Study of magnetization processes in rare-earth-free materials: MnBi and Mn-Fe-P-Si

Magnetic materials are at the base of many innovative devices. In many cases these materials are based on rare-earth (RE) intermetallic compounds, like Nd2Fe14B for permanent magnets and La(FeSi)13 for magnetic cooling. Permanent magnets are employed in a large range of application fields: e.g. in motors, hard disc drives, magnetic resonance imaging, mobile phones, microphones etc. Magnetic materials for permanent magnets need a high magnetocrystalline anisotropy energy (MAE). The MAE is associated with the angle formed by the magnetization vector with the axes of the crystal lattice and its microscopic origin is due to the coupling between the atomic magnetic moments and the crystal lattice. High performance permanent magnets are based on compounds with rare earth elements, like NdFeB-based and SmCo-type magnets. The magnetic refrigeration is based on the magnetocaloric effect (MCE). This cooling technique is 30% more efficient than vapor-compression refrigeration. The MCE is defined as the adiabatic temperature change ΔTad or the isothermal entropy change ΔSiso due to the application of a magnetic field H at constant pressure. For a first order magnetic transition, from an ordered ferromagnetic to a disordered paramagnetic phase with an abrupt change in magnetization, the MCE is maximum. In the last years the research has found many different classes of materials, which exhibit a large magnetocaloric effect. Among these, La(FeSi)13 based compounds have been already optimized, while Fe2P-based compounds still need further investigations. In recent years the problems of rare earth supply have prompted the research in the development of new magnetic materials without RE, as MnBi and Mn-Fe-P-Si alloys. MnBi is a good candidate as permanent magnet for its high magnetocrystalline anisotropy. Both MnBi and Mn-Fe-P-Si compounds have a first order magnetic transition, interesting for magnetic refrigeration. The aim of the present work has been to investigate the contributions of the composition (through the partial substitutions of the elements) and/or of the microstructure (by means of different preparation techniques) on magnetization processes of rare-earth-free MnBi and Mn-Fe-P-Si compounds. In this framework the investigation has regarded especially two topics: (i) the study of the magnetization reversal mechanisms in MnBi compounds with the aim to improve their hard magnetic properties by means of the enhancement of the coercive field, without losing magnetization; (ii) the study of the first order transition and the characterization of MCE of MnBi and Mn-Fe-P-Si compounds, which are promising candidates for cooling applications. The room temperature magnetic refrigeration requires materials with a phase transition around room temperature and a thermal hysteresis as small as possible. However, much research is needed to understand how to minimize the unwanted hysteresis in the phase transitions (for MnBi and Mn-Fe-P-Si) and how to decrease the transition temperature of MnBi (633 K). The research activity described in this thesis has been carried out in collaboration with the National Institute of Metrology (INRIM) in Turin (Italy). The preparation and the characterization of the intermetallic MnBi samples are objects of this activity (both carried out at National Institute of Metrological Research (INRIM) in Turin (Italy)). The Mn-Fe-P-Si samples were prepared at Satie Institute of Cachan (France) and in this work their MCE has been characterized. The main results achieved regard: • the preparation of a unknown metastable (Mn,Ti)Bi phase that exhibits a second order magnetic transition around 180 K; • the increasing of the coercive field of MnBi by means of two different techniques (reduction of the particle size and partial substitution of Mn with Ti); • the decrease of the activation volume of the first order transition of the MnBi by means of the introduction of Ti in the MnBi alloys; • the high value of the isothermal entropy change ΔSiso and the adiabatic temperature change ΔTad of the Mn1.30Fe0.65P0.5Si0.5 compounds at first order transition, prepared by classical sintering. In addition to this results, small thermal hysteresis ΔThyst has been obtained on powders (smaller particle size) of the same compounds