6. Conclusions

The effect of Pd on both magnetic and magneto-optical properties in TbFeCo/Pd multilayers was discussed. An increase of the coercivity was detected when the Pd thickness was increased. The maximum value of the coercivity was found for t_Pd=0.5Å and t_Pd?2Å for t_TbFeCo=30Å and 50Å respectively. This thickness corresponds to a minimum value in the Barkhausen volume, which was measured based on the change in coercivity with sweeping rate of the external field for the magnetization hysteresis curves. A shift in the polarity of the magneto-optical
hysteresis loop was detected when increasing the Pd layer thickness even further. This indicates that the magnetic structure of the TbFeCo/Pd multilayer is believed to be shifted from RE rich to TM rich composition. When investigating the saturation magnetization, an increase was detected for thicker Pd layers. The result suggests that the Pd atoms are being polarized due to the magnetic interaction with transition metals. The polarization is believed to be present up to a distance of about 10Å in the Pd layer and the Pd atoms are supposed to act as TM in the present structure. The detailed mechanism of the polarization of Pd atoms in amorphous TbFeCo films is not known at this moment.

The perpendicular magnetic anisotropy constant, K_u, initially increases with increasing t_Pd and then remains constant, ?2.5*10⁶, for thicker values in the case of t_TbFeCo=50Å, but drops to a level of ~10⁶ in the case of t_TbFeCo=30Å.

As to the magneto-optical polar Kerr effect, for the first time, the dependence of the polar Kerr rotation and ellipticity of TbFeCo/Pd multilayers has been measured over a wide range of photon energy (6.8 to 1.4 eV). An enhancement due to the polarization of Pd atoms takes place at photon energies at around 4.5 eV.  However, it is not clear how the enhancement is taking place in conjunction with the electronic structure of polarized Pd atoms, but this result surely provides a basic information on further studies of this field.

The recording performances of the TbFeCo/Pd multilayer structure were also examined. The signal to noise ratio as high as 50 dB (at 30 kHz bandwidth) was obtained.  This performance is reasonable for practical usage. The recorded bits were observed using MFM. The written bits were found to depend strongly on writing power.  If the writing power is not optimized, not only the written marks are properly made, but also the medium itself would be damaged.

In conclusion, the effect of Pd atoms on both magnetic and magneto-optical properties is evident, which is due to the polarization of Pd atoms through the magnetic coupling with Co and Fe atoms.