Thermal Stability of an Amorphous Material for Magneto-Optical Recording Disks

(A)

(B)

Figure.1.  Schematic view of the relation between average diffusion distance and both temperature T and time t .

  (A): Average diffusion distance in Tb₂₂Fe₇₈ amorphous alloy.

  (B): Average diffusion distance in (Tb_0.22Fe_0.78)₉₄Si₆ amorphous alloy.

  Thermal stability of amorphous alloys for magneto-optical disks is very important, because the amorphous films are pulse-heated very many times during recording. Structural relaxation processes of the alloys proceed through diffusion phenomena even well below the crystallization temperature. Furthermore, the phase transition from a metastable amorphous state to a stable crystalline one is influenced by diffusion processes. In the present work, the thermal stability of amorphous Tb-Fe and Tb-Fe-Si alloys prepared by a dc sputtering was studied from the view point of atomic diffusivities of the components.

  The macroscopic diffusion occurred at a temperature T during time t is characterized by the following average diffusion distance S,

  S = 2 ( D t )^1/2, (1)

  where D is the diffusion coefficient expressed by the Arrhenius relation,

  D = D₀ exp ( - Q / RT ) . (2)

  D₀ is the pre-exponential factor and Q the activation energy. Therefore, S depends strongly on the temperature rather than the diffusion time. The present authors have determined the diffusion coefficients of Co in Dy₂₅Fe₇₅ and Tb₂₂Fe₇₈ amorphous alloys in the temperature range between 570 and 663 K by a radioactive tracer method and an ion-beam sputter-microsectioning technique. The temperature dependence of the diffusion coefficient of Co in both the alloys is represented by the following linear Arrhenius relation,

  D = 5.7×10^-2 exp (- 199 kJ mol^-1 / RT ) m² s^-1 . (3)

  From the close size in the atomic radii between Co and Fe, it is easily expected that the diffusion coefficient of Fe is close to that of Co. Contrary to this relation, the diffusion of Tb is much slower than that of Fe in the amorphous Tb-Fe alloys, because the atomic radius of Tb is 0.1782 nm whereas that of Fe is 0.1274 nm. In amorphous alloys with large difference in the atomic radii of the components such as Zr-Fe alloys, it is well known that the smaller atomsdiffuse much faster than the larger atoms.

  Figure 1(A) is the schematic view of the relation between the average diffusion distance S and both temperature T and time t . In the laser-beams heating process for recording on magneto-optical disks, two factors of heating time span and maximum temperature are apt to discuss as similarly effective. However, the factor of maximum temperature is strongly effective than that of heating time span as show in Fig.1(A). Therefore tighten control of the maximum temperature in recording processes is the key to keep stabilization of the disks.

  Further we investigated the effect of covalent-type bonding on the thermal stability. An addition of Si raises the crystallization temperatures of Tb₂₂Fe₇₈ amorphous alloys. The activation energy of the phase transition from the amorphous to nano-crystalline states for the alloy added Si of 2% increases extremely about 170% up measured by differential scanning calorimetry. Extrapolating this effect to lower temperature ranges of actual laser-beam recording, it can be expected that covalent-type bonding in the amorphous structure ascend the activation energy for the diffusion process. We have confirmed that the diffusion coefficients of Co in (Tb_0.22Fe_0.78)₉₄Si₆ amorphous alloys at temperatures of 600-670 K are about one order of magnitude lower than those of the Tb₂₂Fe₇₈ alloy . The relationship between the average distance in (Tb_0.22Fe_0.78)₉₄Si₆ amorphous alloy and both temperature T and time t is shown in Fig.1(B). In Fig.1(A) and (B) at T = 600K and t = 1.0ns, it is clear that by adding Si the average diffusion distance is reduced from 3×10⁴nm to 10⁴nm. This is due to the strong covalent-type bonding in metal-metalloid type amorphous Tb-Fe-Si alloy in comparison with the metallic bonding in the metal-metal type amorphous Tb-Fe alloy. Consequently, the addition of Si is proved to improve the thermal stability of magneto-optical recording amorphous alloys.