ELECTRONICS & POWER FEBRUARY 1982 185 Ferromagnetic thin-film magnetoresistive transducers The magnetoresistance (MR) effect has been known for more than 100 years, but only recently has it been developed and exploited in thin ferromagnetic films. These developments have led to the wider application of MR transducers by R.M. Jones, AJ. Collins, M. Husni and U. Akin The magnetoresistance and Hall effects are complementary galvanomagnetic phenomena, their behaviour in semicon- ductors being well documented 1 . Fer- romagnetic thin films of suitable com- position and geometry can have sensitive MR responses which can be used for the detection of magnetic fields, from, for example, magnetically recorded tape. The preparation of such transducers, largely by vacuum evaporation, photolithography and etching techni- gues permits batch fabrication and economical production compared with conventional alternatives (e.g. laminated recording heads). In addition, a major advantage of MR-effect transducers is that they are sensitive to magnetic field H, and not its time derivative dH/dt, as would be the case in conventional inductive rcording heads. The magnitude of the output of MR detectors is therefore independent of the speed of any magnetically recorded medium passing it. This is an important advantage in analogue and digital recording systems, and also where a variable speed is involved, e.g. in hand- held 'wands' for labelling and ticket validation or for use in 'swipe readers' for credit cards and access control. The resolution of MR transducers can be superior to that of conventional in- ductive transducers if additional precau- tions are taken to incorporate suitable magnetic shields 2 . These more complex designs find application in high-density magnetic recording on tapes, discs and drums. MR thin-film detectors are two- terminal devices which experience a change in their resistance 6R when acted upon by a suitably oriented magnetic field. The change in resistance is usually detected as a change in voltage across the detector when it is applied with a constant current / (Fig.l). Two arrangements are generally con- sidered, namely Vertical' or 'horizontal' detectors, depending upon their disposi- tion with respect to a magnetically recorded medium (Fig.l). The vertical and horizontal detectors are sensitive to the vertical and horizontal components, respectively, of the external field arising from the magnetic recording. Generally the MR element is within the thickness range 20 to 100 nm. The vertical ar- rangement is usually preferred where high resolution is required and intimate contact between moving parts is involv- ed. Flux guides may be incorporated in the head design for the purpose of displacing the MR element away from the head to medium interface. The ap- plication of horizontal detectors in close- contact situations has in the past been considered impractical owing to the pro- blems of wear. The response of a detector for vertical, horizontal or any intermediate orienta- tion can be calculated to a first approx- imation using the basic relationships given by Wallace. 3 ' 4 Whereas major applications relate to the detection of ef- fectively point-source fields from magnetically recorded media, MR transducers can be applied to the detec- tion and measurement of magnetic fields in general. The basic MR operating characteristic is indicated in Fig. 2. With no applied field (point 0), the single-domain magnetisation of the MR element will be along the element length owing to the in- fluence of the demagnetisation anisotropy field H d and an additional anisotropy field induced during the ele- ment deposition H k . These anisotropies will generally be additive to give an ef- fective anisotropy field of H Q approx- imately egual toH d + H k . In the presence of a small field applied in the plane of vertical horizontal A thickness recorded medium MR element width length 1 Horizontal and vertical MR transducers 0013-5127/82/020185+ 03 $01 -50/0 ©IEE:1982