Cheng-Jian Wang

The Curie temperature of ultra-thin ferroelectric films

The Curie temperature of ferroelectric films described by the transverse Ising model was studied under the mean-field theory. The film layer number, the surface interaction and the surface layer number dependence of the Curie temperature were obtained. There is a critical surface interaction strength, which is JSC=1.25 J for single-surface-layer films, and JSC=1.078 J for multiple-surface-layer films. If the surface interaction strength exceeds the critical value, there exists an optimum film thickness which possesses the maximum Curie temperature; then the surface interaction strength is weaker than the critical value, the Curie temperature decreases monotonically with increasing film thickness, and there exist critical sizes or critical thicknesses at which the ferroelectricity will disappear if the surface interaction is weak enough.

Large room-temperature magnetoresistance in Ag–Ti-added La0.67Ba0.33MnO3

The effects of Ag addition in Ti-doped La0.67Ba0.33MnO3 (LBT) (abbreviated as LBT/Agx, where x is the nominal molar ratio of Ag) on the electric and magnetoresistance (MR) properties have been studied systematically. The results show that the resistivity of the samples decreases dramatically with increasing Ag addition and reaches its minimum for x = 0.27. The room-temperature MR ratio of LBT/Agx increases with increasing x and reaches its maximum at x = 0.27. The MR ratio at 280 K for the x = 0.27 sample at H = 10 kOe is as large as 41%, which is about 3.4 times larger than that for LBT. The Ag addition makes the LBT crystallites more perfect and uniform, and suppresses the magnetic scattering at the grain boundaries. The Ag conduction channels penetrating into the insulating regions can also lower the resistivity. The near room-temperature TC and low resistivity of the Ag-added LBT are responsible for the large intrinsic room-temperature MR.

Ferroelectric thin films described by an Ising model in a transverse field

Ferroelectric films described by an Ising model in a transverse field have been studied under the mean-field approximation. We discuss an N-layer film of simple cubic symmetry with nearest-neighbour exchange in which the exchange strength and transverse field are assumed to be different from the bulk values in Ns surface layers, and we derive and illustrate expressions for the phase diagrams, order parameter profiles, specific heat and susceptibility. In such films, the Curie temperature can shift to either lower and higher temperature compared with the corresponding bulk value. If the surface exchange strength is strong enough, there is still a phase transition to ferroelectricity even when the transverse field is larger than the bulk critical value. In surface-enhanced films with Ns>or=2 the maximum in the order parameter profile occurs in the layers next to the outermost surface layer. The bulk peak of the specific heat becomes a discontinuity at the film Curie temperature. There is a rounded peak at the bulk Curie temperature if the surface exchange strength is higher than the critical value. The dim susceptibility still diverges at the film Curie temperature, as does the bulk susceptibility, but its magnitude is reduced. Also there is a rounded peak at the bulk Curie temperature when the surface exchange strength exceeds the critical value. The bulk-related character of the specific heat and susceptibility is less pronounced and the surface-related character is more pronounced when the film thickness is small or the surface-layer number is large.

Low-field MR behaviour in La0.67Ca0.33MnO3/ZrO2 composite system

La0.67Ca0.33MnO3 (LCMO) nanoparticles were prepared by the sol–gel method. The composite samples of (LCMO)x/(ZrO2)1−x were obtained by mixed LCMO and ZrO2 powders and sintered at 1300°C. Detailed studies of magnetic and magnetotransport properties for composite samples have been performed. The x-ray diffraction and scanning electronic microscopy observations indicated that no reaction happened between LCMO and ZrO2 grains. The low-field magnetoresistance (LFMR) can be greatly improved by ZrO2 addition close to the percolation threshold of x = 60%. The field sensitivity of MR at 77 K in the low field region for (LCMO)0.4/(ZrO2)0.6 is 7%/100 Oe, which is one order of magnitude larger than that obtained by pure LCMO. The enhancement of the spin-dependent tunnelling and scattering of electrons at the interfaces of LCMO and ZrO2 grains is responsible for the enhanced LFMR.

Critical behavior of the Ising model with four-spin coupling

Abstract Critical conditions for the first-order phase transition are determined for a system of 1 2 spins on the Ising model with a four-spin coupling term. The critical point parameters S z > c , T c and E c are found from the mean field theory. Relations between the three critical values and the four-spin coupling strength are discussed.

