Ta Capping Layer Research Articles

The effect of Ta on the magnetic thickness of permalloy (Ni81Fe19) films

The effect of Ta and Ta/Cu seed layers, and Ta and Cu cap layers on the effective magnetic thickness of ultrathin permalloy (Ni81Fe19) was investigated for MRAM applications. The films were deposited by Ion Beam Deposition. The magnetic moment of each as-deposited permalloy film was measured using a B-H looper and a SQUID magnetometer. The films were further annealed at either 525 K for 1/2 h or 600 K for 1 h to study the effect of thermally driven interdiffusion on the magnetic moment of the permalloy film. Our theoretical calculations showed that the presence of 12% intermixing at the interface reduces the Ni moments to zero. Experimentally, it was shown that the tantalum rather than the copper interfaces are primarily responsible for the magnetically dead layers. The Ta seed layer interface produces a loss of moment equivalent to a magnetically dead layer of thickness 0.6±0.2 nm. The Ta metal in the cap layer results in a loss of moment equivalent to a dead layer of thickness 1.0±0.2 nm. Upon annealing, thermally driven interdiffusion is concluded to have a strong effect on the Ta(seed)/ Ni81Fe19 as-deposited interface, based on the doubling of the magnetically dead layer to 1.2±0.2 nm. The Ni81Fe19/Ta(cap) as-deposited interface slightly increases its equivalent magnetically dead layer upon annealing to 1.2±0.2 nm. As-deposited interfaces of Ta(seed)/permalloy and permalloy/Ta(cap) are not chemically equivalent and result in different magnetically dead layers, whereas after annealing to 600 K both interfaces attain comparable intermixing and magnetically dead layers. It was also shown that a half-hour anneal at the lower 525 K annealing temperature, which is closer to the actual processing temperature, results in only slight increase of the magnetically dead layer at both interfaces.

Long-term stability of a quasiperiodic Ta/Al multilayer: Disintegration at room temperature analyzed by grazing angle x-ray scattering and photoelectron spectroscopy

A three-component Fibonacci (3CF) Ta/Al multilayer has been reinvestigated by specular and diffuse x-ray reflectivity and x-ray photoelectron spectroscopy (XPS) after 41 months of storage at room temperature. The specular reflectivity shows drastically suppressed diffraction peaks, whose positions and intensities are explained by severe interdiffusion of the Ta/Al bilayers building the 3CF sequence. Nonspecular reflectivity scans still indicate a high degree of interfacial roughness correlation in the growth direction that is attributed to the long spatial Fourier components of the interface profiles, which are substantially less affected by interdiffusion. The angle-resolved XPS spectra show that the Ta capping layer is completely oxidized and interdiffused by Al, whereas below the oxide layer Ta and Al coexist in metallic form in the same film. Both x-ray reflectivity and XPS yield an oxide layer of ∼30 Å thickness. Despite the severe structural disintegration, the multilayer diffraction spectrum can still be indexed by means of the projection theory for quasiperiodic sequences, which points to a remarkable stability of quasiperiodic properties against significant disorder. We conclude that Ta/Al bilayers are apparently unsuitable for multilayer applications due to the lack of thermal stability even at room temperature, with grain boundary diffusion pointed out as a possible disintegration mechanism. The design of improved 3CF Ta/Al multilayers is discussed with regard to applications in x-ray optics.

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.