الفهرس 
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Abstract
We present, in this thesis, a theoretical study on magnetic systems with either a first-order (FOPT) or a second-order (SOPT) phase transition.
The Bean-Rodbell model is used to investigate the properties of spin-half and general-spin systems in addition to the element Gd. The mean-field theory (MFT) is used to study the R2Fe14B system where R ≡ Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho and B ≡ boron. Magneto-volume coupling, in the Bean-Rodbell model, has a pronounced effect on the studied properties. The temperature and field dependences, of the magnetothermal and the magnetocaloric (MCE) properties for systems that undergo SOPT is drastically different from their counterparts in systems with FOPT. The magnetothermal properties we studied are: the magnetization, the heat capacity and entropy. The calculated MCE are the isothermal change in entropy (ΔSm) and the adiabatic change in temperature (ΔTad). Two figures-of-merit could be calculated from the MCE properties e.g. the relative cooling power RCP(S) and RCP(T). The MCE properties are calculated for magnetic field change ≤ 5T and in a temperature range up to 700K, i.e. above the curie temperatures of the studied compounds. Our calculation showed that a direct MCE is present in the ferromagnetic compounds whereas both direct and inverse MCE were found in the ferrimagnetic compounds (R≡Gd, Dy, Ho, Tb).
The maximum ΔSm and ΔTad values in fields up to 5T are in the range 0.9-6.8 J/mol. K and 1.5-7.7 K respectively. The highest ΔSm and ΔTad are found for R= Pr, namely 6.8 J/mol. K and 7.7 K respectively. The two figures-of-merit i.e. the relative cooling powers RCP(S) and RCP(T) are in the 196-526 J/kg range and the 187.6-403.2 K2 range respectively.
The Arrott-plots and the universal curve were constructed in order to further check the type of the magnetic order. The features of the Arrott plots e.g. the positive slopes of the plots, at different temperature around T¬C, is an evidence of the presence of SOPT. Negative slopes or S-shaped curves are signs for FOPT
Materials.
It was of interest to study the effect of high magnetic fields on the nature of phase transition and to investigate the change of the magnetic order from ferrimagnetism to ferromagnetism in some of the compounds we report on here, e.g. those with R= Gd, Dy. Calculation was performed in magnetic fields up to 300 T in a temperature range 30-430 K. A sharp increase in magnetization has been observed in high fields for both compounds. The Arrott plots in fields up to 300T were calculated for Gd2Fe14B and showed an S-shaped curves at 400 and 430 K.