Epitaxial growth and annealing control of FMR properties of thick homogeneous Ga substituted yttrium iron garnet films

Homogeneous, 30 μm to 70 μm thick Ga substituted yttrium iron garnet films have been grown on Y or Al substituted gadolinium gallium garnet substrates having lattice parameters matched to that of the films. Resonance field and FMR linewidth measurements as a function of frequency and annealing experiments revealed that the magnetization and cubic anisotropy of the films are identical to data from flux grown bulk single crystals, the FMR losses of the films are only slightly higher. For films grown with supercooling ΔT < 50 °C a negative, growth induced, uniaxial anisotropy was found which could be removed by annealing in air at 1100 °C. A compensation of the temperature drift of the FMR frequency can be adjusted in the Ga substituted films by changing the frozen-in Ga-Fe cation distribution by annealing and quenching from different temperatures > 800 °C.     

Tunable In-Plane Anisotropy in Amorphous Sm–Co Films Grown on (011)-Oriented Single-Crystal Substrates

Amorphous Sm–Co films with uniaxial in-plane anisotropy have great potential for application in information-storage media and spintronic materials. The most effective method to produce uniaxial in-plane anisotropy is to apply an in-plane magnetic field during deposition. However, this method inevitably requires more complex equipment. Here, we report a new way to produce uniaxial in-plane anisotropy by growing amorphous Sm–Co films onto (011)-cut single-crystal substrates in the absence of an external magnetic field. The tunable anisotropy constant, k_A, is demonstrated with variation in the lattice parameter of the substrates. A k_A value as high as about 3.3 × 10⁴ J·m⁻³ was obtained in the amorphous Sm–Co film grown on a LaAlO₃(011) substrate. Detailed analysis indicated that the preferential seeding and growth of ferromagnetic (FM) domains caused by the anisotropic strain of the substrates, along with the formed Sm–Co, Co–Co directional pair ordering, exert a substantial effect. This work provides a new way to obtain in-plane anisotropy in amorphous Sm–Co films.     

Growth and characterization of Al-substituted iron garnets for submicron diameter bubble films

Al-substituted rare-earth iron garnet films have been grown that support submicron diameter bubbles. In developing small diameter bubble films, reduction of the saturation induction of a film, 4πMS, is very important because power consumption in drive coils and chip heating increase drastically with higher magnetization. Aluminum ions have been found to be one of the most suitable substitution ions for this purpose. Moreover, owing to the small ionic radius of Al ions, it is easy to meet the lattice matching requirement between films and GGG substrates without lattice adjusting ions such as lutetium. Submicron diameter bubble films are easily grown by the conventional LPE technique in the garnet system (YSmGd)₃ (FeAl)₅ O₁₂. They show good bubble properties, well suited to practical use.     

The origin of the uniaxial anisotropy in thin films of (YLaPb)3 (FeGa)5O12 and its variation along the growth direction

The uniaxial anisotropy of La, Ga: YIG films grown by liquid phase epitaxy has been measured using spin-wave resonance techniques. The results show that in films produced at growth temperatures above 840°C on [111] Gd₃Ga₅O₁₂ substrates the uniaxial anisotropy is stress induced. At lower growth temperatures the Pb incorporation gives rise to a positive growth-induced anisotropy of 4×10⁴ erg/cm³ per Pb atom per formula unit. Around the growth temperature of 840°C, where the Pb and La effects cancel, homogeneous films can be grown. Variations of anisotropy within the filmsthickness will be shown to be due to variations of local growth rates which in turn cause composition changes.     

Gallium concentration effects on magnetic bubble films of EuTm2(Fe,Ga)5O112 garnet grown by liquid phase epitaxy

By varying the Ga concentrations x of EuTm₂Fe_5?xGa_xO₁₂ from x = 0 to about 0.8, thin magnetic films supporting stable bubbles with diameters from 0.5 to about 8μm, respectively, are deposited epitaxially onto gadolinium gallium garnet substrates oriented (111). For low values of x, the Ga segregation coefficient is about 2 and it decreases slightly as x increases. Thus, films contain roughly twice as much Ga as the LPE fluxed melts in which they grow. Both the characteristic length and magnetic bubble stability factor (Q) increase rapidly with x and, to a lesser degree, so does the uniaxial anisotropy field H_A while both 4πM and the uniaxial anisotropy K_u decrease.     

