Magnetic Entropy Change And Critical Phase Transition Of Perovskite Manganites And B20 FeGe Alloy

Phase transition is a kind of phenomenon,where the system is stimulated or disturbed by outside field,will change from one phase to another.In this thesis,the main researches include Mn-based traditional perovskite manganese oxides and The B20 structure alloy with Skyrmion.In this paper,we research the paramagnetic-ferromagnetic phase transition,including the Curie temperature of the phase transformation and the critical exponents of the critical phase transition,which is important for analyzing the system of the magnetic interaction through the critical exponents and explaining the new experimental phenomena in the process of phase transition.Moreover,the paper discusses the magnetic entropy change and the potential application of magnetic refrigeration in the materials perovskite manganese oxides and the B20 structure alloy.The main contents of this paper are as follows:1.Critical behavior and long-range ferromagnetic order in perovskite manganite Nd_0.55Sr0.4₅MnO₃The critical behavior of perovskite manganite Nd_0.55Sr_0.45MnO₃ has been investigated based on the static magnetization measurement around the paramagnetic-ferromagnetic transition temperature 273 K.The critical exponent?=0.4816 and?=1.0846 have been obtained by calculating the magnetic-field dependence of the magnetic-entropy change and the Widom scaling relation.These critical exponents not only obey the scaling hypothesis,but also corroborate the results obtained from the Kouvel-Fisher method.In comparison with the values given by standard models,these obtained exponents are very close to those expected from the mean-field model??=0.5 and?=1?and its magnetic-coupling type belongs to nearly long-range interaction.We suggest that the A-site spin disorder and localized magnetic phase competition are the main reasons for the actual critical exponents to show a slight deviation from the theoretical model.2.Investigation of Magnetic Entropy Change and Griffiths-like Phase in La_0.65Ca_0.35MnO₃ NanocrystallineIn this work,we reported a detailed study of magnetic properties and magnetic entropy change of La_0.65Ca_0.35MnO₃ nanocrystalline,which was prepared by using the sol–gel method.The structure alanalysis shows that the nanocrystalline sample crystalizes in orthorhombic perovskite structure and the average size is about 30 nm.Based on the measurements of magnetization,a larger effective magnetic moment was obtained and an obvious deviation of the inverse magnetic susceptibility was observed,indicating the presence of Griffiths-like phase in paramagnetic region.Around the temperature of paramagnetic–ferromagnetic phase transition,the magnetocaloric effect?as represented by the magnetic entropy change?was determined from isothermal magnetization and calculated with Maxwell relation.Compared with bulk polycrystalline,the obtained magnetic entropy change in nanocrystalline is small.This result clearly reveals that the decrease of the sample's size to nanoscale is detrimental for the increase of magnetocaloric effect of magnetic materials.Besides the particle size and surface effect,the paramagnetic–ferromagnetic phase transition driven from first to second order should be a main reason for the small magnetocaloric effect in La_0.65Ca_0.35MnO₃ nanocrystalline.3.Room-temperature large magnetocaloric effect and critical behavior in La_0.6Dy0.₁Sr_0.3MnO₃The effect of dysprosium incorporation in La_0.7Sr_0.3MnO₃ perovskite manganite on its magnetic properties,magnetocaloric effect and critical behavior was investigated.The temperature dependent magnetization data exhibits a sharp paramagnetic–ferromagnetic transition at T_C=307 K,which has been identified to be a second-order transition by the scaling laws for magnetocaloric effect.The maximum magnetic entropy change and the relative cooling power are found to be,8.314 J/kg K and 187 J/kg,respectively,for a 5 T magnetic field change without a hysteresis loss,making this material a promising candidate for magnetic refrigeration at room temperature.To study the critical behavior of the paramagnetic–ferromagnetic transition,some related critical exponents??,?,and??have been also calculated.The values of critical exponents indicate that the present phase transition does not belong to the common transition classes but shows some abnormal variation.We suggest that the induced lattice disordering and magnetic disordering due to Dysprosium incorporation are essential reasons for the presence of a large magnetocaloric effect and an anomalous ferromagnetic phase transition in the present material.4.Magnetic entropy change and accurate determination of Curie temperature in single-crystalline helimagnet FeGeCubic helimagnet FeGe has emerged as a class of skyrmion materials near room temperature that may be useful in future information technology.Experimentally identifying the detailed properties of skyrmion materials enables their practical application acceleratedly.Here,we study the magnetic entropy change?MEC?of single-crystalline FeGe in its precursor region and clarify its close relation to the critical exponents of a second-order phase transition in this area.The maximum MEC is found to be 2.86 J/kg K for a 7.0 T magnetic-field change smaller than that of common magnetocaloric materials indicating the multiplicity and complexity of the magnetic structure phases in the precursor region.This result also implies that the competition among the multimagnetic phases can partly counteract the magnetic-field–driven force and establishes a stable balance.Based on the obtained MEC and the critical exponents,the exact Curie temperature of single-crystalline FeGe under zero magnetic field is confirmed to be 279.1K,higher than the previously reported 278.2 K.This finding paves the way for reconstruction of the FeGe phase diagram in the precursor region.