Pure Garnet Research Articles

Electronic-cluster model of yttrium iron garnets

We review electronic and magnetic properties of yttrium iron garnets, considered as a superlattice of quantum dots of two different types. Those are the tetrahedral and octahedral iron-oxygen clusters characterized by their respective localized energy levels with the orbital eigenstates, specified by the clusters' respective point symmetries. We discuss two-dot mechanisms of the superexchange coupling between spins, attached to the orbital states, via the $p\text{\ensuremath{-}}d$ tunneling, only (between clusters of different types), or via the $p\text{\ensuremath{-}}d$ tunneling combined with the next-nearest neighbor $p\text{\ensuremath{-}}p$ hopping (between clusters of the same type). The electronic density of states of a superlattice of the clusters is found and analyzed both for pure and valence-uncompensated garnet systems. A scheme of control of the superexchange coupling due to an external magnetic field, applied to the system is proposed.

Synthesis of garnet structure compounds using aqueous sol–gel processing

The sol–gel method based on metal chelates in aqueous solvents has been developed to prepare different oxides having garnet crystal structure. This synthetic approach has been used to prepare rare-earth doped yttrium aluminum garnet Y3Al5O12:Ce, Y3Al5O12:Nd, Y3Al5O12:Ho, and Y3Al5O12:Er samples (YAG:Ln). The polycrystalline powders sintered at 1000 °C are formed as single-phase garnet materials. The formation of pure and neodymium-doped lanthanum aluminum garnets (La3Al5O12 (LAG), and La3Al5O12:Nd (LAG:Nd)) at the same synthesis conditions, however, does not proceed. A systematic study of sol–gel technique synthesized Y3Ga5O12 (YGG) is presented using six different complexing agents. These complexing agents were found to influence the characteristics of the end products, in particular the homogeneity. Finally, some specific features of sol–gel derived mixed-metal Y3ScxAl5−x−yGayO12 (0⩽x, y⩽5) (YSAGG) garnets are discussed in the present paper. The phase purity, composition and microstructural features in the polycrystalline samples were studied by XRD analysis, IR spectroscopy and scanning electron microscopy.

Sol-gel preparation and electrical behaviour of Ln: YAG (Ln = Ce, Nd, Ho, Er)

A sol-gel method has been developed to prepare pure yttrium aluminum garnet Y3Al5O12 (YAG), and rare-earth substituted (Ce-Y3Al5O12, Nd-Y3Al5O12 Ho-Y3Al5O12, and Er-Y3Al5O12) samples. The XRD patterns of the polycrystalline powders sintered at 1000 ?C showed the formation of monophasic garnet materials. The micro-structural features in the polycristalline samples were studied by scanning electron microscopy. A homogeneous distribution of rare-earth do pants in the YAG lattice was achieved in all of the cases. Electrical conductivity measurements were also performed on the compacts of sol-gel derived Ln-YAG samples. In contrast to the expected dielectric behavior, the conducting properties of the examined specimens indicate metallic behavior with the resistivity increasing gradually with increasing temperature.

Sm–Nd disequilibrium in high-pressure, low-temperature Himalayan and Alpine rocks

