Liquid phase epitaxy of magnetic garnets

In designing rare earth iron garnet compositions for magnetic bubble applications it is usually necessary to deal with no fewer than three cations and often as many as six. LPE has become the dominant process for making garnet films from PbO-B₂O₃-Fe₂O₃ fluxed melts. Flux constituents further increase the complexity of the design problem by introducing two more components. Fortunately, thermodynamics reduces the film growth problem to the simpler dimensions of a solid garnet crystallizing in a liquid fluxed melt. For epitaxy, the Pb impurity in films has a significant effect on the lattice match (fit) between a film and its substrate. A mismatch can influence film properties and the growth process as well. This factor is only of concern to the bulk crystal grower when seed crystals are employed. Garnet films can be grown on a variety of gallium garnets which have been pulled and prepared as polished wafers with high perfection only equalled in Si wafer preparation. Isothermal LPE growth is now uniformly practiced and is mainly responsible for the good reproducibility and uniformity of film products. Indeed, some of the most precise and reproducible crystal growth data ever assembled have originated from magnetic garnet studies. Interestingly, more practitioners seem to agree on what happens than on why it happens. Two kinetics models are compared by numerical methods. The relative merits of horizontal versus vertical plane dipping are discussed briefly in terms of both theory and engineering potentials. In summary, the presentation is concerned with the differences among us, the problems we face, and the apparent alternatives.     

Secondary ion mass spectrometry of compositional changes in garnet films

Secondary ion mass spectrometry (SIMS) has been used as the analytical method to obtain directly depth concentration profiles of Pb (Ca and Ga) in magnetic garnet films. The films had the general composition (Y,Pb)₃(Fe,Si,Ga)₅O₁₂ and were grown onto substrates of Ca-doped Y₃Fe₅O₁₂ by means of the LPE technique. The film-substrate interface was found to be reasonably sharp. For dipping without rotation, a transient layer of about 0.28 μm thickness, in which the Pb content changed appreciably, was found arising from the initial non-steady state growth regime. In the case of dipping with rotation, this transient layer was of the order of 0.05 μm and displayed much smaller changes in the Pb content. Changes were also found at the surface of the film due to withdrawal of the specimen from the melt.     

Liquid phase epitaxial growth of Ca,Ge-Substituted garnet films having magnetic bubbles 1.0–3.0 μm in diameter

Ca, Ge-substituted, magnetic garnet films which would support bubbles of 1.0–3.0 μm diameter were grown by isothermal deposition frm supersaturated PbO-B₂O₃ fluxes upon garnet films were yttriumm iron garnet Y₃Fe₅O₁₂, in which rare earths substituted for yttrium and Ge substituted for Fe. Ca was added to charge compensate the Ge. A typical composition of a 2 μm bubble film might be Y_{1.35Lu0.3Sm0.6Ca0.75Fe4.25O12}, but the compositions of the films were varied by modifying the composition of the growth solution and the growth conditions. The magnetic properties of the films were dependent upon film perfection and composition. Bubble mobilities ranged ≈300–1000 cm/sec-Oe. The films, whose thickness varied from 1.0–3.0 μm to match buble diameters, were uniform in thickness to less than 0.30 μm across a 2.54 cm diameter slice. The properties of the films were sufficiently stable with temperature from 0–100°C to indicate that these materials could be used in memory devices.     

On the growth kinetics of Y3Fe5O12: Bi films grown from Bi2O3?PbO?B2O3 melts

LPE isothermal growth of thin films of Y_3−xBi_xFe₅O₁₂ from PbO B₂O₃ Bi₂O₃ melts was used as a tool for studies of structure of high temperature solutions (HTS). Using an anionic model of HTS enthalpy and entropy of dissolving of the garnet phase, the equilibrium constant of Bi₂O₃ dissociation and diffusion coefficients of the growth unit were calculated from experimental data.     

