High-temperature ferromagnetism in laser-deposited layers of silicon and germanium doped with manganese or iron impurities

The paper reports on the results of a study of the synthesis conditions effects on magnetic and transport properties of nanosized layers of high-T_c diluted magnetic semiconductors (DMS), such as Ge:Mn, Si:Mn and Si:Fe, fabricated by laser-plasma deposition over a wide range of the growth temperature, T_g=(20-550) °C on single-crystal GaAs or Al₂O₃ substrates. Ferromagnetism of the layers was detected by measurement data of the magneto-optical Kerr effect, anomalous Hall effect, negative magnetoresistance and ferromagnetic resonance (FMR) at 5-500 K. The optimum growth temperature, T_g, for Si:Mn/GaAs layers with T_c≈400 K is shown to be about 400 °C. The Si:Mn/Al₂O₃ layers with 35% of Mn have the metal-type of conductivity with manifestation of magnetization up to room temperature. Different types of uniformly doped structures and digital alloys have been investigated. In contrast to GaSb:Mn films, Si-based ferromagnetic layers have strongly different magnetic and electric properties in case of uniformly doped structures and digital alloys. Positive results of the Fermi level variation effect on the improvement of Si- and Ge-based DMS layers have been gained on the use of additional doping with shallow acceptor Al impurity which contributes to the increase of the hole concentration and the RKKY exchange interaction of 3d-ions. The Ge:(Mn, Al)/GaAs or Ge (Mn, Al)/Si layers grown at 20 °C feature surprising extraordinary angular dependence of FMR.     

Optical surface polaritons of TM type at the nonlinear semiconductor–nanocomposite interface

TM-polarized optical surface polaritons in a nonlinear semiconductor–nanocomposite guiding structure have been considered. The nanocomposite consists of alternating layers of bismuth-containing garnet ferrite (BIG, Lu_{3 – x}Bi_xFe_{5 – y}Ga_yO₁₂) and gallium–gadolinium garnet (Gd₃Ga₅O₁₂), and the semiconductor (n-InSb) has a cubic nonlinearity and is characterized by two components of the nonlinear susceptibility tensor. With allowance for the anisotropy of the optical properties of the nanocomposite, caused by the magnetization of the BIG layers, the dispersion relation has been obtained and analyzed and its solutions are shown to split into two pairs of high- and low-frequency branches. The influence of the electric field at the interface on the wave characteristics and the existence domains of nonlinear surface TM polaritons has been studied. By solving the inverse problem of finding the profile of the longitudinal electric component of the surface polariton, it has been found that the nonlinearity gives rise to soliton-like wave fields.     

Bismuth-induced enhancement of magneto-optical effects in iron garnets: A theoretical analysis

A semiquantitative model of circular magneto-optical effects in iron garnets is constructed within the concept of charge-transfer transitions and the existing qualitative notions. In the framework of the proposed model, the drastic enhancement of circular magneto-optical effects in R₃Fe₅O₁₂ iron garnets containing impurities of Bi³⁺ or Pb²⁺ ions is explained by the increase in the oxygen contribution to the spin-orbit coupling constant of the (FeO₆)^9? and (FeO₄)^5? complexes (the main magneto-optically active centers in iron garnets). This increase is associated with the covalent admixture of the Bi³⁺ (or Pb²⁺) 6p orbitals (with a giant one-electron spin-orbit coupling constant) to the oxygen 2p orbitals. The influence of the substitution does not reduce to an enhancement of the oxygen spin-orbit interaction alone but also leads to the appearance of the effective anisotropic tensor contributions to the spin-orbit interaction and circular magneto-optical effects. These contributions to the magneto-optical effects in garnets are estimated. The influence of an inhomogeneous bismuth distribution on the magneto-optical effects in Y_3?xBi_xFe₅O₁₂ garnets is investigated using computer simulation. Analysis of the available experimental data on the magneto-optical effects in garnets confirms the validity of the theoretical model proposed.     

