Sputtered equiatomic CoPt magnetic thin films are widely studied due to the fact that CoPt material has some of magnetic properties strictly required for longitudinal magnetic recording media, namely high coercivity and high crystal anisotropy. This potential material has a fcc disordered structure(DOD) in the as-deposited state, but undergoes an ordering transformation to the ordered structure L1o(OD) upon annealing. At that, the magnetic properties of CoPt are dramatically changed. This phenomenon can be explained by the formation of OD phase in DOD matrix during annealing. In this work, the effects of post-annealing and deposition conditions on magnetic properties of CoPt thin films (400 Å) deposited on $SiO_2$/Si(100) substrates were investigated.
The coercivity was increased during isothermal annealing at temperature over 650℃. At 650℃ annealing, the maximum coercivity has been shown at 60 min annealing. And then the coercivity was decreased. The reason of enhanced coercivity was resulted from phase transformation, which was proved by XRD.
To investigate the effects of substrate bias, specimens were prepared by different deposition conditions ; bias 0 V, -200V, -300V. More increasing bias voltage, more decreasing coercivity was shown at initial annealing times. If the enhanced coercivity is mainly due to the fcc → fct phase transformation at initial annealing times, then substrate bias will act on the phase transformation. That is, the decreased coercivity may be attributed to bias effect that forced down the fcc → fct phase transformation at initial annealing time.
To study the effects of substrate temperatures, films were deposited at different temperatures; R.T, 250℃, 300℃, 350℃. The coercivity of films deposited at 350℃ were highly increased to 10 KOe at 120 min annealing while $H_c$ at R.T, 250℃, 300℃ depositions have shown almost the same behavior with annealing times. Also, other magnetic properties $M_s$, $M_r$, $S^*$ were very distinctive at 350℃ deposition compared with other substrate temperatures. Especially, the low $M_rㆍδ$ was obtained at 350℃. It is important to get the appropriate low $M_rㆍδ$ for magnetic recording media. General methods to get a low $M_rㆍδ$ are to reduce thickness of magnetic film. But in this work, the low $M_rㆍδ$ was obtained not by reducing the thickness but by varying the substrate temperature in the same thickness. Also, the high coercivity, approximately 10 KOe, could be obtained.
