Influence of plastic deformation on the magnetic properties of Heusler ${\mathrm{MnAu}}_2\mathrm{Al}$

The Heulser intermetallic ${\mathrm{MnAu}}_2\mathrm{Al}$ is shown to undergo a dramatic change in net magnetization in response to plastic deformation. A mechanism is proposed involving antiferromagnetic interactions in the otherwise ferromagnetic compound (when ordered) that arise due to chemical changes at the antiphase boundaries created by the deformation. The coupling between chemical and magnetic order across antiphase boundaries is likely to occur in other ordered magnetic systems and may provide an explanation for otherwise anomalous magnetic behavior across several systems, including other Heusler intermetallics.

Figure 1

X-ray diffraction patterns and Pawley refinement fits of annealed and ground ${\mathrm{MnAu}}_2\mathrm{Al}$. The annealed sample shows much higher intensity and narrower peak width than after grinding. This peak broadening is due to the nanoscale crystallite size.

Figure 2

(a) Magnetization vs applied magnetic field ($H$) for annealed and ground powders at 2 K. (b) Magnetization vs temperature at $H=0.02\mathrm{T}$ for each. Values of magnetization of the ground powder are multiplied by a factor of 100 so they may be viewed on the same axes. Dotted lines indicate zero-field-cooled measurements.

Figure 3

(a) $M$ vs $H$ at 2 K for an annealed piece and a piece that has been pressed in a hydraulic press after annealing. (b) Magnetization vs temperature at $H=0.02\mathrm{T}$ for each. Dotted lines indicate zero-field-cooled measurements.

Figure 4

(a) Bright field TEM micrograph and the corresponding SAED pattern revealing a fine grained microstructure. The diffraction rings identify the Heusler phase (H) and a secondary phase. (b) Atomic mass contrast HAADF-STEM reveals the secondary phase consists of 20-nm Au deficient particulates located at grain boundaries. (c) Strain sensitive HAADF-STEM identifies intra-granular contrast caused by low angle grain boundaries that have formed during plastic deformation.

Figure 5

Interfacial energy and magnetic configuration for planar faults in ${\mathrm{MnAu}}_2\mathrm{Al}$, as a function of the displacement vector in the (010) or (110) planes. (a) Atomic configuration of the low-energy APBs in the (010) and (110) planes, where the displacement vector in both cases is $a/2\left[001\right]$. (b) Interfacial energy of general planar faults ($\gamma$ surface) in the (010) and (110) planes as a function of the displacement vector. (c) Relative energy of antiferromagnetic and ferromagnetic alignments across these interfaces.

Figure 6

Mn-Mn magnetic exchange as a function of interaction distance. The couplings present in pristine ordered ${\mathrm{MnAu}}_2\mathrm{Al}$ are predominantly ferromagnetic out to long range. At APBs, the change in chemical order results in Mn on the Al sublattice. While the nearest-neighbor ${\mathrm{Mn}}_{\text{Mn}}\text{−}{\mathrm{Mn}}_{\text{Al}}$ interaction is ferromagnetic, the next-nearest-neighbor ${\mathrm{Mn}}_{\text{Mn}}\text{−}{\mathrm{Mn}}_{\text{Al}}$ interaction is antiferromagnetic with a larger magnitude and interaction multiplicity, resulting in a net antiferromagnetic configuration across the APBs.