Yali Xie

Tunable photovoltaic effects in transparent Pb(Zr0.53,Ti0.47)O3 capacitors

We report an investigation on optical, ferroelectric, and photovoltaic properties of transparent Sn-doped In2O3 (ITO)/Pb(Zr0.53,Ti0.47)O3 (PZT)/ITO thin film capacitors. The ferroelectric PZT sandwiched structures grown on glass substrates exhibit a transmittance of 65% in the visible light range. The current-voltage characteristics show that the transparent PZT capacitors possess a significant photovoltaic response under a light illumination. Moreover, the photovoltaic response can be well tuned by an external electrical field, which can be understood by considering the tunable depolarized field in the PZT capacitors.

Positive temperature coefficient of magnetic anisotropy in polyvinylidene fluoride (PVDF)-based magnetic composites

The magnetic anisotropy is decreased with increasing temperature in normal magnetic materials, which is harmful to the thermal stability of magnetic devices. Here, we report the realization of positive temperature coefficient of magnetic anisotropy in a novel composite combining β-phase polyvinylidene fluoride (PVDF) with magnetostrictive materials (magnetostrictive film/PVDF bilayer structure). We ascribe the enhanced magnetic anisotropy of the magnetic film at elevated temperature to the strain-induced anisotropy resulting from the anisotropic thermal expansion of the β-phase PVDF. The simulation based on modified Stoner-Wohlfarth model and the ferromagnetic resonance measurements confirms our results. The positive temperature coefficient of magnetic anisotropy is estimated to be 1.1 × 102 J m−3 K−1. Preparing the composite at low temperature can enlarge the temperature range where it shows the positive temperature coefficient of magnetic anisotropy. The present results may help to design magnetic devices with improved thermal stability and enhanced performance.

Effect of NiO inserted layer on spin-Hall magnetoresistance in Pt/NiO/YIG heterostructures

We investigate spin-current transport with an antiferromagnetic insulator NiO thin layer by means of the spin-Hall magnetoresistance (SMR) over a wide range of temperature in Pt/NiO/Y3Fe5O12 (Pt/NiO/YIG) heterostructures. The SMR signal is comparable to that without the NiO layer as long as the temperature is near or above the blocking temperature of the NiO, indicating that the magnetic fluctuation of the insulating NiO is essential for transmitting the spin current from the Pt to YIG layer. On the other hand, the SMR signal becomes negligibly small at low temperature, and both conventional anisotropic magnetoresistance and the anomalous Hall resistance are extremely small at any temperature, implying that the insertion of the NiO has completely suppressed the Pt magnetization induced by the YIG magnetic proximity effect (MPE). The dual roles of the thin NiO layer are, to suppress the magnetic interaction or MPE between Pt and YIG, and to maintain efficient spin current transmission at high temperature.

Magnetic field induced polarization and magnetoelectric effect of Ba0.8Ca0.2TiO3-Ni0.2Cu0.3Zn0.5Fe2O4 nanomultiferroic

The xBa0.8Ca0.2TiO3-(1 − x)NiCuZn ferrite (x = 0.1, 0.3, 0.5, 0.7, and 0.9) nanocomposites were prepared by using sol-gel method. The densification of these composites was carried out using microwave sintering method. The magnetic field induced changes in the ferroelectric polarization loop may support the possible magnetoelectric coupling between Ba0.8Ca0.2TiO3 and NiCuZn ferrite phases. The observed change in ferroelectric polarization with applied magnetic field proves the coupling between magnetic and ferroelectric order parameters. The loop change is observed with the composition and with magnetic field. The magnetoelectric coefficient of the nanocomposite with x = 0.3 shows a value of 280 mV/cm Oe is obtained.

Electric-field control of magnetic anisotropy in Fe81Ga19/BaTiO3 heterostructure films

We investigate the control of magnetism with an electric field in Fe81Ga19(FeGa)/BaTiO3(BTO) heterostructure films. The as-prepared FeGa/BTO samples present a uniaxial magnetic anisotropy, which is ascribed to be induced by the spontaneous ferroelectric polarization of the BTO substrates. With the electric field applied on the BTO substrates increasing from 0 to 6 kV/cm, the coercivity of FeGa films measured along the BTO[110] direction increases from 28 to 41 Oe, while the squareness of the hysteresis loop decreases from 0.99 to 0.31, which indicates that the easy and hard axes of FeGa films are swapped. The ferroelectric domains of BTO substrates and the magnetic domains of FeGa films exhibit the same dependence on the applied electric fields, manifesting the strong magnetoelectric coupling between the ferroelectricity of BTO substrates and the magnetism of FeGa films.

Thermally assisted electric field control of magnetism in flexible multiferroic heterostructures.

