Comprehension

Directions for Questions: Read the passage carefully and answer the given questions accordingly.

In a modern computer, electronic and magnetic storage technologies play complementary roles. Electronic memory chips are fast but volatile (their contents are lost when the computer is unplugged). Magnetic tapes and hard disks are slower but have the advantage that they are non-volatile so that they can be used to store software and documents even when the power is off.

In laboratories around the world, however, researchers are hoping to achieve the best of both worlds. They are trying to build magnetic memory chips that could be used in place of today’s electronic ones.

These magnetic memories would be non-volatile; but they would also be faster, would consume less power, and would be able to stand up to hazardous environments more easily. Such chips would have obvious applications in storage cards for digital cameras and music-plyers; they would enable handheld and laptop computers to boot up more quickly and to operate for longer; they would allow desktop computers to run faster; they would doubtless have military and space-faring advantages too. But although the theory behind them books solid; there are tricky practical problems and need to be overcome.

Two different approaches, based on different magnetic phenomena, are being pursued. The first, being investigated by Gary Prinz and his colleagues at the Naval Research Laboratory (NRL) in Washington, D.C., exploits the fact that the electrical resistance of some materials changes in the presence of a magnetic field-a phenomenon known as magneto-resistance. For some multi-layered materials this effect is particularly powerful and is, accordingly, called “giant” magneto-resistance (GMR). Since 1997, the exploitation of GMR has made cheap multi-gigabyte hard disks commonplace. The magnetic orientations of the magnetized spots on the surface of a spinning disk are detected by measuring the changes they induce in the resistance of a tiny sensor. This technique is so sensitive that it means the sports can be made smaller and packed closer together than was previously possible, thus increasing the capacity and reducing the size and cost of a disk drive.

Dr. Prinz and his colleagues are now exploiting the same phenomenon on the surface of memory chips rather than spinning disks. In a conventional memory chip, each binary digit (bit) of data is represented using a capacitor reservoir of electrical charge that is either empty of full ---to represent a zero or a one. In an NRL’s magnetic design, by contrast, each bit is stored in a magnetic element in the form of a vertical pillar of magnetizable material. A matrix of wires passing above and below the elements allows each to be magnetized, either clockwise, to represent zero or one. Another set of wires allows current to pass through any particular element. By measuring an element’s resistance you can determine its magnetic orientation, and hence whether it is storing a zero or a one. Since the elements retain their magnetic orientation even when the power is off, the result is Non-Volatile memory. Unlike the elements of electronic memory, a magnetic memory’s elements are not easily disrupted by radiation. And compared with electronic memories, whose capacitors need constant topping up, magnetic memories are simpler and consume less power. The NRL researchers plan to commercialize their device through a company called Non-Volatile Electronics, which recently began work on the necessary processing and fabrication techniques. But it will be some years before the first chips roll the production line.

Most attention in the field is focused on an alternative approach based on the magnetic tunnel - junctions (MTJs), which are being investigated by researchers at chip markets such an IBM, Motorola, Siemens, and Hewlett- Packard. IBM’s research team, led by Stuart Parkin, has already created a 500-element working prototype that operates at 20 times the speed of conventional memory chips and consumes 1% of the power. Each element consists of a sandwich of two layers of magnetizable material separated by a barrier of aluminum oxide just four or five atoms thick. The polarisation of a lower magnetizable layer is fixed in one direction, but that of the upper layer can be set (again, bypassing a current through a matrix of control wires ) either to the left or to the right, to store a zero or a one. The polarisations of the two layers are then in either the same or opposite directions.

Although the aluminum-oxide barrier is an electrical insulator, it is so thin that electrons are able to jump across it via a quantum-mechanical effect called tunneling. It turns out that such tunneling is easier when the two magnetic layers is polarised in the same direction than when they are polarised in opposite directions. So, by measuring the current that flows through the sandwich, it is possible to determine the alignment of the topmost layer, and hence whether it is storing a zero or a one.

To build a full-scale memory chip based on MTJ’s is however no easy matter. According to Paulo Freitas, an expert on-chip manufacturing at the Technical University of Lisbon, magnetic memory elements will have to become far smaller and more reliable than current prototypes if they are to compete with electronic memory. At the same time, they will have to be sensitive enough to respond when a neighboring element is changed. Despite these difficulties, the general consensus is that MTJ’s are the more promising ideas. Dr Parkin says his group evaluated the GMR approach and decided not to pursue it, despite the fact that IBM pioneered GMR in hard disks. . Dr. Prinz, however, contends that his plans will eventually offer higher storage densities and lower production costs.

Not content with shaking up the multi-dollar market for computer memory, some researchers have even more ambitious plans for magnetic computing. In a paper published last month in Science, Russell Cowburn and Mark Welland of Cambridge University outlined research that could form the basis of a magnetic microprocessor - a chip capable of manipulating (rather than merely storing) information magnetically. In place of conducting wires, a magnetic processor would have rows of magnetic dots, each of which could be polarised in one of the two directions. Individual bits of information would travel down the rows as magnetic pulses, changing the orientation of the dots as they went. Dr. Cowburn and Dr. Welland have demonstrated how a logic gate (the basis of a microprocessor) could work in such a scheme. In their experiment, they fed a signal in at one end of the chain of dots and used a second signal to control whether it propagated along the chain.

It is admittedly, a long way from a single logic gate to a full microprocessor, but this was true also when the transistor was first invented. Dr. Cowburn is now searching for backers to help commercialize the technology, says he believes it will be at least ten years before the first magnetic microprocessor is constructed. But other researchers in the field agree that such a chip is the next logical step Dr. Prinz says that once magnetic memory is stored out “the target is to go after the logic circuits.” Whether all magnetic computers will ever be able to compete with other contenders that are jostling to knock electronics off its perch-such as optical, biological, and quantum computing-remains to be seen. Dr. Cowbrun suggests that the future lies with hybrid machines that use different technologies. But computing with magnetism evidently has an attraction all its own.

CAT/2000

Question . 125

In the MTJs approach, it is possible to identify whether the topmost layer of the magnetized memory element is storing a zero or one by