Tuning Of Charge-ordering/antiferromagnetic Coupling In ABO₃-type Manganites

The ABO3-type manganites have abundant physical properties such as phase separation,charge ordering and metamagnetic transition due to the strong coupling between spin,obital,lattice and charge degrees of freedom.There exist two common methods to tune the physical properties of manganites:doping and application of magnetic field.Manganites La0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 exhibit phase separation with the coexisting of ferromagnetic,charge-ordering and antiferromagnetic phases at low temperature.In this doctoral dissertation,we investigate the tuning effects of Mn-site substitution,temperature and external magnetic field on the physical properties of La0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 based on the magnetization measurement system under pulsed high magnetic field.The primary contents are as follows:1.We briefly analyze the importance of pulsed high magnetic field in scientific research and introduce the electrical and magnetic properties of ABO3-type manganites through lattice structure,theory model and research status.In particular,the mechanism of the martensitic-like transformation,the modulation of the charge ordering phase and the mechanism of the spontaneous exchange bias effect are summarized.2.The Ti,Al,Cr,Co and Cu doped La0.5Sr0.5Mn1-xRxO3 and Nd0.5Sr0.5Mn1-xCuxO3 series samples were prepared by solid state reaction method and sol-gel method.We briefly introduce the magnetization measurement system based on the pulsed high magnetic field and related magnetization measurement.3.We selected nonmagnetic Ti4+,Al3+ and magnetic Cr3+ ions to investigate the tuning effects of Mn-site substitution and magnetic field on the magnetic and electrical properties of La0.5Sr0.5Mn1-xRxO3 utilizing steady and pulsed high magnetic field.(1)The Ti4+ doping makes the x = 0.15-0.2 systems exhibiting step-like magnetization behavior below 3 K,which is related to the localized ferromagnetic domains induced by Ti4+ doping.The magnetization jumps are related to the cooling field and field sweep rate,which is a feature of the martensitic-like transition.(2)Nonmagnetic Al3+ ion does not participate in the magnetic exchange interaction and Al3+ doping at the Mn-sites partialy destroys the Mn3+-O2--Mn4+ ferromagnetic double exchange interaction,leading to the enhancement of the antiferromagnetic coupling in the system.The magnetic field-driven sharp metamagnetic transition from the charge-ordering/antiferromagnetic to ferromagnetic phases under pulsed high magnetic field up to 50 T has been observed.The magnetic phase diagram of La0.5Sr0.5Mn1-xAlxO3 has been extended to high magnetic field range.(3)The antiferromagnetic coupling of the doped Cr3+ and its neighboring Mn4+ ions leads to the spin orientation of the Mn ions within the Cr3+-doped "zigzag"chain to be reversed,and ferromagnetic clusters are induced around the doped Cr3+ ions.The Curie temperature decreases with increasing Cr3+ ion doping level due to the enhanced Cr3+-Mn4+,Mn4+-Mn4+ and Cr3+-Cr3+ antiferromagnetic coupling.4.The tuning effects of Cu doping and magnetic field on the charge ordering/antiferromagnetic coupling of Nd0.5Sr0.5MnO3 have been investigated.The experimental results suggest that Cu dopants exist in Cu2+ and Cu3+ ionic state,leading to increase in the amount of Mn4+ and decrease in the Mn3+/Mn4+ ratio,which weakens the Mn3+-O2--Mn4+ferromagnetic double exchange interaction.The eg electrons of Mn3+ participated in the double exchange interaction are localized around the Cu2+ and Cu3+,and the electronic transport mechanism follows the law predicted by small polaron hopping.The collapse of the charge-ordering/antiferromagnetic phases was induced with application of high magnetic field and the magnetic phase diagram of Nd0.5Sr0.5Mn1-xCuxO3 has been extended.5.Co doping-induced spontaneous exchange bias effect in La0.5Sr0.5Mn1-xCoxO3(x ?0.2)has been investigated,which is related to the Co doping-induced cluster-glass state.A transition from positive to negative spontaneous exchange bias has been observed with increasing initial magnetization field and measurement temperature.