In order to decipher the causes of Sm–Nd isotopic disequilibrium in high-pressure, low-temperature rocks, Sm–Nd isotopic analyses were carried out on minerals from four Himalayan (Tso Morari unit) and four Alpine (Dora-Maira, Monte Viso, Sesia Lanzo) eclogitic rocks of different lithologies and different intensities of post-eclogitic metamorphism. In most of these samples, garnets show the following striking features: either 147Sm/ 144Nd ratios lower than whole-rock, or 143Nd/ 144Nd ratios lower than whole-rock for higher 147Sm/ 144Nd ratios. In both cases, no age can be calculated. Two hypotheses can be proposed to explain the strong isotopic disequilibrium in garnet separates: (1) the preservation of the isotopic signature of the plagioclase from which garnet formed, (2) the occurrence in garnet of inherited sub-microscopic crustal inclusions with low 147Sm/ 144Nd and 143Nd/ 144Nd ratios. Knowing the Lu–Hf eclogitisation age for two samples, these two hypotheses are simulated: (1) the composition of plagioclase before eclogitization is recalculated from the measured garnet composition. The plagioclase/whole-rock age obtained with this simulation is geologically meaningless, showing that this hypothesis is invalid, (2) the 143Nd/ 144Nd ratio of sub-microscopic inclusions is calculated considering that the garnet separate, which was analysed, is a mixture between inclusions with low 147Sm/ 144Nd ratios (0.05–0.1) and pure garnet with a 147Sm/ 144Nd of 2. The inclusion/whole-rock ages recalculated for both samples suggest that inclusions have an old, crustal origin. The hypothesis of sub-microscopic inclusions in garnet can therefore explain the observed isotopic disequilibrium provided that the inclusions have a low Sm/Nd ratio, like allanite, apatite (and monazite), and that they are inherited from crustal contamination processes. In the case of a magmatic protolith, contamination can be achieved through crustal assimilation or a first stage of circulation of crustal fluid. In the case of Dora-Maira, the possible sedimentary protolith contains components from an old contaminant crust. In the Himalayan samples, the inverse correlation between the degree of retrogression in blueschist and then amphibolite facies, and the extent of disequilibrium between garnet and other mineral phases suggest a second stage of fluid circulation associated with retrograde metamorphism. This fluid circulation would result in the dissolution of these sub-microscopic crustal inclusions and partial leaching of their rare-earth elements (REEs) from the rock, which would scavenge the crustal 143Nd/ 144Nd isotopic signature from the rock.

Elasticity of the pyrope (Mg 3 Al 2 Si 3 O 12 )-majorite (MgSiO 3 ) garnets solid solution

Dense isotropic polycrystalline specimens of majorite-rich garnets (Py100, Py62Mj38, Py50Mj50, Py21Mj79 and Mj100) along the pyrope (Mg3Al2Si3O12 = Py100)-majorite (MgSiO3 = Mj100) join were fabricated in a 2000-ton uniaxial split-sphere anvil apparatus (USSA-2000) at pressures from 10 to 18.5 GPa and temperatures from 1200 to 1850 °C, within their stability fields in runs of 2–4-h duration, using hot-pressing techniques developed by Gwanmesia et al. (1993). These specimens are single-phased, fine-grained (≤5 mm), free of microcracks, and have bulk densities greater than 99% of the corresponding single-crystal X-ray density. Elastic compressional (P) and shear (S) wave velocities were determined at room pressure and temperature for these polycrystalline garnet specimens by phase comparison ultrasonic interferometry. For Mj100, the P and S wave velocities are within 1% of the Hashin-Shtrikman averages calculated from the single crystal elastic moduli measured by Brillouin spectroscopy. Both the elastic bulk modulus (K) and the shear modulus (G) decrease continuously with increasing majorite content from pyrope garnet (Py100) to pure majorite garnet (Mj100). The compositional dependence of K and G are given by K = 172.3 (40) − 0.085X, and G = 91.6 (10) − 0.038X, where X = mol% majorite), respectively, indicating that substitution of Si for Mg and Al decreases both K and G by about 5% along the solid solution series.

Colour centres investigation in pure and doped yttrium aluminium garnet epitaxial films

Epitaxial films with thickness of 10–250 μm of yttrium aluminium garnet (YAG) doped with Cr were grown by liquid phase epitaxy technique on YAG: Nd substrateds. Co-doping with Mg2+ was used to force the Cr4+ valent state formation. Dependence of absorption spectra of obtained films on melt-solution composition, growth conditiions and thermal treatment in reducing and oxidizing atmospheres is studied. The absorption being characteristic for YAG:Cr4+ crystals is found in co-doped films grown at higher temperatures (1000–1100°C). The chromium entering in the tetravalent state is confirmed by the annealing experiments. A very intensive absorption band in UV region with maximum at 275 nm was found both in co-doped and YAG: Mg2+ epifilms caused probbly by oxygen vacancies compensating the excess charge of Mg2+. Its intensity correlates with Cr4+ content in the film in the following way: it decreases with Cr4+ entering in the film.

