C.P. Yang

The trap state relaxation related polarization in CaCu3Ti4O12

Electrical conditioning properties and dielectric behavior of CaCu3Ti4O12 (CCTO) were investigated. A reversible current-voltage characteristic relaxation effect together with a corresponding dielectric relaxation was found in this material. A trap state model which combines a monopole and dipole relaxation was proposed to be the key element to explain the reported huge permittivity in CCTO. The work also supports the extrinsic origin of the giant dielectric constant.

Large magnetic response in (Bi4Nd)Ti3(Fe0.5Co0.5)O15 ceramic at room-temperature

Ceramics of Nd/Co co-substituted Bi5Ti3FeO15, i.e., (Bi4Nd)Ti3(Fe0.5Co0.5)O15 were prepared by the conventional solid-state reaction method. The X-ray diffraction pattern demonstrates that the sample of the layered perovskite phase was successfully obtained, even if little Bi-deficient pyrochlore Bi2Ti2O7 also existed. The ferroelectric and magnetic Curie temperatures were determined to be 1077 K and 497 K, respectively. The multiferroic property of the sample at room temperature was demonstrated by ferroelectric and magnetic measurements. Remarkably, by Nd/Co co-substituting, the sample exhibited large magnetic response with 2Mr = 330 memu/g and 2Hc = 562 Oe at applied magnetic field of 8 kOe at room temperature. The present work suggests the possibility of doped Bi5Ti3FeO15 as a potential multiferroic.

Influence of oxygen vacancy on the electronic structure of CaCu3Ti4O12 and its deep-level vacancy trap states by first-principle calculation

The electronic structure, formation energy, and transition energy levels of intrinsic defects have been studied using the density-functional method within the generalized gradient approximation for neutral and charged oxygen vacancy in CaCu3Ti4O12 (CCTO). It is found that oxygen vacancies with different charge states can be formed in CCTO under both oxygen-rich and poor conditions for nonequilibrium and higher-energy sintering processes; especially, a lower formation energy is obtained for poor oxygen environment. The charge transition level (0/1+) of the oxygen vacancy in CCTO is located at 0.53 eV below the conduction-band edge. The (1+/2+) transition occurs at 1.06 eV below the conduction-band edge. Oxygen vacancies of Vo1+ and Vo2+ are positive stable charge states in most gap regions and can act as a moderately deep donor for Vo1+ and a borderline deep for Vo2+, respectively. The polarization and dielectric constant are considerably enhanced by oxygen vacancy dipoles, due to the off-center Ti and Cu ...

Voltage dependent capacitances in CaCu3Ti4O12

The capacitance–dc voltage relation has been investigated in the giant dielectric constant material CaCu3Ti4O12. A capacitance drop, rise and even maxima, minima have been found with increasing dc voltage, which could not simply be explained by the back-to-back Schottky barrier theory. A trap charges repositioning between field perpendicular boundaries and field parallel boundaries has been put forward to explain the strange C-V relations. In addition, a first order calculation of C-V has been done based on the charge repositioning model and has led to a successful fitting of the experimental curves, extracting parameters such as a polarisability of about 1.5×10−39 for the induced dipoles.

Spin-dependent varistor-like behavior of Nd0.7Sr0.3MnO3−δ (0⩽δ⩽0.20)

Spin-dependent varistor-like electrical transport is found in Nd0.7Sr0.3MnO3−δ polycrystalline ceramics with slight oxygen deficient δ⩾0.05. The current-voltage I(V) curve shows nonlinear Ohmic characteristics below a critical temperature. Electrical-field-induced changes of the spin array orientations inside of and between the magnetic domains over grain or phase boundaries have to be concluded for the strong electroresistive effect in Nd0.7Sr0.3MnO3−δ series.

Effect of thermal cycle on the interfacial antiferromagnetic spin configuration and exchange bias in Ni-Mn-Sb alloy

Effect of thermal cycle on the interfacial antiferromagnetic (AFM) spin configuration and exchange bias in Ni50Mn36Sb14 alloy has been investigated. The results indicate thermal cycle can induce further martensitic transition from part of arrested FM phase to AFM phase, leading to the reconstruction of interfacial antiferromagnetic spin configuration. The shape of hysteresis loops at 5 K after cooling back can be tuned from a single-shifted loop to a nearly symmetric double-shifted loop gradually accompanied with exchange bias field increasing to peak value and then decreasing. The evolutions can be illustrated intuitively by a simple AFM bidomain model.

Positive to negative zero-field cooled exchange bias in La0.5Sr0.5Mn0.8Co0.2O3 ceramics.

Exchange bias effect obtained after zero-field cooling from unmagnetized state usually exhibits a shift of hysteresis loop negative to the direction of the initial magnetic field, known as negative zero-field cooled exchange bias. Here, positive zero-field cooled exchange bias is reported in La0.5Sr0.5Mn0.8Co0.2O3 ceramics. In addition, a transition from positive to negative exchange bias has been observed with increasing initial magnetization field and measurement temperature. Based on a simple spin bidomain model with variable interface, two type of interfacial spin configuration formed during the initial magnetization process are proposed to interpret the observed phenomenon.

Effect of alloying element Al substitution on Ni-Mn-Sn shape memory alloy by first-principle calculations

The effect of alloying element aluminum on the magnetic properties and martensitic transition in Ni8Mn6Sn2−xAlx shape memory alloys has been studied using the density-functional theory. It is found that the tetragonality, c/a, of the martensite phase increases gradually from 1.30 to 1.34 with increasing Al content. The equilibrium equation of state results show that the alloys Ni8Mn6Sn2−xAlx exhibit an antiferromagnetic-ferromagnetic phase transition in the austenite phase with x increasing. In particular, when x = 2, an unique magnetic transition occurs from ferromagnetic austenite (6.30 μB/f.u) to antiferromagnetic martensite (1.82 μB/f.u), as characterized by the spin polarized electronic density of states.

Microstructure and magnetic properties of (001) oriented FePt∕B4C composite films

FePt∕B4C multilayer composite films have been prepared by magnetron sputtering and subsequent annealing in vacuum. It was found that the B4C layers effectively serve as spacers to separate the FePt layers, enhancing (001) orientation of FePt alloy. Our results show that highly (001) oriented [Fe45Pt55(8 nm)∕B4C (4 nm)]3 film with satisfactory perpendicular coercivity (4.75 kOe) has significant potential as a perpendicular recording medium.

Microstructure evolution, magnetic and mechanical properties of FePt/B4C multifunctional multilayer composite films

FePt/B4C multilayer composite films with a Fe/Pt two-layer structure and a Fe/Pt/Fe sandwich structure were prepared by magnetron sputtering and subsequent annealing in vacuum. When the B4C interlayer thickness of the FePt/B4C films is bigger than 3 nm, a stable value of grain size, coercivity and hardness is observed. The characterizations of the microstructure demonstrate that the diffusion of B and C atoms in the FePt layer is so finite that the multilayer structure of the FePt/B4C films is maintained after annealing at 500 °C for 30 min. The finite diffusion of B and C atoms is responsible for the FePt grain growth confinement, grain and layer separation and eventually induces the stable value mentioned above. The single composite films may find application in substituting the traditional three-layer structure commonly used in the present data storage technology.