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Magnetic Materials


Magnetic Properties of Materials
A History of Superconductivity
Molecule-Based Magnets


Spintronics (short for spin-based electronics), sometimes called magnetoelectronics, is the term given to microelectronic devices that function by exploiting the spin of electrons. The most common use of spintronics today is in computer hard drives. Here memory storage is based on giant magnetoresistance (GMR) a spintronic effect. There is current research focusing on bringing magnetic random-access memory (MRAM) to market. Spintronic based MRAMs should rival the speed and rewritability of conventional RAM and retain their state (and thus memory) even when the power is turned off. Motorola has recently developed a 256-kb MRAM (see Figure 7) based on a single magnetic tunnel junction and a single transistor. This MRAM has read/write cycles of less than 50 nanoseconds.

Figure 7. A 256-kb MRAM based on modern spintronics technology. (Image courtesy of Motorola Corp.)

Spintronics focuses on two types of materials. Ferromagnetic metallic alloys are currently used for magnetoelectronic devices. Ferromagnetic semiconductors, however, are attracting greater attention. If the manufacture of ferromagnetic semiconductors becomes practical, the current microchip industry could switch over to these types of spintronic devices with relatively little change in their infrastructure. The primary barrier to the synthesis of ferromagnetic semiconductors is finding a way to inject spin-polarized currents (spin currents) into a semiconductor.

Most of the work in making semiconductors ferromagnetic has focused on II-VI semiconductors such as CdTe (Aldrich product 25,654-4) or ZnSe (Aldrich products 24,461-9 and 55,301-8).29 Here the semiconductors are doped with magnetic ions (such as manganese) to create small pockets of magnetic character resulting in a diluted magnetic semiconductor (DMS). More recent work has focused on doping III-V semiconductors such as GaAs (Aldrich product 32,901-0) into a DMS state. With III-V semiconductors, however, the magnetic elements are much less soluble than in II-VI semiconductors making them much more difficult to inject into a material like GaAs. Molecular beam epitaxy (MBE) has proven to be an excellent technique in overcoming the difficulty in making DMS III-V materials.30

As new and better techniques for synthesizing ferromagnetics are developed, their prospects for revolutionizing the microelectronic industry increases. Spintronics will sure play a major role in the next generation of information storage devices.

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