$^{11}B$ Nuclear Magnetic Resonance Study of Calcium-hexaborides

Mean, B.J.,Lee, K.H.,Kang, K.H.,Lee, Moo-Hee,Lee, J.S.,Cho, B.K. Korean Magnetic Resonance Society 2003 Journal of the Korean Magnetic Resonance Society Vol.7 No.2

We have performed $^{11}$ B nuclear magnetic resonance (NMR) measurements to microscopically investigate an electronic structure of the ferromagnetic state in three different compositions of calcium-hexaboride single crystals. Although the crystal structure of Ca $B_{6}$ is cubic and three NMR lines may be expected for the nuclear spin 3/2 of $_{11}$ B, a larger number of NMR resonance peaks have been observed. The frequency and intensity of those peaks distinctively changes depending on the angle between crystalline axis and magnetic field. Analyzing this behavior, we find that the electric field gradient(EFG) tensor at the boron has its principal axis perpendicular to the six cubic faces with a quadrupole resonance frequency $v_{Q}$ 600 kHz. Even though the magnetization data highlight the ferromagnetic hysteresis, $^{11}$ B NMR linewidth data show no clear microscopic evidence of the ferromagnetic state in three different compositions of Ca $B_{6}$ single crystals.s.

11B Nuclear Magnetic Resonance Study of Calcium-hexaborides

B. J. Mean,K. H. Lee,K. H. Kang,이무희,J.S. Lee,B. K. Cho 한국자기공명학회 2003 Journal of the Korean Magnetic Resonance Society Vol.7 No.2

We have performed 11B nuclear magnetic resonance (NMR) measurements to microscopically investigate an electronic structure of the ferromagnetic state in three different compositions of calcium-hexaboride single crystals. Although the crystal structure of CaB6 is cubic and three NMR lines may be expected for the nuclear spin 3/2 of 11B, a larger number of NMR resonance peaks have been observed. The frequency and intensity of those peaks distinctively changes depending on the angle between crystalline axis and magnetic field. Analyzing this behavior, we find that the electric field gradient(EFG) tensor at the boron has its principal axis perpendicular to the six cubic faces with a quadrupole resonance frequency Q ≈ 600 kHz. Even though the magnetization data highlight the ferromagnetic hysteresis, 11B NMR linewidth data show no clear microscopic evidence of the ferromagnetic state in three different compositions of CaB6 single crystals 영어논문

[ $^{11}B$ ] Nuclear Magnetic Resonance Study of Spin Structures in Terbium Tetraboride

Mean, B.J.,Kang, K.H.,Kim, J.H.,Hyun, I.N.,Lee, Moo-Hee,Cho, B.K. Korean Magnetic Resonance Society 2006 Journal of the Korean Magnetic Resonance Society Vol.10 No.2

[ $^{11}B$ ] nuclear magnetic resonance (NMR) measurements were performed on the single crystals of $TbB_4$ to investigate local electronic structure and 4f spin dynamics. $^{11}B$ NMR spectrum, Knight shift, spin-lattice and spin-spin relaxation rates were measured down to 4K at 8T. $^{11}B$ NMR shift and linewidth are huge and strongly temperature dependent due to the 4f moments. In addition, both are proportional to magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Tb. Below $T_N$, the single broad resonance peak of $^{11}B$ NMR splits into several peaks reflecting the local magnetic fields due to antiferromagnetic spin arrangements. The longitudinal and the transverse relaxation rates, $1/T_1\;and\;1/T_2$, independent of temperature above $T_N$, decreases tremendously confirming huge suppression of spin fluctuation below $T_N$.

11B NMR Study of the Spin Structures and the Dynamics in GdB4 Single Crystals

이무희,S. K. Nam,B. J. Mean,S. K. Kwon,S. H. Choi,B. K. Cho 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.4

11B nuclear magnetic resonance (NMR) measurements were performed on a single crystal of GdB4 to investigate the 4f spin structure and dynamics. The 11B NMR spectrum, shift, linewidth, spinlattice and spin-spin relaxation rates were measured down to 5 K at a field of 8 T perpendicular to the c-axis. Above TN = 42 K, the 11B NMR shift and linewidth were large due to the 4f moments of Gd. In addition, both were strongly temperature-dependent and proportional to the magnetic susceptibility. These facts confirm that the hyperfine field at the boron site originates from the 4f spins of Gd. Below TN, the single broad resonance peak of the 11B NMR split into various peaks, conrming that internal magnetic fields set in due to an antiferromagnetic phase transition. Assuming that the 4f spins of Gd are aligned antiferromagnetically and noncollinearly along the <110> direction in the basal plane perpendicular to the c-axis, we calculated the dipolar fields at the 16 boron nuclear sites from the 4f spins of Gd. The results show that the various peaks of the 11B NMR spectrum at 5 K are generally consistent with the calculation, suggesting that the noncollinear spin structure is a correct spin configuration. The relaxation rates, 1/T1 and 1/T2, were independent of temperature for temperatures above TN and then decreased tremendously. The vast variation of the relaxation rates indicates a rapid spin fluctuation at temperatures above TN and a huge suppression of the 4f spin fluctuation after the phase transition at TN.

