Moment formation and giant magnetocaloric effects in hexagonal Mn-Fe-P-Si compounds

Limited resources and the wish for improved prosperity call for efficient use of energy. The UN Advisory Group on Energy and Climate Change recommends a target of 40 % improved efficiency by 2030. Materials research can contribute significantly to reach this target. Magnetic refrigeration offers potential to achieve a 50 % higher energy-efficiency compared to vapor-compression refrigeration. This makes magnetic refrigeration a technology that attracts growing attention. Magnetic refrigeration is based on the magnetocaloric effect, i.e., the temperature change of a magnetic material upon the application or removal of a magnetic field in adiabatic conditions. Magnetocaloric materials play an important role in magnetic refrigeration. Giant magnetocaloric materials which are globally-abundant, non-toxic and can be industrially-mass-produced via a simple fabrication method are particularly attractive for magnetic refrigeration applications. Fe2P-based Mn-Fe-P-Si alloys can meet such requirements. The work presented in this thesis is a study of the magnetocaloric effect and related physical properties in the Mn-Fe-P-Si compounds. Some theoretical aspects of the magnetocaloric effect in general, and the origin of the first-order magneto-elastic transition which enhances the magnetocaloric effect in hexagonal Mn-Fe-P-Si compounds in particular are given in Chapter 2. In Chapter 3, a short review is presented of the experimental techniques and set-ups that have been employed for the sample preparation and the characterization of the physical properties of the Mn-Fe-P-Si compounds. Our efforts to optimize the magnetocaloric effect for refrigeration applications are presented in Chapter 4. We show that a giant magnetocaloric effect and a small thermal hysteresis in Mn-Fe-P-Si compounds of hexagonal Fe2P-type structure can be achieved simultaneously. Furthermore, the working temperature can be controlled over a large interval around room temperature by varying the Mn:Fe and P:Si ratios. In Chapter 5, we report on various types of transition found in (Mn,Fe)1.95P0.50Si0.50 when changing the Mn:Fe ratio. Interestingly, we observe a previously unknown first-order magneto-structural transition and a modified first-order magneto-elastic transition favorable for real refrigeration applications. Using high resolution neutron diffraction, x-ray diffraction and high-temperature magnetic-susceptibility measurements, and based on theoretical calculations, a first-order magneto-elastic transition from high-moment to low-moment in the Mn-Fe-P-Si compounds is presented in Chapter 6. This observation supports our proposal that the competition between moment formation and chemical bonding is at the core of giant magnetocaloric effect displayed in the class of hexagonal Fe2P-based materials with first-order magneto-elastic transition. The effect of the replacement of Fe by Mn on the magnetic moments is also discussed. Chapter 7 is devoted to the effects of P:Si ratio on the magnetic and structural properties of the Mn Fe P Si compounds. In chapter 8, we present magneto-elastic coupling in the Mn-Fe-P-Si compounds. Interestingly, hysteresis and magnetic entropy change are found to be correlated with discontinuous changes of the lattice parameters at the transition temperature. Small thermal hysteresis can be obtained while maintaining the giant magnetocaloric effect. A preliminary comparison of the magneto-elastic coupling and magnetocaloric effect for Mn-Fe-P-As/Ge/Si is also given.