nanomagnetism a case history of nanoscience and technology

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This paper overviews the occurrence and studyof magnetism on nanometer-length scales,that is, at sizes where the natural unit on a

ruler would be one-billionth of a meter. Nanomagne-tism has fascinating early origins on planet Earth, and

we must first go back a couple billion years to get tothe beginning of the story. Then we will quickly makesome big jumps forward through time, to get back tothe present. The take-home message is this: we arevery lucky to be here now with the first chance in bil-lions of years to understand nanomagnetism in detail,

using the tools provided by physics.From the perspective of a physicist, magnetism on

nanometer-length scales represents a thoroughly de-veloped, specific example of nanoscience and technol-ogy. The reasons for this include:1) Nanomagnets arent just small, they are also dif-

ferent, making them more compelling objects ofstudy for the physicist.

2) Nanomagnets are the first example of a physics-based nanotechnology to facilitate a >$100 billionannual revenue global industry.

3) Last but not least, a nanomagnetic effect underly-ing the previous point in the autumn of 2007 be-came the first nanotechnological development tobe recognized by a Nobel Prize in physics.

The BeginningsThe first two billion years or so of nanomagnetism

are captured by the transmission electron micrographof Fig. 1(a), an image of the magnetoaerotactic bac-teriumMagnetospirillium magneticum.1,2 These bugs

Nanomagnetism: A Case

History of Nanoscience andTechnologyMark Freeman,University of Alberta and National Institute for Nanotechnology, Edmonton, Canada

grow their own compass needle inside, to passivelyorient themselves in the Earths magnetic field. Theneedle works like a magnetic dip meter in an instruc-tional physics lab, and naturally guides the swimmingdirection toward the mud, where the food is. Themagnetoaerotactic name comes about because, un-like its purely magnetotactic brethren, which swim the

wrong way if the magnetic field direction is reversed(as in visiting the Southern Hemisphere),Magnetospi-

Fig. 1. (a) Transmission electron micro-

graph of magnetoaerotactic bacterium

Magnetospirilium magneticum. (The

TEM image is from Ref. 2.) (b) Photo

using demonstration bar magnets to

illustrate the magnetic configuration

within the chain of magnetosomes.

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rillium magneticumalso follows oxygen concentrationgradients for navigation. The small structures labeledmagnetosome contain (within a protein sheath)single crystals of magnetite, a magnetic iron oxide.

These particles may be the first biominerals ever pro-duced by life on Earth.3

Its worth pausing to reflect on this clever magneticarrangement. Each little particle is a tiny bar magneton its own, and all the little bar magnets line up end-to-end to behave like one bigger bar magnet [Fig.1(b)]. The long skinny shape of a compass needlehelps keep it magnetized. The center-to-center spac-ing of the particles in Fig. 1(a) averages about 57 nmroughly a million times smaller than the scale of themagnets in Fig. 1(b). Segmenting the needle into dis-

crete particles allows the bacterium to share the mag-netosomes with both daughter cells when it divides,passing on the navigation assistance to the next gen-eration. This magnetic design doesnt scale to largerdimensions, however, as discussed below. It is a specialproperty of magnetic nanoparticles that lets it work.

Mankind and Magnetism

Making an enormous jump forward in time, thelast 5000 years of humanitys experience with mag-netism can be summarized as follows. Deposits of

magnetite (lodestone) were found and exploited inChina about 5000 years ago to make the first com-passes. Our name for magnet has its root in the Greekprovince of Magnesia, where the mineral was firstmined in the west. The dramatic action at a distanceproperty of magnets has captivated people from thebeginning, but a couple of thousand years more hadto pass before we began to see serious theories of thephenomenon. The first theory (as opposed to de-scription) is attributed to Ren Descartes, who sug-gested that the orientation of iron filings around a bar

magnet comes about because these materials containdirectional pores through which invisible threadspass and transmit forces.4

