Integrated ab initio modelling of atomic ordering and magnetic anisotropy for design of rare-earth-free permanent magnets
Hard magnetic materials, otherwise known as permanent magnets, are everywhere. Not just useful for making souvenirs to stick on the front of our refrigerators, permanent magnets find a range of essential applications in energy storage and conversion technologies such as electrical motors and generators. Thus, as the world makes the transition away from fossil fuels and towards renewable sources of energy, ever-greater quantities of permanent magnet are required to meet aspirations. A typical electric car drive motor can contain between 1 and 3 kg of permanent magnet, while offshore, direct-drive wind turbines can utilise more than 2 tonnes!
However, there is a problem: the raw materials used to make advanced permanent magnets—the rare-earth elements—are a constrained resource. Relatively abundant as a percentage of the earth’s crust, rare-earth ores require toxic chemistries to process into metals. In combination with other factors, a global desire to reduce our usage of these elements and associated materials translates to discovering and developing alternative magnets!
The desired, chemically ordered L10 phase of FeNi
As cast, FeNi forms a chemically disordered, A1 structure.
Our modelling approach successfully captures the A1/L10 atomic ordering transition in binary FeNi, showing that we are able to successfully predict the experimentally derived magnetic properties of the atomically ordered phase. Crucially, we are able to demonstrate that the FeNi system is only predicted to order favourably when the material is in the ferromagnetic state, suggesting any materials processing must take place below the Curie temperature. We then go on to examine what happens when a range of elements are added to the system, such as Al, Co, and Pt. All of these elemental additions alter ordering tendencies as well as affect the magnetic properties. But it is the case that what is given with one hand is often taken away with the other—for example, while the addition of Co enhances the Curie temperature and saturation magnetisation, it weakens the magnetocrystalline anisotropy energy as well as reduces the atomic ordering temperature. Of particular interest is the addition of Pt: although expensive, Fe-Pt alloys are known to have superb hard magnetic properties. In our work, we suggest that a composition of the form Fe4Ni3Pt will possess magnetic properties superior to FeNi, and exhibit an elevated atomic ordering temperature. These results provide tantalising hints that there may well be as-yet-untested compositions in the Fe-Ni binary system that could help diversify the range of viable permanent magnets, eventually contributing to a more secure global energy economy. The search for the elusive ‘gap’ magnet continues…
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npj Computational Materials
This journal publishes high-quality research papers that apply computational approaches for the design of new materials, and for enhancing our understanding of existing ones.
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