Influence of Cationic Substitutions in Na3Fe2(AsO4)3: Transition from the Garnet to the Alluaudite Structure

The substitution in Na3Fe2(AsO4)3of Na+by either Li+or Ca2+and a vacancy, is able to stabilize an alluaudite structure at the expense of either the garnet or the rhombohedral structure of the parent compound. In the system (Na1−xLix)3Fe2(AsO4)3, pure garnet phases are obtained at 700°C in the 0≤x≤0.333 composition range, whereas pure (or nearly so) alluaudite phases are obtained at 870°C in the 0.333≤x≤0.666 composition range (namely, from Na2LiFe2(AsO4)3to NaLi2Fe2(AsO4)3). In contrast, vacancies (as in (Na1−x□x/2Cax/2)3Fe2(AsO4)3) are hardly accepted in the garnet structure, whereas the range of formation of a pure alluaudite (at 870°C) is more extended (0.166≤x≤0.666), the last composition corresponding to Na□CaFe2(AsO4)3. The stabilization of an alluaudite structure at the expense of a garnet structure has already been observed in previous work and is discussed in relation with the structural role of lithium and vacancies.

Cosmogenic 3He Accumulation in Alluvial Garnets in Bedload from Central Nepal Rivers: Implications for Cosmic-Ray Exposure Dating of Himalayan Erosional Processes

Cosmogenic isotopes, such as 3He, l~ 26A1, are continuously producted at the Earth's surface in rocks and soils by cosmic ray spallation of light isotopes, providing a powerful tool for dating young geomorphological processes. The datation method based on the accumulation of cosmogenic 3He is particulary appealing because: 3Hec (where suffix c refers to 3He produced by cosmic rays) is a stable isotope, its production rate is high and its natural background is very low. Moreover, its analysis is sensitive, relatively simple and rapid. However, the applicability of this method is limited by the high diffusivity of 3He. In practice, 3He~ is quantitatively retained in olivine phenocrysts, allowing datation of young basaltic flows. Dung/ and Roselieb (1996) have argued that the diffusivity of helium is lower in garnet than in olivine. The use of garnet for 3He c dating presents the interesting advantage to extend the field of this method to crustal metamorphic domains, and therefore to the case of major orogens. We have measured3Hec in alluvial garnets sampled in bedload from Central Nepal rivers (Fig. 1). They represent 3 different monolithologic catchments. By analysing fractions of several tenths of grains, we integer the complex history of each individual grain and the results obtained are mostly meaningful for comparison between different positions in a river basin. The garnets were formed in gneisses of the High Himalaya Crystalline (HHC) and schists of the Lesser Himalaya (LH) during high grade metamorphism resulting from the Main Central Thrusting (MCT) during Miocene. In this system, uplift rates estimated using the Ar/Ar method on micas, feldspars and monazite in garnet are high between 2 and 4 mm/yr depending on the position with regard to the MCT (Copeland et al., 1991). For 3Heo dating purpose, pyropes and almandines are more appropriate than grossularites et spessartites (Dung/and Roselieb, 1996). Electron probe analysis showed that the mean composition of the garnets was Aim. 70%, Pyr. 15%, Gros. 7%, Spes. 5%, Andr. 3% except for sample MO38 wich showed a larger contribution of the grossular end-member (Alm. 54%, Pyr. 12%, Gros. 26%, Spes. 6%, Andr. 2%). Helium was extracted using a CO2 laser from mgsized pure garnet fractions and its amount and isotopic ratio measured with a high sensitivity rare gas mass spectrometer in Nancy (SHe = 3.5 • 10 -4 A/Torr, He blank = 2 x 10 15 mol). Helimn-3 present in minerals is the sum of the following potential contributions: 3Her 3He c + 3Hea + 3Hen + 3He m (where suffixes t, a, n and m refer to total, atmospheric, nucleogenic, and mantle-derived). The parent rocks of metamorphic garnets are thought to be marine sediments, mostly turbidites, which makes the possible contribution of mantle-derived 3He m unlikely. In agreement with that, He isotopic ratios measured in hot springs along the MCT are show typical radiogenic values (Marty et al., 1996). 3Hea was estimated from the measured 2~ content. The contribution of nucleogenic 3He in garnet, produced by activation of Li by natural thermal neutrons, was computed from ion probe analysis of Li in garnet and from U and Th analysis &garnets (1CP-MS), or from estimate of U, Th content of parent rocks. In all cases the atmospheric and nucleogenic 3He contributions were found to be small with respect to the total 3He content of the samples.