Site preferences for Nd in YAG films grown by LPE

We have made spin-resonance measurements of the concentration on different, though crystallographically equivalent, sites, of Nd³⁺ dilutely incorporated into thin films of yttrium aluminum garnet (YAG). The films were grown on {001}, {110}, and {111} substrates of pure YAG by liquid phase epitaxy (LPE). Sites which differ only in the orientation of their local axes relative to the growth direction are found to be unequally populated. The site preferences found for {110} films are close to, though larger than, those found previously for {110} facets of bulk crystals grown from the flux. This result supports the view that the growth-induced magnetic anisotropy of LPE films of mixed rare earth iron garnets arises from the same site preference mechanism as the anisotropy found under natural facets of bulk crystals. Very large site preferences, up to 4.5:1, are found in {001} films; but for {111} films they are only slight. Site preferences are not greatly affected by changes in growth temperature (850°C to 1044°C) or in overall concentration (0.1% to 5% atomic). The effective distribution coefficient varies with concentration and substrate orientation.     

The magnetic domain structure in low anisotropy epitaxial garnet films

Single-crystal films of (YCa)₃(FeGe)₅O₁₂ garnets were prepared by the liquid phase epitaxial method on the (111) plane of Gd₃Ga₅O₁₂ substrates. These magnetic bubble domain films have comparable uniaxial anisotropy energy, cubic anisotropy energy and demagnetizing energy. A photographic technique was developed to measure the angles of inclination of the domain magnetization vectors with respect to the normal to the sample surface. Without an applied magnetic field these inclination angles were 58° and 122°. A simple stripe domain model was used to provide an explanation of the experimental results. This model can easily be extended for other types of mixed anisotropy cases.     

The magnetic anisotropy of thin epitaxial CrO<Subscript>2</Subscript> films studied by ferromagnetic resonance

The magnetic anisotropy of thin epitaxial films of chromium dioxide (CrO₂) has been studied as a function of the film thickness by the ferromagnetic resonance (FMR) technique. CrO₂ films with various thicknesses in the range from 27 to 535 nm have been grown on (100)-oriented TiO₂ substrates by chemical vapor deposition using CrO₃ as a solid precursor. In a series of CrO₂ films grown on the substrates cleaned by etching in a hydrofluoric acid solution, the FMR signal exhibits anisotropy and is strongly dependent on the film thickness. The magnetic properties of CrO₂ films are determined by a competition between the magnetocrystalline and magnetoelastic anisotropy energies, the latter being related to elastic tensile stresses caused by the lattice mismatch between the film and the substrate. In the films of minimum thickness (27 nm), this strain-induced anisotropy is predominant and the easy magnetization axis switches from the [ 001] crystallographic direction (characteristic of the bulk magnet) to the [ 010] direction.     

A novel non-Pb flux system for the preparation of yttrium and rare earth iron gallium and aluminum garnets

A flux composed of lithium and rare earth molybdates has been found to be an effective transfer medium for the preparation of yttrium and rare earth iron gallium and aluminum garnets. Magnetic bubble domain properties of epitaxial iron garnet films grown from the molybdate flux are easily controlled by virtue of the transfer process including dimensional control for submicron thick films. The properties of these films compare well to those grown from the PbO·B₂O₃ flux, but the films are contaminant free. This is particularly important for the growth of Nd-doped YAG laser films. Stability region of the garnet phase in the Li₂MoO₄Y₂O₃MoO₃ pseudo-ternary system and solubility are discussed as related to crystal and film growth.     

Features of the coercivity of strained epitaxial garnet ferrite films

We have studied the influence of the relative film-substrate lattice mismatch in the 0.5–0.85% range on the behavior of coercivity in epitaxial garnet ferrite (EGF) films with compositions (Bi, Sm, Lu, Ca)₃(Fe, Sc, Ga, Al)₅O₁₂ grown on (111)-oriented gadolinium gallium garnet (GGG) substrates. As the relative film-substrate lattice mismatch increases, the coercivity of EGF films initially grows, passes through a maximum, and then decreases. The maximum in the coercivity is related to periodic localized stresses caused by the formation of a misfit dislocation network whose period is comparable with the width of domain walls. The period of localized stresses is determined by the relative lattice mismatch between the EGF film and the GGG substrate.     

