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

• Published in: Journal of alloys and compounds 2013-11, Vol.577, p.475-479
• Main Authors: Wang, J.L., Shamba, P., Hutchison, W.D., Md Din, M.F., Debnath, J.C., Avdeev, M., Zeng, R., Kennedy, S.J., Campbell, S.J., Dou, S.X.
• Format: Article
• Language: English
• Subjects: Alloys; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Domain effects, magnetization curves, and hysteresis; Entropy; Exact sciences and technology; Ferromagnetism; Magnetic fields; Magnetic phase boundaries (including magnetic transitions, metamagnetism, etc.); Magnetic phase transitions; Magnetic properties and materials; Magnetically ordered materials: other intrinsic properties; Magnetism; Magnetization; Magnetization curves, magnetization reversal, hysteresis, barkhausen and related effects; Magnetocaloric effect; Magnetocaloric effect, magnetic cooling; Manganese; Neutron diffraction; Neutron diffraction in structure determination; Physics; Structure of solids and liquids; crystallography; Structure of specific crystalline solids
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2013.06.134

•We detected and confirmed two magnetic phase transitions at ∼178K and ∼280K.•Both structural entropy and magnetic entropy contribute to the entropy change around Tstr.•−ΔSMmax can be expressed as −ΔSMmax∝Bn with n=1 around Tstr and n=2/3 around TC. Bulk magnetization measurements (5–320K; 0–8T) reveal that below room temperature Mn0.9Ti0.1CoGe exhibits two magnetic phase transitions at ∼178K and ∼280K. Neutron diffraction measurements (3–350K) confirm that the transition at ∼178K 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 ∼280K originates from the transition from ferromagnetism to paramagnetism. The magnetocaloric behaviour of Mn0.9Ti0.1CoGe around Tstr∼178K and TC∼280K as determined via the magnetic field and temperature dependences of DC magnetisation are given by the maximum values of the magnetic entropy changes −ΔSMmax=6.6Jkg−1K−1 around Tstr∼178K, and −ΔSMmax=4.2Jkg−1K−1 around TC∼280K for a magnetic field change of ΔB=0–8T. 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∼280K 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.