J.M. Robertson

Growth mechanisms and composition in the LPE process for bubble domain materials

A review is presented of some recent results obtained in the LPE growth of magnetic garnet thin films for bubble domain applications by means of vertical dipping and without rotation of the substrate, in a PbO and B 2 O 3 flux. The design of a furnace in which films of the composition Y 3− x La x Fe 5− y Ga y O 12 have been grown reproducibly is described. The homogeneity of the composition of the layers is discussed with respect to film thickness. A clear relation has been found between changes in the lead content and the growth rate for films of Y 3 Fe 5 O 12 . The stoichiometry of these layers is also found to change with lead incorporation, depending on the position of the LPE melt in the phase diagram. Studies of the growth kinetics of Y 3 Fe 5 O 12 films as a function of supercooling, growth temperature and lattice misfit have shown the existence of a transient period of 5–10 min during which the compositional variations take place.

Lead incorporation in thin iron garnet films produced by LPE and chemical analysis by the radioactive tracer technique

The mode of incorporation of lead ions into Y3Fe5O12 single crystal thin films grown by the LPE technique from lead containing fluxes has been carefully studied using radioactive tracers as the analysis technique. In particular, the variation of lead content with film thickness has been studied. It has confirmed the existence of a lead rich layer at the interface of the substrate and the film. The formation of this layer is connected with the transient period during which stable growth is established. The variation of lead with film growth rate is established. The misfit with the substrate lattice parameter has an influence on the grown film, whereas the species of the substrate does not have an effect. There is evidence that rare-earth ions enter the octahedral sites in the garnet.

Strain induced effects in the LPE growth of garnets

Abstract Lattice strains due to the film-substrate lattice misfit and to the incorporation of substituting ions are shown to affect both the rate of growth and the segregation coefficients of solute constituents present during the growth of thin epitaxial layers of substituted iron garnets from PbO-B 2 O 3 fluxes. The previously derived theories are extended to cover the cases in which large amounts of substituents are incorporated and where consequently the mixing entropies and the changes in lattice size cannot be neglected. It is also shown that even with substrate rotation rates of 180 rpm, solute diffusion can produce significant changes in effective segregation coefficients.

Liquid phase epitaxy of garnets

Abstract A short introduction to the technique of liquid phase epitaxy of single crystal thin films of magnetic garnets is given with the emphasis on the relative merits of vertical and horizontal dipping. For vertical dipping results are given for the thickness profiles from top to bottom of the samples and the inhomogeneities within the film thickness. The thickness profile is explained by fluid motion caused by density segregation. The inhomogeneities are explained by growth rate changes due to transient growth at the beginning and nearly steady state growth when the diffusion boundary layer has been formed. It is shown under special conditions these inhomogeneities in composition in LaGa : YIG films can be an advantage and can increase the domain wall velocity. These effects are compared to films grown by horzontal dipping with rotation. Using spin wave resonance it is shown that this process does not produce as homegeneous films as expected. Explanation of this is given in terms of fluid flow during the growth. Because literature data on the diffusion coefficient D has a wide spread of values, an experiment is described in which D has been measured independent of the model using radioactive tracers. These results, combined with the model that the garnet does not diffuse as a molecule but as species like Y2O3 and Fe2O3 and at the growing interface react to form garnet, has lead to a new insight into the solvation of these species in the liquid. Magnetic bubble films of the type CaGe substituted iron garnets are used in large numbers for devices. This makes the reproducibility an important factor. To increase the yields of usuable material it has been found necessary to make the magnetic parameter Hcollapse independent of the growth temperature because of the thermal constants in LPE furnaces. This has been achieved by combining CaGe melts with Ga. Results are given for a 3 μm bubble material. The LPE technique can be used for non-magnetic garnet materials as well as iron garnets. These materials are usually substituted with rare-earths. The incorporation of Ce into Y3Al5O12 is a difficult case. How this can be achieved and difficulties encountered are explained in the last part of the paper.

Induced non-periodic growth striations in flux-grown magnetic oxide single crystals

Abstract A technique is described in which growth striations were introduced purposely into growing orthoferrite crystals by a specific temperature program. In this way it was possible to reveal the growth history of the single crystals; from the results, the size and shape of the crystals at various stages of growth were estimated. The nucleation temperature was calculated and a temperature program designed to obtain larger single crystals of high quality in a shorter time than with normal growth programs.

