Room temperature magnetization measurements of some substituted rare earth iron garnets

The saturation magnetization (M _s) and the initial magnetic susceptibility (χ _in) of substituted yttrium iron garnet (YIG) Ho_3−x Y _x Fe₅O₁₂ where (x=0,0.3,1.5), pure terbium iron garnet Tb₃Fe₅O₁₂, and substituted aluminum iron garnet (AlIG) Ho₃Fe_5−x Al _x O₁₂ where (x=0.05,0.7) at room temperature are reported. It has been observed that M _s and χ _in decrease in a linear manner as Ho³⁺ replaces Y³⁺ in yttrium iron garnet. For substituted (AlIG), a drastic decrease in both M _s and χ _in as Al³⁺ ions replace Fe³⁺ ions. The rate of decrease of M _s and χ _in with the increase of Al³⁺ for substituted (AlIG) is greater than that for substituted (YIG). That is attributed to the decrease in the superexchange interaction (self and mutual iron interactions) with Al substitution.     

Damage processes in Fe 3 O 4 magnetic insulator irradiated by swift heavy ions. Experimental results and modelisation

The behavior of the magnetic properties of magnetite Fe₃O₄ irradiated by swift heavy ions is investigated by magnetization measurements. Although there is no induced structural phase transformation, both coercive field and saturation magnetization are sensitive to ion irradiation and exhibit different behaviors depending on the ion fluence range. In the low fluence regime, the coercive field increases, which is evidence for a strong pinning of magnetic domain boundaries by the induced defects. The magnetization shows a decrease in the saturation value and tends to reorient perpendicularly to the ion track axis. At high fluence, the initial magnetic properties of the sample are nearly restored. The changes in the magnitude and the direction of magnetization are interpreted by magnetostrictive effects related to the stress induced by irradiation. A phenomenological model is applied to reproduce the fluence evolution of the saturation magnetization, assuming relaxation of the stress induced around the core of defects of the tracks by overlapping effects at high fluence. The results are compared to those obtained in the case of yttrium iron garnet Y₃Fe₅O₁₂. Received 18 April 2001 and Received in final form 24 July 2001     

Effect of oxidation dynamics on the film characteristics of Ce:YIG thin films deposited by pulsed-laser deposition

Thin films with different compositions of Ce-substituted yttrium iron garnet (Ce:YIG (Y₂CeFe₅O₁₂)), Ga-doped Ce:YIG (Ce:Ga:YIG (Y₂CeFe_4.25Ga_0.75O₁₂)), and Gd-doped Ce:YIG (Ce:Gd:YIG (Y_1.6CeGd_0.4Fe₅O₁₂)) were deposited on gadolinium gallium garnet (GGG (Gd₃Ga₅O₁₂)) substrates in O₂ or Ar background gas by pulsed-laser deposition (PLD) technique. Crystalline films were obtained at a lower O₂ gas pressure of 20 mTorr or at higher Ar gas pressures of more than 100 mTorr. In addition, the behavior of YO molecules was visualized by two-dimensional laser-induced fluorescence (2D-LIF), in order to investigate the oxidation dynamics in the ablation plume. The oxidation dynamics and the crystallinity had close correlation.     

Single-crystalline films of Ce-doped YAG and LuAG phosphors: advantages over bulk crystals analogues

Luminescent properties of phosphors based on single-crystalline films (SCF) of Y₃Al₅O₁₂:Ce (YAG:Ce) and Lu₃Al₅O₁₂:Ce (LuAG:Ce) garnet have been analyzed in comparison with single-crystal (SC) analogues. It has been shown that the main peculiarity of luminescent properties of YAG:Ce and LuAG:Ce SCF as compared to SC is determined by the extremely low concentration of Y_Al³⁺ and Lu_Al³⁺ antisite defects (AD) in SCF. The advantages of phosphors based on YAG:Ce and LuAG:Ce SCF are caused by the absence in these SCF the additional channels for dissipation of excitation energy connected with AD and vacancy-type defects.     