The first really big step toward understanding thepermanent magnetism embodied in bar magnetscame with the discovery of the electron, and in par-ticular the fact that each electron, in addition to hav-ing electric charge and mass, is itself a tiny magneticdipole. Inside materials there are so many electronsin such close proximity that the organization of their

magnetic moments has to be described according tothe rules of quantum mechanics. Another key pointis that the organization occurs spontaneouslyif thematerial isnt too hot; that is, you dont have to applya magnetic field from the outside to get the electrons

to behave. There are two main flavors of this kindof organization. The best known is ferromagnetism(in honor of iron as the prime example), where manyof the neighboring electron moments align in thesame direction. If this orientation is mostly preservedthroughout a piece of material, we can make a barmagnet or a compass needle. A less well-known, butactually more common, form of the organization hasequal numbers of electron moments pointing up anddown. The material is then not seen to be magnetizedfrom the outside, but the organization can be detected

for example by studying how neutron beams are dif-fracted by the material. It was for that kind of workthat led Clifford Skull, an American physicist fromPittsburgh, and Bertram Brockhouse, a Canadianphysicist born in Lethbridge (Alberta, Canada), toshare the 1994 Nobel Prize in physics.

Computer Models of Nanomagnets

A computer model can be run in order to illustratewhy the magnetosomes have to be so small. Figure 2

Start

Start

Finish

Finish

a)

b)

Fig. 2. Screen captures from two computer

simulations run using the LLG Micromagnetics

Simulator (Ref. 5), showing the initial and

final magnetic configurations. (a) For a

100-nm cube the stable magnetic configura-

tion is very nonuniform and highly demag-

netized. (b) For a 10-nm cube the stable

magnetic configuration is uniform.

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sets up a simulation for a nickel-iron alloy we oftenstudy. Consider a cube of the material 100 nm on aside. In order for the simulation to be reasonably ac-curate, we break it up into 1000 smaller cells, each of

which contain about 100,000 aligned electron mo-ments (that is, within a given smaller cell, all of theelectron spins are assumed to always remain parallel toeach other, thereby behaving as a single giant spin).5Each arrow in the image corresponds to the magneticorientation of one of those cells. In the computer, wecan begin with any starting arrangement that we wish.The simplest is to have all the cells parallel, as in a barmagnet [Fig. 2(a), left panel]. When we turn on theclock and let this system develop, however, we findthat it is very unstable. It produces other interesting

patterns with vortex-like properties that when seenfrom the outside are now almost completely de-magnetized, and would behave as a broken link inthe chain of magnetosomes. Figure 2(a), right panel,shows the arrangement after just 100 ps of simulatedtime (the picosecond is the natural clock unit here, fortypical material parameters).

If we make the particle small enough, however,we get the opposite behavior. Now we can start witha nonuniform initial state [Fig. 2(b), left panel] and

when we start the clock we find that the cells want

to flip around until their moments are all parallel[Fig. 2(b), right panel]. Now the bacterium has a bigsurvival advantage! [The choice of particle size inFig. 2(b) also self-consistently justifies the assump-tion of uniform alignment within the 10-nm cells ofFig. 2(a)]. This is also where the nanoscience aspectbecomes clear: the preferred magnetic configurationof the particle, when simply left to its own devices,changes dramatically when it becomes sufficientlysmallwith the relevant size typically being of nano-meter dimensions.

The phenomenon just illustrated via computermodeling was already known more than 50 yearsago, although it couldnt be studied in gory detailback then. One of my favorite papers from that erais a theoretical work on the effect of placing cavities

within magnetic nanoparticles (the now-fashionableprefix nano was still far in the future).6 It must haveseemed as if research into what we now call nanomag-netism was about to take offbut something elsehappened instead.

The Information Storage Industry

What happened was this: the magnetic hard diskdrive for data storage was invented in California.This was a revolutionary new technology, which alsobegan to improve at a very dramatic rate and cap-tured peoples imagination. The background of Fig.3 shows a photo of a contemporary computer hard

drive, opened to show the working mechanism, rest-ing on a much larger hard drive platter from the early1980s, which stored on the order of 10,000 times lessinformation. In the foreground is a graph showing, ona logarithmic scale, how the density of stored informa-tion has increased over the years. Today, roughly 100million bits of information fit into the same spaceas one original bit from the mid-1950s. To a largeextent, for the past 50 years the majority of activity inmagnetics research has been locked to the size scalesmost relevant to this industry (see scale bar on the

right in Fig. 3), returning only in more recent yearsback to the nanometer range.

