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Unconventional Magnetism in All-Carbon Nanofoam

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Unconventional Magnetism in All-Carbon Nanofoam PHYSICAL REVIEW B 70, 054407 (2004) Unconventional magnetism in all-carbon nanofoam A. V. Rode,1,*,† E. G. Gamaly,1 A. G. Christy,2 J. G. Fitz Gerald,3 S. T. Hyde,1 R. G. Elliman,1 B. Luther-Davies,1 A. I. Veinger,4 J. Androulakis,5 and J. Giapintzakis5,6,*,‡ 1Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia 2Department of Earth and Marine Science, Australian National University, Canberra, ACT 0200, Australia 3Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia 4Ioffe Physical-Technical Institute, Polytechnicheskaya 26, St. Petersburg, Russia 5Foundation for Research and Technology-Hellas, Institute of Electronic Structure and Lasers, P.O. Box 1527, Vasilika Vouton, 71110 Heraklion, Crete, Greece 6Department of Materials Science and Technology, University of Crete, P.O. Box 2208, 710 03 Heraklion, Crete, Greece (Received 11 September 2003; revised manuscript received 28 May 2004; published 17 August 2004) We report production of nanostructured magnetic carbon foam by a high-repetition-rate, high-power laser ablation of glassy carbon in Ar atmosphere. A combination of characterization techniques revealed that the system contains both sp2 and sp3 bonded carbon atoms. The material is a form of carbon containing graphite- like sheets with hyperbolic curvature, as proposed for “schwarzite.” The foam exhibits ferromagnetic-like behavior up to 90 K, with a narrow hysteresis curve and a high saturation magnetization. Such magnetic properties are very unusual for a carbon allotrope. Detailed analysis excludes impurities as the origin of the magnetic signal. We postulate that localized unpaired spins occur because of topological and bonding defects associated with the sheet curvature, and that these spins are stabilized due to the steric protection offered by the convoluted sheets. DOI: 10.1103/PhysRevB.70.054407 PACS number(s): 75.75.ϩa, 78.67.Bf, 81.15.Fg, 61.46.ϩw I. INTRODUCTION ferromagnetism16 and tunable electrical conductivity.17–19 The materials science of carbon has proven to be a rich Such effects have attracted enormous attention and already 20 area of discovery and development in the past decades. Until been proposed for commercial applications. relatively recently, the only known polymorphs of carbon A physical property of particular interest regarding all the were graphite, in which sp2 hybridized carbon atoms form aforementioned carbon allotropes is the magnetic suscepti- planar sheets in a two-layer hexagonal stacking, and dia- bility, ␹, since this bulk probe is related to the low energy mond, in which sp3 carbons form a three-dimensional frame- electronic spectrum. In general, all known carbon allotropes work of cubic symmetry. However, in recent years new car- exhibit diamagnetic susceptibility in the range of ␹ bon allotropes have proliferated. A three-layer rhombohedral =−͑10−5–10−7͒ emu/g Oe with the exception of: (i) poly- 1 stacking modification of graphite was reported in 1956 and merized C60 prepared in a two-dimensional rhombohedral lonsdaleite, the two-layer hexagonal polytype of diamond, phase of ␹=+͑0.25−1.3͒ϫ10−3 emu/g Oe (depending on was discovered in shocked rocks in a meteorite crater in the orientation of the magnetic field relative to the polymer- 1967.2 Fibrous sp1-hybridized carbon has been identified in a ized planes) which shows ferromagnetism;16 (ii) the disor- mineral environment;3 similar “carbynes” have been sug- dered glass-like magnetism observed in activated carbon fi- gested to occur in interstellar dust,4 and similar materials bers due to nonbonding ␲ electrons located at edge states;21 have been synthesized by shock compression of graphite.5–9 and (iii) the unusual magnetic behavior observed in single A broad spectrum of “turbostratic graphites” are also known, wall carbon nanohorns ascribed to the Van Vleck paramag- in which individual graphene layers show rotational and tilt netic contribution.22 Although ferromagnetism in polymer- disorder relative to their neighbors. A recent extreme ex- ized C60 is noteworthy, the exceptionally large magnetic sig- ample is provided by cores of carbon spherules in the nal in carbon nanostructures remains a case of special Murchison meteorite, which are aggregates of randomly ori- interest.23 ented single layers with additional structure defects.10 This Superparamagnetic and/or ferromagnetic-like behavior in particular specimen is among the oldest known naturally oc- carbon-based material has been previously observed in amor- curring material, since it solidified before the condensation phous carbon materials obtained by chemical synthesis or of the Solar System. In contrast, recently synthesized