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SINGLE-MOLECULE MAGNETS EVOLVE Transition-metal clusters are ideal systems to explore nanoscale magnetism and quantum effects S T E P H E N K. R I T T E R , C & E N
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technology needs today is to find more efficient ways to store and process digital information. There are a couple of possibilities: One is to squeeze more data onto storage devices by making currently used magnetic nanoparticles even smaller. Another is to develop fundamentally different ways to process information, such as quantumbased computing. For any of these poten-
have been shown to exhibit quantum tunneling of magnetization and quantum phase interference—key properties needed for materials to function as quantum bits (qubits). The past couple of years have seen a burst of activity in SMM research, as fine details of the properties of the tiny magnets have become better understood and the synthesis of more diverse compounds has evolved.
HAPPY HOLIDAYS Wreath-shaped Mn84 cluster, the largest single-molecule magnet synthesized so far, crystallizes in hexagonal arrays (Mn = blue, 0 = red, C = gray). Multiple layers align to form nanotubular stacks that offer a variety of possible future applications. tial applications, the use of chemistry to develop new materials will be critical. That's where single-molecule magnets (SMMs) may fit in. Conventional magnets rely on the collective behavior of the unpaired electron spins of hundreds of thousands or millions of individual metal centers in a particle or bulk material. SMMs, on the other hand, are transition-metal clusters that individually exhibit the classical properties of a magnet below a critical temperature called the blocking temperature, which is currrently limited to about 4 K. Because of their small size, SMMs also HTTP://WWW.CEN-ONLINE.ORG
"We have many types of magnets today made from iron, cobalt, and nickel, and metal alloys and metal oxides," notes University of Florida chemistry professor George Christou, one of the pioneers of the SMM field. "Magnetic materials make up a multi-billion-dollar-per-year industry So we're not trying to replace the magnets we already have—they are already fantastic. But single molecules are much smaller than even the smallest magnetic particles, and if a molecule can function as a magnet, then you can use a molecule to store information—one bit of information per molecule."
The smallest metal nanoparticles used in research are about 3 nm in diameter, which is in the neighborhood of 1,000 atoms, Christou says. "The most information that can be stored on hard drives and other devices currently is 3 billion bits, or 3 gigabits, in 1 cm 2 area of a cobalt-based magnetic material," he notes. "The much smaller size of our molecules means we could get 30 trillion of them into 1 cm 2 , and thus a storage density of 30 trillion bits, or 30 terabits, is feasible. This is 10,000 times greater than the current best by computer manufacturers. One of the research challenges now is to find better SMMs that function at higher temperatures." The most studied SMMs so far have been clusters with manganese oxide cores surrounded by a sheath of organic carboxylate ligands. The oxygen atoms bridge the manganese atoms to allow interactions between the large number ofunpaired electrons, leading to the magnetic properties, Christou explains. BESIDES THEIR size, SMMs have a number of other advantages over traditional magnets, Christou says. They are soluble in organic solvents because of the peripheral ligands, while magnetic particles in general are insoluble. The alterable shell of organic ligands that helps solubilize SMMs also makes them highly crystalline and prevents the magnetic cores of the molecules from making contact with neighboring molecules. Unlike magnetic particles made by breaking down larger bulk materials, SMMs have a uniform size. They also are air-stable for long-term storage. "There is a lot of operator control, or synthetic manipulation, we can do with these molecules that is not really possible with traditional magnetic particles," he adds. Christou and his group have prepared about three-fourths of the SMMs reported thus far. One of his primary collaborators in studying SMM properties during the past 15 years has been University of California, San Diego, chemistry professor David N . Hendrickson. Christou and Hendrickson have been working since 2001 with physicists Stephen O. Hill of the University of Florida and Andrew D. Kent of New York University, as well as physical chemist Naresh S. Dalai of Florida State University, under a National Science Foundation Nanoscale Interdisciplinary Research Team grant. The remainder of the synthesis and investigation of the properties of SMMs has C&EN
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EARLY BIRD Manganese cluster, Mn12012(02CC6H5)16(H20)4, was one of the first compounds to be identified as a single-molecule magnet in the early 1990s (Mn = blue, Ο = red, C = white). The cluster, shown with its crystals, has an Mn12012 core surrounded by 16 benzoate groups. been carried out by several groups around the world, and at one time or another near ly all the primary researchers in the field have worked together at some level. "The SMM field is truly an interdisciplinary area," Christou says.
