A
universal memory that is fast, power-efficient and non-volatile would
allow new designs that avoid this bottleneck. Hao Meng and co-workers at
the A*STAR Data Storage Institute have now shed new light on how to
manufacture such a memory
Computers
often do not run as fast as they should because they are constantly
transferring information between two kinds of memory: a fast, volatile
memory connected to the CPU, and a slow, non-volatile memory that
remembers data even when switched off. A universal memory that is fast,
power-efficient and non-volatile would allow new designs that avoid this
bottleneck. Hao Meng and co-workers at the A*STAR Data Storage
Institute have now shed new light on how to manufacture such a memory.
The
researchers explored a special class of universal memory called
spin-transfer torque magnetic random access memory (MRAM). A
spin-transfer torque MRAM typically comprises two magnetic films that
are separated by an insulating layer. The resistance between the two
films is low if the magnetization direction in each film is parallel,
and high if it is anti-parallel. Information is stored in the relative
magnetization between the two films, and read out by measuring
resistance. The magnetization directions can be switched by applying
spin torque to the films’ magnetic domains (using a spin polarized
electric current).
High-temperature
annealing is a key step in the manufacture of an MRAM cell. Annealing
alters the crystal structure of the cell materials, which in turn
changes the degree of magnetization and how the cell functions. In
particular, the greater change in resistance between parallel and
anti-parallel magnetizations, the better the memory will function.
Previous studies have shown that this resistance change increases as the
annealing temperature increases, but drops if the annealing temperature
rises too much.
Meng
and co-workers extended this analysis to other critical MRAM
characteristics. They focused on a cell made with CoFeB magnetic films,
which has a natural magnetization direction outside of the plane of the
film. They found that the annealing temperature that yielded maximum
resistance variation exceeded the temperature necessary for maximum
thermal stability. This is critical information for design engineers,
who must balance these two metrics against each other.
Meng
and co-workers also found that the minimum current density necessary to
change the film magnetization increased with annealing temperature. A
lower current is desirable for practical cell operation. The current
density could be lowered by reducing the thickness of the magnetic
films. However, lower thicknesses also produced an undesirable reduction
in resistance variation. By explicitly demonstrating the trade-offs
necessary in the design of spin torque MRAMs, the data is expected to
help engineers design the next generation of these promising devices.
Source: A*STAR
