Size Dependence of the Read Voltage and Electrical Diameter of STT MRAM Cells

We present an experimental study of the size-dependent tunneling magnetoresistance ratio (TMR) and voltage read signal in perpendicular spin transfer torque magnetoresistive random-access (MRAM) memory cells, which shows that the maximum read signal is mostly independent of the size, while TMR decreases with decreasing size. Our analysis shows that this is due to a size-dependent parasitic resistance specific to the nanofabrication process and that the intrinsic $\Delta \text{RA}$ of the cells is size-independent. As a consequence, we show that the electrical diameter of an MRAM cell can be reliably extracted down to sub-20 nm assuming that $\Delta \text{RA}$ does not depend on the cell size.