Magnetic structure of ion-implanted bubble garnets

The surface magnetic structure of bubble garnet films implanted with 80 keV Ne⁺ ions has been investigated by conversion-electron Mössbauer spectroscopy in conjunction with backscattered X-ray Mössbauer spectroscopy. For lower doses (～1–3 × 10¹⁴Ne⁺cm^-2) a ferrimagnetic component with in-plane magnetization coexists with a smaller paramagnetic phase in the implanted layer; for a dose of 5 × 10¹⁴Ne⁺cm^-2 only a paramagnetic phase is observed.     

On the temperature dependence of coercivity in bubble garnets films

Accurate measurements of magnetic aftereffect and coercivity in a magnetic bubble garnet are presented in the temperature range (-35°C, +60°C). The bubble material is an epitaxial Y_2.6Sm_0.4Ga_1.1Fe_3.9O₁₂ composition supporting 4 μm bubbles. The duration of the drive field pulse is varied between 100 ns and 2 ms. The curves taken at different temperatures of the minimum drive field duration versus its amplitude show similar hyperbolic shape. They are shifted towards small magnetic fields when the temperature increases. The experimental data are compared with a phenomenological model based on thermal activation. A good fit is obtained by taking the values at 20°C as a reference and comparing the experimental data with the calculated values for the different temperatures.     

Optical absorption in cobalt-containing single-crystal epitaxial films of gadolinium gallium garnets

Cobalt-containing single-crystal garnet films are grown by liquid-phase epitaxy from supercooled PbO-B₂O₃-and Bi₂O₃-B₂O₃-based solution melts on Gd₃Ga₅O₁₂ substrates. It is shown that cobalt in the films is in a trivalent state. Upon introduction of GeO₂ into the initial solution melt, cobalt transforms into a bivalent state. It is revealed that the absorption spectrum of the grown films exhibits two broad bands in the wavelength ranges 450–800 and 900–1800 nm. Each of these bands contains three components. The spin-orbit splitting of the observed bands is determined.     

Superconductivity in non-cubic crystal structures

Superconductivity has been observed in alloy samples of the zirconium-germanium hafnium-germanium, yttrium-germanium and lanthanum-germanium systems. The samples were prepared on heated substrates, under varying conditions by r.f. sputtering technique. The composition span was from 20 to 90 at.% Ge. Superconductivity was found in all four of these systems and was identified with the digermanide phase by X-ray analysis. Four terminal resistive measurements show the following superconducting transitions: 8 K (ZrGe₂), ≈ 2.2 K (HfGe₂), 3.8 K (YGe₂) and 2.65 K (LaGe₂). Annealing studies indicate that the superconducting ZrGe₂ and HfGe₂ phases are metastable, and depend strongly on the conditions of synthesis.     

Evidence for liquid phase reactions during single bubble acoustic cavitation

We extended the recent experiment by Lepoint et al. [Sonochemistry and Sonoluminescence, NATO ASI Series, Series C 524, Kluwer Academic Publishers, Dordrecht/Boston/London, 1999, p. 285], involving a so-called single bubble sonochemistry process, to a three-phase system. We have found experimental evidence that a single cavitating bubble can activate the oxidation of I- ions after the injection of a CCl4 liquid drop in the bubble trapping apparatus. The solvent drop (CCl4 is almost water insoluble) is pushed towards the bubble position and forms a thin film on the bubble surface. When the acoustic pressure drive is increased above 100 kPa, the three-phase system gives rise to a dark filament, indicating the complexation reaction between starch (added to the water phase) and I2. I2 species is the product of surface reactions involving bubble-induced decomposition of CCl4. Further increase of the acoustic drive causes the thin CCl4 film to separate from the bubble and stops I2 production. The study of the chemical activity of this three-phase system could give new advances on dynamics of the bubble collapse.     

Charged walls in implanted garnets

The domain structure around non-implanted discs in implanted garnets has been investigated as a function of the strength of in-plane fields, H_| is in a hard direction, the observed domain structures can be explained by the theory of Shir and Lin [14]. When H_| is in easy direction or at 30° to it, charged walls are still present for values of H_| for which the theory predicts saturation of the implanted layer. This results is attributed to demagnetizing effects. The dependence of the length of charged walls on the diameter of non-implanted discs and on H_| has been studied.     

Evidence for replacing zinc with cobalt and iron atoms in epitaxial ZnO:CoFe-diluted magnetic semiconductors

The structure and local environment of ZnO:CoFe were investigated. An epitaxial structure of ZnO thin film doped with CoFe on Al₂O₃ substrate was confirmed by XRD. By fitting the Zn EXAFS spectra, the bond distances and coordination numbers of Zn atoms in the ZnO:CoFe show that Co and Fe ions replace some positions of zinc atoms. The XANES study of Zn and O also show that CoFe cluster formation in this system can be excluded.     

