Proton NMR wipeout effect due to slow fluctuations of the magnetization in single molecule magnets

Abstract

We report a detailed 1 H NMR study on the spin dynamics of single molecule magnets as a function of temperature and external magnetic field. A gradual loss of the 1 H NMR signal intensity ͑wipeout effect͒ is observed on decreasing the temperature for all the investigated ferromagnetic clusters. This effect is accompanied by a simultaneous enhancement of the spin-spin and spin-lattice relaxation rate T 2 −1 and T 1 −1 , respectively. The complications entered in the interpretation of the signal loss by the wipeout effect are overcome, and the information about the spin dynamics is retrieved, by implementing a simple and intuitive model that captures the main physical characteristics of the problem and reveals a universal behavior of the spin dynamics for all the clusters. According to our analysis the origin of the wipeout effect as well as the enhancement of the relaxation rates T 1 −1 and T 2 −1 in the FM clusters is related to a decrease of the lifetime broadening parameter of the magnetic energy levels, down to the range of the 1 H Larmor frequency. The temperature dependence of the lifetime broadening can be described at intermediate temperatures by a power law dependence on T similar to that observed in antifferomagnetic rings ͓S. H. Baek et al., Phys. Rev. B 70, 134434 ͑2004͔͒.

Relaxation dynamics in a Fe7nanomagnet

“…M H xy (0)T decreases by lowering the temperature due to the so-called wipeout effect, experimentally observed in other MNMs and qualitatively explained in Ref. 28.…”

“…As mentioned above, 1/T 1 data on Fe 7 are also affected by the wipeout effect, 28 i.e., a gradual loss of the 1 H NMR signal intensity is observed upon decreasing the temperature. This signal loss is associated with the enhancement of the transverse relaxation rates of the probed protons over the limit fixed by the experimental setup; they become so fast that the transverse nuclear magnetization decays irreversibly before it can be observed in a pulsed NMR experiment.…”

“…In this work we have measured the temperature dependence of 1/T 1 for two different values of the applied magnetic field. NMR measurements on MNMs are often affected by the so-called wipeout effect: 28 At low temperatures it is possible to have an NMR signal loss, due to the enhancement of the protons transverse relaxation rate 1/T 2 over the limit of the experimental signal detection capability. As a consequence, not all the protons contribute to the molecular 1/T 1 .…”

Relaxation dynamics in the frustratedCr9antiferromagnetic ring probed by NMR

“…1 H NMR measurements are often affected by the so-called wipeout effect. It consists of a signal loss in the NMR signal intensity, due to the enhancement of the transverse relaxation rates 1/T 2 of the protons over the limit fixed by the experimental setup (around 67−100 (ms) −1 in our experimental conditions, corresponding to T 2 ∼ 10 − 15 µs ) 30 . Thus, to calculate the spin-lattice relaxation rate of Cr 9 we take into account also the wipeout effect.…”

“…Therefore, we have investigated Cr 9 by 1 H-NMR, by measuring the temperature-dependence of the proton spin-lattice relaxation rate 1/T 1 (T ) for different values of the applied magnetic field 26,29 . We have developed a theoretical model for Cr 9 including magnetoelastic couplings which allows us to reproduce 1/T 1 data, taking also into account the subtle wipeout effect 30 . The interpretation of these measurements has allowed us to estimate the coupling between electron spins and phonons, and to study the decay of the molecular magnetization.…”