Determination of magnetic anisotropy constants for bubble garnet epitaxial films using field orientation dependence in ferromagnetic resonances

Cubic magnetocrystalline anisotropy constant K₁ as well as uniaxial anisotropy constant K_u and gyromagnetic ratio γ are precisely determined for (111) magnetic garnet epitaxial films from measurements on FMR field orientation dependence. Strict FMR conditions are derived from total free energy expressions, where the differences between magnetization direction and applied field direction are taken into consideration. By applying magnetic field in (110) plane, FMR is measured to obtain the three best fitting parameters of K₁, K_u and γ. Present analysis is compared with Cronemeyer et al's analysis. Influence of sample misalignment on measurement accuracy is also presented. Similarly, for (110) garnet films with orthorhombic magnetic anisotropy, measurements are carried out for two crystallographic planes of (001) and (110), and the four best fitting parameters of K₁, K_u, δ and γ are determined.     

Transport properties of textured Bi2Sr2CaCu2O8+y,thin films

Superconducting films of the high-T _c compound Bi₂Sr₂CaCu₂O_8+y , have been grown on (111)-oriented gadolinium gallium garnet substrates by a liquid-phase technique. The films show a very high degree of preferential orientation with thec-axis perpendicular to the substrates. The onset of the resistive transition was 85 K while zero resistance was obtained at 78 K. Results concerning the critical current properties of the films are described. Measurements of the paraconductivity effects on the electrical resistivity above the superconducting transition due to thermodynamic fluctuations are also reported.     

LPE growth of lithium ferrite on spinel substrate crystals

Preliminary results on the growth of lithium ferrite by LPE (liquid phase epitaxy) are described. A PbO-B₂O₃ flux was used with nonmagnetic spinel-structured substrate crystals. Compared with growth of magnetic garnets, the LPE melt was found to be relatively unstable with respect to undercooling. Nevertheless, LPE growth was achieved. All of the epitaxial films grown so far are cracked; however, ferrimagnetic resonance linewidths as low as 7.8 Oe at 9.07 GHz have been measured.     

Comparison of Bi3Fe5O12 film giant Faraday rotators grown on (111) and (001) Gd3Ga5O12 single crystals

Bismuth iron garnet (Bi₃Fe₅O₁₂, BIG) epitaxial thin films were grown on single crystal (Gd₃Ga₅O₁₂, GGG) (111) and (001) substrates by rf-magnetron sputtering technique. Processing parameters have been optimized to obtain high deposition rate (2.74 μm/h) and the surface rms roughness less than 10 nm. X-ray diffraction reveals films epitaxial quality: exclusive (111) or (001) orientation with narrow rocking curves and strong in-plane texture. Films possess low optical loss and magneto-optical Faraday rotation (FR) as high as 5 deg/μm at 677 nm wavelength. Comparative analysis of films grown on (111) and (001) substrates clearly shows significant superiority of BIG/GGG(001) film. For this film, the coercive field ∼100 Oe appears to be 2.5 times lower while the optical transmission to be 10% higher than that for BIG/GGG(111) film. Enhanced magneto-optical performance of BIG/GGG(001) films relies upon better accommodation of the film-to-substrate mismatch strain through the tetragonal BIG lattice distortions compared to the rhombohedral one in BIG/GGG(111) films.     

Substrate facet replication by epitaxial magnetic garnet films

Epitaxial ferrimagnetic garnet films were deposited on Czochralski grown single crystal gadolinium gallium garnet substrates containing faceted regions. Films grown by both chemical vapor deposition (CVD) and liquid phase epitaxy (LPE) were studied. Lattice parameter distributions were determined by the method of X-ray double crystal topography with rocking curve analysis. The demagnetized domain strip width, magnetization, and characteristic length were measured in a CVD film in regions inside and outside the substrate facets. It was determined that replication of the substrate facets by the epitaxial film is accomplished by a difference in film stress. This stress difference arises from the lattice parameter difference between the faceted and unfaceted regions of the substrate. These results lead to the establishment of a criterion for allowable lattice parameter variation in substrates to be used for magnetic bubble domain films with stress induced anisotropy.     

