We studied Mn 12 -acetate by inelastic neutron scattering and diffraction. We separated the energy levels corresponding to the splitting of the lowest S multiplet (S 10 ground state). The irregular spacing of the transition energies unambiguously shows the presence of high-order terms in the spin Hamiltonian [D 20.457͑2͒ cm 21 , B 0 4 22.33͑4͒ 3 10 25 cm 21 ]. The relative intensity of the lowest energy peaks is very sensitive to the small transverse term that is responsible for quantum tunneling, providing the first determination of this term in zero magnetic field ͓B 4 4 63.0͑5͒ 3 10 25 cm 21 ͔. PACS numbers: 75.25. + z, 75.45. + j, 78.70.Nx 0031-9007͞99͞83(3)͞628(4)$15.00

TV and T/v (Neel) have been measured for a series of YBa2(Cuo.96Zno.o4)306+x samples. The T variations of the homogeneous susceptibility Xs of the Cu02 planes, given by the shift of the 89 Y NMR line, are found to be nearly unchanged with respect to pure samples for x > 0.5, which implies that the charge transfer is negligibly modified by Zn, and that the magnetic pseudogap is not associated with superconducting pairing. Detection of an unusual Curie contribution to the 89 Y NMR width for x > 0.5 provides evidence that Zn induces magnetic moments in the CuC>2 planes, which play a role in the depression of 7V. PACS numbers: 74.70.Hk, 75.20.Hr, 75.30.Kz, 76.60.Cq The sizable reduction of T c induced by Zn substitutions in cuprates has stimulated a large number of experimental studies [1] to try to understand the changes of the electronic properties induced by Zn. A major problem is to clarify whether the hole content of the conducting CUO2 planes is modified in the presence of zinc. We insist here on the use of microscopic magnetic probes [2,3], such as the 89 Y NMR, which allows a separation of the modifications of the 1:2:3 phase from those associated with the existence of minute amounts of impurity phases. In the course of a detailed study on the phase diagram of pure YBa2Cu3C>6+jc we have evidenced that Zn does not induce an appreciable reduction of Tj\ in YBa2Cu306 [2]. The present work is an attempt to compare the modifications of the magnetic properties over the whole range of oxygen content. We shall see here that both T c and T/v are very sensitive to oxygen doping. This will allow us, among other things, to resolve an apparently conflicting experimental report from Mossbauer data [4] indicating that JT/V is largely reduced by Zn substitution, in YBa2Cu 3 06. But the main concern of this Letter is to provide indications of the changes of the magnetic properties in the metallic state. We show here that the phase diagram versus oxygen content and the determination of the uniform susceptibility % s of the CuC>2 planes from 89 Y NMR data give direct evidence that the charge-transfer processes are not markedly modified by Zn in the metallic state. We rather clearly demonstrate for the first time on microscopic grounds that Zn induces local moments which are certainly responsible for a pair-breaking mechanism or an electronic localization. The fact that the moments are localized in the Q1O2 planes is demonstrated here by comparison with data taken on Co-substituted samples.

Mesoscopic ͑500-2000 Å͒ and microscopic ͑5-20 Å͒ phase segregation with temperature and magnetic field was studied in the model manganite Pr 0.7 Ca 0.3 MnO 3 by high-resolution neutron diffraction and inelastic neutron scattering. Intragranular strain-driven mesoscopic segregation between two insulating phases, one of which is charge ordered ͑CO͒, sets in below the CO temperature in zero field. The CO phase orders antiferromagnetically, while the other insulating phase shows spin-glass behavior. After field-induced metallization, the CO phase coexists with a ferromagnetic metallic phase.

