X-ray and neutron diffraction studies of coupled structural phase transitions in DyBaCo2_{2}O5.5_{5.5}

A structural transition at $T\approx 322$ K from the $Pmmm$ to $Pmma$ phase is found to coincide with an anomaly of resistivity. Another structural phase transition doubling the lattice parameter $c$, which has been postulated earlier to accompany a low-temperature magnetic transition in TbBaCo$_{2}$O$_{5.5}$, is observed in a single crystal DbBaCo$_{2}$O$_{5.5}$ by means of the X-ray and neutron diffraction. The low temperature phase does not belong to the space group $Pcca$ that has been chosen earlier as the highest subgroup of the $Pmma$. The transition is of the first order with the temperature hysteresis, between $T\approx 100$ and $T\approx 200$ K, which probably explains anomalous magnetic properties in this temperature range.Comment: 6 pages, 4 figure

Elucidating Individual Magnetic Contributions in Bi-Magnetic Fe3O4/Mn3O4 Core/Shell Nanoparticles by Polarized Powder Neutron Diffraction

Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of Fe3O4/Mn3O4 core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the Fe3O4 and Mn3O4 magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the Mn3O4 shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the Fe3O4 cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials

ON THE POLARIZED NEUTRON RESEARCH OF THE CRITICAL DYNAMICS OF FERROMAGNETS ABOVE THE CURIE POINT

Il est discuté la question de l'étude de la dynamique critique de ferromagnetiques dans des expériences sur la diffusion de neutrons polarisés à petits angles. Sont présentés les résultats expérimentaux de l'étude de la dynamique de spin sur les échantillons de 57Fe, effectuée à l'aide de l'analyse traditionnelle du vecteur de la polarisation de neutrons diffusés dans la zone de température de τ = 10-4 ÷ 10-1 et impulsions de transfert kθ = 10-2 ÷ 10-1 Å-1. Les résultats de l'expérience sont comparés avec les conclusions théoriques basées sur le comportement asymptotique de la fonction de spin de Green du ferromagnétique aux énergies élevées.Small angle polarized neutron scattering experiments are discussed as applied to the investigation of the critical dynamics of ferromagnets. The results of an experimental study of spin dynamics in 57Fe samples by a three-dimansional analysis of the scattered neutron polarization vector over the temperature range τ = 10-4 ÷ 10-1 and momentum transfer interval k θ = 10-2 ÷ 10-1 Å-1 are presented. The experimental data are conpared with theoretical predictions obtained from the asymptotic behavior of the spin Green function of a ferromagnet at high energies

THE ASYMMETRY OF POLARIZED NEUTRON CRITICAL SCATTERING FROM Fe ABOVE Tc IN MAGNETIC FIELD

Il a été révélé expérimentalement l'asymétrie de la diffusion critique de neutrons polarisés dans les ferromagnétiques au-dessus de Tc en présence du champ magnétique. La nature de cette asymétrie est comprise et il est montré que l'effet étudié est lié avec les corrélations dynamiques triples de spins. Les résultats obtenus pour le fer qualitativement sont en accord avec les prédictions théoriques de la dynamique critique à trois spins.An asymmetry in the critical scattering of polarized neutrons from ferromagnets above Tc in a magnetic field has been detected experimentally. The nature of this asymmetry was showed to originate from three-spin dynamic correlations. The results obtained for iron agree qualitatively with the theoretical predictions of the three-spin critical dynamics

EXPERIMENTAL OBSERVATION OF LEFT-RIGHT ASYMMETRY OF POLARIZED NEUTRON SCATTERING FROM Fe ABOVE Tc

Il est révélé que l'intensité de la diffusion critique des neutrons dans le fer au-dessus de Tc dépend de l'orientation relative de la polarisation initiale de neutrons et de la normale au plan de diffusion. La valeur obtenue de l'asymétrie est de PA = (1,5 ± 0,2). 10-4 à T - Tc = 3,5°. Les résultats sont en accord avec la théorie qui prédit le phénomène pareille, du aux corrélations dynamiques triples de spins.The intensity of the neutron critical scattering from iron above Tc was found to depend on the relative orientation of the initial neutron polarization and the normal to the scattering plane. The magnitude of the asymmetry obtained is PA = (1.5+0.2) x 10-4 at T-Tc=3.5°. The results are in agreement with the theory predicting this phenomenon which is due to three-spin dynmic correlations