Fred, I am hoping for an upturn in ENER stock so I can get off my diet of beans and coffee - and buy some TP. Here is today's "john" report.
The 27 Nov 98 Science Magazine review article " Magnetoelectronics" is by Gary A. Prinz, of the US Naval Research Lab. It covers several devices based on electron spin alignment effects on the electrical resistance of multilayer, alternating films of ferromagnetic materials (FMM) and nonmagnetic materials (NMM). Unfortunately, the article provides little in the way of quantitative characteristics.
For nonvolatile memory chips, two devices are briefly reviewed. One development is apparently being led by Honeywell. They use what are called "spin valve" elements. In these, the resistance of a layer of conductive NMM is sandwiched between two layers of FMM. When the spin orientation of the two FMM layers is aligned, the element exhibits less resistance. Elements of this type are contacted at the edges - i.e., the current flows parallel to the layers - the reason being that the element would have too little resistance as measured perpendicular to the layers (which are very thin). One FMM layer is magnetically "soft" so its spin orientation can be changed by a current in a nearby wire (electrical currents always generate magnetic fields). There are fancy "addressing" schemes that allow the elements, arranged in arrays, to be written and read with the same lines (leads).
The second device described is the one being pioneered by IBM - the "TMJ" element we have been discussing. The article, again, gives very little quantitative information. Physically, the element is a layer of non-conducting NMM sandwiched between two layers of FMM. The layer of NMM is so thin that electrons can "tunnel" through (thanks to the quantum mechanical nature of the universe). This type of element is contacted by lines on opposite sides of the element - i. e., the current flows perpendicular to the layers - the reason being that the resistance along the layers would be too much, just the opposite problem compared to the "spin valve" elements. A few comments by the author are revealing:
" A detailed physical understanding of (this element) is still being actively researched, but the large changes in device impedance (~30%) at room temperature already permit application for device technology." (30% is large?, The OUM changes by about 5000%!)
After noting that TMJs carry low currents, desirable for portable electronics, Prinz says: "However, the high impedance of a (TMJ) may prove to be unattractive in terms of response time or noise. This challenge increases as device sizes are reduced, because the tunneling devices carry their current perpendicular to the plane of the films and, as the area of the device shrinks, the resistance increases".
Prinz notes that TMJ elements in an array can be written and read using the same lines, in a grid, but "such an array is multiply shorted by the elements". "The solution to this old problem with grid arrays is to place a diode at every intersection so that the current can pass in only one direction, . . ." "It is a technological challenge to fabricate these diodes in an integrated manner with the tunnel junction storage elements, but its solution could permit the construction of extremely high density memory."
I feel better and better about our OUM - or is it just that I am always "relieved" by the time I finish my "john" reading. |
