welcome to the AGF!
garageband has a decent piano that is basically the same as logic's that i use. but, garageband is not free.
this is a google search of iphone piano apps. you may want to check out a few and see if one works for you.
https://www.google.com/search?q=ipho...hrome&ie=UTF-8
otherwise, you may want to check keyboard sites or a site that reviews keyboard apps.
good luck
play music!