Authored by ML Corró*
Abstract
In the last years the Mechanomagnetic Spectroscopy technique has been developed from the PUCOT, improved and successfully used to study magnetic materials. In such technique, based in the reversible Villari effect, we study the magnetostriction at ultrasonic frequencies and different experimental conditions of temperature, polarizing field and stress. We will review the history, the set-up of the technique and some results obtained in different materials as rare earth Dy and ferromagnetic shape memory alloy Ni-Fe-Ga-Co.
Keywords: Magnetism; Magnetostriction; Ferromagnetic shape memory alloy; Dy
Abbreviations: MMS: Mechanomagnetic Spectroscopy; PUCOT: Piezoelectric Ultrasonic Composite Oscillator Technique; RVE: Reversible Villari Effect; FSMA: Ferromagnetic Shape Memory Alloy
Introduction: The History
The Piezoelectric Ultrasonic Composite Oscillator Technique (PUCOT) [1-3] is used to study the internal friction in materials. In such technique, quartz transducers are used to induce resonant oscillations at ultrasonic frequencies in bar-shaped samples. In our laboratory, the basic experimental PUCOT system was improved with several additions. We designed a cryostat to change the temperature during the experiments. Besides, a coil to apply polarizing fields was introduced in order to measure magnetic effects in the internal friction, as the magneto mechanical damping. Lately it was thought that, due to the reversible Villari Effect (RVE), the use of PUCOT under polarizing applied fields should produce measurable stress-induced induction in materials. To measure it, the experimental set-up of PUCOT was modified by the addition of a pick-up coil around the samples. Such experiments were successful, and the new developed technique was called Mechanomagnetic Spectroscopy (MMS) [4]. First measurements of stress-induced induction showed only positive values and some unexpected zero values. It was thought that we recorded the magnitude (modulus, absolute value) of the signal and some of such zeros could correspond to the transition between positive and negative values. To check this hypothesis a lock-in amplifier was introduced to measure the phase of the signal with reference to the oscillatory applied stress. Successfully, 180° shifts in the phase were measured. Such result implicated a change of the sign at zero points, as it was expected [5]. The last step in the development of the MMS was to relate the measured stress-induced induction to the reversible inverse and direct magnetostriction [6]. Nowadays we can consider the MMS as a fully developed technique.
Set-Up
As it was stated, we apply an oscillatory stress (in the order of 90 kHz) to bar-shaped samples (typical size of 1x1x15 mm3) by means of quartz transducers. The length of the sample depends on the Young Modulus in the following way:
Equation (1)
E=4ρl2f2,
where ρ is the density, l the length, and f the fundamental resonant frequency of the sample, respectively. The oscillatory strain in the sample produces a stress-induced induction in magnetic samples Citation: ML Corró. Mechanomagnetic Spectroscopy: A Review. Mod Concept Material Sci. 3(3): 2020. MCMS. MS.ID.000561. DOI: 10.33552/MCMS.2020.03.000561 Page 2 of 4 due to the RVE. To measure it, it was conceived to place a pickup coil around the maximum strain section of the sample since a voltage would be generated due to the Faraday’s law. Such voltage can be estimated in the following way.
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