The Study Of Ordered Phases Competition And Quantum Transition In Strongly Correlated Manganites Systems

Manganites with pervoskite structure show complicated and spectacular properties, which draw many academicians’attention and research in recent years. However, most study work focused on the A site doping, the study on B site (Mn site) doping with A site half doping is still little. The Mn site doping can destroy the charge order of Mn4+ and Mn3+. Especially at low temperatures, the Mn site doping strongly affects the nature of the sample. This is of great significance for basic research and has potential applications. So, we synthesized the doping type manganites of Pr0.5Ca0.5Mn1-xAlxO3series samples. As nonmagnetic cations, Al3+can counterbalance the Jahn-Teller distortion of Mn3+and tend to render the structure more symmetric around them. Furthermore, ferromagnetic clusters can be induced in the spite of lack of magnetic interactions.The paper carried out a systematic study on crystal, electrical transport and magnetic properties of Pr0.5Ca0.5Mn1-xAlxO3series samples. The samples we have made are orthorhombic perovskite structure. The highest synthesis temperature is1300℃, and the samples showed significant anti-ferromagnetic, which have not been reported such exotic magnetic phenomena in Pr0.5Ca0.5Mn1-xAlxO3series samples. Below the freezing temperature, magnetization measured in zero field cooled and field cooled conditions does not coincide. At low temperatures, ferromagnetic component distributes randomly in the basement of the antiferromagnetic, competing with each other results in the formation of ferromagnetic clusters and antiferromagnetic clusters. And the macro performance is field cooled and zero field cooled curves bifurcate at the freezing temperature. Tco decreases with increasing Al content, at the same time, the magnetization at Tco increases with increasing Al content. The hysteresis loops at2K shows significant hysteresis, two steps appear in the first branch. With the external field of9T, the magnetization is still not saturated. This is the result of superposition of the ferromagnetic components and antiferromagnetic components, perform obvious cluster glass state. Changes at steps can be attributed to the transition of charge ordered antiferromagnetic phase into ferromagnetic phase in the samples.The spin freezing temperature of ac magnetic susceptibility moves to the higher temperature with frequency increasing, is a function of frequency. With the increase of Al doping amount, the value of the real part of ac susceptibility shows an increasing trend, which is similar to the behavior of dc field magnetization. The ground state of the samples x=0,0.01,0.02is cluster glass state. The peaks at Tf of samples x=0.05,0.07decrease with increasing frequency, which is the phase separation of ferromagnetic clusters in the charge ordered antiferromagnetic system. The ground state of the x=0.03,0.04samples is the transition state from cluster glass for the x=0.02sample to phase separation for the x=0.05sample. The ground state of x=0.02sample is cluster glass, of x=0.05sample is phase separated state.The resistivity of samples series of Pr03.5Ca035Mn1-xAlxO3was measured in zero field and5T magnetic field,2K to300K temperature regions, shown a semiconducting behavior. At low temperatures, the CMR effect is very obvious. For all the samples with different Al doping amount, the CMR value get larger with Al3+doping amount. The reason is that Al3+doping and the field render ferromagnetic clusters increase, thus, the resistivity decrease.