Ti substitution for Mn in MnCoGe - The magnetism of Mn0.9Ti0.1CoGe

Bulk magnetization measurements (5-320 K; 0-8 T) reveal that below room temperature Mn0.9Ti0.1CoGe exhibits two magnetic phase transitions at ∼178 K and ∼280 K. Neutron diffraction measurements (3-350 K) confirm that the transition at ∼178 K is due to the structural change from the low-temperature orthorhombic TiNiSi-type structure (space group Pnma) to the higher temperature hexagonal Ni2In-type structure (space group P63/mmc), while the transition at ∼280 K originates from the transition from ferromagnetism to paramagnetism. The magnetocaloric behaviour of Mn0.9Ti0.1CoGe around Tstr ∼ 178 K and TC ∼ 280 K as determined via the magnetic field and temperature dependences of DC magnetisation are given by the maximum values of the magnetic entropy changes −ΔSMmax = 6.6 J kg−1 K−1 around Tstr ∼ 178 K, and −ΔSMmax = 4.2 J kg−1 K−1 around TC ∼ 280 K for a magnetic field change of ΔB = 0-8 T. Both structural entropy - due to the unit cell expansion of ∼4.04% - and magnetic entropy - due to an increase in the magnetic moment of ∼31% - are found to contribute significantly to the total entropy change around Tstr. Critical analysis of the transition around TC ∼ 280 K leads to exponents similar to values derived from a mean field theory, consistent with long-range ferromagnetic interactions. It was found that the field dependence of −ΔSMmax can be expressed as −ΔSMmax ∝ Bn with n = 1 for the structural transition around Tstr and n = 2/3 for the ferromagnetic transition around TC, thereby confirming the second order nature of this latter transition.