The Mn-Ga binary system contains a phase with the L10 structure, which occurs at compositions between MnGa and Mn2Ga. Due to the large separation of the planes of Mn atoms along the c-axis, ferromagnetic coupling of the Mn moments occurs. The phase is only stable at compositions richer in Mn than MnGa and the excess Mn atoms necessarily occupy Ga sites in the unit cell. The Mn atoms on Ga sites couple anitferromagnetically with Mn atoms on Mn sites and this reduces the magnetisation of the material. The magnetic properties are therefore sensitive to composition and further investigations are needed in order to determine if L10 Mn-Ga alloys fulfil the technical requirements for rare earth free permanent magnets.
Figure 1: The L10 structure and magnetic order of equiatomic MnGa. The very small magnetic moment of the Ga atoms is not shown.
The L10 structure of the Mn-Ga system was synthesised in the composition range of 55-65 at% Mn by annealing the quenched high temperature phase. XRD measurements were used to show that single phase L10 samples were obtained. The characterisation of the magnetic properties with SQUID and high field VSM (max. applied field of 14 T) confirmed that the magnetisation decreased with increasing Mn content, which was attributed to the antiferromagnetic coupling of the overstoichiometric Mn atoms siting on Ga sites. This decrease in magnetisation was accompanied by an increase of the magnetocrystalline anisotropy and coercivity (Fig. 2) [Mix2015].
Figure 2: XRD patterns of Mn-Ga alloys with different Mn content showing the single phase L10 structure (left). Room temperature SQUID measurements of the L10 Mn-Ga compounds showing the increasing coercivity and decreasing magnetisation with higher Mn contents (right).
Attempts to increase the coercivity of the samples by producing powders were successfully carried out. Thereby the magnetisation of the magnetic powder was reduced but could be retrieved by post milling annealing procedures. With this method an enhanced coercivity of up to 10 times could be achieved. Furthermore it was possible to hot compact the milled powders with increased coercivities and obtain packing densities of 83 % to 99 %. The coercivity could be retained during the procedure and first evidence for a partial alignment of the powders could be seen (Fig. 3). The possibility of aligning the powders under the action of an applied magnetic field was investigated. With fine powders a degree of texure of 0.45 was achieved (a value of 0 represents a randomly oriented material and 1 represents a perfectly textured material). The limitation of the alignment process could be seen in the polycrystalline highly twined nature of the powder particles (Fig. 3) [Mix2015].
Figure 3: SQUID measurements of the Mn60Ga40 compound in bulk, powder and hot compacted state (left). Secondary electron image of the hot compacted sample showing the polycrystalline and highly twinned nature of the powder particles (right).