The magnetic and structural properties of polycrystalline Co 4−x Ni x Nb 2 O 9 (x = 1, 2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x = 1, the compound crystallizes in the trigonal P3c1 space group. Below T N = 31 K it develops a weakly noncollinear antiferromagnetic structure with magnetic moments in the ab plane. The compound with x = 2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T C = 47 K. For this compound a weakly noncollinear ferrimagnetic structure with two possible configurations in the ab plane was derived from neutron diffraction study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provides further information on the magnetic structure of the compounds.
Structural, magnetic, and magnetocaloric properties of the phase separated La0.5Ca0.5–xSrxMnO3 (x = 0, 0.2, 0.3, 0.4, and 0.5) manganites have been studied. The results show that the phase coexistence state can be investigated by magnetocaloric studies (especially the field dependence of magnetic entropy change at constant temperature). Magnetic entropy change (△SM) shows positive (negative) peak at the vicinity of TN (TC). However, in the intermediate temperatures between TN and TC, both the sign and magnitude of △SM are strongly dependent on temperature and magnetic field, manifesting the competition of ferromagnetic (FM) (negative △SM) and non-FM (positive △SM) phases. This behavior is more pronounced in the parent compound, La0.5Ca0.5MnO3, in which the intermediate phase separation ranges between 160 and 225 K. The substitution of Ca by Sr enhances the ferromagnetic state, weakens the phase separation, and thus narrows the temperature range in which the field related effects (such as the sign change of △SM) are observed.
To understand the dielectric breakdown in a polycrystalline ferroelectric, a thorough phase field simulation has been performed by introducing a new degradation function. Time and position evolution of breakdown path, electric field dependence of breakdown time, and the effect of several parameters such as grain orientation, dielectric constant of grain and grain‐boundaries (GBs), thickness of GBs (dGB${d}_{\text{GB}}$), and grain size (Gnormala${G}_{\text{a}}$) on the threshold breakdown electric field, EBT${E}_{\text{BT}}$, are investigated. The results indicate that EBT${E}_{\text{BT}}$ is improved with decreasing dielectric constant. The dependence of EBT${E}_{\text{BT}}$ on grain size and GB's thickness obeys a power function Gnormala−n${G}_{\text{a}}^{-n}$ (n=0.42$n=0.42$) and dGBn${d}_{\text{GB}}^{n}$ (n=0.3$n=0.3$), respectively. The results help to engineer the grains and GBs properties and achieve a high breakdown electric field, which is very important in energy‐storage applications.
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