Investigation On The Magnetic Property Under Pressure And Magnetoresistance Effect Of La_1-xCa_xMnO₃(x=0.1,0.3,0.5)

The perovskite manganite possess lots of unique phyical properties,such as the colossal magnetoresistance(CMR),metal-insulator transition(MIT),Jahn-Teller(J-T)distortion,charge ordering and phase separation,etc.,enabling them to be used in fields of magneto-electric coupling,magnetic storage and the tunneling junction,etc.The manganite with the formula R_1-xA_xMnO₃ is an important member of the perovskite manganites,where R represents trivalent rare earth metal ions(such as La³⁺,Nd³⁺),A denotes the divalent alkali earth metal ions(such as Ca²⁺,Sr²⁺and Ba²⁺),and Mn ions exhibits the mixture valences of Mn³⁺and Mn⁴⁺.Due to the interaction of charge-lattice-spin-orbit freedom degrees,R_1-xA_xMnO₃ exhibits the rich physics,such as the Mn³⁺-O-Mn⁴⁺double-exchange(DE)ferromagnetism,the Mn³⁺-O-Mn³⁺or Mn⁴⁺-O-Mn⁴⁺super-exchange antiferromagnetism and the J-T distortion due to the interaction between e_g electrons of Mn³⁺and the crystal lattice.The physical properties of R_1-xA_xMnO₃ intensively depend on the doping level.The ground state of LaMnO₃ is an A-type antiferromagnetic and insulating state.The substitution of Ca²⁺at Lasites produces the Mn with the+4 valence,resulting in the significant changes of physical properties of R_1-xA_xMnO₃.For examples,La_0.9Ca_0.1MnO₃ with x=0.1 exhibits the phase separation with the coexistence of ferromagnetic metallic,ferromagnetic insulatiing and(canted)antiferromagnetic insulating states in the temperature region below T_c.When the doping amount is 30%,La_0.7Ca_0.3MnO₃ is a ferromagnetic metal state below T_c.For La_0.7Ca_0.3MnO₃ with x=0.3,it exhibits the ferromagnetic metallic state below T_c;with increasing the temperature,the competition between Mn³⁺-O-Mn⁴⁺and the J-T distortion induces the MIT.For La_0.5Ca_0.5MnO₃with x=0.5,it locates the phase boundary between the charge-ordered antiferromagnetic and ferromagnetic metallic states.Moreover,as a result of the approximate interaction energy among charge,lattice,spin and orbit,a tiny external stimulation such as the magnetic field/electric field/pressure can significantly change physical properties,such as the CMR effect and the MIT induced by the magnetic field,the electro-resistance effect induced by the electric field and the reduction of resistivity and MIT induced by the pressure.With respect to the pressure(P),just like the temperature and magnetic field,acting as clean and effective means,on one hand,it can manipulate the electric/magnetic properties of La_1-xCa_xMnO₃;on the other hand,theoretically,it can manipulate the J-T distortion,as a result,it can be used to find novel phenomena and to reveal the physical mechanism.There are many reports on Raman,electrical transport properties and neutron diffraction under the pressure,but there are relatively few studies on magnetic properties under the pressure.In this thesis,we use the pressure as a means to study the influence of pressure on the magnetic properties of La_1-xCa_xMnO₃.This thesis is divided into six chapters,and the content of each chapter is as follows:Chapter 1:We firstly introduce the crystal structure and basic physical properties of La_1-xCa_xMnO₃.Then,the pressure effect and magnetoresistance effect were reviewed.Finally,the research purpose and significance of this thesis are pointed out.Chapter 2:We mainly introduce the methods of preparation and characterization of La_1-xCa_xMnO₃ samples.Chapter 3:We investigated the magnetic properties under the pressure and the electrical transport under the magnetic field for the bulk La_0.9Ca_0.1MnO₃ polycrystalline.At the ambient pressure,La_0.9Ca_0.1MnO₃ exhibits a paramagnetic(PM)-ferromagnetic(FM)transiton at the Curie temperature(T_c)of 191 K and a canted-antiferromagnetic(C-AFM)transiton at 68 K.The sample exhibits the PM insulating state above T_c,but the phase separated FM insulating,FM metallic and the C-AFM states.The pressure enhances the antiferromagnetic coupling of ferromagnetic clusters with the concomitant variation of anisotropy in the PM region.Around T_c,the pressure suppresses the large J-T distortion,linearly increasing T_c with the pressure coefficient of 15.9 