Fe-doped Crystals Research Articles

Ultrafast all-optical modulation in Fe-doped GaN at 1.31 and 1.55 μm with high contrast and ultralow power

We demonstrate the possibility of all-optical modulation at 1.31 and 1.55 μm optical communication wavelengths by interband pumping of Fe-doped GaN crystals using femtosecond pump-probe techniques. Considering the lower photon energy of near-infrared probe pulses, switching time was well controlled by Fe doping from the nanosecond range to a value as fast as 10 ps for Fe concentration of 1 × 1019 cm−3 arising from the carrier trapping effect of deep Fe acceptors, which suggests a modulation speed performance of ∼50 GHz. Simultaneously, about 50% of modulation contrast was achieved by means of optical excitation at an ultralow pump fluence of 0.5 mJ/cm2. Moreover, almost no degradation of the modulation contrast and speed was observed due to Fe doping.

Effects of sintering atmospheres on phase transformation, oxygen vacancy and photoabsorption behaviors of highly Fe-doped titania crystals

Fe-doped TiO2 crystals were successfully prepared using a sol–gel technique in reducing and oxidizing atmospheres. The effects of sintering atmosphere on phase transformation, oxygen vacancy concentration and photoabsorption behaviors were investigated. The results indicate that upon sintering in reducing atmosphere, Ti and Fe ion valences were decreased and highly Fe ions (12 mol%) were entirely dissolved into titania crystals, increasing oxygen vacancy concentration and leading to increased photoabsorption capability. In contrast, sintering in oxidizing atmosphere causes precipitation of the Fe2O3 phase, which is detrimental to the photoabsorption capability. The best photoabsorption performance is obtained by sintering 12 mol% Fe-doped TiO2 in reducing atmosphere, resulting in an absorption edge of approximately 435 nm, which is much higher than that of undoped TiO2 in the oxidizing atmospheres with the absorption edge 352 nm.

Comparative study of structural, optical, and magnetic properties of Fe-, Co-, Gd-, Cu-doped Na2WO4 nanocrystals: effect of treatment in hydrogen gas

Nanocrystalline samples of sodium tungstate (Na2WO4) doped with different ions (Fe, Co, Gd or Cu) were synthesised. The investigation objective is to study the effect of doping on the structural and optical properties of Na2WO4 including the possibility of creation and develop ferromagnetic (FM) properties. Moreover, the effect of annealing in hydrogen gas (hydrogenation) on the created magnetic properties was also studied. Therefore, the structural, optical and magnetic properties of the prepared samples were systematically studied. The structural studies confirm the doping of (Fe, Co, Gd or Cu) ions in Na2WO4 lattice. The optical study showed that the band gap of host Na2WO4 red-shifted with doping and blue-shifted with hydrogenation. It was observed that the hydrogenation creates FM properties in undoped Na2WO4 nanocrystals. For Gd-doped Na2WO4, the hydrogenation creates FM component superimposed on the original PM behaviour. However, relatively strong FM properties were created in hydrogenated Co− and Fe-doped Na2WO4 crystals. Thus, the main finding of this work is the oxygen vacancy-enhanced RT-FM, which could be achieved by both doping and hydrogenation. The saturation magnetisation attained ~ 0.1–0.2 emu/g, demonstrating that Fe− or Co-doped Na2WO4 nanocrystalline are suitable to use in practical applications. In general, the results show that it is possible to use Na2WO4 crystals doped with magnetic ions in optoelectronic applications that include magnetic actions.

A charge-based deep level transient spectroscopy measurement system and characterization of a ZnO-based varistor and a Fe-doped SrTiO3 dielectric

A charge-based deep level transient spectroscopy (Q-DLTS) method is applied to provide insights into the electronic behavior near grain boundaries and may provide new insights into mechanisms such as fatigue, degradation, dielectric aging, and dielectric breakdown. Here, we tested the Q-DLTS in both a ZnO varistor material and Fe-doped SrTiO3 materials. Comparisons are made to other data on ZnO varistors, and we obtain very good agreement for the energy levels. The status of deep traps in Fe-doped SrTiO3 dielectrics has been investigated where the relaxation was contrasted in a single crystal and polycrystalline ceramic materials. The relaxation is only observable in the polycrystalline materials, and was absent in single crystal Fe-doped crystals indicating that the deep traps originating from the Schottky barriers at the grain boundaries provide the DLTS signals. The energy associated with this grain boundary trap was found to be 1.26 eV.

Specific features of the influence of high-energy electron beams on the luminescent properties of undoped and Nb, Fe-doped Al₂O₃ crystals.

The influence of 10 MeV high-current electron beams accelerated by the M-30 microtron on the luminescent properties of the α-Al₂O₃, Al₂O₃:Nb and Al₂O₃:Fe crystals has been studied. The effect of the long-term phosphorescence at room temperature has been found that can be used to monitor electron and gamma accelerator beams.

