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(a) A schematic diagram of a magnetoresistive tunneling and magnetoresistive device. (b) A schematic diagram of the crystal of the studied metastable body-centered cubic cobalt-manganese alloy. (c) A schematic diagram of the face-centered cubic structure, which is one of the thermodynamically stable phases of cobalt-manganese alloys. Credit: Tohoku University

A group of Tohoku University researchers has unveiled a new material that exhibits enormous magnetoresistance, paving the way for the development of non-volatile magnetoresistive memory (MRAM).

The details of their unique discovery were published in Journal of Alloys and Compounds.

Today, the demand for advances in hardware that can efficiently process large amounts of digital information and sensors has never been greater, especially with governments deploying technological innovations to build smarter societies.

Much of this hardware and sensors rely on MRAM and magnetic sensors, and tunnel magnetoresistive devices make up the majority of such devices.

Tunneling magnetoresistive devices use the tunneling magnetoresistive effect to detect and measure magnetic fields. This is related to the magnetization of the ferromagnetic layers in the magnetic tunnel junctions. When the magnets are aligned, a state of low resistance is observed and the electrons can easily pass through the thin insulating barrier between them.

When the magnets are misaligned, electron tunneling becomes less efficient and leads to increased resistance. This change in resistance is expressed as the magnetoresistive ratio, a key figure in determining the efficiency of tunneling magnetoresistive devices. The higher the magnetoresistance ratio, the better the device.

Current magnetoresistive tunneling devices include magnesium oxide and iron-based magnetic alloys, such as iron-cobalt. Iron-based alloys have a body-centered cubic crystal structure under ambient conditions and exhibit a huge tunneling magnetoresistance effect in devices with a rock salt-type magnesium oxide.

The thermodynamically stable crystal structure in cobalt-manganese-iron ternary alloys showing the composition of the material in which the enormous magnetoresistance ratio was discovered and the magnetoresistance data collected at low and room temperature. These features were achieved due to the body-centered cubic structure in a metastable state. Credit: Tohoku University

There have been two notable studies using these iron-based alloys that have resulted in magnetoresistive devices exhibiting high magnetoresistance ratios. The first in 2004 was from the National Institute of Advanced Industrial Science and Technology in Japan and IBM; and the second came in 2008, when researchers at Tohoku University reported a magnetoresistance ratio of over 600% at room temperature, something that jumped to 1000% at temperatures near zero Kelvin.

Since those discoveries, various institutes and companies have invested considerable effort in refining the physics, materials and processes of the devices. However, apart from iron-based alloys, only a few Heusler-type ordered magnetic alloys have exhibited such enormous magnetoresistance.

Dr Tomohiro Ichinose and Professor Shigemi Mizukami of Tohoku University recently began exploring thermodynamically metastable materials to develop a new material that can demonstrate similar magnetoresistance ratios. To do so, they focused on the strong magnetic properties of cobalt-manganese alloys, which have a body-centered cubic metastable crystal structure.

“Cobalt-manganese alloys have face-centered cubic or hexagonal crystal structures as thermodynamically stable phases. Because this stable phase exhibits weak magnetism, it has never been studied as a practical material for tunneling magnetoresistive devices,” Mizukami said.

In 2020, the group reported on a device that used a cobalt-manganese alloy with a metastable body-centered cubic crystal structure.

Using data science and/or high-throughput experimental methods, they built on this discovery and succeeded in achieving enormous magnetoresistance in devices by adding a small amount of iron to the body-centered cubic metastable cobalt-manganese alloy. The magnetoresistance ratio was 350% at room temperature and even exceeded 1000% at low temperature. Furthermore, the manufacturing of the device used the sputtering method and a heating process, something compatible with current industries.

“We have produced the third instance of a new magnetic alloy for tunneling magnetoresistive devices that exhibit enormous magnetoresistance and establish an alternative travel direction for future improvements,” adds Mizukami.

More information:
Tomohiro Ichinose et al, Large tunnel magnetoresistance in magnetic tunnel junctions with metastable body-centered cubic CoMnFe alloy magnetic electrodes, Journal of Alloys and Compounds (2023). DOI: 10.1016/j.jallcom.2023.170750

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