Influence of Cr doping on magnetocaloric effect and physical properties of slowly cooled NiMn1−xCrxGe

Abstract

Slowly cooled NiMn1−xCrxGe (x = 0.04, 0.11, 0.18 and 0.25) has been studied by X-ray (90–480 K) and neutron diffraction (1.6–400 K), differential scanning calorimetry (300–480 K) and magnetic (2.0–400 K, magnetic field up to 90 kOe) measurements. The compounds undergo a martensitic phase transition changing their crystal structure between the orthorhombic low temperature phase (space group Pnma) and the hexagonal high temperature one (space group P63/mmc). The temperature of the structural phase transition significantly decreases with increasing x (TS = 464 K for x = 0.04 and 373 K for 0.25 on heating and similarly TS = 445 K for x = 0.04 and 355 K for 0.25 on cooling). Antiferromagnetic helicoidal ordering with the propagation vector k = (kx, 0, 0) for x = 0.04 and 0.11 (TN = 357 and 353 K, respectively) and ferromagnetic one for x = 0.25 (TC = 360 K) has been found. The sample with x = 0.18 shows a coexistence of a helicoidal antiferromagnetic structure and the ferromagnetic one below ∼170 K while at higher temperatures the ferromagnetic ordering remains stable up to 362 K. For all investigated samples the magnetocaloric effect is observed. Maximum entropy change (−ΔS) increases with increasing Cr concentration from about 8 J/(kg K) at 90 kOe, found for x = 0.04 and 0.11 at the Néel temperature, up to 29 J/(kg K) observed for x = 0.25 in cooling regime at the magnetostructural phase transition temperature.