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

Abstract

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 DSmax M = 6.6 J kg 1 K 1 around Tstr - 178 K, and DSmax M = 4.2 J kg 1 K 1 around TC 280 K for a magnetic field change of DB = 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 DSmax M can be expressed as DSmax M / 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.