Bairu Zhao

Leakage mechanism of (Ba0.7Sr0.3)TiO3 thin films in the low-temperature range

Pt/(Ba0.7Sr0.3)TiO3(BST)/YBa2Cu3O7−x(YBCO) capacitors on SrTiO3 substrates were fabricated, and the temperature dependence of their current–voltage characteristics was studied in the temperature range from 100 to 300 K. It was found that the leakage current strongly depends on the voltage polarity and temperature. In the case of a Pt electrode negatively biased at a high electric field (>400 kV/cm), the Pt/BST interface acts as a Schottky barrier with the barrier height of 0.47 eV in the temperature range from 200 to 300 K. When the YBCO electrode was biased negatively and the electrical field was maintained higher than 64 kV/cm, the leakage mechanism shows space-charge-limited-current behavior.

Re-entrant spin glass behavior in Mn-rich YMnO3

We use magnetism and specific heat measurements to investigate the hexagonal Mn-rich YMnO3. It is found that upon cooling from a high temperature, the compound first orders antiferromagnetically at TN∼72K and then undergoes a re-entrant spin glass (RSG) transition at TSG∼42K. This RSG behavior results from the competition between the ferromagnetic interaction and the antiferromagnetic interaction, which is related to the intrinsic geometric magnetic frustration in this system.

Change in photovoltage due to an external magnetic field in a manganite-based heterojunction

The effect of magnetic field on the photovoltaic effect (PVE) has been investigated for a manganite-based heterojunction composed of a La0.7Ce0.3MnO3 film and a 0.5 wtu2009% Nb-doped SrTiO3 substrate. A linear decrease in the photovoltage (Voc) with magnetic fields is observed and the relative variation of Voc [defined as 1−Voc(H)∕Voc(0)] is larger than 10% under a field of 0.5 T at a temperature T=16.5K. There is a proportional relation between ΔVoc and the magnetoresistance of the heterojunction. We attribute the modulation of PVE to variation of magnetization and resistance of depletion layer by external magnetic fields.

Temperature- and field-dependent leakage current of Pt/(Ba0.7Sr0.3)TiO3 interface

Pt/(Ba0.7Sr0.3)TiO3(BST)/YBa2Cu3O7−x(YBCO) capacitors on SrTiO3 substrates were fabricated. The temperature and field dependence of the leakage current of Pt/BST interface were studied in the temperature range from 100 K to 320 K and in an electric field up to 3 MV/cm. For a middle electric field ( 1.8 MV/cm), the leakage mechanism is governed by the Fowler–Nordheim tunneling in the whole measured temperature range.

Enhanced dielectric properties of Ba1-xSrxTiO3 thin film grown on La1-xSrxMnO3 bottom layer

The Ba{sub 0.7}Sr{sub 0.3}TiO{sub 3} (BST)/La{sub 0.67}Sr{sub 0.33}MnO{sub 3} (LSMO) heterostructure has been fabricated by pulsed-laser deposition on (001) SrTiO{sub 3} single crystal substrate. The crystallization and surface morphology of the heterostructure have been characterized by x-ray diffraction and atomic force microscopy. The composition distribution along the depth of the heterostructure is analyzed by Auger electron spectroscopy. It is founded that the LSMO bottom layer shows a marked effect on the dielectric properties of the BST films. Comparing with BST/YBCO (YBCO-YBa{sub 2}Cu{sub 3}O{sub 7}) heterostructure grown by similar process, the dielectric loss exhibits much lower and less frequency dispersive, especially in the high frequency region. The loss tangent at 100 kHz is about 0.012 and the figure of merit factor is about 25 under the condition of 200 kV/cm. Ferroelectric hysteresis measurement shows that the remanent polarization and coercive field of the heterostructure are 3.4 {mu}C/cm{sup 2} and 39 kV/cm, respectively. These parameters are all better than BST/YBCO heterostructure. The good surface morphology and the element diffusion between BST and LSMO layers may be suggested to be responsible for the above-improved parameters of BST/LSMO heterostructure.The Ba0.7Sr0.3TiO3(BST)∕La0.67Sr0.33MnO3(LSMO) heterostructure has been fabricated by pulsed-laser deposition on (001) SrTiO3 single crystal substrate. The crystallization and surface morphology of the heterostructure have been characterized by x-ray diffraction and atomic force microscopy. The composition distribution along the depth of the heterostructure is analyzed by Auger electron spectroscopy. It is founded that the LSMO bottom layer shows a marked effect on the dielectric properties of the BST films. Comparing with BST∕YBCO (YBCO—YBa2Cu3O7) heterostructure grown by similar process, the dielectric loss exhibits much lower and less frequency dispersive, especially in the high frequency region. The loss tangent at 100kHz is about 0.012 and the figure of merit factor is about 25 under the condition of 200kV∕cm. Ferroelectric hysteresis measurement shows that the remanent polarization and coercive field of the heterostructure are 3.4μC∕cm2 and 39kV∕cm, respectively. These parameters are all better than...

Positive temperature coefficient of resistivity in Pt/(Ba0.7Sr0.3)TiO3/YBa2Cu3O7-x capacitors

Pt∕(Ba0.7Sr0.3)TiO3(BST)∕YBa2Cu3O7-x(YBCO) capacitors were fabricated on (001) LaAlO3 substrates. The leakage current density-electric field characteristics were measured in the temperature range from 20 to 300K. In the electric field region of >0.7MV∕cm for positively biased Pt electrode and >1.4MV∕cm for negatively biased Pt electrode, the leakage current shows space-charge-limited-current behavior from 20 to 250K. Meanwhile a positive temperature coefficient of resistivity (PTCR) in the temperature range from 20 to 150K was observed and attributed to the strong negative temperature dependence of the dielectric constant of BST thin films.

Abnormal temperature dependence of dielectric constant in (Ba0.7Sr0.3)TiO3 thin films

Pt∕(Ba0.7Sr0.3)TiO3∕YBa2Cu3O7−x capacitors were fabricated on LaAlO3 substrates. Dielectric and ferroelectric properties were measured in the temperature range from 20 to 250K. It was found that the dielectric constant decreased with decreasing temperature from 250 to ∼150K but then increased as the temperature continued to decrease to 20K. Considering this phenomenon along with the irregular temperature dependence of spontaneous polarization, it is indicated that such abnormal behavior originates in a second-order structural phase transition in the (Ba1−xSrx)TiO3 thin film in the vicinity of 150K.