Investigation On The Multiferroicity?magnetocaloric And Electrocaloric Effects Of YMn_0.5Cr_0.5O₃

Transition metal oxides(TMOs)have been extensively studied in decades because of their novel physical properties,such as superconductivity,colossal magnetoresistance,multiferroicity and thermoelectricity,etc,as well as their promise for future applications.Generally speaking,these abundant properties can be ascribed to the complicated and subtle interplay between spin,orbit,lattice and charge degrees of freedom.Despite of the common sense,researchers are still far from an in-depth understanding for the nature of these oxides.In our previous study,we investigated the structural and magnetic properties of the half-Cr-doped yttrium manganite YMn0.5Cr0.5O3(YMCO).It was found that the YMCO oxide had an orthorhombic structure with a = 5.2513 A,b= 5.6417 A,and c ?7.4699 A(in Pbnm setting)and a strong magnetic transition at TN? 75 K.On the basis of the molecular field theory of ferrimagnetism,a detailed analysis of the temperature dependence of inverse susceptibility(?-1-T)at high temperatures above TN revealed that the oxide is in a ferrimagnetic ground state at low temperatures.It also shows ferroelectricity at low temperatures.Therefore,it is essential to explore the other physical properties of the YMCO compound and its element substitution effects.Chapter 1.We first give a brief introduction to the crystal structure of rare earth transition oxides RMO3 with perovskite structure,focusing on the orthorhombic structure with the space group Pbnm.We then summarize various magnetic exchange interactions including the super-exchange and the double exchange,etc;magnetic orders and spin wave excitation;mechanisms for ferroelectricity in type-I and type-II single phase multiferroic materials including the exchange striction interaction,Inverse Dzyaloshinskii-Moriya interaction and Single ion model.Next,we give a detailed description of mechanisms for negative magnetization,such as the competition between magnetic sublattices,the Dzyaloshinsky-Moriya(DM)interaction and the magnetocrystalline anisotropy etc.At last,we briefly summarize the magnetocaloric and electrocaloric effects in oxide samples.Chapter 2.Firstly,we describe the preparation methods for polycrystalline(Conventional solid-state reaction method)and single crystalline(flux method)samples and the structural characterization(XRD).Next,we summarize the experimental methods for researching the physical properties of the samples,including SEM(EDS)for quantitative composition analysis,SQUID and VSM for magnetic measurements,as well as PC and BE methods for ferroelectric measurements.Chapter 3.In this chapter,Y1-xEuxMn0.5Cr0.5O3(0.1 ?_x ? 1.0)oxides are synthesized in a polycrystalline form by a conventional solid-state reaction method.X-ray diffraction patterns reveal that the whole series of compounds featured a simple distorted perovskite structure with the orthorhombic space group Pbnm.With increasing Eu content,the shorter a and c axes increase,while the longest b axis changes little.A clear correlation between TN and the negative molecular field coefficient within the Cr3+ sublattice indicates that the antiferromagnetic exchange interaction between the Cr3+ cations plays a dominant role in the magnetic ordering.A pyroelectric current peak appears at the temperature around TN,implying possible ferroelectricity induced by magnetic orders in the YMCO compound.Chapter 4.We studies magneto-and electro-caloric effects in multiferroic YM0.5Cr0.5O3 compound,which has a ferrimagnetic transition at TN?75 K.Two peaks in the temperature dependence of isothermal magnetic entropy change ?SM can be observed at TN and lower T0,respectively.While the first maximum of ?SM corresponds to the ferrimagnetic transition as expected,the second one might be related to a metamagnetic transition.Using the Maxwell's relation,an electric field driven entropy change(-?SE)has also been obtained at temperatures around TN,revealing the multicaloric effect in the YMCO sample.By fitting the specific-heat and magnetization data at low temperatures according to the spin-wave excitation model,the nearest neighboring exchange integral J12 has been estimated to be around 10-4 eV,which is in agreement with the value gained from the molecular field theory.Chapter 5.We report an intensive study on negative magnetization under zero-field-cooled(ZFC)mode in YM0.5Cr0.5O3 polycrystalline sample.It has been found that the magnetization reversal in ZFC measurements is strongly related to a giant coercivity of the oxide.The giant coercivity may result from the cooperative effect of magnetocrystalline anisotropy and the Dzyaloshinsky-Moriya interaction,especially at temperatures below 10 K.By fitting the high-temperature paramagnetic data under nominal zero field,the value of the trapped field in a superconducting magnet has been derived to be around several Oe,which further demonstrates that the negative ZFC magnetization is an artifact caused by negative trapped field in combination with the giant coercivity.Chapter 6.Y0.76Bi0.24Mn2O5 and Y0.75BiO25Mn1.74Al0.26O5 single crystal samples are synthesized by the flux method.We identify the a,b and c crystal axis of the samples by using XRD technique.And then the anisotropic magnetic and ferroelectric properties for the two samples are initially studied.It has been found that the Y0.76Bi0.24Mn2O5 single crystal yields also simultaneous antiferromagnetism and ferroelectricity as its prototype compound YMn2O5.It is demonstrated that the magnetic transition temperature of the Y0.76Bi0.24Mn2O5 single cystal is higher than that of Y0.75Bi0.25Mni.74Al0.26O5 but similar to that of YMn2O5(TN?45 K).The results imply that the larger Bi3+ ion substitution for Y3+ has little influence on the exchange interactions between the magnetic Mn ions,whereas the Al3+ ion substitution for Mn3+weakens the exchange interactions,resulting in the lower magnetic transition temperature.Further studies on the single crystals are still under way.