A review of mechanomagnetic spectroscopy. Case study: Dy polycrystals

• A method is proposed for estimating the direct magnetostriction from the reversible Villari effect.
• The method operates at extremely low magnetic fields (e.g. the Earth's magnetic field).
• The temperature and field dependences of the direct magnetostriction agree with data available.

The current state and capabilities of mechanomagnetic spectroscopy (MMS) are reviewed. Such technique, based on the Piezoelectric Ultrasonic Composite Oscillator Technique (PUCOT) has been developed to study the magnetoelastic coupling through the reversible inverse magnetostriction (reversible Villari effect, RVE) over wide ranges of periodic stress, temperature and polarizing magnetic field H  . The technique allows one to study also elastic and anelastic properties simultaneously with RVE. Measurements of RVE can be performed as a function of cyclic applied field H(t)=H0sinωt at constant temperature or as a function of temperature at fixed H. In contrast to other macroscopic techniques, MMS is not based on the measurement of the strain or length of the sample with and without applied field. This important advantage allows us to study very weak magnetoelastic coupling, like in the antiferromagnetic phase of Dy for magnetic fields as low as the Earth's one, which requires an extremely high sensitivity. Based on the Maxwell relation, an algorithm was developed to estimate the direct longitudinal magnetostriction versus H from the RVE.