Crystal Field Analysis Of The Magnetic Properties Of Rare-earth-Fe Compounds And Their Hydrides

A rare-earth-transition inter-metallic compound is a candidate for excellent materials when it fulfils the following conditions: it must have a large spontaneous magnetization, strong uniaxial magnetocrystalline anisotropy and high Curie temperature. Since the nineties, it had been found that the insertion of hydrogen atoms within the crystal structure induces a significant modification of the physical properties. A moderate unit cell increase and a large increase of the Curie temperature is found. Other properties such as the magnetocrystalline anisotropy are also affected by H insertion within the lattice. In this thesis, the magnetic properties of several R-T compounds and their corresponding inter-metallic compounds are studied by comparison of calculations with a series of experiment .The experiments include the magnetization curves along the crystal axis at a series of temperature and pressure, the magnetization curves for aligned sample at deferent temperature. The magnetic properties of RFe11Ti and their hydrides RFe11TiH have attracted much attention in the last decades. The insertion of interstitial hydrogen element in RFe11Ti results in an apparent change of their magnetic properties. It is known that the magneto-crystalline anisotropy (MCA) of rare-earth R ion in the R-Fe inter-metallic compounds originates from crystalline-electric-field (CEF) interaction and the value of CEF parameters are usually obtained by fitting the magnetization curves for the single crystal or the magnetically aligned powder sample. The values of CEF parameters Anm for RFe11Ti and RFe11TiH(R=Sm,Tb,Ho) are evaluated by fitting calculations to the magnetization curves measured on the single crystals or on magnetically aligned powder samples at 4.2 K and higher temperatures. It has been found that the insertion of H significantly change the values of CEF parameters Anm ,which show that the insertion of H significantly change the anisotropy of R ions. The values of CEF parameters Anm for HoFe11Ti under high-pressure are evaluated by fitting calculations to the magnetization curves measured on the single crystals in contrast to its hydrogen which cause to an increase in the cell volume . This increase may reduce the hybridization/exchange interaction between the Fe 3d and rare earth 5d electrons. It retain the crystal structure (ThMn12 type) of the parent alloys under high-pressure. The value of 2μBHex is evaluated by fitting the data that temperature dependence of the saturation magnetization at 0 and 8 kbar. It demonstrates that the value of 2μBHex is practically constant under pressure from 0 to 8kbar. With the parameters of exchange field 2μBHex, the values of CEF parameters Anm for HoFe11Ti are evaluated by fitting calculations to the magnetization curves measured on the single crystals under 0, 4 and 8kbar. It shows that the pressure in RFe11Ti has been found to have significant effect on CEF parameters Anm. A decrease of the MCA of rare-earth R ion and the absolute value of A20 with the increasing of pressure is observed. There is different effect on the magnetic properties of the compounds by hydrogen and high-pressure. It demonstrates that...