Magnetic phase transitions in garnets

The phenomenological theory broadly applicable to magnetic transitions in ferrimagnetic garnets is discussed briefly. The experimental techniques, particularly nuclear magnetic resonance and Mössbauer effect spectroscopy, are then reviewed. Finally, there is a review of the results on specific garnets which undergo such transitions. Some remaining problems are pointed out.     

Cation distribution in Eu-Sc-Fe garnets

The cation distribution has been studied with ⁵⁷Fe Mössbauer spectroscopy in the garnet system Eu_3?ySc_2+yFe₃O₁₂ with y = 0.0, 0.2 and 0.5. It is shown that the previously proposed cation distribution is not correct. The problem of possible impurities in the investigated system is discussed in detail. Several possible cation distributions are considered compatible with Mössbauer data. Mössbauer results combined with the composition dependence of lattice constants show that the tetrahedral sites are accessible to Sc³⁺ ions. The system studied is a second example of a garnet structure in which Sc³⁺ ions are found at the tetrahedral sites. A small fraction of Sc³⁺ ions for the samples y = 0.2 and 0.5 is also found at the dodecahedral sites.     

Recent developments in the study of quadrupolar gradients in non-cubic metals

This paper surveys the existing experimental data on quadrupolar coupling constants in metals emphasizing the principal trends emerging from these data and their significance to theoretical models on electric field-gradients in non-cubic metals.     

Diamagnetic domains and magnetostriction in beryllium

Magnetostriction was for the first time studied under the conditions of formation of diamagnetic domains (Condon domains). Transverse magnetostriction oscillations on a beryllium single crystalline plate oriented normally to magnetic field were measured in magnetic fields up to 7 T at temperatures down to 1.5 K. The relative amplitude of oscillations increased almost as the square of magnetic field and reached 10^?5. The signal had a sawtoothed shape corresponding to alternation of homogeneous and inhomogeneous (domain) states in the region of the existence of magnetic domains. The arising of domains was accompanied by singularities in the observed signals which is explained by an anomalous increase in the compressibility coefficient of the domain state: coefficient oscillations were more than 100 times larger than the value predicted by the standard theory. The observed relation between magnetization current and deformation led us to conclude that the compressibility of the metal was fully determined by conduction electrons. Magnetostriction then exactly compensated Fermi level oscillations. The position of the Fermi level therefore remained constant under magnetic field variations. In addition, the domain wall thickness had to increase as the plate grew thicker.     

Crystal field interaction in erbium garnets

A crystal field analysis has been performed for erbium aluminum garnet (ErAG) and erbium gallium garnet (ErGG). Good agreement between the measured and calculated energy levels was obtained.     

Computation of inverse of magnetostriction model for Terfenol-D

The use of a non-linear model or non-linear inverse model in a control configuration has been shown to significantly improve the performance of actuators that use hysteretic materials to drive them. A method to calculate the inverse of a model for the hysteretic material Terfenol-D is presented here, to be used as part of a full control configuration for Terfenol-D actuators. This inverse is based on a magnetostriction model that has been shown to characterize the behavior of Terfenol-D well. Initially unbounded, accumulating errors in the inverse are analyzed and a method to bound the error for periodic input is presented. Simulations show steady-state tracking of strain waveforms that reach 95% of saturation to within 2% in an open-loop configuration. For use with inputs as high as 95% of saturation, though, the ratio of the input frequency to the sampling rate must be kept below specified levels. With inputs that reach only 85% of saturation, the error from inversion is below 0.2% and can be controlled by a parameter choice.     

Transient magnetostriction in ferro- and ferri-magnetic materials

An acoustic impulse excited by a laser pulse generates some transient magnetostriction effects in various ferromagnetic and ferrimagnetic materials. A crystal of small magnetic anisotropy energy gives a steady magnetoelastic oscillation which is resonant with the sample dimension. The other materials show the surface effect only. The temperature dependence is studied up to the Curie temperature. Around the phase transition point the long-wavelength fluctuation of magnetization is excited by the elastic impulse, and some critical phenomena are observed. These effects are discussed by the usual theory of magnetoelastic interaction.     

Evidence of structural strain in epitaxial graphene layers on 6H-SiC(0001)

The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC (0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows us to resolve submonolayer growth, including individual, localized C=C dimers in a diamondlike carbon matrix for AES C/Si ratio of approximately 3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio >6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.