Improved Thermal Stability of Gmr Spin Valve Films
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ABSTRACT In order to improve the thermal stability of magnetic multilayer "spin valve" structures, we have measured the magnetic and magnetoresistive properties of a number of samples with the general structure of NiO/Co/Cu/Co/Cu/Co/NiO as a function of annealing time at 250 'C. The magnetoresistance (MR) of the samples annealed in air decreases proportionally to the square root of the annealing time. For samples annealed in a vacuum, the decrease in magnetoresistance is reduced, but not eliminated. Magnetometry of a vacuum annealed NiO/Co/NiO sample shows a magnetization reduction and a coercivity increase which suggest oxidation of the NiO-biased "outer" Co layers of the spin valve structure. For increasing NiObiased Co layer thickness, we show enhanced thermal stability and even increasing MR with
annealing time for samples with the thickest outer Co layers. INTRODUCTION Spin valves 1-3, which are based on the giant magnetoresistance effect, are rapidly finding applications in devices such as magnetic field sensors and read heads for high density magnetic recording. In recent work 4, symmetric spin valves 5"6 of the general structure NiOfCo/CufCo/Cu/Co/NiO were optimized for maximum MR as a function of the thicknesses of the Co and Cu layers. As-deposited MR values exceeding 21% in fields less than 10 mT (100 G) were obtained. In processing these films into devices, there is often a baking step where the temperature is raised to 240 TC for periods up to 24 hours. Therefore, it is important not only that the spin valve films should be able to endure the processing, but also that the films should be optimized to give the best performance after the anneal. The basic symmetric spin valve structure is shown in fig. 1. The structure consists of NiO three Co layers separated by two Cu layers, with Co an
underlayer
and
an
overlayer
of
antiferromagnetic NiO. Through an interfacial
interaction between NiO and Co, the outer Co layers have a large coercivity, Hcouter, typically on the order of 20-50 mT (200-500 G) and a
smaller exchange bias field, which shifts the hysteresis loop of the outer films by 6-8 mT.
The center Co layer has a much lower coercivity,
Cu
........ .................
Co Cu Co NiO Fig. 1. Schematic structure of NiO-biased symmetric spin valve.
Hccenter. The sign and magnitude of the coupling
between the Co layers through the Cu layers is characterized by a coupling field, Hexcenter, and depends sensitively on the Cu layer thickness 5, which is chosen to produce the minimum coupling between the Co layers. The MR response of a spin valve with the field and the current parallel and in the plane
of the film is shown in fig. 2. The resistance of the film depends on the relative orientations of the magnetizations in the Co films, with the lowest resistance obtained when the magnetizations are parallel. In a negative saturating field, the magnetizations of the layers are 397
Mat. Res. Soc. Symp. Proc. Vol. 384 0 1995 Materials Research Society
aligned in the field direction and the electrical
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