Magnetic Random Access Memory
Figure 9.1 Three types of memory cell structure (a) SRAM (b) DRAM (c) Flash.Recently a lot of research has been done in magnetic random access memory (MRAM) because of the development of novel highly magnetoresistive (MR) thin film magnetic structures, namely giant magnetoresistance (GMR) and the related spin-valve sandwich in metallic multilayered structures , and magnetic tunnel junction (MTJ) structures [10-14]. MRAM is the most promising candidate for a memory with all advantages including fast speed, high density and low power consumption, that’s why referred as universal memory. It combines non-volatility with relatively high read and write speeds and unlimited endurance. Furthermore, the MRAM storage element resides in the metal interconnect layers, well above the silicon, allowing its process to be optimized independently from the underlying semiconductor process. MRAM is therefore cost effective to integrate and is ideally suited for embedded-memory applications. State-of-the-art MRAM combines a magnetoresistive magnetic tunnel junction with a single-pass transistor for bit selection during read process (Fig. 9.2) . In a tunnel junction there is a free magnetic layer, a tunneling barrier and a fixed magnetic layer. The magnetization of the fixed layer is pinned and prevented from rotating, whereas the free layer magnetization orientation is not fixed, it can be switched and is used for information storage. The relative magnetization orientation of free layer with respect to the fixed layer is responsible for the high or low resistance of the magnetic
tunnel junction. For tunnel junctions with an aluminum oxide tunneling barrier, the maximum difference in resistance (the magnetoresistance) can be up to 70% .