Structure-magnetism correlations in the ε --> τ transformation in nanostructured MnAl magnets: a low-energy route for permanent magnet processing

Abstract

A promising strategy for realization of new concepts in permanent magnetic materials is to revisit the fundamentals governing structure-magnetic property behavior in ‘classic’ hard magnetic alloys that were discovered before the advent of the rare-earth-based supermagnets [i] and employ advanced fabrication and analysis methods to improve their magnetic performance. Among the list of hard magnetic materials, the intermetallic MnAl compound with the tetragonal L10 type crystal structure (τphase) combines favorable aspects such as low density, large corrosion resistance, and low costwith good permanent magnetic performance. Advanced fabrication methods for MnAl that provide a refined microstructure with small crystallites are envisioned to provide enhanced surface-to-volume ratios to promote formation of the intermetallic L10–typeτ-phase from the metastable-MnAl parent phase, with lowered phase transformation temperatures. Non-equilibrium processing techniques, such as melt-spinning and mechanical milling, allow access to thermodynamically-metastable phases and microstructures that can be beneficial, including nanoscaled features and enhanced magnetic properties. Results are reported here that support the hypothesis that the temperature required for t-phase formation in the MnAl system is dependent on details of the microstructure. Mn 55Al45 ribbons were synthesized by rapid solidification methods (melt-spinning), using a silica crucible and a wheel speed of 64 m/s. The structural properties of the produced ribbons, characterized by x-ray diffraction, confirm a majority hexagonal-MnAl phase of average grain size 30-60 nm which displays a strong antiferromagnetic character at low temperatures (TB<95 K). Fluctuations in the local Mn content and interactions between Mn-rich regions and Mn-poor areas render large values of exchange bias in the magnetization loops at low temperatures (HE∼13kOe at T= 10 K). [ii] The annealing-induced transformation from-MnAl to τ-MnAl has been characterized using Superconducting Quantum Interference Device (SQUID) magnetometry, x-ray and neutron diffraction, and differential scanning calorimetry. The e-MnAl to τ-MnAl transformation is revealed by the appearance of a ferromagnetic signal after annealing at 568 K (295◦C), a temperature that is 50-100 K lower than that previously reported for the onset temperature of the–>τ transformation in quenched MnAl alloys. [iii] The large reduction in phase transition temperature is attributed to the small size of the e-MnAl parent phase grains. Understanding the nature of the -MnAl –>τ -MnAl transformation could enable further reduction of the transformation temperature, which may play an impactful role for improving fabrication and processing techniques and strategies, and engineering of L10-MnAl permanent magnets.