CHAPTER 00
Magnetism of fullerene charge-transfer
complexes
Ales Omerzu1 and Madoka Tokumoto2
1
Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
2
Nanotechnology Research Institute, Natl Inst. of Advanced Industrial Science and
Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
1 Introduction
The research on magnetic properties of fullerene charge-transfer (CT)
complexes was sparked in 1991 when Fred Wudl’s group in Santa Barbara, in their
course of studying a reduction of fullerenes with strong organic donors, discovered a
compound tetrakis(dimethylamino)ethylene-C60 (TDAE-C60), which surprisingly
showed a ferromagnetic transition at 16 K1. That was a temperature, which exceeded
Curie temperatures of any other pure organic material known so far by more then order
of magnitude. Since 1991 a lot of experimental and theoretical work has be done
towards understanding of the ferromagnetic ordering in TDAE-C60. The most important
results will be presented in Section 2.
The discovery of TDAE-C60 motivated researchers from several laboratories
around the world to try to synthesize new organic, fullerene-based ferromagnets. Their
approaches could be divided into four main groups: (1) reduction of higher fullerens
with TDAE, (2) reduction of C60 with different organic or organometallic donors, (3)
functionalization of C60 and subsequent doping and (4) complexing C60 with rare earth
elements. Each of those approaches and their results will be presented in following
sections.
2 TDAE-C60
In the original work1 the authors synthesized TDAE-C60 by adding TDAE
(liquid) to a toluene solution of C60. The result was black, microcrystalline powder,
which precipitated from the solution almost instantaneously after adding of TDAE. The
material was highly air sensitive and all manipulation had to be done in a glove box
with an inert atmosphere. According to elemental analysis it was claimed that
stoichiometry was 1:1.16, which was later proven to be false. TDAE and C60 react in a
simple 1:1 ratio to form a charge transfer salt TDAE+C60-. Measurements of temperature
and field dependence of magnetisation demonstrated a clear transition to the
ferromagnetic phase below Tc = 16 K. The magnetisation increased abruptly below Tc
and Tc increased with the measuring field. The field dependence of magnetisation below
Tc displayed an S curve, characteristic to ferromagnets but with no observable
hysteresis. Magnetic susceptibility measurements at high temperatures (T > 30 K)
revealed almost temperature-independent behavior, quite distinct from the Curie-Weiss
behavior  = C/(T-), which is expected for systems of localised magnetic moments.
Furthermore, the electric conductivity measured on a compressed pellet turned out to be
quite high, 10-2 Scm-1. From those findings the authors concluded that TDAE-C60 might
be an itinerant soft ferromagnet.
Soon after discovery, the first X-ray diffraction study on TDAE-C60 was
performed in Brookhaven National Laboratory2. It was demonstrated that TDAE and
C60 crystallize in 1:1 stechiometric ratio, indeed. The structure was determined to be ccentered monoclinic (space group C2/m). The intramolecular C60-C60 separation is
shortest along the c axis (9.98 Å) and much greater in the a-b plane (10.25 Å). The twofold axis of TDAE molecules also orients itself along c axis. From the structural
parameters it could be concluded that TDAE-C60 has an anisotropic, low dimensional
band structure, which could account for unusual electronic and magnetic properties.
Little later, experiments of Tanaka’s group showed a slightly different picture3.
From ESR measurements they found that unpaired spins reside mostly on C60 molecules
(the g-value 2.0003 of TDAE-C60 is much closer to 1.999 for electrochemically
prepared C60- then 2.0036 for TDAE+). They also predicted the Jahn-Teller distortion of
C60- or, in other words, polaron formation in C60. In contrast to original findings they
observed the Curie-Weiss behavior of the magnetic susceptibility and from the shape of
M(H) curves at 4.5 K they calculated that magnetic moments at low temperatures form
clusters with an average size of 170 spins per cluster. Evidently, TDAE-C60, at least in
it’s powder form, showed magnetic properties of a superparamagnet.
