Antiferromagnetic coupling of anti-phase domains

NiMn-based Heusler compounds show a variety of interesting functional properties such as the ferromagnetic shape memory effect or the magnetocaloric effect. Since the magnetic properties are very sensitive to the degree of structural order in these systems [1], understanding the correlation of these two ordering parameters is crucial.

In these systems the magnetic moments are mainly carried by the Mn atoms, which interact ferromagnetically in the L2₁-ordered state and antiferromagnetically for neighbouring Mn spins. Using TEM and Lorentz TEM a reversal of magnetization at structural anti-phase domain boundaries (APBs) has been observed in Ni₂MnAl_0.5Ga_0.5 implying that structural and magnetic domains are identical [2]. To clarify the reason of this relation and to study the mechanism of coupling of ferromagnetic domains across APBs, we investigated Ni₂MnAl and Ni₂MnAl_0.5Ga_0.5 powder samples in distinct ordering states via magnetic small-angle neutron scattering, giving access to the magnetization microstructure. With an external magnetic field applied we observe a redistribution of intensity along the field direction, implying that the domains couple antiferromagnetically.

[1] P. Neibecker et al., Appl. Phys. Lett. 105, 261904 (2014)

[2] H. Ishikawa, Acta Mater. 56, 4789 (2008)