MAGNETORESISTIVE SENSOR WITH IMPROVED ANTIFERROMAGNETIC FILM

Abstract: An improved thin film magnetoresistive (MR) sensor uses an alloy comprising Fe, Mn and Cr as an antiferromagnetic layer to provide a longitudinal exchange bias in the ferromagnetic MR layer. Sufficient exchange biasing is provided and the FeMnCr layer exhibits excellent corrosion resistance.

Description: This invention relates to magnetoresistive (MR) read sensors, and in particular to a MR read sensor in which an improved antiferromagnetic film provides a longitudinal bias field in the ferromagnetic MR film of the sensor. A general description of the principle of operation of MR sensors in magnetic recording systems is provided by Tsang in "Magnetics of Small Magnetoresistive Sensors", J.Appl.Phys. , Vol. 55(6), 15 March 1984, pp. 2226-2231. A MR sensor for reading information signals from a magnetic recording medium is described in US-A-4,103,315 disclosing the features of the preamble of claim 1. The patent describes a MR read sensor which utilizes antiferromagneticferromagnetic exchange coupling to produce a uniform longitudinal bias in the MR layer of the sensor. The exchange coupling between the antiferromagnetic and ferromagnetic layers creates a single domain state in the ferromagnetic layer and thereby suppresses the so-called Barkhausen noise associated with domain activity. The materials suggested by the '315 patent are nickel-iron (NiFe) as the ferromagnetic MR layer and a face-centered-cubic (FCC) phase (gamma phase) of manganese (Mn) alloy as the antiferromagnetic layer. The patent also suggests that FeMn alloys with cobalt (Co), copper (Cu), germanium (Ge), nickel (Ni) and rhodium (Rh), may produce a stable gamma phase Mn alloy when deposited on NiFe, but indicates that chromium-manganese (CrMn) and manganese-palladium (MnPd) alloys failed to produce a stable gamma phase. Of the possible gamma Mn alloys, iron-manganese (FeMn) appears to exhibit the greatest ability to exchange couple with the NiFe layer. The strength of the exchange bias field for exchange coupled films of sputter deposited NiFe/FeMn and FeMn/NiFe has been studied by Tsang, et al. in "Exchange Induced Unidirectional Anisotropy at FeMn-Ni₈₀Fe₂₀ Interfaces", J.Appl.Phys. Vol. 52 (3), March 1981, pp. 2471-2473. One of the problems with the use of FeMn as the antiferromagnetic layer is its susceptibility to corrosion. The material is exposed to corrosive environments during the thin film fabrication processes and during operation of the magnetic recording system. Thus it is desirable to improve the MR sensor by utilising an antiferromagnetic material which provides corrosion resistance properties comparable to those of NiFe and sufficient exchange coupling with the NiFe MR layer. Accordingly the present invention provides a magnetoresistive sensor of the type having a layer of antiferromagnetic material in direct contact with a magnetoresistive layer of ferromagnetic material for inducing a longitudinal exchange bias field in the magnetoresistive layer, the antiferromagnetic layer being made of an alloy comprising manganese and iron, characterised in that: the antiferromagnetic material also comprises chromium in an amount greater than approximately 3 atomic percent and less than approximately 12 atomic percent. The improved antiferromagnetic layer composition produces both a sufficient exchange bias and a corrosion resistance comparable to that of the NiFe layer. For a fuller understanding of the present invention reference should be made to the following detailed description taken in conjunction with the accompanying drawings. Fig. 1 is a schematic representation of a prior art thin film MR sensor; Fig. 2 is a graph of loop shift or exchange bias (H UA ) in a NiFe layer as a function of the percentage solute for various solutes present in a FeMn based alloy antiferromagnetic layer; and Fig. 3 is a graph of corrosion rate as a function of percentage solute for various solutes in the FeMn alloy. As illustrated in Fig. 1, a typical thin film MR sensor comprises a substrate 10, a transverse bias layer 12, a spacer layer 14, a MR layer 16 and an antiferromagnetic layer 18. The MR layer 16 which is formed of ferromagnetic material such as Ni₈₀Fe₂₀, is attached to electrical conductors 20 and provides an output current. The output current from MR layer 16 is a signal which enables a separate sensing circuit to determine resistance changes in the MR layer. The resistance changes are a function of changes in the magnetic fields intercepted by MR layer 16 from recorded data on the magnetic storage medium. In order to assure that MR layer 16 has uniaxial anisotropy an antiferromagnetic layer is formed on MR layer 16. The antiferromagnetic layer, which in the prior art may be the gamma phase of Mn alloys, creates an interface exchange coupling with the ferromagnetic MR layer 16. This results in a longitudinal exchange bias field (H UA ) in MR layer 16 and creates a single magnetic domain state in MR layer 16. The existence of a single magnetic domain state in MR layer 16 is essential to suppress Barkhausen noise which is associated with MR materials which exhibit multiple magnetic domain states. The transverse bias layer 12 provides a magnetic field oriented generally perpendicular to the medium so as to slightly bias the magnetic field in MR layer 16 in a direction non-parallel to the medium. This transverse bias maintains the MR layer 16 in a linear response mode such that the current output is essentially a