cules, ended up synthesizing the Mn12 ben zoate derivative in the late 1980s. Christou sent some of the benzoate to Dante Gatteschi of the Laboratory for Molecular Magnetism at the University of Florence, in Italy. Gatteschi's group was already working in the area of 3-D molecule-
in Gatteschi's group, along with Gatteschi, Christou, Hendrickson, and others. The paper reported the low-temperature mag netic susceptibility of the clusters, estab lishing them as the first known SMMs [J. Am. Chem. Soc, 115,1804 (1993)]. Sessoli, Gatteschi, and coworkers also pub lished a paper describing the magnetic hys teresis of the compounds [Nature, 365,141 (1993)]. Hysteresis is the ability of a mate rial to remain magnetized after an applied magnetic field is removed. It's required to keep the spin magnetic moment from ran domly flipping and losing stored informa tion—that is, the ones and zeros of com puter binary code. Those two papers set off a gold rush for physicists to study the magnetic proper ties of the Mn 12 compounds, Christou says. "It became a physics playground. These compounds were crystalline, solu ble, and monodisperse. And because they are so small, they show a lot of the quan tum properties that theoretical physicists predicted should be exhibited by small magnetic particles but that no one had seen before. "The physicists wanted to un derstand quantum tunneling, the factors that control it, and the quantum mechanical mathemati cal expressions to describe the ef fects," Christou continues. "There are hundreds of papers onMn 12 ac etate now, and the physicists prob ably know more about it than the chemists."
THE EARLY TYPES of moleculebased magnets, first developed in the late 1960s, consist of three-di mensional arrays of inorganic or organic molecules, particularly organometallic complexes. In these compounds, some of which are magnets at room temperature, the magnetism arises from the col lective unpaired electrons of the bulk material, similar to iron or cobalt metal particles. A lot of progress has been made on devel A KEY THEORIST working on oping these 3-D molecule-based QUANTUM PAIR The single-molecule magnet SMMs has been Philip Stamp, di magnets since the late 1980s. MnA03Cl4(02CCH2CH3)3(pyNdine)3 crystallizes in rector of the Pacific Institute of pairs held together by hydrogen bonds between The first identified single-mole Theoretical Physics at the Univer chlorine and hydrogen atoms (Mn = green, 0 = cule magnet was a manganese ox sity of British Columbia, Vancou yellow, Ν = blue, CI = red, C and Η = gray). ide cluster with acetate ligands, ver. His work was among the first Electronic coupling between the two molecules Mn 12 0 12 (0 2 CCH3) 16 (H 2 0) 4 . It has to focus on magnetic and quantum shuts down quantum tunneling in the absence of eight Mn(III) and four Mn(IV) properties of nanoparticles. An an applied magnetic field that is observed in the metal centers in a M n 1 2 0 1 2 core other key figure —and one many individual molecules, indicating that the dimer that is surrounded by 16 acetate SMM researchers have worked or other SMMs might function as qubits for groups. The molecule, first report with—is Wolfgang Wernsdorfer of quantum computing. ed in a paper in 1980, has 20 un the National Center for Scientific paired electrons. Research's Louis Neél Laboratory in Grenoble, France. He is an expert on Not much happened with this com based magnets. the low-temperature study of magnetic pound for a decade, Christou recalls. His Studies of the magnetic properties of properties of SMMs. group, which had initially been interested the Mn 12 compounds led to a paper by in studying manganese-based biomoleRoberta SessoH, then a graduate student SMMs that contain metals other than
"There is a lot of operator control, or synthetic manipulationp we can do with these molecules that is not really possible with traditional magnetic particles." 3 0
