Iron Nitrides: Powerful Magnets Without The Rare Earth Elements

Since their relatively recent appearance on the commercial scene, rare-earth magnets have made quite a splash in the public imagination. The amount of magnetic energy packed into these tiny, shiny objects has led to technological leaps that weren't possible before they came along, like the vibration motors in cell phones, or the tiny speakers in earbuds and hearing aids. And that's not to mention the motors in electric vehicles and the generators in wind turbines, along with countless medical, military, and scientific uses.

These advances come at a cost, though, as the rare earth elements needed to make them are getting harder to come by. It's not that rare earth elements like neodymium are all that rare geologically; rather, deposits are unevenly distributed, making it easy for the metals to become pawns in a neverending geopolitical chess game. What's more, extracting them from their ores is a tricky business in an era of increased sensitivity to environmental considerations.

Luckily, there's more than one way to make a magnet, and it may soon be possible to build permanent magnets as strong as neodymium magnets, but without any rare earth metals. In fact, the only thing needed to make them is iron and nitrogen, plus an understanding of crystal structure and some engineering ingenuity.

To start things off, what even is a permanent magnet? Like many simple questions about nature, there's no easy answer that doesn't require a fair amount of hand-waving. Even physicists eventually get to a point where their answer comes down to, "We just don't know." But that doesn't mean magnetism is a complete mystery, and the things that we do know about it are pretty straightforward, and actually help in understanding how both rare earth magnets and their alternatives work.

We’ve taken a stab at the basics of magnetism before, but to summarize, any charged particle, like an electron, has what's known as an intrinsic magnetic moment, meaning they act like little magnets. In atoms with filled electron shells, these magnetic moments cancel each other out because each pair of electrons have moments that point in opposite directions. But in atoms with unpaired electrons in their outer shells, there's nothing to cancel out the magnetic moments, which means these elements are magnetic. These elements tend to come from two specific areas of the periodic table: the d-block metals like cobalt, nickel, and iron, and the f-block actinides lanthanides, which include the rare-earth metals like samarium, neodymium, and praseodymium.

There's more to a magnet than just where its ingredients came from on the periodic table, though. Magnetism is about getting all those intrinsic magnetic moments lined up and acting in the same direction. Just like the electrons in an atom of a magnetic element have to not fight each other, the atoms must also arrange themselves so that their magnetic moments are all pointing the same way. This is referred to as having a high magnetic anisotropy, and is one of the characteristics of strong magnets. Rare-earth metals like neodymium have very high magnetic anisotropy, which contributes to the strength of rare-earth magnets.

But rare-earth metals by themselves actually make pretty poor magnets, at least on a practical level. This is due to their relatively low Curie point, which is the temperature above which a substance loses its magnetic properties. At room temperature, a pure bar of neodymium wouldn't be a magnet at all. In fact, it would need to be chilled to below 20 K to have any magnetic properties. To get around that, rare-earth metals are mixed with other ferromagnetic elements to form alloys that have a strong magnetic coercivity while also having a decent Curie point. The most common rare-earth magnet alloy, a combination of iron, neodymium, and boron, has a Curie temperature in the range of 300-400°C, depending on the exact mix of elements.

Going further down the rabbit hole of magnetism requires getting comfortable with the concepts of crystallography. This is a fiendishly complicated subject, with nomenclature and terminology that's confusing because it seems like it's the same as standard chemical formula notation, but it's clearly not. A full understanding of how adding neodymium to iron makes a powerful permanent magnet, and how making a powerful magnet without any rare earth is possible, would require a deeper dive into crystallography than we have space for here. Luckily, the basics will suffice, along with a little hand waving. And credit is due here to my friend Zachary Tong, who pitched in and helped me get my head around these difficult topics.

The crystal structure of a substance is all about how its atoms pack together into ordered arrangements. The building block of crystals is called the unit cell, which is the smallest possible repeating unit of the crystal. For neodymium magnets, the unit cell formula is Nd2Fe14B. This is confusing when you look at schematics of the crystal structure, which show far more than two neodymium atoms and fourteen irons. But the important thing here is that the unit cell shape of Nd2Fe14B is what's known as simple tetragonal (ST), which sounds like it should be a pyramid but is actually a cube that has been stretched along one axis. This axial asymmetry gives each crystal a high degree of magnetic anisotropy, which is part of the reason that neodymium magnets are so strong. The other factor is that the neodymium boosts the number of unpaired electrons in the alloy compared to plain iron alone, which makes for a stronger overall magnetic moment.

