NMR Studies of Magnetism in Borides with Low Carrier Density.

低载流子密度硼化物磁性的核磁共振研究。

基本信息

批准号：
13640355
负责人：
TAKIGAWA Masashi
金额：
$ 2.18万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2001
资助国家：
日本
起止时间：
2001 至 2002
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-13640355/
关键词：
Borides Low carrier density NMR Kondo insulator High temperature weak ferromagnetism

项目摘要

YbB_<12> is known as a typical Kondo insulator. Since Igaw et al. at the Hiroshima University succeeded to grow large single crystals, progress have been made in the measurements of thermal and magnetic properties, transport properties, spectroscopies, and neutron scattering. In this project, we have performed nuclear magnetic resonance (NMR) experiments at the Yb, B and Lu sites for single-crystal samples of pure and 1% Lu doped YbB12. In undoped YbB12, the nuclear spin-lattice relaxation rate at the Yb nuclei shows an activated temperature dependence, indicating an energy gap in the spin excitation spectrum due to either f-p hybridization or Kondo singlet formation. The nuclear spin-lattice relaxation rate at the B nuclei shows similar temperature dependence as the Yb nuclei at high temperatures. However, it increases below 20 K and shows a broad peak near 6 K. The cause for this anomalous temperature dependence was identified to be localized moments induced near dilute defects. It h … More as been theoretically suggested that in Kondo insulators, non magnetic impurities or defects of rare earth elements could lead to localized magnetic moments induced.near defects. The results of B NMR appear to be related to such defects-induced local moments. In order to further confirm this, we measured the relaxation rate at the B and. Lu nuclei in the 1%-Lu doped sample. Both nuclei show similar temperature dependence of the relaxation rate, but the Lu nuclei show 10 times larger relaxation rate than the B nuclei. This indicates that the localized moments are induced near Lu sites.We also found a sharp peak in the relaxation rate at the B nuclei near 30 K in undoped YbB_<12>, which is due to cross relaxation between the Yb and B nuclei near 30 K where the resonance frequency for Yb and B nuclei become equal. This enables us to determine the Yb resonance frequency through the measurements on B nuclei.Strong recent attention has been paid on CaB_6 with very dilute La doping, since the ferromagnetism was reported for this system in spite of the absence of any "magnetic elements" such as transition metal elements. However, extremely small magnetization made it difficult to clarify whether the ferromagnetism is intrinsic to this system or due to impurity phases. If it is intrinsic, the ferromagnetic moment should reside mainly on the 2p orbitals of B sites, which we should be able to detect by B NMR. We have, measured anisotropy of the local field at the B sites in 0.5 %-La doped CaB_6 single crystals with high accuracy. We found that three crystals among five examined show finite internal field at B nuclei, suggesting intrinsic ferromagnetic moments. However, the values of the internal field are not correlated well with the bulk magnetization of each crystal. Less

YBB_ <12>被称为典型的近托绝缘子。由于Igaw等。在广岛大学，在测量热和磁性特性，传输性能，光谱和中子散射方面已经取得了进展。在这个项目中，我们在YB，B和LU位点进行了核磁共振（NMR）实验，用于纯和1％lu掺杂的YBB12的单晶样品。 YB核的核自旋晶格松弛速率未依存的YBB12显示了活化的温度依赖性，表明由于F-P杂交或近代杂交形成而导致自旋兴奋谱的能量间隙。 B核的核自旋晶格松弛速率显示出与高温下YB核的相似温度依赖性。然而，它增加了20 k以下，并显示在6 K附近的较大峰值。这种异常温度依赖性的原因被认为是在稀释缺陷附近诱导的局部力矩。它……从理论上提出的是，在临床绝缘子中，稀土元素的非磁杂质或缺陷可能导致诱导的局部磁矩。 B NMR的结果似乎与这种缺陷引起的局部力矩有关。为了进一步确认这一点，我们测量了B和B和。 1％lu掺杂样品中的lu核。两种核的弛豫率都显示出相似的温度依赖性，但是LU核显示的弛豫率是B核的10倍。这表明在LU位点附近诱导了局部矩。我们还发现，在未掺杂的YBB_ <12>中，B核接近30 K的松弛速率急剧峰值，这是由于Yb和B核之间的交叉松弛，在30 K接近30 K之间，其中YB和B核I的共振频率与Buiti相等。这使我们能够通过B核里的测量来确定YB共振频率。由于据报道，尽管没有任何“磁性元素”（例如过渡金属元素），但该系统的急性la掺杂量非常稀释。然而，极小的磁化使得难以阐明铁磁性是该系统固有的还是由于杂质阶段而引起的。如果它是固有的，那么铁磁矩应主要驻留在B位点的2p轨道上，我们应该能够通过B NMR检测到。我们已经以高精度测量了B位置的局部场各向异性。我们发现，在五个检查中，三个晶体在B核上显示有限的内部场，表明内在的铁磁矩。但是，内部场的值与每个晶体的块状磁化良好。较少的