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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>是典型的近藤绝缘体。自从广岛大学的Igaw等人成功地生长出大单晶以来，在热性质和磁性质、输运性质、光谱学和中子散射的测量方面取得了进展。在这个项目中，我们进行了核磁共振（NMR）实验在YB，B和Lu网站的单晶样品的纯和1%的Lu掺杂的Yb B 12。在未掺杂的YbB 12中，Yb核处的核自旋-晶格弛豫速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在自旋激发谱中存在能隙。在高温下，B核的核自旋-晶格弛豫速率显示出与Y B核相似的温度依赖性。但在20 K以下，它增加，并在6 K附近出现一个宽峰.这种异常的温度依赖性的原因被确定为附近的稀释缺陷引起的本地化的时刻。这是 ...更多信息在近藤绝缘体中，稀土元素的非磁性杂质或缺陷可导致在缺陷附近感生的局部磁矩。B NMR的结果似乎与这种缺陷引起的局部矩有关。为了进一步证实这一点，我们测量了在B和。1%-Lu掺杂样品中的Lu核。两个核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。在未掺杂的Yb B_3中，在30 K附近，B核的弛豫速率出现一个尖峰，<12>这是由于在30 K附近，YB和B核的共振频率相等时，YB核和B核之间的交叉弛豫引起的.这使得我们能够通过对B核的测量来确定Y B的共振频率。最近，人们对La掺杂很低的CaB_6体系给予了很大的关注，因为尽管没有任何“磁性元素”，如过渡金属元素，CaB_6体系仍被报道具有铁磁性。然而，极小的磁化强度使得难以澄清铁磁性是该系统固有的还是由于杂质相。如果它是内禀的，铁磁矩应该主要位于B位的2 p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掺La 0.5%的CaB_6单晶中B位局域场的各向异性。我们发现，在五个检查晶体中的三个显示有限的内部磁场在B核，这表明内在的铁磁矩。然而，内部场的值与每个晶体的体磁化强度不相关。少