Structure And Magnetic Properties Of The Doped Perovskite-like Heavy Rare Earth Manganites At A And B Sites

The perovskite-like rare earth manganites have been investigated intensively in material science and condensed matter physics due to its rich physical phenomina, such as colossal magnetoresistance effects, gaint magnetocaloric effect, metal-insulator transition, etc., and the application potentials in electronics, communications and so on. In this dissertation, series of polycrystalline samples DyMn1-x CoxO₃, Dy1-x LuxMn0.7Co0.3O₃, LuMn1-x CoxO₃,Lu1-x LaxMn0.7Co0.3O₃, and Ho1-x LaxMn0.7Co0.3O₃ have been prepared by the sol-gel method.The crystal structure and the magnetic properties are investigated precisely. The main experimental results are listed as follows:1. The crystal structure of DyMn1-x CoxO₃（0≤x≤0.8） meaintains orthorhombic phase with group Pbnm while the lattice parameters decrease as the Co concentration x increases. A magnetic phase transition from antiferromagnetic to ferromagnetic state occurs at x ? 0.2. The magnetization enhances as x increases, reaches its maximum value at x = 0.5, and decreases with the further increasing of x. The magnetic variation of of the samples might be attributed to the competition between the superexchange coupling and the double exchange interaction among the transition metal ions.2. The crystal structure of Dy1-x LuxMn0.7Co0.3O₃（0≤x≤0.7） meaintains orthorhombic phase with group Pbnm while the lattice parameters tend to decrease as the Lu concentration x increases. Both Curie temperature Tc and the magnetization at 10 K decrease with the increasing of x. It is found that the Curie temperature Tc is approximately linear to the volume of the unit cell, which indicates the ferromagnetic interaction in the orthorhombic Pbnm phase is related not only to the double exchange among the transition metal ions but also to the crystal distortions.3. The undoped Lu MnO₃ has a hexagonal structure with group P63 cm. With the Co dopants（the doping concentration x ￡ 0.4）, orthorhombic phase with group Pbnm appears.Both P63 cm and Pbnm phases are antiferromagnets. With the La concentration x increasing,the structure of Lu1-x LaxMn0.7Co0.3O₃ changes from the biphasic antiferromagnets（P63cm and Pbnm） at x < 0.2 to triple phases（P63cm, Pbnm and Pm3m） at intermediate concentration and finally to a pure ferromagnetic Pm3 m phase as x 3 0.9. The lattice parameters of all phases increase when the La concentration x increases. The magnetic enhancement of Lu1-x LaxMn0.7Co0.3O₃ with a increasing x is due to the increasement of the Pm3 m ferromagnet concertration.4. With an increasing La concentration x, the Ho1-x Lax Mn0.7Co0.3O₃ sample changes from the ferromagnetic Pbnm phase（x ￡ 0.2） to the biphasic（Pbnm and Pm3m） ferromagnets（0.3 ￡ x < 0.8） and ends at the ferromagnetic Pm3 m phase（x 3 0.8）. The lattice parameters of both phases increase when x increases. The magnetization（at 10K） of the samples increases approximately linearly with an increasing doping concerterations x since the magnetization of Pm3 m phase is much large than that of Pbnm phase. Silimalr to Dy1-x LuxMn0.7Co0.3O₃ samples, the Curie temperature of ferromagnetic Pbnm is found approximately linear to the volume of the unit cell.