So with all this in mind, how can the addition of nitrogen to iron make magnets that have properties comparable to rare-earth magnets? Again, it's partly to do with the crystal structure, and partly to do with the electronic structure of the elements in the alloy. Iron normally has a unit cell that's either body-centered cubic (BCC), where eight iron atoms are centered on the corners of a perfect cube and one atom is at the dead center, or face-centered cubic (FCC), with an atom at each corner and one in the center of each face. But when nitrogen is alloyed with iron, the cubic unit cell structure gets distorted in what's known as a body-centered tetragonal (BCT) structure. What happens is that the nitrogen atoms get incorporated into the interstitial space of the crystal, elongating one side. This asymmetry is similar to the tetragonal crystal structure of neodymium magnets. Coupled with the ferromagnetic properties of iron, the result is a strongly magnetizable alloy without the need for rare-earth metals.

Iron nitrides are nothing new. Nitriding processes, such as gas nitriding by exposing heated steel to ammonia, have been used for steel finishing for more than a century. The more complex iron nitride α"-Fe16N2 was first discovered in 1951; its magnetic properties were explored in the early 1970s and again in the 1990s as part of the search for new and better heads for hard drives and other magnetic recording media.

This alloy showed promise in magnetics but proved difficult enough to work with that results weren't easily reproducible, so interest in α"-Fe16N2 waned until the late 2000s, when methods of producing thin films of the material were developed. These experiments showed that these films may have two to three times the magnetic energy product, a key measurement in determining the strength of a magnet, than neodymium magnets. Along with all the other properties that have been discovered, this makes iron nitride an excellent candidate for a new kind of magnet without rare earth elements.

With most scientific discoveries, there's a long way between the lab and a practical commercial product, and this is true with iron nitride magnetics. A lot of the recent advances in iron nitride permanent magnets have come from the lab of Jian-Ping Wang in the Department of Electrical and Computer Engineering at the University of Minnesota. Four different methods for synthesizing bulk α"-Fe16N2 material have been developed there, some of which show some promise in the industrial environment.

The earliest methods of making α"-Fe16N2 required a high-temperature process with rapid quenching of the nitrided sample, which doesn't lend itself to scaling up to industrial production. One of the first attempts to get around this was the use of ion implantation. This technique, in which ions are accelerated in a vacuum by a strong electric field and slammed into a target substrate, is common in semiconductor manufacturing, where it is used to dope silicon wafers. To make iron nitride magnets, pure iron foils 500 nm thick are mounted on a silicon substrate and bombarded with atomic nitrogen ions. This is followed by a series of annealing steps, which activate the implanted nitrogen and produce a thermal strain in the material that traps the nitrogen inside the foil's crystal structure, producing the distortion necessary. Foils of α"-Fe16N2 made this way show hard magnetic behavior, and practical magnets can be made by stacking the foil into layers and binding them into a single structure.

Low-temperature nitridation is also possible, using iron oxide nanoparticles as a starting material. In this method, the particles are treated with ammonia gas to get the nitrogen into the crystal structure. Alternatively, iron oxide can be mixed with ammonium nitrate in a planetary ball mill; after a few days of milling at 600 rpm, the stainless steel balls decompose the ammonium nitrate into elemental nitrogen, which diffuses into the iron nanoparticles. The resulting α"-Fe16N2 is then separated by magnet and can be formed into solid shapes. This method seems like it would easily scale up to an industrial process.

High-temperature nitridation of iron foils and wires is also possible. This method uses ribbons of an iron-copper-boron alloy and exposes it to an atmosphere of ammonia and hydrogen at 550°C for 28 hours, followed by a rapid treatment at 700°C and an ice-water quench. A variation on this method is the strained-wire approach, where high-purity iron is melted in a crucible with urea. The nitrogen that decomposes from the urea diffuses into the iron, and the mixture goes through heat treatment and quenching steps before being hammered flat and cut into strips. The strips are put into a straining device and stretched during an annealing step, which serves to elongate the crystal structure and trap the diffused nitrogen.

Strong permanent magnets aren't the only thing that iron nitrides might be good for. Soft magnetics, which are materials with lower coercivity and are good for things like the cores of transformers and inductors, or for read-write heads of magnetic media, may also be possible by doping α"-Fe16N2 with elements like carbon, oxygen, or boron. These dopants reduce the magnetic anisotropy of the crystal structure, making it harder to permanently magnetize them while maintaining high saturation magnetization.

There's a lot of promise to so-called "clean-earth" magnets — so much so that the University of Minnesota has spun off a company, Niron Magnetics, to turn the concepts and processes into products. We’re keen to see where this technology goes, and look forward to powerful magnets made with nothing but rust and fertilizer.