Electromagnetic acoustic transducers (EMATs) are widely used in various industries and scientific research applications for non-destructive testing of materials, including those in which they need to operate at elevated temperatures. The magnets used in EMATs to produce a bias magnet field can be either permanent magnets or electromagnets, and these magnets work in conjunction with magnetic coils to generate an alternating magnet field to induce eddy currents in the material. These interactions, in turn, generate ultrasonic waves for testing and evaluating materials.
But the challenge when designing permanent magnets used at high temperatures is excessive thermal energy can lead to self-generated demagnetization, where a magnet's coercivity and remanence are reduced, leading to a loss of magnetization.
Newly published research, co-authored by University of Cincinnati researchers and Lake Shore Application Scientist Cosmin Radu, looks at the design of permanent magnet EMATs to better address this challenge. The NDT & E International paper (subscription required) first describes a way to predict magnetic losses associated with exposure to elevated temperature, then looks at a method for evaluating the demagnetizing field while using experiments with various magnetic configurations.
For these experiments, a Lake Shore Cryotronics Model 8607 vibrating sample magnetometer (VSM) equipped with a high-temperature option was used to measure the magnetic properties of the permanent magnet samples, including Alnico and rare earth SmCo and NdFeB magnets, at various temperatures.
Specifically, second quadrant demagnetization curve measurements (a standard measurement capability of the 8600 Series VSM) were performed at variable temperatures — in turn, providing insights that aid in the determining the effectiveness of the EMAT magnets at maintaining their magnetic strength under high-temperature conditions.
Why the 8600 Series VSM is ideal for such applications
For magnetic characterization applications where one needs to understand how permanent magnets perform at elevated temperatures, the 8600 Series VSM provides certain benefits, including:
- The necessary field strength (3.22 T on the Model 8607 described in the paper, and up to 3.62 T on the Model 8610 system)
- Easy-to-install variable temperature (86-SSVT) and high-temperature oven (86-OVEN) accessories, allowing one to measure magnetic properties at elevated temperatures, as high as 1273 K with the oven option (however, in some cases, the 86-SSVT option is preferred due to it having a smaller diameter, allowing its use in smaller electromagnet gap and in higher field, and a wide temperature range of 77 K to 950 K).
- Software containing, as standard, the second-quadrant magnetization routine, which focuses on the second quadrant of the hysteresis loop and is designed to measure the demagnetization characteristics of materials — which is particularly helpful for evaluating the performance and stability of permanent magnets where high temperatures can cause demagnetization to occur.
For more information about SSVT option for our VSM, relative to typical noise performance at variable temperatures, read our application brief:
