Suppression of Ferromagnetic Order by Uniaxial Anisotropy and Its Influence on Nuclear Magnetic Relaxation for Li9V3(P2O7)3(PO4)2

Abstract

The spin dynamics of the Li9V3(P2O7)3(PO4)2 insertion electrode system is explored with the results of magnetization and nuclear magnetic resonance for Li ions. The magnetic susceptibility at high temperatures exhibits a Curie–Weiss law with a ferromagnetic Weiss temperature of about 3 K, while at low temperatures, the enhancement and suppression of susceptibility appear in the absence of long-range order. This aspect is understood on the basis of the XXZ model with the single-ion uniaxial anisotropy. Thus, the slowing down of ferromagnetic spin fluctuations without the long-range order occurs at temperatures below 10 K. In the vicinity of the virtual transition temperature, the spin–lattice relaxation rate \(T_{1}^{ - 1}\) can be expressed by the theoretical asymptotic form with the spin susceptibility χs, \(T_{1}^{ - 1} \propto \chi _{\text{s}}^{1/2}\). At temperatures above 150 K, on the other hand, a quadrupole relaxation by spin–phonon coupling through the two-phonon Raman process may become dominant, which is likely caused by the significantly oscillated Li ions in the large tunnel of the characteristic crystal structure.