Magneto-optical properties of the iron garnet Tb3Fe5O12 near the magnetic compensation temperature

The magneto-optical susceptibility and magnetic hysteresis loops of the Faraday effect, which accompany the technical magnetization of the iron garnet Tb₃Fe₅O₁₂, have been investigated experimentally in the temperature region near the magnetic compensation point of this ferrimagnet T _c = 249 K. It has been found that, during the technical magnetization as the temperature approaches the magnetic compensation point T _c , the velocity of domain walls increases, whereas the magneto-optical susceptibility has local maxima to the right and to the left from T _c . Mentioned features of magneto-optical properties of the iron garnet Tb₃Fe₅O₁₂ are associated with the resonance natural frequency of the domain wall oscillations in the crystal with the frequency of the alternating magnetic field. It has been shown that the used theoretical model of the magnetic resonance of domain walls makes it possible to consistently describe (at the qualitative level) the revealed regularities of the variation of the Faraday effect in the iron garnet Tb₃Fe₅O₁₂ near T _c .     

Synthesis and characterization of Co7(OH)12(C2H4S2O6)(H2O)2—a single crystal structural study of a ferrimagnetic layered cobalt hydroxide

A novel layered hydrotalcite-like material, Co₇(H₂O)₂(OH)₁₂(C₂H₄S₂O₆), has been prepared hydrothermally and the structure determined using single crystal X-ray diffraction (a=6.2752(19) Å, b=8.361(3) Å, c=9.642(3) Å, α=96.613(5)°, β=98.230(5)°, γ=100.673(5)°, R1=0.0551). The structure consists of brucite-like sheets where 1/6 of the octahedral sites are replaced by two tetrahedrally coordinated Co(II) above and below the plane of the layer. Ethanedisulfonate anions occupy the space between layers and provide charge balance for the positively charged layers. The compound is ferrimagnetic, with a Curie temperature of 33 K, Curie-Weiss θ of −31 K, and a coercive field of 881 Oe at 5 K.     

Enhanced magneto-optical Kerr effects in nanocrystalline Sc-doped CoFe2O4 thin films

Nanocrystalline CoFe_2−xSc_xO₄ (x=0-0.4) thin films were prepared on silicon substrates at reduced temperature by a sol-gel process, and the doping effects of scandium on the microstructure, magnetism and polar magneto-optical Kerr effects of the as-deposited films were examined. It was shown that the intensities of both of the Kerr rotation peaks increase with the doping content x of Sc³⁺. The increase for the peak at 540 nm is due to the decrease of the electrostatic polarization of O²⁻ resulting from the relatively large radius of Sc³⁺, and that for the peak at 620 nm was a result of the migration of Co²⁺ from octahedral to tetrahedral sites in the presence of the dopant of Sc³⁺.     

A sensitive detection method for magnetization-induced second-harmonic generation under an externally applied field

We describe the instrumentation related to the first observation of magnetization-induced enhancement of surface second-harmonic generation (SHG) from the paramagnetic Si(111)-7×7 surfaces. A judicious choice of polarization and sample orientation enabled us to isolate the magnetic-field-dependent tensor element of the nonlinear susceptibility. A conductive liquid-nitrogen system, coupled to an ultrahigh-vacuum system that is immune to the high magnetic field of 10 T, cooled the sample to about 120 K. A high extinction that is necessary to detect the magnetization-induced SHG (MSHG) was accomplished by minimizing stray optical effects such as Faraday rotation, photoluminescence and thermal birefringence in all optical components, with extra care taken for the UHV window. Consistent and stable operation of this sensitive measurement system permitted experiments involving MSHG at multiple wavelengths and temperatures. Probing the magnetization-induced optical nonlinearity was validated by quenching the surface states by oxidation of the surface layers. From MSHG measurements at two wavelengths resonant with different surface-state transitions, we were able to suggest that the observed MSHG is proportional to the number of dangling bond states of Si(111)-7×7. Received: 15 January 2002 / Published online: 2 May 2002     

Pressure dependence of dielectric constant,elastic constants,and lattice parameters of the Y3(Ga,Al)5O12 host

Atomistic simulations were performed to investigate the lattice parameters, dielectric constant, and elastic constants of Y₃(Ga_xAl_5−x)O₁₂ (x = 1, 2, 3, 4, 5) structures. The calculated lattice parameters and elastic constants are in good agreement with those in available experimental results. The pressure dependence of all studied quantities was investigated. In general, a change in the behavior of all studied quantities is found when the Ga concentration becomes more than that of the aluminum (Al) in Y₃(Ga_xAl_5−x)O₁₂ (x = 1, 2, 3, 4, 5) structures.     

