V. DAVYDOV, (c)
by Valery DAVYDOV, senior researcher, L.F. Vereshchagin institute of Physics of High Pressures, Russian Academy of Sciences
This is hard to believe: a magnet attracting a couple of pieces of black matter caused research scientists to pause and wonder. But it's a fact. This photograph appeared in a supplement to an article published by the journal Nature (Vol. 413) in October 2001, and it drew interest from those involved with magnetic phenomena professionally. The point is that the attracted material was carbon, not metal.
For centuries the idea of magnets was associated with compounds of iron and other metals, rare-earths too. The very word - "magnet" - comes from the name of the ancient town of Magnesia in Asia Minor, where odd "stones" (actually, chunks of iron ore) were found; odd, because they could attract metal objects. Something perceived as a wonder then.
In time, with the progress of science, the idea of magnets as metal-containing materials attained to a canonical form as the Latin name for iron - Ferrum - came to be attached to various magnetic phenomena, such as ferro-, antiferro- and ferromagnetism, related to different types of orderliness of the magnetic moments of atoms or ions under the effect of exchange interaction forces. In the case of ferromagnetism, these moments are aligned unidirectionally, in parallel, at temperatures below critical-this critical temperature is known as the Curie temperature (T c ). It varies with different substances: say, for iron it is equal to 1,043 С, for cobalt - 1,403 С, for nickel - 631 C.
Since the phenomenon of magneto-ordered state is characteristic of elements related by chemical classification to d-transitional (iron, cobalt, nickel, etc.) or to rare-earth f-elements (gadolinium, terbium, dysprosium, etc.), containing incomplete d- and f-electron shells*, the corresponding
Gibbs energy (G) of graphite (1), diamond (2), glass carbon (3), amorphous carbon (4) and fullerite C 60 at pressures of 1 atm. and 10 GPa, and at temperatures of 300 and 1,000 С.
* Electron shells of atoms are designated by Latin characters depending on their energy level and sublevel. -Ed.
Diagram of atomic (A), monomer molecular M (1, 2, 3) and polymolecular Mp (О, Т, R, 3D) carbon structures formed when fullerite C 60 is treated under conditions of quasihydrostatic compression. Rhombs indicate p, Т parameters of samples synthesis for isobaric sections at 2.5 and 6.0 GPa; red rhombs marked with arrow correspond to a temperature range wherein samples with ferromagnetic properties were obtained; 3 - vitreous state, 4 - diamond formation region.
magnetic phenomena came to be called d- and f-magnetism, respectively.
As the nature of the bulk of available magnetic materials had been explained toward the close of the 1980s, it seemed it was the presence of d- and f- elements, be it only as one component of a particular substance, that was a necessary condition for spontaneous magnetization. But, as it often happens, new and rather surprising results were obtained at the same time. Magneto- ordered structures, it turned out, could likewise appear in pure organic compounds containing only hydrogen, carbon, nitrogen and oxygen - chemical elements which, considering the structure of their electron shells, are assigned to what is known as p-elements.
The first example of a metal-free organic magnet was a chemical compound discovered in 1991 by a team of Japanese researchers under M. Tamura. It was nitronyl, a derivative of nitrooxide. Even though its Curie temperature proved to be very low (-0.65 С), this discovery showed there could be molecular ferromagnets whose magnetic properties were coupled to the electrons of atomic p-orbitals. That is, yet another kind of magnetic phenomena, the p- magnetism, was discovered, a phenomenon considered to be outright impossible twenty years before. As good as simultaneously with the discovery of the first ferromagnet, Pierre M. Allemand and co-authors (USA) published data on fullerene C 60 , a newly discovered molecular form of carbon; its compound with tetradimethyl amine ethylene, the communication said, is also a ferromagnet, and with a higher Curie temperature at that - 16.1 С.
The phenomenon of p-magnetism found in multicomponent systems made one wonder: can magneto-ordered state arise in a mono-component system on the basis of pure carbon, a typical p-element remarkable, however, for the striking diversity of its forms?
The first positive answer to this question appears to have come from Alexander Ovchinnikov, RAS Corresponding member, and coauthors who, in 1991, hypothesized about a carbon structure with ferromagnetic characteristics. Then, in 1996, a group of Japanese and American research scientists headed by K. Nakada and M. Dresselhaus demonstrated the possibility of other magneto- ordered states of pure carbon (they did this by computing the zonal structure of nanodimensional graphite particles). And yet, despite such theoretical forecasts, it was not clear whether one could obtain carbon magnets in practice. Attempts to synthesize them from conventional carbon-containing materials did not come off. However, the discovery of fullerene and, in particular, the effective method developed by V. Kremcher and coworkers in 1990 for synthesizing fullerene C 60 opened up fresh opportunities.
Let's recall that a molecule of C 60 is a closed lattice composed of 60 carbon atoms situated in the vertices of a truncated icosahedron* and forming hexa- and pentagonal cells. In a condensed state under normal conditions these molecules form a face-centered
* Icosahedron - regular polyhedron having 20 triangular faces, 30 arms, 12 vertices (with 5 arms converging into each vertex). - Ed.
cubic packing which, unlike a single molecule of C 60 , is called fullerite C 60 .
From a thermodynamic standpoint, fullerite C 60 means yet another metasteady state of solid carbon - a state which, among other things, is also a "vertex" of metastability in the carbon system. Before the discovery of fullerenes the energy level width, corresponding to the range of solid carbon forms known to date, was about 10 kJ/mole under normal conditions. The discovery of fullerite C 60 meant a dramatic upward shifting of the upper boundary of the energy level. Under like conditions the use of fullerite C 60 as the initial state of the system made it possible to attempt obtaining a family of new carbon materials that could evolve as intermediate states in the carbon system's transition from the extreme metasteady state of the initial fullerite to graphite and diamond; such intermediate states correspond to absolute energy minima of the system.
