Creating a very strong EM field using DC power

[Damname101]

I have been thinking about creating an EM field as of late. I did
briefly play with a neodymium magnet and a dc motor and was
able to generate an 8+ milli gauss field at about 3 foot.

What are some of my options for creating a more powerful EM
field and extending it out to 15 foot using DC voltage?

thanks in advance