Instability on Magnetic Field Calculation
 

When I put a core, this is what happens to the magnetic field. I dont have a clue to what I should do to solve this. Both of the boundaries on vertical and horizontal directions are normal (symmetric) and the external boundarie is a vector potential with 0 Webberm. The core is solved by parametric solution, as the ferromagnetic plate around the conductors. If someone knows what is happening please answer the topic.

https://uploaddeimagens.com.br/image...magnetico1-png
https://uploaddeimagens.com.br/image...magnetico2-png
https://uploaddeimagens.com.br/image..._magnetico-png