Magnetic tunnel junctions (MTJs) are a critical sensor technology employed in magnetoresistive random access memory (MRAM) and hard-disk drives to read data. MTJs are composed of two magnetic films separated by an insulating barrier, or quantum mechanical "tunnel barrier". This configuration gives MTJs the desirable property of having a large tunneling magnetoresistance, resulting in high sensitivity and rapid response to magnetic field changes. MTJs thereby have an ability to read magnetic data stored at ultrahigh densities.
Conventional MTJs exhibit an undesirable property called "orange-peel coupling" that degrades sensitivity to small magnetic fields. Orange-peel coupling is a consequence of interfacial roughness (like the bumpiness of an orange-peel) at the tunnel barrier. Although the roughness is only on the scale of a few atoms, these small magnetic protrusions or bumps create magnetic fields that cross the insulating tunnel barrier and create a magnetic coupling between the magnetic thin films above and below. Orange-peel coupling makes the device less sensitive to the data the MTJ is trying to read.
Recent work in MML's Metallurgy Division has shown a way to avoid this problem. We have found that manipulating the MTJ's exposure to oxygen preferentially oxidizes the little bumps, preventing orange-peel coupling. In this manner the high sensitivity required of the next generation of MTJs is preserved. The treatment should be valuable in optimizing the performance of hard-disk drives.