Digital magnetoresistive sensor based on the giant magnetoresistance effect

摘要

A magnetoresistive (MR) sensor based on the giant magnetoresistance (GMR) effect provides a digital output signal. The multilayer stack of alternating ferromagnetic layers and nonferromagnetic metal spacer layers in the GMR sensor has an essentially single crystalline structure so that each of the ferromagnetic layers exhibits uniaxial magnetic anisotropy, i. e. the magnetic moments of the ferromagnetic layers can lie only parallel or antiparallel to a single axis. Unlike GMR multilayers where all of the magnetic moments are affected simultaneously by the external magnetic field, in the present GMR sensor each ferromagnetic layer has its magnetic moment responsive to an external magnetic field strength that is different from the magnetic field strengths at which the magnetic moments of the other ferromagnetic layers are responsive. This allows each ferromagnetic layer to switch its magnetization direction from parallel to antiparallel, or vice versa, independently of the other ferromagnetic layers. This unique property of each ferromagnetic layer is accomplished by either selecting each ferromagnetic layer to have a different uniaxial magnetic anisotropy energy, such as by varying the strain during crystalline growth, or by subjecting each ferromagnetic layer to a different value of antiferromagnetic exchange coupling energy, such as by varying the thicknesses of the nonferromagnetic metal spacer layers. As a result, the resistance of the GMR sensor changes in stepped increments as the external magnetic field is varied, thereby providing a digital signal output. The digital GMR sensor can be used as a read head in a multiple data layer magnetic recording data storage system. In one disk drive embodiment, the magnetic recording disk has two magnetically isolated and decoupled magnetic data layers. The digital GMR sensor reads the written data bits from both data layers simultaneously, and it's digital output signal is then decoded by conventional logic circuitry to provide the separate data recorded in each of the data layers.