X. L. Zhong

Raman scattering and high temperature ferromagnetism of Mn-doped ZnO nanoparticles

Raman scattering has been used to study the influence of manganese, an effective dopant to obtain ZnO diluted magnetic semiconductors, on the lattice dynamics of ZnO. It is found that Mn doping increases the lattice defects and induces two Raman vibration modes of 275 and 526cm−1. On the other hand, high temperature (TC higher than 350K) ferromagnetism is observed in Zn1−xMnxO (x⩽0.02) nanoparticles. It is found that the ferromagnetism of Zn1−xMnxO nanoparticles is strongly related to defects in ZnO.

Multiferroic nanoparticulate Bi3.15Nd0.85Ti3O12-CoFe2O4 composite thin films prepared by a chemical solution deposition technique

Multiferroic xBi3.15Nd0.85Ti3O12–(1−x)CoFe2O4 composite thin films with x=0.5, 0.6, and 0.7 were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates by a chemical solution deposition technique. X-ray diffraction shows that there are no other phases but bismuth-layered perovskite Bi3.15Nd0.85Ti3O12 and spinel CoFe2O4 phases in the films. Scanning electron microscopy reveals that CoFe2O4 aggregates locally into nanoparticles and embeds in the Bi3.15Nd0.85Ti3O12 matrix. The composite films exhibit both good ferroelectric and magnetic properties at room temperature, as well as distinct magnetoelectric coupling behaviors, which are comparable with those of multiferroic composite films with conventional Pb-based ferroelectric as a ferroelectric component.

Cathodoluminescence and room temperature ferromagnetism of Mn-doped ZnO nanorod arrays grown by chemical vapor deposition

Mn-doped ZnO nanorods are prepared by a chemical vapor deposition method using zinc and MnO2 powders as source materials of Zn and Mn, respectively. Cathodoluminescence (CL) characterization indicates that the Mn-doped ZnO nanorods grown at different growth temperatures have different oxygen vacancy concentrations. Room temperature ferromagnetism with a saturation magnetization of 0.87μB∕Mn has been observed in Mn-doped ZnO nanorods grown at 650°C, in which moderate oxygen vacancy concentration is observed by CL characterization. It is also found that oxygen vacancies play an important role in the appearance of room temperature ferromagnetism in Mn-doped ZnO nanorods.

Structure evolution and ferroelectric and dielectric properties of Bi3.5Nd0.5Ti3O12 thin films under a moderate temperature annealing

Bi3.5Nd0.5Ti3O12 (BNT) ferroelectric thin films were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates by chemical solution deposition. Structure evolution and ferroelectric and dielectric properties of the as-prepared thin films under a moderate temperature (600–750°C) annealing were studied in detail. The experimental results showed that the BNT thin films annealed at 700°C exhibit preferred (00l) orientation, and the remnant polarization (2Pr) and dielectric constant (er) are higher (the values of 2Pr and er at 100kHz are 54μC∕cm2 and 448, respectively) than those of the deposited films annealed at other temperatures. Additionally, the mechanism concerning the dependence of electrical properties of the BNT ferroelectric thin films on the annealing temperature was discussed.

Improved ferroelectric properties of bismuth titanate films by Nd and Mn cosubstitution

Thin films of Nd and Mn cosubstituted bismuth titanate, i.e., Bi3.15Nd0.85(Ti3−xMnx)O12 (BNTM) (x=0, 0.005, 0.01, 0.03, 0.05, and 0.1), were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates at 700°C by a chemical solution deposition technique. The structures of the films were analyzed using x-ray diffraction and Raman spectroscopy. These films possessed preferred (117) and (00l)-oriented polycrystalline structures. The ferroelectric properties of BNTM films were systematically investigated as a function of the Mn content. It is found that a low concentration substitution with manganese ions in Bi3.15Nd0.85Ti3O12 greatly enhances the remnant polarization (2Pr) and reduces the coercive field (2Ec) of the film. The 2Pr and 2Ec are 78μC∕cm2 and 205kV∕cm, respectively. No fatigue phenomenon is also observed for the BNTM film with x=0.01 up to 1.5×1010 switching cycles.

Room temperature electrocaloric effect on PbZr0.8Ti0.2O3 thin film

A phase field model based on the time-dependent Ginburg–Landau equation (TDGL) is developed to investigate the electrocaloric effect (ECE) of PbZr0.8Ti0.2O3 (PZT) thin film with a multidomain structure. The rhombohedral domain morphology is obtained through numerically solving the TDGL equation with periodic boundary conditions at room temperature. Then the ECE of PZT thin film is investigated by the thermodynamics analysis. It is shown that a great ECE exists with an applied ac electric field at room temperature. The magnitude of ECE greatly depends on the external applied electric field and becomes a periodic time-dependent quantity. The theoretical calculations also reveal that both compressive and tensile misfit strains caused by the mismatch between the film and the substrate can largely affect the electrocaloric properties. It reveals that the compressive substrate strain enhances the ECE of PZT thin film while the tensile substrate strain suppresses it.

Magnetism mechanism in ZnO and ZnO doped with nonmagnetic elements X (X = Li, Mg, and Al): A first-principles study

First-principles calculations are performed to study the magnetism mechanism in undoped ZnO and ZnO doped with nonmagnetic elements X (X = Li, Mg, and Al). In undoped ZnO, it is found that the Zn vacancy is spin-polarized with a magnetic moment of 1.54 μB. The ferromagnetic coupling between two Zn vacancies is favorable in energy and a band coupling model is used to explain the magnetic coupling. In ZnO doped with nonmagnetic elements X, the magnetism is also attributed to the presence of Zn vacancies. The introduction of X reduces the formation energy of Zn vacancy and stabilizes it to some extent.

The coexistence of the negative and positive electrocaloric effect in ferroelectric thin films for solid-state refrigeration

The coexistence of the negative and positive electrocaloric effect (ECE) caused by an applied electric field direction different from the dipole direction of 180° domain structures is firstly found in ferroelectric thin films using the phase-field–based simulation. The calculation results reveal that a negative adiabatic temperature change (ΔT = −3.4 K) and a positive adiabatic temperature change (ΔT = 3.1 K) coexist in the PbTiO3 thin film with 180° domain structures under the dimensional electric field ΔE = 0.1. The coexistence of negative and positive ECE in a thin film can provide a new way to design solid-state refrigerators.

Spatial distribution of manganese and room temperature ferromagnetism in manganese-doped ZnO nanorods

Monochromatic cathodoluminescence image of Mn-doped ZnO nanorods shows that Mn elements are mainly distributed on the surface of nanorods. The intensity of ultraviolet luminescence decreases with the increment in Mn-doped concentration. Mn-doped ZnO nanorods exhibit a room temperature ferromagnetic characteristic with a saturation magnetization (MS) of 0.39 μB/Mn and a coercive field of 50 Oe.

The electrical and switching properties of a metal-ferroelectric (Bi3.15Nd0.85Ti3O12)-insulator (Y2O3-stabilized ZrO2)-silicon diode

Metal-ferroelectric-insulator-silicon diodes with Bi3.15Nd0.85Ti3O12 (BNT) as the ferroelectric layer and Y2O3-stabilized ZrO2 (YSZ) as the insulating buffer layer have been fabricated. Measurements revealed that the memory window of the fabricated diode reduces with increasing operating temperature, which is due to the decrease of coercive field and polarization of the BNT film. The diode demonstrates excellent retention and fatigue characteristics because of the good interface and high barrier height of YSZ/Si. In addition, the switching of the diode corresponds to an approximate constant-current process, and the complete switching time displays an obvious decrease as the gate voltage increases.