Multilayer Structure with Improved Magnetoresistance Ratio Could Increase the Sensitivity of Magnetic Field Sensors

Applications requiring efficient electric energy management can be enhanced by magnetic field sensors.

If the magnetic field sensors are improved to below the picoTesla range, they can enable a method—known as magnetic encephalography—–to determine the activity of the brain at room temperature with millisecond resolution, without the need for superconducting quantum interference device (SQUID) technology, which necessitates cryogenic temperatures to work.

Now, a research team from Japan’s National Institute of Materials Science at the University of Tsukuba and LG Japan Lab Inc. has explored the possibility of improving the magnetoresistance ratio in a current-perpendicular-to-plane giant magnetoresistance, or CPP-GMR, device through a half-metallic Heusler CoFeAl0.5Si0.5 (CFAS) alloy. The alloy has 100% spin-polarized conduction electrons, which allow extremely high spin-asymmetry of electron scattering and lead to a large magnetoresistance ratio. The study findings have been reported in the Journal of Applied Physics, from AIP Publishing.

Magnetoresistance refers to a change in electrical resistance in response to an externally applied magnetic field. This phenomenon is significant for all magnetic field sensor applications. The sensitivity of the magnetic field sensors can be increased by first increasing their magnetoresistance ratio (a value defined as a change in electrical resistance against magnetization or magnetic field).

We were able to demonstrate further enhancement of the magnetoresistance ratio by making multilayer stacks of CFAS and silver (Ag). By precisely controlling the interfacial roughness of the multilayers, we obtained antiparallel interlayer exchange coupling between each of the CFAS layers, up to six, and achieved not only a high magnetoresistance ratio but also high linearity of resistance change against the magnetic field.

Yuya Sakuraba, National Institute for Materials Science

Sakuraba is the leader of the Magnet Materials Group at the National Institute for Materials Science (NIMS). Studies performed in the past have shown that half-metallic Heusler alloys are especially suitable for improving the magnetoresistance ratio in CPP-GMR devices.

Heusler-based alloys are expected to be the next-generation read head for hard disk drives with high areal recording density over 2 terabits per square inch. And our work has demonstrated that further enhancement of the magnetoresistance ratio is possible by creating a multilayer structure, which now really opens up the potential of Heusler-based CPP-GMR for highly sensitive magnetic field sensor applications.

Yuya Sakuraba, National Institute for Materials Science

The scientists developed a fully epitaxial device on one crystalline magnesium oxide (MgO) substrate. In case an analogous property is achieved in a polycrystalline device, it can well become a candidate for a novel magnetic field sensor that has more sensitivity than a traditional tunnel magnetoresistance sensor or Hall sensor.

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