Understanding the origin of magnetic moment enhancement in novel alloys

Magnetic materials have become indispensable to various technologies that support our modern society, such as data storage devices, electric motors, and magnetic sensors.

High-magnetization ferromagnets are especially important for the development of next-generation spintronics, sensors, and high-density data storage technologies. Among these materials, the iron-cobalt (Fe-Co) alloy is widely used due to its strong magnetic properties. However, there is a limit to how much their performance can be improved, necessitating a new approach.

Some earlier studies have shown that epitaxially grown films made up of Fe-Co alloys doped with heavier elements exhibit remarkably high magnetization. Moreover, recent advances in computational techniques, such as the integration of machine learning with ab initio calculations, have significantly accelerated the search for new material compositions.

Iridium (Ir)-doped Fe-Co alloy (Fe-Co-Ir) is one such material, identified through machine learning, that has been shown to possess large magnetic moments, representing the strength and orientation of magnetic fields, exceeding even those of conventional Fe-Co alloys.

However, identifying the source of these enhanced magnetic properties has been a significant challenge. In particular, the effect of Ir-doping on the magnetic properties of Fe-Co alloys remains poorly understood.

To overcome this challenge, a research team led by Assistant Professor Takahiro Yamazaki from the Department of Material Science and Technology at Tokyo University of Science (TUS) implemented a novel approach. They utilized high-throughput X-ray magnetic circular dichroism (XMCD) on compositionally graded single-crystal thin films.

Their study is published in the journal Physical Review Materials.

Asst. Prof. Yamazaki explains, “Unlike previous studies which used polycrystalline thin films, we utilized compositionally graded single-crystal Fe-Co-Ir thin films, offering a more controlled environment for probing the mechanisms behind their enhanced magnetic properties. Furthermore, using the world’s largest synchrotron radiation facility, SPring-8, we performed XMCD measurements to systematically investigate their magnetic properties.”

The team also included Mr. Takahiro Kawasaki and Prof. Masato Kotsugi from TUS, Dr. Yuma Iwasaki and Dr. Yuya Sakuraba of the National Institute of Materials Science (NIMS), Dr. Naomi Kawamura of the Japan Synchrotron Radiation Research Institute, and Prof. Takuo Ohkochi of the University of Hyogo.

Using the advanced technology at NIMS, the team first fabricated compositionally graded thin films in which the amount of Ir doping increased linearly from one end, consisting of pure Fe-Co alloy, to the other end, consisting of Fe-Co alloy with 11 at% Ir.

The team then performed X-ray magnetic circular dichroism (XMCD) measurements on these films, using both soft and hard X-rays.

Soft X-rays have lower energy than hard X-rays and are therefore better suited for studying lighter metals like Fe and Co, while hard X-rays are more suitable for studying heavy metals like Ir. This approach provided a more detailed understanding of each element’s contribution to the material’s magnetic behavior.

The results revealed significant improvements in the magnetic moments of both Fe and Ir due to Ir doping. The magnetic moment of Fe increased by 1.44-fold and Ir by 1.54-fold at 11 at% Ir concentration compared to that at 1 at% Ir concentration. To further validate and understand the origin of these enhancements, the team conducted ab initio calculations.

Fe and Co belong to a class of elements known as 3d transition metals, where their outermost electrons occupy the 3d atomic orbitals, while Ir belongs to 5d transition metals.

The theoretical analysis supported the experimental findings and revealed that Ir addition leads to increased electron localization and stronger spin-orbit coupling between 3d electrons of Fe and Co and 5d electrons of Ir. This interaction results in enhanced magnetic moments, primarily through increased contributions of orbital magnetic moments.

“The findings highlight the critical role of Ir in enhancing the magnetic properties of Fe-Co-Ir alloys,” notes Asst. Prof. Yamazaki.

“Our efficient, high-throughput materials evaluation workflow and theoretical analysis method will serve as a foundation for designing high-performance ferromagnetic materials. This could lead to the development of highly efficient electric motors and next-generation high-density data storage devices, which could ultimately reduce environmental impact and contribute to a more sustainable society,” he concludes.

Also, Fe-Co alloy with Ir could help in designing efficient electronic devices that can be made commercially available, with essential testing phases. This alloy has potential implementation in developing cost-effective data storage devices.