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manganese, including iron, vanadium, the individual SMMs. The data show that phosphorus ligands on the Re(II) ions \J. cobalt, and nickel, have been reported. quantum tunneling still occurs in the usu Am. Chem. Soc, 126,15004 (2004)]. The Christou and his collaborators lately have complex is important, as it is the first ex al steplike manner, but it's the collective been exploring SMMs that have various ample of a cube-shaped SMM cluster and behavior of the dimer, rather than a single numbers of manganese atoms. They now the first SMM to contain a 5d element, molecule. The results show that even weak have synthesized compounds ranging from electronic exchange interactions can have Dunbar notes. The effect of the magneti Mn 4 to Mn 8 4 , including an Mn 2 5 cluster cally anisotropic low-spin Re(II) ions on a large influence on the quantum proper with a record 51 unpaired electrons. They ties of SMMs, Christou says, and suggest the high-symmetry environment induces also are looking for new methods an unexpected type of magnetic to synthesize SMMs, including sol behavior, she says. Magnetic anisouble Mn(III) starting materials and tropy is the nonuniform distribu combining preformed manganese tion of magnetic properties in a clusters of different sizes to make molecule or particle. clusters not available from the usu The findings on Mn 4 Re 4 , and al manganese starting materials. related clusters made by graduate While many of the structures of student Curtis R Berlinguette, SMMs are impressive to view, per "are important to the field, as they haps the most striking is an Mn 8 4 lend insight into how first-order cluster shaped like a holiday wreath single-ion anisotropy of a metal that was reported earlier this year ion can lead to SMM behavior of ATTRACTIVE CHAINS Single-chain magnets, by Christou, Wernsdorfer, postdoc such as this manganese-nickel complex with an a different origin than that ob Anastasios J. Tasiopoulos, and co Mn-0N-Ni-N0-Mn-0 2 repeat unit, are served for Mn 12 acetate," Dunbar workers [Angew. Chem. Int. Ed., 43, expanding the scope of molecule-based magnets says. These single-ion effects were 2117 (2004)]. This cluster, the (Mn = dark red, Ni = orange, 0 = light red, Ν = first observed for SMMs last year largest SMM made so far, has a blue, C s yellow). The 1-D chains are not discrete by Naoto Ishikawa and coworkers M n 8 4 0 7 2 core that was built up by molecules like single-molecule magnets, nor do ofTokyo Institute ofTechnology, the reaction of Mn 12 acetate with a they form 3-D polymeric structures like in Japan, for a set of lanthanide permanganate salt. traditional molecule-based magnets. phthalocyanine double-decker How does Mn 84 's size compare complexes \J, Am. Chem. Soc, 125, to other nanomagnets? The giant ring of that this approach could be a chemical way 8694(2003)]. 1,032 atoms has an outside diameter of 4.2 to tune the quantum tunneling. Dunbar's group is planning to prepare nm and is 1.2 nm thick, making it a little and study a series of molecular cubes, larger than the smallest known cobalt SEVERAL RESEARCH groups are starting squares, and trigonal bipyramids, and use magnetic particles, Christou says. The to branch out in other directions to create them as building blocks for larger assemMn 8 4 cluster crystallizes in a hexagonaldifferent families of SMMs and study their close-packed arrangement resembling properties. For example, groups led by graphite sheets, with multiple layers align chemistry professors Kim R. Dunbar at ing to form nanotubes. This architecture Texas A&M University andJeffrey R. Long offers a variety ofpossibilities for new ma at the University of California, Berkeley, terials, such as inserting guest molecules have been independently synthesizing into the tubes. "There is something gor SMMs with cyanide bridging groups, geous about a large molecular cluster like rather than the standard oxide bridging Mn 8 4 , the way it forms these beautiful groups. nanotubular stacks," Christou observes. "I "Christou and Hendrickson's research in think the structural aesthetics of SMMs the area of manganese carboxylate SMMs, are not to be underestimated." as well as that of other groups such as Gatteschi and Sessoli, has had a tremendous Getting to a point where the elec impact on the field of molecular magne tronic