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MAGNETIC SYSTEMS 2009NMR evidence for long zero-quantum coherence in antiferromagneticmolecular wheels NaFe 6 and LiFe 6Molecular magnetic clusters of nanometer size have receivedenormous attraction because of their spectacularquantum phenomena. A unique class of these clusters arethe antiferromagnetic (AFM) molecular wheels, in whichmagnetic metal ions are assembled in a ring-like structure.The dominant AFM Heisenberg interaction J between themagnetic metal ions leads to a nonmagnetic S = 0 groundstate and a first excited S = 1 state in zero magnetic field.In strong magnetic fields the Zeeman splitting induces levelcrossings (LCs), where the ground state of the moleculechanges from S = 0, M = 0 to S = 1, M = −1, and furtherto S = 2, M = −2, etc. Due to the hyperfine couplingbetween the nuclear spin and the Fe III ions, the nuclear relaxationrate T −11is very sensitive to the spin state and thespin dynamics of the ferric wheel. Most NMR studies havebeen carried out by means of 1 H NMR, since protons providea very strong signal, and their T −11is sensitive to thedynamics of the Néel vector ⃗n = ∑ 6 i=1 (−1)i ⃗s i . However, amajor drawback is a huge amount of inequivalent protonson each ring complicating the analysis of the spin dynamics,especially around the level crossings. In order to overcomethis problem we performed NMR measurements onthe nuclei which are located in a single-site position of highsymmetry, in the center of the ferric wheel–here 23 Na or 7 Linuclei. Their hyperfine interaction is proportional to the totalspin of the ferric wheel ⃗S = ∑ 6 i=1 ⃗s i leading to importantdifferences as far as T −11is concerned.Here we present the first central alkali NMR study atlow temperature and at high field, focusing on the magneticfield dependence at the level crossings [L. Schnelzeret al., submitted to EPL]. NMR measurements on 23 Naand 7 Li nuclei were carried out on single crystals of [Na/Li⊂Fe 6 {N(CH 2 CH 2 O) 3 } 6 ]Cl·5CHCl 3·0.5H 2 O (Na/LiFe 6in short). They were performed in 17 and 20 T superconductingmagnets in Grenoble and Karlsruhe. Single crystalswere mounted in the mixing chamber of a dilution refrigeratorfor very low temperatures and in pumped 4 He for measurementsat 2 K. Figure 110 shows the measured T −11ratesat 220 mK and 2 K. The low temperature measurements ofNaFe 6 at 220 mK reveal a strong increase of T1 −1 towardsthe LC at 12 T. The LC is not characterized by an additionalenhancement of T1 −1 as expected for proton NMR,but by a small reduction of T1 −1 (inset to figure 110a). Thisis attributed to the insensitivity of the central alkali nucleito the fluctuations of the Néel vector. At 2 K T1 −1 shows abroad maximum around LC and, surprisingly, a reductionby three orders of magnitude of T −11in-between the 1 st andthe 2 nd LC. Similar results have been obtained for LiFe 6 ;plotted on a reduced field scale (B/J) the measurements onLiFe 6 and NaFe 6 are almost identical. Measurements of23 Na T1 −1 at the first LC, at temperatures down to 80 mK,reveal the existence of a very small gap 0.06 K, in spiteof high symmetry of the molecule. This implies the existenceof a small perturbation reducing the symmetry of themolecule. The most striking phenomena is a spectaculardecrease by three orders of magnitude of T1 −1 occurring inthe middle between the first and the second LC.The relaxation data have been described by calculating thecorresponding spectral density of spin fluctuations by amethod of moments. It turns out that the observed strongextinction of relaxation is not at all visible in the intensity ofthe zero frequency resonance (the zero moment), but is entirelydue to its width, calculated as the second moment m 2 .For the calculation of m 2 we used the secular (i.e., energyconserving) part of the dipolar intermolecular interaction.Between the first and the second LC there is a broad andvery deep minimum of m 2 . In this field range the groundstate is a S = 1 state whereas the first excited state changesfrom S = 0 to S = 2 through a broad anti-LC. Since theminimum of m 2 results from the balance of the matrix elementsof these states, it extends over the same field range asthe anti-LC. It is remarkable that √ m 2 can become smallerby two orders of magnitude than the nuclear Larmor frequency.This quite unusual situation leads to the reductionof T1 −1 by three orders of magnitude. One should also realizethat the correlation time τ c = 1/ √ m 2 corresponds tothat of a zero quantum coherence. τ c can be as long as ≃0.2 µs, which is very unusual for an electronic spin system.Figure 110: (a) Field dependence of 23 Na T1 −1 in NaFe 6 atT = 220 mK (white squares) and 2 K (black squares), and (b) 7 LiT1 −1 in LiFe 6 at T = 2 K (black squares) and 1 H T1 −1 (dots). Theresults of the calculations are given by solid lines. Inset displays azoom on the field dependence close to the LC at 220 mK.M. Horvatić, C. BerthierL. Schnelzer, B. Pilawa, M. Marz, H. von Löhneysen (Physikalisches Institut, Universität Karlsruhe,Germany)82