Crystallographic defects strongly influence alloy’s magnetic properties

An alloy of manganese, aluminum and carbon, MnAl-C is a good candidate to replace scarce and expensive rare earth materials in permanent magnets. Gusenbauer et al. show that crystallographic defects in this alloy strongly affect its magnetization properties.

Nano-sized twinning defects are found to be particularly important in determining this alloy’s coercivity, the ability of a magnetic material to withstand the demagnetization effect of an external field.

The investigators prepared alloys with the chemical composition Mn₅₃Al₄₅C₂ using a hot-extrusion technique. Transmission electron microscopy demonstrated that the resulting crystals transformed to regions with fine recrystallized grains and some non-recrystallized regions. The latter contained a large amount of crystallographic twins in a lamellar arrangement.

Twinning occurs when a single crystal has two or more regions with different orientations, which may be related by rotation about a symmetry axis. In this experiment, the lamellar regions were mostly found to be oriented 75.6^o to one another, so were true crystallographic twins.

Experimental studies and simulations confirmed the existence of twin defects makes saturating the magnet more difficult. They also cause a reduction in coercivity as well as the permanent magnet’s strength, a quantity known as the energy density product.

“When twins cannot be avoided, numerical optimization suggests that small grains and thin lamellas will improve the energy density product,” said author Markus Gusenbauer.

For these reasons, the authors found MnAl-C can be a suitable magnetic material, especially if twinning defects can be suppressed.

“If they cannot be completely removed, it is advisable to try reducing the grain size and the twin width of the lamellar structures,” said Gusenbauer.

Source: “Insights into MnAl-C nano twin defects by micromagnetic characterization,” by Markus Gusenbauer, Alexander Kovacs, Harald Oezelt, Johann Fischbacher, Panpan Zhao, Thomas G. Woodcock, and Thomas Schrefl, Journal of Applied Physics (2021) The article can be accessed at https://doi.org/10.1063/5.0035387 .