Researchers found materials that exhibit great magnetoresistance

June 09, 2023

(Nanowerk News) A group of researchers from Tohoku University have unveiled a new material that exhibits enormous magnetic resistance, paving the way for the development of non-volatile magnetoresistive memory (MRAM).

Details of their unique find are published in Journal of Alloys and Compounds (“Large tunnel magnetoresistance at the junction of the magnetic tunnel with the magnetic electrode of a body-centered cubic CoMnFe alloy”). (a) Schematic diagram of the tunnel magnetoresistive device and the magnetoresistance. (b) Schematic diagram of the metastable body-centered cubic cobalt-manganese alloy crystals studied. (c) Schematic diagram of the face-centered cubic structure, which is a thermodynamically stable one of the phases of the cobalt-manganese alloy. (Image: Tohoku University)

Today, the demand for hardware advancements that can process large amounts of digital information and sensors efficiently has never been greater, especially with governments implementing technological innovations to achieve a smarter society.

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

Tunnel magnetoresistive devices exploit the effect of tunnel magnetoresistance to detect and measure magnetic fields. This is related to the magnetization of the ferromagnetic layers at the junctions of the magnetic tunnels. When the magnets are aligned, a state of low resistance is observed, and electrons can easily penetrate the thin insulating barrier between them. When the magnets are misaligned, the tunneling of electrons becomes less efficient and causes a higher resistance. This change in resistance is expressed as the magnetoresistive ratio, a key number in determining the efficiency of a tunneling magnetoresistive device. The higher the magnetoresistance ratio, the better the device.

Current tunnel magnetoresistive devices consist of magnesium oxide and an iron-based magnetic alloy, such as iron-cobalt. The iron-based alloy has a body-centered cubic crystal structure under ambient conditions and exhibits a large tunnel magnetoresistance effect in devices with rock salt-type magnesium oxide.

There have been two important studies using this iron-based alloy to produce magnetoresistive devices featuring high magnetoresistance ratios. The first was in 2004 by the National Institute of Advanced Industrial Science and Technology in Japan and IBM; and the second came in 2008, when researchers from Tohoku University reported a magnetoresistance ratio exceeding 600% at room temperature, something that jumps to 1000% at temperatures close to zero kelvin.

Since that breakthrough, various institutes and companies have invested a lot of effort in honing device, materials and process physics. But apart from the iron-based alloys, only a few Heusler-type ordered magnetic alloys have shown very large magnetoresistance. The crystal structure is thermodynamically stable in a cobalt-manganese-iron ternary alloy A thermodynamically stable crystal structure in a cobalt-manganese-iron ternary alloy featuring a material composition in which large magnetoresistance ratios are found, and magnetoresistance data are collected at low temperature and room temperature. This characteristic is obtained due to the body-centered cubic structure in the metastable state. (Image: Tohoku University)

Dr. Tomohiro Ichinose and Professor Shigemi Mizukami of Tohoku University have recently started exploring thermodynamically metastable materials to develop new materials capable of exhibiting a similar ratio of magnetic resistance. To do so, they focused on the strong magnetic properties of the cobalt-manganese alloy, which has a body-centered cubic metastable crystal structure.

“The cobalt-manganese alloy has a face-centered cubic or hexagonal crystal structure as a thermodynamically stable phase. Because this stable phase exhibits weak magnetism, it has never been studied as a practical material for tunnel magnetoresistive devices,” said Mizukami.

Back in 2020, the group reported a device using 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 managed to obtain enormous magnetoresistance in the device by adding small amounts of iron to a metastable body-centered cubic cobalt-manganese alloy. The magnetoresistance ratio is 350% at room temperature and also exceeds 1000% at low temperature. In addition, device manufacturing uses sputtering methods and heating processes, something that is compatible with today’s industry.

“We have produced a third example of a new magnetic alloy for a magnetoresistive tunneling device that exhibits large magnetoresistance, and establishes an alternative direction of travel for future improvements,” Mizukami added.

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