Control of Properties of Magnetic Thin Films for Shifting Registers

Abstract

The operation of a domain wall motion shifting register depends on the fact that the critical threshold field Hs, required to introduce a reverse domain into a saturated, uniaxial, anisotropic, magnetic thin film is greater than the critical threshold field H0, required to move walls of already exiting domains. Thus, an important parameter characterizing the performance of the register is the nucleation factor η defined as η=Hs/H0. This must be large to obtain wide tolerance in the shifting field, H, as H0≤H≤Hs. It must also be large in order to obtain high speed, as wall velocity varies as H − H0. Normally it is difficult to produce films with η>2, but here it is shown how the difficulty can be overcome by the use of thick films (>2000 Å) of high anisotropy (anisotropy field Hk>15 Oe. Experimental results are given for Ni-Fe-Co alloys showing that Hs and Hk both increase with the cobalt concentration, whereas H0 decreases with increasing film thickness. Hence the two fields Hs and H0 can be controlled independently, and in this way values of η of up to 12 have been obtained. The ratio of nickel to iron is adjusted for zero magnetostriction. Measurements were made by observation by the Kerr effect of domains in an actual shifting register. There is a tendency for domains to nucleate at the edges of the film when shifting fields are applied, the nuclei growing to represent spurious information. It is shown how this is prevented by gradually reducing the thickness of the film at the edges. When domains are propagated along the film another difficulty is observed, that occasional remanent nuclei from the trailing edge walls remain unshifted, and grow during subsequent shifting cycles, to represent spurious information. It is shown that this can be prevented by applying a biassing field of about 1 Oe which tends to erase the remanent nuclei. Also, when the shifting fields are readjusted to operate with the bias field the effect is further enhanced. However, this field reinforces the demagnetizing field of the domains and so causes them to break up into narrow bars in order to reduce their total field energy. This break up reduces the magnetic flux which can be detected by the output sense conductor. When either a thicker film is used to increase the output signal, or the register is reduced in size to increase the operating speed, the effect of the demagnetizing field is accentuated and the domains become very narrow in the hard direction. This effect can be reduced by using a film with a higher value of H0. Shift registers with a packing density of 40 bits/in., cycle time of 1 μsec, shifting fields of about 14 Oe, and biasing fields of about 1 Oe have been produced.