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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>被称为典型的近藤绝缘体。自从伊高等人。广岛大学成功生长出大型单晶，在热磁特性、输运特性、光谱学和中子散射的测量方面取得了进展。在本项目中，我们对纯 YbB12 和 1% Lu 掺杂的 YbB12 单晶样品的 Yb、B 和 Lu 位点进行了核磁共振 (NMR) 实验。在未掺杂的 YbB12 中，Yb 核处的核自旋晶格弛豫率显示出激活的温度依赖性，表明由于 f-p 杂化或近藤单线态形成而导致自旋激发光谱中的能隙。 B 核处的核自旋晶格弛豫率显示出与高温下 Yb 核相似的温度依赖性。然而，它在 20 K 以下增加，并在 6 K 附近显示出宽峰。这种异常温度依赖性的原因被确定为在稀缺陷附近引起的局部力矩。理论上表明，在近藤绝缘体中，非磁性杂质或稀土元素的缺陷可能导致在缺陷附近产生局部磁矩。 B NMR 的结果似乎与这种缺陷引起的局部力矩有关。为了进一步证实这一点，我们测量了B和处的弛豫率。 1%-Lu 掺杂样品中的 Lu 核。两个核都显示出相似的弛豫速率的温度依赖性，但 Lu 核显示出比 B 核大 10 倍的弛豫速率。这表明局域矩是在Lu位点附近引起的。我们还发现未掺杂的YbB_<12>中B核在30 K附近的弛豫率出现尖峰，这是由于Yb和B核在30 K附近的交叉弛豫，此时Yb和B核的共振频率变得相等。这使我们能够通过对 B 核的测量来确定 Yb 共振频率。最近，具有非常稀的 La 掺杂的 CaB_6 受到了强烈的关注，因为尽管不存在任何“磁性元素”（例如过渡金属元素），但该系统仍具有铁磁性。然而，极小的磁化强度使得很难澄清铁磁性是该系统固有的还是由于杂质相造成的。如果它是本征的，那么铁磁矩应该主要存在于 B 位点的 2p 轨道上，我们应该能够通过 B NMR 检测到。我们高精度地测量了 0.5%La 掺杂 CaB_6 单晶 B 位点的局域场各向异性。我们发现，所检查的 5 个晶体中的 3 个晶体在 B 核处显示出有限的内部场，表明存在固有铁磁矩。然而，内部磁场的值与每个晶体的体磁化强度没有很好的相关性。较少的