Effect of Mg and Fe Doping on Optical Absorption of LiNbO3 Crystal through First Principles Calculations

Using first principles calculations, we investigate the structural, optical, and electronic properties of LiNbO3 (LN) and M doped LN (M=Mg, Fe). The density of states are calculated to analyze the effect of doping Mg and Fe ions on the absorption spectra and electronic properties of LN. The results show an ultraviolet shift in the optical absorption edge of Mg-doped LN compared with that of intrinsic LN. On the contrary, the absorption edge of Fe-doped LN crystal reveals a red shift. The optical absorption spectra show an improved optical response in the visible range for Mg-doped LN, which significantly differs from that obtained for Fe-doped LN. The electronic excitations from the valence band to the conduction band of LN leads to an improved optical absorption response in the visible region as observed experimentally. The obvious changes of the doped LN crystal are found in some cases, which provide a helpful guide for preparing doped LN crystal.

Remarkable enhancement in optical and thermal properties of sodium hydrogen phthalate hemihydrate crystals due to Fe3+ doping

Effects of Fe-doping on the solution grown semi-organic non-linear optical sodium phthalate hemihydrate (SP) single crystals have been reported. The structure and morphology of pure and doped crystals were analyzed. The degree of dopant inclusion was ascertained by Atomic Absorption Spectroscopy. The vibrational bands of the crystals were elucidated by Fourier transform infrared and Raman spectra. Fe inclusion in SP crystals resulted in an absorption band at 370nm in UV region and increase in transmittance in entire visible region with no appreciable change in optical band gap. Intensity of photoluminiscent peaks at 412nm and 522nm was enhanced due to Fe-doping with inhomogeneous broadening. Dielectric studies showed lower dielectric tensor component (ε33) and higher dielectric loss in Fe-doped crystals compared to pure SP crystals. Melting point and thermal stability enhanced due to the introduction of small ionic radius Fe3+ ion in the pure SP crystal. SHG efficiency of pure SP crystal was doubled after Fe-doping and became comparable to that of KDP.

Optical Properties of Mg, Fe, Co-Doped Near-Stoichiometric LiTaO₃ Single Crystals.

Mg, Fe co-doped near-stoichiometric lithium tantalite (SLT) single crystals were grown by employing the zone-leveling Czochralski (ZLCz) technique. The optical properties, holographic parameters, as well as the composition of the grown crystals were measured. It was found that the Li/Ta ratio decreased with the doping of Mg and Fe ions. A red shift was observed in absorption spectrum for the Mg, Fe co-doped crystals compared to the undoped and Mg-doped ones. The effect of the iron ions (Fe2+ and Fe3+) was further discussed based on the specified absorption bands. Moreover, the occupation mechanism for the defects was discussed by using the IR absorption spectrum, which was attributed to the FeTa3− defects in the highly Fe-doped crystal. In addition, the holographic parameters were also found to be improved with a higher Fe/Ta ratio in the crystals.

Structure, electrical and magnetic property investigations on dense Fe-doped hexagonal BaTiO3

Hexagonal Ba(Ti1−xFex)O3−δ (x = 1/6, 1/3) ceramics treated with post annealing are specifically synthesized to explore the origin mechanism of the unusual ferromagnetism in the doped system. X-ray diffraction refinements and transmission electron microscope experiments reveal that their structures are incommensurately modulated owing to simultaneous oxygen vacancies at both O1 and O2 sites. Consequently, coexisting weak ferrielectricity and weak ferromagnetism are presented at room temperature. Analysis on their leakage current plot reveals that their conduction follows trap-filled limit model. In combination with the magnetism studies on 5 mol% Fe-doped crystals [Phys. Rev. B 83, 144407 (2011)], the reduction of ferromagnetism with an increase of conductivity suggests that dynamic exchanges of trapped electrons among the bound magnetic polarons attribute to the intrinsic ferromagnetism.

D– d Transitions of Fe 3+ ions in Fe-doped K 2Al 2B 2O 7 crystal

Single crystal of K 2Al 2B 2O 7 (KABO) with 6.3% aluminum substituted by iron was grown by top-seeded solution method using KF-B 2O 3 as flux. Optical transmittance, reflection and photoluminescence spectra of the Fe-doped KABO crystal were investigated by comparing with those of the Fe-free crystal. The absorption and fluorescence bands in the visible and near infrared regions were assigned to d– d transitions of tetrahedrally coordinated Fe 3+ ions. The results of theoretical calculation based on the crystal-field approximation show a good agreement with the observed optical absorption peak positions. These evidences reconfirmed that the Fe 3+ impurity causes the UV absorption in KABO crystal which prevent it from being used as efficient nonlinear optical materials for the fourth harmonic generation of Nd-based lasers.

