Acousto-Optics, Optical Computing, and Signal Processing - Applied Optics Fundamentals and Device Applications - page 141

concept was similar to the serial propagation of bubbles in a magnetic bub-
ble memory, but promised faster performance. The serial crosstie memory
presented numerous development problems, however, illustrating the
need for an alternative approach [48]. The random access approach was
successfully demonstrated in early 1982 by Mentzer, Schwee et al. [49]. Its
unique characteristics as a faster, nonvolatile, radiation- and temperature-
insensitive, random access device make the crosstie memory a viable
alternative for many applications. More recently [50], this technology is
being developed for alternative flash memory, fast-start computer systems,
biomedical storage devices, and dynamic RAM replacement. Significant
efforts are underway as of this writing at IBM, Infineon, Naval Research
Laboratory, and elsewhere.

3.12.2 Fabrication and Operation of the Crosstie Memory

Thin film permalloy (81-19 Ni-Fe) patterns on a silicon substrate support
stable magnetization domain states which can be switched rapidly and
detected nondestructively. The geometry of the basic memory cell is shown
in Figure 3.30 [51]. The contours represent the direction of magnetization in
the memory cell. The cell on the left represents a digital zero (Neel wall); and
the cell on the right represents a digital one state (crosstie Bloch pair on Neel
wall). The crosstie wall constitutes a transition state between Bloch walls of
thick films and Neel walls of very thin films. The fundamental cell geometry
supports two states—the Neel wall and the Bloch line-crosstie pair on the
Neel wall.
Vacuum deposition of the permalloy alloy is performed in a magnetic
field resulting in approximately 400 Å isotropic films with H k < 10 Oe.

Neel wall: ˝Zero˝ Crosstie-Bloch

line pair: ˝One˝

FIGURE 3.30
Magnetic domain pattern in CRAM storage element.