A high-energy inelastic neutron scattering investigation of the Gd-Co exchange interactions in GdCo4B: Comparison with density-functional calculations

International audienceInelastic neutron scattering is used to quantify the Gd-Co exchange interaction in GdCo4B. A significant reduction is observed in comparison with the GdCo5 compound. A mean value of 130 T is obtained for the exchange field on the two Gd sites in GdCo4B. The experimental results are compared with density-functional calculations. The local atomic magnetic moments calculated using the LSDA+U approximation are reported for each atomic site of the GdCo4B crystal structure. These calculations demonstrate that the two nonequivalent Gd crystal sites experience a significantly different exchange interaction, a difference that is discussed in the light of the local atomic environment. The observed reduction of the exchange field occurring upon substituting B for Co in GdCo5 is mainly caused by the decrease of the Co magnetic moment, whereas the Gd-Co coupling constant is found to be almost the same in both GdCo5 and GdCo4B

Interplay of structural distortions, dielectric effects and magnetic order in multiferroic GdMnO3

Multiferroic materials are characterized by simultaneous magnetic and ferroelectric ordering making them good candidates for magneto-electrical applications. We conducted thermal expansion and magnetostriction measurements in magnetic fields up to 14 T on perovskitic GdMnO3 by highresolution capacitive dilatometry in an effort to determine all longitudinal and transversal components of the magnetostriction tensor. Below the ordering temperature T (N) = 42 K, i.e., within the different complex (incommensurate or complex) antiferromagnetic phases, lattice distortions of up to 100 ppm have been found. Although no change of the lattice symmetry occurs, the measurements reveal strong magneto-structural phenomena, especially in the incommensurate sinusoidal antiferromagnetic phase. A strong anisotropy of the magnetoelastic properties was found, in good agreement with the type and propagation vector of the magnetic structure. We demonstrate that our capacitive dilatometry can detect lattice expansion effects and changes of the dielectric permittivity simultaneously because the sample is housed inside the capacitor. A separation of both effects is possible by shielding the sample. Dielectric transitions could be detected by this method and compared to the critical values of H and T in the magnetic phase diagram. Dielectric changes measured at 1 kHz excitation frequency are detected in GdMnO3 at about 180 K, and between 10 K and 25 K in the canted antiferromagnetic structure which is characterized by a complex magnetic order on both the Gd- and Mn-sites