NEWS & VIEWS

JAGADEESH S. MOODERA ANDPATRICK LECLAIRare at the Francis Bitter Magnet Laboratory,MassachusettsInstitute of Technology,Cambridge,Massachusetts 02139,USA.

e-mail: moodera@mit.edu

S emiconductor-based electronics has successfullyruled the world of integrated circuits andcomputers for nearly 60 years.In recent years,

however,there has been an enormous interest inmagnetoelectronic devices1,which use the spin ratherthan the charge of electrons — hence the term ‘spinelectronics’or simply ‘spintronics’.The realization ofspin-based electronics has taken a leap forward with anew proposal for reprogrammable ‘chameleon’processors using magnetoresistive elements2.The ‘on-the-run’reprogrammable capability,low powerconsumption,and non-volatile built-in memory ofthese magnetoresistive devices promise to revolutionizemicroprocessor and computation hardware.

In magnetic materials, the ‘spin up’and ‘spin down’electron populations are unequal,and the electriccurrent is spin polarized; this spin-polarized electricalcurrent can be manipulated with a small magnetic field.One of the crucial properties of magnetic materials istheir intrinsic hysteresis — once a spintronic device isset in a particular state, it remains in that stateindefinitely.This non-volatile property has profoundimplications for data storage,because it represents agreat advantage over conventional semiconductordevices,which require a constant voltage to ‘remember’their state.One highly visible example of this advantageis the phenomenal capacity of hard drives,whichoperate through submicrometre,ultrasensitive spin-dependent read sensors.

The proposal of Ney and colleagues2 forreprogrammable computing is based on simple trilayerelements using either the giant magnetoresistance(GMR) effect3 or the tunnel magnetoresistance (TMR)4

effect. In its simplest form (Fig. 1),a GMR or TMRdevice consists of two ferromagnetic layers (metallic orsemiconducting) separated by a very thin non-magnetic spacer (a metal or insulator). In such devices,the current flow (or resistance) depends on the relativemagnetization orientation of the two magneticelectrodes,which can be parallel or antiparallel.So far,the most clearly visible application of TMR and GMRdevices is in magnetic random-access memories(MRAMs),which exploit the existence of two non-volatile resistance states5,6,12. Information stored inMRAM cells can be held indefinitely without power,with switching speeds and densities projected to beat

conventional memories5,7.However,whereas MRAMshave the potential to revolutionize memory incomputing devices,Ney and colleagues have addressedthe even more exciting promise of spin-basedelectronics for reprogrammable computing.Most importantly, this functionality can be achieved

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SPIN ELECTRONICS

A quantum leapA new design for reprogrammable microprocessors based onsingle magnetoresistive elements has the potential to thrustmagnetoelectronics from journal concept to everyday product.

Figure 1 The operation of tunnel magnetoresistance (TMR) and giant magnetoresistance (GMR)devices. Both devices consist of two ferromagnetic layers separated by a spacer, and have essentiallythe same functionality (although the GMR and TMR effects have fundamentally different microscopicorigins). In both cases, two stable zero-field resistance states are possible, corresponding to parallel andantiparallel magnetizations of the ferromagnetic layers. a,A TMR device consists of two ferromagneticlayers (red) separated by an insulating spacer (blue). For a sufficiently thin spacer (less than about 2 nm),a tunnel current can flow between the ferromagnets. Because spin is conserved in the tunnellingprocess, the current is larger for parallel than for antiparallel magnetizations — an effect of up to about70% change in resistance (D.Wang, unpublished work). b, In a GMR device, the insulating spacer isreplaced with a non-magnetic metal (yellow). Electrons of a given spin orientation are scattered muchmore strongly when travelling through a ferromagnet with a majority of its electrons in the opposite spinstate. For parallel magnetizations, one spin channel (in blue) has almost no scattering, whereas the other(in green) is more strongly scattered. For antiparallel magnetizations, both channels have somescattering.This again leads to differing currents, and a resistance change of about 15% in this case.

TMR

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NEWS & VIEWS

with a relatively minor extension to current MRAMschemes (Fig. 2).

