Kerstin Brachwitz

Effect of rare-earth ion doping on the multiferroic properties of BiFeO3 thin films grown epitaxially on SrTiO3(1 0 0)

High-quality epitaxial Bi1?xRExFeO3 (RE=La, Nd, Gd; x?=?0, 0.05, 0.15) thin films were prepared on SrTiO3(1?0?0) substrates using pulsed laser deposition. X-ray diffraction and RBS-channelling spectroscopy showed that the films are single-phase perovskite, free of additional phases and textured with preferential orientation along the [1?0?0] direction. The dependences of magnetization on temperature and field showed that the films exhibit weak ferromagnetic properties. Among the studied rare-earth doping ions, Bi3+ substitution by Gd3+ most considerably enhanced the ferromagnetic properties. Substitution by La3+ smoothened out the surface morphology, which is important for different potential applications. Both undoped and doped films showed clear ferroelectric response in piezoresponse force microscopy, thus confirming their multiferroic nature. The doping was found to promote a preferential ferroelectric poling of the domains.

Fresnoite thin films grown by pulsed laser deposition: photoluminescence and laser crystallization

Fresnoite (Ba2TiSi2O8–BTS) thin films were grown on fused quartz, silicon (100), MgO (100), and a-plane sapphire by pulsed laser deposition, and crystallized by subsequent thermal or flash lamp annealing. The corresponding texture evolution of the BTS thin films was studied by X-ray diffraction. The preferential (001) texture of the crystallised BTS films was found to be most pronounced on sapphire substrates. The broad photoluminescence band of the BTS thin films depends only weakly on temperature. The intensity of the BTS luminescence can be as high as that of the most efficient oxide scintillator materials. In order to qualify the fresnoite thin films for photonic applications, we demonstrate infrared-laser direct writing in amorphous BTS films which allows a local crystallisation and patterning. A subsequent considerable enhancement of luminescence intensity can be applied for UV-sensitive marking of nearly any object.

On the transition point of thermally activated conduction of spinel-type MFe2O4 ferrite thin films (M=Zn, Co, Ni)

Thin films of zinc ferrite (ZnFe2O4), cobalt ferrite, and nickel ferrite have been grown on SrTiO3(100) substrates. By changing the growth temperature TG, the electrical conductivity σ of the thin films can be switched from insulating to conducting. An increased lattice constant of thin films grown at low temperatures (e.g., TG(ZnFe2O4)<450 °C) indicates the formation of defects, which seems to promote the hopping conductivity. Temperature-dependent measurements of σ reveal two thermally activated conduction processes. Interestingly, the transition point of these two processes at Tx=107 K−140 K is close to the Verwey transition known from structurally similar magnetite thin films.

Highly textured fresnoite thin films synthesized in situ by pulsed laser deposition with CO 2 laser direct heating

Fresnoite Ba2TiSi2O8 (BTS) thin films were grown and crystallized in situ using pulsed laser deposition (PLD) with CO2 laser direct heating of the a-plane sapphire (1 1 0) substrates up to 1250 ◦ C. Starting with 775 ◦ C growth temperature, (0 0 1)- and (1 1 0)-textured BTS and BaTiO3 phases, respectively, could be assigned in the films, and the typical fern-like BTS crystallization patterns appear. For higher process temperatures of 1100 to 1250 ◦ C, atomically smooth, terraced surface of the films was found, accompanied by crystalline high-temperature phases of Ba–Ti–Si oxides. HAADF micrographs taken in both scanning transmission electron microscopy and energy-dispersive x-ray spectrometry mode show details of morphology and elemental distribution inside the films and at the interface. To balance the inherent Si deficiency of the BTS films, growth from glassy BTS × 2 SiO2 and BTS × 2.5 SiO2 targets was considered as well. The latter targets are ideal for PLD since the employed glasses possess 100% of the theoretical density and are homogeneous at the atomic scale.