Growth and characterization of Fe-doped Pb(Mg<sub>1/3</sub>Nb<sub>2/3</sub>)O<sub>3</sub>-0.26PbTiO<sub>3</sub> single crystals

0.3 mol% Fe-doped 0.74Pb(Mg1/3Nb2/3)O3-0.26PbTiO3 (PMN-0.26PT) single crystals were grown by a modified Bridgman technique. The structure, dielectric and pyroelectric properties of the Fe-doped crystals were investigated and compared with pure PMN-0.26PT crystals. The investigations showed that small concentrations of Fe substitution increased the coercive field and remnant spontaneous polarization. Two dielectric anomaly peaks were observed under Curie temperature in the dielectric constant versus temperature curve for Fe-doped crystals, which is closer to the room temperature compared with the pure one. The results demonstrate that the domain walls can be pinned by the dopant dipolar defects 2Fe′Ti-Vo, resulting in an increase of coercive field and a decrease of dielectric constant and dielectric loss, which led to the improvement of the detectivity figure of merit (FOM) and kept high voltage responsivity FOM of doped crystals for pyroelectric devices. Such investigation will provide a better understanding of the nature in the complex perovskite materials with high performance.

Light-induced absorption change in Fe-doped GaN

The properties of a light-induced absorption (LIA) change were investigated in a semi-insulating Fe-doped GaN crystal. The temporal profile and a spectrum of the LIA were measured in the temperature range from 15K to 300K. The LIA was almost proportional to the light intensity and showed slight saturation behavior. The relaxation time constant, in the range of milliseconds, was equal to the lifetime of infrared photoluminescence related to the Fe3+ internal transition Fe3+(4T1)→Fe3+(6A1). The LIA was attributed to an excited-state absorption from Fe3+(4T1). Some of the peak energies observed in the LIA spectrum showed good agreement with transition energies expected from an energy diagram of Fe-doped GaN crystal.

Simulation of iron impurity in BaTiO3 crystals

Abstract Iron-doped barium titanate (BaTiO 3 ) has been simulated taking into account cubic and tetragonal crystallographic lattices of the crystal. A quantum-chemical method based on the Hartree–Fock formalism has been used throughout the study. The calculated equilibrium structures of Fe-doped crystals reveal the defect-inward displacements of the Ti and O atoms whereas the shifts for the Ba atoms are encountered to be away with respect to the Fe impurity. According to the analysis of electron density population and electron band structure it is found that some unusual chemical bonding might take place between the Fe atom and its six adjacent O atoms. The role of Fe impurity in the ferroelectric polarization of the tetragonal BaTiO 3 crystal has been discussed too.

Fe-doped 0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 single crystals

Pure and 0.3 at% Fe-doped 0.71PMN–0.29PT single crystals were grown by a modified Bridgman method. The dielectric and ferroelectric properties of the ⟨1 1 1⟩-oriented crystals were investigated. Two dielectric anomaly peaks were observed in the dielectric constant versus temperature curve for both pure and Fe-doped crystals. The dielectric constant and loss of the Fe-doped crystal were 460 and 0.001 at 1 kHz, respectively; those of pure crystal were much larger than the Fe-doped crystal. The coercive field of the Fe-doped crystal achieved ∼0.8 kV mm−1, about twice that of pure crystal. Also the domain structure of the poled crystals was observed. In this paper, the effects of the Fe doping and doping mechanism are discussed.

Dielectric constant and EPR spectra of Fe doped TlInS2 crystal near the structural phase transitions

Dielectric constant and electron paramagnetic resonance (EPR) spectra of Fe-doped TlInS2 crystal has been investigated in the temperature range of 5–300 K. The influence of Fe impurities on dielectric properties of TlInS2 crystal has been considered in comparison with earlier observed results from pure TlInS2 compounds. Considerable decrease of the dielectric constant, as well as the change of the shape of its temperature dependence in a result of influence of doped iron atoms, has been observed. It has been established that EPR lines exhibit remarkable splitting and appearance of additional resonance lines at the temperatures lower than 200 K, which is attributed earlier as the temperature of the ferroelectric phase transition. The simulation for low-temperature EPR spectra has been performed taking into account the appearance of four additional centers in addition to four equivalent centers observed from EPR spectra at room temperature. Such transformations are considered as result of nonequivalent displacements of different groups of Tl atoms during the structural phase transitions.

