6-7-35Kitashinagawa, Shinagawaku, Tokyo. 141 Japan Inaoduction Direct ovenvritability and higher density am desiderata for the next generation of magnetooptical disks. Overwriting by light intensity modulation, which is possible using an exchangecoupled magnetic multilayer disk, is more suitable for modulation at a high frequency than overwriting by magnetic field modulation. In fact, light intensity modulation is the only way to record on a double-sided disk and to use a multibeam optical pickup. Magnetically induced Super Resolution (MSR) using a magnetic multilayer disk is a very promising technique for increasing recording density, because the current optics can be used. This paper reports multilayer disks capable of ovewiting and super resolution.
Direct Overwriting bv Lieht Intensitv Modulation using Mametic Triule LaversDirect overwriting by light intensity modulation was first reported by using magnetic double-layer film'. We adopted magnetic triple-layer film to improve overwriting characteristics'. Practically, it is necessary that the disk can be overwritten with a low error rate and with a wide power margin for the high and low power levels. coating the aluminum layer was found to be effective in expanding the power -gin. Figure ](a) and (b) show the overwritable region of high and low power for magnetic triple-layer disks without and with aluminum coating, respectivel;. A short light pulse width for the high powcr was used to achieve a linear recording density of 0.8 pm/ bit, which is higher than the current density of 1.0 Cun/bit. A wide power margin of E30 96 was obtained for the low power by using the aluminum coated disk and the short pulse. Moreover, 3Tand 8T pattern by (2.7) Codc mark-position wording with a linear density of 0.8 w i t was overwritten at a low e m rate on a triple-layer disk with a track pitch of 1.4 pm, which is narrower than 1.6 pm for an IS0 MO disk'. The high linear density of 0.8 w i t and m w hack pitch of 1.4 pm will permit a double-density overwritable disk when used with zoning technique.Ma~neticallv Induced Suoer Resolution using Mametic Ouadri Lavers Two detection methods, Front Aperture Detection(FAD) and Rear Aperture Detection(RAD), have been reponed for MSR using magnetic multilayer disks6.T'he cut-off spatial frequency for both methods is more than t w i c e that in conventional detection. Super resolution was analyzed using RAD and a double-mask structure in the readout light spot was found'. Figure 2 shows spin states of the magnetic quadri-layer disk for MSR by RAD with double masks. The aperture is created between the down-spin mask and the up-spin mask. High resolution and a high UN of 49 dE3 at a mark length of 0.4 pm arc made possible by the narrow apenure generated by double masks. A high density of 0.3 pm/bit was achieved in the quadri-layer disk by RAD with a byte error rate less than IO4 by using mark-length rccordhg'. (a) 1 ' q ,4,, r , , , & 100 9 0 3 0 4 0 5 0 6 0 7 0 8 0 down-spin mask recorded mark apenure . . disupspin mask 9.0 3.0 4.0 5.0 6.0 ...