Research On Magnetic And Magneticdielectric Properties In R₂Cu₂O₅ System

Magnetoelectric effect usually exists in multiferroic materials with coexisted ferroelectric and magnetic properties,the materials with strongly coupled magnetic and electronic degrees of freedom provide new possibilities for practical applications.But,ferroelectricity and magnetism seldom coexist due to mutual exclusiveness.In recent years,however,other form of magnetoelectric effect has been discovered-magnetodielectric effect,and this effect also exists in some special magnetic materials,which become the hot topic in condensed state and material physics.In this thesis,we have investigated the magnetic property,dielectric property and magnetodielectric effect in Dy₂Cu₂O₅,Ho₂Cu₂O₅ and Yb₂Cu₂O₅ polycrystalline samples,which possess a non-centrosymmetric polar structure with space group Pna2₁.In Dy₂Cu₂O₅ and Ho₂Cu₂O₅,R³⁺and Cu²⁺sublattices order simultaneously,exhibiting a typical paramagnetic to antiferromagnetic transition at 10.6 K and 13.1 K,respectively.While for Yb₂Cu₂O₅,two magnetic transitions which originate from the orderings of Yb³⁺（7.8 K）and Cu²⁺（13.5 K）sublattices are observed.The Curie-Weiss temperature of three samples obtained by Curie-Weiss fitting is negative.A magnetic field induced metamagnetic transition is obtained in these three cuprates below Néel temperature（T_N）,and compare with the Y₂Cu₂O₅,the critical field becomes smaller.By means of dielectric measurement,distinct magnetodielectric effect is demonstrated by the alteration of dielectric anomaly with the increase of the external magnetic field at T_N.Moreover,two dielectric relaxation-related steps are observed in Dy₂Cu₂O₅,the low-temperature relaxation and high-temperature relaxation can be understood in terms of the unique zigzag chains of Cu²⁺and the deviation of the oxygen stoichiometry The H-T phase diagram is constructed for Dy₂Cu₂O₅,Ho₂Cu₂O₅ and Yb₂Cu₂O₅ based on the results of isothermal magnetic and dielectric hysteresis loops,which means that the dielectric anomaly is directly related to the metamagnetic transition.Due to the specific spin configuration and different spin evolution in the magnetic field,a positive magnetodielectric effect is formed in Ho₂Cu₂O₅,and a negative one is observed in Yb₂Cu₂O₅.But in Dy₂Cu₂O₅,there are both positive and negative magnetodielectric effects,and the results are discussed in terms of the magnetic fieldinduced change of spin configuration and spin–phonon coupling,and understand the dielectric behavior and the magnetodielectric effect in materials with complicated spin structures.At the end of this thesis,the magnetodielectric effect of Ho₂Cu₂O₅ was changed by Sr²⁺doping,providing a feasible idea for regulating the magnetodielectric effect of the R₂Cu₂O₅system.