Preliminary atomic moment results for new iron-cobalt-manganese thin film potentially exceed decades-old record by 50% – ScienceDaily


The burgeoning field of spintronics harnesses the spins of electrons – as opposed to their charge – to improve semiconductor devices such as hard drives and cell phone components by extending battery life. Spintronic developments, however, increasingly come up against a barrier known as the Slater-Pauling limit, the maximum for how a material can pack its magnetization. Today, a new thin film is about to shatter that decades-old benchmark.

A team of researchers from Montana State University and Lawrence Berkeley National Laboratory announce this week in Letters of Applied Physics, from AIP Publishing, that they built a stable thin film based on iron, cobalt and manganese that has an average atomic moment potentially 50% greater than the Slater-Pauling limit. Made with a technique known as Molecular Beam Epitaxy (MBE), the Centered Cubic Ternary Alloy (bcc) exhibits a magnetization density of 3.25 Bohr magnetons per atom, exceeding the previously considered maximum of 2, 45.

“What we have is a potential breakthrough in one of the most important parameters of magnetic materials,” said Yves Idzerda, author of the article from Montana State University. “Great magnetic moments are like the force of steel – the bigger the better.”

The Slater-Pauling curve describes the magnetization density of alloys. For decades, binary iron-cobalt (FeCo) alloys have reigned supreme, displaying a maximum average atomic moment of 2.45 Bohr magnetons per atom and setting the current limit for a stable alloy magnetization density. Previously, researchers mixed FeCo alloys with high magnetic moment transition metals, such as manganese. However, when these ternary alloys are made, they lose much of their bcc structure, a key part of their high magnetism.

Instead, this team turned to MBE, a meticulous technique that involves draping a substrate with beads of individual metal atoms, one layer at a time, to create a 10 to 20 nanometer film of Fe9Co62Mn29. . About 60 percent of the available compositions retained the bcc structure as a thin film, compared to only 25 percent in bulk.

To better understand the composition and structure of the alloy, the group used X-ray absorption spectroscopy and high-energy electron diffraction by reflection. X-ray magnetic circular dichroism results showed that the new material produced an average atomic moment of 3.25 Bohr magnetons per atom. When tested with more standard vibrating sample magnetometry, even though this magnetization density dropped, it was still significantly above the Slater-Pauling limit – 2.72.

Idzerda said the gap would provide areas for future research, adding that the interface between manganese and the substrate in the crystal could explain the gap.

“I kept the optimism for this because the technique we used is a bit non-standard and we have to convince the community of the performance of this material,” Idzerda said.

Idzerda and his team will now investigate the strength of iron-cobalt-manganese alloys and more efficient fabrication techniques. They also plan to explore how molecular beam epitaxy could lead to other highly magnetic thin films, potentially mixing four or more transition metals.

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