Intra-cavity second harmonic generation with Nd:YVO4/BIBO laser at 542 nm

We report, for the first time, an efficient intra-cavity second-harmonic generation (SHG) at 1084 nm in a nonlinear optical crystal, BiB₃O₆(BIBO) at the direction of (θ, ?) = (170.1°, 90°), performed with a LD end-pumped cw Nd:YVO₄ laser. With 590 mW diode pump power, a continuous-wave (cw) SHG output power of 19 mW at 542 nm yellow-green color has been obtained using a 1.5 mm-thick BIBO crystal. The optical conversion efficiency was 3.22%. It was found that the output wavelength could be 532 nm, 537 nm or 542 nm according to regulating the angle of BIBO.     

High-throughput characterization of BixY3−xFe5O12 combinatorial thin films by magneto-optical imaging technique

Bi_xY_3−xFe₅O₁₂ thin films have been grown on GGG (Gd₃Ga₅O₁₂) (1 1 1) substrates by the combinatorial composition-spread techniques under substrate temperature (T_sub) ranging from 410 to 700 °C and O₂ pressure of 200 mTorr. In order to study the effect of substrates on the deposition of Bi_xY_3−xFe₅O₁₂ thin films, garnet substrates annealed at 1300 °C for 3 h were also used. Magneto-optical properties were characterized by our home-designed magneto-optical imaging system. From the maps of Faraday rotation angle θ_F, it was evident that the Faraday effect appears only when T_sub = 430-630 °C. θ_F reaches to the maximum value (∼6°/μm, λ = 632 nm) at 500 °C, and is proportional to the Bi contents. XRD and EPMA analyses showed that Bi ions are easier to substitute for Y sites and better crystallinity is obtained for annealed substrates than for commercial ones.     

Influence of mobile space charges on electrical properties of ferroelectric Bi3.5La0.5Ti3O12 thin films

Lanthanum-substituted bismuth titanate, Bi_3.5La_0.5Ti₃O₁₂ (i.e., x=0.5 in Bi_4−xLa_xTi₃O₁₂), thin films have been grown on Pt/Ti/SiO₂/Si substrates using pulsed laser deposition. The frequency dependence of the real part ε′(ω) and the imaginary part ε″(ω) of the dielectric constant has been studied. The ε′(ω) does not show any sudden change within the frequency range of 10²-10⁶ Hz. In contrast, the ε″(ω) shows a large dispersion as frequency decreases. The observed relaxation behavior in ε″(ω) can be explained in terms of a migration of oxygen vacancies in (Bi₂O₂)²⁺ layers, not in Bi₂Ti₃O₁₀ perovskite layers.     

Synthesis and crystal structure of [3,5-(CH3)2C6H3NH3]4P4O12·3H2O

Chemical preparation and crystal structure are given for a new cyclotetraphosphate: [3,5-(CH₃)₂C₆H₃NH₃]₄P₄O₁₂·3H₂O. This compound is triclinic P with the following unit-cell parameters: a=8.298(3), b=8.299(3), c=17.242(7)Å, α=97.13(3), β=102.72(3), γ=64.55(3)°, Z=1 and V=1045.2(8)ų. The crystal structure has been solved and refined to R=0.040 using 6086 independent reflections. The atomic arrangement can be described as layers organization. Layers built by P₄O₁₂ ring anions, ammonium groups and water molecules parallel to the plan (001), between which the organic groups are located. Characterization by X-ray diffraction, IR absorption, and thermal analysis are described.     

Spinel-type solid solutions involving Mn and Ti:Crystal chemistry, magnetic and electrochemical properties

