  The origin of incommensurate structural modulation in Ni-Mn based Heusler
type magnetic shape memory alloys (MSMAs) is still an unresolved issue inspite
of intense focus on this due to its role in the magnetic field induced
ultra-high strains. In the archetypal MSMA Ni2MnGa, the observation of
non-uniform displacement of atoms from their mean positions in the modulated
martensite phase, premartensite phase and charge density wave as well as the
presence of phason broadening of satellite peaks have been taken in support of
the electronic instability model linked with a soft acoustic phonon. We present
here results of a combined high resolution synchrotron x-ray powder diffraction
(SXRPD) and neutron powder diffraction (NPD) study on Ni2Mn1.4In0.6 using
(3+1)D superspace group approach, which reveal not only uniform atomic
displacements in the modulated structure of the martensite phase with
physically acceptable ordered magnetic moments in the antiferromagnetic phase
at low temperatures but also the absence of any premartensite phase and phason
broadening of the satellite peaks. Our HRTEM studies and first principles
calculations of the ground state also support uniform atomic displacements
predicted by powder diffraction studies. All these observations suggest that
the structural modulation in the martensite phase of Ni2Mn1.4In0.6 MSMA can be
explained in terms of the adaptive phase model. The present study underlines
the importance of superspace group analysis using complimentary SXRPD and NPD
in understanding the physics of the origin of modulation as well as the
magnetic and the modulated ground states of the Heusler type MSMAs. Our work
also highlights the fact that the mechanism responsible for the origin of
modulated structure in different Ni-Mn based MSMAs may not be universal and it
must be investigated thoroughly in different alloy compositions.
