Abstract
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.
Highlights
During the last decade, physicists have paid considerable attention to Molecular Nano Magnets (MNMs) as model systems to study fundamental quantum and magnetic properties.1,2 In many MNMs, the intermolecular magnetic interactions are negligible as compared to the intra-molecular exchange interactions, the bulk magnetic properties reflect the microscopic features of individual molecular units
The peaks at the first and second level crossing (LC) in the nuclear spin-lattice relaxation rate are dominated by inelastic scattering and a value of Γ ∼ 1010 rad/s is inferred for the life time broadening of the excited state of the open ring, due to spin phonon interaction
Antiferromagnetic Molecular (AFM) rings are an interesting subgroup of MNMs, which are formed by a finite number of transition-metal ions arranged in a cyclic structure
Summary
Physicists have paid considerable attention to Molecular Nano Magnets (MNMs) as model systems to study fundamental quantum and magnetic properties. In many MNMs, the intermolecular magnetic interactions are negligible as compared to the intra-molecular exchange interactions, the bulk magnetic properties reflect the microscopic features of individual molecular units. Metallic ions in place of the transition metal magnetic ions, one can manipulate the topology of the interactions which determines the magnetic properties and LCs. One particular situation of interest in the present work is when the homometallic ring is interrupted by the presence of a diamagnetic metal ion, as in the case of Cr8Cd, recently investigated by polarized neutron diffraction.. The aim of the present work is the investigation, by thermodynamic measurements and nuclear magnetic resonance (NMR) relaxation and spectroscopy, of the multiple LCs between ground and excited states in a single crystal of the Cr8Zn finite spin segment, with an even number of magnetic ions.
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