At this point it was not yet clear if TDAE-C60 is a “proper” ferromagnet, but
following experiments cleared that issue. Suzuki et al.4 and Dunsch et al.5 found that
TDAE-C60 could exhibit a hysteresis curve although with a very small coercive field,
Hc~ 2 Oe and a remanent magnetic moment Mr ~ 3x10-4 emu. In addition, Suzuki et al.4
also measured the AC susceptibility with a nonzero imaginary part, i.e. energy losses,
related to the hysteresis. The final proof for the ferromagnetic state in TDAE-C60 came
from a zero field SR experiment by Lappas et al.6 The authors demonstrated the
existence of an internal magnetic field of 68 Gauss with very broad distribution (48
Gauss) reflecting spatial inhomogenity.
The SR experiment wasn’t the only one, which indicated inhomogenities,
structural or magnetic, in the system. Even in the early stage of the research some
experimental results appeared to be in contradiction with the hypothesis of a long-range
ferromagnetic ordering in TDAE-C60. In the first place, the temperature dependence of
the ESR line-width showed a relatively small line broadening and no frequency shift7
with a non-exponential and very slow decay of the magnetization8. Both of these
features are characteristic of random magnetic systems without a long-range order.
Hence, it was suggested that TDAE-C60 could be a spin glass. After experiment by
Mihailovic et al.9, which established a direct connection between orientational degrees
of freedom of C60 molecules and magnetic interactions in the system, this hypothesis
seemed to be even more plausible (this connection was latter demonstrated also by
theoretical calculations10,11). By freezing C60 molecules in random orientations one can
obtain a distribution of exchange interactions in the system, and consequently, magnetic
disorder and frustration – two essential conditions for a spin glass. Later measurements
of linear and non-linear susceptibilities12 partly confirmed the spin glass hypothesis.
The linear susceptibility 1 exhibited a broad peak centered at 10 K and the non-linear
susceptibilities 3, 5 and 7 diverged at the same temperature. The only feature, which
deviated from the spin-glass behavior, was the absence of any shift of the peak position
with frequency, which is so characteristic for spin glasses. Obviously, TDAE-C60 was
showing some characteristics of spin glasses and some of ferromagnets and it wasn’t
inconceivable that both phases coexist in a sample.
At that stage of research it was evident that lot of questions on the nature of
TDAE-C60 (ferro)magnetism remained, which couldn’t be answered by experiments on
powder samples. A reproducibility of physical properties for powder samples was
unsatisfactory even for the samples from one and the same group. Due to different
sample purity, which was mainly affected by solvent inclusion into the crystal structure
and oxygen contamination, as well as varying grain sizes in the powder samples,
TDAE-C60 exhibited an inconsistent behaviour. The samples were changing their
properties even by aging, usually by increasing their ferromagnetic signal. It was clear
that for making any progress in understanding of ferromagnetism in TDAE-C60
monocrystals were essential. The first attempt in growing single crystals was done by
Suzuki et al.13, but their crystals were rather small (0.3 mm in length, 0.05 mm in
diameter) and of poor quality. So, it was impossible to determine a crystal structure.
Finally, the crystals even didn’t show the ferromagnetic transition.
A similar approach for the single crystal growth by diffusion method was
adopted by one of the authors (A.O.). That time results were much better. Firstly, the
crystals showed a ferromagnetic transition at 16 K. Secondly, by improving the method,
which included a reducing of diffusion speed by smarter design of a crystal-growing
cell and a temperature control, it was possible to obtain high quality single crystals of
millimeter size. That breakthrough paved the way for experiments, which have
followed.
Figure 1. Three different views on the TDAE-C60 crystal structure along b, a and c
axis.
Having macroscopic single crystals available, one of the first questions, which
should be answered, was a mechanism of the electrical conductivity. Although
microwave conductivity measured by Schilder et al.14 and optical conductivity
measured by Bommeli et al.15 confirmed an insulating behavior of TDAE-C60, their
experiments where performed on microcrystalline samples with grains typically 10 –
100 nm in size. Large surface to volume ration and material’s high air sensitivity
obviously impair a clear discrimination between the insulating and the metallic intrinsic
conducting state of TDAE-C60. Omerzu et al.16 circumvented that problem by
measuring the AC and the DC conductivity on single crystals of TDAE-C60 with direct
electrical contacts. They found that conductivity could be decomposed into two
components: frequency-dependent, temperature-independent tunneling and temperaturedependent phonon-assisted hopping. A dynamic, rotational disorder of C60 molecules
plays a key role in the conductivity. The conductivity shows a crossover at T0 = 150 K.
