Magnetic benchmark: Record-breaking thin film

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  • Published: Feb 15, 2018
  • Author: David Bradley
  • Channels: NMR Knowledge Base
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Thin film magnet

US researchers have developed a new magnetic thin film that could finally break through the Slater-Pauling limit on magnetic density in a material. With this decades-old benchmark circumvented new applications and devices might be possible. Credit: AIP/Appl Phys Lett, Yves Idzerda et al

US researchers have developed a new magnetic thin film that could finally break through the Slater-Pauling limit on magnetic density in a material. With this decades-old benchmark circumvented new applications and devices might be possible.

Spintronics, magnetic storage media, and applications such as nuclear magnetic resonance (NMR) spectroscopy could all benefit from new insights into the nature of magnetism and ultimately new ways to make magnetic materials. One of the limitations is the Slater-Pauling limit, the maximum for how tightly a material can pack its magnetization. This is particularly pertinent in the development of spintronics in devices that would use not only electron charge but electron spin as the currency of computation.

The team from Montana State University and Lawrence Berkeley National Laboratory describe details of their stable, thin film composed of iron, cobalt and manganese in the journal Applied Physics Letters. The thin film has an average atomic moment putatively 50 percent above the Slater-Pauling limit. The material was fabricated using molecular beam epitaxy (MBE), which generated a ternary body-centred cubic (bcc) alloy with a magnetization density of 3.25 Bohr magnetons per atom. This beats the previous maximum of 2.45.

Barrier breakthrough

“What we have is a potential breakthrough in one of the most important parameters of magnetic materials,” explains Montana State University's Yves Idzerda. “Large magnetic moments are like the strength of steel - the bigger the better," he adds.

The Slater-Pauling limit puts a boundary on the magnetization density achievable in an alloy. For many years, iron-cobalt (FeCo) binary alloys have been considered the maximum under this limit, with a maximum average atomic moment of 2.45 Bohr magnetons per atom. Before that, mixed iron-cobalt alloys with added high magnetic moment transition metals, such as manganese, had been tested but in reality they lose their bcc structure and fail at the first test of high magnetism.

By utilizing MBE, the team has demonstrated a more precise way to make thin films that is akin to draping beads of individual metal atoms on a thin film substrate rather than hammering them together in the crystal. They can build up a new alloy one layer at a time, to create a thin film just 10-20 nanometres thick with the formula Fe9Co62Mn29. This approach sustained the bcc structure in the film in approximately two times out of every three attempts at optimizing the composition, as opposed to just one in four in the bulk.

MBE for magnets

The team used X-ray absorption spectroscopy and reflection high-energy-electron diffraction to investigate the alloy’s composition and its structure. X-ray magnetic circular dichroism data revealed that the new thin film material had an average atomic moment of 3.25 Bohr magnetons per atom. When they tested the film with the more conventional technique of standard vibrating sample magnetometry, they saw that although the magnetization density dropped, it was still well above the Slater-Pauling limit - 2.72.

Idzerda suggests that rather than being a hindrance this discrepancy in the values of magnetic density will actually open up new areas of research. He adds that the interface between the manganese atoms and the substrate within the crystal might account for the gap and that this could be improved or exploited in other ways to fine-tine the material for particular applications. “I have guarded optimism for this because the technique we used is a little bit non-standard and we have to convince the community of this material’s performance,” Idzerda adds.

The team will next investigate just how strong and stable are the iron-cobalt-manganese alloys. They will also look at making the fabrication technique more efficient. It might even be possible to use MBE to make highly magnetic thin films containing four or more transition metals, which would again open up new areas of research.

Related Links

Appl Phys Lett 2018, 112, 072403: "Large moments in bcc FexCoyMnz ternary alloy thin films"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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