Exchange Coupled CoPd/TbCo Magneto-Optic Storage Films

Abstract

Films of CoPd with weak perpendicular anisotropy have been shown to exchange couple to square loop TbCo films on both the Tb and Co sides of compensation. The exchange is sensitive to reactive impurities at the interface and is broken under conditions that produce as little as one monolayer of paramagnetic compound. Even when the coupling at the interface is strong, we show that only a limited thickness of the CoPd layer is spin oriented perpendicular. Many materials with high magnetooptic rotation are not suitable as a storage medium because they do not have perpendicular anisotropy, high coercivity and good square loop properties. By exchange coupling such a low anisotropy but high MO rotation material, the read-out layer (RO), to an appropriate storage layer (ST) it may be possible to obtain stable storage bits in the exchange coupled system with improved carrier to noise ratios (CNR). It has been shown that in the system Nd(FeCo)/Tb(FeCo) that exchange coupling can produce films with high rotation and good square loop properties¹. Furthermore, this system has been shown to follow the model of Kobayashi et al² with respect to the thickness dependence of the switching field¹. One of the assumptions of the model is that there is a wall at the interface between the layers when any subnetwork moments are opposing across the interface. This wall energy tends to prevent the subnetworks from aligning in opposition. The interlayer wall energy, σ<inf>w</inf>can be expected to depend on the exchange stiffness (A) and the anisotropy energy (K<inf>u</inf>) as for an ordinary Bloch wall, σ<inf>w</inf> = AK<inf>u</inf>. In the CoPd/TbCo system, the wall energy is expected to be lower in the CoPd layer because of its relatively low anisotropy energy. The anisotropy depends somewhat on deposition conditions and microstructure as with many Co alloys^3,4. Since the wall energy is lower in the CoPd layer we can expect the wall to form in this layer. Also the wall width is given by δ= A/K<inf>u</inf> so we can expect the wall to be wider in the CoPd than in the previously studied high anisotropy RETM alloys. CoPd is of interest for short wavelength MO applications because it has been shown to have a broad peak in Kerr rotation at 450 nn⁵. In many respects, the CoPd/TbCo system can be considered as a model for the study of exchange coupling. Because of its low anisotropy it is similar to the more technologically interesting system of PtMnSb/ThFe⁶. The CoPd/TbCo system however is easily prepared in a three source evaporator by sequential deposition of the binary alloys onto room temperature substrates. We have studied CoPd films coupled to TbCo films on both sides of compensation. We have also studied the effects of deposition sequence on the strength of the coupling. © 1989 IEEE