Y B Zhang

High temperature stress-induced "double loop-like" phase transitions in Bi-based perovskites

Polycrystalline 0.94(Bi1/2Na1/2)TiO3–0.06BaTiO3 samples were tested under uniaxial mechanical compression at various temperatures in the vicinity of the polar tetragonal to nonpolar tetragonal phase boundary. They are shown to display double loop-like stress-strain behavior, marked by a closed ferroelastic hysteresis loop. Thus, it forms a mechanical analog to the polarization-electric field hysteresis behavior of barium titanate above the Curie temperature. As temperature is increased there is an apparent loss of macroscopically observable ferroelasticity, despite the persistence of tetragonality. Macroscopic experimental results are discussed in conjunction with temperature-dependent and stress-dependent high-energy x-ray diffraction data. This reveals a phase transition below the Curie temperature, marked by a discontinuous change in lattice parameters and octahedral tilting during compressive mechanical loading.

First-principles calculations of enhanced ferromagnetism in ZnO codoped with cobalt and nitrogen

Using first-principles calculations based on density functional theory, N-codoped ZnO:Co has been demonstrated to be potentially a p-type diluted magnetic semiconductor. By investigating 13 geometrically distinct configurations, Co and N dopants are found to have a tendency toward staying close to each other with most stable –O–Co–N–Co–O– complexes. The dominant ferromagnetic interaction is due to the hybridization between N 2p and Co 3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. The ferromagnetic coupling strongly relies on the distance of N from Co, while it weakly depends on the direction of aligned Co ions.

Intrinsic ambient ferromagnetism in ZnO:Co induced by Eu codoping

We manipulate the interaction of Co’s 3d and Eu’s 4f electrons to design and fabricate ZnO:Co+Eu, which possesses intrinsic ferromagnetism at ambient temperature. The results show that the Eu ions tend to neighboring Co ions in order to eliminate the lattice distortion caused by the larger Eu ions via strain coupling. It was also revealed that the preference of parallel spin alignment between Eu and Co ions results in ferromagnetism. The theoretical analyses and our experimental results evidenced that the induced ferromagnetism in the Eu and Co codoped ZnO is intrinsic at ambient temperature.

Enhancement of Co substitution induced by Eu codoping in ZnO-based diluted magnetic semiconducting thin films

To avoid the occurrence of doped magnetic ion clustering is a challenge in fabrication of diluted magnetic semiconductors (DMSs). In this work, we report the intrinsic ferromagnetic behavior in Co-doped ZnO DMSs induced by Eu codoping. Both structural parameters and magnetic properties demonstrate the existence of an interaction between Co and Eu ions. The observation of multiplet structures for the localized Co 3d states in x-ray absorption and x-ray magnetic circular dichroism characterization evidences that the codoped Eu plays an important role in facilitating the Co substitution of Zn, leading to intrinsic ferromagnetism.

Substantial stabilization of ferromagnetism in ZnO:Mn induced by N codoping

Electronic structures and magnetic properties of ZnO:Mn and ZnO:Mn+N systems are investigated using first-principles density functional calculations with generalized gradient approximation. The results provide a fundamental theoretical understanding in the substantial ferromagnetic stability induced by N codoping in the ZnO:Mn system observed experimentally. They demonstrate that the ferromagnetic interaction is due to the hybridization between N 2p and Mn 3d states and is very sensitive to the geometrical configurations of dopants in the ZnO host lattice. The most stable ferromagnetic configuration corresponds to the Mn-N-Mn cluster, energetically strong enough to lead to hole-mediated ferromagnetism at room temperature.

Electric-field-induced phase transitions in co-doped Pb(Zr1−xTix)O3 at the morphotropic phase boundary

Abstract The strain- and polarization-electric field behavior was characterized at room temperature for Pb0.98Ba0.01(Zr1−xTix)0.98Nb0.02O3, 0.40  x  0.60. The investigated compositions were located in the vicinity of the morphotropic phase boundary, giving insight into the influence of crystal structure on the hysteretic ferroelectric behavior. The remanent strain of particular compositions is shown to be larger than theoretically allowed by ferroelectric switching alone, indicating the presence of additional remanent strain mechanisms. A phenomenological free energy analysis was used to simulate the effect of an applied electric field on the initial equilibrium phase. It is shown that electric-field-induced phase transitions in polycrystalline ferroelectrics can account for the experimental observations. The experimental and simulation results are contrasted to neutron diffraction measurements performed on representative compositions in the virgin and remanent states.

The feeble role of oxygen vacancies in magnetic coupling in ZnO based dilute magnetic semiconductors

The role of the oxygen vacancy (V(O)) as the dominant defect in ZnO in magnetic interactions of ZnO based dilute magnetic semiconductors (DMSs) was examined in detail using density functional theory. It was found that V(O) does not lead to a thermally activated carrier mediated magnetism or form magnetic centers in the ZnO lattice. However, neutral V(O) may facilitate the ferromagnetism, but has a limited influence on the original antiferromagnetic coupling of the magnetic ions in oxygen stoichiometric ZnO DMSs. As a result, the ferromagnetism observed in previous experiments should be attributed to other defects such as hydrogen contamination or zinc interstitials.

Hydrogen multicenter bond mediated magnetism in Co doped ZnO

Magnetism in Co doped ZnO (ZnO:Co) is strongly affected by the presence of the ZnOs extrinsic impurities, such as unintentional hydrogen dopants. Our ab initio investigation reveals that in ZnO:Co the formation of substitutional H (H(O)) with four-fold hydrogenic bonds is favored over interstitial hydrogen (H(I)) by 0.4 eV. It is found that H(O) is trapped by Co ions to form highly stable Co-H(O)-Co complexes. H(O) also mediates a strong short-ranged ferromagnetic interaction between Co dopants via short-range exchange interaction which induces room temperature ferromagnetism.

The ageing and de-ageing behaviour of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free piezoelectric ceramics

Ageing behaviour usually occurs in acceptor-doped piezoelectric materials (e.g., hard lead zirconate titanate) and exhibits the development of a pinched or shifted hysteresis loop over time. Although no pinched hysteresis loop was observed for lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 material, this study showed that the piezoelectric properties change over time in the poled state. The shift of the hysteresis loop along the electric field axis and the development of asymmetry in strain and permittivity hysteresis loop were observed during the ageing process. The origin of this ageing behaviour is proposed to be local defect dipoles and the migration of the charged defects to the grain boundaries. The reorientation of the defect dipole contributes to a fast but unstable ageing mechanism in this material while the migration of the charged defects contributes to a slow but more stable mechanism.

ZnO:Co diluted magnetic semiconducting single crystalline films by hydrothermal epitaxy

Epitaxial ZnO:Co films with room-temperature ferromagnetism have been hydrothermally grown in water at 90°C using a ZnO seed layer on MgAl2O4 (111) substrates. The ZnO:Co films are single crystalline with out-of-plane orientations of ZnO:Co⟨001⟩∥MgAl2O4⟨111⟩ and in-plane orientations of ZnO:Co[110]∥MgAl2O4[1¯1¯2] and ZnO:Co[1¯10]∥MgAl2O4[1¯10] in spite of numerous threading dislocations at the boundaries of crystal mosaic. Saturation magnetizations of 0.83, 0.77, and 0.08emu∕cm3 and coercivities of 40, 22, and 22Oe were obtained for the epitaxial Zn1−xCoxO (x=0.02, 0.05, and 0.10) films, respectively, at room temperature. Hydrogen unintentionally incorporated in the hydrothermal synthesis may contribute to the ferromagnetic properties in ZnO:Co by enhancing carrier concentration as a shallow donor.