Metal particle tape (MP)
Magnetic tape whose layer is made of finely divided metallic iron particles rather than oxide — higher density but more prone to ageing.
Until the 1980s magnetic tape was coated with iron oxide, the same element as rust. Metal particle tape switched to pure metallic iron grains, which have much higher coercivity and can hold a more densely written signal. That made the formats built on it small and capable: DAT/DDS for data, Video8 and DV for pictures, and the first generations of LTO.
The price is durability. Pure metallic iron wants to oxidise, and an MP tape left in a humid archive for twenty years can have lost a measurable amount of signal. That is exactly why the tape industry moved to barium ferrite, which is already an oxide and can't rust further. A related variant, metal-evaporated (ME) tape, laid the metal down as a thin evaporated film instead of loose particles.