The design of high temperature EMATs to avoid irreversible magnetic losses

Many applications demand Electromagnetic Acoustic Transducers (EMATs) withstand exposure to elevated temperature without performance degradation. Permanent magnet EMATs are typically optimized on the basis of maximizing magnetic bias and mode purity, but there is a gap in the literature to-date about magnetic design to limit signal degradation through irreversible demagnetization. For practical purposes, recoverable irreversible demagnetization, which can only be recovered through active remagnetization, represents permanent damage to an EMAT. The extent of recoverable irreversible demagnetization depends on a combination of magnetic material selection and the physical configuration of the magnet (including magnet geometry, interaction with other ferromagnetic materials, and interaction with other magnetic fields). This paper presents the background theory on self-generated demagnetizing fields, which lead to recoverable irreversible losses, and presents an approach to predict the resulting magnetic losses. Thermal exposure experiments on different magnetic configurations are presented, illustrating the predictive method, and showing the importance of magnetic design to avoid magnetic losses. A further experiment, where EMATs are built with different magnetic configurations, shows that a favorable design results in negligible loss in post-exposure ultrasonic signal amplitude, whereas an unfavorable design led to a 65 % loss. Consequently, it is proposed that when designing an EMAT for elevated temperature, minimizing the demagnetizing field should be included as an important aspect of optimization.