Designing Sustainable Permanent Magnets: Alloying Additions and Magnetocrystalline Anisotropy in L1₀ FeNi

We report results from an holistic approach for modelling both atomic ordering and subsequent magnetocrystalline anisotropy energy (MAE) of magnetic materials with application to the design of novel, rare-earth-free permanent magnets. This computationally efficient technique allows for fast exploration of the materials design space and could open up a new route for materials discovery and manufacturing. Here we report recent results concerning the class of magnetic materials which chemically order into the L1₀ structure. These materials typically have large MAE values appropriate for advanced permanent magnets, with prime examples being FePt and CoPt that contain elements of high criticality. <i>Ab initio</i> theory has previously confirmed that it is the tetragonal, L1₀ order that produces the high MAE values measured in these materials [1]. There is therefore a desire to discover new L1₀ materials which are made using more-abundant elements, but which still retain desirable magnetic properties. One such candidate material is L1₀ FeNi, found in meteoritic, tetrataenite samples. This material is known to have a high uniaxial anisotropy, but a low chemical ordering temperature and sluggish kinetics make it challenging to manufacture in a laboratory setting.<br/><br/>Here, we consider introducing a third element into the binary Fe-Ni composition to promote ordering tendencies and enhance its predicted MAE. Systems with the general formula Fe_50-xNi_50-yX_x+y (X=Pd, Co, Pt, Al) were investigated. Crucially, our modelling predicts the nature of any chemical order [2] and goes on to predict the MAE for a given system [1], using the same ab initio formalism for both aspects of the modelling approach. The ordering behaviour predicted on adding these dopants is rich and a variety of chemical orderings are obtained. Interestingly, we find that it is often the addition of light elements such as Al which enhance the MAE the most. We are also able to study the impact of magnetic order on predicted atomic order and show that annealing samples in an applied magnetic field may enhance chemical ordering [3].<br/><br/><b>Acknowledgements</b><br/>We gratefully acknowledge the support of the UK EPSRC, Grant No. EP/W021331/1. C.D.W. is supported by a studentship within the UK EPSRC-supported Centre for Doctoral Training in Modelling of Heterogeneous Systems, Grant No. EP/S022848/1. This work was also supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE SC0022168 (for atomistic insight) and by the U.S. National Science Foundation under Award ID 2118164 (for advanced manufacturing aspects).<br/><br/><b>References</b><br/>[1] J. B. Staunton et al., Phys. Rev. B <b>74</b>, 144411 (2006).<br/>[2] C. D. Woodgate and J. B. Staunton, Phys. Rev. B <b>105</b>, 115124 (2022).<br/>[3] C. D. Woodgate D. Hedlund, L. H. Lewis, J. B. Staunton, Phys. Rev. Mater. <b>7</b>, 053801 (2023).