Magnetic state dynamics in itinerant paramagnet UM3B2 (M= Co, Ir) probed by 11B NMR
arXiv (Cornell University)(2008)
摘要
We have carried out the $^{11}$B NMR measurement on the itinerant
paramagnetic systems U$M_{3}$B$_{2}$ ($M =$ Co, Ir) to investigate the
low-dimensional characteristics of the $5f$-electrons due to the structural
anisotropy. The recent X-ray analysis suggests that UIr$_3$B$_2$ has a
different structure modulated from the ever-known superlattice. The azimuth
angle variation of NMR spectrum within the $ab$-plane clarified that B atoms
occupy the single site, and a certain ligands arrangement surrounding B atom
turns to the same orientation as the another one through the three- or six-fold
rotation around the c-axis. These results have been consistent with the X-ray
proposition. To evaluate the temperature ($T$) development of general
susceptibility ($\chi_{q,\omega}$), Knight shift and nuclear spin-lattice
relaxation rates measurements were performed and the similar variations of
$\chi_{q,\omega}$ were identified in both UCo$_{3}$B$_{2}$ and
UIr$_{3}$B$_{2}$. Above a crossover point defined as $T^{*}\simeq50$ K, the
evolution of $\chi_{q,\omega}$ is dominant at $q=0$, suggesting that
ferromagnetic correlations develop in high-$T$ regimes; meanwhile, below
$T^{*}$, the $q=0$ part in $\chi_{q,\omega}$ shows the saturation tendency, and
a different class of dispersion at finite-$q$ suddenly emerges. This particular
magnetic correlations are interpreted as the antiferromagnetic correlations,
and notable feature of the magnetic state dynamics in low-$T$ regimes is that
the antiferromagnetic correlations arise together with the ferromagnetic
component at the same time. The unique magnetic correlations obtained from NMR
experiment will be discussed by the possible low-dimensionality of
U$M_{3}$B$_{2}$ lattice.
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关键词
superlattices,spectrum
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