Freezing of molecular rotation in a paramagnetic crystal studied by ${}^{31}$P NMR

Abstract

We present a detailed ${}^{31}$P nuclear magnetic resonance (NMR) study of the molecular rotation in the compound [Cu(pz)${}_2$(2-HOpy)${}_2$](PF${}_6$)${}_2$, where pz = C${}_4$H${}_4$N${}_2$ and 2-HOpy = C${}_5$H${}_4$NHO. Here, a freezing of the PF${}_6$ rotation modes is revealed by several steplike increases of the temperature-dependent second spectral moment, with accompanying broad peaks of the longitudinal and transverse nuclear spin-relaxation rates. An analysis based on the Bloembergen-Purcell-Pound (BPP) theory quantifies the related activation energies as $E_a/k_B$ = 250 and 1400 K. Further, the anisotropy of the second spectral moment of the ${}^{31}$P absorption line was calculated for the rigid lattice, as well as in the presence of several sets of PF${}_6$ reorientation modes, and is in excellent agreement with the experimental data. Whereas the anisotropy of the frequency shift and enhancement of nuclear spin-relaxation rates is driven by the molecular rotation with respect to the dipole fields stemming from the Cu ions, the second spectral moment is determined by the intramolecular interaction of nuclear ${}^{19}$F and ${}^{31}$P moments in the presence of the distinct rotation modes.

arXiv comment9 pages, 5 figures; additionally 2 pages with 3 figures of supplemental material; typos corrected, references added, supplemental material added

References in corpus (9)

• ^{63}Cu, ^{35}Cl, and ^{1}H NMR in the S=1/2 Kagomé Lattice ZnCu_{3}(OH)_{6}Cl_{2}
• Inhomogeneous Low Frequency Spin Dynamics in La_{1.65}Eu_{0.2}Sr_{0.15}CuO_4
• Similar glassy features in the NMR response of pure and disordered La1.88Sr0.12CuO4
• Spin Dynamics in the LTT Phase of ~1/8 Doped Single Crystal La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4
• Progressive slowing down of spin fluctuations in underdoped LaFeAsO${}_{1-x}$F${}_x$
• Magnetic field induced anisotropy of ${}^{139}\mathrm{La}$ spin-lattice relaxation rates in stripe ordered ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$
• Defect-induced magnetism in SiC probed by nuclear magnetic resonance
• Extremely well isolated 2D spin-$1/2$ antiferromagnetic Heisenberg layers with small exchange coupling in the molecular-based magnet CuPOF
• Competing effects of Mn and Y doping on the low-energy excitations and phase diagram of La${}_{1-y}$Y${}_y$Fe${}_{1-x}$Mn${}_x$AsO${}_{0.89}$F${}_{0.11}$ iron-based superconductors

Cited by in corpus (2)

• Field-tunable Berezinskii-Kosterlitz-Thouless correlations in a Heisenberg magnet
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