Pulsed-field magnetization, electron spin resonance, and nuclear spin-lattice relaxation in the｛Cu_3｝ spin triangle

摘要

We report on pulsed-field magnetization, Q-band electron spin resonance (ESR), and ~(23)Na NMR measurements of the S= 1/2 spin triangle clusters Na_9[Cu_3Na_3(H_2O)_9(α-XW_9O_(33)_2] (X=As and Sb). The pulsed-field magnetization shows pronounced hysteresis loops and magnetization steps including the half-step magnetization. The detailed magnetization behavior depends substantially on the diamagnetic heteroatom X. The angular dependence of ESR parameters necessitates Dzyaloshinskii-Moriya interaction. The temperature dependence of the ~(23)Na spin-lattice relaxation rate, 1/T_1 scales well to χ(T)T, where χ(T) is the static susceptibility. The spin-spin relaxation rate, 1/T_2, increases rapidly for temperatures below 15 K due to dipolar interactions between the ~(23)Na nuclei and Cu~(2+) spins. The two clusters exhibit a markedly different field dependence of 1/T_1 at antilevel crossing points. The enhancement of 1/T_1 is noticeable only for X=Sb. Since the spin configurations of both clusters are nearly the same, the dependence of magnetization and 1/T_1 on X is ascribed to the strong coupling of the spins to a lattice vibration, leading to an enhanced mixing of the S= 1/2 chiral state.