K/GPa and simultaneously increasing the magentization.But the pressure reduces the magnetization below T_c,attributable to the suppression of pressure to the weakly distorted MnO₆ octrhedron in the FM metallic phase,making the Mn ionic moment canted.The pressure suppresses the C-AFM state,gradually reducing the anisotropy related to the C-AFM transition,indicating that the pressure changes the FM and C-AFM phase-separated state below T_c to a more homogeous C-AFM state.Moreover,the magnetic field causes a negative magnetoresistance effect.Magnetoresistance exhibits peaks at 191 K and 127 K,attributable to the suppression of spin-related scattering by the magnetic field and the transition of the antiferromagnetic insulating phase to the ferromagnetic metal phase.Chapter 4:We investigated the magnetic properties under the pressure and the electrical transport under the magnetic field for the bulk La_0.7Ca_0.3MnO₃ polycrystalline.The pressure induces the antiferromagnetic(AFM)coupling of the FM clusters in the PM region above T_c;suppresses the J-T distortion around T_c and enhances the DE interaction,hence increasing T_c;enhances the distortion of MnO₆ octahedral in the ferromagnetic metallic region below T_c,resulting in the decrease of magnetization.MR appears a peak at 255 K,due to the enhanced DE interaction by the magnetic field and the increase in the kinetic energy of the e_g electrons.The rise in the magnetic field decreases the activation energy of polarons in the para-magentic region.Chapter 5:Effects of pressure on magnetic properties of La_0.5Ca_0.5MnO₃ were investigated.Results showed that the application of pressure reduced PM-ferromagnetic FM transition temperature T_cfrom 226 K to 223 K,as well as FM-AFM transition temperature T_N from 180K to 175 K.Meanwhile,the applied pressure also increased charge/orbital ordering transition temperature T_OO from 70 K to 74 K.Below T_N,inhomogeneous AFM phases,including FM and AFM phases,coexisted in La_0.5Ca_0.5MnO₃.The application of pressure led to changes in zero-cooling magnetization(?M_ZFC),with?M_ZFC?0 in the PM region above 285 K,?M_ZFC>0 at T^+/-<T<285 K,and?M_ZFC<0 at T<T^+/-(where T^+/-corresponding to temperature at which?M_ZFC changed from the positive to negative value).Analysis of the data revealed that?M_ZFC>0 did not result from enhanced double-exchange coupling by the applied pressure but most-likely from pressure-induced AFM-to-FM transition.On the other hand,?M_ZFC<0 in the low temperature region below T^+/-may be attributed to the pressure-induced FM-to-AFM transition,the variation in AFM type,and enhancement of J-T distortion.The difference of pressure-induced effects below and above T^+/-was related to the formation and the evolution of charge/orbital ordering in the low temperature region.Chapter 6:The CoFe₂O₄/La_0.7Ca_0.3MnO₃ polycrystalline bulk is chosen as the studying object.The pressure is applied during the magnetic measurements in order to tune the interparticle distance,and hence change the inter-grain exchange-coupling,dipolar interaction and the anisotropy,for the purpose to study their effects on the magnetic property of the composite.The main results are as follows.At the ambient pressure,the step-like spontaneous magnetizing is observed due to the difference of anisotropy of the particles.The magnetic transition temperature T_c of the La_0.7Ca_0.3MnO₃ in the composite is lower than that of pure La_0.7Ca_0.3MnO₃,due to the lattice mismatch and the Co(Fe)-O-Mn super-exchange,occurring at the interface between CoFe₂O₄ and La_0.7Ca_0.3MnO₃.In the low temperature region,the pressure reduces the magnetization,due to the enhanced distortion of Mn³⁺O₆ octahedral in La_0.7Ca_0.3MnO₃,induced extra anisotropy and the enhanced dipolar interaction by the pressure.The coercivity H_c of the composite at 10 K monotonically increases with the pressure,due the induced anisotropy by the pressure,while the remanence ratio M_r/M_s decreases with the pressure,because of,on one hand,the reduced remanence magnetization M_r due to the enhanced distortion of Mn³⁺O₆octahedral and induced anisotropy by the pressure,on the other hand,the larger saturation magnetization M_s of the composite than that of the pure CoFe₂O₄.Summary and outlook.