Iron Nitride Magnets: History, Theory, and Commercial Prospects

In 1972, two scientists announced the discovery of a remarkably strong magnet, claiming it possessed the highest saturation magnetization at room temperature among all known magnetic materials. This material, composed of common elements like iron and nitrogen, sparked a four-decade-long debate about the reality of its magnetism.

Understanding Magnetism

Magnetism, specifically ferromagnetism, originates from the magnetic moments of electrons. These moments arise from an electron's intrinsic spin and its orbital motion around the atomic nucleus. These tiny magnetic moments act as individual magnets. When many of them align properly, they create a strong net magnetic field.

The challenge lies in maintaining this alignment. In materials like iron, cobalt, and nickel, special quantum effects help align electron magnetic moments into "magnetic domains." However, these domains are often unordered, causing their magnetism to cancel out, resulting in a weak or zero net magnetic field.

While an external magnet can temporarily align iron's domains, creating a magnetic field, this effect is not permanent. Heat energy eventually disorganizes these domains. Permanent magnets, in contrast, have atomic structures that force and maintain the alignment of these moments in one direction. Most powerful permanent magnets combine iron, nickel, or cobalt with rare earth elements like neodymium or samarium.

The Discovery of Iron Nitride Magnetism

In 1971, Japanese scientist Mikaku Takahashi, while researching magnetic properties of evaporated thin films at the University of Tokyo, made an unexpected discovery. His team, including Mr. Hatayama and Mr. T.K. Kim, was experimenting with thin nickel films using vacuum evaporation deposition. This process involves heating a metal in a vacuum until it vaporizes, then condensing it onto a target as a thin film.

During their experiments, they observed that lowering the vacuum pressure introduced more oxygen and nitrogen into the chamber. With nickel, this led to the formation of nickel oxide, which is antiferromagnetic, reducing the overall magnetism of the film. This was expected.

However, when the vacuum reached approximately 2 mTorr, a level considered low, the magnetization measurement unexpectedly surged. This anomaly prompted Takahashi to ask his graduate student, Mr. Kim, to repeat the experiment with iron. To their surprise, the iron films also showed a sudden increase in magnetism at around 3 mTorr vacuum.

The most astonishing aspect was the sheer strength of this magnetism. The material exhibited a saturation magnetization of 1800 to 1900 Gauss, significantly higher than pure iron's 1700 Gauss. This was comparable to iron 65 cobalt 35, then considered the most highly magnetized permanent magnet.

Analysis revealed that the iron films contained a mixture of alpha iron and a phase of iron nitride called alpha double prime Fe16N2. This specific crystal structure, modified by nitrogen atoms, showed an average magnetic moment of about three Bohr magnetons per iron atom, far exceeding ordinary iron's 2.2 Bohr magnetons per atom. Takahashi envisioned a new permanent magnet made from iron and atmospheric elements, eliminating the need for expensive cobalt.

Decades of Doubt and Rediscovery

After confirming their measurements through multiple methods, Takahashi's team published their findings in 1972, titled "New Magnetic Material Having Ultrahigh Magnetic Moment." However, the scientific community struggled to replicate these results consistently. A leading Japanese physics theoretician, believed to be Junjiro Kanamori, found the calculations for such magnetism impossible. A German professor also failed to obtain the Fe16N2 giant magnetic moment. Even Takahashi's own lab found it difficult to consistently reproduce their earlier results, leading to "scattered data." Over the next decade, Takahashi himself began to doubt his initial findings.

The crystal structure of alpha double prime Fe16N2 was not new; it had been discovered by Kenneth Henderson Jack in 1951. However, Jack and other researchers had never reported any unusual magnetic properties.

The mystery persisted until the mid-1980s when scientists from Hitachi, led by Sugita, approached Takahashi. Hitachi was seeking more powerful magnets for hard disk drives. Utilizing advanced tools like molecular beam epitaxy, the Hitachi team evaporated and deposited a thin film of iron in the presence of nitrogen gas onto an indium gallium arsenide substrate.

In 1991, Hitachi published their results, reporting a 34-nanometer thick film with a magnetic field strength of 2.8 to 3 Tesla, translating to an average magnetic moment of 3.2 to 3.5 Bohr magnetons per atom. This reignited interest, but again, consistent replication proved elusive. Despite two symposiums dedicated to iron nitride magnetism in 1994 and 1995, no definitive conclusions were reached, and research interest waned in the 2000s.

