A magnetic cover letter varies from person to person. Like an ordinary letter, it explains whether you have received financial aid or are applying, and if successful, when and how long you will go abroad. What do you want to learn from foreigners (what research are you interested in) and so on.
Attach a resume, indicating personal basic information (preferably with photos), study and work resumes, research fields, scientific research projects, awards, works, patents, academic part-time, international academic exchange experience, etc.