Magnetic relaxation in spinel Mo-ferrite and Ti substituted Mo-ferrite

A compensation temperature of 138 K was observed in the temperature-dependent magnetization curves of MoFe₂O₄. Relatively slow magnetization relaxation characterized the transitions between different spin states (compensated and uncompensated). Large magnetic after effect was found in time-dependent magnetization curves after heating or cooling from different characteristic temperatures for different spin states. The magnetic relaxation was nearly independent on magnetic field, supporting the presence of spin states and no involvement of domain structure. For the Ti substituted Mo_0.6Ti_0.4Fe₂O₄ sample, there were a compensation at ∼ 100 K and a maximum of magnetization at ∼ 175 K. Similar results of anomalous magnetic relaxation was observed in Ti substituted Mo-ferrite (Mo_0.6Ti_0.4Fe₂O₄). If the Mo_0.6Ti_0.4Fe₂O₄ sample was heated from 100 K to 235 K, the time-dependent magnetization curve could be considered as a combination of two magnetic relaxation processes. However, if the sample was heated from 100 K to 295 K, the time- dependent magnetization curve became complex. Received 30 October 2001 and Received in final form 21 January 2002     

Synthesis and laser patterning of Bi-doped Y3Fe5O12 crystals in germanosilicate glasses

New germanosilicate glasses giving the crystallization of yttrium iron garnet Y₃Fe₅O₁₂ (YIG) and Bi-doped YIG, 23Na₂O-xBi₂O₃-(12−x)Y₂O₃-25Fe₂O₃-20SiO₂-20GeO₂ (mol%), are developed, and the laser-induced crystallization technique is applied to the glasses to pattern YIG and Bi-doped YIG crystals on the glass surface. It is clarified from the Mössbauer effect measurements that iron ions in the glasses are present mainly as Fe³⁺. It is suggested from the X-ray diffraction analyses and magnetization measurements that Si⁴⁺ ions are incorporated into YIG crystals formed in the crystallization of glasses. The irradiations (laser power: 32-60 mW and laser scanning speed: 7 μm/s) of continuous wave Yb:YVO₄ fiber laser (wavelength: 1080 nm) are found to induce YIG and Bi-doped YIG crystals, indicating that Fe³⁺ ions in the glasses act as suitable transition metal ions for the laser-induced crystallization. It is suggested that YIG and Bi-doped YIG crystals in the laser irradiated part might orient. The present study will be a first step for the patterning of magnetic crystals containing iron ions in glasses.     

Magnetic behavior and role of the antiphase boundaries in Fe3O4 epitaxial films sputtered on MgO (001)

Magnetite Fe₃O₄ films were grown on single crystal MgO (001) substrates using facing target sputtering technique. Conversion Electron M?ssbauer Spectroscopy and magneto optical polar Kerr spectra have confirmed the stoichiometric repartition of Fe cations corresponding to the inverse spinel structure and the electronic structure characteristic of bulk Fe₃O₄. Hysteresis loops carried out at room temperature show that, in a 1 T applied magnetic field, only 60% of the saturation magnetization is detected. This behavior is discussed in correlation to the antiphase boundaries (APBs) observed by electron microscopy. Magnetic force microscopy studies show that magnetic domains are larger than the mean distance between APBs. Received 2 July 2001     

