Ta Capping Research Articles

Time and temperature dependence of high thermal stability in NiO/Co/Cu/Co/M spin valves

The thermal stability of Cu/Co 'bottom' spin valve structures deposited on NiO has been assessed as a function of temperature, time, and cap layer compositions including Au, Ta and Ta/sub x/O/sub y/. Samples with 5.0 nm thick cap layers of Au showed a reduction in giant magnetoresistance (GMR) from 9.3% to 3.6% after a 0.5 hr anneals at 250/spl deg/C. As a function of time, the sample resistance, R(t), showed an exponential saturation with a time constant, /spl tau/, which follows an Arrhenius law, /spl tau/=/spl tau//sub 0/ exp[/spl Delta/E/kT], with /spl Delta/E=1.29/spl plusmn/0.03 eV and /spl tau//sub 0/ on the order of 10/sup -8/ s to 10/sup -9/ s. These parameters, which fit R(t), correlate with the time and anneal temperature dependence of GMR decay in the Au-capped spin valves. In contrast, samples with Ta or Ta/sub x/O/sub y/ cap layers showed good stability for 0.5 hr anneals up to 325/spl deg/C. Decreases in GMR from 12.7% to 3.9% following a 20 h anneal at 325/spl deg/C are accompanied by significant changes in the hysteresis loops which appear to be associated with the pinned layer. With a 5.0 nm Ta cap, R(t) follows a logarithmic-like time dependence characteristic of a distribution of activation energies.

Microstructural study of ion-beam deposited giant magnetoresistive spin valves

Detailed microstructural investigation of ion beam deposited giant magnetoresistance (GMR) spin valves has been carried out using various techniques of transmission electron microscopy (TEM) and x-ray diffraction. Two fcc phases, i.e., FeMn and NiFe/Co/Cu/Co/NiFe have been identified in ion-beam deposited Ta (50 Å)/FeMn (80 Å)/NiFe(30 Å)/Co(15 Å)/Cu(33 Å)/Co(15 Å)/NiFe(60 Å)/Ta(25 Å)/Si(001) spin valves. The Ta buffer layer is amorphous, while the 50-Å-thick Ta cap layer consists of a 25-Å-thick amorphous layer and on top of which a Ta oxide layer. The lattice constants of the fcc FeMn and the fcc NiFe/Co/Cu/Co/NiFe increase with the ion-beam voltage. Both the FeMn and the NiFe/Co/Cu/Co/NiFe layers are (111) textured. The misfit strain between the FeMn layer and the pinned NiFe layer is released by the formation of dome shape FeMn surface rather then by the formation of misfit dislocations at the interface between the two layers. The peak to valley height of the domes seems to have little effect on the GMR properties of the ion-beam deposited spin valves. It was found that large columnar grain width gives large exchanged field and ΔR/R of the spin valves.

Effect of corrosion on magnetic properties for FeMn and NiO spin valves

Spin-valve multilayer structures are promising candidates for next generation read heads in high-density magnetic recording applications. Two commonly used antiferromagnetic layers that exhibit effective pinning for spin valves are FeMn and NiO. Previously, the only corrosion studied of the two materials with respect to magnetic recording are concerned with immersing thin films into acidic and basic solutions. In a realistic corrosion study of spin valves constructed with these pinning materials, we find a high humidity environment is catastrophic to the FeMn spin valves, even with protective Ta capping layers. Scanning electron microscopy along with magnetic measurements on edge protected samples gives convincing evidence that edge effects seem to dominate. Protection of the edges with an epoxy film merely slows down the degradation. In contrast, NiO spin valves show remarkable resistance to corrosion. While magnetoresistance ratio decreases for FeMn spin valves with increased corrosion, coercivity remains relatively constant and pinning field actually increases for both material systems.

Effect of annealing on the interfaces of giant-magnetoresistance spin-valve structures

Si/Ta/NiFe/Cu/NiFe/FeMn/Ta spin-valve layered structures were analyzed by x-ray reflectivity, before and after annealing at 240, 320, and 360 °C. Specularly reflected x-ray data were collected using a high-resolution reflectometer and were analyzed by least-squares refinement. The thicknesses of the individual layers in the NiFe/Cu/NiFe/FeMn magnetic sandwich remained essentially unchanged. With the exception of the FeMn/Ta interface, the widths of the buried interfaces increased rapidly with annealing temperature. The increase in widths at each of the NiFe/Cu and Cu/NiFe interfaces from 6.8 to 22.6 Å caused a fivefold increase in the magnetically inactive layer in NiFe and a tenfold decrease in magnetoresistance. An increase in the total film thickness with annealing temperature was found to be caused by the surface oxidation of the Ta capping layer and the growth of a Ta silicide layer between the Ta buffer layer and the Si substrate.

Effect of surface composition observed by Auger electron spectroscopy on magnetization and magnetostriction of NiFe and NiFeRh thin films

The saturation magnetization, 4πMs, and the magnetostriction constant, λs, of Ni81Fe19, Ni81Fe19/Ta, and Ni72Fe17Rh11/Ta thin films were studied as a function of film thickness before and after annealing at 250 °C in vacuum for 2 h. For films of thickness t<200 Å, 4πMs and λs were found to be strongly dependent on film thickness with an even larger variation after annealing. For all films, at t=60Å, 4πMs decreases by 20%–30% compared to bulk value. The value of λs is in the range of 10−6 for all films and shows a complex behavior, including a change of sign, depending on film composition as well as Ta capping. Auger depth profiling of uncapped films has shown the presence of an Fe-rich surface oxide layer whose composition changed and thickness increased after annealing. The composition and thickness of the Ta-film interface layer in the Ta-capped films did not change with annealing. The presence of such layers, at the film surface, Ta-film interface, and film-substrate interface, of different composition and magnetic properties from the film bulk, can explain the observed behavior of 4πMs and λs.