Controlled lattice constant mismatch by compositional changes in liquid phase epitaxially grown single crystal films of rare earth yttrium iron gallium garnets on gadolinium gallium garnet substrates

Liquid phase epitaxy (LPE) by dipping substrates into supercooled fluxed solution provides a convenient way of changing the melt and film composition. The apparatus, method and composition of the melts that we use will be described in detail. The composition of the layer is essentially Y₃GaFe₄O₁₂, which has a smaller lattice constant than the Czochralski-grown gadolinium gallium garnet substrate. By substitutions such as gadolinium, samarium or lanthanum for yttrium the lattice constant mismatch can be controlled. The effect of these substitutions on film properties such as cracks, stresses and magnetic domain pattern as well as the interaction of substrate defects such as dislocations with the epitaxially-grown film are discussed.     

New flux systems for the LPE growth of thin garnet films

A new flux system is presented for the growth of bismuth substituted iron garnet thin films by liquid phase epitaxy. The fluxes do not contain lead and are based on the system Bi₂O₃, RO₂ where R = Si, Ge, Ti or Ce. Data is presented that shows the viscosities of these melts are below 16 cp at temperatures ∽800°C when the eutectic compositions are chosen and that they have low freezing points. The growth conditions and melt compositions are given for the growth of various rare-earth iron garnets with high bismuth substitutions. Results of measurements of optical and magnetic properties of some of the from Pb containing fluxes. Thus these materials are more promising for use in magnetic bubble displays. It is also shown that the uniaxial anisotropy was also less in the films grown from the new system than those grown from Pb containing fluxes.     

LPE growth of YLaTm and YLaEu garnet films

The growth of films of the composition Y_3?(x+yLa_xR_yFe_5?zGa_zO₁₂, where R = Tm or Eu, on GGG substrates by liquid phase epitaxy from a PbO-B₂O₃ fluxed melt is described. Highly uniform, low defect density films have been grown by means of horizontal dipping using intermittent substrate rotation in conjunction with a novel substrate holder. A furnace with three separate temperature zones was used to optimize the vertical temperature profile. Incorporation of La into the films allows compositions containing Tm to be made whose lattice parameters are matched to GGG and compositions which contain Eu to be made with smaller Eu concentrations than were previously possible. The compositions grown, which contain only trivalent cations, exhibit growth induced anisotropy, high mobility, low coercivity, and sufficient magnetostriction to allow ion implantation to effectively suppress hard bubble formation. Detailed measurements of saturation temperature as a function of oxide concentration show that the La ion contributes differently to the garnet phase than do other commonly used rare earth cations. The kinetics of melt equilibration have been studied and procedures to equilibrate the melt to ensure reproducible film growth are described. Electron microprobe analysis has been used to determine the La distribution coefficient as a function of La concentration, the growth rate of the film, and the melt temperature.     

LPE growth kinetics of CaGe-Substituted EuTm2Fe5O12 garnet films

The diffusion-reaction growth melts fluxed with PbO-B₂O₃-Fe₂O₃, has been extended to include a thermally-activated diffusion term. The fluxed melt had a lower PbO : B₂O₃ ratio (11 : 1) than that generally used (≈16 : 1) for garnet LPE films. Measured growth rates for LPE films, produced from melts with four different concentrations of CaGe-Substituted EuTm₂Fe₅O₂ garnet, fit the model with: diffusivity, $\text{D (cm}{}^2\text{/s) = 0.06 exp(}\text{-1.0 eV}\text{kT}\text{)}$; reactivity, $\text{K (cm/s) = 8000 exp(}\text{-1.61 eV}\text{kT}\text{)}$; viscosity, $\text{v (}\text{cm}{}^2\text{s}\text{) = 0.01}$; and concentration, $\text{c}{}_{\mathrm{e}}\text{(}\text{g}\text{cm}{}^3\text{) = 4220 exp ?}\text{1.0678 eV}\text{kT}$ for rotation rates ω (rpm) = 36 to 196 in the growth temperature range 890 to 965°C. It is shown that the garnet melt concentration terms are the most critical ones in determining the growth rate, hence the activation energy (heat of solution) for the equilibruim concentration is reported to five significant figures. Evaluating the D and the K effects by both a numerical (results given above) and an analytic method demonstrates that these parameters are least critical in determining the growth rate in this case.     

