S. B. Palmer

Oxide superconductor and magnetic metal thin film deposition by pulsed laser ablation: a review

A practical review of the use of laser ablation for the deposition of thin films of oxide superconductors and metals is presented. Details of the design and operation of a laser ablation deposition system are discussed, together with the influence of operating parameters such as laser wavelength and power density on the film growth, with the aim of assisting the researcher beginning to use and/or study laser ablation deposition.

Narrow band ultrasonic frequency attentuation bone measurement system

In the apparatus disclosed herein, a bone containing body member to be tested is placed between a pair of transducers and a predetermined sequence of tone signals having frequencies spanning a range from 200 to 600 kilohertz is transmitted through the body member, and the set of values representing the amplitudes of the corresponding received signals are stored. The set of values obtained with the body member between the transducer is normalized using a set of values obtained from the same sequence without the body member in place thereby to generate a third set of values which are compensated for the response characteristics of the transducer and related interfaces. A value corresponding to the rate of change of attenuation with respect to frequency is then calculated from the third set of values and is adjusted for the bone thickness, this adjusted value being related to characteristics of the body member. The bone thickness is determined by a broadband pulse echo measurement.

High accuracy non-contact ultrasonic thickness gauging of aluminium sheet using electromagnetic acoustic transducers

Aluminium sheet thickness has been calculated from ultrasonic data obtained using a send-receive, radially polarised electromagnetic acoustic transducer (EMAT). Sheets in the thickness range between 0.1 and 0.5 mm have been measured using this non-contact approach at a stand-off of up to 1.5 mm. Normal incidence shear waves generated and detected in the sheet and the resultant waveforms have been processed using transit time measurements and Fourier analysis. Two broad band EMAT systems have been used to perform the measurements with centre frequencies of approximately 5 MHz and frequency content up to 10 and 20 MHz respectively. The most accurate measurements of thickness on thin sheets have been made using Fourier analysis and have yielded measurements accurate to within 0.2% (or 0.4 microm) for 280 microm thick aluminium sheets. Discrete shear wave echoes can be observed for sheets down to a thickness of 250 microm using the higher frequency EMAT system. However temporal measurements of these signals yield lower accuracy results when compared to the Fourier analysis method which is capable of sub-micron accuracy.

Pulsed laser ablation deposition of yttrium iron garnet and cerium-substituted YIG films

Yttrium iron garnet (YIG) thin films were grown on gadolinium gallium garnet substrates using pulsed laser ablation deposition (PLAD) with a XeCl excimer laser. Films were grown up to over 2 μm thick, however cracking proved to be a problem for films over 1 μm thick. The lattice parameter(s) of the films and the substrates were measured and indicated that the film/substrate structure was bending to accommodate strain due to the lattice mismatch. The films had saturation magnetisation values close to that of bulk YIG and were isotropic in the film plane. The magnetisation data also indicate stress-induced uniaxial isotropy. The ablation conditions were varied to produce uncracked films with low droplet densities. YIG melts incongruently during the laser ablation process and cone-like structures form on the ablation target lowering the ablation rates. Cerium-substituted YIG films were also grown in both oxygen and argon atmospheres, substituting cerium into YIG increases the lattice parameter and hence reduces the strain. The Ce-YIG film grown in argon was greenish indicating that cerium was in the desired oxidation state.

A laser-EMAT system for ultrasonic weld inspection

Non-destructive testing of welded metal components is an area where significant cost savings can be made if preparation costs can be minimised. The largest cost associated with preparation is grinding off the weld cap in order to facilitate the detection of defects that may be present just under the cap. In addition, there are also potential savings to be made if the actual testing time can be reduced or if the tests can be performed at elevated temperatures beyond which conventional contacting ultrasonic techniques can be practically used. This paper describes a non-contacting NDT method using a pulsed laser and an electromagnetic acoustic transducer (EMAT) which has the potential to interrogate the entire cross-section of a weld.

Texture measurements of metal sheets using wideband electromagnetic acoustic transducers

The group velocity of the zero-order symmetric Lamb wave mode (S0) has been measured on metal sheet as a function of angle to the rolling direction. Generation and detection of the Lamb wave was achieved using wideband non-contact ultrasonic electromagnetic acoustic transducers (EMATs). The EMATs were relatively small in size and employed permanent NdFeB magnets. The angular dependent velocity of the S0 mode was measured on both aluminium and stainless steel (316) sheet. The entire system was computer controlled and the data were processed as acquired. The generation EMAT was driven with a current pulse of 270 A (peak value). This high current level gave rise to a more powerful generation source than is usually obtained with EMATs, enabling measurements to be made on stainless steel sheet which is much less efficient for EMAT generation of S0 mode Lamb waves than aluminium sheet.

Fermi surface as the driving mechanism for helical antiferromagnetic ordering in Gd-Y alloys

The first direct experimental evidence for the Fermi surface (FS) driving the helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported. The presence of a FS sheet capable of nesting is revealed, and the nesting vector associated with the sheet is found to be in excellent agreement with the periodicity of the helical ordering.

In-plane and out-of-plane particle velocity measurement using electromagnetic acoustical transducers

The electromagnetic acoustic transducer (EMAT) is a particle velocity sensor. EMATs can be used to preferentially measure particle velocities in a particular direction (such as in-plane or out-of-plane) by arranging a static magnetic field perpendicular to the vector plane constructed by the wire direction and the specific direction of interest without interfering with the incident ultrasonic field. A receiving EMAT model is given for both infinite and finite sized magnets and ultrasonic waves, explaining the influence of EMAT size, ultrasonic wave size on induced coil voltage and the sensitivity to liftoff. Rayleigh waves and Lamb waves are measured using in-plane and out-of-plane EMATs, and are validated by comparison to the theoretical predictions. More specifically predicted and measured phase and amplitude measurements for Rayleigh and Lamb waves are compared.

TEA-CO2 laser generation of ultrasound in non-metals

Abstract An industrial TEA-CO 2 laser, operating at a wavelength of 10.6 μm, has been used to produced broadband ultrasonic pulses in polymers. The generation mechanism falls into three categories. At low power densities ≤ 10 7 W cm −2 a thermoelastic regime predominates. As the power density is increased in the range (1–5) × 10 7 W cm −2 ablation of the material surface plays an increasingly important role in the acoustic generation. Thirdly, at greater power densities, plasma breakdown just above the material surface serves as the means of generation. This paper describes the acoustic sources for these types of generation mechanism and presents theoretically calculated acoustic waveforms to match those recorded experimentally.

Ultrasonic generation with a pulsed TEA CO2 laser

The authors have demonstrated that air breakdown produced just above a metal surface by a pulsed CO2 TEA laser, is an efficient source of ultrasound within the metal. The normal force source produces both longitudinal and shear waves in the bulk and Rayleigh surface waves. No damage is produced on the metal surface. Electromagnetic acoustic transducers provide complementary noncontacting detectors.