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Tetrataenite Magnets

June 19, 2023

Iron is, by mass, the most common element on Earth, but iron oxidizes (rusts) rapidly, and metallic iron is rarely found on Earth's surface. Pure iron is a very soft material with about the same flexibility as aluminum. However, addition of less than two percent carbon produces carbon steel, and some carbon steels are up to 1000 times harder than pure iron. Another important property of iron and some iron alloys is their ferromagnetism. Pure α-iron, the room temperature body-centered cubic (BCC) crystal form of iron, is magnetic below its Curie temperature of 770 °C (1,420 °F; 1,040 K), and it can be made into a permanent magnet by cooling through its Curie temperature in the presence of a magnetic field.

Some iron alloys, in particular Alnico 5, an alloy of iron (51 at-%), aluminum (8 at-%), nickel (14 at-%), cobalt (24 at-%), and copper (3 at-%), were commonly used as permanent magnets in the mid-20th century. One popular use was as loudspeaker magnets. Such magnets were replaced by the superior rare-earth magnets in the 1970s. An important figure of merit of a magnet is its maximum energy product, the maximal value of the product of magnetic induction (B) and applied magnetic field (H) on its hysteresis curve. The maximum energy product of Alnico 5 is 5.5 mega-gauss-oersted (MGOe, 43.8 kJ/m3), while that of most NdFeB rare earth magnets is greater than 60 MGOe (~500 kJ/m3).

As I've written in several previous articles (including Rare Earths from Coal Waste, April 18, 2022, Rare Earth Metals from Fly Ash, July 7, 2016, and Materials Supply Chain, July 14, 2014), most rare earth mining and refining is in China. China has a near monopoly on global production, producing an estimated 60% of the world's rare earths in 2021, as compared with the 15% of the United States.[1]

2021 World Rare Earth Production (estimated)

Estimated rare earth production by country in 2021.

China produced 168,000 US tons of rare earth oxide equivalent in 2021, up from 140,000 the previous year.

(Chart created using Gnumeric from data in ref. 2.[2] Click for larger image.)

A non-rare earth permanent magnet would be welcome, especially one made from inexpensive elements such as iron. Laboratory synthesis of an unique iron-nickel alloy with promising magnetic properties was announced at the end of 2022 in an open access paper in Advanced Science.[2-6] The alloy, tetrataenite, is found in meteorites, but rarely found in nature. It was thought that it could be formed only by a very slow cooling process, estimated to be a few degrees per million years, that allowed diffusion of atoms into its L10 layered crystal structure. Prior laboratory synthesis was achieved by irradiation by neutrons or electrons that nudges its atoms into place.

Tetrataenite specimen

A specimen of tetrataenite, recovered from Zacatecas, Mexico. This is a portion of an H5 chondrite meteor that fell on December 15, 1978. Its size is 2.7x2.0x2.0 centimeters. The tetrataenite crystals appear as silvery patches.

(Wikimedia Commons image by Robert M. Lavinsky.)

The 2022 study, by materials scientists from the Istituto Italiano di Tecnologia (Genova, Italy), the Austrian Academy of Sciences (Leoben, Austria), the Montanuniversität Leoben (Leoben, Austria), and the University of Cambridge (Cambridge, UK) found that a small addition of phosphorus allowed creation of tetrataenite by usual smelting techniques.[2-6] Tetrataenite can be made permanently magnetic with a high coercivity that's essential for maintaining a permanently magnetic state. It's theoretical magnetic energy product is greater than 42 MGOe (335 kJ/m3), nearly on par with rare earth magnets.[7] However, all theoretical predictions should be taken with a grain of phosphorus until experimentally proven.

The research paper begins with a nice summary of the prior art with information from a review by Wasilewski.[8] This is summarized in the following table.

