Core memory was the standard memory on PDP-8 computers (and all earlier DEC machines) until the PDP-8/A which used either core or semiconductor memory. This transition happened in the mid to late 1970's when semiconductor memory chips became cheaper than core. My PDP-8/E has a 16kx8 SRAM third party (non DEC) memory board from for half of its memory. Core memory is non volatile, some of the semiconductor memory boards had the ability to use backup batteries to allow them to appear non volatile like core when power was lost.

The largest core memory DEC offered for the PDP-8 was 16k 12 bit words on one core memory board. I only have the 4k word boards. Donations are always welcome.

How core works
"Magnetic Core Memories" Mullard Educational Service
Core memory and concepts

PDP-8/E Core information in PDF
PDP-9 core memory fundementals
Core memory history
Core memory history