  A remarkable decrease of the structural transition temperature of MnNiSi from
1200 K to <300 K by chemically alloying it with MnFeGe results in a coupling of
the magnetic and structural transitions, leading to a large magnetocaloric
effect near room temperature. It was found that the magnetostructural
transition is highly sensitive to external (hydrostatic) pressure: relatively
low hydrostatic pressures (~2.4 kbar) lead to an extraordinary enhancement of
the isothermal entropy change from $-$\Delta $S$ = 44 to 89 J/kg K at ambient
(atmospheric) and 2.4 kbar applied pressures, respectively, for a field change
of \Delta $H$ = 5 T. This giant entropy change is associated with a large
relative volume change of about 7% induced by 2.4 kbar applied pressure during
the magnetostructural transition. The pressure-enhanced magnetocaloric effects
are accompanied by a shift in transition temperature, an effect that may be
exploited to tune the transition to the required working temperature, and
thereby eliminate the need for a given material to possess a large
magnetocaloric effect (i.e., entropy change) over a wide temperature range.
Furthermore, this material also possesses negligible hysteresis losses.
