Adelnia Fatemeh, Chiesa Alessandro, Bordignon Sara, Carretta Stefano, Ghirri Alberto, Candini Andrea, Cervetti Christian, Evangelisti Marco, Affronte Marco, Sheikin Ilya, Winpenny Richard, Timco Grigore, Borsa Ferdinando, Lascialfari Alessandro
Dipartimento di Fisica, Università degli Studi di Milano and INSTM, I-20133 Milano, Italy.
Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Parma, I-43124 Parma, Italy.
J Chem Phys. 2015 Dec 28;143(24):244321. doi: 10.1063/1.4938086.
A detailed experimental investigation of the effects giving rise to the magnetic energy level structure in the vicinity of the level crossing (LC) at low temperature is reported for the open antiferromagnetic molecular ring Cr8Zn. The study is conducted by means of thermodynamic techniques (torque magnetometry, magnetization and specific heat measurements) and microscopic techniques (nuclear magnetic resonance line width, nuclear spin lattice, and spin-spin relaxation measurements). The experimental results are shown to be in excellent agreement with theoretical calculations based on a minimal spin model Hamiltonian, which includes a Dzyaloshinskii-Moriya interaction. The first ground state level crossing at μ0Hc1 = 2.15 T is found to be an almost true LC while the second LC at μ0Hc2 = 6.95 T has an anti-crossing gap of Δ12 = 0.19 K. In addition, both NMR and specific heat measurements show the presence of a level anti-crossing between excited states at μ0H = 4.5 T as predicted by the theory. In all cases, the fit of the experimental data is improved by introducing a distribution of the isotropic exchange couplings (J), i.e., using a J strain model. The peaks at the first and second LCs in the nuclear spin-lattice relaxation rate are dominated by inelastic scattering and a value of Γ ∼ 10(10) rad/s is inferred for the life time broadening of the excited state of the open ring, due to spin phonon interaction. A loss of NMR signal (wipe-out effect) is observed for the first time at LC and is explained by the enhancement of the spin-spin relaxation rate due to the inelastic scattering.
报道了对低温下开放反铁磁分子环Cr8Zn在能级交叉（LC）附近产生磁能级结构的效应进行的详细实验研究。该研究通过热力学技术（扭矩磁强计、磁化强度和比热测量）和微观技术（核磁共振线宽、核自旋晶格和自旋-自旋弛豫测量）进行。实验结果与基于最小自旋模型哈密顿量的理论计算结果非常吻合，该哈密顿量包括Dzyaloshinskii-Moriya相互作用。发现第一个基态能级交叉处μ0Hc1 = 2.15 T几乎是一个真正的LC，而第二个LC处μ0Hc2 = 6.95 T的反交叉间隙为Δ12 = 0.19 K。此外，核磁共振和比热测量都表明，如理论所预测的，在μ0H = 4.5 T时激发态之间存在能级反交叉。在所有情况下，通过引入各向同性交换耦合（J）的分布，即使用J应变模型，实验数据的拟合得到了改善。核自旋晶格弛豫率中第一和第二个LC处的峰值主要由非弹性散射主导，由于自旋-声子相互作用，推断出开环激发态寿命展宽的Γ值约为10(10) rad/s。在LC处首次观察到核磁共振信号的损失（消除效应），并通过非弹性散射导致的自旋-自旋弛豫率增强来解释。
