الفهرس 
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Abstract
This thesis is concerned with a meanfield theory (MFT) study on selected amorphous and crystalline alloys of the rareearth transition metal based systems. These systems are of interest due to both technological merit and richness of the magnetic behavior. The systems under study are the amorphous GdFe and ErFe alloys. Three alloys with 42, 52 and 60 at % Gd and five alloys with 8.5, 10, 16.5, 21.5 and 28 at % Er are considered in the former and latter systems respectively. The third system is the crystalline RFe10V2 compound with R= Y, Lu, Nd, Gd, Tb, Dy, Ho, Er and Tm. We used MFT to calculate the magnetization dependence on temperature in a wide temperature range from close to 0 K to the critical temperature and above. We have used Mathematica software package to write our own program, which was used in all our symbolic and numeric analyses and also for plotting all the graphs of this thesis. The MFT study on the systems worked on in this thesis shows that the twosublattice system provides a suitable description of the RETM alloys within the framework of MFT. In the fixed composition RFe10V2 compounds the light rare earths e.g Nd couples ferromagnetically to the Fe sublattice whereas the heavy ones (Gd and heavier rareearths) couple ferrimagnetically. The Curie temperatures of these compounds reach a maximum for R= Gd and DROP as one moves to either the lighter or heavier rareearths. The largest magnetization at 0 K is found for R= Nd and the smallest for R= Ho. The maximum magnetic entropy is proportional to the multiplicity of the rare earth which is largest for Ho. The magnetic field effects on both the specific heat and entropy were studied for the three systems and showed a sign for second order magnetic phase transition at Tc and entropy saturation above Tc. The magnitudes of the specific heat anomaly and maximum entropy were calculated using the angular momenta and concentrations of the magnetic species. The agreement between our simulated results and the calculated ones is less than 16 %. When compared with available experimental data our results showed fair agreement and consistency.