Orientation of a MgB2 Powder in a Magnetic Field

강기혁,B. J. Mean,김성훈,B. Ndiaye,조병기,J. Y. Kim,이무희 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.61

We have attempted magnetic orientation of MgB2 powder grains in an epoxy resin under a 7-T horizontal magnetic field. The horizontal field was crucial to get rid of gravitational sedimentation by rotating the mixture of MgB2 powder grains and the epoxy resin along the horizontal axis. Since we did not know the principal axes and the magnitude of the magnetic anisotropy tensor for MgB2, we devised different alignment schemes assuming two cases: case I for the easy axis along the c-axis and case II for the two easy axes perpendicular to the c-axis. For each case, we built different rotation devices: namely, for case I, the rotation axis was parallel to the magnetic field whereas, for case II, it was perpendicular to the magnetic field. For both cases, the powder grains should be aligned along the rotation axis. For assessment of the c-axis alignment, X-ray diffraction (XRD) and 11B nuclear magnetic resonance (NMR) measurements were utilized. However, the XRD and 11B NMR data for both cases were the same as those for the powder sample and showed no evidence of orientation. For possible scenarios, we suggest two possibilities: negligible anisotropy of the magnetic susceptibility tensor and the large size of the ground MgB2 powder particles still in multi-grains.

Orientation of a MgB2 Powder in a Magnetic Field

Lee, Moohee,Kang, Kihyeok,Mean, B. J.,Kim, Sung Hoon,Ndiaye, B.,Kim, J. Y.,Cho, B. K. Korean Physical Society 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.6

¹¹B NMR Study of the Spin Structure and Dynamics in TbB4 Single Crystals

S. K. Kwon,Moohee Lee,B. J. Mean,S. H. Choi,S. K. Nam,조병기 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.2

¹¹B nuclear magnetic resonance (NMR) measurements have been performed on single crystals of TbB₄ to investigate the 4f spin structure and dynamics. ¹¹B NMR spectrum and shift, as well as the spin-lattice and the spin-spin relaxation rates, have been measured down to 5 K at 8 T perpendicular to the (110) plane. ¹¹B NMR shift and linewidth are huge and strongly temperature-dependent due to the 4f moments of Tb. In addition, both are proportional to the magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Tb. Below TN = 43 K, the single broad resonance peak of ¹¹B NMR splits into several peaks reflecting the onset of internal magnetic fields due to the antiferromagnetic spin arrangements. Assuming that the 4f moments are aligned antiferromagnetically and collinearly in the basal plane perpendicular to the c-axis, we have calculated the dipolar fields at the 16 boron nuclear sites from the 4f spins of Tb. The results show that the various peaks of the ¹¹B NMR spectrum at 5 K are generally consistent with the calculation, confirming that the collinear spin arrangement is correct. The longitudinal and the transverse relaxation rates, 1/T₁ and 1/T₂, are independent of temperature above TN, but decrease tremendously below TN, indicating a huge suppression of the spin fluctuations. ¹¹B nuclear magnetic resonance (NMR) measurements have been performed on single crystals of TbB₄ to investigate the 4f spin structure and dynamics. ¹¹B NMR spectrum and shift, as well as the spin-lattice and the spin-spin relaxation rates, have been measured down to 5 K at 8 T perpendicular to the (110) plane. ¹¹B NMR shift and linewidth are huge and strongly temperature-dependent due to the 4f moments of Tb. In addition, both are proportional to the magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Tb. Below TN = 43 K, the single broad resonance peak of ¹¹B NMR splits into several peaks reflecting the onset of internal magnetic fields due to the antiferromagnetic spin arrangements. Assuming that the 4f moments are aligned antiferromagnetically and collinearly in the basal plane perpendicular to the c-axis, we have calculated the dipolar fields at the 16 boron nuclear sites from the 4f spins of Tb. The results show that the various peaks of the ¹¹B NMR spectrum at 5 K are generally consistent with the calculation, confirming that the collinear spin arrangement is correct. The longitudinal and the transverse relaxation rates, 1/T₁ and 1/T₂, are independent of temperature above TN, but decrease tremendously below TN, indicating a huge suppression of the spin fluctuations.

$^{13}C$ Nuclear Magnetic Resonance Study of Graphite Intercalated Superconductor $CaC_6$ Crystals in the Normal State

김성훈,강기혁,민병진,이무희,김준성,Kim, Sung-Hoon,Kang, Ki-Hyeok,Mean, B.J.,Ndiaye, B.,Lee, Moo-Hee,Kim, Jun-Sung The Korean Superconductivity Society 2010 Progress in superconductivity Vol.12 No.1

$^{13}C$ NMR (nuclear magnetic resonance) measurements have been performed to investigate the local electronic structure of a superconducting graphite intercalation compound $CaC_6$ ($T_c$ = 11.4 K). A large number of single crystals were stacked and sealed in a quartz tube for naturally abundant $^{13}C$ NMR. The spectrum, Knight shift, linewidth, and spin-lattice relaxation time $T_1$ were measured in the normal state as a function of temperature down to 80 K at 8.0 T perpendicular to the c-axis. The $^{13}C$ NMR spectrum shows a single narrow peak with a very small Knight shift. The Knight shift and the linewidth of the $^{13}C$ NMR are temperature-independent around, respectively, +0.012% and 1.2 kHz. The spin-lattice relaxation rate, $1/T_1$, is proportional to temperature confirming a Korringa behavior as for non-magnetic metals. The Korringa product is measured to be $T_1T\;=\;210\;s{\cdot}K$. From this value, the Korringa ratio is deduced to be $\xi$ = 0.73, close to unity, which suggests that the independent-electron description works well for $CaC_6$, without complications arising from correlation and many-body effects.