Zhi-Wei Jiao

Strain driven enhancement of ferroelectricity and magnetoelectric effect in multiferroic tunnel junction

The strain effect on the ferroelectric and magnetoelectric coupling in multiferroic tunnel junction (MFTJ) Co/BaTiO3/Co has been investigated systematically by using first-principles calculations within density functional theory. It is found that both in-plane compressive strain and uniaxial tensile strain lead to the enhancement of ferroelectric polarization stability and intensity of magnetoelectric coupling in the MFTJ. There is a transition from the paraelectric phase to the ferroelectric phase for the BaTiO3 layer in MFTJ when the loaded in-plane compressive strain increases up to -2.8% and the corresponding average ferroelectric polarization is about 0.13 C m(-2). Meanwhile, the calculated surface magnetoelectric coefficients increase with increasing in-plane compressive strain. Similar phenomena have been also observed in the case of uniaxial tensile strain implemented in MFTJ. The results suggest that the ferroelectric polarization and magnetoelectric coupling in multiferroic tunnel junctions can be controlled by strain and we expect that this study can provide a theoretical basis for the design of spintronic devices.

THICKNESS DEPENDENCE OF BUCKLING PATTERNS OF Ta FILMS SPUTTERED ON GLASS SUBSTRATES

Tantalum (Ta) films deposited on glass substrates have been prepared by a direct current magnetron sputtering method, and buckling patterns induced by residual compressive stress are investigated in detail. When the film thickness increases, the buckling morphologies evolve from straight-sided buckle network to wavy or wormlike wrinkles gradually, and finally change into telephone cord buckles. The geometrical parameters of the buckling patterns are found to increase linearly with the film thickness. Based on the geometrical parameters of the buckling patterns, the mechanical properties of the Ta films are also discussed in the frame of continuum elastic theory.

Tuning competition between metallic and insulating phases with strain, light and electric field in thin films of La0.39Pr0.24Ca0.37MnO3

The electric field and photo illumination effects in thin films of La0.39Pr0.24Ca0.37MnO3(LPCMO)/SrTiO3 with different thicknesses (15, 40 and 120 nm) have been systematically investigated. X-ray diffraction measurements demonstrate that these grown films are epitaxial and c axis oriented. Due to strain relaxation, these films are in different strain states. It is found that the strain state can greatly change not only the electrical properties but also the responses to external perturbations. The thinner film experiences a larger tensile in-plane strain and has a more robust insulating state. The low-temperature insulating state in the 120 nm sample is readily destroyed by a small electric field or light illumination without a bias voltage. Both the critical light density and the critical electric field used to collapse the insulating state in the 40 nm LPCMO film are much larger than those in the 120 nm sample. It is revealed that the bias electric field and light illumination play complementary roles. Light illumination can reduce the critical bias electric field. The 15 nm sample exhibits a robust insulating state, which cannot be destroyed by a combination of a bias voltage of 200 V (E = 4 × 105 V m−1) and light illumination (λ = 532 nm, P = 2 mW cm−2). Large transient photoconductivity has been observed in the 40 nm sample under low bias voltages and in the 15 nm sample under all bias voltages. The observed transient photoconductivity and persistent photoconductivity are believed to have different origins.

Photoluminescence of n-type GaN film in an argon plasma

The photoluminescence of n-type GaN thin films in an argon plasma condition has been studied by in situ measurement. The characteristic near-band-edge (NBE) peak and yellow luminescence (YL) of GaN films are observed in the spectra. However, compared to the luminescence of GaN in air and in a vacuum, in the plasma of 80 W power, the NBE peak and YL intensity first increase dramatically and then decrease with extended time. The effects are attributed mainly to an increasing temperature induced by the plasma, but also to a lesser extent by damage to the film caused by the plasma.

Characterization of barrier heights in Nd0.7Sr0.3MnO3/Nb : SrTiO3 junctions with current–voltage characteristics

Barrier heights in heterojunctions composed of Nd0.7Sr0.3MnO3 and Nb doped SrTiO3 have been characterized with current–voltage characteristics by using various methods. It is revealed that an inappropriate assumption of the transport mechanism would usually lead to the underestimation of the barrier height. Provided that the transport mechanism is correctly identified the barrier height deduced with current–voltage characteristics could be in accordance with Andersons model and typical internal photoemission results. It is suggested that for manganite–titanate heterojunctions introducing the ideality factor in the thermionic emission saturation current density equation might largely counteract the underestimation of the barrier height.