(Zn,Cd)O thin films for the application in heterostructures: Structural and optical properties

We report on (Zn,Cd)O thin films, grown by pulsed laser deposition on a-plane sapphire substrates with high Cd-contents up to 0.25. By incorporating Cd in ZnO and by applying a low growth temperature of about 300°C, the (Zn,Cd)O related luminescence redshifts to an energy of 2.46 eV as a result of the large Cd-content of 0.25. The redshift of the bandgap energy was additionally proven by transmission measurements. By fitting the transmission curves, the spectra of the absorption coefficient and the index of refraction are calculated. The (Zn,Cd)O thin films are single phase and exhibit the wurtzite crystal structure. An increasing a- and c-lattice constant is observed with increasing Cd-content.

Modeling the electrical transport in epitaxial undoped and Ni-, Cr-, and W-doped TiO2 anatase thin films

Electrical transport in undoped and Ni-, Cr-, and W-doped TiO2 thin films on SrTiO3(001) is modelled either with the sum of two thermally activated processes with exponential temperature dependence of conductivity, or with the sum of three-dimensional Mott variable-range hopping (VRH) and an activated process with low activation energy. The latter is interpreted for both models as small polaron hopping (<θD/4). According to reduced chi-square values, the double activated model is superior for data of higher ordered films grown at 540 and 460 °C. For lower growth temperature, VRH plus activated conductivity fits partly better. For all dopants, n-type conductivity is observed.

Exchange bias and magnetodielectric coupling effects in ZnFe2O4–BaTiO3 composite thin films

Multiferroic composite thin films prepared from ferrimagnetic zinc ferrite (ZFO) and ferroelectric barium titanate (BTO) show sizable magnetic exchange bias. After field cooling in +3 T, an exchange-bias field of about −37 mT for a 65% ZFO/35% BTO and of about −34 mT for a 35% ZFO/65% BTO composite is observed at 10 K. Exchange biasing is accompanied by a significant vertical loop shift of about 10% of the total saturation magnetization after field cooling in 3 T. The composite films show simultaneous ferromagnetic, ferroelectric and magnetocapacitance effects. Depending on the ZFO to BTO ratio of the target for pulsed laser deposition, the composite films show either preferential spinel- or perovskite-like X-ray diffraction patterns. Raman spectra of the composites are dominated by broadened peaks at positions near the phase-pure BTO or doped ZFO. The composite films show nm-size amorphous precipitates in a ZFO-like matrix, which are most probably responsible for the observed exchange bias effects.

Vacuum ultraviolet dielectric function of ZnFe2O4 thin films

The dielectric function of ZnFe2O4 thin films has been studied in a wide spectral range from 0.5 eV to 9 eV. The observed optical transitions are identified as charge transfer and ligand field transitions. We propose a parametric model based on known model dielectric functions that fits well to the experimental data. The assignment of optical transitions is justified by a thorough comparison with optical data from other materials exhibiting similar transitions. The main contributions to the optical response are found to arise from charge transfer transitions from the O2p to the metal ion 4s and 3d bands. Furthermore weak crystal field transitions are observed. The evolution of the dielectric function as well as the crystal quality of the thin films with respect to growth temperature is further analyzed. A blue shift of the observed optical transitions is related to a relaxation of the lattice toward the bulk and an increase of the crystal quality with increasing growth temperature.

Magnetic activity of surface plasmon resonance using dielectric magnetic materials fabricated on quartz glass substrate

The magnetic activity of surface plasmons in Au/MFe2O4 (M = Ni, Co, and Zn) polycrystalline bilayer films fabricated on a quartz glass substrate was studied for future magnetic sensor applications using surface plasmon resonance. The excitation of surface plasmons and their magnetic activity were observed in all investigated Au/MFe2O4 films. The magnetic activity of surface plasmons of the polycrystalline Au/NiFe2O4 film was larger than those of the other polycrystalline Au/MFe2O4 films, the epitaxial NiFe2O4 film, and metallic films. The large magnetic activity of surface plasmons of the polycrystalline film is controlled by manipulating surface plasmon excitation conditions and magnetic properties.