Magnetic structures of magnetocaloric (Mn1-xNix)CoGe and Mn(Co1-xNix)Ge alloys

The magnetocaloric effect (MCE) - a significant temperature change around the magnetic transitions in materials driven by magnetisation or demagnetisation - has emerged as an increasingly important topic in condensed matter physics in the past two decades. This development is due primarily to potential applications in refrigeration as an alternative to gas-based compression-expansion refrigeration [1]. A large MCE occurs generally around a magnetic transition, especially when the magnetic transition coincides with a structural transition (magneto-structural transition) [1]. MnCoGe-based compounds offer particular scope for MCE applications, particularly for cooling around room temperature with previous studies having shown that it is relatively straightforward to engineer the structural transition temperature and thereby produce a magneto-structural transition [2]. In the present work, a series of (Mn1-xNix)CoGe (x = 0.0-0.07) and Mn(Co1-xNix)Ge (x = 0.14-1.00) samples have been prepared in order to investigate the effects of doping the Mn and Co sites of MnCoGe with Ni. The crystal structures and magnetisation were measured using XRD (20-310 K) and PPMS (5-320 K) with the magneto-structural transitions studied using neutron powder diffraction and polarised neutron diffraction (5-450 K; WOMBAT, OPAL). Magneto-structural transitions from ferromagnetic-orthorhombic (FM-Orth) structure to paramagnetic-hexagonal (PM-Hex) structure were obtained in both (Mn1-xNix)CoGe and Mn(Co1-xNix)Ge around room temperature. A spiral antiferromagnetic (SP-AFM) structure was also observed in the orthorhombic structure of Mn(Co1-xNix)Ge (x ≥ 0.55) at low temperature, following by a magnetic transition from SP-AFM to FM at higher temperature. In addition, the influence of magnetic field on the FM-Orth/PM-Hex magneto-structural transition was studied using field dependent neutron diffraction (5-320 K; 0-9 T). Our investigations show that normal (inverse) MCE are obtained around the FM-Orth/PM-Hex (SP-AFM/FM) transitions in (Mn1-xNix)CoGe and Mn(Co1-xNix)Ge.