the variation of the value of the magnetic field H is stance records where the bits do not exceed a maxi
shown in FIG. 3. When the value of H reaches the value mum width of 5 microns with magnetic domain inter
of the field of anisotropy of the material of the layer, vals not exceeding 15 microns.
Hk, the layer is saturated in the direction of its hard In order to eliminate such a drawback and, on the
magnetization axis. In order to determine the direction 5 other hand, to greatly improve the accuracy of re
of the applied external magnetic field, it is necessary to sponse of a magnetoresistive member according to the
provide a shift of the zero of the ordinate axis from 0 invention, it is further provided, as shown in FIG. 2, to
to 01 and the shift is conventionally ensured by applying arrange the magnetoresistive layer 1 between two high
an additional permanent magnetic field to the layer 1. permeability layers 2 and 3 thicker than the layer 1.
10 Preferably though not imperatively, the layers 2 and 3 A magnetoresistive device according to the present are made of anisotropic character. Each such layer may invention does not require such an additional and trou- for instance have a thickness of at least 1,000 A up to blesome magnetic field in that, as shown in FIG. 5, the 5 /jl or more when the thickness of the layer 1 is beeasy axis of magnetization of the magnetoresistive layer tween 200 and 300 A. Such layers as 2 and 3 are mag1 is inclined at an angle 0 with respect to the direction 15 netostatically coupled to the magnetoresistive layer 1 of the flow of electrical current through the layer. The and insulated therefrom by means of thin dielectric layangle 6 may advantageously be about 45°. The curve of ers or films of a material such as Si 02. Each dielectric variation of the magnetoresistive effect is as shown in film only need be a few hundreds of Angstroms in FIG. 6, When the value of the external field H equals thickness.
the value Hc cos 8, Hc being the coercive field of the 20 More than one such layer as 2 or 3 may be provided
magnetic material of the layer 1, the magnetization in on one side or on both sides of the magnetoresistive the layer is oriented perpendicular to the direction of layer 1. When needed, a stack may be provided by
the current I. When, on the other hand, H = Hc sin 0, placing one more magnetoresistive layer on the sides of
the magnetization of the layer is parallel to the direc- the layers 2 and 3. Layers such as 2 and 3 are estab
tion of the electrical current I. FIG. 12 shows the corre- 25 lished over such additional magnetoresistive layers and
sponding variation of resistance of the magnetoresistive so forth. Such a stack may be formed on a mechanically
device, R, plotted against the variation of the magnetic resistant substrate. The material of such layers as 2 and
field H. When H equals O, the value of the magnetore- 3 may be the same as the material of the layer 1 when
sistive layer 1 is, for instance, Ro. When H = —Hc sin needed.
8, the value of the resistance is +RS with respect to Ro. 30 The layers 2 and 3, which are of high premeability When H = Hc cos 0, the value of the resistance is -Rs due to their increased thickness, act as guiding memwith respect to Ro. It then suffices to take the value Ro bers for the lines of intensity of the magnetic field from as a reference value in any load circuit for the "signal" the source 4 so that the magnetoresistive layer 1 refrom the magnetoresistive device for obtaining both the ceives a substantially uniform magnetic field over its value and the direction of the external magnetic field 35 whole height, the magnetoresistive effect is optimized H to which the magnetoresistive device is subjected. and the localizing of the source 4 occurs with a fair acThe accuracy of the response of magnetoresistive curacy in the device. As the rotation of the magnetizalayer depends not only upon the magnitude of the mag- tion vector is coherent within the layers 2 and 3, the ronetoresistive effect in the layer 1 but also, and more im- tation of the magnetization vector is also coherent and portantly, upon the uniformity of the rotation of the 40 actually constant over the whole height of the layer 1 vector of magnetization in the layer in the direction of when the driving field H is of the same order of magnithe "height" h of the layer, FIG. 2. FIG. 1 shows in dot- tude as the coercive field of the material of the layer 1. ted lines the distribution of the lines of intensity of the It may be said that the high permeability structure of magnetic field generated by the source 4 with respect the device acts as a magnetic field "transformer." The to the layer 1 and it is apparent that the value of the overall breadth of the magnetic structure defines the field is not uniform along the "height." Consequently width of a "window" for localization of the source 4 the rotation of the vector of magnetization will not be with respect to the magnetoresistive device and the unicoherent throughout the layer and the response of the form magnetic flux applied to the layer 1 is maximum magnetoresistive device will be subject to a substantial when the mid-plane of the source coincides with the attenuation. As stated above, the value of the external vertical mid-plane of the device. Further, such a magfield which produces a complete rotation of the mag- netoresistive device short-circuits any demagnetizing netization in the layer is about equal to the value of the fields which may be generated by the magnetoresistive field of anisotropy of the material of the layer when the layer proper.
demagnetizing fields in the h direction are small, When 55 The presence of the high permeability layers further a localized source of magnetic field generates a field of reinforces the action of the angular orientation of the a few hundreds of oersteds, which is quite normal for axis of easy magnetization of the magnetoresistive layer digitally recorded members such as tapes, disks or with respect to the direction of the electrical current, drums, or even such as "magnetir rules," the mag- As shown in FIG. 11, when no external magnetic field netoresistive layer is activated by an isofield line of a 60 is applied to the device, the magnetization vectors of value substantially equal to 3 oersteds for the Fe-Ni the layers 2 and 3 align on the easy magnetization axis alloy of the layer. Such a line is relatively far from the of the magnetoresistive layer 1, one of them being howsource 4 and consequently the localization of the ever of opposite orientation from the two other ones, source is very indefinite. The device could only be used as shown for instance in the layer 3. When the external in a readout transducer if the magnetic record to be 65 field is applied, the magnetization vectors of the layers read is of a low density of digits or marks. On the other 2 and 3 rotate by an angle depending on their thickness hand it is desired to use such a magnetoresistive device though this rotation is coherent throughout their for a readout of high density records such as for in- heights. Because of the magnetostatic coupling existing