Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Intra-molecular origin of the spin-phonon coupling in slow-relaxing molecular magnets

Abstract: We perform a systematic investigation of the spin-phonon coupling leading to spin relaxation in the prototypical mononuclear single molecule magnet [(tpaPh)Fe]−. In particular we analyze in detail the nature of the most relevant vibrational modes giving rise to the relaxation. Our fully ab initio calculations, where the phonon modes are evaluated at the level of density functional theory and the spin-phonon coupling by mapping post-Hartree–Fock electronic structures onto an effective spin Hamiltonian, reveal that acoustic phonons are not active in the spin-phonon relaxation process of dilute SMMs crystals. Furthermore, we find that intra-molecular vibrational modes produce anisotropy tensor modulations orders of magnitude higher than those associated to rotations. In light of these results we are able to suggest new designing rules for spin-long-living SMMs which go beyond the tailoring of static molecular features but fully take into account dynamical features of the vibrational thermal bath evidencing those internal molecular distortions more relevant to the spin dynamics.

Optical modification of a single impurity molecule in a solid

Abstract: THE possibility of obtaining information about solids on a truly microscopic scale has stimulated several recent advances in the optical detection and spectroscopy of single impurity centres in solids. For the system composed of pentacene impurity molecules in the crystal p-terphenyl, absorption1and fluorescence excitation2 studies at liquid-helium temperatures have led to direct observations of the lifetime-limited homogeneous linewidth of a single pentacene molecule3, as well as the surprising observation of spontaneous spectral diffusion in a crystal4. Spectral diffusion—changes in the resonance frequency of an impurity molecule with time as a result of structural relaxation processes in the molecular environment—is generally expected in amorphous hosts. We report here the observation of optical spectra of single perylene impurity molecules in a polymeric (polyethylene) host. At 1.5 K, individual perylene molecules show the expected spectral diffusion; further-more, we observe light-induced changes in resonance frequency (that is, persistent spectral hole-burning5) for certain single molecules. These observations suggest the possibility of optical storage at the single-molecule level.

Light ion irradiation of Co/Pt systems: Structural origin of the decrease in magnetic anisotropy

Abstract: We study the structural properties of Pt/Co/Pt systems submitted to ${\mathrm{He}}^{+}$ ion irradiation, in order to understand why the magnetic anisotropy can be decreased in a controlled way. It is shown by grazing x-ray reflectometry that the irradiation-induced Pt and Co atom displacements can be largely accounted for by a simple ballistic recoil mechanism model. Our results indicate that even in these nm-thick films, irradiation may affect the upper and lower interfaces differently. Specifically, the upper Co interface undergoes short-range mixing, resulting in roughness, whereas the lower Co interface mostly evolves by longer-range mixing, leading to alloy formation. Irradiation also releases strain in these Co-Pt systems, but has no chemical ordering effect. Together with slow asymmetric interface roughening, the cobalt tensile strain relaxation at low fluences accounts for the magnetic anisotropy decrease. The type of analysis we propose could be useful to understand why other magnetic properties, such as interlayer exchange coupling, can be controlled by light ion irradiation.

Proton Spin Relaxation in Aqueous Solutions of Paramagnetic Ions. II. Cr+++, Mn++, Ni++, Cu++, and Gd+++

Abstract: Nuclear magnetic relaxation times for protons in dilute aqueous solutions of chromic, manganous, nickel, cupric, and gadolinium ions were measured in the frequency range 1.9 to 60 Mc/sec. Results were interpreted in terms of Solomon's formulation of electron‐nuclear dipole‐dipole interaction and Bloembergen's expression for scalar coupling of electron and nuclear spins. In large magnetic fields relaxation times were found to be shorter than those expected on the basis of low field values, suggesting that the effective ion magnetic moments, electron spin relaxation times, and/or electron‐nuclear spin exchange constants are field‐dependent.

Electron‐spin relaxation and ordering in smectic and supercooled nematic liquid crystals

Abstract: We report on careful line shape studies of slow motional and orientation dependent ESR spectra of a deuterated liquid‐crystal‐like spin probe dissolved in a benzilidene‐derivative (40,6) and in cyanobiphenyl derivative (S2 and 5CB) liquid crystals. The simulation of the ESR spectra is based on the Lanczos algorithm recently applied by Moro and Freed in a general and efficient formulation of slow motional and ordering effects on ESR line shapes. With 40,6 which exhibits monolayer smectic phases, we find that the main change in the spin relaxation upon passing from the nematic to the smectic A phase consists of changes occuring in ordering attributable to packing forces on functional groups. Such ordering effects appear to be further enhanced in the SB phase with consequent alterations in dynamics. With S2, which exhibits an interpenetrating bilayer smectic A phase, we find unusual ESR spectra in that phase which may be simulated on the basis of a model of cooperative distortions static on the ESR time scale, and superimposed on individual molecular reorientation. This mode is interpreted as a collective chain distortion which affects the orientational distribution of the piperidine ring of the spin probe. A similar phenomenon is observed in the supercooled nematic phase of 5CB, which is aligned by an electric field, and evidence is also found that the reorientational dynamics of this ring are affected by interaction with local cooperative modes in the liquid crystal (i.e., a SRLS mechanism previously proposed by Freed and co‐workers). Some microscopic characteristics of liquid crystals revealed by this and previous ESR spin probe studies are discussed.