Mitsunori Mochida

Relaxation time and thermal stability in magneto-optical recording media

The time dependence of the magnetization process in amorphous TbFeCo thin films and (TbFeCo/Pt) multilayers is studied. By assuming a Gaussian distribution of relaxation times for magnetic aftereffect, the non-Arrhenius behavior of the magnetization decay is found in almost all samples. From a fitting of experimental data one can determine the mean value of relaxation time /spl tau//sub 0/ and also its width /spl Delta//spl tau//sub 0/. The multilayers of TbFeCo/Pt show a high stability compared to single layers of TbFeCo with also a small activation volume.

Thermal stability dependence on Tb content in TbFeCo recording media

Thermal stability of magnetic domain size in magneto-optical recording media TbFeCo is discussed. Shrinkage in domain size under accelerated conditions using irradiating laser light is found more significant for rare earth (RE) rich disk than transition metal (TM) rich disk. The reason for this difference is because the coercivity Hc of the RE-rich type is higher than the TM-rich type at the time of laser irradiation, which is effective against a wall field for minimizing domain size shrinkage.

Thermal stability in magneto-optical recording media : Analysis of magnetization decay

Thermal stability of amorphous TbFeCo monolayers and {TbFeCo/Pt} multilayers is studied by magnetic viscosity measurements M(t). Using, isothermal remanence measurements the relationship between the magnetization reversal process and its time dependence has been investigated. The non-linear evolution of magnetization with ln(time) happens when the mechanism responsible for magnetization reversal process is mainly domain nucleation. On the other hand domain wall motion process induces a linear M(t) behavior. For multilayers with very small Pt thickness (samples with high square hysteresis loop), magnetization time decay can be described easily by a single energy barrier EB model. As Pt thickness increases, distribution of EB becomes wider leading to almost linear magnetization decay with ln(time) as in longitudinal recording media. The activation volume is determined for these media and shows a strong correlation with reversing field, thus yielding variations of stability over a written bit.

Thermal Stability in Magnetic and Magneto-Optical Recording Media

First, a brief review of thermal decay of magnetization is given. Discussions on activation volume are presented in detail for longitudinal recording media. It is noted that the activation volume estimated based on waiting time methods is found to decrease with increasing reversing field for all the media under consideration. This result suggests that an activation volume in a written bit changes from place to place, depending upon on stray field due to a bit-transition. The recording noise is closely related to activation volume as well. Thermal stability of amorphous TbFeCo mono-layers and {TbFeCo/Pt} multilayers is studied by magnetic viscosity measurements M(t). Using, isothermal remanence measurements the relationship between the magnetization reversal process and its time dependence is investigated. The non-linear evolution of magnetization with In(time) takes place when the mechanism responsible for magnetization reversal process is mainly domain nucleation. On the other hand domain wall motion process induces a linear M(t) behavior. For multilayers with very small Pt thickness (samples with high square hysteresis loop), magnetization time decay can be described by a single energy barrier E B model. As Pt thickness increases, the distribution of E B becomes wider leading to almost linear magnetization decay with In(time) as in longitudinal recording media. Finally, the life time of written domains is discussed in terms of domain size. It is concluded that the difference between a pinning field and wall-field is a decisive factor for governing a life time.