Dear Maryam Akbari thank you very much for your interesting technical question. As an organometallic chemist I'm not an expert in this field. However, I think that the following research article could be helpful answering your question:
A novel approach to melt purification of magnesium alloys
Article A novel approach to melt purification of magnesium alloys
In this study the authors managed to reduce the Fe content in the AZ61 alloy melt to 20 ppm. The article has been posted as public full text on RG and can be freely downloaded as pdf file.
Frank T. Edelmann Thank you very much for your attention to my question and sharing this article. I studied this article and another research report that was similar process and did in our lab, but not get good result and Fe constant was upper of 50 ppm that not good for us.