This study reports the structural and magnetic properties of spinel systems Li₄Mn_5−xTi_xO₁₂ (“4-5-12” series) and LiNi_0.5Mn_1.5−xTi_xO₄ (“LNMTO” series), both based on Mn⁴⁺ substitution by Ti⁴⁺. Intermediate compositions covering the whole range of compositions (0≤x≤5 and 0≤x≤1.5, respectively) were prepared by solid state reaction. The 4-5-12 system forms a continuous spinel solid solution, whereas the spinel phase range in LNMTO stops before the end member “LiNi_0.5Ti_1.5O₄”, which is multi-phased with a major hexagonal phase component. Cell parameters and (Mn,Ti)-O distances increase monotonically with titanium content in both series. In the LNMTO series, the end member LiNi_0.5Mn_1.5O₄ is known to form a superstructure with Ni/Mn cation ordering. Neutron diffraction and Raman spectroscopy show that this order is lost when Ti is substituted, even at low level (x=0.15). The LNMTO crystal chemistry is also complicated by the presence of partial cation inversion, and the presence of a secondary rocksalt-type phase that modifies the spinel stoichiometry. Magnetic properties are characterized by a competition between ferromagnetic and antiferromagnetic interactions; no magnetic ordering is achieved, in agreement with B-site cation frustration and disorder. Electrochemical measurements show that the Ti^3+/4+ and Mn^3+/4+ redox couples behave independently in the 4-5-12 series, and that titanium decreases the high-potential electrochemical redox activity of LNMTO because of its blocking character for electron transfer to and from the nickel sites in the spinel structure.     

Properties of exchange interaction in Yb3Fe5O12 under extreme conditions

In Yb₃Fe₅O₁₂, the exchange effective field can be expressed as H_eff=−λ·M_Fe=−λχ_eff·H_e=−γ·H_e where γ is named as the exchange field parameter and H_e is the external magnetic field. Then, in this paper, by the discussions on the characteristics of the exchange field parameter γ, the properties of exchange interaction in ytterbium iron garnet (Yb₃Fe₅O₁₂) are analyzed under extreme conditions (high magnetic fields and low temperatures). Our theory suggests that the exchange field parameter γ is the function of the temperatures under different external magnetic fields, and γ=a+b·T+c·T², where the coefficients a, b, c are associated with the external magnetic fields and the magnetized directions. Thus, the temperature-dependence, field-dependence and anisotropic characteristics of the exchange interaction in Yb₃Fe₅O₁₂ are revealed. Also, excellent fits to the available experiments are obtained.     

Electrical and electrochemical properties of molten salt-synthesized Li4Ti5−xSnxO12 (x=0.0, 0.05 and 0.1) as anodes for Li-ion batteries

Submicron-sized polyhedral Li₄Ti_5−xSn_xO₁₂ (x=0.0, 0.05, and 0.1) materials were successfully prepared by a single-step molten salt method. The structural, morphological, transport and electrochemical properties of the Li₄Ti_5−xSn_xO₁₂ were studied. X-ray diffraction patterns showed the formation of a cubic structure with a lattice constant of 8.31 Å, and the addition of dopants follows Vegard's law. Furthermore, FT-IR spectra revealed symmetric stretching vibrations of octahedral groups of MO₆ lattice in Li₄Ti₅O₁₂. The formation of polyhedral submicron Li₄Ti_5−xSn_xO₁₂ particles was inferred from FE-SEM images, and a particle size reduction was observed for Sn-doped Li₄Ti₅O₁₂. The chemical composition of Ti, O and Sn was verified by EDAX. The DC electrical conductivity was found to increase with increasing temperature, and a maximum conductivity of 8.96×10⁻⁶ S cm⁻¹ was observed at 200 °C for Li₄Ti₅O₁₂. The galvanostatic charge–discharge behavior indicates that the Sn-doped Li₄Ti₅O₁₂ could be used as an anode for Li-ion batteries due to its enhanced electrochemical properties.     

Formation of submicron-sized SrFe12−xAlxO19 with very high coercivity

Submicron-sized SrFe_12−xAl_xO₁₉ (x=1.3) was formed in glass-ceramic matrix using controlled thermocrystallization of the SrO–Fe₂O₃–Al₂O₃–B₂O₃ glass and the hexaferrite powder was obtained by removing the matrix phases. The samples were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray (EDX) analysis and magnetization measurements. The glass-ceramic material exhibits very high coercivity value up to 10.18 kOe which approaches a theoretically estimated maximum value for the compound. The hexaferrite powder consists of well faceted single crystals, which adopt the shape of a truncated hexagonal bipyramid. The powder saturation magnetization value is close to the theoretically estimated one for bulk material. Crystal structure of the powder was refined by Rietveld method and distribution of Al atoms on Fe sites was determined. Al atoms occupy 41% of 2a sites, 14% of 12k sites and 5% of 4e(1/2) sites, while 4f sites are not affected.     