This graph is the magnetic version of Moores law,the self-fulfilling prophecy from semiconductor tech-nology that forecast the number of transistors on achip would increase over time in a similar way.

The most remarkable thing about this curve is thefact that there have been a couple of moments wherethe rate of improvement actuallyincreased sharply.These came about from the introduction of new ways

Fig. 3. The magnetic Moores law curve, superim-

posed on a photo of two items of disk hardware sepa-rated by about a human generation.

(Based on a chart by Ed Grochowski at IBM Almaden.)

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to read the data off the disks, which naturally becomesvery challenging as the bits become very small. Thesecond subtle kink, in 1997, is now legendary be-cause it corresponded to the adoption of a brand-new

(from the point of view of technology) nanomagneticphenomenon known as giant magnetoresistance(GMR), discovered in 1988 and honored with the2007 Nobel Prize in physics.

Giant Magnetoresistance

In brief, giant magnetoresistance is an effect inwhich the electrical resistance of a metal film com-posed of very thin layers alternating between magneticand nonmagnetic composition depends stronglyon whether the magnetic layers have their moments

aligned parallel or antiparallel. The reason is that elec-trons coming out of one layer with their magnetic mo-ments aligned one way encounter more resistance ifthey enter another layer where their moments want tobe aligned the other way. This can make the electricalresistance very sensitive to an applied magnetic field,

which controls the orientation of the layers. The layershave to be very thin for this to be effective; in the car-toon of Fig. 4, the little balls represent individual at-oms. Since the effect works best with nanometer-thicklayers, it can be used in sensors for very tiny magnetic

bits. This eliminated a bottleneck in the informationstorage industry.

We tend to be unconscious of the fact that wevealready been buying nanomagnetic technology at theelectronics commodity store for more than 10 years.The second kink may not look like much, but it wastruly disruptive. Companies that could not bringGMR into production soon went under. AppliedMagnetics in Santa Barbara, at about 40 years one ofthe oldest companies in the business, plunged fromrecord profits to bankrupt in just a few years during

the latter half of the 1990s.The development of giant magnetoresistance really

had legs. It led to intensive consideration of questionssuch as, what is the effect of the electron magneticmoment scattering on the magnetization itself? Itturned out even to be possible to reverse the magne-tization direction at the high current densities, whichcan be achieved in nanocontacts.7GMR can be seenas the jumping-off point for a massive amount of on-going research in magnetoelectronics (also known as

spintronics).8Indeed, GMR itself has already beensurpassed and replaced in the technology by tunnel-ing magnetoresistance (TMR).

Magnetism and Microscopy

Reading the data is not the only challenge in im-proving the performance of disk drives. The electron

micrograph (Fig. 5) shows the business end of a read-write device. The thin layers to the left are a GMR sen-sor (the reader), and the bulkier structure to the rightof that is the writer. Evolving experimental techniquessuch as stroboscopic microscopy have provided insightinto the physical limitations for writing, especially athigh speed. Microscopy development is an area wherethere is still great return on invested effort, for nano-science and technology. The GMR sensor itself can beviewed as a kind of magnetoresistance microscope. In

Fig. 4. Giant magnetoresistance cartoon.

The green balls indicate atomic layers of iron;

the grey balls atomic layers of chromium.

From: http://nobelprize.org/nobel_prizes/

laureates/2007/phyadv07.pdf.

Fig. 5. Scanning electron microscope

image of the air-bearing surface of an

early GMR magnetic recording device (the

device is built onto a slider that flies

only nanometers above the surface of the

spinning hard disk). The thin layers atthe bottom make up the reader, while the

thick middle layer and large block at the

top are the two poles of the writer.