Y2O3-Al2O3-Nd2O3 phase diagram and the growth of (Y,Nd)3 Al5O12 single crystals

The optimum compositions of the melts used for the growth of yttrium-aluminum garnet (YAG) single crystals with different neodymium contents are determined using the phase diagram of the ternary system Y₂O₃-Al₂O₃-Nd₂O₃ with the binary sections Y₃Al₅O₁₂-Nd₂O₃ and Y₃Al₅O₁₂-Nd₃Al₅O₁₂. A number of melt compositions characterized by one garnet phase, namely, (Y,Nd)₃Al₅O₁₂, are established. Single crystals of yttrium-aluminum garnets with a high content of the activator (up to 2.6 wt % Nd) are grown by the Czochralski method. __________ Translated from Kristallografiya, Vol. 48, No. 5, 2003, pp. 945–949. Original Russian Text Copyright ? 2003 by Soboleva, Chirkin. Dedicated to the 60th Anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences     

On the Formation of the Garnet Phase in Oxidic High-Temperature Solutions

The paper informs on the formation of the garnets Y₃Fe₅O₁₂, Y₃Al₅O₁₂ and Gd₃Ga₅O₁₂ as primary phase in the ternary solvent mixtures PbO PbF₂ B₂O₃ and PbO MoO₃ B₂O₃. There are correlations with formerly measured oxygen ion concentrations (OIC) in the solvents:

• i each garnet grows in a special region of the OIC, the phase boundaries are parallel to the lines of constant OIC and
• ii the garnet solubility increases distinctly with decreasing OIC. This emphasizes the important role of the oxygen ions for the garnet formation.

Solubility models are discussed taking into consideration both previous and present results. We conclude that the garnet formation is described best by a model reaction of the type RE rare earth elements, Y; M Al, Fe, Ga.     

Spectroscopy of Rare-Earth Elements in Single-Crystal Films of Yttrium-Aluminium Garnets (YAG)

The utilization of liquid-phase epitaxy for growing thin yttrium-aluminium garnet films is discussed. By transfer method from lead solvents films were obtained with a composition Y₃Al₅O₁₂: Nd³⁺ on substrates with composition Y₃Al₅O₁₂, and films Y₃Al₅O₁₂: Er³⁺, Y₃Al₅O₁₂: Er³⁺, Ga³⁺ (about 40 wt% of erbium) on substrates Y₃Al₅O₁₂: Nd³⁺ Characteristics are given for simultaneous induced emission generation of the system film-substrate at 77 K Er³⁺ ion (λ_gen = 1.6602 μ) being a component of the film, and Nd³⁺ ion, being a component of the substrate (λ_gen = 1.061 μm). The results obtained are discussed.     

A rietveld study of the cation substitution between uvarovite and yttrium-aluminum synthetic garnets,obtained by sol-gel method

A series of synthetic garnets solid solutions is compositions between Y₃Al₂Al₃O₁₂ (Y-Al garnet) (YAG) and Ca₃Cr₂Si₃O₁₂ (uvarovite) was synthesized using the sol-gel method. The expected general formula is (Ca_xY_1-x)₃ (Cr_xAl_1-x)₂ (Si_xAl_1-x)₃O₁₂ where x = 0 to x = 1. The cation distribution in dodecahedral, octahedral and tetrahedral sites and bond distances in these synthetic garnets were determined using the Rietveld method. It shows the incomplete substitution in small sites. The smaller site the smaller substitution there is.     

Ab initio calculation and spectral properties of nano- and bulk materials

This paper presents the development of ab initio calculation of the electronic structure of either clusters, nano-crystals, doped and unperfected bulk crystals. In addition, analysis of selected experimental data for γ- or plasma irradiated pure and doped wide-band gap oxides such as sapphire, α-Al₂O₃, garnet, Y₃Al₅O₁₂, Gd₃Sc₂Al₃O₁₂ and perovskites YAlO₃, SrTiO₃ is presented. Change in the crystals surface morphology and spectroscopic properties of sapphire, perovskites, garnets as well as ion oxidation state in pure and doped γ- and plasma irradiated crystals are discussed in detail using the optical and X ray spectroscopy experimental results.     

Untersuchungen zum Kristallwachstum in der Hochtemperaturlösung mit Hilfe der differentiellen Thermoanalyse

A DTA-apparatus is described which is suitable for solubility measurements of garnets in flux melts. It is important that the temperature will be measured as near as possible to the crystallization region. Solubility temperatures are measured with an uncertainty ≦ 10 deg. A mixture of PbO, PbF₂ and B₂O₃ was used as solvent. The following garnet systems were investigated: Y₃Fe_5−xM_xO₁₂ (M = Ga, Al, In, Cr; x = 0 …︁ 5) and Gd₃Ga₅O₁₂. If results of other authors are available they are in good agreement with our data. Habit and chemical analysis of crystallites give informations about the grown phases and their composition. In connexion with the solubility thus it is possible to select appropriate solvents.     

