Investigation Of Magnetocaloric Effect And Critical Behavior Based On Magnetic Compounds

In contrast to conventional refrigeration methods,magnetic refrigeration offers a number of environmental benefits,an alternative to common techniques.This new technology exploits the magnetocaloric effect（MCE）,i.e.,the temperature variation occurring in materials when they are subjected to a magnetic field,to produce refrigeration without requiring the compression and expansion of a gas.Refrigeration can be achieved with the MCE,which occurs when a magnetic material experiences a change in temperature as a result of an applied magnetic field.This technology is economically feasible and ecologically friendly in comparison with traditional vapor compression technology,in addition,it can save around 20-30%of the needed energy,and hence the effect attracts the attention of many researchers.This thesis describes the research into the MCE and the critical exponent of suitable candidates of magnetic material for magnetic refrigeration.The document is organized into5 chapters.Chapter 1 is an overview of the history of the MCE and its theoretical aspects and scientific context.All the definition of MCE will be introduced.Also,the important characterization of MCE is introduced and explained.The phase transition and its variety of it are described and the different ways to distinguish the nature of phase transition are introduced.Some theoretical aspects related to MCE such as Landau theory,Griffith phase,critical exponent and universal entropy change are described in detail.In chapter 2,the main experimental techniques used in this Thesis are described,including the sample preparation and characterization.In chapter 3,polycrystalline samples of La_0.6-xEr_xSr_0.4Mn O₃（x=0.0125,0.05,0.1）have been prepared using the solid-state reaction method.X-ray powder diffraction confirmed a single-phase state and revealed a rhombohedral structure with the Rˉ3c space group in all samples.It is shown that the unit cell volume gradually decreases by replacing La with Er.The Curie temperature（_CT）decreases as the Er content rises.A downturn of the reciprocal value of the magnetic susceptibility versus temperature curve proves the presence of the Griffith phase above _CT.An occurrence of two overlapped transitions,from the ferromagnetic phase to the Griffith phase and from the latter one to the paramagnetic phase,in La_0.6-xEr_xSr_0.4Mn O₃ samples caused the significant expansion of the total transition temperature range with increasing x.In turn,a widening of the transition temperature span gives rise to the spreading of the magnetic field induced entropy maximum over a broad temperature interval.In chapter 4,we study the magnetocaloric properties of a Ca Ba Co₄O₇（CBCO）polycrystalline cobaltite has been undertaken along with research on the nature of magnetic phase transition.The magnetization as a function of temperature identifies the ferrimagnetic（FIM）to paramagnetic（PM）transition at a Curie temperature of 60 K.Moreover,a Griffiths-like phase is confirmed in a temperature range above _CT.The compound undergoes a crossover from the first to second-order ferrimagnetic transformation,as evidenced by the Arrott plots,scaling of the universal entropy curve,and field-dependent magnetic entropy change.The maximum of entropy change is 3 J/kg.K for?H=7 T at_CT,and a broadening of the entropy peak with increasing magnetic field indicates a field-induced transition above _CT.The analysis of the magnetic entropy change using the Landau theory reveals the second-order phase transition and indicates that the magnetocaloric properties of CBCO are dominated by the magnetoelastic coupling and electron interaction.The corresponding values of refrigerant capacity（RC）and relative cooling power（RCP）are estimated to be 33 and 42 J/kg,respectively.In chapter 5,the critical properties and the nature of the ferromagnetic–paramagnetic phase transition in the 2D organic-inorganic hybrid（CH₃NH₃）₂Cu Cl₄ single crystal have been investigated by dc magnetization in the vicinity of the magnetic transition.Different techniques were used to estimate the critical exponents near the ferromagnetic–paramagnetic phase transition such as modified Arrott plots,the Kouvel–Fisher method,and the scaling hypothesis.Values ofβ=0.22,γ=0.82,andδ=4.4 were obtained.These critical exponents are in line with their corresponding values confirmed through the scaling hypothesis as well as the Widom scaling relation,supporting their reliability.It is concluded that this 2D hybrid compound possesses strong ferromagnetic intra-layer exchange interaction as well as weak interlayer ferromagnetic coupling that causes a crossover from 2D to 3D long-range interaction.In the chapter 6,the results obtained from the studies have been collected and the suggestions for the next researches are present.This work has studied the MCE of Manganite and Cobaltite materials and critical effects of new magnetic compounds,which is a frontier topic in the field of solid-state refrigeration in recent years.Using Landau Theory,KF and other methods,the paper explores the relationship between"structure-magnetism-phase transition behavior and MCE"of new magnetic compounds.