Programmable logic is essentially based on the ideaof generic hardware programmed for a specificfunctionality.For instance,one can make an array ofgates that can be programmed individually as ‘on’or‘off ’so that a single cell can function as an OR,AND,NAND or NOR gate,depending on what is desired.Taken on a larger scale,one can envisage entire chipswhose end functionality is completely configurable —what Ney and colleagues call the ‘chameleon’processor.Many advantages grow out of this multiplefunctionality.Prototyping new logic designs to find themost efficient one becomes trivial.More striking is thepossibility of reconfiguring a chameleon processor ‘onthe fly’to handle tasks more efficiently — if a problem isbetter suited to parallel processing, the chip can beconfigured accordingly.Additionally, information-processing elements and information-storage elements are in fact the same,and interchangeable.This allows a much closer integration of components,potentially allowing marked increases in speed andcomputational efficiency.

Even more exciting is the possibility of usingchameleon processors to create truly universalmagnetoelectronic devices.There are many devices onthe market today with multiple functionalities — acombined cellular phone,MP3 music player andorganizer, for example — but these all have a cost interms of size and complexity.But with areprogrammable spin-logic element the situation couldbe very different.A chip that is one momentprogrammed as a decoder for playing music could,mere nanoseconds later,be reprogrammed as a cellular

phone signal decoder or as a processor dedicated tomathematical calculations. In essence,one couldenvisage devices not so far removed from the fanciful‘tricorders’familiar from Star Trek — a small hand-helddevice whose capabilities are limited only by thecleverness of the programmer.

Programmable logic itself is not new,and designsbased on GMR and TMR devices have been proposedbefore8,9.What sets the current work apart is that thedesign is based on existing MRAM cells and requiresonly two additional current-carrying leads.The biggestadvantage of this idea is that several potential problems(such as cell size, switching field distribution,magneticcoupling between layers,and cost) are already beingsolved in the development of MRAM.However,moreresearch will be needed to show whether Ney’s proposalis indeed practicable and scalable.Current MRAMdesigns are based on the use of one ‘free’and one‘pinned’magnetic layer for magnetic stability10,whereasNey’s scheme relies on two free layers with differentswitching fields. It is uncertain whether a practicalMRAM logic array can be made in this way; perhaps onecan use two cells to generate all four logic functions.Another often-cited fault of magnetic logic is the ratherlow ‘on/off ’ratio of magnetic devices (about 1.5,compared with thousands or more for semiconductortransistors). Improving on this ratio requires bothimproved device designs and more fundamentalresearch into new materials. In particular, so-called halfmetals — which have only one spin-state available forthe current carriers11 — might allow near-infinite on/offratios.Spin-based devices owe their existence to basicresearch3,4,12,and their potential use in novelarchitectures will almost certainly follow this trend.

References1. Prinz, G. A. Science 282, 1660–1663 (1998).

2. Ney, A., Pampuch, C., Koch, R. & Ploog, K. H. Nature 425, 485–487 (2003).

3. Baibich, B. N. et al. Phys. Rev. Lett. 61, 2472–2475 (1988).

4. Moodera, J. S., Kinder, L. R., Wong, T. M. & Meservey, R. Phys. Rev. Lett. 74,

3273–3276 (1995).

5. http://www.motorola.com/content/0,1037,307,00.html

6. http://www.hpl.hp.com/news/storage.html

7. Wolf, S. A. et al. Science 294, 1488–1495 (2001).

8. Black, W. C. Jr & Das, B. J. Appl. Phys. 87, 6674–6679 (2000).

9. Richter, R. et al. J. Magn. Magn. Mater. 240, 127–129 (2002).

10.Gider, S., Runge, B.-U., Marley, A. C. & Parkin, S. S. P. Science 281, 797–799

(1998).

11.de Groot, R. A., Mueller, F. M., van Engen, P. G. & Buschow, K. H. J. Phys. Rev.

Lett. 50, 2024–2027 (1983).

12.Gallagher, W. J. et al. J. Appl. Phys. 81, 3741–3746 (1997).

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Figure 2The ‘chameleon’processor design for a magneticlogic gate cell, composed of amagnetoresistive element andthree independent current-carrying leads (A,B and C)2.The magnetization direction offerromagnetic layer 1 isswitched when there is currentflowing simultaneously in both A and B.Similarly, themagnetization of ferromagneticlayer 2 can be switched byhaving current flow in all threeleads A,B and C. In this way,various magnetic configurations(and hence the output voltage ofthe cell) are set by controlling theinitial current flow in leads A,Band C.Each of the magneticstates is non-volatile until actedon by a combination of currentpulses in the leads.

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© 2003 NaturePublishing Group

© 2003 Nature Publishing Group