Magnetic collapse in yttrium iron garnet Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> at high pressure

The effect of high pressures up to 70 GPa on single-and polycrystalline samples of yttrium iron garnet Y₃⁵⁷Fe₅O₁₂ is studied by Mössbauer absorption spectroscopy (for the ⁵⁷Fe nucleus) in a diamond-anvil cell. It is found that the hyperfine magnetic field H_hf at ⁵⁷Fe nuclei vanishes abruptly at a pressure of 48 ± 2 GPa, which indicates the transition of the crystal from the ferrimagnetic state to nonmagnetic one. The magnetic transition is irreversible. When the pressure decreases, the magnetic state is not recovered and the garnet remains nonmagnetic until zero pressure. The behavior of the quadrupole splitting and isomer shift shows that, simultaneously with the magnetic transition, irreversible electron and possibly spin transitions occur with changes in the local crystalline structure. The mechanisms of the magnetic collapse are discussed.     

Magneto-elastic properties of Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> (TGG)

The temperature dependence of the elastic constants in Tb₃Ga₅O₁₂ was measured and analysed with a simple crystal field model. The magneto-elastic coupling constants have been deduced from this experiment. The coupling constant g_Γ5, related to the c₄₄ mode, is anomalously large. These coupling constants are important for the interpretation of the phonon Hall effect.     

Specific features of magneto-optical spectra of Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript>

The spectra of magnetic circular dichroism in the range of the ⁷ F ₆→⁵ D ₄ absorption band and the spectra of magnetic circular polarization of luminescence in the range of the ⁵ D ₄→⁷ F ₅ band in the terbium-gallium garnet Tb₃Ga₅O₁₂ are studied at a temperature of 80 K. The optical transitions between the Stark sublevels of the ⁷ F ₆, ⁷ F ₅, and ⁵ D ₄ multiplets are identified based on the analysis of the magneto-optical and optical spectra. It is shown that the experimentally determined symmetry and energy of the Stark sublevels of these multiplets confirm the results of numerical calculations of the energy spectrum of the Tb³⁺ rare-earth ion in terbium-gallium garnet.     

Deposition and stoichiometry control of Nd-doped gadolinium gallium garnet thin films by combinatorial pulsed laser deposition using two targets of Nd:Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> and Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>

We have demonstrated pulsed laser deposition of Nd-doped gadolinium gallium garnet on Y₃Al₅O₁₂ by the simultaneous ablation of two separate targets of Nd:Gd₃Ga₅O₁₂ (GGG) and Ga₂O₃. Such an approach is of interest as a method of achieving stoichiometry control over films whilst the growth parameters are kept constant and optimal for high quality crystal growth. We show here how the stoichiometry and resultant lattice parameter of a film can be controlled by changing the relative deposition rates from the two targets. Films have been grown with enough extra Ga to compensate for the deficiency that commonly occurs when depositing only from a GGG target. We have also grown crystalline GGG films with an enriched Ga concentration, and this unconventional approach to film stoichiometry control may have potential applications in the fabrication of films with advanced compositionally graded structures.     

Faraday rotation and the magnetocaloric effect in the Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> terbium gallium garnet at low temperatures in strong magnetic fields

The magnetocaloric effect and the Faraday rotation in a paramagnetic cubic crystal of terbium gallium garnet in strong magnetic fields oriented along different crystallographic directions are calculated theoretically. It is demonstrated that, in strong magnetic fields, the magnetocaloric effect and the Faraday effect are characterized by strong anisotropy, which disappears in weak magnetic fields.     

Zeeman effect and stark splitting of the electronic states of the rare-earth ion in the paramagnetic terbium garnets Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> and Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>

The Zeeman effect in the ⁷ F ₆ → ⁵ D ₄ absorption band of the Tb³⁺ ion in the paramagnetic garnets Tb₃Ga₅O₁₂ and Tb₃Al₅O₁₂ was studied. The field dependences of the Zeeman splitting of some absorption lines are found to exhibit unusual behavior: as the magnetic field increases, the band splitting decreases rather than increases. Symmetry analysis relates these lines to 4f → 4f electron transitions of the doublet-quasi-doublet or quasi-doublet-doublet type, for which the field dependences of the splitting differ radically from the well-known field dependences of the Zeeman splitting for quasi-doublet-quasi-doublet or quasi-doublet-singlet transitions in a longitudinal magnetic field.     

Gas sensor based on nanosize In<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> film

Nanosize films of In₂O₃:Ga₂O₃ (96:4 weight %) have been deposited on a glassceramic substrate by the method of rf magnetron sputtering. The surfaces of fabricated films were studied with use of a scanning electron microscope; sizes of grains were determined and the thicknesses of films were measured. In order to prepare a gas-sensitive structure, a thin catalytic palladium layer and ohmic comb contacts were deposited on the In₂O₃:Ga₂O₃ film surface by the method of ion-plasma sputtering. The sensitivity of sensors based on the glassceramic/In₂O₃:Ga₂O₃ (96:4 weight %)/Pd structure to different concentrations of propane and butane gas mixture, as well as to methane was investigated at temperatures of working substance from 250 to 300°C.     

