Fe57NMR and relaxation by strong collision in the tunneling regime in the molecular nanomagnet Fe8

Abstract

Fe NMR measurements have been performed in single crystal and oriented powder of enriched 57 Fe8 molecular cluster in the temperature range 0.05-1.7 K in zero external field and with small perturbing field up to 1 T for both transverse and longitudinal orientation of H with respect to the anisotropy axis. The 57 Fe NMR spectrum is analyzed in terms of a dominant contribution due to the hyperfine interaction arising from core polarization. The measured temperature dependence of the resonance frequency is explained well by calculating the local average magnetic moment of the Fe 3+ ion with a simple model which incorporates the effects of thermal average in the low lying energy states. Nuclear spin-lattice relaxation rate ͑1/T 1 ͒ and spin-spin relaxation rate ͑1/T 2 ͒ were investigated via temperature and field dependences. The obtained results are analyzed in terms of both intrawell thermal fluctuations of the hyperfine fields due to spin-phonon interaction, and interwell fluctuations due to phonon assisted quantum tunneling of the magnetization. It is argued that in zero external field and at low T the 57 Fe and the proton 1 / T 1 is dominated by a strong collision relaxation mechanism due to the fact that phonon assisted tunneling transitions generate a sudden reversal of the local quantization field at the nuclear site. The data could be explained satisfactorily by assuming that the 57 Fe 1 / T 1 measures directly the effective tunneling rate. However, in order to fit the data we had to assume a larger in-plane anisotropy than previously reported, resulting in a bigger tunneling splitting in zero field. A comparison with published data of 55 Mn in Mn12 indicates that a strong collision relaxation mechanism may apply also in Mn12. Finally the H and T dependence of 57 Fe 1 / T 2 is well explained simply in terms of thermal fluctuations of the magnetization without any tunneling contribution. At very low T the 1 / T 2 approaches a limiting value which can be explained in terms of the dipolar interaction between proton and 57 Fe nuclei in the quasistatic regime.

Dynamics and thermalization of the nuclear spin bath in the single-molecule magnetMn12−ac: Test for the theory of spin tunneling

“…To understand the details of the nuclear spin fluctuations, one should then look directly at the nuclear spins by means of low-temperature NMR experiments, performed under different regimes for the quantum dynamics of the electron spin. These experiments have been carried out by several groups, 23,24,25,26,27 but an accurate analysis of their implications for the more general theory of nuclear-spin mediated quantum tunneling is still lacking.…”

Probing spin dynamics and quantum relaxation inLiY0.998Ho0.002F4viaGraf1, Lascialfari2, Borsa3 et al. 2006Phys. Rev. BSelf Cite

“…[12][13][14][15] Performing NMR measurements on the 165 Ho nucleus would be extremely difficult due to the very large nuclear quadrupolar moment (Q ≈ 2 barn) which is likely to complicate the nuclear energy level scheme in a non cubic crystal environment. Thus we chose to use the 19 …”

Quantum Nanomagnets and Nuclear Spins: An Overview

“…the influence of quantum tunneling of the nanomagnets on the nuclear spin dynamics, was observed short thereafter for 1 H [43] and 57 Fe in Fe 8 [4], and 55 Mn in Mn 12 [25,27] [Fig. 3(c)].…”

“…9), and T -independent relaxation at low-T [48]. (c) Crossover to T -independent nuclear relaxation rate T −1 1 driven by quantum tunneling in Fe 8 [43,4] and Mn 12 -ac [27], Eq. [25,27].…”

“…The picture has been completed by experiments looking at the "other side of the coin", i.e. studying directly the dynamics of the nuclear spins by NMR experiments [17,11,14,25,27,4], and finding that such dynamics is indeed profoundly entangled with the quantum fluctuations of the giant spin. At the present stage, much of the efforts in the field are directed towards using quantum nanomagnets as spin qubits for quantum information purposes, by pushing the giant spins into a regime where the tunneling can be made coherent [29].…”