOe, for the sample without Ag, to 4.4 kOe, for the sample with 3 nm Ag. Silver insertion also reduced the magnetic grain size and affected the magnetic reversal mode in these films.

The anisotropy constant of L10 ordered FePt is larger than that in the disordered phase.  Since the anisotropy energy mainly arises from the electronic structure, it is therefore essential to understand the electronic nature of disordered and L10 ordered FePt. Spin magnetic moment and orbital magnetic moment are important parameters for understanding the electronic structure and magnetic properties.  The magnetic moments of disordered and ordered L10 Fe50Pt50 films were investigated using magnetic circular dichroism and spin polarized full relativistic Korringa-Kohn-Rostoker (SPRKKR) calculations. The measurements showed that the spin magnetic moments of Fe in both ordered and disordered films were similar with a lower value than that obtained by SPRKKR calculations.  Both films however showed larger orbital moments of Fe compared to the calculations.  The results suggested that the spin magnetic moment of Fe in FePt thin films was insensitive to L10 ordering.

The phase miscibility of Co, Cr and Pt in oriented nanostructured CoCrPt magnetic thin films was investigated using anomalous x-ray scattering (AXS) from the (002) reflection and extended x-ray absorption fine structure (EXAFS) at Co K-, Cr K- and Pt LIII-edges (Fig. 2). The AXS measurements at Co K-edge clearly showed the presence of Co in the crystalline region. However, Cr was not detected in the lattice. The EXAFS at Co K-edge indicated that the nearest neighboring atoms of Co were mixed with 80 % Co and 20 % Pt, consistent with the results of EXAFS at Pt LIII-edge. The results showed that only Pt and Co were at the Co (002) lattice of the nanotextured CoCrPt thin films.

Fig. 2 AXS spectra of the (002) reflection in the vicinity of absorption edges of (a) Cr K-edge, (b) Co K-edge, and (c) Pt LIII-edge of CoCrPt crystals.

Principal investigator: Chow Gan Moog

E-mail: msecgm@nus.edu.sg

Tel : 6516 3325