M. F. Gillies

Plasma oxidation of thin aluminum layers for magnetic spin-tunnel junctions

This article presents results of a study initiated to characterize the plasma-oxidation process of very thin Al films, a technology commonly used to produce good barrier layers for magnetic spin-tunnel junctions. The behavior of oxygen in the oxidizing Al layer is determined using both quantitative (Rutherford backscattering spectrometry, transmission electron microscopy) and qualitative (x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry) analytical techniques. We have applied in situ XPS and experimented with 18O2 to unravel details of the oxidation mechanism. In addition, the influence of the oxygen pressure on the oxidation rate was established, both with and without a plasma being present. From optical emission spectra it is concluded that this pressure has a minor effect on the relative abundance of excited species in the oxygen plasma. When combined, these data constitute the basis of a model that distinguishes several steps in the plasma oxidation of Al. At the start, oxygen p...

Direct observation of magnetization reversal processes in micron-sized elements of spin-valve material

Simple calculations suggest that when continuous films of spin-valvematerial are patterned into micron-sized elements the magnetic properties should change markedly, depending on the element shape and size. We have used the differential phase contrast imaging mode of transmission electron microscopy to study directly the magnetization distributions supported by such elements in zero field and when subjected to an applied field in the pinning direction. For elements whose long axis is parallel to the pinning direction a parallel alignment of the free and pinned layers is favored. When subjected to a field a complex domain structure evolves and different irreversible paths are followed as the element is taken from negative to positive saturation and back again. By contrast, when the pinning direction is parallel to the short axis an antiparallel arrangement, where the magnetostatic contribution to the energy is effectively suppressed, can be preferred and simpler reversal mechanisms, with a higher degree of reversibility, are frequently seen.

Influence of the noble gas mixture composition on the performance of a plasma display panel

The influence of the noble gas mixture composition on the plasma display panel performance is investigated in test panels with a design which resembles the one used in commercial panels. Single gases and binary and ternary mixtures of He, Ne, Ar, Kr, and Xe are applied, where the Xe concentration is varied from 0% to 100%. The performance is characterized in terms of the panel luminance, efficacy, and discharge voltages. It is found that while an increase in efficacy and luminance can be achieved in several multicomponent mixtures it is necessary to examine the associated increase in the firing voltage, Vf. If one considers the luminance versus Vf dependence, then binary NexXe1−x mixtures are optimal to achieve the highest efficacy values at the lowest Vf. The maximum efficacy gain factor in high Xe partial pressure mixtures is about a factor of 3 with respect to the mixture applied in default commercial panels.The influence of the noble gas mixture composition on the plasma display panel performance is investigated in test panels with a design which resembles the one used in commercial panels. Single gases and binary and ternary mixtures of He, Ne, Ar, Kr, and Xe are applied, where the Xe concentration is varied from 0% to 100%. The performance is characterized in terms of the panel luminance, efficacy, and discharge voltages. It is found that while an increase in efficacy and luminance can be achieved in several multicomponent mixtures it is necessary to examine the associated increase in the firing voltage, Vf. If one considers the luminance versus Vf dependence, then binary NexXe1−x mixtures are optimal to achieve the highest efficacy values at the lowest Vf. The maximum efficacy gain factor in high Xe partial pressure mixtures is about a factor of 3 with respect to the mixture applied in default commercial panels.

Enhancement of the giant magnetoresistance in spin valves via oxides formed from magnetic layers

An enhancement of the giant magnetoresistance effect is investigated in spin valves where oxide layers, which are formed from magnetic layers, are incorporated in the structure. Information about Co–Fe based nanooxide layer (NOL) is obtained via x-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Cross-section transmission electron microscopy is also used to explore the effect of an NOL on the polycrystalline structure of the spin valve.

Domain processes in the magnetization reversal of exchange-biased IrMn/CoFe bilayers

We have used transmission electron microscopy to study directly the way magnetization reversal proceeds in the ferromagnetic layer for sets of bilayers in which the ferromagnetic layer (CoFe) was of constant thickness while the thickness of the antiferromagnetic layer (IrMn) was varied. The first set studied was in the as-deposited state while the second was subjected to rapid thermal processing. For IrMn thicknesses of 20 A, no shift fields were observed, although significant coercivities (∼70 Oe) were recorded and reversal involved rather simple domain processes. By contrast, complex small-scale domains dominated the reversal processes for samples where the IrMn thickness exceeded 60 A; here, strong exchange biasing and higher coercivities were the norm. For all thicknesses of IrMn, an unexpected variation in the dominant orientation of domain walls on the outward and return parts of the magnetization cycle tended to be observed. A possible origin of this, together with the differences between the as-de...

