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Title: Magnetoelastic coupling and tricritical metamagnetism
Author: Baumfeld, Oliver Lukas
ISNI:       0000 0004 6422 8039
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Magneto-elastic coupling plays a significant role in magnetocaloric materials for magnetic refrigeration at room temperature. It can lead to the existence of a so-called tricritical point where a metamagnetic phase transition changes from first order to second order. Controlling tricriticallity allows us to control the hysteresis of the metamagnetic transition which is of relevance for cyclic application of a magnetic field in a cooling device. This thesis investigates the two incommensurate antiferromagnets CoMnSi and NiMn(Ge,Si) by means of powder neutron diffraction and magnetisation measurements. The correlation between structural parameters and the magnetic structure is established. In CoMnSi a very broad magneto-elastic phase transition is found to go hand in hand with the changes in thermomagnetic hysteresis and a new cycloidal ground state is proposed. In NiMn(Ge,Si) a first order transition between two antiferromagnetic states in zero field is responsible for the changes in the metamagnetic critical field. The antiferromagnetic exchange interaction decreases with the silicon content and it is found that this is correlated with a decrease in the strength of the magneto-elastic coupling. Furthermore this thesis investigates the peak splitting in the metamagnet La(Fe,Si)13H due to hydrogen diffusion which is a direct consequence of strong magneto-volume coupling. By means of time- and temperature-dependent magnetisation measurements the diffusion constant D is estimated to be approximately between 10E-15 and 10E-16 m^2s^-1 at room temperature. It is shown that the thermodynamic driving force behind hydrogen diffusion and phase segregation may be attributed to the lower free energy of hydrogen interstitials in the ferromagnetic state relative to the paramagnetic state.
Supervisor: Sandeman, Karl G. ; Cohen, Lesley F. Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral