EXCHANGE COUPLING AND SWITCHING FIELDS OF RE-TM DOUBLE AND TRIPLE-LAYER STACKS FOR DIRECT OVERWRITE

The recording properties of common single layer magneto-optical (MO) d isks can be improved by introducing exchange coupled rare-earth (RE) t ransition-metal (TM) films, both from the point of view of capacity an d data transfer rate. Direct overwrite (DOW) is a method to double the data transfer rate during writing, because the write process of new b its and the erase process of old bits is performed simultaneously. MO multilayer stacks must exhibit exchange coupling in order to be suitab le for DOW. The switching fields of each layer depend on coercive ener gy, magnetic field energy and the energy of the interface wall between coupled layers. In this paper we discuss the dependence of switching fields on layer thickness and wall energy sigma(W) for several double and triple layer stacks. The memory and reference layers are Tb-FeCo a nd Dy-FeCo, respectively. Triple layer stacks have an intermediate lay er (GdFe and GdFeCo) to adjust the wall energy. DOW presupposes a weak coupling at 300 K and a strong coupling at higher temperatures. This requires a very thick reference layer in double layer stacks, causing excessive heat capacities. However, for triple layers the DOW demands are met in much thinner stacks as the wall energy is adjusted through the intermediate layer. The wall energy of the intermediate layer is d etermined by its anisotropy. We studied the anisotropy of evaporated G d-Fe, Gd-Co and Gd-FeCo films as a function of composition. Gd-Co show ed only in-plane anisotropy, while Gd-Fe was perpendicularly magnetize d for Fe contents up to 87 at%. Adding Co to Gd-Fe leads to a layer wi th a temperature dependent easy axis. The carrier-to-noise ratio (CNR) of a series of magneto-optical disks, both double and triple layer st acks, was determined. The maximum CNR of 51.4 dB is comparable to resu lts on a single layer disk with the same memory layer. On triple layer stacks a CNR > 45 dB could be written at a laser power below 9 mW.