Methods of Tetrataenite Synthesis
Technique Material
Neutron irradiation (>1 MeV) Fe50Ni50
Electron irradiation (1 MeV) Fe29-49Nibalance
Cyclic oxidation and reduction of Ni-coated Fe particles Fe50Ni50
Molecular beam epitaxy Fe50Ni50
Reverse martensitic transformation Fe40-50Nibalance
High pressure torsion & annealing Fe-Ni-Co alloys
Crystallization of glassy ribbons Fe-Ni-Si-B-P-Cu
Cold rolling to 86% reduction in thickness Fe54Ni46 and Fe52Ni46Ti2
Cryo-milling of melt-spun ribbons Fe52Ni46Ti2
Nitriding and denitriding Fe50Ni50
Cyclic oxidation and reduction of nanopowder Fe50Ni50

Global demand for rare earth magnets is estimated to reach $37 billion by 2027.[6] Aside from China's putative control of the rare earth market, there are environmental problems as well. Vast quantities of ore need to be mined to extract small quantities of these elements.[4] Furthermore, the subsequent processing of the ore is environmentally hazardous.[6] The tetrataenite research team was not intentionally on a quest to synthesize this material. They were doing research on the mechanical properties of glassy Fe-Ni-P-B alloys when they discovered dendrites in their specimens.[4-5] Careful measurements using X-ray diffraction and transmission electron microscopy revealed the L10 tetrataenite crystal phase.[4-5]

In their experiments, the alloying elements were cast at 1123 K in copper molds and then rapidly cooled at a rate of 10-104 K/sec.[2,5] Adjustment of the element ratios showed that the rapid creation of the L10 phase was caused by the phosphorus.[5] Rods cast with a larger diameter, and therefore at a lower cooling rate, had a smaller volume fraction of the dedritic tetrataenite.[2] The L10 phase had lattice constants of a = 0.255±0.001 nm, and c = 0.363±0.001 nm.[2]

SEM image of tetrataenite dendrites

scanning electron micrograph of a lateral section of a 1 mm diameter rod of Fe50Ni30P13C7 showing tetrataenite dendrites in an alloy matrix.

(Fig. 1b of ref. 2, licensed under a Creative Commons License.)

Says study author and postdoctoral research associate at the University of Cambridge, Yurii Ivanov, "The presence of phosphorus seems to be critical in permitting formation of tetrataenite without such treatments as neutron irradiation."[5] The role of phosphorous is to stabilize vacancies in the crystal lattice and accelerate atomic mobility.[2]. Calculations indicate that the atomic mobility at 573 K is accelerated by a factor of more than 10,000 by the addition of 1 at-% P.[2]

A patent application has been filed, but the study authors did not attempt to create or characterize permanent magnets from their material.[2,4] Some experts see no prospect of these alloys replacing rare-earths in any permanent magnet application.[5] The Cambridge researchers agree that this alloy might not match high-performance neodymium-based magnets.[5] Says study author and professor of materials science at Cambridge, Lindsay Greer,
"The analogy here would be that we have shown we can make a brick - a piece of tetrataenite - but not yet a house - a magnet."[5]
The research was funded in part by the European Research Council and the Austrian Science Fund.[4]


  1. Rare Earths Statistics and Information, National Minerals Information Center, U.S. Geological Survey, 2022.
  2. Yurii P. Ivanov, Baran Sarac, Sergey V. Ketov, Jürgen Eckert, and Lindsay A. Greer, "Direct Formation of Hard‐Magnetic Tetrataenite in Bulk Alloy Castings," Advanced Science, vol. 10, no. 1 (October 25, 2022), Article no. 2204315, doi:10.1002/advs.202204315. This is an open access article with a PDF file here.
  3. Laura H. Lewis, Ian J. Mcdonald, Sahar Keshavarz, and R. William McCallum, "Method of tetratenite production and system therefor," US Patent No. 11,462,358, October 4, 2022.
  4. New approach to 'cosmic magnet' manufacturing could reduce reliance on rare earths in low-carbon technologies, University of Cambridge Press Release, October 25, 2022.
  5. Sarah Wells, "Making Cosmic Magnets on Earth," Physics, vol. 15, no. 182 (November 23, 2022).
  6. Tanner Stening, "Accelerating the Production of Tetrataenite as Alternative to Rare-Earth Magnets," Northeastern University Press Release, October 7, 2022.
  7. E. Dos Santos, J. Gattacceca, P. Rochette, G. Fillion, and R.B. Scorzelli, "Kinetics of tetrataenite disordering," Journal of Magnetism and Magnetic Materials, vol. 375 (February 1, 2015), pp. 234-241, https://doi.org/10.1016/j.jmmm.2014.09.051.
  8. Peter Wasilewski, "Magnetic characterization of the new magnetic mineral tetrataenite and its contrast with isochemical taenite," Physics of the Earth and Planetary Interiors, vol. 52, no. 1-2 (October, 1988), pp. 150-158, https://doi.org/10.1016/0031-9201(88)90063-5.

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