Search

We report on the magnetic properties of the neptunium based ferromagnetic compound NpAl 2 . We used magnetization measurements and 27 Al NMR spectroscopy to access magnetic features related to the paramagnetic and ordered states ( T C =56 K). While very precise DC SQUID magnetization measurements confirm ferromagnetic ordering, they show a relatively small hysteresis loop at 5 K reduced with a coercive field H Co ~3000 Oe. The variable offset cumulative spectra (VOCS) acquired in the paramagnetic state show a high sensitivity of the 27 Al nuclei spectral parameters (Knight shifts and line broadening) to the ferromagnetic ordering, even at room temperature.

The anisotropy of magnetic fluctuations propagating along the high-symmetry directions in cubic systems with 3- k magnetic order is analyzed within the random-phase approximation assuming anisotropic exchange interactions. Both transverse and longitudinal structures are considered, with reference to the UO 2 and USb compounds, respectively. In the case of UO 2 , the spin-waves polarizations calculated for acoustic and optical branches are favorably compared with three-dimensional polarization analysis experiments carried out on a triple axis spectrometer. The overall spin-waves polarization behavior emerges as a consequence of the 3- k nature of the magnetic order, whatever the strength of the exchange coupling assumed.

The linear and non-linear AC magnetic susceptibilities of the perovskite La1−xSrxCoO3, with 0.15 ⩽ x ⩽ 0.30 , are presented. It is observed that, for x 0.20 , a freezing temperature is defined by AC magnetic susceptibility, pointing to a glassy behaviour whose origin is the phase separation of ferromagnetic metallic-like clusters embedded in an insulating matrix. At this temperature, the non-linear magnetic susceptibility shows critical divergence, similar to spin-glasses, although the width of the diverging peak does not compare well with canonical spin-glasses.

The results of linear and non-linear magnetic AC susceptibility measurements are reported for polycrystalline LaMn 0.5 Ga 0.5 O 3 . The observation of an asymmetric peak in the linear AC response at 59 K, accompanied by the development of a sizeable second-harmonic component and of a double-peak third-harmonic signal, suggests the establishment of a long-range ordered magnetic ground state with unconventional dynamics. The results are compatible with a picture assuming a canted antiferromagnetic structure or the presence of ferromagnetic domains embedded in an antiferromagnetic matrix.

Uranium oxysulphide is an ionic compound whose magnetic and thermodynamic properties have been extensively studied in the past. UOS becomes antiferromagnetic below TN = 55 K with an ordered moment μord ≅ 2μB. A previous neutron crystal field (CF) spectroscopy experiment was performed to observe the splitting of the J = 4 ground manifold. Two possible CF level schemes were proposed in order to interpret the results, differing essentially for the presence of the intense Г(1)t5 → Гt4 transition at low energy (

In this paper we report on a resonant magnetic X-Ray scattering (RMXS) study of the low dimensional antiferro-magnetic system KCuF 3 . Azimuthal scans performed on the quadrupolar feature discovered in the spectrum of the magnetic peaks is analysed: the presence of a F E 2 ( 3 ) term poses serious doubts on the local symmetry description at the Cu site.

The results of low-temperature neutron diffraction measurements on a Li 2 VOSiO 4 powder sample are reported. Neutron diffraction confirms the presence of collinear magnetic order, with a propagation vector ( 1 2 1 2 0) . The refined ordered moment at 1.44 K is 0.62(3) μ B , in excellent agreement with the value expected for the square-lattice Heisenberg model (∼0.6 μ B ). No evidence for a structural distortion between 2 and 5 K has been found.

Small angle neutron scattering experiments performed on a powder of γ-Fe 2 O 3 nanoparticles reveal the existence of a temperature-dependent magnetic signal, increasing with decreasing temperature. The extra scattering appearing below 275 K is fitted to the sum of a Lorentzian and a Lorentzian squared. The first contribution is attributed to the spin correlations in the particle core, with a correlation length of 2.6 nm at T =4 K. The second contribution indicates the presence of frozen clusters of coupled particles, with a correlation length that increases with decreasing temperature, up to 60 nm.

The heat capacity of a molecular Cr 8 ring was measured as a function of temperature and magnetic field in order to follow the evolution of the Schottky anomaly through the first ( S =0 to |1,−1〉) level crossing occurring at B c1 =6.9 T. The results are interpreted within the framework of a spin Hamiltonian approach and they nicely fit the pattern of the energy levels derived by inelastic neutron spectroscopy. At B c1 , the Schottky anomaly almost vanishes suggesting that a true crossing occurs in Cr 8 in contrast to what was observed for ferric wheels.

We present the results of neutron spectroscopy for the tetranuclear iron(III) cluster of formula Fe4(OCH3)6(dpm)6, having an S=5 spin ground-state. At T=16.3 K two peaks appear at 7.3 and 8.3 meV . They have been attributed to the inter-multiplet transitions between the ground state and the first two excited S=4 multiplets. The experimental data, which have been interpreted in the framework of a microscopic model Hamiltonian, show that an exact C3 magnetic symmetry is not adequate to describe this system. The Q-dependence of the cross section is well reproduced by this model.

We have studied the AC and DC magnetic properties of the lamellar compounds La 1− x Sr 1+ x CoO 4 (0⩽ x ⩽0.4). These materials are found to evolve towards an anomalous “cluster-glass”-like behaviour as x increases. The onset of the ferromagnetic order in the bidimensional phases is seen to occur at around 150 K, followed by a blocking process around 125 K. Also, present in the magnetic behaviour of these materials is the signal coming from small intergrowths of a perovskite phase, also detected by TEM, although invisible for powder-X-ray diffraction.

Epitaxial La 0.7 Ca 0.3 MnO 3 (LCMO) thin films have been deposited by high pressure DC sputtering technique on SrTiO 3 substrates. As-grown films exhibit metal–insulator transition temperature ( T MI ) with values ranging from 131 to 220 K. The maximum value of the magnetoresistance is of about −77% in a magnetic field of 0.5 T for T MI =131 K. The same films, annealed in a pure oxygen atmosphere, exhibit transitions to a metallic state at temperatures ranging from 210 to 284 K. Transmission electron microscopy analyses show a coherent growth between film and substrate.

237 Np Mossbauer spectroscopy between 77 and 4.2 K was carried out in connection with neutron diffraction studies between 300 and 2 K on the tetragonal intermetallic NpCu 4 Al 8 . The Mossbauer spectra give an isomer shift of + 14.3 mm/s vs. NpAl 2 indicating the non-Kramers Np 3+ ( 5 I 4 )-ion to be present. Below 45 K the onset of magnetic hyperfine splitting is observed. It develops into a fully resolved Zeeman pattern at 4.2 K with B eff ≈330 T. Neutron diffraction finds no evidence for magnetic order over the whole temperature range scanned. All these results can consistently be explained by paramagnetic relaxation phenomena involving a low lying Γ t 5 doublet closely followed by a Γ t 4 singlet as the tetragonal crystalline field states of the Np 3+ ion.

Crystal field excitations in UOS have been measured by neutron spectroscopy up to 200 meV. Two main peaks have been observed at about 76 and 84 meV. The overall range of the spectrum agrees with the crystal field model used to analyze specific heat data. The change of peak intensities and positions upon ordering is also accounted for.