It is a temperature, which separates the high-temperature orientationally disordered state
from the low-temperature ordered state as it was demonstrated by 13C NMR
measurements17. The hopping mechanism prevails at T > T0 where the hopping
probabilities are higher, but at T < T0 the tunneling is a more efficient conducting
channel.
Figure 2. The DC conductivity of TDAE-C60 single crystal as a function of
temperature. The full squares were measured for cooling at rate 0.1 K/min, while the
open circles were measured in near-quench conditions, 33 K/min.
Figure 3. Temperature dependence of the second (a) and the first (b) moment of the
13
C NMR spectra in powdered TDAE-C60.
Single crystals, in contrast to powder samples, have reproducible physical
properties and offer a possibility for another intricate property of TDAE-C60 to be also
explained. Namely, the powder samples frequently showed much lower magnetization
as expected, and even worse, it’s value for particular sample changed with aging. To
resolve that intricacy, Mrzel et al.18 choosed TDAE-C60 crystals grown at 10C, which
when fresh show no ferromagnetic signal at low temperatures. They treated the samples
in several heating cycles at temperatures between 50C and 110C. After each heating
cycle they measured temperature dependence of the ESR signal. They observed a sharp
increase in the intensity of the ferromagnetic signal after the sample was treated at 70C
or higher. The ferromagnetic signal eventually disappears when the sample was heated
above 100C. From then on it has been clear that TDAE-C60 can exist in at least two
crystal modifications: the usual or -TDAE-C60 which has a ferromagnetic phase below
16 K and newly discovered ’-TDAE-C60 modification without the ferromagnetic
phase. The ’ modification is the metastable one and can be irreversibly transformed
into the stable  modification by thermal treatment.
0.002
0.0015
0.001
M (emu)
0.0010
0.000
0.0005
-0.001
T=5K
H = 10 Oe
-0.002
0.0000
-200-150-100 -50
0
50 100 150 200
H (Oe)
2
4
6
8
10 12 14 16 18 20
T (K)
Figure 4. The field dependence (left) and the temperature dependence (right) of
magnetisation of -TDAE-C60.
When the existence of two different modifications of TDAE-C60 was firmly
established, researchers started with experiments, which would determined the nature of
the two TDAE-C60 modifications magnetic ground states. Arcon et al.19 measured a
ferromagnetic resonance in -TDAE-C60. By using the low-field ESR technique they
showed a nonlinear variation of the resonance frequency,  with resonance field, H and
proved the existence of long range magnetic order. From the  versus H dependence
they were able to rule out an antiferromagnetic behavior as well as a paramagnetic or a
spin-canted one. From an extremely low value of anisotropy field (29 Gauss) they
concluded that -TDAE-C60 is an example of an easy axis Heisenberg ferromagnet with
the easy axis along crystallographic c-axis, the axis of the closest C60- approach.
Another insight to the nature of the ferromagnetic transition in -TDAE-C60
offered measurements of the critical behavior near the ferromagnetic phase transition
point by Omerzu et al.20 The authors presented results of independent measurements of
the static critical exponents for susceptibility (T) ~ (T/Tc-1)-, spontantenuos
magnetization Ms ~ (1- T/Tc) and critical isotherm H ~ M in the vicinity of the
transition temperature Tc. The obtained results  = 1.22  0.02,  = 0.75  0.03 and  =
2.28  0.14 differed significantly from those expected for a 3D Heisenberg ferromagnet,
 = 1.38,  = 0.36 and  = 4.8. In addition, the exponents didn’t obey the scaling relation
 =  ( - 1). The authors found an explanation for such discrepancy in a reduced
effective dimensionality of the system caused by additional degrees of freedom coming
from C60 molecular rotation. Those induce an important degree of randomness into the
system and alter the nature of the ferromagnetic transition.