Effect of lanthanum ions on magnetic properties of Y3Fe5O12 nanoparticles

Lanthanum ion (La³⁺)-substituted garnet nanoparticles Y_3?x La _x Fe₅O₁₂ (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) were fabricated by a sol–gel method. Their crystalline structures and magnetic properties were investigated by using X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Mössbauer spectrum. The XRD results show that samples of Y_3?x La _x Fe₅O₁₂ (0.0 ≤ x ≤ 0.8) are all single phase and the sizes of particles range from 32 to 65 nm. Those of Y₂LaFe₅O₁₂ consisted of peaks from garnet and LaFeO₃ structures. Compared to pure YIG, the saturation magnetization is larger when the La concentration x = 0.2. However, with increasing La concentration (x), it decreases obviously. Meanwhile, may be due to the enhancement of the surface spin effects, the saturation magnetization rises as the particle size is increased. Different from the pure YIG, the Mössbauer spectra of Y_2.8La_0.2Fe₅O₁₂ and Y_2.2La_0.8Fe₅O₁₂ are composed of four sets of six-line hyperfine patterns. The results tell us that the substitution of La³⁺ ions with large ionic radius (1.061 Å) will give rise to a microscopic structure distortion of the a- and d-sites to different degrees, and the Zeeman sextets from a- and d-sites begin to split into two sub-sextets, which is helpful to explain the phenomenon observed in the study of the magnetic property.     

Magnetostriction constants of epitaxial La,Ga: YIG films measured by microwave resonance

The magnetostriction constants of Y_2.85La_0.15Fe_3.75Ga_1.25O₁₂ epilayers have been measured by observing the shift of the resonance line of a thin film which is stressed by three-point bending. The result is λ₁₁₁=−(0.75±0.15) × 10⁻⁶ and λ₁₀₀=−(0.4±0.1) × 10⁻⁶ which is in agreement with the measurements on bulk single crystals. This result indicates that there is no growth induced contribution to the magnetostriction in thin films of this garnet grown by liquid phase epitaxy.     

Damage induced in garnets by heavy ion irradiations: a study by optical spectroscopies

The damage induced by heavy-ion irradiation has been studied in yttrium iron garnet (Y₃Fe₅O₁₂ or YIG) films, doped with Ca, Tb and Tm, grown by liquid-phase epitaxy on gadolinium gallium garnet (Gd₃Ga₅O₁₂ or GGG) substrates. Irradiations of doped-YIG epitaxial films and GGG substrates with 36-MeV ¹⁸³W and 12-MeV ¹⁹⁷Au ions were applied for fluences between 1 × 10¹³ and 3 × 10¹⁵ cm^–2 near room temperature. The radiation damage was monitored by micro-Raman spectroscopy and UV–visible optical absorption spectroscopy. Raman spectra revealed that amorphisation was achieved in YIG for both ions, whereas a high lattice disorder was induced in GGG without reaching amorphisation for the Au ion irradiation. Raman spectra also showed that a major damage of the tetrahedral sites was induced in GGG, as previously found for YIG. It is concluded that with such ions reaching the stopping power threshold of track formation in YIG and GGG the observed rate of amorphisation may result from a combination of electronic and nuclear energy losses as calculated using the unified thermal spike model.     

Conversion-electron Mössbauer study of sputtered Bi3Fe5O12

Temperature dependence of the magnetization measurements and Conversion Electron Mössbauer Spectroscopy (CEMS) have been performed on sputtered Bi₃Fe₅O₁₂ (BiIG), Y₃Fe₅O₁₂ (YIG) and (Bi, Y)IG films. Under the effect of the diamagnetic dilution of the diamagnetic yttrium sublattice the magnetization of BiIG is decreased with respect to YIG. The hyperfine field at the tetrahedral iron sites is increased, indicating a decrease in the intra-sublattice exchange interaction rather than the decrease of the tetrahedral iron moment.     

Effect of composition disorder on the electron paramagnetic resonance spectrum of the thermally populated excited state of Er<Superscript>3+</Superscript> ions in YLuAG mixed garnets

The results of investigation of electron paramagnetic resonance of Er³⁺ ions in the thermally populated first excited state in (Y_{1 − x} Lu _x )₃Al₅O₁₂ (YLuAG) mixed yttrium-lutetium garnet single crystals (0 ≤ x ≤ 1) are considered. In composition-disordered YLuAG, a number of new (as compared to Y₃Al₅O₁₂ (YAG)) Er³⁺ paramagnetic centers are detected; these centers appear due to a change in the crystal field symmetry and magnitude upon isomorphic substitution of Lu³⁺ for Y³⁺ in the yttrium sublattice of garnets. The origin of new paramagnetic centers is established and their formation probability is calculated.     

