Effects Of Grain Size On The Magnetic Properties Of La_xCa_1-xMnO₃

Various materials crystallize in perovskite structure, which are widely investigated and found application as functional materials in life. The La doping Ca Mn O3 system has been studying as a hot topic in the field of condensed matter physics due to the potential application in device. However, new challenge of size emerges from the viewpoint of application. When material is in small size, its properties change significantly due to the size effect, as well as the strong correlation nature. To study the effect of size is necessary for realizing device miniaturization.The effect of size on the magnetic properties of Lax Ca1-x Mn O3 system, where x= 0.1, 0.18, was studied. Bulk La doping Ca Mn O3 samples of different doping level were synthesized by using solid state reaction, nanoparticles using sol-gel method, and thin film using pulsed laser deposition. The XRD results suggest that samples were synthesized successfully. The size of nanoparticles is about 10 nm, and the thickness of thin film is 12 nm. Finally, the effect of size on the magnetic properties of bulk ceramic, nanoparticles and thin film were investigated.For nanoparticles and bulk ceramic, when La doping concentration is 0.1, the magnetization is large in the former. The ferromagnetic clusters emerge in antiferromagnetic Ca Mn O3 matrix due to double exchange interactions caused by La doping, and the sample exhibits ferromagnetism. With decreasing size, long antiferromagnetic ordering breaks, and the net spin moments existing on surface enhances ferromagnetism.When La doping concentration is 0.18, the ceramic sample is different from nanoparticles in nature. Ceramic sample exhibits the phenomenon of charge ordering which does not appear in nanoparticles, because the condition of long period for developing charge ordering does not meet in nanoparticles due to lattice distortion. Charge ordering hinders the development of ferromagnetism, and the magnetization is low even at low temperature and strong magnetic field, when La doping concentration is high.The Curie temperature of the thin film moves to lower temperature compared with ceramic and nanoparticles, for film endures tensile stress comes from substrate, which suppresses the motion of itinerant electrons and reduces band width, at the same time electronic-phonon interaction strengthens, and the Curie temperature is low. As a result, the magnetization of thin film of 0.18 La doping is lower than that of 0.1 doping one, possibly due to the charge ordering exists in the former.