An in situ x-ray photoelectron spectroscopy study of AlOx spin tunnel barrier formation

Thin aluminum oxide barrier layers have been studied by in situ x-ray photoelectron spectroscopy to determine their oxidation degree dependence on oxidation time and method (thermal or plasma oxidation). A 1.5 nm thick Al layer is found to completely oxidize by exposure to an oxygen plasma for a time in the interval 30–60 s, i.e., using the conventional plasma oxidation method. For times less than 30 s, however, we observed not only a metallic-Al peak but the formed oxide was substoichiometric. The composition of the formed oxide increased towards Al2O3 as the oxidation continued. It was also found possible to oxidize up to 1 nm of Al, at room temperature, upon deposition on Co previously exposed to 9.3 Pa (70 mTorr) oxygen for 10 s. Annealing junctions with the idealized structure Co/Al2O3/Co at up to 275 °C was found to increase their magnetoresistance (up to 35%) and resistance (up to a decade), if the Al was deposited on an oxidized Co bottom electrode.

Magnetization reversal of NiFe films exchange-biased by IrMn and FeMn

We have used the transmission electron microscope to study how the magnetization reversal mechanism of thin NiFe layers exchange-biased by IrMn and FeMn varies over a wide temperature range. The reversal behaviour was qualitatively similar for layers biased by both types of antiferromagnet. At room temperature and below the most striking feature was the scale of the domain structures observed. Very high density domain structures with micron (or sub-micron) wall separations developed. By contrast at elevated temperatures, the reversal mechanism simplified. This is consistent with there being a strong local variation of the pinning strength between the NiFe and the antiferromagnetic layer. The overall temperature variation of the pinning changes much more rapidly than the magnetic properties of an isolated NiFe layer over a similar temperature range. 3Current address: Sun Microsystems, Springfield, Linlithgow, West Lothian, EH49, UK. 4Current address: Veeco Instruments, 80 Las Colinas Lane, San Jose, CA 95119, USA.

Compositional, structural, and electrical characterization of plasma oxidized thin aluminum layers for spin-tunnel junctions

In this paper we present results on how the plasma oxidation of a thin (1.5 nm) Al layer proceeds. Transmission electron microscopy of a Co/Al–oxide multilayer was used to determine the thickness of the oxides and Rutherford backscattering spectrometry and elastic recoil detection were utilized in order to determine the oxygen content. The oxide was also characterized via ac impedance measurements. These measurements indicated that the oxidation of Al on Co occurs in three discrete steps.

Effect of thin oxide layers incorporated in spin valve structures

The enhancement of the magnetoresistance effect, induced by incorporating nano-oxide layers (NOLs) in a bottom-type spin valve, was studied for various preparation conditions. The effect of a NOL in the Co90Fe10 pinned layer was found to depend critically on the oxygen pressure applied to form the thin oxide film. Pressures over 10−3 Torr O2 yield oxides thicker than about 0.7 nm, which apparently deteriorate the biasing field which exists over the oxide. The magnetoresistance values can further be raised by forming a specular reflecting oxide on top of the sense layer. Promising results were obtained with an Al2O3 capping layer formed in a solid-state oxidation reaction that occurs spontaneously when a thin Al layer is deposited on the oxidized surface of the Co90Fe10 sense layer.

The optimum oxidation state of AlO/sub x/ magnetic tunnel junctions

In this paper we present results on how both the resistance and the magnetoresistance of magnetic spin-tunnel junctions depend on the oxidation time used to form the AlO/sub x/ barrier. The bias voltage dependence of junctions with barriers created with different oxidation times is measured and found to be optimal when the oxidation time is chosen to give sufficient oxygen for the barrier to be stoichiometric Al/sub 2/O/sub 3/. The oxygen content of the barrier was determined by Rutherford backscattering spectrometry and elastic recoil detection.