Calcium gallium germanium garnet as a substrate for magnetic bubble application

Czochralski-grown calcium gallium germanium garnet has been investigated as a possible substitute for gadolinium gallium garnet. The melting temperature of Ca3Ga2Ge3O12 (1380°C as compared with 1740°C for Gd3Ga5O12) allows the use of platinum as a crucible material. The compositional range of melts that form a single garnet phase on solidification was determined by differential thermal analysis. The preferential evaporation rate of germanium oxide from solid garnet as a function of temperature and oxygen partial pressure was determined. The smaller lattice constant of this material (a = 12.25 A as against the conventional 12.38 A) necessitates a film composition with a correspondingly smaller lattice parameter. Films of the type Lu(3−x)CaxFe(5−x)SixO12 have been grown on [111] substrates by liquid phase epitaxy from a PbO,B2O3 melt. The parameters of a 4.6 μm thickness film at 20°C are: bubble diameter =4.8 μm, 4 π MS = 160 Gauss and Hcoll = 95 Oe. The temperature coefficient of the anisotropy field and the collapse field are 0.30 and 0.25 0e/°C respectively.

Growth of spinel ferrite films by liquid phase epitaxy

Thin films of MgFe2O4, NiFe2O4, ZnFe2O4, Ni1-xZnxFe2O4 have been grown on MgO substrates by liquid phase epitaxy from a PbO-B2O3 flux. Discussion of the quality of the films examined by interference microscopy and X-ray topography is presented. Preliminary magnetic measurements, such as ferrimagnetic resonance, are given for some of the layers. With substitution of Ga, Sn and Cr in MgFe2O4 films the segregation coefficients were in the range 1.11-1.32; 0.81-0.97; 1.06-1.56 and 0.005-0.012 for Ga, Fe, Sn and Cr respectively.

Faraday Rotation and Optical Absorption of Epitaxial Films of Y3−xBixFe5O12

Thin films of Y3−xBixFe5O12 (xu2009≤u20090.3) have been grown on Gd3Ga5O12 and Sm3Ga5O12 substrates by liquid phase epitaxy. Faraday rotation and optical absorption measurements between 0.7 and 0.4 micron are reported. Near 0.5 the FR has a large negative value of −3 degrees/micron for xu2009=u20090.26 as compared with +0.3 degrees/micron for YIG. Both the optical absorption and the figure of merit (rotation/absorption) are higher for the Bi containing garnet films. The promising properties of these materials for magneto‐optic applications are discussed.Thin films of Y3−xBixFe5O12 (xu2009≤u20090.3) have been grown on Gd3Ga5O12 and Sm3Ga5O12 substrates by liquid phase epitaxy. Faraday rotation and optical absorption measurements between 0.7 and 0.4 micron are reported. Near 0.5 the FR has a large negative value of −3 degrees/micron for xu2009=u20090.26 as compared with +0.3 degrees/micron for YIG. Both the optical absorption and the figure of merit (rotation/absorption) are higher for the Bi containing garnet films. The promising properties of these materials for magneto‐optic applications are discussed.

Preparation of barium lead hexa-aluminate single crystal layers by the liquid phase epitaxy technique

In this paper we show that it is possible to grow the hexagonal material (Ba, Pb)Al1 2O1 9 on single crystal substrates by the liquid phase epitaxy technique. The layers have been characterized using optical microscopy and X-ray topography. We show that the quality of films grown on c-orientated substrates are of good quality, whilst those grown on α-orientated substrates suffer from cleavage cracks perpendicular to the c-direction. Chemical analysis was done using the electron probe microanalysis and electron excitation techniques. In this study the flux PbO/PbF2 was used. After about one week the high evaporation rate of the PbF2 at ≈ 1000°C lead to irreproducible growth.

Control of the growth-induced magnetic anisotropy in ferrimagnetic garnet films grown by liquid-phase epitaxy

Abstract The influence of melt composition and growth temperature Tg on the growth-induced magnetic anisotropy constant Kgu in Bi substituted LPE ferrimagnetic garnet films has been investigated. In films grown from Bi2O3 based and from PbO B 2 O 3 based melts the value of Kgu increased upon decreasing Tg, associated with an increasing incorporation of B1. However, at given Bi content the films grown from Bi2O3 based fluxes generally exhibited smaller values of Kgu than those from PbO B 2 O 3 based melts. The addition of small amounts of Ca2+ to the Bi2O3 based melts resulted in (Y,Bi)-garnet films changing from n- to p-type electrical conductivity at a minimum value of Kgu. Additions of Si4+ to a PbO B 2 O 3 based melt reduced the uniaxial magnetic anisotropy of La-YIG-Ga films accompanied by a change from p- to n-type conductivity. Similarly, (Gd,Bi)-garnet films from PbO B 2 O 3 based melts changed from n- to p-type conductivity upon decreasing Tg where Kgu changes sign from negative to positive. From the temperature dependence of the electrical conductivity the electron and hole concentrations in some of these films were estimated suggesting that the sign change of Kgu in the investigated (Gd,Bi)-garnet films and the minimum of Kgu in the investigated (Y,Bi)-garnet films and (La,Y)-garnet films occur at donor-acceptor compensation. This experimental evidence led us to conclude that the growth induced magnetic anisotropy may be correlated with the presence of donor and acceptor centers in the garnet structure.