The energy levels of these new carbon states are found within an energy range which was quite out of reach before the discovery of fullerenes; in actual fact, this means the making of new forms of carbon. But since the density of fullerite C 60 (1.68 g/cm 3 ) is much lower than that of graphite (2.26 g/cm 3 ) and diamond (3.52 g/cm 3 ), pressure becomes the most effective tool for initiating fullerite conversions.
Systematic studies of high-pressure states arising as a result of fullerite C 60 treatment in a wide range of pressures and temperatures show that the pressure- and temperature-initiated conversions of fullerite do indeed give rise to new forms of carbon. Even a rule-of-thumb classification that proceeds from the characteristics of the main structure-forming element (atom, molecule, polymolecular cluster) allows to identify at least four carbon states within the system.
Fragments of unidimensional (a) and two types of bidimensional (b and c) polymers of C 60 which are the structure-forming elements of the orthorhombic, tetragonal and rhombohedral phases, respectively.
First, these are the molecular (M) phases based on monomer molecules of C 60 . Second, these are the polymolecular (Mp) phases which are packings of different types of С 60 polymers formed through unidimensional (ID) or bidimensional (2D) polymerization of C 60 . A packing of linear polymers gives rise to the orthogonal (0) phase, while packings of bidimensional polymers with the quadrangular and hexagonal topology produce the tetragonal (T) and rhombohedral (R) polymer phases, respectively.
Next come the three-dimensional (3D) polymer structures formed at pressures above 9 GPa - very, very hard structures.
And last, we have the different atomic (A) carbon states originating at temperatures above the limit of the thermal stability of the C 60 cluster and tending to graphite and diamond with a further temperature increase.
The structural diversity of the new forms of carbon obtained through the pressure-initiated conversions of fullerite made necessary a systematic study of the complex of their physical characteristics. Of special interest in this respect was the evolution of the system's electrophysical and magnetic properties during its transition from different ID, 2D and 3D polymolecular states to atomic states accounting for the most radical restructuring of the system. For this purpose we synthesized several series of samples corresponding to a variety of isobaric sections p, T of the C 60 diagram in a temperature range of 900 - 1,150 С.
We made a close study of isobaric sections at 2.5 and 6.0 GPa. The synthesis was effected at high-pressure units of the Toroid type, with twice sublimated fullerite C 60 (Term USA) used as the source material. The experiment involved these steps: loading of the high-pressure chamber; heating; isothermal allowance of the sample at assigned pressure and temperature values for 1,000 seconds; and subsequent hardening (tempering) of the sample under pressure to room temperature. The high-pressure hardening allowed to keep the state of high pressure under normal conditions and thus study it with the use of different physical methods.
Comprehensive studies of electrical and magnetic characteristics of the synthesized samples - these studies were carried out by T. Makarova of the St. Petersburg-based A.F. Joffe Physical-Technical Institute (RAS), together with physicists from research laboratories of Sweden, Germany and Brazil - did produce remarkable results indeed. Studying magnetization of samples (synthesized at 6.0 GPa) in a constant magnetic field, they found that five of the six samples obtained in a temperature range of 1,010-1,050 С demonstrated a manifest ferromagnetic behavior. The Curie temperature of these samples was equal to 500 С. Since the initial fullerite C 60 is a diamagnetic material, the above effect could be explained either by a significant presence of ferromagnetic impurities in the samples, or by some conversions of the carbon material proper which initiated magneto-ordered states. An ultimate analysis of the source fullerite and the samples obtained showed that the overall concentration of magnetic admixtures (Fe, Co, Ni, and so on) was not above 0.0025 percent.
But such an amount of impurities could result in saturation magnetization about a thirtieth of one observed under experimental conditions. According to X-ray phase analysis data, our magnetic samples were a rhombohedral phase of C 60 with traces of partial disintegration. Depolymerization of the samples at atmospheric pressure and temperatures above 650 С led to a complete loss of their ferromagnetic properties. These facts invited the conclusion that the cause of the observed magnetic behavior must be related to the structural features of the carbon states obtained.
Since the appearance of magneto-ordered states was observed only in samples obtained within a narrow temperature range corre-
Magnetization curves and hysteresis loops of a magnetic sample. Data obtained at temperature 10 С (triangles) and 300 С (black circles).
Temperature curves of magnetization of a ferromagnetic sample in a magnetic field of 0.2 Т (upper curve, triangles) and remanent magnetization in a zero magnetic field (lower curve, circles). Curie temperature, ~ 500 С.
spending to the limit of thermal stability of the rhombohedral polymer phase at 6.0 GPa, we postulated that the appearance of constant magnetic moments in the carbon system could be put down to defects in the polymer layers of the rhombohedral phase; these defects were responsible for the formation of uncoupled electrons. Yet, as a matter of fact, the defective structure based on the rhombohedral phase is not the only cause of a state determining the ferromagnetic behavior of the samples. Among other putative candidates to this role could be also ferromagnetic structures based on sp 2 and sp 3 - the hybridized states of carbon suggested by Dr. A. Ovchinnikov, or else these could be graphite particles formed under our experimental conditions. At this stage we cannot exclude such a possibility.
To ascertain the nature of induced magneto-ordered states, researchers are carrying out detailed studies of new batches of magnetic samples at laboratories both here in Russia and abroad. The previous results, if confirmed independently, would enable us to state with better certainty that a ferromagnetic material on the basis of pure carbon does exist indeed.
The author of the present article would like to acknowledge with gratitude the generous help from the Russian Foundation of Basic Research (grants Nos. 00- 03-32600 and IR-97-1015).
Опубликовано на Порталусе 10 сентября 2018 года
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