【作者】 石峰军；

【导师】 唐成春；

【作者基本信息】 华中师范大学 ， 凝聚态物理， 2005， 硕士

【摘要】 以钙钛矿结构锰氧化物为代表的巨磁电阻效应材料,由于它们所表现出来的超大磁致电阻效应（colossal magnetoresistance）在提高磁存储密度及磁敏感探测元件上具有十分广阔的应用前景,因而受到人们广泛的关注。普遍认为双交换作用对钙钛矿锰氧化物中CMR效应起关键作用,故本论文通过对Mn位元素的替代来研究掺杂后对双交换作用的影响。本论文的主要工作和结果如下: 研究了Mn位替代氧化物La_2/3Ca（₁/3）Mn_1-xCr_xO₃和La_0.8Ca_0.2Mn_1-xCr_xO₃的结构和磁性。通过结构分析,掺入少量Cr后体系的晶胞收缩,体积变小;Jahn-Teller畸变增大,Mn-O-Mn的平均键角也随之减小,使得体系的居里温度Tc降低,从这个侧面都支持了双交换作用模型。通过样品的磁相图还发现随着Cr含量的增加,体系经历了铁磁金属、铁磁绝缘体、反铁磁态转变。并讨论了x=0.67作为体系结构和磁性的临界点。从结构和磁性两个方面,证实了双交换模型在解释锰氧化物的铁磁金属性是合理的。 通过测量La_0.8Ca_0.2Mn_1-xCr_xO₃的热磁曲线（ZFC—FC）、交流磁化率曲线研究了体系中的自旋玻璃态特征。发现体系存在典型的自旋玻璃态（SG）和团簇玻璃态（CG）特征,且其热磁性质可以用温度和磁场的双势阱模型来解释;通过不同频率下的交流磁化率曲线,讨论了其自旋玻璃态的冻结温度T_f与最大弛豫时间τ间的关系,得到了其特征时间常数。更多还原

【Abstract】 Perovskite manganites have attracted enormous attention in recent times due to their colossal magnetoresistance （CMR） property and potential applications in magnetic information store and low-field magnetic sensors. Nowadays, it has been extensively accepted that the double exchange interaction plays a key role in perovskite manganites. Therefore, the structure and magnetic properties of perovskite manganites have been studied by doping on Mn sites in this thesis, the main results are as following:The structure and magnetic properties have shown that the substitution of Cr for Mn in La_2/3Ca_1/3Mn_1-xCr_xO₃ and La_0.8Ca_0.2Mn_1-xCr_xO₃ decrease the Curie temperature. The unit cell volume and the average angler Mn-O-Mn of the sample decrease as a function of the Cr content. There is a general tnedencey for the distortion of MnO₆ octahedra to increase as the Cr content increases. With increasing Cr content, ferromagnetic metal-insulator（M-I） transition and ferromagnetic-antiferromagnetic （FM-AFM） transition were observed in the magnetic phase diagram. The above results support the double exchange model in manganite.We have also studied the spin glass （SG） behavior of La_0.8Ca_0.2Mn_1-xCr_xO₃ in definite conditions, which has been investigated from different aspects and differentmethods（ZFC-FC, ac susceptibility）. Using the concept of spin glass and thedouble-potential-well model, we have plotted the relation between the freezing temperature and the relaxation of M time, and calculated the characteristic time constant of the spin glass.更多还原