Thermal and electrical control of magnetic anisotropy were investigated in flexible Fe81Ga19 (FeGa)/Polyvinylidene fluoride (PVDF) multiferroic heterostructures. Due to the large anisotropic thermal deformation of PVDF (α1 = −13 × 10−6 K−1 and α2 = −145 × 10−6 K−1), the in-plane uniaxial magnetic anisotropy (UMA) of FeGa can be reoriented 90° by changing the temperature across 295 K where the films are magnetically isotropic. Thus, the magnetization of FeGa can be reversed by the thermal cycling between 280 and 320 K under a constant magnetic field lower than coercivity. Moreover, under the assistance of thermal deformation with slightly heating the samples to the critical temperature, the electric field of ± 267 kV cm−1 can well align the UMA along the two orthogonal directions. The new route of combining thermal and electrical control of magnetic properties realized in PVDF-based flexible multiferroic materials shows good prospects in application of flexible thermal spintronic devices and flexible microwave magnetic materials.

Anisotropic magnetoresistance in polycrystalline La0.67(Ca1−xSrx)0.33MnO3

We investigated the anisotropic magnetoresistance (AMR) effects in polycrystalline La0.67(Ca1−xSrx)0.33MnO3. An anomalously large AMR of 19.1% was observed near the metal–insulator transition temperature (TMI) in polycrystalline La0.67Ca0.33MnO3, but slight Sr doping could significantly depress the anomalous AMR around TMI. By studying the temperature, magnetic field, current direction dependence of AMR and the phase transition process, it was suggested that the anisotropic spin-polarized transport and the demagnetization effect together with the metamagnetic transition around TMI play important roles in the anomalous AMR effects in polycrystalline perovskite manganites.

Extraordinary Hall resistance and unconventional magnetoresistance in Pt/LaCoO3 hybrids

We report an investigation of transverse Hall resistance and longitudinal resistance on Pt thin films sputtered on epitaxial LaCoO3 (LCO) ferromagnetic insulator films. The LaCoO3 films were deposited on several single crystalline substrates [LaAlO3, (La, Sr)(Al, Ta)O-3, and SrTiO3] with (001) orientation. The physical properties of LaCoO3 films were characterized by the measurements of magnetic and transport properties. The LaCoO3 films undergo a paramagnetic to ferromagnetic (FM) transition at Curie temperatures ranging from 40 to 85 K, below which the Pt/LCO hybrids exhibit significant extraordinary Hall resistance up to 50 m Omega and unconventional magnetoresistance ratio Delta rho/rho(0) about 1.2 x 10(-4), accompanied by the conventional magnetoresistance. The observed spin transport properties share some common features as well as some unique characteristics when compared with well-studied Y3Fe5O12-based Pt thin films. Our findings call for new theories since the extraordinary Hall resistance and magnetoresistance cannot be consistently explained by the existing theories.

Strain induced tunable anisotropic magnetoresistance in La0.67Ca0.33MnO3/BaTiO3 heterostructures

In this paper, we investigated the influence of strain on anisotropic magnetoresistance (AMR) in La0.67Ca0.33MnO3 (LCMO) films epitaxially grown on BaTiO3(001). For 250-nm-thick LCMO film, the AMR shows a peak near the metal-insulator transition (MIT) temperature, which is similar to that in bulk LCMO. When the thickness of LCMO is decreased to 150 nm, the AMR value achieves a maximum at low temperature. For 80-nm-thick LCMO film, in addition to the appearance of the maximum AMR at low temperature, the symmetry and sign of AMR are also changed, associated with interface strain in the different phases of BaTiO3. In comparison, the AMR for the reference LCMO films grown on SrTiO3(001) shows a maximum value near the MIT temperature regardless of the thickness of film. Our experiment results suggest that not only the strain value but also the distortion type can considerably tune the AMR of LCMO films.

Effect of epitaxial strain and lattice mismatch on magnetic and transport behaviors in metamagnetic FeRh thin films

We grew 80 nm FeRh films on different single crystals with various lattice constants. FeRh films on SrTiO3 (STO) and MgO substrates exhibit an epitaxial growth of 45° in-plane structure rotation. In contrast, FeRh on LaAlO3 (LAO) displays a mixed epitaxial growth of both 45° in-plane structure rotation and cube-on-cube relationships. Due to the different epitaxial growth strains and lattice mismatch values, the critical temperature for the magnetic phase transition of FeRh can be changed between 405 and 360 K. In addition, the external magnetic field can shift this critical temperature to low temperature in different rates for FeRh films grown on different substrates. The magnetoresistance appears a maximum value at different temperatures between 320 and 380 K for FeRh films grown on different substrates.