First-ever rare-earth free permanent magnets with 20 MGOe energy product

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Figure 8. Microstructure of the prepared sample, observed by high resolution transmission electron microscopy (HRTEM). A 5 x 1017/cm2 fluence generates a microstructure with a clear boundary.

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A team of researchers from the University of Minnesota, Los Alamos National Laboratory, and Oak Ridge National Laboratory has prepared the first-ever rare-earth-free-magnet with a magnet performance of 20 MGOe (megagauss-oersted). The stored energy in a magnet is called magnet performance or maximum energy product. The journal Scientific Reports published the research.

Demand for permanent magnets is increasing due to their higher efficiency and smaller size than the electromagnets in motors and generators. The most powerful, durable, and useful permanent magnets contain the rare earth elements neodymium or samarium. However, rare earths present challenges: 1) they are expensive, 2) the extraction process is energy intensive and hazardous for the environment, and 3) a specific region of the world dominates the supply chain. All these pose a major problem when the world depends on rare-earth-based permanent magnets for running motors, generators, sensors, and many other machines and devices.

The group of researchers synthesized a rare-earth-free permanent magnet based on an iron (Fe) and nickel (Ni) α″-Fe16N2 martensite (metastable iron phase) structure. The raw material is abundant and cheap, and the excellent magnetic properties of the material (in theory up to 130 MGOe) had been predicted. However, the material had been very difficult to make previously. The desirable α″-martensite crystal structure is only stable below 214 ^oC, but a temperature greater than 300 ^oC is needed to give the material the correct grain microstructure. Therefore, the α″-Fe₁₆N₂ martensite microstructure cannot be achieved via traditional annealing.

The team bonded an iron layer to a silicon wafer. They prepared a 500 nm α″-Fe₁₆N₂ foil by first implanting a high fluence of nitrogen ions in a pure iron matrix, followed by a two-step thermal annealing process to engineer the foil for the high energy product. The investigators used the tunable capability of ion implantation fluence and energy to construct a microstructure with grain size of 25–30 nm on the FeN foil sample. Ion implantation is a commonly used technology in the semiconductor industry. The authors suggest that this method of materials processing could be developed for the bulk scale to manufacture the rare-earthfree magnetic material.

This is the first-ever demonstration of energy product of 20 MGOe from a rare-earth-free magnet. In comparison, neodymium and samarium magnets score 10-48 and 16-33 MGOe, respectively. The new material possesses a giant saturation magnetization and a large magnetic coercivity. Magnetic coercivity is a measure of the ability of a material to withstand an external magnetic field without becoming demagnetized. A large magnetic coercivity differentiates permanent (“hard”) magnets from “soft” magnetic materials that cannot support a permanent field. Thus the rare-earth-free-magnet demonstrates excellent properties for a permanent magnet.Reference: “Synthesis of Fe16N2 compound Free-Standing Foils with 20 MGOe Magnetic Energy Product by Nitrogen Ion-Implantation,” Scientfic Reports 6, 25436 (2016); doi: 10.1038/srep25436. Authors: Yanfeng Jiang, Md Al Mehedi, and Jian-Ping Wang (University of Minnesota – Minneapolis); Engang Fu and Yongqiang Wang (Materials Sciencein Radiation and Dynamics Extremes, MST-8); and Lawrence F. Allard (Oak Ridge National Laboratory).

Reference: “Synthesis of Fe16N2 compound Free-Standing Foils with 20 MGOe Magnetic Energy Product by Nitrogen Ion-Implantation,” Scientfic Reports 6, 25436 (2016); doi: 10.1038/srep25436. Authors: Yanfeng Jiang, Md Al Mehedi, and Jian-Ping Wang (University of Minnesota – Minneapolis); Engang Fu and Yongqiang Wang (Materials Sciencein Radiation and Dynamics Extremes, MST-8); and Lawrence F. Allard (Oak Ridge National Laboratory).

Electronics Weekly highlighted the work.

The Center for Integrated Nanotechnologies (CINT), a DOE Office of Science nanoscience user facility operated by Los Alamos and Sandia national laboratories, sponsored the Los Alamos participation. The work supports LANL’s Energy Security mission area and Materials for the Future science pillar through development of materials for clean energy. Technical contact: Yongqiang Wang

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Figure 9. Characterization results of a bulk Fe16N2 free-­‐standing foil prepared by the nitrogen ion implantation method with 5 × 1017/cm2 fluence (a) In-­‐plane hysteresis loops for the sample at room temperature, showing Hc = 1910 Oe, Ms = 245 emu/g, Mr = 216 emu/g; (b) The calculated energy product, indicating maximum value 20 MGOe; (c) The X-­‐ray diffraction spectrum, showing the Fe16N2 phase generated in the foil; (d) The HRTEM diffraction pattern of FeN sample with 5 × 1017/cm2 fluence, showing Fe16N2, Fe4N/Fe4−xNixN and Fe phases.