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，在热岛大学中，在热和磁性特性，传输特性，光谱和中子散射中成功地生长了大型单晶。 YB YBB12的磁共振（NMR）在B核处的核自旋晶格速率相似的温度依赖性与高温下的YB核相似。几乎是稀释的缺陷。在理论上，它在近相传中，稀有地球元素的非磁性杂质或缺陷可能会引起磁性时刻。。在30 K附近的B核的速率接近30 K中的30 K，其中Yb和B核的sthequencience是相等的。 Cab_6带有枢轴的掺杂，因为没有任何“磁性元素”（例如过渡金属元素）或由于杂质阶段而据报道，该系统据报道了该系统。 B的2p轨道坐着，Wee Shoud能够检测到，测量了0.5％ACY的B位点的局部场各向异性。，内部磁场的值较小