South Korea's Kospi sinks 7.57% as chip fears compound a brutal week

A 500% jump in RAM prices was supposed to be good news for Samsung and SK Hynix. Instead their stock has been in free fall all week, and Thursday was the worst day yet.

South Korea's benchmark Kospi index dropped 7.57% on October 15, closing at 5,554.22 and marking its seventh straight losing session, according to a report from the Seoul Economic Daily that dubbed it "Black Thursday." Bloomberg put the intraday decline at as much as 8%, the sharpest drop since the index entered what traders are now calling a rare bear market. Samsung Electronics closed down 9.48% at 210,000 won. SK Hynix fell 11.06% to 1,262,000 won. Samsung Electro-Mechanics, a smaller supplier further down the chain, lost almost 17% in a single session.

This isn't a one-day scare. It's the back half of a story that started two days earlier, on October 13, when CXMT, the Chinese state-backed memory chipmaker, told an industry conference in Shanghai that it plans to launch a DRAM chip built on a new cell architecture called 4F² before the end of 2026. Samsung shares fell 7.4% that day and SK Hynix fell 10%, according to Bloomberg, in what Saxo strategist Charu Chanana called "a clear trigger for the selloff in memory stocks." Thursday's plunge piled a second, unrelated shock on top of a market that hadn't recovered from the first.

4F² refers to the physical footprint of a single memory cell: four times the minimum line width a manufacturer's lithography tools can etch, squared. Smaller footprint per cell means more cells packed onto the same wafer, which means more storage without a more expensive manufacturing process. Samsung and SK Hynix have not gotten there yet in mass production. If CXMT genuinely has, a company still barred from buying ASML's most advanced lithography machines under US export controls would have beaten both Korean giants to a milestone neither has cracked, despite having no such restrictions.

That's a big if. CXMT has published no yields, no capacities, no shipping date beyond "before the end of 2026," and no independent verification of the claim. Morningstar analyst Jing Jie Yu told Bloomberg that memory-equipment makers have flagged real difficulty in the jump to 4F² architecture, and that the gap between unveiling a design and shipping it at commercial yields can run for years, not months. CXMT's global DRAM market share sat at roughly 10% as of September, according to Bloomberg's reporting, still a fraction of Samsung's or SK Hynix's. Frankly, a conference announcement with no hardware behind it is a strange thing to erase tens of billions of dollars of market value over. But fear doesn't wait for proof.

What makes the CXMT claim land so hard right now is the backdrop it's landing on. DRAM prices have been spiking for months on the back of AI data center demand, with 128GB DDR5 modules reportedly climbing as much as 500% to around $3,399, a surge Micron has called the worst memory supply shortage in 40 years. Samsung and SK Hynix have been able to raise prices because supply is tight and most of their new capacity is going to high bandwidth memory for AI chips, not to the consumer RAM market. A credible Chinese competitor flooding that same supply with cheaper, denser chips threatens the one thing propping up their stock multiples: pricing power. That's the trade investors are making, whether or not CXMT ever ships a working chip.

Rate fears piled on top

Thursday's drop had its own, separate trigger. US consumer price data released overnight showed inflation running at 3.7% year over year, above the 3.6% forecast, according to the Korea JoongAng Daily. That reading pushed back market expectations for near-term Federal Reserve rate cuts, which hits richly valued AI-linked stocks particularly hard, the same stocks that had carried the Kospi to records earlier this year. Foreign investors sold a net 3.63 trillion won, about $2.7 billion, of Kospi shares on Thursday alone, extending a selling streak to eight straight sessions.

So the Kospi is absorbing two shocks that have nothing to do with each other but happen to hit the same stocks at once: a long-term structural fear about Chinese competition in memory chips, and a short-term repricing of US interest rate odds. Untangling how much of Thursday belongs to each is close to impossible, and that's exactly the problem for anyone trying to call a bottom. The CXMT threat, real or not, isn't going away on its own timeline. The Fed's next move isn't either.

Also read: TSMC posts record profit as AI chip demand defies market debt fears • South Korea's KOSPI Plunges 8% as Samsung and SK Hynix Lead a Chip Rout • Copper's record 2026 rally is the AI infrastructure trade nobody priced in

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