Ultra-High Density Magnetic Memory Cells

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We have investigated magnetic properties and magnetization processes of sub-micron magnetic memory cells in order to find a breakthrough that increases a density of MRAM (magnetic random access memory).(1) We have established a microfabrication process that makes rectangular memory cells (70 nm x 200 nm). The size corresponds a memory volume of 16 Gbits.(2) Magnetic states of the Ni-Fe(5 nm)/Hf(5 nm) rectangular cells and Ni-Fe(5 nm)/Hf(2 nm)/Ni-Fe(5 nm)/Hf(5 nm) rectangular cells were observed using magnetic force microscopy. The observation showed that the magnetic flux closed through the two magnetic layers in the Ni-Fe/Hf/Ni-Fe/Hf cells, and the closed magnetic flux decreased the switching field of the Ni-Fe/Hf/Ni-Fe/Hf cells. The micromagnetics simulation showed that the switching field of the Ni-Fe/Hf/Ni-Fe/Hf cells is about 1/2 of the Ni-Fe/Hf cells for the cell size of 50 nm x 100 nm.(3) The micromagnetics simulation showed that the magnetization direction of the asymmetric ring dots (material: 5-20 nm thick Ni-20at%Fe, diameter. 500-1000 nm) that have partly planed outer sides were controlled between clockwise and counterclockwise by in-plane magnetic fields, even though the ordinary ring dots needed the clockwise and counterclockwise magnetic fields caused by the perpendicular current.