Thayer Watkins
Silicon Valley

Metallic Magnetic Superlattices

Source: L.M. Falicov, "Metallic Magnetic Superlattices," Physics Today, (October 1992), pp. 46-51

Metallic magnetic superlattices (also known as multilayers) are alternating thin layers of magnetic and nonmagnetic metals constructed with precise dimensions and composition. They have interesting properties with respect to resistance and magneto-optics. The magnetic layers may have their direction of magnetization parallel (ferromagnetic coupling or antiparallel (antiferromagnetic coupling). The nature of the coupling between the magnetic layers depends upon the thickness and nature of the nonmagnetic spacers between the magnetic layers. For very thin nonmagnetic layers the natural arrangement is antiferromagnetic coupling. This is related to the tendency for a ferromagnetic material to partition itself into domains with antiparallel orientation of the directions of magnetization. It is energetically advantageous for a ferromagnetic material to divide into such domains. But there is a natural unit of length for such domains and if the thickness of the nonmagnetic spacer layers gets to twice the natural width of the ferromagnetic domains the coupling of the magnetic layers will be parallel rather than antiparallel.

The nature of the coupling of the magnetic layers affects resistance as well as other properties of the metallic superlattice. Thus the magnetic arrangement or coupling can be monitored by several means including:

Falicov cites the creation of a wedge-shaped spacer with a very low wedge angle as an important technical achievement. The slope of the wedge is between 10nm per mm and 40 nm per mm or 1x10-5 and 4x10-5. The reciprocal of this slope is the ratio of the accuracy required in the horizontal dimension to achieve a given accuracy in the vertical dimension; i.e., 105 to 2.5x104.

Technical Application of Negative Magnetoresistance

Multilayers with giant negative magnetoresistance can be used to read magnetically recorded information. The circuitry can be simpler and the read times lowered by a factor of 100. This means the density of stored information can increase substantially. This will make MRAM, Magnetic Random Access Memory, feasible.