The Arsenates NaCa2M2+2(AsO4)3(M2+=Mg, Ni, Co): Influence of Cationic Substitutions on the Garnet–Alluaudite Polymorphism

Cationic replacements in the arsenates NaCa2M2+2(AsO4)3(M2+=Mg, Ni, Co) have a strong influence on their garnet–alluaudite polymorphism. The preferential formation of an alluaudite phase (at the expense of the garnet phase) is induced by the following cationic modifications: (i) The replacement of Na+by larger cations such as Ag+, K+, or Tl+. The cation distribution is partly disordered in the Na and Ag compounds and ordered in the K and Tl compounds. (ii) The formation of vacancies through the (Na+or Li+)–Ca2+replacement, namely (Na or Li)1−x□x/2Ca2+x/2Mg2(AsO4)3. The formation of a pure defect garnet phase is limited to smallxvalues (x=0.1 or 0.2) whereas a full series of defect alluaudite phases (terminating at Ca2.5Mg2(AsO4)3,x=1) can be obtained. (iii) The replacement of Ca2+by Mg2+or Co2+, leading to the new alluaudite-like compounds NaCaMg3(AsO4)3, NaCaCo3(AsO4)3, and NaCo4(AsO4)3. Most of these results can be explained by the existence, in the alluaudite structure, of three crystallographic sitesX(2),X(1), andM(1), which are respectively larger, approximately equal to, or smaller than the dodecahedral sites of the garnet structure. A garnet–alluaudite transformation has not been observed in the corresponding vanadates, in which the garnet structure appears very stable.

Densification behaviour of Y/sub 3/Fe/sub 5/O/sub 12/ materials prepared by microwave sintering process

The microwave sintering (ms) process is observed to be superior to conventional sintering (cs) in the densification of Y/sub 3/Fe/sub 5/O/sub 12/ materials. A density of 97.3% theoretical density (T.D.) with pure garnet phase and small grain size (5/spl sim/7 /spl mu/m) was obtained for materials microwave sintered at 1300/spl deg/C for 20 minutes. By contrast, only 94.3% of T.D. can be reached for materials sintered by conventional processing at 1425/spl deg/C for 8 h and the grains are large (/spl sim/25 /spl mu/m). A higher densification rate in the microwave sintering process, as compared with that in conventional sintering processing, is ascribed to the lower activation energy of densification (i.e., (E/sub a/)/sub ms/=83.3 kcal/mol and (E/sub a/)/sub cs/=163.2 kcal/mol). On the other hand, small grain size in microwave sintered samples is attributed to the fact that the (E/sub a/)/sub ms/-value is smaller than the activation energy of grain boundary migration (E/sub g/=94.8 kcal/mol).

Spin reorientation in substituted iron garnets investigated by Mössbauer spectrometry

Substituted iron garnets exhibiting very high magnetic permeability properties were investigated by 57Fe Mössbauer spectrometry. The temperature dependences of the hyperfine field and the quadrupole shift at the different iron sites suggest the existence of a distribution of spin reorientation transitions, distribution attributed to the cationic substitution. This phenomenon which is in qualitative agreement with permeability measurements, is attributed to the introduction of scandium and terbium cations in the iron and yttrium sublattices, respectively. The hyperfine data are consistent with a gradual change of the easy magnetization axis from the [100] to [111] directions with increasing temperatures. The cationic distribution resulting from the octahedral and tetrahedral iron site occupancies differs from that encountered in pure iron garnets.

