Frustration driven structural distortion in VOMoO4

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements and electronic structure calculations in VOMoO4 are presented. It is found that VOMoO4 is a frustrated two-dimensional antiferromagnet on a square lattice with competing exchange interactions along the side J1 and the diagonal J2 of the square. From magnetization measurements J1+J2 is estimated around 155 K, in satisfactory agreement with the values derived from electronic structure calculations. Around 100 K a structural distortion, possibly driven by the frustration, is evidenced. This distortion induces significant modifications in the NMR and EPR spectra which can be accounted for by valence fluctuations. The analysis of the spectra suggests that the size of the domains where the lattice is distorted progressively grows as the temperature approaches the transition to the magnetic ground state at Tc=42 K

Strong electronic correlations in Lix_xZnPc organic metals

Nuclear magnetic resonance, electron paramagnetic resonance and magnetization measurements show that bulk Li$_x$ZnPc are strongly correlated one-dimensional metals. The temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ and of the static uniform susceptibility $\chi_S$ on approaching room temperature are characteristic of a Fermi liquid. Moreover, while for $x\simeq 2$ the electrons are delocalized down to low temperature, for $x\to 4$ a tendency towards localization is noticed upon cooling, yielding an increase both in $1/T_1$ and $\chi_s$. The $x$-dependence of the effective density of states at the Fermi level $D(E_F)$ displays a sharp enhancement for $x\simeq 2$, at the half filling of the ZnPc lowest unoccupied molecular orbitals. This suggests that Li$_x$ZnPc is on the edge of a metal-insulator transition where enhanced superconducting fluctuations could develop.Comment: 5 pages, 4 figure

Magnetic properties and spin dynamics in single molecule paramagnets Cu6Fe and Cu6Co

The magnetic properties and the spin dynamics of two molecular magnets have been investigated by magnetization and d.c. susceptibility measurements, Electron Paramagnetic Resonance (EPR) and proton Nuclear Magnetic Resonance (NMR) over a wide range of temperature (1.6-300K) at applied magnetic fields, H=0.5 and 1.5 Tesla. The two molecular magnets consist of CuII(saldmen)(H2O)}6{FeIII(CN)6}](ClO4)38H2O in short Cu6Fe and the analog compound with cobalt, Cu6Co. It is found that in Cu6Fe whose magnetic core is constituted by six Cu2+ ions and one Fe3+ ion all with s=1/2, a weak ferromagnetic interaction between Cu2+ moments through the central Fe3+ ion with J = 0.14 K is present, while in Cu6Co the Co3+ ion is diamagnetic and the weak interaction is antiferromagnetic with J = -1.12 K. The NMR spectra show the presence of non equivalent groups of protons with a measurable contact hyperfine interaction consistent with a small admixture of s-wave function with the d-function of the magnetic ion. The NMR relaxation results are explained in terms of a single ion (Cu2+, Fe3+, Co3+) uncorrelated spin dynamics with an almost temperature independent correlation time due to the weak magnetic exchange interaction. We conclude that the two molecular magnets studied here behave as single molecule paramagnets with a very weak intramolecular interaction, almost of the order of the dipolar intermolecular interaction. Thus they represent a new class of molecular magnets which differ from the single molecule magnets investigated up to now, where the intramolecular interaction is much larger than the intermolecular one

Mesoscopic phase separation in Nax_xCoO2_2 (0.65≤x≤0.750.65\leq x\leq 0.75)

NMR, EPR and magnetization measurements in Na$_x$CoO$_2$ for $0.65\leq x\leq 0.75$ are presented. While the EPR signal arises from Co$^{4+}$ magnetic moments ordering at $T_c\simeq 26$ K, $^{59}$Co NMR signal originates from cobalt nuclei in metallic regions with no long range magnetic order and characterized by a generalized susceptibility typical of strongly correlated metallic systems. This phase separation in metallic and magnetic insulating regions is argued to occur below $T^*(x)$ ($220 - 270$ K). Above $T^*$ an anomalous decrease in the intensity of the EPR signal is observed and associated with the delocalization of the electrons which for $T<T^*$ were localized on Co$^{4+}$ $d_{z^2}$ orbitals. It is pointed out that the in-plane antiferromagnetic coupling $J\ll T^*$ cannot be the driving force for the phase separation.Comment: 14 figure

Effects of reaction atmosphere on physico-chemical properties of V-doped FeNb11O29

FeNb11O29, pure or doped with metal transition ions, is a very promising material with advanced multifunctionalities and interesting applicative perspectives. It is isostructural with Nb12O29, one of the rare compounds in which Nb displays a local magnetic moment and shows both antiferromagnetic ordering and metallic conductivity at low temperatures. In this work we have synthesized and studied Fe0.8V0.2Nb11O29 monoclinic powders. In particular we monitored the effects on structural, electronic and magnetic properties in samples produced in different atmospheres to stabilize cations with different oxidation states. We have demonstrated that the reaction atmosphere influences the phase homogeneity, the crystallite size and the amount of paramagnetic centres, with a transformation of Fe3+ in Fe2+ when an inert atmosphere is used, as proved by the absence, in this case, of any electron paramagnetic resonance signal. Also the Raman spectra result to be affected due to the change of coordination polyhedra. Subsequent re-oxidation of reduced powders brings to the monophasic iron niobate again containing Fe3+ demonstrating the reversibility of redox process. This reversibility is accompanied by a complete restoring of monoclinic structure evidenced by X-ray diffraction data and by Raman measurements, which allowed also to follow in situ the spectral changes induced by laser heating

Third-molar extraction with ultrasound bone surgery: a case-control study.

The aim of this case-control study was to evaluate the postoperative period and healing between 2 surgical methods (traditional and ultrasound bone surgery) that are used for mandibular third-molar extraction.Fifteen patients with impaction of both of the lower third molars and indications for their extractions were used in this study. Bilateral-mandibular third-molar extractions were performed at the same surgical time: traditional surgery with burrs was used on 1 side (control site), and ultrasound surgery was used on the other side (test [T] site). After surgery, the patients were examined at 7 and 14 days and at 1 and 3 months to evaluate tissue healing. The following was assessed at every follow-up: pain, trismus, swelling, and alveolar bone level.The study included 15 patients, and 30 mandibular third-molar extractions were performed. We found only 1 postoperative complication: 1 patient had alveolitis in the control site. Complete recoveries without any complications were reported in all of the patients at the T sites.Complete recoveries without any complication were reported in all patients at the T sites. The only disadvantage of the piezoelectric technique was the length of operation time, which was increased by approximately 8 minutes; however, this effect was offset by reducing the morbidity.Our preliminary study showed that Piezosurgery is an excellent tool for reducing the risk of complications and improving the postoperative period