The presence of intrinsic randomness in -TDAE-C60 was clearly demonstrated
in measurements of linear and non-linear AC susceptibilities by Omerzu et al.21 It is
know that for ferromagnetic systems with a relatively low degree of disorder in
magnetic interactions a re-entrant spin glass (RSG) transition follows the ferromagnetic
transition at a lower temperature TRSG < TFM. Measurements of odd and even harmonics
of AC magnetic response in TDAE-C60 revealed an additional broad peak centered at 7
K, but only for odd harmonics. The reason is that at the spin glass transition the time
reversal symmetry is not broken in contrast to the ferromagnetic transition. Indeed, the
measurements showed a divergence in both odd and even harmonics at T = TFM. The
frequency dependence of the peak in the imaginary part of the linear susceptibility at 7
K gave an additional confirmation for the reentrant spin glass transition. Thus, the riddle
of the coexistence of the long-range ferromagnetic order and the short-range spin-glass
disorder was resolved.
10
9
8
8
6
''(a.u.)
'(a.u.)
7
4
6
5
4

3
2
2
1
0
0
-1
4
6
8
10 12 14 16 18 20 22
T(K)
4
6
8
10
12
14
16
18
20
T(K)
Figure 5. The temperature dependence of the real (left) and the imaginary (left) parts of
the linear AC susceptibility of -TDAE-C60 measured at difrent frequencies between 33
Hz and 3 kHz.
Magnetic properties of ’-TDAE-C60 are much simpler. Measuring the
macroscopic magnetic properties i.e. the temperature and the field dependence of the
magnetisation, Omerzu et al. 22 showed that ’TDAE-C60 is a paramagnet. The fieldtemperature dependence of the magnetisation exactly follows the Brillouin formula
M = N tanh ( H / kBT),
where N is the number of spins in a sample and  is the magnetic moment ( in the case
of S = ½,  = B – the Bohr magneton). From the formula and the measured
magnetisation and the mass of the sample it was possible to calculate an effective
number of spins per formula unit Neff. It turned out that Neff equals the number of C60ions in the sample. That notion immediately posed a question on missing contribution of
TDAE+ spins. Additional measurements at higher temperatures showed that Neff
increases from 1 to 2 per formula unit as temperature approaches 100 K. A mechanism
which could account for such behavior might be an antiferromagetic correlation among
TDAE+ spins, which causes the TDAE+ subsystem of spins to “freeze out” from the
bulk magnetization at low temperatures. It could also explain why antiferromagnetic
correlations were frequently observed in measurements of high-temperature
susceptibility. However, the role of the TDAE+ spins in the TDAE-C60 magnetism
remains provocative until now.
0.025
M (emu)
0.020
0.015
0.010
T=2K
0.005
0.000
0
10
20
30
40
50
H (kOe)
Figure 6. Magnetisation of ’-TDAE-C60 as a function of an applied field at T = 2 K.
The solid line is the Brillouin function, M = N tanh ( H / kBT).
(emu/Oe)
1.4x10
-8
1.2x10
-8
1.0x10
-8
8.0x10
-9
6.0x10
-9
4.0x10
-9
2.0x10
-9
0.0
0
50
100
150
200
250
300
T (K)
Figure 7. Magnetisation of ’-TDAE-C60 as a function of temperature measured in an
external field of 10 kOe. The solid line is the Curie-Weiss function,  = C /(T-).
The circumstance that TDAE-C60 appears in two modifications with completely
different magnetic properties was a clue for the microscopic understanding of its
magnetism. An irreversible transition from the metastable, nonferromagnetic form ’TDAE-C60 into the stable, ferromagnetic form -TDAE-C60 can be performed in a
controlled way. Usually, crystals of ’-TDAE-C60 are sealed into glass or quartz
capillaries under He. The transformation takes place at 70ºC. It needs 6 hours for
completion and any excess heating can gradually degrade the samples. The whole
procedure can be controlled by measuring magnetization curves at low temperatures
before and after the annealing.