BackgroundParietal fibrinous peritonitis (PFP) and generalised peritonitis (GP) are two postoperative complications in cows, characterised by fluid and fibrin accumulation throughout the peritoneum (GP) or in an encapsulated cavity (PFP). Unlike GP, PFP is scarcely documented.MethodsTwenty-one GP cases and 12 PFP cases were confirmed by ultrasound in cows referred to the Veterinary Clinic (Liège University) for complications after caesarean section. All cows underwent a standardised examination protocol. Blood samples were analysed for metabolic and inflammatory markers. Bacteriology was performed on peritoneal fluid samples. Treatment consisted of surgical drainage of the abdominal cavity (GP) or the encapsulated cavity (PFP). Variables concerning anamnesis, clinical findings and treatment outcomes were compared.ResultsPerioperative complications had occurred in 9/21 GP cows but 0/12 PFP cows (P<0.05). Biochemical analysis indicated pronounced inflammation and did not differ between groups. Peritoneal fluid samples of both groups were contaminated and contained similar bacteria (Trueperella pyogenes and Escherichia coli). While 11/12 PFP cows were discharged, all patients with GP died or were euthanased (P<0.05).ConclusionsWe hypothesise that PFP and GP are two different manifestations of perioperative peritoneal contamination. The severity and spread of the contamination determine the clinical presentation and the prognosis.

Quasi-elastic neutron scattering (QENS) was employed to study the molecular dynamics of three structurally related sterols, namely, cholesterol, lanosterol, and ergosterol. Oriented bilayers of dipalmitoylphosphatidylcholine (DPPC) were investigated at 40 mol % sterol content and at three temperatures (20, 36, and 50 degrees C) for two energy resolutions. Data analysis was concentrated on a direct comparison of the out-of-plane and the in-plane high-frequency motions of the three sterols in terms of their rates and amplitudes. The (spatially restricted) diffusive motion of the three sterols in the two directions was characterized by diffusion constants in the range of (5-30) x 10(-12) x m(2) x s(-1), with a significantly faster rate of diffusion along the membrane normal, resulting in a diffusional anisotropy, D(a). At low temperature (20 degrees C), cholesterol showed the highest value (D(a) = 4.5), while lanosterol gave the lowest one (D(a) = 2.0). At high temperature (50 degrees C), ergosterol diffusion had the highest diffusion anisotropy (D(a) = 2.0) compared to lanosterol (D(a) = 1.8) and cholesterol (D(a) = 1.6). Most interestingly, cholesterol showed at all three temperatures an amplitude of its out-of-plane-motion of 1.0-1.1 nm, more than a factor of 3 higher than measured for the other two sterols. This finding suggests that the short alkyl chain of the cholesterol molecule may cross at high frequency the bilayer midplane, while the other two sterols remain confined within the geometrical limits of each monolayer leaflet. The results provide an example of how slight structural alterations of sterols can affect their molecular dynamics in bilayers, which in turn may be relevant to the membrane micromechanical properties.

Quasielastic neutron scattering (QENS) at two energy resolutions (1 and 14 microeV) was employed to study high-frequency cholesterol motion in the liquid ordered phase (lo-phase) of oriented multilayers of dipalmitoylphosphatidylcholine at three temperatures: T = 20 degrees C, T = 36 degrees C, and T = 50 degrees C. We studied two orientations of the bilayer stack with respect to the incident neutron beam. This and the two energy resolutions for each orientation allowed us to determine the cholesterol dynamics parallel to the normal of the membrane stack and in the plane of the membrane separately at two different time scales in the GHz range. We find a surprisingly high, model-independent motional anisotropy of cholesterol within the bilayer. The data analysis using explicit models of molecular motion suggests a superposition of two motions of cholesterol: an out-of-plane diffusion of the molecule parallel to the bilayer normal combined with a locally confined motion within the bilayer plane. The rather high amplitude of the out-of-plane diffusion observed at higher temperatures (T >/= 36 degrees C) strongly suggests that cholesterol can move between the opposite leaflets of the bilayer while it remains predominantly confined within its host monolayer at lower temperatures (T = 20 degrees C). The locally confined in-plane cholesterol motion is dominated by discrete, large-angle rotational jumps of the steroid body rather than a quasicontinous rotational diffusion by small angle jumps. We observe a significant increase of the rotational jump rate between T = 20 degrees C and T = 36 degrees C, whereas a further temperature increase to T = 50 degrees C leaves this rate essentially unchanged.