In situ electric properties of Ag films deposited on rough substrates

Silver (Ag) films have been deposited on rough substrates (including frosted glass and silicone grease), and for comparison on flat glass, by DC-magnetron sputtering, and their sheet resistances measured in situ during deposition. It is found that the growth of Ag films proceeds through three distinct stages: discontinuous, semi-continuous, and continuous regimes. The sheet resistance on rough substrates jumps in the vicinity of the percolation threshold, whereas the resistance on flat substrates decreases monotonically during deposition. The abnormal in situ electric properties on rough substrates are well explained based on the differences of the growth mechanism and microstructure of Ag films on different substrates.

Positive Exchange Bias in Tb/Cr Bilayers with TC < TN

The exchange bias phenomena in Tb/Cr films, where the Curie temperature (TC) of ferromagnetic Tb is much less than the Néel temperature (TN) of antiferromagnetic Cr, are systematically investigated. For the Tb/Cr bilayers, the exchange bias field HE changes from negative to positive as the temperature goes up. The unusual behavior of HE(T) is due to the spin-density waves spin structure in the Cr layer. For the Tb/Cr/Tb/Cr film, however, HE is negative throughout the temperature range in our experiment and its absolute value decreases monotonously with the temperature, which is ascribed to the coexistence of the interlayer coupling between Tb layers and the interfacial exchange coupling between Tb and Cr layers.

Experimental Observations Of Orderly Structures In Copper Films Deposited On Liquid Substrates

In this paper, an optical microscopy study of orderly structures, namely bands, which are observed in a nearly free sustained copper (Cu) film system, is presented. The band is composed of a large number of parallel key-formed domains with different width w but nearly uniform length L. This study shows that the morphologies of the Cu films are very susceptible to the deposition rate, i.e. with the increase in the deposition rate f, the bands with rectangular domains first become irregular gradually and then disappear completely. The experiment indicates that the growth mechanism of the orderly patterns can be explained in terms of the relaxation of the internal stress in the films, which is related to the characteristic boundary condition of the films on the liquid substrates and the nearly zero adhesion of the solid–liquid interface.

Anomalous Magnetic Properties of Tb/Cr/Ta Film Above Curie Temperature

Magnetic properties of Tb/Cr/Ta film, for which the Curie temperature (TC) of ferromagnetic Tb is much lower than the Néel temperature (TN) of antiferromagnetic Cr, have been investigated. An apparent TC beyond that of pure Tb film was observed. The origin of this enhancement of TC is due to a magnetic proximity effect between Tb and Cr. In the temperature range of TC < T < TN, the coercivity HC and exchange bias field HE of the film show unusual temperature dependence. As the temperature increases from TC to TN, HE still exists and its sign changes from negative to positive with a low cooling field. At T > TN, HE turns to vanish. The coercivity HC exists not only at TC < T < TN, but also above TN. These unusual results can be discussed in terms of the pinned uncompensated spins and rotatable uncompensated spins in the interface. Additionally, the magnitude of the cooling field has a strong effect on HE, which probably results from the competition between the exchange coupling energy at the Tb/Cr interface and the Zeeman energy of the Cr surface magnetization.

Interfacial structure, ferroelectric stability, and magnetoelectric effect of magnetoelectric junction FeCo/BaTiO3/FeCo with alloy electrode

The interfacial structures, ferroelectric instability, and magnetoelectric coupling effect of FeCo/BaTiO3/FeCo junction with alloy electrode have been studied systematically using the first-principles calculations within density functional theory. Owing to different interfacial coupling between the electrode and BaTiO3 barrier layer, there are four possible interfacial structures, including TiO2/Fe, BaO/Fe, TiO2/Co, and BaO/Co interfaces. Among the four interfacial structures, the TiO2/Fe interface is the most stable one. With the decreasing thickness of BaTiO3 barrier layer in the junction, there exists a critical size of ferroelectricity with a thickness of ~4.5 formula unit cells. However, the ferroelectric polarization of magnetoelectric junction with the alloy FeCo electrode is larger than that of the junction with the Co electrode using a same size due to the stronger screening ability of alloy FeCo electrode. Meanwhile, we find that the induced magnetic moments of Ti atoms are nearly from the contribution of the first TiO2 layer adjacent to the interfaces irrespective of the thickness of ferroelectric barrier. In other words, the induced magnetic moments of Ti atoms only maintain an atom-layer thickness. Moreover, the induced magnetic moments of Ti atoms and local magnetic moments of Fe atoms at the two interfaces of tunneling junction depend on the polarization orientation, leading to a sizable ME coupling effect.