Second-harmonic generation in the thermal/electrical poling (100−x)GeS2·x(0.5Ga2S3·0.5CdS) chalcogenide glasses

Utilizing Maker fringe (MF) method, second-harmonic generation (SHG) has been observed within the GeS₂-Ga₂S₃-CdS pseudo-ternary glasses through thermal/electrical poling technique. The SHG phenomenon was considered to be the result of breakage of the glassy macroscopic isotropy originated from the reorientations of dipoles during the thermal/electrical poling process. Under the same poling condition conducted with 5 kV and 280 °C for 30 min, the maximum value of second-order nonlinear susceptibility χ⁽²⁾ of the poled (100−x)GeS₂·x(0.5Ga₂S₃·0.5CdS) glasses was obtained to be ≈4.36 pm/V when the value of x is equal to 30. Nonlinear dependence of χ⁽²⁾ on compositions of these glasses can be well explained according to the theory related to the reorientation of dipoles.     

Magneto-optical and magnetic properties of (YBiSmTm)3(FeGa)5O12 ion-implanted films

A strong dependence of the meridional Kerr effect on the implantation regime that is different in different segments of the spectrum has been observed in He⁺-implanted (YBiSmTm)₃(FeGa)₅O₁₂ films. In addition to the Kerr effect, we have also investigated the dependence of the magnetization, the magnetic anisotropy, and the Néel temperature on the conditions of implantation. Correlations were noted in the behavior of the magnetic, magneto-optical, and electrical properties of the implanted layers. Fiz. Tverd. Tela (St. Petersburg) 39, 1267–1269 (July 1997)     

A compounded rare-earth iron garnet single crystal exhibiting stable Faraday rotation against wavelength and temperature variation in the 1.55 μm band

The Bi, Tb and Yb partially substituted iron garnet bulk single crystals of Tb_3−x−yYb_yBi_xFe₅O₁₂ were grown by using Bi₂O₃/B₂O₃ as flux and accelerated crucible rotation technique for single-crystal growth. Faraday rotation (FR) spectra showed that the specific FR of the (Tb_0.91Yb_1.38Bi_0.71)Fe₅O₁₂ crystal under magnetic field at saturation was measured to be about −1617°/cm at λ=1.55 μm, Faraday rotation wavelength coefficient (FWC, 0.009%/nm) in the wavelength range of 1.50–1.62 μm and Faraday rotation temperature coefficient (FTC, 3.92×10⁻⁵/K) at λ=1.55 μm were even smaller than that of YIG. It is proven that through combining two types of Bi-substituted rare-earth iron garnets with opposite FWC and FTC signs, the compound rare-earth iron garnets with low FWC and FTC may be obtained due to the compensation effect. The saturation magnetization of (Tb_0.91Yb_1.38Bi_0.71) Fe₅O₁₂ crystal is 0.48×10⁶ A/M and is also much smaller than that of YIG. We have found empirically that there is a simple relationship between the FR θ_f(x) and Bi content x for Tb_3−x−yYb_yBi_xFe₅O₁₂, which is given by θ_f(x)=(−2759x+400)°/cm.     

Photocontrol of magnetization in Al-substituted Fe3O4 thin films

Thin films of (111)-oriented spinel ferrite Al_0.5Fe_2.5O₄ have been prepared by a pulsed-laser deposition (PLD) technique on α-Al₂O₃ (0001) substrates. The films exhibit cluster-glass behaviors with a spin-freezing temperature, T_g, near or above room temperature. The magnetization was found to increase following light irradiation below T_g, which indicates the photoinduced melting of cluster-glass states. An analysis comparing the dynamic behavior of magnetic response to light irradiation between zero-field-cooled (ZFC) states and field-cooled (FC) states at 10 K under various light intensities, I, revealed that the direct photoexcitation of spins occurs when I≤0.78 mW/mm², while the thermal heating effect following the light absorption of the samples also contributes to the enhancement of magnetization when I≥1.22 mW/mm². The magnetization of the films could be controlled by light irradiation even at room temperature. This suggests the possibility of utilizing these films in the development of novel magneto-optical memory devices.