GMR requires that an antiparallel configuration of the magnetizations
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GMR requires that an antiparallel configuration of the magnetizations in the multilayers can be switched into parallel by applying a magnetic field, but AF interlayer exchange is not the only way to obtain an antiparallel configuration. GMR effects can also be obtained with multilayers combining hard and soft magnetic layers (Barnas et al. 1990; Shinjo and Yamamoto 1990; Dupas et al. 1990). As the switching of the magnetizations of the hard and soft magnetic layers occurs at different fields, there is a field range in which they are antiparallel and the resistance is higher, as illustrated in fig. 5 and fig. 6.
The best known structure in which interlayer exchange is not used to obtain an AF configuration and GMR, is the spin valve structure, introduced by Dieny et al. (1991a). A spin valve structure, in its simplest form shown in fig. 7, consists of a magnetically soft layer separated by a nonmagnetic layer from a second magnetic layer, which has its magnetization pinned by an exchange biasing interaction with an antiferromagnetic (FeMn) or ferrimagnetic layer. The nonmagnetic layer is thick enough to minimize the magnetic coupling between the ferromagnetic layers. The operation of the spin valve can be understood from the magnetization and magnetoresistance curves in fig. 8. One of the permalloy layers has its magnetization pinned by the FeMn in the negative direction. When the magnetic field is increased from negative to positive values, the magnetization of the free layer reverses in a small field range close to H = 0, whereas the magnetization of the pinned layer remains fixed in the negative direction. Consequently, the resistance increases steeply in this small field range. Only by applying a large positive field can the exchange biasing interaction be overcome; then the pinned permalloy layer switches too and the resistance comes back to its initial value. The steep slope of the resistance variation in a small positive field is now used for many low field applications (sensors, read heads, MRAM).
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Several types of spin valve structures have been developed and figSeveral types of spin valve structures have been developed and fig. 7 represents what is called a simple exchange-biased spin valve. Copper is generally used for the nonmagnetic layer and the ferromagnetic layers can be layers of permalloy (Ni80Fe20), cobalt or NiFeCo alloys. An ultrathin layer of Co can be inserted between permalloy and Cu to enhance the GMR ratio (Parkin et al. 1993). The pinning layer can be an antiferromagnetic compound such as FeMn, a ferrimagnetic alloy of Tb and Co (Freitas et al. 1994a, 1994b) or an antiferromagnetic oxide such as NiO (Anthony et al. 1994; Swagten et al. 1996; Egelhoff et al. 1997b). The highest GMR ratios have been obtained with magnetic layers of Co and NiO for the pinning layer. The advantage of NiO comes from the increased specularity of the reflections of electrons at the metal/oxide interface. A GMR ratio of 15% at room temperature has been obtained by Swagten et al. (1996) with a NiO/Co/Cu/Co/Cu/NiO structure in which the bottom NiO layer is the pinning layer while a NiO over-layer is introduced to improve the specularity of reflections at the top surface. Recently there have been made efforts to further improve the specularity of the reflections and enhance GMR.
For example, as shown by Egelhoff et al. (1997a), growth in the presence of a partial pressure of 02 (acting as a surfactant) creates surfaces and interfaces that scatter the electronsmore specularly, and also reduce the ferromagnetic bridging across the copper layer. In this way GMR ratios of 19% at room temperature have been obtained (Egelhoffet al. 1997b), For applications at a very low field, spin valves with permalloy give the best resuits. In fig. 9, we show an example of GMR curves for NisoFe20/Cu/NisoFe2o/FesoMnsospin valves prepared by sputtering in a magnetic field so as to induce a uniaxial magnetic anisotropy. When the anisotropy directions of the two permalloy layers are parallel (fig. 9(a)), the field sensitivity, defined as (1/R)(dR/dH) at the steepest point of the GMR curve, reaches 33 %/(A/m); however a drawback for applications is a relatively large hysteresis (0.3 kA/m). In fig. 9(b) with crossed anisotropies obtained by rotating the magnetic field before sputtering the second permalloy layer, the hysteresis is negligible and the sensitivity is still 8%/(kA/m). Curves (c) and (d) are for samples that have been annealed in an applied field, the hysteresis is small and the sensitivity is 18%/(kA/m).
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Add:Unit H, 4F Rihua Mansion, No. 8 Xinfeng 2nd road, Torch Hi-Tech Zone, Xiamen, China.
Tel:0086-592-5781916
Fax:0086-592-5123653
E-mail:info@china-magnet.net
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