  Magnetic, transport, and x-ray diffraction measurements of ferromagnetic
shape memory alloy Ni$_{2.16}$Mn$_{0.84}$Ga revealed that this alloy undergoes
an intermartensitic transition upon cooling, whereas no such a transition is
observed upon subsequent heating. The difference in the modulation of the
martensite forming upon cooling from the high-temperature austenitic state
[5-layered (5M) martensite], and the martensite forming upon the
intermartensitic transition [7-layered (7M) martensite] strongly affects the
magnetic and transport properties of the alloy and results in a large thermal
hysteresis of the resistivity $\rho$ and magnetization $M$. The
intermartensitic transition has an especially marked influence on the transport
properties, as is evident from a large difference in the resistivity of the 5M
and 7M martensite, $(\rho_{\mathrm{5M}} - \rho_{\mathrm{7M}})/\rho
_{\mathrm{5M}} \approx 15%$, which is larger than the jump of resistivity at
the martensitic transition from the cubic austenitic phase to the monoclinic 5M
martensitic phase. We assume that this significant difference in $\rho$ between
the martensitic phases is accounted for by nesting features of the Fermi
surface. It is also suggested that the nesting hypothesis can explain the
uncommon behavior of the resistivity at the martensitic transition, observed in
stoichiometric and near-stoichiometric Ni-Mn-Ga alloys.
