Publications (163) View all
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Article: Proton NMR wipeout effect due to slow fluctuations of the magnetization in single molecule magnets
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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 effect is observed on decreasing the temperature for all the investigated ferromagnetic clusters. This effect is accom-panied by a simultaneous enhancement of the spin-spin and spin-lattice relaxation rate T 2 −1 and T 1 −1 , respec-tively. 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.Phys. Rev. B. ; 70. -
Article: NMR-D study of the local spin dynamics and magnetic anisotropy in different nearly monodispersed ferrite nanoparticles.
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ABSTRACT: We present a systematic experimental comparison of the superparamagnetic relaxation time constants obtained by means of dynamic magnetic measurements and (1)H-NMR relaxometry, on ferrite-based nanosystems with different composition, various core sizes and dispersed in different solvents. The application of a heuristic model for the relaxivity allowed a comparison between the reversal time of magnetization as seen by NMR and the results from the AC susceptibility experiments, and an estimation of fundamental microscopic properties. A good agreement between the NMR and AC results was found when fitting the AC data to a Vogel-Fulcher law. Key parameters obtained from the model have been exploited to evaluate the impact of the contribution from magnetic anisotropy to the relaxivity curves and estimate the minimum approach distance of the bulk solvent.Journal of Physics Condensed Matter 01/2013; 25(6):066008. · 2.55 Impact Factor -
Article: Local spin density in the Cr(7)Ni antiferromagnetic molecular ring and (53)Cr-NMR.
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ABSTRACT: We present (53)Cr-NMR spectra collected at low temperature in a single crystal of the heterometallic antiferromagnetic (AF) ring Cr(7)Ni in the S = 1/2 ground state with the aim of establishing the distribution of the local electronic moment in the ring. Due to the poor S/N we observed only one signal which is ascribed to three almost equivalent (53)Cr nuclei in the ring. The calculated spin density in Cr(7)Ni in the ground state, with the applied magnetic field both parallel and perpendicular to the plane of the ring, turns out to be AF staggered with the greatest component of the local spin 〈s〉 for the Cr(3+) ions next to the Ni(2+) ion. The (53)Cr-NMR frequency was found to be in good agreement with the local spin density calculated theoretically by assuming a core polarization field of H(cp) =- 11 T/μ(B) for both orientations, close to the value found previously in Cr(7)Cd. The observed orientation dependence of the local spin moments is well reproduced by the theoretical calculation and evidences the importance of single-ion and dipolar anisotropies.Journal of Physics Condensed Matter 09/2012; 24(40):406002. · 2.55 Impact Factor -
Article: Evolution of spin relaxation processes in LiY_ {1− x} Ho_ {x} F_ {4} studied via ac-susceptibility and muon spin relaxation
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ABSTRACT: We present measurements of magnetic field and frequency dependence of the low-temperature (T = 1.8 K) ac-susceptibility and temperature and field dependence of the longitudinal field positive muon spin relaxation (μSR) for LiY1−xHoxF4 with x = 0.0017, 0.0085, 0.0408, and 0.0855. The fits of numerical simulations to the susceptibility data for the x = 0.0017, 0.0085, and 0.0408 show that Ho-Ho cross-relaxation processes become more important at higher concentrations, signaling the crossover from single-ion to correlated behavior. We simulate the muon spin depolarization using the parameters extracted from the susceptibility, and the simulations agree well with our data for samples with x = 0.0017 and 0.0085. The μSR data for samples with x = 0.0408 and 0.0855 at low temperatures (T < 10 K) cannot be described within a single-ion picture of magnetic field fluctuations and give evidence for additional mechanisms of depolarization due to Ho3+ correlations. We also observe an unusual peak in the magnetic field dependence of the muon relaxation rate in the temperature interval 10–20 K that we ascribe to a modification of the Ho3+ fluctuation rate due to a field induced shift of the energy gap between the ground and the first excited doublet crystal field states relative to a peak in the phonon density of states centered near 63 cm−1.Phys. Rev. B. 07/2012; 86(1). -
Article: Evolution of spin relaxation processes in LiY$_{1-x}$Ho$_x$F$_4$ with increasing x studied via AC-susceptibility and muon spin relaxation
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ABSTRACT: We present measurements of magnetic field and frequency dependences of the low temperature (T = 1.8 K) AC-susceptibility, and temperature and field dependences of the longitudinal field positive muon spin relaxation ({\mu}SR) for LiY$_{1-x}$Ho$_x$F$_4$ with x = 0.0017, 0.0085, 0.0408, and 0.0855. The fits of numerical simulations to the susceptibility data for the x = 0.0017, 0.0085 and 0.0408 show that Ho-Ho cross-relaxation processes become more important at higher concentrations, signaling the crossover from single-ion to correlated behavior. We simulate the muon spin depolarization using the parameters extracted from the susceptibility, and the simulations agree well with our data for samples with x = 0.0017 and 0.0085. The {\mu}SR data for samples with x = 0.0408 and 0.0855 at low temperatures (T < 10 K) cannot be described within a single-ion picture of magnetic field fluctuations and give evidence for additional mechanisms of depolarization due to Ho$^{3+}$ correlations. We also observe an unusual peak in the magnetic field dependence of the muon relaxation rate in the temperature interval 10 - 20 K that we ascribe to a modification of the Ho$^{3+}$ fluctuation rate due to a field induced shift of the energy gap between the ground and the first excited doublet crystal field states relative to a peak in the phonon density of states centered near 63 cm$^{-1}$.07/2012;
