Study On Magnetic Properties Of Nano-sized Mixed Valence Manganites

Since the discovery of manganites,it has been a hot material in condensed matter physics research.Manganites have strong electromagnetic correlation properties?During the phase transition,manganites exhibit very rich properties,such as colossal magnetoresistance,magnetocaloric,and magnetostrictive effects.As a multifunctional material,manganites is also an excellent electrocatalyst,and it can be used in hyperthermia to treat cancer.When manganites is reduced to nanometer size,its structure,morphology,specific surface area,magnetic properties and magnetic phase transition process can be effectively controlled.In addition,the properties of manganites are also very sensitive to their stoichiometry,defect chemistry and external pressure,which provides the possibility to tune their functional properties.This paper is based on the magnetic properties of nano-sized manganites,using a variety of methods to control its functional properties to establish the relationship between the stoichiometry-size-morphology-functional properties of manganites,so as to explore possibility of application in interdisciplinary research.The main content of the thesis is divided into four chapters.The first chapter mainly introduces the basic structure,theoretical model and functional properties of manganites,including colossal magnetoresistance effect,magnetocaloric effect,and application of hyperthermia therapy.The nano-size effect of manganites,non-stoichiometric effects and properties under hydrostatic pressure were also investigated in detail.The second chapter introduces the multifunctional properties of La_0.6Sr_0.4Mn_O3nano-sized manganites.Near the Curie temperature,the sample exhibits excellent magnetocaloric properties.Under the alternating magnetic field,La_0.6Sr_0.4Mn_O3 also has a high specific loss power,and the relaxation hysteresis plays an important role,indicting the prospect of magnetic hyperthermia application.Electrochemical tests show that the smaller size of the sample will show better electrocatalytic performance due to its larger specific surface area,which proves that La_0.6Sr_0.4Mn_O3 nanoparticles have better electrocatalytic performance.These prove the possibility of La_0.6Sr_0.4Mn_O3nanoparticles in interdisciplinary application.The third chapter introduces the magnetic and magnetocaloric properties of non-stoichiometric La_0.8-xK_0.2Mn_1+xO₃(0?x?0.2)nanoparticles.The nanoparticles are annealed at high temperature to introduce defects,change the internal chemical pressure,increase the Mn-O bond length,and reduce the Mn-O-Mn bond angle,thereby reducing the T_C and improving the magnetocaloric effect.The application of hydrostatic pressure and chemical pressure have opposite regulation effects on the properties of manganites.Under high pressure,the ferromagnetism of manganites is enhanced,but the maximum value of magnetic entropy change is reduced,which proves that the change of crystal structure has a significant effect on the magnetocaloric properties of manganites.Chapter 4 introduces the magnetic,electrical and magnetic phase transition theories of non-stoichiometric La_0.8-xNa_0.2Mn_1+xO_3-?(0?x?0.2)nanoparticles.XPS experiments proved that there are three different valence states of Mn ions in the samples,Mn²⁺,Mn³⁺,Mn⁴⁺.With the increase of temperature,Mn²⁺will participate in the multiple double exchange of Mn³⁺?O^2-?Mn²⁺?O^2-?Mn⁴⁺.The La_0.8-xNa_0.2Mn_1+xO_3-?nanostructured sample exhibits the characteristics of a second-order phase transition with the change of temperature,but the critical magnetic phase transition is very complex and cannot be described by a single model.At the same time,it is found that the superparamagnetic state of the samples has a very important contribution to the low-field magnetocaloric of manganites.The results show that chemical defects and superstoichiometric manganese doping can modulate the magnetic phase transition process and functional properties of manganites,thereby expanding the understanding of manganites and providing new ideas for the synthesis of new type manganites.