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 Mn 0.9 Ti 0.1 CoGe 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 Ni 2 In-type structure (space group P 63/mm c), while the transition at ∼280 K originates from the transition from ferromagnetism to paramagnetism. The magnetocaloric behaviour of Mn 0.9 Ti 0.1 CoGe around T str  ∼ 178 K and T C  ∼ 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 −Δ S M max  = 6.6 J kg −1  K −1 around T str  ∼ 178 K, and −Δ S M max  = 4.2 J kg −1  K −1 around T C  ∼ 280 K for a magnetic field change of Δ B  = 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 T str . Critical analysis of the transition around T C  ∼ 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 −Δ S M max can be expressed as −Δ S M max  ∝  B n with n  = 1 for the structural transition around T str and n  = 2/3 for the ferromagnetic transition around T C , thereby confirming the second order nature of this latter transition.