Patent GB-1533460-A (GB)
Title: Magnetoresistive head assembly for sensing magnetically recorded information
IPC: G01R33/09
Filed: 1976-08-25
Abstract: 1533460 Magnetoresistive heads INTERNATIONAL BUSINESS MACHINES CORP 25 Aug 1976 [8 Sept 1975] 35294/76 Heading G5R [Also in Division H3] A magnetoresistive head assembly comprises a plurality of magnetoresistive (MR) elements 34, 36 to read information from a magnetic record, such as a tape, disc or magnetic bubble structure, and providing signals indicative of the resistance of each element, a thermal drift compensation element 52 to provide a reference signal indicative of its resistance, or common bias and signal return path conductor 38 providing one input to each of the elements 34, 36, 52, and electrical conductors 40, 42, 44 for supplying a bias current to elements 34, 36, 52 and for carrying the information and reference signals, so that difference signals can be derived by differentially sensing the information signals and the reference signal. Temperature sensitive resistance material, particularly magneto resistive material, is used for the compensation elements. In an embodiment having a plurality of pairs of MR read elements 34, 36, 58, 60, and MR compensation elements 52, 74 each associated with a respective pair of read elements, one A.C. bias generator 55 biased half of the read elements and their associated compensation elements via buffer amplifier 48, 50, 56 and a second A.C. bias generator 65 biases the remaining elements via buffer amplifiers 66, 67, 68. The outputs of generators 55 and 65 are 180 degrees out of phase so that the bias current through lead 38 is substantially zero. Differential amplifiers 44, 46, 70, 72 are each connected to a respective sense element and the associated compensation element and act by common mode rejection to eliminate the bias current (without use of a low pass filter as provided in the prior art-Fig. 1, not shown) and noise produced by thermal and other fluctuations from the amplified data signals. The assembly of elements 34, 36, 58, 60, 52, 74 and their connecting leads may be produced by thin film techniques, Figs. 5A, 5B (not shown), with each pair of MR sensing elements being together formed by a respective centre-topped MR stripe (73), (75), and with each MR compensation element-lying parallel, Fig. 5A, or perpendicular, Fig. 5B, to the associated sensing elements. Each of the MR sensing and compensation elements may have a permalloy MR layer which is magnetically biased due to the bias current and the provision of a superimposed shunt bias layer. The bias layers may be of titanium dioxide, possibly also with a layer of MR material or of magnetically soft permalloy. An alternative embodiment, Fig. 7 (not shown), has N MR sensing elements, with even numbered sensing elements associated with a first MR compensation element (112) and a first A.C. bias generator (111), and odd numbered sensing elements associated with a second MR compensation element (100) and a second A.C. bias generator (109) which is out of phase with generator (111) to provide zero bias current in a common return lead (122). A further embodiment has only one bias generator (80), Fig. 6 (not shown), and one MR compensation element associated with all the MR sensing elements. In these two embodiments the or each MR compensation element is aligned with the row of MR sensing elements and located at one end, or a respective end, of the row, Fig. 8 (not shown).