Spin dependent EXAFS in pure metals and garnets

Magnetic EXAFS are studied using circularly polarized synchrotron radiation in the energy range of 6000–9000eV at the L-edges of pure Gd metal, the L 2,3-edges of Eu in Eu 3Fe 5O 12 and the K-edges of Fe, Co and Ni metal. On the basis of a two-step model the dichroic absorption effects are described in terms of an additive contribution to the backscattering amplitude caused by the exchange interaction, which is directly correlated to the spin-moment of the neighboring atom. The damped low-frequent background found in the dichroic L 2,3-spectra of Gd and Tb metal is interpreted as an atomic x-ray fine structure of strong magnetic character.

Effect of bismuth doping on thermal expansion and misfit dislocations in epitaxial iron garnets

The growth of high quality, bismuth-substituted, rare earth iron garnet epitaxial films on rare earth gallium garnet substrates is limited by the effects of stress between the film and substrate. Thick films in particular have a low tolerance for lattice mismatch at room temperature, so the film and substrate must be closely matched to prevent cracking. However, there is a difference in thermal expansion between rare earth iron garnets and rare earth gallium garnets that is increased by bismuth substitution in the epitaxial film. This extrapolates to a thermal expansion for pure bismuth iron garnet of 12.9 X 10 -6°C -1, which is 37% greater than that of rare earth gallium garnets and 26% greater than that of pure rare earth iron garnets. Consequently the thermal expansion mismatch at the growth temperature is increased, which requires that, to maintain a lattice match at room temperature, the films must be grown highly in compression. The critical misfit strain required to form misfit dislocations during film growth also decreases with increasing bismuth content and extrapolates to zero at a bismuth concentration of 2.6 atoms per formula unit. These effects combine at the film growth temperature of 800°C to create a limiting value of the bismuth content, beyond which it is difficult or impossible to grow high quality dislocation-free epitaxial films.

Ferromagnetic resonance in epitaxial iron-garnet magnetically coupled layers

We present results of ferromagnetic resonance investigations of the double-layer magnetic films. The epitaxial structures examined are formed by a layer of doped garnet (YEuTmCa) 3(FeGe) 5O 12 with the easy axis along the surface normal and a sub-layer of pure yttrium-iron garnet Y 3Fe 5O 12 with easy plane anisotropy. Peculiarities are revealed in the dependences of the FMR resonance fields and the line width on the angle between magnetic field and the surface plane. These features are due to the interlayer magnetic coupling. The temperature range is determined in which the coupling effects in high-frequency features of these films can be studied.

Magnetostatic backward volume wave-base-guided-wave magnetooptic Bragg cells and application to wide-band light beam scanning

The realization of high-performance magnetostatic backward volume wave (MSBVW)-based magnetooptic (MO) Bragg cells in pure and bismuth-doped yttrium iron garnet-gadolinium gallium garnet (YIG-GGG) waveguides is discussed, and the demonstration of wide-angle light beam scattering using such MO Bragg cells is reported. The MSBVW-based MO Bragg cells have been shown to possess the following advantages over the magnetostatic forward volume wave (MSFVW)-based counterparts: (1) the center frequency of the former is higher by some 4.9 GHz at the same DC magnetic field; (2) the first passband in which the MO Bragg cells commonly operate is located at the high-frequency end for the former, but at the low-frequency end for the latter; and (3) the Bragg diffraction or mode-conversion efficiency of the MSBVW-based MO Bragg cells can be significantly higher than that of the MSFVW-based counterparts.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Coprecipitation of yttrium and aluminium hydroxide for preparation of yttrium aluminium garnet

