Interest on thin film media showing high coercivity, high crystal anisotropy, small grain size and low media noise is growing because of high density longitudinal magnetic recording. One of the promising materials is the equi-atomic CoPt alloy of $L1_0$ ordered structure. This alloy shows a fcc disordered structure in the as-deposited state. Upon annealing at above 600℃, this alloy undergoes an ordering transformation and becomes hard ferromagnetic showing a large increase in its coercivity. In this work, we have investigated the effect of insitu annealing, including the carbon and Cr addition effects, on the magnetic properties and microstructure of CoPt thin film(50nm), which was magnetron-sputter deposited on the $SiO_2/Si(100)$ substrate.
When the CoPt film, deposited at room temperature, was ex-situ (post-) annealed at 560℃ in a vacuum furnace, the ordering kinetics was sluggish and its coercivity increased very slowly with annealing time. In contrast, when the CoPt film was deposited either at room temperature or at 560℃ and in-situ annealed at 560℃, the ordering kinetics was fast and its coercivity reached 9 kOe for 120 min of annealing. Two factors were considered to be responsible for this in-situ annealing effect. The first is the difference in the surface state. The Ag capping experiments showed that Ag capping produced a much more pronounced decrease of the coercivity anisotropy $(H_{∥} / H_⊥)$ in the ex-situ annealing than in the in-situ annealing. This suggested that the surface effect is more important for ex-situ annealing. The second is the difference in the initial grain size distribution. The grain size distribution of initial grain structure was wider and bimodal-like in the case of in-situ deposition and annealing; this was more favorable for the ordering transformations.
The change of CoPt film thickness affected the structure and magnetic properties of the film. At the film thickness of 5nm, the film consisted of isolated island and became continuous at about 10nm. The increasing film thickness led to the promotion of (111) texture and to a decrease of the coercivity anisotropy ratio ($(H_{∥} / H_⊥)$.
Inserting the carbon layer in the form of $CoPt/C_n(n=2)$ (vol.% of carbon = 5%) decreased the coercivity from 9 kOe to 8kOe. The intensity of (111) peak decreased and (111) peak shifted to high angle with carbon addition and the c/a ratio of L10 CoPt lattice increased with carbon addition. The addition of 5 vol.% carbon led to an abrupt reduction of grain size from 96 nm to 50 nm on in-situ annealing the film for 120min at 560℃.
The addition of 25 vol% Cr to CoPt film in the form of $[CoPt(7.5nm)/Cr(2.5nm)]_{n=3}$ resulted in a significant reduction of its grain size and of its coercivity to about 3kOe. However, the Cr addition had an effect to significantly increase its coercivity anisotropy ratio by up to 150%.
높은 보자력과 작은 결정립크기, 낮은 미디어 노이즈를 갖는 자성 미디어에 대한 관심이 증대되고 있다. 이러한 물질중에 유력한 것은 equi-atomic 조성을 갖는 L10 CoPt이다. 이 물질은 상온에서 증착직후, 불규칙 구조를 갖는다. 그러나 600도 이상에서는 규칙상으로 변화하여 큰 보자력값을 갖는다. 이러한 특성을 갖는 CoPt 를 이용하여, 고온에서 in-situ 열처리에 따른 자성특성의 변화와 미세조직의 변화, 그리고 Carbon 및 Cr 의 첨가효과에 대하여 알아보았다. In-situ annealing 의 경우 ordering kinetics가 빨라졌으며, 보자력은 9kOe까지 증가하였다. In-situ효과에 대한 2가지 요인을 고려해 볼수 있었다. 하나는 표면효과이며, 또 하나는 grain size distribution의 차이이다. Carbon을 5 vol% 첨가하였을 때 보자력은 약간 감소하였으나. 결정립의 크기는 크게 줄일 수 있었다. Cr을 25vol% 첨가하였을때, 높은 보자력 이방성 값을 얻을 수 있었다.