mate- pyrolysis.24,25 The origin of ferromagnetism was suggested rials include many further distinct forms of carbon such as to be attributed to the mixture of carbon atoms with sp2 and fullerenes,11,12 multiwalled carbon nanotubes,13 and single- sp3 bonds and resulted ferromagnetic interaction of spins walled carbon nanotubes,14 which are based on a mixture of separated by sp3 centers.26 An increase in saturated magne- sp2 and sp3 hybridized carbon atoms. The structural phase tization of amorphous-like carbon prepared from different space of carbon between graphite-like and diamond-like hy- hydrogen-rich materials indicated the importance of hydro- bridization states now has many occupants. There is even gen in the formation of the magnetic ordering in more variety in the electronic properties of various carbon graphite.27,28 Very recently, ferro- or ferrimagnetic ordering allotropes, which range from superconductivity15 to was discovered in proton-irradiated spots in highly oriented 1098-0121/2004/70(5)/054407(9)/$22.5070 054407-1 ©2004 The American Physical Society A. V. RODE et al. PHYSICAL REVIEW B 70, 054407 (2004) pyrolytic graphite (HOPG).29 It was demonstrated that pro- with plastic tweezers and placed into a glass vial for further tons implanted in HOPG triggered ferro- (or, ferri-) magnetic analysis. Full details regarding the experimental conditions ordering with a Curie temperature above room temperature. can be found elsewhere.31–33 Here, we briefly explain some To check the importance of hydrogen, spots of similar topog- unique features of the synthesis conditions, which are differ- raphy were produced with helium ion irradiation. Those ent from any previously used nanocluster synthesis method. spots showed no significant magnetic signal in magnetic The carbon vapor temperature in the laser plume, where force microscopy measurements, which stressed the signifi- the formation process takes place, is in the range 1–10 eV cance of hydrogen in the formation of magnetic ordering in ͑10 000–100 000 K͒, i.e., the formation takes place in a graphite. These findings were supported by theoretical partly ionized plasma.38 The high-repetition-rate laser abla- predictions,30 which show that hydrogenated graphite can tion creates an almost continuous inflow of hot carbon atoms display spontaneous magnetization originating from different and ions with an average temperature of ϳ2 eV into the numbers of mono- and double-hydrogenated carbon atoms. experimental chamber. This vapor heats the ambient gas and We have recently synthesized a new form of carbon: increases the partial density of carbon atoms in the chamber. cluster-assembled carbon nanofoam, by laser ablation of The process of formation of carbonaceous clusters begins glassy carbon in an argon-filled chamber using a high- when the carbon density reaches the threshold density, at repetition rate high-power laser.31–33 The synthesis technique which the probability of collisions between carbon atoms is to our knowledge unique. We characterized the carbon becomes sufficiently high. nanofoam by several electron microscopy techniques, which The consumption rate of carbon atoms due to the cluster revealed a web-like structure containing randomly intercon- formation significantly exceeds the evaporation rate by laser nected clusters, 4–9 nm in diameter. The nanofoam exhibits ablation in the experimental conditions of high-repetition- some remarkable physical properties, including ultralow den- rate laser ablation.32,38 Thus, the formation is a nonequilib- sity (ϳ2–10 mg/cm3, the lowest gravimetric density ever rium periodical process. We suggest that the formation pro- reported for a solid), and a large surface area (comparable to cess comprises periodic heating and cluster formation stages, zeolites) of 300–400 m2 /g.31,32 with the time period dependent on the initial Ar density, the Here we report the equally unusual magnetic properties of evaporation rate, and reaction rate, which in turn is a func- the cluster-assembled nanofoam. The foam shows strong tion of the temperature and density of the atomic carbon. It is positive magnetization, some of which is lost in the first few essential that during the period of cluster formation, short in hours after synthesis, but much of which is persistent. This comparison to the heating period, the argon gas does not cool paper presents a detailed study of one sample, which dis- down, but maintains its high temperature for cluster forma- played a saturation magnetization of Ms = +0.42 emu/g at tion. Hence, the argon (and carbon vapor) temperature from 1.8 K 12 months after synthesis. We assess and eliminate the second formation cycle onward is higher than the forma- impurities as significant contributors to the measured prop- tion threshold temperature, which is
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