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

近年来，人们对著名的CMR锰氧化物进行了各种各样和大量的研究。最近合成了A位有序锰氧化物。我们通过测量^&lt；139&gt；La和^&lt；55&gt；Mn核磁共振研究了A位有序的LaBaMn_2O_6和YBaMn_2O_6。在LaBaMn2O6的基态，我们可以观察到铁磁区和反铁磁区的锰核的核磁共振信号，以及LaBaMn2O6基态的铁磁区和反铁磁区的La核磁共振信号。因此，从微观角度证明了LaBaMn2O6中的铁磁区和反铁磁区共存，尽管LaBaMn2O6层在晶体结构上是有序的。通过比较Mn内场和铁磁区的中子散射强度随温度的变化关系，证明了在200K以下的铁磁区中出现并长大了反铁磁区，并且在基态时反铁磁区约占50%。也有证据表明，铁磁区不是纳米级的，而是中级级的，这是因为磁畴壁的存在使核磁共振信号有很大的增强。从自旋-自旋弛豫速率对温度的依赖关系，证明了LaBaMn_2O_6中与反铁磁区共存的铁磁态与无序La_&lt；0.5&gt；Ba_&lt；0.5&gt；MnO_3中的铁磁态相同。我们还在TRIUMF和KEK测量了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.