properties of clusters can be finetism," Dunbar says. "Our work on cyanidetuned to make SMMs with desired prop based SMMs complements the research erties is a direction Christou and others DOUBLE FEATURE Cyanideon metal oxide-based analogs and demon would like to move toward. One system bridged complexeSp such as this strates that the SMM phenomenon is developed by Christou's group is a manMn4Re4 cluster with tripodal more general." Cyanide chemistry offers ganese S M M dimer p r e p a r e d from phosphorus ligands, provide the advantages of being easier to control Mn 4 03Cl 4 (0 2 CCH 2 CH 3 )3(pyridine)3. different influences on the than cluster self-assembly reactions based The dimer is held together by six hydro magnetic behavior of SMMs than on oxides, and it's more predictable in gen bonds between chlorine and hydro the traditional oxide-bridged terms of the nature of the spin coupling gen atoms [Nature, 416, 4 0 6 (2002)]. compounds (Mn = yellow, Re = between metal centers, she adds. orange, Ρ = purple, Ν = blue, CI = Single-crystal studies carried out by green, C = gray). This complex is Wernsdorfer using superconducting quan One of Dunbar's compounds, recently also important as the first example tum interference devices reveal that the reported by graduate student EricJ. Schelof a cubic SMM cluster and the first hydrogen bonding leads to coupling of the ter and postdoc Andrey V Prosvirin, is a SMM to contain a 5d element. magnetic moments of the two Mn 4 units Mn 4 Re 4 cyanide cluster with chloride liand different quantum behavior than for gands on the Mn(II) ions and tripodal HTTP://WWW.CEN-ONLINE.ORG
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SCIENCE & TECHNOLOGY tetramer version [Inorg Chem., 43, 5486 (2004)]. Clérac and Miyasaka, in collaboration with Wernsdorfer and others, studied the magnetic properties of their original manganese-nickel SCM, observing that the manganese-nickel repeat unit displays the properties of an SMM [Phys. Rev. B., 69, 132408 (2004)]. "Our point in this work is not only to reveal the correlation between SMM and SCM systems, but also to search for the variation in quantum effects from the atomic quantum regime to the classical bulk regime," Miyasaka says. The type of interaction between SMM units shown to affect quantum tunneling in Christou's Mn 4 dimer might also be a key interaction affecting the quantum behavior of SCMs, he adds. A few companies have been interested in developing SMMs into commercial applications, Christou says, but no one in industry has jumped in yet because the operating temperature is still too low. In fact, he and his collaborators ADIFFERENT approach to making have yet to patent any of their lanthanide SMMs that uses macroCROWN JEWEL Manganese-dysprosium work. cyclic chemistry is being pursued cluster is one of the first SMMs to contain a by graduate student Curtis M. Zalanthanide element (Dy = green, Mnllll) = blue, "It's still kind of early," Christou leski and chemistry professors Vin Mn(IV) = yellow, 0 = red, Ν = purple). The framework observes. The first thing that has cent L. Pecoraro of the University of the complex is a hexagonal ring of dysprosium to be done is to get blocking temof Michigan and Martin L. Kirk of atoms capped on either end by a manganese peratures above 5 K, he says. But the University of New Mexico and trimer. The metal atoms along with the bridging the temperature shouldn't be an their coworkers. The team has fo ligands form a metallacrown-type structure. economic barrier to their applicacused on developing SMMs that, tion, as the use of superconducting like the 3d-5d and other 3d-4f complexes, magnet coils for nuclear magnetic resostructure like traditional iron or cobalt provide more anisotropy nance applications has shown, Christou metal magnets or organometallic-based points out. molecular magnets. There are now a few The researchers have concentrated on examples of SCMs, the first one being a making metallacrowns, the inorganic As far as applications go, some acacobalt-based molecule synthesized in analogs of crown ethers that "are particu demics are working to deposit Mn 12 