0.008
0.006
T=2K
0.004
M (emu)
0.002
0.000
-0.002
-0.004
-0.006
-0.008
-50
-40
-30
-20
-10
0
10
20
30
40
50
H (kOe)
Figure 8. The field dependence of the magnetisation of TDAE-C60 before (open
squares) and after (filled squares) annealing. The measurements were performed on the
same single crystal.
Since the transformation occurs at mild conditions one would suppose only
minor structural differences between ’and  TDAE-C60. As it was expected,
Narymbetov and co-workers23 found the two modifications to be structurally
indistinguishable at room temperature. Differences appeared at temperatures below 50
K as additional diffuse lines in a diffraction pattern. By further cooling down to 7 K
those lines disappeared in the case of paramagnetic (PM) ’-TDAE-C60 and in the case
of ferromagnetic (FM) -TDAE-C60 they coalesced into additional sharp diffraction
spots. Those additional diffraction spots for -TDAE-C60 correspond to a primitive unit
cell, indicating that the crystal transformed from a C-centered structure to a primitive
one. A refinement of the crystal structure was possible only after introducing additional
degree of freedom – a relative rotation of C60 molecules around their three-fold axis by
±60º with 50% occupancy. In the PM sample, the relative C60 orientations are similar to
those encountered in other C60 solids: the 6-6 double bond faces the center of the
hexagon on the neighbouring molecule. In the FM sample on the other hand, a new
orientation appears (±60º), which leads to three possible relative configurations of the
C60s. However, only configuration in which the neighbouring C60 are rotated relative to
each other by ±60º is compatible with 50% occupancy of two rotations determined from
the structural refinement. In that configuration, the double bond on one molecule faces
the center of the pentagon of its neighbour, leading to C60 molecules ordered along the
c-axis with alternating orientations.
’-TDAE-C60
-TDAE-C60
Figure 9. A schematic diagram of the C60 molecular orientations in the a-b plane for
the PM (left) and the FM (right) structures. The corresponding C-centered and primitive
unit cells in the a-b plane are shown.
a)
b)
c)
d)
Figure 10. Projections of two neighbouring C60 units along the c-direction. a) The PM
phase. b) to d) Three possibilities of mutual orientations in the FM phase.
Kambe et al.24 explored a temperature dependence of -TDAE-C60 structure in
more detail. They found that additional diffraction spots, which correspond to the
primitive lattice start to appear at 180 K. They followed an increase of the new Bragg
reflection as the samples temperature decreased. From the smooth increase of the
intensity they concluded that the C-centered to primitive lattice structural transition in
-TDAE-C60 is of the second order.
When it seemed that the relation between TDAE-C60 structure and its magnetic
properties was satisfactory resolved a new discovery appeared. In their investigation of
pressure effect in TDAE-C60 Mizoguchi et al.25 found not only that Tc of the
ferromagnetic transition in -TDAE-C60 decreased with increasing pressure and
eventually disappeared at 9 kbar but also that above 10 kbar at 300 K -TDAE-C60
polymerised. The new, polymer -TDAE-C60 phase consist of 1D C60 chains covalently
interconnected by [2+2] cycloaddition in a similar way as in polymer o-Rb1C60. The
new phase was stable even after pressure release. Garaj et al.26 measured the
temperature dependence of the -TDAE-C60 ESR signal above 300 K and found that TDAE-C60 depolymerised at 520 K. This process was irreversible and the
depolymerized samples showed magnetic properties similar to the ferromagnetic TDAE-C60. There is another interesting property of -TDAE-C60: it is a paramagnet
with the magnetic susceptibility showing the Curie-Weiss temperature dependence but
with twice as many spins as in -TDAE-C60. From a shift in the ESR g-factor from
2.0005 in -TDAE-C60 to 2.0028 in -TDAE-C60, which is much closer to 2.0036 in
TDAE+ cation radical, the authors concluded that the missing TDAE+ spins revived in
-TDAE-C60. TDAE+ spins, which are mutually cancelled in ’ and -TDAE-C60 (the
physical origin of that is still unknown) appeared to be localized and noninteracting in
the polymer -TDAE-C60.
