Non-contact ultrasonic measurements of the elastic constants of magnetic materials

Abstract

Ultrasonic testing using contacting transducers such as quartz or PZT is well established. However, standard measurement techniques used require physical contact of the sample and ultrasonic transducer and some sort of couplant between the two. With this configuration there is a possibility of damaging the sample, transducer or bond during testing, thermal cycling, or removal of the transducer. We present results taken using recent advances in non-contact methods of ultrasound generation and detection using electromagnetic acoustic transducers (EMATs), which offer some significant benefits over contact ultrasonic techniques. Circumventing the need for couplant removes the possibility of contaminating the system, which is an issue for some material property measurements, and allows easier measurements over a wider range of temperatures. An automated data analysis system has been developed which allows the velocity of sound in the sample, and hence the elastic constants, to be determined to a high accuracy. This technique is illustrated using measurements of the alloy Gd64Sc36.

Magnetic phase transitions in Gd64Sc36 studied using non-contact ultrasonics

“…Sample temperature was measured using a Cernox probe in thermal contact with the sample. Despite relatively high peak output power, EMATs can be used for low temperature measurements without causing significant problems with heating, because of their low duty cycle [15,20]. The base temperature during experiments did not differ, within errors, for generation by quartz or EMAT.…”

“…The experimental setup and automated data collection and handling has been described in detail in [15], and is summarised here. The ultrasonic measurements were done in a pulse-echo configuration [8,15] using a MATEC 6600 pulse generator-receiver, a commercially available X-cut quartz transducer and a custom-designed EMAT. The EMAT coil was of the single layer spiral (pancake) type, 4.5 mm in diameter, wound using 0.14 mm diameter enamel-coated copper wire.…”

“…Three parameters were monitored at different magnetic fields and temperatures; elastic constant C 33 , attenuation of ultrasound, and the efficiency of ultrasound generation (for EMAT only). The elastic constant was obtained by measuring the time between echoes using cross-correlation of the first echo with the echo train to improve precision [15,24]. Attenuation was obtained by fitting echo amplitudes to an exponential decay curve.…”

“…EMATs consist of a coil of wire and an applied (bias) magnetic field, and their design has been described in detail elsewhere [13,15,16]. A current is pulsed through the coil, leading to a time-varying magnetic field, and when placed near a sample a mirror current or magnetisation change may occur within the sample; ultrasonic generation is therefore within the sample itself, rather than within the transducer.…”

“…Conventional ultrasonic measurements use piezoelectric transducers such as quartz or PZT to generate and detect ultrasound, however, these require the use of couplant and direct physical contact with the sample, [5][6][7][8] and with repeated thermal cycling the coupling between the transducer and the sample may quickly deteriorate. Electromagnetic acoustic transducers (EMATs) [10][11][12] offer a number of advantages, being non-contact, with narrowband or broadband operation possible through careful transducer design, and have previously been successfully employed to study phenomena such as giant quantum oscillations [13] and magnetic phase transitions [14,15], as well as being common in non-destructive testing applications [16].…”