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Rare Earth free permanent magnets

Rare Earth free permanent magnets

Among currently investigated rare-earth-free magnets, ferromagnetic τ-MnAl is a potential candidate having promising intrinsic magnetic properties. Mn(Fe)AlC was synthesized by mechanical alloying method. Effects of carbon on microstructure and magnetic properties were systematically investigated. It was found that high purity of τ-MnAl(C) could be obtained at 2 at.% C doping, showing clearly stabilizing effect of carbon. Mn54.2Al43.8C2 has the best magnetic properties: magnetization at 2T M2T = 414 kA.m-1, remanent magnetization Mr = 237 kA.m-1, coercivity HC = 229 k.Am-1, and |BH|max = 11.2 kJ.m-3. HC increased inversely with the crystallite size of τ phase and proportionally with C content. Moreover, first principle calculation showed both stabilizing effect and preferable interstitial positions of carbon in tetragonal τ-MnAl. Mn51-xFexAl47C2 (x= 0.25, 0.5, 1, 2, 4, 6) alloys were also synthesized by mechanical alloying method, showing the high purity of τ phase up to 2 at.% Fe doping. Adding of Fe on MnAl(C) reduced both magnetization and TC but likely increased slightly HC. 57Fe Mössbauer spectrometry at 300K was used to probe local environment in ε-, τ-, β-, and γ2-MnFeAl(C). In which, γ2-, ε-, and β-MnFeAl(C) exhibited a quadrupolar structure while τ -Mn50.5Fe0.5Al47C2 spectrum showed a rather complex magnetic hyperfine splitting. The interaction between Fe and Mn examined by in-field Mössbauer measurement at 10 K and 8 T showed a non-collinear magnetic structure between Fe and Mn with different canting angles at different sites. Hyperfine field of MnFeAl alloy calculated by Wien2k supported both magnetic properties and Mössbauer results.

Changes in magnetic moment with substitution. Mössbauer spectra showing ferrimagnetic order. Magnetization curve of substituted phases

Related papers

  • Tang Nguyen, F. Calvayrac, A. Bajorek and N. Randrianantoandro, Mechanical alloying and theoretical studies of MnAl(C) magnets, Journal of Magnetism and Magnetic Materials, 2018, 462, 96–104
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