clus2001 by Andrea Caneschi, Sessoli, and larly attractive for SMMs because they al ters on surfaces, but that too is not very Gatteschi at the University of Florence low for a high density of metal ions," Zaadvanced, Christou says. "We have been and their coworkers. leski says. Their collaborator, Dimitris P. avoiding putting Mn 12 on surfaces in our Kessissoglou and his group at Aristotle Uni One type of S CM is a family of man- lab because two dimensions might not be versity ofThessaloniki, in Greece, had ear the future of information storage," he ganese(III)-nickel(II) complexes synthe lier made the first metallacrown-based notes. "A lot of us believe the future of sized by chemists Hitoshi Miyasaka of SMM, a Mn 2 6 cluster that has a Mn 1 6 0 1 2 SMMs and information storage is going to Tokyo Metropolitan University and core surrounded by strands of manganese be three-dimensional. And Mn 12 is probMasahiro Yamashita of Tohoku Universi atoms and ligands to give a metallacryptate ably not going to be the future of SMMs ty, Sendai, both in Japan, and Rodolphe structure—a 3-D version of a metallacrown either. It's the best at the moment, but we Clérac of Paul Pascal Research Center, Pes[Angew. Chem. Int. Ed, 42,3763 (2003)]. need better compounds." sac, France. The chains are made up of a Mn 2 complex with salen-type bidentate In a different approach, Zaleski, Peco One of the things Christou likes to ligands linked to a nickel complex conraro, and their coworkers turned to com point out as giving the SMM field high taining pyridine ligands. binations of lanthanide ions with transi potential is that it's an area of materials tion metals to make a class of Ln 6 Mn 6 research where scientists have all the adThe repeat unit of the chain is clusters [Angew. Chem. Int. Ed., 43, 3912 vantages of molecular solution chemM n - O N - N i - N O - M n - 0 2 , and the chain (2004)]. The complexes have a hexagonal istry—solubility room-temperature synlengths are approximately 110 repeat units Ln 6 core with two Mn 3 "wings." The dys thesis, and easy control of molecular long, or about 140 nm, Miyasaka notes. prosium analog shows SMM behavior, but structure. "The great thing about SMMs The chains are magnetically isolated from gadolinium and terbium analogs don't, they is that the molecule is not the precursor each other in the crystal structure by void note. The team is continuing to synthesize to the material, the molecule is the interspaces occupied by inorganic counterions. mixed-metal clusters with metallacrown esting material," he concludes. • The team recently reported a Ni-Mn 2 -Ni
blies. The chloride ligands on the Mn 4 Re 4 complex, for example, provide ideal sites for substitution, Dunbar says. Continuing the exploration of the low er part of the periodic table, a few re searchers are beginning to make mixed transition-metal and lanthanide SMMs. The first reported examples are Cu 2 Tb 2 and Cu 2 Dy 2 clusters synthesized by chem istry professor Naohide Matsumoto of Kumamoto University in Kumamoto, Japan, and coworkers [J.Am. Chem. Soc, 126,420 (2004)]. The complexes were shown to have SMM behavior, although mag netic hysteresis was not observed. Graduate student Abhudaya Mishra in Christou's group more re cently synthesized an SMM with a M n n L n 4 O s core, where Ln = Nd, Gd, Dv, Ho, or Eu [J.Am. Chem. Soc, 126, 15648 (2004)}. The dyspro sium cluster, described in detail in the paper, is the first mixed 3d-4f SMM to exhibit hysteresis and quantum tunneling, Christou notes.
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networks and has yet to complete full char acterization of the magnetic properties. Another new type of molecular mag net includes metal complexes that form extended linear structures, like a chain of beads. The magnetic properties of these single-chain magnets (SCMs), as they are called, come from coupling interactions between one link in the chain and the next. The one-dimensional chains are a fundamentally different type of magnet because they aren't discrete molecules like SMMs and they don't have a 3-D network
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