Magnetocaloric effect in the Ru-doped MnNiGe system

We investigated the magnetic properties of the Ru-doped MnNiGe system. The temperature dependence of magnetization, M(T), of Mn0.8Ru0.2NiGe shows a step-like anomaly at around Tt= 173 K (heating process) with a hysteresis of 9 K between the cooling and heating processes. The Mn0.8Ru0.2NiGe compound exhibits a first-order martensite structural change coupled with the ferromagnetic (FM) transition, as seen in the MnNiGe–CoNiGe system. Tt increases linearly with increasing the magnetic field B. The T dependence of the isothermal magnetic entropy change, ΔSM(T), peaks at around Tt, with values of −4.7, −15.1, −24.4, and −32.1 J kg−1 K−1 for ΔB=1, 3, 5, and 7 T, respectively. The three refrigerant capacities estimated from the ΔSM(T) curve show a linear dependence on ΔB. The calculated density of states, DOS, of Mn0.75Ru0.25NiGe in the paramagnetic-hexagonal austenite phase shows a peak at the Fermi energy, EF, and the Mn 3d orbitals contribute significantly to this peak. On the other hand, the DOS at the EF for the compound in the ferromagnetic-orthorhombic martensite phase is smaller than that of the compound in the austenite phase. As the total energy of the electron system, Ee, decreases with the structural change, the phase transition of this system at Tt is driven by the band-Jahn–Teller effect, where the Mn 3d orbitals play the main role.