[Paper Review] Designing rare-earth free permanent magnets in Heusler alloys via interstitial doping

Qiang Gao, Ingo Opahle+2 more|arXiv (Cornell University)|Sep 7, 2019

Magnetic Properties of Alloys35 references4 citations

TL;DR

This study proposes a strategy to design rare-earth-free permanent magnets in cubic Heusler alloys by doping light interstitial atoms (H, B, C, N), which induce strong tetragonal distortions and significantly enhance uniaxial magneto-crystalline anisotropy energy (MAE). Using high-throughput density functional theory (DFT) calculations, the authors identify 32 candidates with MAE > 0.4 MJ/m³, attributing the enhancement to local chemical bonding changes and symmetry breaking around interstitials, offering a pathway to competitive permanent magnets without rare-earth elements.

ABSTRACT

Based on high-throughput density functional theory calculations, we investigated the effects of light interstitial H, B, C, and N atoms on the magnetic properties of cubic Heusler alloys, with the aim to design new rare-earth free permanent magnets. It is observed that the interstitial atoms induce significant tetragonal distortions, leading to 32 candidates with large ($>$ 0.4 MJ/m$^3$) uniaxial magneto-crystalline anisotropy energies (MAEs) and 10 cases with large in-plane MAEs. Detailed analysis following the the perturbation theory and chemical bonding reveals the strong MAE originates from the local crystalline distortions and thus the changes of the chemical bonding around the interstitials. This provides a valuable way to tailor the MAEs to obtain competitive permanent magnets, filling the gap between high performance Sm-Co/Nd-Fe-B and widely used ferrite/AlNiCo materials.

Motivation & Objective

• To develop rare-earth-free permanent magnets with high performance to overcome the cost and supply risks of Sm-Co and Nd-Fe-B magnets.
• To investigate whether light interstitial atoms (H, B, C, N) can induce large magneto-crystalline anisotropy energy (MAE) in cubic Heusler alloys.
• To identify stable Heusler compounds with substantial uniaxial MAE through high-throughput DFT screening.
• To understand the origin of enhanced MAE by analyzing local distortions and chemical bonding changes induced by interstitials.

Proposed method

• High-throughput density functional theory (DFT) calculations were performed to evaluate the effects of interstitial H, B, C, and N atoms on magnetic properties in cubic Heusler alloys.
• Interfacial and tetragonal distortions were analyzed by calculating the c/a ratio and structural relaxation under interstitial doping.
• Magneto-crystalline anisotropy energy (MAE) was computed using the total energy difference between out-of-plane and in-plane magnetization states.
• The MAE was further decomposed using orbital moment analysis and chemical bonding analysis via the integrated crystal orbital Hamilton population (ICOHP).
• Theoretical models, including Bruno’s model and perturbation theory, were applied to interpret the origin of enhanced MAE.
• Stability criteria, including negative formation energy and site preference, were used to filter viable candidates.

Experimental results

Research questions

• RQ1 Can interstitial doping in Heusler alloys induce sufficient uniaxial magneto-crystalline anisotropy energy (MAE) to qualify as high-performance permanent magnets?
• RQ2 What is the role of local chemical bonding and lattice distortion in enhancing MAE due to interstitial atoms?
• RQ3 Which interstitial elements (H, B, C, N) are most effective in inducing large MAE in cubic Heusler compounds?
• RQ4 How do interstitial atoms influence the orbital magnetic moments and anisotropy contributions of neighboring Fe and Ni atoms?
• RQ5 Can the MAE enhancement be attributed primarily to symmetry breaking or to specific chemical bonding changes?

Key findings

• A total of 32 Heusler compounds exhibited uniaxial MAE exceeding 0.4 MJ/m³ after interstitial doping, indicating strong potential for permanent magnet applications.
• The highest uniaxial MAE observed was 0.95 MJ/m³ in Ni₂FeGa with nitrogen interstitial doping, significantly exceeding the 0.03–0.04 MJ/m³ of conventional ferrite and AlNiCo magnets.
• Interstitial atoms such as C and N induced strong local chemical bonding with Fe and Ni atoms, particularly forming strong bonds with Fe-iii atoms below the interstitial, which enhanced their orbital moment contribution to MAE by over 200%.
• The MAE enhancement was primarily driven by local distortions and changes in chemical bonding, not just global tetragonal distortion, as evidenced by the strong correlation between ICOHP values and MCA contributions.
• H interstitials induced only minor MAE enhancement due to weak bonding (ICOHP = -0.63 eV), whereas C and N interstitials showed significantly stronger effects due to stronger bonds (ICOHP = -2.88 eV and -2.38 eV, respectively).
• The contribution of Fe-iii atoms to MAE increased to 0.681 meV/atom with N doping and 0.508 meV/atom with C doping—more than double the 0.260 meV/atom in the pristine compound—confirming the dominant role of local chemical environment.