(La,Sr)3Mn2-2xA2xO7(A=Al,Ti) 和R2-xSrNaxMn2O7(R=Pr,Sm)層狀鈣鈦礦錳氧化物的磁電傳輸性質

Abstract

Because of the potential applications of CMR effect in magnetic random access memories, and many physical properties observed in the manganite with a perovskite structure, it has been attracted a lot of attention in condensed matter physics since the discovery of the colossal magnetoresistance(CMR) phenomenon. We study the structures, magnetic and electrical transport properties of the perovskite manganites through doping on the A-site and B-site independently. The main results are generalized as follows. A series of polycrystalline Al-doped and Ti-doped Ruddlesden-Popper manganates (La,Sr)_3 Mn_(2-2x) A_2x O_7 (A=Al or Ti) are synthesized by the solid state reaction method. The effect of Al doping or Ti doping on the magnetic and transport properties is investigated. With the decrease of temperature, the system undergoes a transition from paramagnetism to the long-range ferromagnetic order and further to spin glass state. It is found that Al substitution for Mn greatly affects the magnetic and electrical properties of the parent phase (La,Sr)_3 Mn_2 O_7. A little Al dopant can lead to the samples showing semiconductor or insulator behaviour in the whole observed temperature range while the parent phase has a metal-insulator transition. We also prepare polycrystalline layered perovskite R_(2-x) SrNa_x Mn_2 O_7，(R=Pr,Sm), and investigate its magnetic properties. At low temperature, the FM and AFM phases coexist in the sample. Substitution of Sm^(3+)or Pr^(3+) by Na^+ results in the change of the average radius of the cation on A-site and the related position of manganese ion and oxygen ion, and then weakens the double exchange interaction (DE) of Mn^(4+)-O-Mn^(3+)couple. A first-order phase transition near Curie temperature Tc and metamagnetic transition at certain temperatures above Tc are observed in the sample.