Temperature effect on the elastic properties of yttrium garnet ferrite Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript>

The Young’s moduli along the [100] and [110] crystallographic directions and the shear modulus along the [100] direction in a high-purity yttrium garnet ferrite single crystal are measured in the temperature range from 20 to 600°C. All the independent elastic constants are calculated for this temperature range. The behavior of the elastic moduli and elastic anisotropy factor is analyzed in the vicinity of the critical temperature of the magnetic phase transition.     

Catalytic growth and characterization of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires

β-Ga₂O₃ nanowires have been synthesized using Ga metal and H₂O vapor at 800 °C in the presence of Ni catalyst on the substrate. Remarkable reduction of the diameter and increase of the length of the Ga₂O₃ nanowires are achieved by separation of Ga metal and H₂O vapor before they reach the substrate. Transmission electron microscopy analyses indicate that the β-Ga₂O₃ nanowires possess a single-crystalline structure. Photoluminescence measurements show two broad emission bands centered at 290 nm and 390 nm at room temperature. Received: 27 June 2002 / Accepted: 7 October 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +886-6/234-4496, E-mail: wujj@mail.ncku.edu.tw     

Luminescence properties of phosphors based on Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> (TbAG) terbium-aluminum garnet

The processes of excitation energy transfer in phosphors based on single-crystal Tb₃Al₅O₁₂:Ce (TbAG:Ce) and Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet films have been investigated. These films are considered to be promising materials for screens for X-ray images and luminescence converters of blue LED radiation. The conditions for excitation energy transfer from the matrix (Tb³⁺ cations) to Ce³⁺ and Eu³⁺ ions in TbAG:Ce and TbAG:Ce,Eu phosphors have been analyzed in detail. It is established that a cascade process of excitation energy transfer from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions and from Ce³⁺ ions to Eu³⁺ ions is implemented in TbAG:Ce,Eu via dipole-dipole interaction and through the Tb³⁺ cation sublattice.     

Temperature-controlled synthesis of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanobelts and nanosheets

We have investigated the effect of growth temperature on structural morphology and photoluminescence (PL) properties of as-synthesized gallium oxide (Ga₂O₃) nanostructures. The products consisted of Ga₂O₃ nanobelts and nanosheets (i.e. wider nanobelts), which had monoclinic crystalline structures. The average width of structures grown at 1000 °C was relatively greater than those at 800 °C, revealing that higher temperature favored the formation of nanosheets. PL measurements of 800 °C- and 1000 °C-grown samples indicated that both samples exhibited a broad emission band peaked around the blue-light region, while only the 800 °C-grown sample showed a red peak. PACS 81.07.-b; 81.05.Je; 61.10.Nz; 68.37.Hk; 68.37.Lp     

Field-induced magnetic transition in a mixed rare-earth aluminum garnet Er<Subscript>2</Subscript>HoAl<Subscript>5</Subscript>O<Subscript>12</Subscript>

The temperature dependence of the ac magnetic susceptibility of a single-crystal mixed rare-earth garnet Er₂HoAl₅O₁₂ has been investigated within the range from 1.8 to 300 K in a zero constant field and in applied bias fields of up to 9 T. In the absence of a constant magnetic field the magnetic susceptibility followed the Curie–Weiss law. The application of a constant magnetic field caused a magnetic phase transition, the temperature of which increased with increasing magnetic field. The temperature of the maximum of the ac magnetic susceptibility, which is a characteristic of the phase transition, did not show a noticeable dependence on the frequency of the alternating magnetic field.     

Investigation of luminescence spectra of the Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> and Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce single-crystal films

The luminescence spectra of single-crystal films and bulk crystals of yttrium-aluminum garnet Y₃Al₅O₁₂ and Ce³⁺-activated Y₃Al₅O₁₂ were investigated. It was shown that the room-temperature luminescence intensity of the Ce³⁺-free single-crystal Y₃Al₅O₁₂ film was considerably lower than that of the bulk crystals, while the luminescence intensity of the Ce³⁺ ions in the Y₃Al₅O₁₂:Ce films was considerably higher than that one for the corresponding bulk crystal.     