3 Higher fullerens reduced with TDAE
In the early stage of research on TDAE-C60 magnetism it was interesting to
compare it with higher fullerenes reduced with TDAE. It turned out that C70, C84, C90
and C96 readily form charge-transfer (CT) complexes with TDAE27. Their magnetic
properties were characterised mainly with ESR. The g-factor and line-width of the ESR
line for all of TDAE-higher fullerene samples were almost temperature independent and
the intensity of the ESR line, which is proportional to the spin susceptibility, followed
the Curie low I ~ C/T. Hence, the TDAE-higher fullerenes CT complexes are simple
paramagnets. From the g value the authors concluded that the unpaired spins reside
mainly on fullerene units. Later, Oshima et al.28 were able to crystalize TDAE-C70toluene complex and to obtain its crystal structure. In their samples C 70 molecules
formed singly bonded dimers. Magnetically, the crystals were paramagnets down to 1
K. It was supposed that spins on C70 dimers form spin singlets, so that magnetic signal
of TDAE-C70 could only originate from TDAE+ cation radicals. That would suggest that
also in other TDAE-higher fullerenes complexes TDAE+ spins are not silent.
Tanaka et al.29 succeeded to synthesise molecular alloys TDAE-(C60)1-x(C70)x in
a broad x range from 0.1 to 0.9. The low temperature magnetic properties of the alloys
were monotonically changing from a ferromagnetic for TDAE-C60 to a paramagnetic for
TDAE-C70. Interestingly, the Curie temperature, Tc also linearly decreased as the
content of C70 increased. This is consistent with the mean field result for Tc: Tc =
2JzS(S+1)/3kB, where z is the effective number of nearest neighbours. This result is
important because if the magnetic interactions in TDAE-C60 would be only along chains
in the c direction, any amount of impurities (C70 substitutions) would brake the
ferromagnetic order.
4 Reduction of C60 with different organic or organometallic donors
Although C60 is a weak electron acceptor it combines with many organic or
organometallic donors to form charge-transfer salts. Here we will mention only those,
which are relevant for magnetism of fullerene-based charge-transfer compounds.
Klos et al.30 were stimulated by the discovery of ferromagnetism in TDAE-C60
to try reduction of C60 with other amines similar to tertiary amine TDAE. For that
purpose they synthesised tertiary amines diazobicyclononene (DBN) and
diazobicycloundecene (DBU). In contrast to TDAE, where its eight methyl groups
sterically hinder direct reaction with C60, DBN and DBU are not so well protected.
DBN+ and DBU+ reacted with C60- forming covalent bonds and only a few percent of
nominal spin survived. Nevertheless, those residual spins showed in the case of DBUC60 a short range magnetic order, which evolves below 70 K.
In 1994 Wang and Zhou31 published results of magnetic measurements on the
charge transfer complex [1, 1’, 3, 3’-tetramethyl-2,2’-bi(imidazolidine)]+-C60- (TMBIC60). The complex showed ferromagnetic behavior up to 140 K with large hysteresis
loop (coercive field 1000 Oe!). However, very soon Schilder et al.32 showed that it was
false. In fact, TMBI even hardly made a CT complex with C60 and resulting product was
mainly diamagnetic (the ESR signal came only from impurities).
Otsuka et al.33 used a variety of electron donors including aromatic amines,
phenothiazines, phenazines, tetratianofulvalene derivatives and metallocenes to form
charge-transfer complexes with C60. Among them, only CT complexes with
metalocenes: decamethyferrocene (Cp2*Fe), cobaltocene (Cp2Co) and nicklocene
(Cp2Ni), showed ferromagnetic characteristics. All three complexes exhibited an Sshaped M(H) curve even at room temperature with a narrow hysteresis. Similarly, the
charge transfer complex with 1,1’-biferrocene34 showed signs of ferromagnetism at 20
K. Unfortunately, synthesis and magnetic properties of charge-transfer complexes of C60
with the metallocenes were not reproducible.
5 Charge transfer complexes of C60 derivatives
Another approach in the synthesis of novel fullerene-based molecular magnets
was based on functionalizing of C60 by covalently attaching different adducts to the
fullerene cage. Subsequently, such derivatives of C60 would be combined with organic
or organometallic donors to form charge-transfer complexes. The idea was to slightly
alter the fullerene electronic properties, e.g. electronic affinity, as well as to hinder C 60s
rotational degrees of freedom.
