Magnetoresistive Random Access Memory

Figure 30.10 [30.15] is a schematic diagram of a mag-netoresistive random-access memory (MRAM) that acts as a non-volatile storage device. As with Flash Memory, there are word lines and bit lines at the intersection of which is a cell with a switchable property. Whereas with Flash Memory the electric potential of a storage cell is modified by electron tunneling, here it is the magnetic domain orientation that is switched by the current flowing through the electrodes. Passage of electrons through the magnetic domain of the cell depends on the state of polarization of the spins of the injected electrodes, much the same as the operation of giant magnetoresistance elements used to read data in a hard disk drive. In contrast to HDD and Flash Memory, MRAM is still in a stage of development and, as such, is not yet commercially available. One of its main attributes is its high speed of programming, which is faster than that of Flash Memory. The main challenges confronting MRAM technology are: (i) the size of the cell is still too large, (ii) the power consumption required during the write mode is still too high, and (iii) there are manufacturing difficulties associated with high temperature steps that may damage the magnetic layers.

"0" State

Magnetization direktion

Magnetization direktion

Word current

Sense current

Sense line

Sense current

Sense line

Magnetization direktion

Magnetization

Word current

"1" State

Magnetization direktion

Word current

Sense current

Sense line

Fig. 30.10 A diagram of a MRAM cell

Sense current

Sense line

Magnetization

Word current

To exemplify the status of this technology, note that Motorola demonstrated a 1-Mbit MRAM test chip using a 0.6-^m process, yielding a cell size of 7.2 ^m2. Projections are that using a 0.18-^m process will shrink the cell size to about 0.7 ^m2, and thus be competitive with Flash Memory. Also, Sony demonstrated an MRAM test chip using a 0.35-^m process that yielded a cell size of 5.8 ^m2. Other companies like IBM, which is partnering with Infineon, are also active in MRAM research.

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