10.4 Magnetic Random Access Memories (MRAM)
237
10.4.4 About MTJ-Based Magnetic Random Access
Memories (MRAM)
Recently, there is a strong interest in the development of non-volatile memory
devices based on magnetic materials, i.e., Magneto-resistive Random Access Memories (MRAMs). The key attributes are (i) non-volatility; (ii) low operating voltage;
(iii) limitless endurance of reading and writing capacity; (iv) high-speed read and
write operation; (v) radiation hardness and (vi) very large (>10
15 ) read–write cycle
capability (Granley et al. 1996). Combination of these attributes in MRAM provides
performance, cost-effectiveness and several exclusive characteristics for diversified
applications. MRAM memory technology merges MR device, with MTJs as storage
element and standard Si-based microelectronics to yield the functionality. Some basic
characteristics of MTJs are as follows:
(a) As already discussed, operation of MTJ is governed by quantum mechanical
tunnelling of spin-polarized electrons through a very thin insulating spacer
layer.
(b) Resistance of the MTJ depends on the relative magnetization directions of two
ferromagnetic layers separated by an insulating spacer layer.
(c) MRAM cells are constructed in such a way so that it possesses two stable
magnetic states corresponding to high or low resistance values. More important
point is that those high or low resistance values should be retained without
application of any external power.
(d) High value of MR, obtained in MTJs, promises implementation in case of large
read signals for MRAM cells.
(e) Moreover, higher resistance of the cell and enhanced MR value causes large
output signal of cell voltage and optimal matching of impedance with peripheral
sensing circuitry.
These advanced attributes allow MTJ-based MRAM to be commercially competitive. This approach is quite different from usually obtainable commercial memories,
such as (dynamic) DRAM and Flash memory. Their working principle is generally
based on stored charge. It has been found that MRAMs could be as fast as Dynamic
Random Access Memories (DRAMs) and almost as tiny as Static Random Access
Memories (SRAM) in the dimension of the cell.
10.4.5 Basic Cell Operation
In case of MTJ-based MRAM device, a single MTJ defines the memory cell. MTJ
cells are arranged in array as shown in Fig. 10.8a. In order to read a single bit, a
bias voltage is applied across that particular bit line where the target bit is located.
In the following manner, the memory state of the bit is decided from the amount of
current passing through the bit. It is well understood that a MTJ bit consists of a
237
10.4.4 About MTJ-Based Magnetic Random Access
Memories (MRAM)
Recently, there is a strong interest in the development of non-volatile memory
devices based on magnetic materials, i.e., Magneto-resistive Random Access Memories (MRAMs). The key attributes are (i) non-volatility; (ii) low operating voltage;
(iii) limitless endurance of reading and writing capacity; (iv) high-speed read and
write operation; (v) radiation hardness and (vi) very large (>10
15 ) read–write cycle
capability (Granley et al. 1996). Combination of these attributes in MRAM provides
performance, cost-effectiveness and several exclusive characteristics for diversified
applications. MRAM memory technology merges MR device, with MTJs as storage
element and standard Si-based microelectronics to yield the functionality. Some basic
characteristics of MTJs are as follows:
(a) As already discussed, operation of MTJ is governed by quantum mechanical
tunnelling of spin-polarized electrons through a very thin insulating spacer
layer.
(b) Resistance of the MTJ depends on the relative magnetization directions of two
ferromagnetic layers separated by an insulating spacer layer.
(c) MRAM cells are constructed in such a way so that it possesses two stable
magnetic states corresponding to high or low resistance values. More important
point is that those high or low resistance values should be retained without
application of any external power.
(d) High value of MR, obtained in MTJs, promises implementation in case of large
read signals for MRAM cells.
(e) Moreover, higher resistance of the cell and enhanced MR value causes large
output signal of cell voltage and optimal matching of impedance with peripheral
sensing circuitry.
These advanced attributes allow MTJ-based MRAM to be commercially competitive. This approach is quite different from usually obtainable commercial memories,
such as (dynamic) DRAM and Flash memory. Their working principle is generally
based on stored charge. It has been found that MRAMs could be as fast as Dynamic
Random Access Memories (DRAMs) and almost as tiny as Static Random Access
Memories (SRAM) in the dimension of the cell.
10.4.5 Basic Cell Operation
In case of MTJ-based MRAM device, a single MTJ defines the memory cell. MTJ
cells are arranged in array as shown in Fig. 10.8a. In order to read a single bit, a
bias voltage is applied across that particular bit line where the target bit is located.
In the following manner, the memory state of the bit is decided from the amount of
current passing through the bit. It is well understood that a MTJ bit consists of a