Fast ionic conduction in cubic hafnium garnet Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Hf<Subscript>2</Subscript>O<Subscript>12</Subscript>

A new member of the family of garnets with fast lithium ion conduction has been found with the composition Li₇La₃Hf₂O₁₂. The anion arrangement corresponds to the oxygen framework in garnets, e.g., in Ca₃Fe₂Si₃O₁₂. Hafnium is coordinated octahedrally while the lanthanum environment can be described as a distorted cube. Lithium occupies a large number of positions with tetrahedral, trigonal planar, and metaprismatic coordination. Li₇La₃Hf₂O₁₂ shows a lithium bulk ion conductivity of 2.4 × 10⁻⁴ Ω⁻¹ cm⁻¹ at room temperature with an activation energy of 0.29 eV.     

Intense White Luminescence from Combustion Synthesized Ca<Subscript>12</Subscript>Al<Subscript>14</Subscript>O<Subscript>33</Subscript>: Yb<Superscript>3+</Superscript>/Yb<Superscript>2+</Superscript> Single Phase Phosphor

The Ca₁₂Al₁₄O₃₃: Yb³⁺/Yb²⁺ single phase nano-phosphor has been synthesized through combustion route and its luminescence and lifetime studies have been carried out up to 20 K using 976 and 266 nm excitations. The samples heated in open atmosphere have shown the presence of Yb in Yb³⁺ and Yb²⁺ states. The 976 nm excitation results a cooperative upconversion emission at 486 nm due to the Yb³⁺ state and a broad band in the blue region and has been assigned to arise from the defect centers. The 266 nm excitation on the other hand results a broad emission band even from as-synthesized phosphor without doping of Yb, the width of which increases in presence of Yb due to the emission from Yb²⁺ ions formed in heated samples. The white emission covers almost whole visible region with bandwidth 190 nm. The ions in Yb²⁺ state has been found to increase with the increase in heating temperature up to 1,273 K. A back conversion of Yb²⁺ to Yb³⁺ has been observed for higher temperatures. Effect of boric and phosphoric acids as flux on the emission properties of Yb³⁺ and Yb²⁺ states have been examined and discussed. Quantum yield of emission has also been determined for different samples.     

Laser performance of holmium-doped Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> and (Lu,Y)<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> crystals pumped by Tm:fiber laser

Optical properties of Ho³⁺-doped Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂ crystals were investigated and compared. Substitution of Y for Lu in the host garnet Lu₃Al₅O₁₂ results in broad absorption and emission spectra, and improvements in the laser behavior of Ho³⁺. Pumped by Tm:fiber laser, a maximum output power of 5.02 and 5.73 W of Ho-doped Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂ have been obtained, respectively. The center lasing wavelength are 2124.5 and 2123.0 nm for Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂, respectively.     

Special features of linear magnetic birefringence in an α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga single crystal

Experimental investigations of the field dependence of the linear magnetic birefringence (LMB) in an α-Fe₂O₃:Ga single crystal are presented. It is established that during crystal magnetization in the basal plane near directions perpendicular to the С₂ axes in the region of magnetic field saturation, the LMB changes nonmonotonically. The observed special feature of the field LMB dependence is due to the reorganization of the magnetic α-Fe₂O₃:Ga structure during magnetization.     

CaCuMn<Subscript>6</Subscript>O<Subscript>12</Subscript> vs. CaCu<Subscript>2</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript>: A comparative study

The ferrimagnetic compounds Ca(Cu_xMn_3?x)Mn₄O₁₂ of the double distorted perovskites AC₃B₄O₁₂ family exhibit a rapid increase of the ferromagnetic component in magnetization at partial substitution of square coordinated (Mn³⁺)_C for (Cu²⁺)_C. In the transport properties, this is seen as a change of the semiconducting type of resistivity for the metallic one. The evolution of magnetic properties of Ca(Cu_xMn_3?x)Mn₄O₁₂ is driven by strong antiferromagnetic exchange interaction of (Cu²⁺)_C with (Mn³⁺/Mn⁴⁺)_B coordinated octahedra. The competing interactions of (Mn³⁺)_C with (Mn³⁺/Mn⁴⁺)_B lead to the formation of noncollinear magnetic structures that can be aligned by magnetic fields.     

Magnetic and magneto-lattice dynamics in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript>

The dynamics of magnetoelectric RMn₂O₅ crystals (R=Eu and Gd) was studied in the frequency and temperature ranges 20–300 GHz and 5–50 K, respectively. The crystals possessed magnetic and ferroelectric long-range order and had close transition temperatures, T_{N, C}?36 and 30 K for R=Eu and Gd, respectively. Mixed magneto-lattice excitations were observed in GdMn₂O₅; the excitations were most intense near the transition temperature T?30 K at frequencies close to the antiferromagnetic resonance frequencies of the Mn subsystem. Along with the antiferromagnetic resonance of the Mn subsystem, the ferromagnetic resonance of the Gd subsystem was observed in GdMn₂O₅ in an external magnetic field. No such dynamics was characteristic of EuMn₂O₅.