Who put the magnets in Captain America's shield (bee bop a lu bop) Cap was revived from comic book limbo in THE AVENGERS# 4, March 1964. Two issues later, the story opened with a surprising scene where he is making his shield fly around and change direction. It seems Iron Man decided to beef up the shield by putting "sub-miniature transistors" in it that Cap could operate through magnets attached to his glove. (Stan Lee had an interesting idea of what transistors were, that is he thought they were magic.) I'd guess that it was Jack Kirby who dreamed this up, he absolutely loved adding gadgets that were never seen again; the man's imagination was always in high gear.
This "remote control shield" gimmick only lasted a few months, though. By TALES OF SUSPENSE# 62 (February 1965) a minor bad guy called the Deacon plots to get a hold of the shield to use its magnetic whatzits, only to be enlightened by Captain America that he "threw away all of Iron Man's magnetic gimmicks! They ruined my shield's delicate balance." Again, I don't know who decided to get rid of the magnet angle, and we're not likely to ever find out at this late date. But much of the character's appeal was that he had no super-powers, he was a normal human being in peak condition and it made more sense for his control of the shield to come from sheer skill.
Also, it's worth noting that all during the 1940s and 1950s, and up until the mid-1960s stories, that Captain America's shield was just steel. Obviously high-quality steel, but not vibranium or adamantium or unobtainium or whatever. In TALES OF SUSPENSE# 93, AIM scientists are messing around with the darn thing, and find that titanium buzz saws and hydraulic pressure drills can't scratch it. "Whatever it is composed of, the substance was never mined here on Earth! it is undoubtedly some form of alien, extra-terrestrial metal!" Well, not quite. But it was the start of the idea that Cap's shield is essentially indestructible.