Magnetocaloric effect in the Ru-doped MnNiGe system

Abstract 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 T t = 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. T t increases linearly with increasing the magnetic field B . The T dependence of the isothermal magnetic entropy change, Δ S M ( T ) , peaks at around T t , with values of − 4 . 7 , − 15 . 1 , − 24 . 4 , and − 32 . 1 J ⋅ kg − 1 ⋅ K−1 for Δ B = 1 , 3, 5, and 7 T, respectively. The three refrigerant capacities estimated from the Δ S M ( T ) curve show a linear dependence on Δ B . The calculated density of states, DOS, of Mn0.75Ru0.25NiGe in the paramagnetic-hexagonal austenite phase shows a peak at the Fermi energy, E F , and the Mn 3 d orbitals contribute significantly to this peak. On the other hand, the DOS at the E F 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, E e , decreases with the structural change, the phase transition of this system at T t is driven by the band-Jahn–Teller effect, where the Mn 3 d orbitals play the main role.