Magnoresistive Random Access Memories (MRAMs)

Magnetoresistive Random Access Memory (MRAM) combines a magnetic device with standard silicon-based microelectronics to obtain the combined attributes of non-volatility, high density, high-speed operation and unlimited read and write endurance, which are not found in any other existing memory technology. MRAM typically employs magnetic tunnel junctions (MTJs) as storage elements. The MTJ is a device based on quantum mechanical tunneling of spin-polarized electrons through a very thin dielectric. The relative magnetization orientation of two ferromagnetic layers separated by this dielectric layer determines the resistance of the MTJ structure. MRAM cells are designed to have two stable magnetic states that correspond to high or low resistance values and retain those values without any applied power. The cells are read by sensing the resistance to determine if the state is high or low, while the writing is carried out by the magnetic fields generated from the current flowing in the bit and word lines.

Fig. 1 Concept of MRAM.

This resistance-based approach is distinctly different from commonly available commercial memories such as DRAM and Flash memory that are based on stored charge. To date, the development of MRAM technology has faced many challenges impacting the manufacturability of high-density devices. These include achieving adequate write switching and disturb margins to support reliable operation, and adequate read margins to operate at a competitive access speed.