Magnetic film domain-wall motion devices

Magnetic domain-wall motion memory devices have been under development for a number of years. Several techniques have been devised for controllably introducing a pattern of magnetic domains into a saturated background material and for shifting these domains between input and output locations. A particularly flexible version of this class of device, lending itself as well to the performance of magnetic logic, is channeled domain tip propagation. The technique is reviewed. Channeled domain-tip propagation requires some means of defining a pattern of narrow low coercive-force paths within a background material of high coercivity. Several techniques are described which include the use of the surface roughness of an underlying metallic film and the direct coupling between hard and soft magnetic layers for locally modifying coercive force in an appropriate pattern. The variety of magnetic channel structures which can be produced are described along with the resulting magnetic properties and their application to memory and logic devices. Their properties are functions of the size and shape of the channel structures and depend on their orientation with respect to the easy axis of the film and on the composition of the latter. Theory and experiment are presented which describe the dependence of channeled domain-tip coercive force and velocity of propagation on the variables mentioned above. The operation of various magnetic logic elements, including AND, OR, and NOR gates and nonreciprocal propagation paths, i.e., diodes, is discussed. The use of inductive and magnetoresistive schemes for detecting output signals is described. Several specific examples of the application of domain-tip propagation techniques to different memory and logic devices are given. The design and operation of counters, long shift registers, a last-in first-out (LIFQ) list memory, and a first-in first-out (FIFO) serial data buffer memory are discussed. These examples serve to illustrate the criteria used in the design of domain-tip propagation memory devices as well as to demonstrate the use of the bidirectional shifting and magnetic logic capability of the technique.