Helical magnetic order in Ni2MnAl

Magnetic susceptibility, X-ray diffraction and neutron diffraction measurements have been made on quenched and slow-cooled samples of Ni 2 MnAl alloy. Above 300K the susceptibility is Curie-Weiss but on cooling below this temperature the susceptibility becomes field dependent. Neutron diffraction patterns indicate that the compound is chemically ordered into the B2 structure with some additional L2 1 (Heusler alloy) type of order, that is enhanced by slow cooling. At 4.2K magnetic scattering was observed in fundamental reflections and in pairs of satellite reflections which were displaced by +or-0.111a* from the reciprocal lattice points corresponding to the nuclear reflections. The positions and intensities of the magnetic reflections are consistent with an antiferromagnetic cone spiral structure associated with the Mn atoms. The collinear components are coupled ferromagnetically in (111) sheets with antiferromagnetic coupling between adjacent planes.