Heusler alloy
![In the case of the full Heusler alloys with formula X2YZ (e. g., Co2MnSi) two of them are occupied by X-atoms (L21 structure), for the semi-Heusler alloys XYZ one fcc sublattice remains unoccupied (C1b structure).](/Images/godic/202501/18/Heusler_alloy_-_structure3551.png")
![Electron microscope images of Cu-Mn-Al Heusler alloy showing magnetic domain walls tied to APB's (a) L21 antiphase boundaries by <111> dark-field imaging - the remaining micrographs are in bright-field so that the APB's are not in contrast (b) magnetic domains by Foucault (displaced aperture) imaging, and (c) magnetic domain walls by Fresnel (defocus) imaging.](/Images/godic/202501/18/Magnetic_Domains_on_Antiphase_Boundaries_in_Heusler_Alloy3551.jpg")
A Heusler alloy is a ferromagnetic metal alloy based on a Heusler phase. Heusler phases are intermetallics with particular composition and face-centered cubic crystal structure. They are ferromagnetic as a result of the double-exchange mechanism between neighboring magnetic ions. The latter are usually manganese ions, which sit at the body centers of the cubic structure and carry most of the magnetic moment of the alloy. (See the Bethe-Slater curve for more info on why this happens.)