Margin degradation in the long-term testing of 3 µm bubble devices

Considerable degradation of the bias threshold ranging from 0.2 to 0.6 Oe/decade in its collapse side was found in the long-term testing of 3 μm bubble 80 kbit memory chips using (YEuYbCa)
3
(FeGe)
5
O
12
films. Investigating this problem in all its aspects, it was concluded that this margin degradation was caused by the fluctuation of the bias threshold, which closely corresponded to the fluctuation in the collapse field of bubbles statically trapped under the permalloy bars. To clarify the cause of this fluctuation, the permalloy film, the garnet film ion-implanted layer etc. were investigated, and it was found that this fluctuation depended on the garnet film thickness, the garnet film composition and on the arrangement of the surrounding permalloy patterns. The instability of the in-plane domains in the ion-implanted layers was proposed as a possible explanation of this fluctuation. Satisfactory results were obtained in long-term operation of the 80 kbit chips fabricated using the (YEuYbCa)
3
(FeGe)
5
O
12
and (YSmLuCa)
3
(FeGe)
5
O
12
films with the increased thickness of 3.4 to 3.6 μm, and it was found that the (YSmLuCa)
3
(FeGe)
5
O
12
films exhibited better characteristics compared to (YEuYbCa)
3
(FeGe)
5
O
12
films.