Characterization of strain relaxation of (0 0 1) oriented SrTiO3 thin films grown on LaAIO3 (1 1 0) by means of reciprocal space mapping using x-ray diffraction

The strain relaxation of SrTiO₃ r.f. magnetron sputter-deposited thin films on LaAlO₃ substrates have been studied by x-ray diffraction mapping. An investigation of different x-ray optics shows that a, so called, hybrid mirror monochromator in combination with a triple-bounce analyser crystal provides very good conditions for characterization of thin distorted films grown epitaxially onto substrates with high structural order. The in-plane and out-of-plane lattice parameters of the SrTiO₃ films could accurately be determined since the x-ray diffraction optics enabled the splitting of substrate peaks, caused by the twinning in the rhombohedral LaAlO₃ to be resolved and, provided film peak intensities are high enough, to precisely establish their positions. Films in the thickness range 9.3–144.0 nm were found to be partially relaxed, having a tetragonal distortion due to in-plane strain that was found to decrease with increasing film thickness, approaching an undistorted SrTiO₃ lattice parameter of 0.3927 nm. This value is 0.6% larger than the bulk indicating that the compositions of the films were slightly non-stoichiometric. The strain relaxation of the grown films was found to follow the general trend of a predicted strain–thickness relation based on energy density balance considerations regarding misfit dislocations and lattice strain.     

Growth and temperature characteristics of small bubble YSmLuCaFeGe garnets

Several hundred YSmLuCaFeGe garnet films supporting bubbles 1–3 μm in diameter were grown by LPE, and their static bubble properties were measured from ?50 to 100°C and compared with those of similar films supporting 5 μm bubbles. Bubble mobilities in the range ～ 300–1000 cm/sec-0e and q values as high as 4.7 were achieved, along with saturation magnetizations from 330 to 550 G and collapse-field temperature coefficients in the range ?0.20 to ?0.27%/°C. Static coercivities were generally less than 0.3 0e at 25°C and 1 0e at ?25°C. The growth-induced component of the uniaxial anisotropy field, when scaled by bubble diameter, was found to vary in proportion to the relative concentrations of Sm³⁺ and Y³⁺ in the film, i.e., H_K/d ∝ Sm/Y.     

Transport properties of textured Bi2Sr2CaCu2O8+y, thin films

Superconducting films of the high-T _c compound Bi₂Sr₂CaCu₂O_8+y , have been grown on (111)-oriented gadolinium gallium garnet substrates by a liquid-phase technique. The films show a very high degree of preferential orientation with thec-axis perpendicular to the substrates. The onset of the resistive transition was 85 K while zero resistance was obtained at 78 K. Results concerning the critical current properties of the films are described. Measurements of the paraconductivity effects on the electrical resistivity above the superconducting transition due to thermodynamic fluctuations are also reported.Supported by Ansaldo S.p.A Divisione Ansaldo Ricerche, Via Corso Perrone 25, I-16100 Genova, Italy.     

Magnetic Resonance Studies of Half-Metallic Epitaxial CrO2 Thin Films Grown on Differently Oriented TiO2 Substrates

The results of structural and ferromagnetic resonance (FMR) investigations of epitaxial half- metallic chromium dioxide (CrO₂) thin films of thicknesses between 58?nm and 540?nm grown by chemical vapor deposition (CVD) on (100) and (110)-oriented TiO₂ single crystal substrates are presented. The angular dependences of the FMR spectra in different experimental geometries were obtained. Effective magnetic anisotropies of epitaxially grown CrO₂ films of different thicknesses on strain and strain-free surfaces of TiO₂ substrates were estimated. The results indicate that the magnetic behavior of the CrO₂ films results from a competition between the magnetocrystalline and strain anisotropies. It has been revealed that the strain anisotropy dominates in the films grown onto (100)-oriented TiO₂ substrates. On the contrary, the films grown onto (110)-oriented substrates demonstrate strain-free magnetic anisotropy behavior.     

Characteristics of YLaTm-garnet films for magnetic bubble applications

Films of Y_1.6La_0.3Tm_1.1(FeGa)₅O₁₂ have been grown by liquidphase epitaxy on Gd₃Ga₅O₁₂ substrates. This is a new film composition whose high mobility (～1000 cm/sec/Oe) and low temperature coefficient of bubble diameter (～0.1%/°C) make it attractive for bubble domain devices. Films supporting 3 to 8μm-diameter bubbles have been prepared with typical defect densities less than 5/cm². Data are presented to show that this material can be grown reproducibly despite a rather large La segregation coefficient. Measurements of the uniaxial anisotropy as a function of lattice mismatch are presented along with the results of ion implantation studies which show that a dosage of 3 × 10¹³ cm^?2 of Ne is sufficient to suppress hard bubbles in this material. Also, bubble velocity measurements are discussed in detail as they reveal a number of interesting effects. These include the existence of a perpendicular velocity component which can be virtually eliminated by applying an in-plane field and the absence of extreme dynamic conversion effects in high gradient fields.