Large room-temperature magnetoresistance in Pd-added manganites

A series of La0.67(Ca0.65Ba0.35)0.33MnO3∕Pdx composites were synthesized using a sol-gel method followed by a conventional solid-state reaction route. The results show that the resistivity of the composites decreases dramatically with Pd addition, while the room-temperature magnetoresistance (MR) increases remarkably. A large MR of about 170% is obtained at room temperature and 1T applied magnetic field for x=0.27 sample. The large enhancement of the MR can be attributed to the decrease in resistivity caused by the good conductive metal Pd, which improves the disordered atomic structure and magnetic property on the grain surfaces/boundaries. In addition, the polarization of Pd atoms near the Mn ions on the grain surfaces/boundaries also plays a very important role on the enhancement of the MR, which induces a large number of spin clusters.

A microscopic model of first order phase transition in squaric acid

Abstract The contribution of four-body interaction is introduced into the pseudospin model in order to treat the first order phase transition in squaric acid. Using the Greens function method, we study the phase transition properties, the conclusion is that the first order phase transition is induced by the four-body interaction.

Electronic transport and extra large magnetoresistance in LCBMO/Pdx composites

Two-phase composites LCBMO/Pdx were synthesized using the sol?gel technique followed by the solid-state reaction. Pd addition induces a remarkable decrease in resistivity, which is mainly related to the improvement of grain boundaries/surfaces caused by the segregation of Pd. The resistivity data for all samples follow the adiabatic small-polaron-hopping model at high temperature above the Curie temperature TC, while in the low temperature region, they are proportional to T2, reflecting that the electron?electron scattering mechanism is dominant in the ferromagnetic metallic state. In addition, Pd addition induces a large enhancement of room temperature magnetoresistance (MR). Especially for the x = 0.27 sample, an extra large magnetoresistance over 170% is obtained at 10?kOe and 289?K. This is the largest MR obtained at room temperature in all kinds of colossal magnetoresistance (CMR) materials. The good conductivity and polarizing effect of Pd are responsible for the enhancement of MR. The former leads to the decrease in resistivity and the latter induces a large number of spin clusters.

Electrical properties and room temperature magnetoresistance of La0.67(Ca0.65Ba0.35)0.33MnO3/Agx composites

The electrical properties of the La0.67(Ca0.65Ba0.35)0.33MnO3/Agx composite system are systematically investigated as a function of Ag-added content. Ag addition induces a decrease in resistivity (ρ) and an increase in temperature TP where the ρ peak is located. This is due to the improvement of grain boundaries caused by the segregation of Ag on the grain surfaces. The T2 dependence of the resistivity data in the low temperature region of T < TP reflects that the conductive mechanism in the ferromagnetic (FM) metallic state mainly arises from electron–electron scattering. At high temperatures, above the Curie temperature TC, the resistivity data for all samples follow the adiabatic spin polaron hopping model. Moreover, the ρ –T curves for x = 0.27 and 0.30 samples fit well to the phenomenological percolation model based on phase segregation. This result confirms the coexistence of FM clusters and paramagnetic regions near TC, and indicates the prominent intrinsic behaviour of the La0.67(Ca0.65Ba0.35)0.33MnO3 matrix. Note that a 27% molar ratio of added Ag induces a large room temperature magnetoresistance (MR) ratio of up to 41%. This value is attractive for the study of applications. The good agreement of experimental data with the Brillouin function indicates that the MR behaviour in this composite system accounts for the spin-dependent hopping of the electrons among the spin clusters.

Substituting effects of Sm in polycrystalline La-Ca-Mn-O

The Sm substituting effects for La in La0.67Ca0.33MnO3 have been studied systematically. With increasing Sm doping amount, the metal-isolator phase transition temperature for the samples decreases monotonically, the corresponding peak resistivity increases dramatically and the Curie temperature decreases monotonically. The substitution of La-Ca-Mn-O with 0.13% Sm for La improved the magnetoresistance ratio by an order of magnitude. The effects of doping with Sm can be explained in terms of the lattice effects. An irreversible MR behaviour was observed in the Sm-doped compound. This effect was enhanced with increasing Sm doping amount.