P. Carretta

Spin Dynamics in the Negatively Charged Terbium (III) Bis-phthalocyaninato Complex

The experimental and theoretical study of the electron spin dynamics in the anionic form of a single-ion molecule magnet (SIMM), the bis-phthalocyaninato terbium (III) molecule [Pc(2)Tb](-)[TBA](+), has been addressed by means of solid state (1)H NMR spectroscopy. The magnetic properties of the caged Tb(3+) metal center were investigated in a series of diamagnetically diluted preparations, where the excess of tetrabutylamonium bromide ([TBA]Br)(n) salt was used as diamagnetic matrix complement. We found that a high temperature activated spin dynamics characterizes the systems, which involved phonon-assisted transitions among the crystal field levels in qualitative agreements with literature results. However, the activation barriers in these processes range from 641 cm(-1) for the diamagnetically diluted samples to 584 cm(-1) for those undiluted; thus, they exhibit barriers 2-3 times larger than witnessed in earlier (230 cm(-1)) reports (e.g., Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. J. Am. Chem. Soc. 2003, 125, 8694-8695). At cryogenic temperatures, fluctuations are driven by tunneling processes between the m = +6 and -6 low-energy levels. We found that the barrier Delta and the tunneling rates change from sample to sample and especially the diamagnetically diluted [Pc(2)Tb](-) molecules appear affected by the samples magneto/thermal history. These observations emphasize that matrix arrangements around [Pc(2)Tb](-) can appreciably alter the splitting of the crystal field levels, its symmetry, and hence, the spin dynamics. Therefore, understanding how small differences in molecular surroundings (as for instance occurring by depositing on surfaces) can trigger substantial modifications in the SIMM property is of utmost importance for the effective operation of such molecules for single-molecule data storage, for example.

Li2VO(Si,Ge)O4, a prototype of a two-dimensional frustrated quantum heisenberg antiferromagnet

NMR and magnetization measurements in Li2VOSiO4 and Li2VOGeO4 are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings J1 along the sides of the square and J2 along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J2/J1>0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.

Charge Segregation, Cluster Spin-Glass and Superconductivity in La1.94Sr0.06CuO4

A {sup 63}Cu and {sup 139}La NMR/NQR study of superconducting (T{sub c}=8 K) La{sub 1.94} Sr{sub 0.06} CuO{sub 4} single crystal is reported. Coexistence of spin-glass and superconducting phases is found below {approximately}5 K from {sup 139}La NMR relaxation. {sup 63}Cu and {sup 139}La NMR spectra show that, upon cooling, CuO{sub 2} planes progressively separate into two magnetic phases, one of them having enhanced antiferromagnetic correlations. These results establish the antiferromagnetic-cluster nature of the spin glass. We discuss how this phase can be related to the microsegregation of mobile holes and to the possible pinning of charge stripes. {copyright} {ital 1999} {ital The American Physical Society }

Magnetic and thermodynamic properties ofLi2VOSiO4:A two-dimensionalS=1/2frustrated antiferromagnet on a square lattice

NMR, muSR, magnetization and specific heat measurements in Li(2)VOSiO(4) powders and single crystals are reported. Specific heat and magnetization measurements evidence that Li(2)VOSiO(4) is a frustrated two-dimensional S=1/2 Heisenberg antiferromagnet on a square lattice with a superexchange coupling J_1, along the sides of the square, almost equal to J_2, the one along the diagonal (

NMR as a Probe of the Relaxation of the Magnetization in Magnetic Molecules

J_2/J_1=1.1\pm 0.1

Glassy spin freezing and NMR wipeout effect in the high-TcsuperconductorLa1.90Sr0.10CuO4:Critical discussion of the role of stripes

with

Correlated trends of coexisting magnetism and superconductivity in optimally electron-doped oxypnictides.

J_2 + J_1 = 8.2\pm 1

Frustration-driven structural distortion in VOMoO 4

K). At

NMR-MRI, μSR and Mössbauer spectroscopies in molecular magnets

T_c\simeq 2.8

Single‐Molecule‐Magnet Behavior in the Family of [Ln(OETAP)2] Double‐Decker Complexes (Ln=Lanthanide, OETAP=Octa(ethyl)tetraazaporphyrin)

K a phase transition to a low temperature collinear order is observed. T_c and the sublattice magnetization, derived from NMR and muSR, were found practically independent on the magnetic field intensity up to 9 Tesla. The critical exponent of the sublattice magnetization was estimated