In 1994 Venturini et al.35 presented the first successful synthesis of doped
fullerene derivative ferromagnet. It was dinitro-spiromethanofullerene (C61”No2”)
doped with bicyclopentadiene cobalt (Cp2Co or cobaltocene). It showed paramagnetic
to ferromagnetic transition at 8 K. Transition temperature was lower then in TDAE-C60
but it was encouraging sign for a further exploration. The second breakthrough
happened in 1998 when Mrzel et al.36 reported ferromagnetic transition in a cobaltocene
doped C60 derivative at 19 K - significantly higher then in TDAE-C60. The derivative
was 1-(3-aminophenyl)-1H-methanofullerene[C60]. The important feature in both
compounds was that cobaltocene in its oxidised state Cp2Co+ has spin S = 0 and by no
means could contribute to the magnetic signal. Only spins S = ½ on fullerene moieties
contributed to the magnetic ordering. That was essential discovery having in mind that
the role of TDAE+ spin S = ½ in TDAE-C60 ferromagnetic ordering was unknown.
Figure 11. Fullerene derivative 1-(3-aminophenyl)-1H-methanofullerene[C60] (left)
and cobaltocene Cp2Co (right).
For cobaltocene-doped fullerene derivatives it was found that the temperature at
which the doping was performed plays a crucial role in determining the low-temperature
magnetic properties of these materials. A detailed study37 revealed the optimum
conditions, particularly the temperature for the synthesis of ferromagnetic material. The
magnetisation of samples differed markedly both in a magnitude and in a critical
temperature Tc. The magnetisation was highest when the synthesis was performed in the
vicinity of 45C and fell off rapidly on either side of that temperature. The critical
temperatures ranged from 13 K to 17 K. The low-temperature magnetisation in a weak
external field (the spontaneous magnetisation) could vary approximately by a factor of
three among different samples. The samples in ref. 36 showed also a hysteretic behavior
in their magnetic curves below Tc with a coercive field Hc ~ 100 Oe and a remanent
magnetisation Mr which is about 0.1 percent of the expected saturation magnetisation
Ms. The magnetisation did not show saturation in fields up to 1 kOe.
6 Intercalation of magnetic ions
Because various atom and molecules can be intercalated into C60 crystal, it was
expected that new magnetic C60 compounds could be synthesised by intercalation in
which magnetic moments would be carried by intercalants. For that purpose Eu was an
obvious choice38. Europium has a magnetic moment 7B in the divalent state, while it is
nonmagnetic in the trivalent state. In a fulleride Eu6C60 europium ions are in the
divalent state and they order ferromagnetically below 12 K39. Substitution of Eu with
nonmagnetic Sr ions in Eu6-xSrxC60, (x = 1 –5) had little effect on the transition
temperature, Tc. In addition, Eu6C60 showed a huge negative magnotoresistance at and
below Tc. Evidently, there exist a strong interaction between conduction carriers and
localized magnetic moments; namely, the strong -f interaction in Eu6C60. This fact
indicates that the ferromagnetism in Eu6C60 comes from the indirect exchange
interaction via C60 molecules, which is quite in contrast with the case of magnetic
semiconductor EuO.
Second fullerid with rare earth intercalated ions, which showed a magnetic
behavior was CexC6040. Cerium has outermost electronic configuration 4f15d16s2. In the
case of CexC60 cerium ion is in a trivalent state Ce3+ with unpaired 4f1 electron.
Magnetic properties of CexC60 were rather controversial. When cooled in a zero
magnetic field the system showed transition to the superconducting state below 13.5 K.
In contrary, cooling even in a very low magnetic field (2 Oe) destroyed
superconductivity and the system exhibited a ferromagnetic transition at 15 K. The
ferromagnetic state was also characterised by a hysteresis loop in a M(H) curve.
Although proximity of two different ground states in CexC60 hasn’t been explained yet,
we could roughly ascribe superconductivity to the doped C60 subsystem and
ferromagnetism to superexchange interactions between Ce+ ions.
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