H. Lefakis

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 A 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.

CAN SPIN VALVES BE RELIABLY DEPOSITED FOR MAGNETIC RECORDING APPLICATIONS?(INVITED)

The tolerance of the expected read-back signal of spin valve giant magnetoresistance based structures to varying deposition and process conditions are described. We determine if spin valves can be produced reliably, and evaluate which thicknesses and properties are most critical. First, the dependence of spin valve properties on layer thickness are experimentally determined. Next, the variation of read-back signal and transfer curve characteristics with spin valve properties is calculated from micromagnetic modeling. Finally, these are convolved with the expected reproducibility of layer thickness to obtain an effective “yield” of structures within 10% of the mean amplitude. We find that spin valves can be reliably deposited, with “yields” well in excess of 90% likely.

X‐ray reflectivity analysis of giant‐magnetoresistance spin‐valve layered structures

The amount of intermixing at the interfaces of sputter‐deposited spin‐valve layered structures, comprising Si/Ta (50 A)/NiFe (75 A)/Cu (22.5 A)/NiFe (50 A)/FeMn (110 A)/Ta (50 A), were obtained from least‐squares refinement of x‐ray reflectivity data. The observations were modeled by layers of nominal composition with compositional inhomogeneity at the interfaces. Layer thicknesses deduced from x‐ray analysis were generally within a few percent of the nominal values. Interface widths between the two NiFe layers and the Cu spacer were 6.2‐7.4 A, indicating intermixing of atoms of about three monolayers at the interfaces. A 10.0‐A interface width was found at the FeMn interface suggesting a mixed‐phase layer of α‐ and γ‐FeMn. Layer densities, except those of the less dense Ta underlayer and the oxidized Ta surface, agreed to within 10% of bulk values. The results were in agreement with those obtained from closely related Si/Ta/NiFe/Cu/NiFe/Ta, Si/Ta/NiFe/Cu/Ta, Si/Ta/NiFe/Ta, and Si/Ta films.

The role of interfaces in NiFe/Cu/NiFe spin values

Abstract A sharp decrease of the giant magnetoresistance amplitude by interface roughening is observed in NiFe/Cu/NiFe/FeMn spin valves, which we attribute to the presence of high resistivity, compositionally intermixed regions at the NiFe/Cu interfaces. These results show that interfacial spin independent scattering must be included in the theory of GMR.

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 A, 4πMs and λs were found to be strongly dependent on film thickness with an even larger variation after annealing. For all films, at t=60A, 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...

Surface-oxidation-induced phase separation in FeMn thin films

Films of Fe50Mn50, 700 A thick, were deposited on glass substrates by diode rf sputtering. Auger electron spectroscopy, and x‐ray fluorescence analysis verified the film composition and x‐ray diffraction determined the crystalline phase to be α‐Mn(Fe), a complex bcc‐like structure and a considerable deviation from that dictated by the equilibrium binary phase diagram. The films were annealed for 1 h at 260 °C and 10−7, 10−5, and 10−2 Torr. Auger depth profile analysis showed that (a) while no further oxidation (than that induced by ambient exposure) occurred during annealing at the lowest pressure, severe film oxidation took place at the highest pressure and (b) preferential surface oxidation of Mn resulted in a layered structure consisting of a surface Mn‐ and oxygen‐rich layer followed by a region (deeper into the film) depleted in Mn and Fe enriched. The crystalline phases present were determined by x‐ray diffraction analysis to be MnO (fcc) and α‐Fe(Mn) (bcc). Magnetic measurements of the oxidized fil...

Structure and magnetism of Ta/Co/Ta sandwiches

Abstract HRTEM and RT magnetometry were used to characterize Ta/Co/Ta sandwiches sputter-deposited on Si(001) wafers. The sandwiches show an intermixed Ta/Co interface region of about 10 A thickness, which is connected to the presence of a non-ferromagnetic interfacial layer of 6.6 A thickness per Ta/Co interface. The Co layer has a textured morphology, composed of columns with diameters of about 10–100 A, and a marked 〈111〉 fcc structure with numerous stacking faults. Repetitions of hcp stacking sequences are mainly localized in the narrowest columns. The Ta layer shows a short-range ordering characterized by randomly oriented stacks of a few atomic planes about 10–20 A large.

Alumina-Ti Interface Reactions Studied By AES And UPS

The interaction of Ti with A1 2 O 3 under UHV conditions has been studied by AES and UPS. Ti was deposited by iterated and successively thicker evaporations (up to a total thickness of 91 A) under UHV conditions onto the alumina substrate at room temperature. The oxide substrate was grown on Al/Si(111) in an adjacent VHV preparation chamber. The last deposition was followed by in situ annealing. It was found that, contrary to indications of thermodynamic considerations using bulk equilibrium data, Ti interacts strongly with alumina at room temperature, dissociating it to metallic Al and forming TiO 2 at the interface. This reaction is limited, resulting in an interface width of ∼10 A and allowing subsequent evaporations to homogeneously cover the products. Annealing to 500°C does not produce any evidence of outdiffusion or extensive interface reactions, implying that the interface oxide is stable and an effective barrier to Al (and/or Ti) diffusion up to this temperature.

Structural characterization of Co in sputtered Ta/Co/Ta thin-film sandwiches

Abstract The morphology and structure of sputtered Ta/Co/Ta thin-film sandwiches were investigated by cross-sectional high-resolution transmission electron microscopy. Although the Co layer shows an overall 〈111〉-oriented fcc structure, a high density of hcp stacking faults in present in the layer. The Co structure can thus be described as a random distribution of hcp stacking sequences in an overall fcc structure.

Distribution of current in spin valves (abstract)

We present a model describing the distribution of spin up and spin down currents within the layers of spin valve structures. With this model and experimentally determined bulk mean free paths we successfully describe a variety of experimental results, including the variation of resistance and magnetoresistance with both ferromagnetic and nonferromagnetic layer thicknesses, and the current‐induced field acting on the ferromagnetic layers versus overall current. Our model is based on an approximate path integral solution of the Boltzmann equation for in‐plane transport in a multiply layered structure. For a given orientation of the magnetizations it calculates the current density jσ(r) at each point r for each spin σ throughout the structure. From jσ(r) it is straightforward to obtain the current in each layer for parallel versus antiparallel magnetizations, or of the current density near interfaces relevant to electromigration. Included in our model are both spin dependent bulk scattering as well as scatte...