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In search for the origins of the extraordinary low twinning stress ofNi-Mn-Ga magnetic shape memory alloys we studied the thermally induced changesof structure in Ni$_{50}$Mn$_{25+x}$Ga$_{25-x}$ ($x$=2.7--3.9) single crystalsamples and compared them with twinning stress dependences. The alloysexhibited transformation to five-layered (10M) martensite structure between 297to 328 K. All samples exhibited magnetic shape memory effect. Just below thetransformation temperature the samples had very low twinning stress of about0.1--0.3 MPa, which increased with decreasing temperature. The structuralchanges were monitored using X-ray diffraction in the temperature range173--343 K. The 10M structure was approximated by monoclinic lattice with theunit cell derived from the cubic unit cell of the parent L2$_{1}$ phase. Withdecreasing temperature, the lattice parameters $a$ and $\gamma$ increased, $c$decreased, while $b$ was nearly constant. For $x\leq3.5$, sudden sharp changesin $a$ and $b$ parameters additionally occurred, resulting in $a=b$ in someregions of the phase diagram, which might be related to the refinement of twinstructure of 10M martensite on nanoscale. The temperature dependences oflattice parameter $\gamma$ (and $c$ or $c/a$) correlate well with thetemperature dependences of twinning stress in agreement with the prediction bya microstructural model of twin boundary motion. On the contrary, there is nocorrelation between $(a-b)$ and twinning stress. This indicates no significantrole of $a/b$ twins or laminate in twin boundary motion mechanism and lowtwinning stress.

Thermally induced changes of structure in Ni$_{50}$Mn$_{25+x}$Ga$_{25-x}$ magnetic shape memory single crystals with very low twinning stress