Using instrumentals, copyright infringement?

k. so i'm an aspiring rapper type person. If i use the instrumentals from a song that's been copyrighted, and i use it for my debut mixtape, would this be copyright infringement? i plan on the mixtape being used as a promotional tool, just to get my name out, and keep in mind it is completely FREE. and i would state that i just sampled the song and i didnt actually make it or anything. but the lyrics would be my own. thank you for your help