Coprecipitation of yttrium and aluminium hydroxide for the preparation of pure yttrium aluminium garnet (YAG) powder with small grain size is the subject of this study. Starting materials are sulphates and chlorides of yttrium and aluminium. To obtain pure YAG (Y 3 Al 5 O 12 ), the pH during flocculation of the precursor must be chosen carefully. The presence of water increases the degree of agglomeration. To minimize agglomeration, the influence of dispersion liquids has been studied, leading to optimized conditions for precipitation. Das Thema dieser Arbeit ist die Kopräzipitation von Yttrium und Aluminium zur Darstellung von reinem Yttrium-Aluminium-Granat-Pulver mit kleiner Korngröße. Ausgangsmaterialien sind die Sulfate und Chloride von Yttrium und Aluminium. Um reines YAG (Y 3 Al 5 O 12 ) zu erhalten muss der pH-Wert während der Fällung des Zwischenproduktes sorgfaltig gewählt werden. Die Anwesenheit von Wasser hat einen nachteiligen Effekt auf den Grad der Agglomeration. Um diesen nachteiligen Effekt beseitigen zu können wird der Einfluß verschiedener Dispersionsflüssigkeite untersucht. Damit ist es möglich geworden die optimale Bedingungen für die Fällung festzustellen. On décrit la coprécipitation d'yttrium et d'aluminium pour obtenir une poudre d'yttrium aluminate très fine et de haute pureté. Comme materiaux de départ sont employés des sulphates et des chlorures d'yttrium et d'aluminium. La préparation de YAG (Y 3 Al 5 O 12 ) pur demande un choix précis du pH pendant la floculation de produit intermédiaire. En ce qui concerne l'hydrolyse, la quantité relative d'eau détermine le degré d'agglomérisation. L'influence des liquides de dispersion différentes est étudiée ce qui a conduit aux conditions optimalisées pour la floculation.

Elasticity of grossular and spessartite garnets by Brillouin spectroscopy

The single‐crystal elastic moduli of natural samples of grossular (99.0 mol % Ca3Al2Si3O12) and spessartite (94.8 mol % Mn3Al2Si3O12) have been measured by Brillouin spectroscopy under ambient conditions. From these results the adiabatic bulk moduli Ks and shear moduli μ are calculated to be Ks = 168.4 ± 0.7, μ = 108.9 ± 0.4 for grossular, and Ks = 178.8 ± 0.8, μ = 96.3 ± 0.5 for spessartite (all in units of gigapascals). A calibration of our spectrometer by determination of the elastic properties of MgO indicates that these modulus values are accurate to within twice the stated rms error. Of the major rock‐forming silicate garnets, spessartite has the largest bulk modulus, and grossular has the largest shear modulus, with the possible exception of majorite (a high‐pressure form of pyroxene). Approximate values for the bulk and shear moduli of common aluminosilicate garnet solid solutions can be obtained with an estimated uncertainty of less than 3% from a linear molar average of the end‐member properties. The elastic properties of pure almandite garnet (Fe3Al2Si3O12), which have thus far not been measured, are estimated as Ks = 177 ± 3 and μ = 97 ± 1 GPa by a linear regression analysis of extant measurements on garnet solid solutions and end‐members. These same data imply, however, that a linear modulus‐composition relationship may not be appropriate for all silicate garnet compositions.

Specific heat of pure and yttrium-doped dysprosium aluminum garnet (DAG) powder near TN.

We have measured the specific heat of both pure and yttrium-diluted powder specimens of the Ising-like three-dimensional antiferromagnet dysprosium aluminum garnet (DAG) near ${T}_{N}$. No unusual behavior is found as a result of varying yttrium concentration but pronounced rounding of the peaks is observed even for the pure powder. These results are in qualitative contradiction with recent neutron scattering measurements where sharp critical behavior is observed in the same temperature region and for the same sample as used in the present study. A simple-finite-size rounding mechanism does not explain the observed discrepancy.

MAGNETOOPTICAL ROTATION OF PrIG AND NdIG

Bulk single crystals and epitaxial films of Pr3Fe5O12 and Nd3Fe5O12 have recently been grown for the first time by Fratello et al. Magnetooptical (Faraday) rotation of these two compositions has been measured at room temperature over the wavelength range 0.5 to 2.5 μm. Of all the pure rare earth iron garnets, only the rotation of Pr3Fe5O12 approaches that of the Bi containing garnets now being used in magnetooptical devices.

Magnetostatic wave devices for UHF band applications

Magnetostatic surface wave (MSSW) devices made with pure and gallium-substituted yttrium-iron garnet (Ga:La-YIG) films are described. These devices include nondispersive and dispersive delay lines, band-pass filters, oscillators, and resonators. By controlling the magnitude of the bias magnetic field and the temperature of operation, it is possible to tune these devices over a wide frequency range extending from 0.3 to 4 GHz and from 3 to 18 GHz using Ga:La-YIG and pure YIG films, respectively.