linear function of the resistance changes. As is known in the art, the transverse bias can be provided by shunt biasing, soft film biasing, or permanent magnet biasing. A more comprehensive description of the MR sensor of Fig. 1 and alternative embodiments of it are described in the US-A-4103315. In order to provide an improved antiferromagnetic layer in the MR sensor various ternary FeMn based alloys were prepared as thin film structures by RF sputter deposition in a uniform magnetic film of 100 Gauss. The power density, system pressure and substrate temperature were fixed at 2.6 W/cm², 3,33 Pa (25 millitorr) and 50°C, respectively. The base pressure of the system was approximately 6,6 x 10⁻⁷ Pa (5.10⁻⁹ Torr). The substrates were semiconductor grade single crystal silicon, glass or quartz which were first solvent cleaned and then cleaned by exposure to a glow discharge prior to deposition. Films of nickel-iron (Ni₈₁Fe₁₉) were deposited on the substrates at a rate of approximately 1 Angstrom per second. The thickness of the NiFe films varied from approximately 20 nm (200 Å) to 60 nm (600 Å). A bias voltage of -50 volts was used to reduce the residual gas contamination. Ternary alloys with FeMn were then deposited on the NiFe layers by cosputtering a third target and an Fe₅₀Mn₅₀ target. The sample materials prepared in accordance with this process were then tested to measure the exchange bias (H UA ) in the NiFe MR layer and the corrosion resistance of the antiferromagnetic layer. The exchange bias field of the NiFe MR layer is illustrated in Fig. 2 as a function of various percentages of the ternary additives of Rh, Ni, Al and Cr to FeMn in the antiferromagnetic layer. The H UA value for Fe₅₀Mn₅₀, normalized to the conditions of Fig. 2, is approximately 1920 A/m (24 Oe). As shown in Fig. 2, Rh has the least effect on reducing the exchange bias. The data shown in Fig. 2 indicates that all of the solutes are capable of use in a FeMn-based ternary alloy antiferromagnetic layer, provided the atomic percent (at.%) concentration of the ternary additive is relatively small. In order to determine the corrosion resistance of the various ternary alloy films, the films were exposed to an atmosphere of SO₂, NO₂, H₂S and HCl at 70% relative humidity and 28°C for 100 hours. The corrosion rates, which are given as the amount of weight gain of the films per surface area, for NiFe, FeMn, and the various FeMn-based ternary alloy films are shown in Fig. 3. The corrosion rate of Ni₈₁Fe₁₉ is about 11 micrograms/cm² and Fe₅₀Mn₅₀ about 37 micrograms/cm². The addition of Cr to Fe₅₀Mn₅₀ decreases the corrosion rate of Fe₅₀Mn₅₀ from about 37 micrograms/cm² (for 3 at.% Cr) to about 15 micrograms/cm² (for 10 at.% Cr). The addition of approximately 10 at.% Ni decreases the corrosion rate to a level below that of FeMn but significantly higher than that achieved with 10 at.% Cr. As shown by Fig. 3, the addition of Rh or Al to FeMn had little or no effect on the corrosion rate. Since it is known that Rh additions to NiFe cause a significant decrease in the corrosion rate, the negligible effect of Rh on the corrosion rate of FeMn is somewhat surprising. Similarly, Al was initially a likely candidate for increasing the corrosion resistance of FeMn because of its use in passivating steel. However, the FeMnAl alloy actually had little effect on the atmospheric corrosion rate of FeMn. Based upon the experimental results obtained, as shown in Figs. 2 and 3, an antiferromagnetic layer of FeMnCr, with Cr present in an amount greater than approximately 3 at.% and less than approximately 12 at.%, results in a MR sensor with sufficient exchange bias in the MR layer and with a significantly improved corrosion resistance, a corrosion resistance which approaches that of the NiFe MR layer. While it has been previously suggested in US-A-4 103 315 that a CrMn alloy would not result in a gamma phase Mn alloy, the results of Fig. 2 indicate that with a Cr concentration of less than approximately 12 at.% in a (Fe₅₀Mn₅₀)Cr antiferromagnetic film, there is sufficient exchange coupling, indicating that the FeMnCr alloy is in the gamma phase. With the discovery that small additions of Cr to FeMn result in a corrosion resistant antiferromagnetic layer which is capable of inducing a longitudinal bias in a NiFe layer, it is possible to conclude that yttrium (Y), titanium (Ti), or silicon (Si), either as ternary or quaternary additions to FeMn or as a quaternary addition to FeMnCr, may also perform as well as Cr because all of these elements are known to produce stable passivating oxides.

Claims: A magnetoresistive sensor of the type having a layer of antiferromagnetic material (18) in direct contact with a magnetoresistive layer of ferromagnetic material (16) for inducing a longitudinal exchange bias field (H ua ) in the magnetoresistive layer (16), the antiferromagnetic layer being made of an alloy comprising manganese and iron, characterised in that: the antiferromagnetic material (18) also comprises chromium in an amount greater than approximately 3 atomic percent and less than approximately 12 atomic percent. A sensor as claimed in claim 1 further comprising means (12) for producing a transverse bias field in said magnetoresistive layer (16). A sensor as claimed in claim 1 or claim 2, wherein the magnetoresistive layer (16) is an alloy comprising nickel and iron.

Inventors: HOWARD, JAMES KENT
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1988
Main classification: G11B5
IPC codes: G11B5, H01L43, G01R33