The Cluster Plus Atom Theory and Commercialization

In 2010, Professor Jian-Ping Wang and his team at the University of Minnesota published a theory called "cluster plus atom" to explain the quirky behavior of iron nitride magnets. This theory's timing was fortuitous, as China's ban on rare earth exports to Japan that same year highlighted the vulnerability of rare earth-dependent permanent magnets.

Professor Wang further developed his theory and pioneered a method to manufacture these magnets. In 2013, he founded Niron Magnetics, a startup funded by the US government, to commercialize the technology.

How Iron Nitride Magnets Work: The Cluster Plus Atom Theory

The cluster plus atom theory posits that iron nitride magnets are still soft iron ferromagnets, but nitrogen atoms reconfigure electron flow in a unique way.

1. Ordinary Iron: In pure iron, electrons are spread out and travel evenly throughout the crystal lattice, creating a strong but not exceptionally high net magnetism.
2. Nitrogen's Role: When small nitrogen atoms are introduced into the alpha double prime Fe16N2 crystal structure, they get "jammed" between iron atoms, forming unique octahedral clusters.
3. Electron Behavior in Clusters: Within these nitrogen clusters, electrons behave differently. They become more subject to Hund's rule, a quantum principle that organizes electrons to create stronger magnetic moments.
4. Roaming Glue: The remaining iron atoms outside these clusters act as a "roaming glue," flowing between the nitrogen clusters and collectively organizing the cluster magnetic moments to create the crystal's overall strong magnetic field.

Professor Wang's team provided evidence for this theory through two instrument measurements:

• X-ray Magnetic Circular Dichroism (XMCD): This chemical tool, which measures magnetic properties, showed two distinct "fingerprints" for the iron nitride magnet, indicating its dual nature.
• Polarization Dependent X-ray Near Edge Spectroscopy (XANES): This spectroscopy measurement, when applied to different parts of the crystal, revealed that iron atoms near nitrogen clusters were "trapped," as predicted by the cluster plus atom theory.

The difficulty in consistently replicating these giant magnetic moments stems from the delicate nature of the cluster plus atom state. It requires the iron crystal to be "strained," meaning the atoms are literally stretched apart. This strain is crucial for free electrons to travel properly and is a key factor in achieving consistent Fe16N2 results.

Weaknesses and Commercial Potential

Despite its promise as an environmentally friendly alternative to rare earth permanent magnets, Fe16N2 has significant weaknesses:

1. Lack of Coercivity: This is its greatest fundamental shortcoming. While Fe16N2 has high saturation magnetization, it is easily demagnetized when exposed to an opposing magnetic force. Neodymium iron boron magnets have a coercivity of 10 kOe or higher, whereas iron nitride ranges from 1.29 to 3 kOe, depending on its physical form.
2. Unstable Crystal Structure: Fe16N2 decomposes at temperatures between 200 and 500 degrees Celsius into ordinary iron and Fe8N, with its magnetic power degrading even before these temperatures. This instability limits its use in high-temperature applications like electric vehicles or turbines. Alloying with a third element like vanadium is being explored as a potential solution.
3. Difficulty in Manufacturing: Producing these magnets is complex, requiring processes like nitrogenation to stuff nitrogen atoms into the iron lattice. Achieving large blocks of iron nitride magnets, similar to neodymium iron boron magnets, remains elusive.

Niron Magnetics has attracted investments from major partners like GM, Stellantis, Western Digital, Samsung, and Volvo, and is building a US factory with an annual capacity of 1,500 tons of magnets.

The scientific community generally acknowledges the reality of iron nitride magnets but cautions against calling them "game-changing." A 2025 paper by Mantas Derba and John Ormerod suggests that these magnets have commercial value primarily in niche applications that do not require high heat or coercivity, such as stereo speakers. Their commercial potential is seen as similar to upscaled Alnico magnets (aluminum, nickel, cobalt alloys), valuable but not comparable to the widespread impact of neodymium iron boron magnets.

The search for true rare earth-free permanent magnets continues, with other candidates like iron-cobalt or iron phosphide alloys, and tetrataenite (found in meteorites), also being explored.