A multitechnique approach has allowed the first experimental determination of single-ion anisotropies in a large iron(III)-oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6ClO4 (1) in which Hpmdbm = 1,3-bis(4-methoxyphenyl)-1,3-propanedione. High-frequency EPR (HF-EPR). bulk susceptibility measurements, and high-field cantilever torque magnetometry (HF-CTM) have been applied to iron-doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12-(pmdbm)6]ClO4, whose synthesis and X-ray structure are also presented. HF-EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard-axis type anisotropy with DFe = 0.43(1) cm(-1) and EFe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = DFe[S2(z) - S(S + 1)/3] + Fe[S2(x) - S2(y)]. HF-CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster, Z. The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Z are negative, DFe(ZZ)= -0.164(4) cm(-1). Due to the dominant antiferromagnetic coupling, a negative DFe(ZZ) value determines a hard-axis molecular anisotropy in 1, as experimentally observed. By adding point-dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D1 = 4.70(9) cm(-1), is in excellent agreement with that determined by inelastic neutron scattering experiments at 2 K, D1 = 4.57(2) cm(-1). Iron-doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm = dibenzoylmethane), have also been investigated by HF-EPR at 525 GHz. The single-ion anisotropy is of the hard-axis type as well, but the DFe parameter is significantly larger [DFe = 0.770(3) cm(-1). EFe = 0.090(3) cm(-1)]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single-ion terms can contribute significantly to the magnetic anisotropy of iron(III)-oxo clusters, which are currently investigated as single-molecule magnets.

A systematic inelastic neutron scattering study of the superexchange interaction in three different undoped monolayer cuprates (La 2 CuO 4 , Nd 2 CuO 4 and Pr 2 CuO 4 ) has been performed using conventional triple axis technique.We deduce the in-plane antiferromagnetic (AF) superexchange coupling J which actually presents no simple relation versus crystallographic parameters.The absolute spectral weight of the spin susceptibility has been obtained and it is found to be smaller than expected even when quantum corrections of the AF ground state are taken into account.PACS numbers: 75.40. Gb, 75.30.Et, 25.40.Fq, 74.72.Dn Typeset using REVT E X 1The copper spins properties of the insulating cuprates are of particular interest as they give insights into the microscopic description of the high-T C superconductors. Undoped parent compounds of many high-T C cuprates are usually described as Mott-Hubbard insulators.They exhibit an antiferromagnetic ordering below a Néel temperature ranging between 250 K and 420 K. This Néel state is well accounted for by a spin- and Z c ≃ 1.18 represents quantum corrections of the AF ground state). Unfortunately, due to the large steepness of the in-plane spin wave dispersion (related to the large value of J ≈ 100-150 meV), the spin-wave velocity is not easily deduced from INS experiments. Therefore, a precise knowledge of J is still needed in parent compounds of cuprates. Another essential magnetic parameter is the spectral weight of copper spin susceptibility which has been, so far, only reported in La 2 CuO 4 [2,3]. The importance of these two parameters has been recently emphasized in doped materials as J is found to be renormalized compared to the undoped case and the spectral weight is shifted to lower energy [3,4].Here, we present, by systematic neutron scattering measurements, the spin wave excitations of three different parent compounds of single-CuO 2 layer cuprates. Especially, using an adapted focalised neutron scattering geometry, we are able to determine their spin-velocity with accuracy and to deduce J. Furthermore, we have determined the spectral weight of the spin susceptibility in absolute units and the perpendicular spin susceptibility, χ ⊥ . χ ⊥ can be also obtained as a consequence of sum-rules by applying the hydrodynamics relation, A special scattering geometry [6] was used in order to align the resolution spectrometer ellipsoid along the AF line, i.e. the (001) direction. Namely, this focalisation allows us to separate counterpropagating spin-waves at relatively low energies as compared with standard geometries [7,8]. We extend this technique down to 15 meV. For such a geometry, only one q c value is accessible for a fixed energy transfer and a fixed final neutron energy. To be powerful, this geometry also requires very good sample mosaicities.We now present q-scans (constant energy transfer scan) along the (110) direction in the three different monolayer cuprates: La 2 CuO 4 , Nd 2 CuO 4 and Pr 2 CuO 4 . Figure 1 depicts qscans measured at an energy tr...