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the case of magnetics, a partial list of evolving tech-niques might include: magnetic force microscopy;magnetic resonance force microscopy; scanning elec-tron microscopy with polarization analysis; transmis-sion electron microscopy (Lorentz and holographicmodes); x-ray photoelectron emission microscopy;and spin-polarized scanning tunneling microscopy.9

An ultrafast stroboscopic magneto-optical micro-scope lets us perform an analog of magnetic resonanceimaging on small magnets. Figure 6 shows on the

right a montage of stroboscopic movie frames from anexample wherein a thin-film magnetic dot has beenstruck with a pulse of magnetic field, causing the mag-netization within it to vibratemuch like a drum-head would after being hit by a drum stick (left pan-el). These are small-angle ferromagnetic resonanceoscillations (angular excursions on the order of onemilliradian). The same approach can also be used tostudy large-angle motions, including the ones leadingto magnetic switching (and used almost innumerabletimes per year now around the world to store data).

A major goal of this work is to run the problemall the way to the end, that is, to study the staticsand dynamics of the magnetization down to the levelof the atomic lattice. We cannot yet say whether theconceptual limit of the Moores law curve, magneticinformation storage with a bit footprint of just afew atoms, can be realized. (A famous example ofnonmagnetic and very slow information storage onthis scale is exploited in Fig. 7.) In any case, many pasttheoretical arguments for a limiting storage density

have already gone by the wayside as the industry hascontinued to create work-arounds.

With more than two billion years of nanomagne-

tism history on Earth already in the bag, the mostremarkable aspect has to be the understanding andcontrol that has developed very suddenly over the pastcentury, thanks to physics.

Acknowledgments

I am very grateful for extensive assistance from LynnChandler in the preparation of this article, and to

AAPT for stimulating it via the wonderful summermeeting in Edmonton in July 2008. It has beengreat fun to work with all present and former group

members and collaborators in this area over the past15 years, and with the support of NSERC, iCORE,CIFAR, CRC, CFI, the National Institute forNanotechnology, and the University of Alberta.

References1. Richard P. Blakemore, Magnetotactic bacteria,Ann.

Rev. Microbiol.36, 217-238 (1982).

2. Arash Komeili, Hojatollah Vali, Terrance J. Beveridge,and Dianne K. Newman, Magnetosome vesicles arepresent before magnetite formation, and MamA is

required for their activation, PNAS 101(11), 3839-3844 (2004).

3. Robert E. Kopp and Joseph L. Kirschvink, The iden-tification and biogeochemical interpretation of fossilmagnetotactic bacteria, Earth Sci. Rev.86, 42-61(2008).

4. Gerrit L.Verschuur, Hidden Attraction: The History andMystery of Magnetism(Oxford University Press, U.S.,1996).

5. M.R. Scheinfein, LLG Micromagnetics Simulator,available at http://llgmicro.home.mindspring.com/.

Fig. 6. Ferromagnetic resonance imaging. A thin film

magnetic dot vibrates with magnetic oscillations

after being hit with a fast pulse of magnetic field.

(Snare drum photo used with permission from Jupiter

images.)

Fig. 7. The letters A-A-P-T rendered in Times

New Monoxide font [medium: carbon monox-

ide on Cu(111)]. For more information and to

typeset your own phrases, see the Times New

Monoxide typewriter at www.phys.ualberta.

ca\~dfortin\nano.

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6. William Fuller Brown Jr. and A.H. Morrish, Effect ofa cavity on a single-domain magnetic particle, Phys.Rev.105(4), 1198-1201 (1957).

7. E.B. Myers, D.C. Ralph, J.A. Katine, R.N. Louie, andR.A Buhrman, Current-induced switching of domains

in magnetic multilayer devices, Sci.285,

867-870(1999).

8. S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M.Daughton, S. von Molnr, M.L. Roukes, A.Y. Chtch-elkanova, and D.M. Treger, Spintronics: A spin-basedelectronics vision for the future, Sci.294, 1488-1495(2001).

9. M.R. Freeman and B.C. Choi, Advances in magneticmicroscopy, Sci.294, 1484-1488 (2001).

PACS codes: 40.00.00, 80.00.00

Mark Freeman is a professor of physics at the University

of Alberta and a research officer at the National Institute

for Nanotechnology.

www.phys.ualberta.ca/~freeman; mark.freeman@

ualberta.ca

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nanomagnetism a case history of nanoscience and technology

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