Allan E. Schultz

Oxidation of tunnel barrier metals in magnetic tunnel junctions

The oxidation of an ultrathin metal layer (<1nm) to form an oxide tunnel barrier is of critical importance for the fabrication of magnetic tunnel junctions (MTJs) with low product of resistance and area (R×A). Nonuniform and excessive or insufficient oxidation will occur by using conventional plasma, air, or O2 and noble gas mixtures as oxidation methods. An oxidation method was investigated to oxidize only an ultrathin layer of metal (such as Y) without oxidizing adjacent ferromagnetic thin film layers. We have now demonstrated that a gas mixture of H2O∕H2 with a fixed chemical potential of oxygen determined by the relative amounts of the two gases can oxidize Y and Ta thin layers while simultaneously keeping a Co ferromagnetic layer completely unoxidized. This universal method can be used to preferentially oxidize a host of other metals with high tendency to form oxides, such as Zr, Hf, Nb, rare earth metals, etc. and may allow us to access the feasible lower limit of barrier thickness in MTJs.

Intermixing and phase separation at the atomic scale in Co-rich (Co,Fe) and Cu multilayered nanostructures

Despite the fact that Co-rich (Co,Fe) alloys and Cu are immiscible materials in bulk form, evidence of thermally induced mixing at the atomic scale has been observed in thin-film multilayers of (Co,Fe) and Cu. However, long term anneals at lower temperatures produced a breakup of the multilayers into a two-phase mixture of (Co,Fe) and Cu particles. The observations were made with the use of the three-dimensional atom probe technique, with supporting evidence from differential scanning calorimetry and x-ray diffraction. Besides their scientific importance, these results are of interest where these (Co,Fe) and Cu thin films are used to produce the giant magnetoresistive effect.

Quantitative analysis of transition curvature by magnetic force microscopy

A technique, based on magnetic force microscopy (MFM), has been developed to quantify the curvature of recording written transitions. By evaluating the phase deviation and coherence of MFM imaged recording tracks, the on-track transition curvature can be precisely measured to nanometer level or less while minimizing relevant imaging noise associated with MFM tips or recording media. The magnetization curvature properties are characterized quantitatively using the proposed technique at different recording conditions.

Thermodynamic Evaluation of the Interface Stability Between Selected Metal Oxides and Co

For an interface to be considered thermodynamically stable, the phases in contact must be in equilibrium with each other (connected by a stable tie-line) and have negligible mutual solubility on the phase diagram. The stability of Co based magnetic tunnel junctions (MTJs), with Co/M x O 1- x /Co structures (M = Al, Gd, Hf, La, Mg, Si, Ti, Ta, Y and Zr), were evaluated with regard to these two conditions. Specifically, low temperature ternary isothermal phase diagrams were calculated and evaluated for the Co–M–O systems. All of these systems have at least one oxide in equilibrium with Co and thus have at least one thermodynamically stable tunnel barrier candidate for use in Co based MTJs. In light of the assumptions made in this analysis, along with the uncertainty in applying bulk enthalpy data to thin films, the current evaluation of interfacial stability serves as a first step in identifying suitable stable tunneling barrier materials in MTJs for detailed study.

Selective oxidation of an individual layer in a magnetic tunnel junction through the use of thermodynamic control

Oxidation of an ultrathin metal layer (less than 1 nm) to form a tunnel barrier oxide, without oxidizing adjacent layers, is of critical importance in making nanoscale devices such as magnetic tunnel junctions. It is extremely difficult, if not impossible, to achieve this objective using conventional methods that rely on kinetic control of the oxidation process. We present an alternative approach using a gas mixture with a fixed chemical potential of oxygen as the oxidizing medium. This mixture, chosen with thermodynamic calculations, tends to uniformly oxidize the tunnel barrier to the thermodynamically favored stoichiometry without oxidizing the adjacent layers. Experiments on a model system show that a thin-film layer such as Al can be oxidized without oxidizing common ferromagnetic alloys, such as Co–Fe, using a mixture of CO2∕CO or H2∕H2O. The chemical states of the Al and Co–Fe based example were characterized using x-ray photoelectron and synchrotron-source Fourier transform infrared spectroscopy.

Thermal stability of the interfaces between Co-, Ni-, and Fe-based ferromagnets in contact with selected nitrides MN (M=Al, B, Nb, Ta, Ti, and V)

Nitride tunnel barriers have potential applications in magnetic tunnel junctions (MTJs). Thermal stability of the interfaces between Co-, Ni-, and Fe-based ferromagnets and these nitride tunnel barriers is critical to device performance. With guidance from low-temperature ternary isothermal phase diagrams of the Co–M–N, Ni–M–N, and Fe–M–N systems (M=Al, B, Nb, Ta, Ti, and V), the interfaces in Co∕MN, Ni∕MN, and Fe∕MN structures were evaluated in terms of two criterions: the phases in contact must (1) be in equilibrium with each other (i.e., connected by a stable tie line) and (2) have negligible mutual solubility in the phase diagram at the temperatures of interest. Of the investigated interfaces, Co∕AlN, Co∕BN, Co∕NbN, Co∕TaN, Co∕TiN, Ni∕BN, Ni∕TaN, Fe∕BN, Fe∕NbN, Fe∕TaN, and Fe∕TiN were found to be thermodynamically stable. However, in light of some simplifications made in this analysis, the current evaluation of interfacial stability serves as a useful step in preselecting candidate nitride-based MTJ t...