Equation of state and high-pressure irreversible amorphization in Y3Fe5O12

The change of crystal structure in yttrium iron garnet Y₃Fe₅O₁₂ was studied at room temperature at high pressures up to ∼55 GPa by the x-ray diffraction technique in diamond anvil cells. At a pressure of about ∼50 GPa, a drastic change in the x-ray diffraction pattern was observed indicating the transition into an amorphouslike state. When the pressure was increased, the bulk modulus of YIG was found to be 193 ± 4 GPa. It was also found that the amorphous state was retained after decompression down to ambient pressure. From the shape of x-ray patterns in the “amorphous” phase, it was concluded that the local atomic structure consists of iron-oxygen FeO₆ octahedral complexes with disordered orientations of local axis and of randomly arranged others ion fragments with the overall Y₃Fe₅O₁₂ composition. For the amorphous phase, it was evaluated that the bulk modulus of FeO₆ octahedral complexes is about 260 GPa. The text was submitted by the authors in English.     

Tailored photoluminescence of YAG:Ce phosphor through various methods

Trivalent cerium Ce³⁺ (Ce) activated yttrium aluminum garnet Y₃Al₅O₁₂ (YAG) phosphor was synthesized by two methods: solid state reaction (SS), and combustion (CB) with urea, respectively. The crystallization and luminescent properties of the phosphors were studied. Factors influencing on the intensity of luminescence and the location of emission band of YAG:Ce, such as the type of flux used in SS, the reaction atmosphere, the concentration of activator, were investigated in detail. We accomplished red or blue shift of Ce emission band by a number of techniques in order to match with the variable emission wavelength of blue light emitting diodes. The change of emission in color coordinates was illustrated by chromaticity. Co-doping other rare earth ions with Ce³⁺ ions into YAG was attempted to increase the color rendering index.     

High-frequency excitations of stripe-domain lattices in magnetic garnet films

Magnetic garnet films of composition (Y,Bi)₃(Fe,Al)₅O₁₂ have been grown by liquid phase epitaxy on [111] and [110] oriented substrates of gadolinium gallium garnet. The domain wall resonance and the two branches of the domain resonance of periodic stripe domains are measured as function of the bias induction applied in the film plane parallel to the stripes. Resonance frequencies up to 7.5 GHz are observed. An improved version of the hybridization model is developed to describe these resonances. It turns out that hybridization of the domain resonance branches is determined by the cubic anisotropy for [111] oriented films, while for [110] oriented films coupling of the domain resonances is mainly caused by the orthorhombic anisotropy. The theoretical model is in excellent agreement with experiments, no fitting parameters are used. It is also used to derive the phase relation between the precessing magnetizations of neighbouring domains.     

Fabrication and magnetic properties of Ce1Y2Fe5O12 thin films on GGG and SiO2/Si substrates

Microstructure and magnetic properties of crystalline Ce₁Y₂Fe₅O₁₂ thin films prepared on GGG and on SiO₂/Si substrates by pulsed laser deposition were studied. The results show that highly textured Ce₁Y₂Fe₅O₁₂ film with (4 4 4) preferred orientation prepared on GGG (1 1 1) shows strong paramagnetism superimposed by a weak ferromagnetism. However, polycrystalline Ce₁Y₂Fe₅O₁₂ thin films on SiO₂/Si, which can only be obtained after post-annealing, show strong ferromagnetism with easy axis of magnetization lying in the plane of the film. With post-annealing temperature increasing, CeO₂ segregates from Ce₁Y₂Fe₅O₁₂; then YIG continues to be decomposed, forming Fe₂O₃. Consequently, the saturation magnetization of Ce₁Y₂Fe₅O₁₂ films decreases first and then increases correspondingly, which indicates that the magnetic properties of Ce₁Y₂Fe₅O₁₂ films are mainly related to the microstructure.     

