Ll_o Phase Formation in CoPt Thin Films

Abstract

The high magnetic anisotropy and high coercivity of equiatomic CoPt thin films make them attractive as potential materials for magnetic recording applications. Magnetic coercivity (H_c) over 10 kOe has been measured in films in which the as-deposited fee phase has been partially transformed to the atomically ordered Ll₀ phase. Very high H_c has been related to high volume fraction and small size of the Ll₀ precipitates. A better understanding of the Ll₀ phase formation and quantification of volume fraction is critical to optimizing the magnetic properties of this material. As we have previously reported, an increase in H_c was observed with an increase in Ll₀ volume fraction in 10 nm thick, equiatomic CoPt films. In our current investigation we have observed that, at anneal temperatures far from the order/disorder transition temperature, e.g. at T = 0.6 T_c, numerous, very fine Ll₀ precipitates are seen, some of which cluster at the parent phase grain boundaries. At T = 0.85 T_c, very few, larger Ll₀ regions were seen. As precipitates of the ordered phase grow to impingement, antiphase boundaries (APB) are formed. This is consistent with a heterogeneous nucleation and growth mechanism for the formation of the Ll₀ phase throughout the temperature range studies.