Fe-doping-induced evolution of charge-orbital ordering in a bicritical-state manganite

Impurity effects on the stability of a ferromagnetic-metallic state in a bicritical-state manganite, ${({\mathrm{La}}_{0.7}{\mathrm{Pr}}_{0.3})}_{0.65}{\mathrm{Ca}}_{0.35}\mathrm{Mn}{\mathrm{O}}_{3}$, on the verge of metal-insulator transition have been investigated by substituting a variety of transition-metal atoms for Mn ones. Among them, Fe doping exhibits the exceptional ability to dramatically decrease the ferromagnetic transition temperature. Systematic studies on the magnetotransport properties and x-ray diffraction for the Fe-doped crystals have revealed that charge-orbital ordering evolves down to low temperatures, which strongly suppresses the ferromagnetic-metallic state. The observed glassy magnetic and transport properties as well as diffuse phase transition can be attributed to the phase-separated state where short-range charge-orbital-ordered clusters are embedded in the ferromagnetic-metallic matrix. Such a behavior in the Fe-doped manganites form a marked contrast to the Cr-doping effects on charge-orbital-ordered manganites known as the impurity-induced collapse of charge-orbital ordering.

A study of the effect of annealing on Fe-doped LiNbO3by HRXRD, XRT and FT–IR

The annealing effect on the structural perfection of Fe-doped LiNbO3single crystals has been studied by high-resolution X-ray diffractometry (HRXRD), X-ray topography (XRT) and Fourier transform infrared (FT–IR) spectroscopy. The single crystals, prepared by mixing Li2CO3and Nb2O5powders in the molar ratio 48.6:51.4 with 0.05 mol% of iron at 1415 (1) K, were grown by the Czochralski (CZ) method along the [001] direction in air and poled during crystal growth by the application of a DC field. Two low-angle (tilt angle ∼1 arc minute) structural grain boundaries were observed in as-grown specimens. FT–IR spectra revealed that these crystals contain OH−and CO32−ionic defects. Grain boundaries and CO32−ionic defects were successfully removed, while the concentration of OH−ions was considerably reduced by post-growth thermal annealing at elevated temperatures.

Relaxor properties of Fe-doped TlInS2

The relaxor properties of Fe-doped TlInS2 crystals are investigated. It is shown that Fe-doped TlInS2 compounds exhibit all features inherent in relaxor ferroelectric materials. The temperature range of existence of the microdomain (relaxor) state and the temperature of the transition from this state to a macro-domain state are determined.

Order-disorder transition inBi2.1Sr1.9CaCu2O8+δsingle crystals doped with Fe and Pb

The magnetic field ${H}_{\mathrm{dis}}(T)$ where an order-disorder transition of the vortex lattice in high-${T}_{c}$ superconductors occurs, is investigated by measurements of the magnetization $M(H)$ in ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) single crystals doped with iron and lead. Comparative studies are made of the temperature dependences of the field ${H}_{\mathrm{peak}}(T),$ where the second peak occurs in $|M(H)|,$ and the fields ${H}_{\mathrm{min}}(T),$ and ${H}_{\mathrm{infl}}(T)$ where a minimum and an inflection point occur at the low-field side of this peak. It is proposed that ${H}_{\mathrm{dis}}(T)$ lies close to ${H}_{\mathrm{infl}}.$ In ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{Ca}}_{1.0}({\mathrm{Cu}}_{1\ensuremath{-}y}{\mathrm{Fe}}_{y}{)}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals with Fe concentration $y=0,$ 0.005, 0.016, and 0.022, a pronounced peak in the derivative $|dM/dH|$ is observed, whose position ${H}_{\mathrm{infl}}(T)$ is independent of temperature T. We relate this peak to the field ${H}_{\mathrm{dis}}(T),$ which separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. In heavily Pb-doped single Bi2212 crystals, ${H}_{\mathrm{infl}}(T)$ decreases with increasing T. For the same crystals, a minimum in the normalized relaxation rate $S(H)$ is observed at ${H}_{\mathrm{infl}},$ indicating two different flux-creep mechanisms above and below that field and two different solid vortex phases. It is argued that the negative slope of ${H}_{\mathrm{dis}}(T)$ in heavily-Pb-doped Bi2212 crystals is related to the enhanced c axis conductivity caused by the Pb sitting between the ${\mathrm{CuO}}_{2}$ layers and causing three-dimensional vortex lines, while in Fe-doped Bi2212 crystals the Fe ions sit on the ${\mathrm{CuO}}_{2}$ planes and thus do not enhance the coupling between pancake vortices.

Spontaneous polarization of Fe-doped SbNbO4 crystals

The temperature dependence of spontaneous polarization in SbNbO4: Fe crystals has been studied. The temperatures of the ferroelectric and the structural phase transitions (537 ± 1) K and (876 ± 1) K are determined from the permittivity data. The anomalies in the temperature dependence of Ps provided by the presence of the FeSb and (OiVSb)-type impurity centers are observed in the whole range of ferroelectric ordering. In the SbNbO4 structure, the macrofragments—two types of mesotetrahedra formed by Sb ions and NbO6-octahedra-are established, which are responsible for the spontaneous polarization of the crystal.