Role of twinning in plastic deformation

A new concept of the structure of yttrium-aluminum garnet (Y₃Al₅O₁₂) has demonstrated the possibility of forming inversion twins in the (111) plane. The twinning processes are shown to be responsible for stress relaxation during the growth and plastic deformation of Y₃Al₅O₁₂ single crystals. This way of stress relaxation can be used to describe the plastic deformation of other crystals, at least cubic ones.     

Defect generation in LPE garnet films during annealing

The defects induced in LPE garnet films during annealing processes near 1300°C were investigated by X-ray topography, etching, scanning electron microscopy and Nomarski interference microscopy. The results show that besides loops and more complicated dislocation configurations point defects play an essential role for the stress relaxation mechanism in the film. The observed coarsening effects on the film surface are due to the high surface free energy of {111} faces on garnets. A consistent explanation of the various experimental observations is given by assuming the creation of oxygen vacancies during annealing.     

Determination of Concentration of Rare Earth Ions Yb3+, Er3+, Tb3+, Ho3+, Tm3+ in Monocrystals of Yttriumaluminum Garnets on Basis of Magnetic Susceptibility

Crystals of solid solutions (R_xY_1-x)₃Al₅O₁₂ (where R is rare earth ion Er³⁺, Yb³⁺, Tb³⁺, Ho³⁺, Tm³⁺) with garnet structure were grown. The temperature dependencies of magnetic susceptibility for these crystals were obtained. On the basis of measurement of magnetic susceptibility a non-destructive technique for determining the concentration of rare earth ions in yttrium-aluminum garnets was developed.     

Crystal growth of Ca3−xYxMn2Ge3O12 and phase relations in the PbO?B2O3?GeO2 based melts

The bulk crystals of Ca_3−xY_xMn₂Ge₃O₁₂ garnets were grown on a seed from the PbO B₂O₃ GeO₂ based melts, where the primary crystallization field of garnets was found. The saturation temperatures of the melts were determined before each growth run. The dependence of Y incorporation into the crystals versus the melt composition was also evaluated.     

The importance of the oxygen ion concentration for oxidic high-temperature solvents

Using e.m.f. measurements we determined relative oxygen ion concentrations of the systems PbO—PbF₂—B₂O₃, PbO—MoO₃—B₂O₃, PbO—Bi₂O₃—V₂O₅, 3 PbO · MoO₃—Fe₂O₃, and 3 PbO · MoO₃—Ga₂O₃. Furthermore the alteration of the relative oxygen ion concentration of pure PbO was observed after stepwise addition of the solvent components PbF₂, B₂O₃ and of the garnet forming oxides Fe₂O₃ and Y₂O₃ giving complete high temperature solutions for the Y₃Fe₅O₁₂ preparation. Applying the Lux' acid-base concept the behaviour of these oxide mixtures is discussed. Combinations which contain weak bases and weak acids allow the adjustment of a desired oxygen ion concentration that is little changed by added oxides for crystal growth. These combinations show favourable properties as solvents for oxidic substances with complicated compositions, e.g. perovskites, garnets, ferrites.     

X-ray quantitative determination of the phases in a batch for the synthesis of poly- and monocrystal yttriumaluminium garnet

For the synthesis of ceramics and single crystals of yttriumaluminium garnet, a batch consisting of Y₂O₃ and Al₂O₃ is first treated thermally in order to obtain the garnet phase Y₃Al₅O₁₂. It is possible to make a choice of the optimum technological parameters of the batch if the amounts of all oxide phases, present during the synthesis, are well known. A combined X-ray quantitative method is suggested for this purpose. The initial oxides and a garnet phase are determined by the method with external standard. The rest two impurity oxide phases are determined as binary system. The specific diffraction characteristics of the present compounds were taken into account. The amounts of all phases in the batch, treated thermally from 1373 to 2075 K at every 100 K are shown.