NMR Study of Spin Dynamics in V 7 Zn and V 7 Ni Molecular Rings
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Applied Magnetic Resonance
ORIGINAL PAPER
NMR Study of Spin Dynamics in V7Zn and V7Ni Molecular Rings F. Adelnia1 · P. Arosio1 · M. Mariani2 · F. Orsini1 · A. Radaelli3 · C. Sangregorio4 · F. Borsa2 · J. P. S. Walsh5 · R. Winpenny6 · G. Timco4 · A. Lascialfari1,2,7 Received: 1 July 2020 / Revised: 25 September 2020 / Accepted: 30 September 2020 © The Author(s) 2020
Abstract We present a 1H NMR investigation of spin dynamics in two finite integer spin molecular nanomagnetic rings, namely V7Zn and V7Ni. This study could be put in correlation with the problem of Haldane gap in infinite integer spin chains. While V7Zn is an approximation of a homometallic broken chain due to the presence of s = 0 Zn2+ ion uncoupled from nearest neighbor V 2+ s = 1 ions, the V 7Ni compound constitutes an example of a closed periodical s = 1 heterometallic chain. From preliminary susceptibility measurements on single crystals and data analysis, the exchange coupling constant J/kB results in the order of few kelvin. At room temperature, the frequency behavior of the 1H NMR spin–lattice relaxation rate 1/T1 allowed to conclude that the spin–spin correlation function is similar to the one observed in semi-integer spin molecules, but with a smaller cutoff frequency. Thus, the high-T data can be interpreted in terms of, e.g., a Heisenberg model including spin diffusion. On the other hand, the behavior of 1/T1 vs temperature at different constant fields reveals a clear peak at temperature of the order of J/kB, qualitatively in agreement with the well-known Bloembergen–Purcell–Pound model and with previous results on semi-integer molecular spin systems. Consequently, one can suggest that for a small number N of interacting s = 1 ions (N = 8), the Haldane conjecture does not play a key role on spin dynamics, and the investigated rings still keep the quantum nature imposed mainly by the low number of magnetic centers, with no clear topological effect due to integer spins.
1 Introduction The ground state of an infinite antiferromagnetic (AFM) spin 1D chain is gapless for the case of semi-integer spins [1]. On the other hand for integer spins, the AFM ground state of an infinite chain is separated by a finite gap from the first excited * M. Mariani [email protected] Extended author information available on the last page of the article
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state (Haldane prediction) [2]. This is an important result both for the general standpoint of topological effects in statistical mechanics and for the specific applications to magnetism. On the other hand, in the last decades, chemists and physicists have developed the field of single molecule magnets (SMM) [1] that represent ideal models to study magnetism on a limited and controlled finite number of spins, mainly in zero and one dimension. Their investigation has led to the discovery of new fundamental physics phenomena, at the border between quantum and classical framework, e.g., quantum tunneling [1, 3–7] and coherence, quantized transport and quan
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