Magnetic ICs

[maniacminer]

Something I never thought of before and I only noticed it because I was being super lazy and McGuyver-ing up a heatsink from these right sized, easy reach, super strong neodymium magnets to rest on a roasting 74LS139 in a Beeb. I was worried that they might fall off and short something out, but blow-me-down but they just leapt out of my fingers and stuck firmly to the IC...

So, my inner physicist was plopping magnets on all the ICs in the Beeb and most are seriously attracted to the magnet, however, the AM26LS30, DS88LS120, 81LS95's, 74LS244's, LM319's, 75159, 74F245 and 68B54 aren't though, but the 6502, 6522, 6850, 6845, OSROM, BASIC2, SAA5050, video ULA, serial ULA, an old 74LS245 and all the Hitachi 4816 DRAMs are... Is it steel inside? What part is attracting the magnet?

[1024MAK]

The lead frame I suspect has an iron-nickel alloy in it. Only the very centre part is the actual chip. See the Wikipedia page

Mark