Microscopic Research of Spin, Charge and Orbital Ordering in A-Site Ordered Manganites

近年来，人们对著名的CMR锰矿石进行了各种各样的研究。最近合成了a位有序氧化锰。我们通过测量^<139>La和^<55>Mn的NMR来研究a位有序的LaBaMn_2O_6和YBaMn_2O_6。在LaBaMn_2O_6基态下，在铁磁区和反铁磁区均可观测到Mn核的^<55>Mn核磁共振信号，在铁磁区和反铁磁区均可观测到La核的^<139>La核磁共振信号。因此，从微观角度证明了LaBaMn_2O_6中的铁磁区和反铁磁区是共存的，尽管在晶体学上LaO、BaO和MnO_2层是有序的。通过比较Mn内场对温度的依赖关系和铁磁区对中子散射强度的依赖关系，证明了反铁磁区在200 K以下的铁磁区出现并生长，在基态约占50%。由于畴壁的作用，^<55>Mn的核磁共振信号显示出非常大的增强，因此也证明了铁磁区不是纳米级的，而是中尺度的。LaBaMn_2O_6中的铁磁态与反铁磁区共存，与无序La_<0.5>Ba_<0.5>MnO_3中的铁磁态相同。从相同温度下自旋-自旋弛豫速率的依赖。我们也在TRIUMF和KEK测量μSR。

In these years various kind of and a lot of researches have been made concerning the well-known CMR manganites. Recently A-site ordered manganese oxides have been synthesized. We have been investigating the A-site ordered LaBaMn_2O_6 and YBaMn_2O_6 by measuring ^<139>La and ^<55>Mn NMR. We could observe the ^<55>Mn NMR signals from Mn nuclei both in ferromagnetic and antiferromagnetic regions as well as ^<139>La NMR signals from La nuclei both in ferromagnetic and antiferromagnetic regions at the ground state of LaBaMn_2O_6. Therefore it is evidenced that the ferromagnetic and antiferromagnetic regions are coexisting in LaBaMn_2O_6 from a microscopic point of view although LaO,BaO and MnO_2 layers are ordering crystallographically. The fact that the antiferromagnetic region appears and grows in the ferromagnetic region below 200 K and that it occupies about 50 % at the ground state has been shown by comparing the temperature dependence of Mn internal field with that of the neutron scattering intensity by the ferromagnetic region. It has been also evidenced that the ferromagnetic region is not nano-scale but mezzo-scale because ^<55>Mn NMR signals show a very big enhancement due to the domain walls. The ferromagnetic state coexistent with antiferromagnetic region in LaBaMn_2O_6 is proved to be the same as that in disordered La_<0.5>Ba_<0.5>MnO_3. from the same temperature dependence of spin-spin relaxation rate. We have been also measuring μSR at TRIUMF and KEK.