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The defence will be in French.
Abstract: R-M compounds (where R is a rare earth atom and M is a 3d transition metal) have numerous applications and can exhibit original behaviours (Kondo effect, metamagnetism, etc.) that motivate in-depth fundamental studies. Among these remarkable behaviours, the itinerant electron metamagnetic transitions (IEM) are the most spectacular example. Here we explore the magnetism of itinerant electrons in the RM12B6 (B = boron) phases, whose singular behaviour has attracted the attention of the community.
In the first part, we studied 3d magnetism by the substitution of Fe for Co in YCo12-xFexB6. This substitution causes an anisotropic increase of the lattice. The solubility limit of Fe was determined. The magnetic properties of this series of compounds show great sensitivity to the presence of Fe in the structure. These were studied using various models (molecular field, Kuz’min, Inoue-Shimizu, and Takahashi spin fluctuation theory). It has been shown that the substitution of Fe for Co increases the degree of delocalisation of the magnetism of itinerant electrons in this family of compounds, while promoting the gradual establishment of an antiferromagnetic order. IEM transitions were observed for the first time for compounds with x = 1.5, 2 and 2.5. Magnetic phase diagrams were constructed for these compositions. Neutron powder diffraction measurements were used to determine the preferred Fe occupancy pattern in YCo12-xFexB6 and to resolve the magnetic structures of the compounds YCo12B6 and YCo10Fe2B6. Massive single crystals of these two compositions were also synthesised for the first time, and magnetisation measurements were used to identify the different anisotropy coefficients for YCo12B6.
In the second part, we studied the effects of chemical subsitutions on the compound LaFe12B6, whose magnetic behaviour is highly original. The study of the substitution of Pr for La in La1-xPrxFe12B6 enabled us to determine the solubility limit of Pr in this crystal structure, and to show the strong sensitivity of the magnetic properties of the LaFe12B6 system to this substitution. The IEM transitions of this system were studied in detail, enabling magnetic phase diagrams of different compositions to be constructed. The various physical effects associated with these transitions were also studied (magnetocaloric effects, significant magnetostrictive effects and giant magnetoresistance). X-ray diffraction measurements at low temperature enabled us to observe a structural transition associated with magnetic ordering for Pr contents with x > 0.05. This lowering of symmetry was identified as the transition from the rhombohedral R-3m phase to the monoclinic C2/m phase. Low-temperature neutron diffraction measurements showed that the magnetic order of the monoclinic phase is that of a ferromagnetic compound. The substitution of C for B in LaFe12B6-xCx was studied by synchrotron X-ray diffraction, and the small decrease in lattice parameters was quantified precisely. The effect of the substitution of C for B on the magnetism of the LaFe12B6 compound was identified as analogous to the application of a positive external pressure. IEM transitions therefore require stronger external magnetic fields to initiate. Magnetic phase diagrams were constructed for different carbon compositions by combining laboratory magnetisation measurements with pulsed high-field measurements.
