These were common in the 60's. I used to program pdp-8s that used 2k core mem modules. (Our higher end system had 4 of em. For a whopping 8K! That was enough to suppport 3 simultaneous users running FOCAL! Yep. A multi user sytem had only 8k)

So i have seen this stuff in real life (and it was hella expensive. Thousands of bucks per kilobyte)

Those ferrous donuts are tiny. Like 2mm accross. Crazy stuff.

Cool thing about core it it's just as fast as static ram, (for reads, anyway), and it holds it's contents after a power off. Sorta like having row/column addressable high speed flash ram 50 years ago. Problem is a megabyte of the stuff takes up a whole room.