Low-field magnetoresistance in magnetic tunnel junctions prepared by contact masks and lithography: 25% magnetoresistance at 295 K in mega-ohm micron-sized junctions (abstract)

Interest in magnetic tunnel junctions (MTJ) has been increased by the recent observation of large room-temperature magnetoresistance (MR) in structures comprised of sandwiches of two ferromagnetic (FM) layers with different coercivities separated by a thin ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ tunnel barrier. Using an ultrahigh vacuum dc magnetron sputtering system, a variety of MTJ structures have been explored. Junctions were fabricated directly using computer-controlled placement of successive contact masks, and by patterning using optical lithography techniques and processes, compatible with MR head structures. Structures were prepared in which one of the FM layers is exchange biased with an antiferromagnet $\mathrm{MnFe}$ layer. Thus, the junctions exhibit two well-defined magnetic states in which the FM layers are either parallel or antiparallel to one another. The tunnel barrier was prepared by in situ plasma oxidation of thin $\mathrm{Al}$ layers sputter deposited at room temperature. Using FM layers comprised of $\mathrm{Co}$ or permalloy $({\mathrm{Ni}}_{\mathrm{81}}{\mathrm{Fe}}_{\mathrm{19}})$ , MR values exceeding $\text{30\%}$ were obtained at room temperature in low fields $\text{(5–10\hspace{0.5em}}\mathrm{Oe})$ . Similar magnetic and MR properties are found in junctions ranging in size from $\text{80×80\hspace{0.5em}μ}{\mathrm{m}}^2$ (contact masks) to $\text{2×2\hspace{0.5em}μ}{\mathrm{m}}^2$ (lithography). The smallest junctions, of area $\text{2×2\hspace{0.5em}μ}{\mathrm{m}}^2$ , have resistance values of $\text{∼5\hspace{0.5em}}\mathrm{M}\Omega$ and $\text{25\%}$ MR. Thus, changes in resistance of $\text{∼1\hspace{0.5em}}\mathrm{M}\Omega$ are found. Such structures may form an excellent magnetic switch. However, for many device applications, much lower resistances are required. The junction resistance can readily be varied by several orders of magnitude by varying the thickness and oxidation conditions of the $\mathrm{Al}$ layer as well as by varying the thicknesses and nature of the layers within the electrodes themselves. Note that it is important that the voltage drop laterally across the electrodes must be small compared to that across the tunnel barrier itself. This requires measurement of junctions with low sheet resistances in lithographically patterned devices.