Combinatorial thin film synthesis of Gd-doped Y3Al5O12 ultraviolet emitting materials

Multi-layer Y₃Al₅O₁₂:Gd thin films were deposited in a combinatorial fashion using radio-frequency magnetron sputtering. Initially, Y₃Al₅O₁₂ (yttrium aluminum garnet or YAG) films were deposited in a combinatorial fashion to optimize the Y and Al sputtering conditions. Subsequently, alternating layers of uniform Y₃Al₅O₁₂ and graded Gd were deposited and the film composition was homogenized by post-deposition 1000 °C 10-h thermal treatment. A composition range of 1–12 at% Gd was produced in two films. Ultraviolet emission with a peak wavelength at 312 nm was observed from the gadolinium ⁸S_7/2-⁶P_7/2 transition via cathodoluminescence (CL) excitation. The CL-induced intensity was found to be maximum at 5.5 at% Gd. PACS 81.15.Cd, 78.60.Hf, 02.10.Ox, 78.66.-w, 81.15.-z, 78.67.Pt     

Laser ceramics: Mechanisms of solid-state reactions

The role of twinning in synthesizing Y₃Al₅O₁₂ from Al₂O₃ and Y₂O₃ powders during milling and compacting is shown by the example of obtaining laser ceramics based on yttrium-aluminum garnet. Simulation of twins in aluminum and yttrium oxides revealed the possibility of forming fragments of the garnet structure near the twin boundary. It is concluded that process of solid-state reactions is mainly associated with twinning, which leads to local variation in the symmetry and/or phase transitions.     

Growth and characterization of cerium-substituted yttrium iron garnet single crystals for magneto-optical applications

This paper is concerned with the preparation and characterization of cerium-substituted yttrium iron garnets, which are known to be oxides having a large Faraday rotation effect. Using the improved flux method we successfully grew bulk single crystals of iron garnet doped with Ce³⁺ ions with maximum substitutions up to 0.349. Here we investigate different solution compositions for maximum Ce³⁺ substitution. The Faraday rotation and optical absorption spectra were measured in the near infrared region for different Ce³⁺ ion substituted iron garnets. The specific Faraday rotation of Ce_0.349Eu_0.195Y_2.456Fe₅O₁₂ was found to be 1430 deg/cm at a wavelength of 780 nm and –1280 deg/cm at 1150 nm. The Ce substitution prominently enhances the Faraday rotation effect, and Yb³⁺ and Eu³⁺ ions substituted for Y³⁺ in the dodecahedral sites of YIG can increase the concentration of Ce³⁺ ions, depressing the formation of nonmagnetic Ce⁴⁺ ions by charge compensation. Received: 24 January 2001 / Accepted: 2 March 2001 / Published online: 27 June 2001     

Effect of post-annealing temperature on the microstructure and magnetic properties of Ce:YIG thin films deposited on Si substrates

Amorphous Ce₁Y₂Fe₅O₁₂ (Ce:YIG) thin films deposited on single crystal Si(1 0 0) and thermally oxidized Si(1 0 0) substrates by pulsed laser deposition were annealed in the temperature range of 700-1000 °C in air. The annealing temperature dependence of microstructure and magnetic properties of Ce:YIG films was studied using X-ray diffraction combined with vibrating sample magnetometer. The results show that single phase of polycrystalline Ce:YIG thin films can be obtained by the post-annealing of as-deposited films at the temperature of 700 °C. However, two steps of phase segregation of Ce:YIG occur as the post-annealing temperature increases: at first, Ce:YIG is decomposed into YIG and non-magnetic CeO₂ when annealed at 800 °C; then YIG continues to be decomposed forming Fe₂O₃ when the temperature is increased up to 900 °C. Consequently, the saturation magnetization of Ce:YIG films decreases first and then increases with the post-annealing temperature going up, which indicates that the saturation magnetization of Ce:YIG films is mainly related to the phase composition of the films. Meanwhile, the presence of SiO₂ buffer layer can significantly enhance the saturation magnetization of Ce:YIG films.