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A highly sensitive torsional vibration technique is used to study the magnetic properties of fullerite C60 (99.98%) at temperatures of 77–300 K in dynamic and static experiments. Vibrational energy absorption peaks associated with phase transitions and realignment of the magnetic structure of the fullerite are detected at T = 152, 195, 230, and 260 K. Relaxation magnetic processes in fullerite C60 at room temperature are studied. “Spontaneous” rotation of a motionless sample of fullerite freely suspended on an elastic filament is observed when external longitudinal or transverse magnetic fields are switched on. The direction of the “spontaneous” rotation changes with time. It is proposed that these phenomena are related to relaxation processes in the rotational subsystem of C60 molecular rotators, as well as to magnetic flux trapped in the fullerite and weakly damped eddy currents induced in the sample by the applied field.

We studied the intragrain magnetic hysteresis of the superconductor (Hg 1− x Re x )Ba 2 Ca 2 Cu 3 O 8+ δ ( x =0.10, 0.18 and 0.25) close to T c . From the magnetization curves we determined the temperature dependence of the irreversibility field ( H irr ( T )), and the field dependence of the magnetization curve width ( ΔM ( H )). After irradiation to a fluence of 2×10 21 m −2 ( E >0.1 MeV), the magnetization curves moderately increase their symmetry, and sizeable enhancements of H irr ( T ) and ΔM ( H ) are observed. Although the substitution x =0.18 shows the best pinning properties before irradiation, x =0.10 matches—or even surpasses—them afterwards. This suggests that, within our temperature and field window, neutron irradiation plays a far more important role than Re-substitution in enhancing the irreversible behavior in the Hg-1223 system.

A detailed study of the supercurrent flow in ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$ single crystals is presented. From dc magnetization and ac susceptibility measurements we obtain the sample dimension over which the superconducting currents flow. We find that this dimension is identical to the sample size and, therefore, demonstrates the bulk nature of superconductivity in fullerenes in an unambiguous way.

We report on detailed magnetic investigations of big ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$ single crystals with sizes of $\ensuremath{\sim}1\mathrm{mm}.$ The samples were characterized by x-ray analysis, neutron diffraction, and ac magnetization measurements. The temperature dependence of the upper and lower critical fields was obtained, the latter by the trapped magnetic-moment method. The coherence length and the penetration depth at $T=0$ and close to the transition temperature were evaluated. The magnetic field and the temperature dependence of the critical current density were studied and the irreversibility lines determined. Long-term magnetic relaxation, measured in a wide range of temperatures and magnetic fields, allowed us to obtain the temperature dependence of the relaxation rate and the flux creep activation energy at different magnetic fields. The influence of sample inhomogeneities on all these properties was investigated. The results are compared to those obtained on powder and polycrystalline samples.

Magnetic relaxation processes in superconducting fulleride K3C60 are investigated on monocrystalline samples in a wide range of temperatures and magnetic fields. A logarithmic time dependence M(t) of magnetization is observed in samples with 100% of superconducting phase. The normalized relaxation rate increases with temperature in the entire range of magnetic fields. The flux creep activation energy ranges from 10 to 80 meV, and its temperature dependence has a peak. It has been concluded from the results of measurements on samples with nonideal stoichiometry that inhomogeneities strongly affect the relaxation processes and can mask the logarithmic dependence M(t) completely.

Long-term magnetic relaxation in the fullerene superconductor K{sub 3}C{sub 60} was measured on crystalline samples in a wide range of temperatures and magnetic fields. A logarithmic {ital M}({ital t}) dependence is observed on single crystals with 100{percent} shielding fraction. The relaxation rate at different magnetic fields increases progressively with temperature. The flux-creep activation energy is found to be in the range from 10 to 80 meV with a peak in its temperature dependence. From measurements on samples with nonperfect stoichiometry we show that inhomogeneities strongly affect the relaxation process and may mask a logarithmic behavior. {copyright} {ital 1996 The American Physical Society.}

A review of the superconducting magnetic properties of doped fullerenes is presented. Experimental results on the main superconducting properties, such as the critical fields and the characteristic lengths, are critically discussed. Different methods to evaluate the lower critical field are discussed. Finally, experimental data on properties connected to flux pinning, such as the critical current density, the irreversibility line and the pinning force, are summarized and discussed.

High resolution magnetization measurments on C60, C70 and iodine doped C60 fullerenes are presented. An irreversibility of the magnetization at T < 100 K and a strong extremum at T ⋍ 60 K are clearly observed for C60, C70 and IC60. It is shown that these effects can be explained in terms of a model for a transition into a frozen orientational disorder state, which has the character of a glass transition.

The temperature dependence of the upper critical magnetic field ${\mathit{H}}_{\mathit{c}2}$ of the superconducting ${\mathrm{RbCs}}_{2}$${\mathrm{C}}_{60}$ fullerene has been obtained. The value of ${\mathit{H}}_{\mathit{c}2}$(0) at zero temperature is estimated to be 17 T and the coherence length \ensuremath{\xi}(0)=4.4 nm. The lower critical field ${\mathit{H}}_{\mathit{c}1}$ has been studied using three different methods. The ${\mathit{H}}_{\mathit{c}1}$(T) dependence obtained from the procedures based on the deviation from perfect diamagnetism and on Bean's critical-state model has a positive curvature with decreasing temperature. It is shown from a high-field analysis that this saturationless dependence does not reflect the intrinsic ${\mathit{H}}_{\mathit{c}1}$ of the superconductor. The value of ${\mathit{H}}_{\mathit{c}1}$(0) is estimated to be 5.5--8.1 mT and the penetration depth \ensuremath{\lambda}(0)=300--370 nm.

From the experimentally obtained H(T) dependence of the transition boundary between short-range and long-range vortex order for ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ superconductors, the temperature dependences of the effective potential barrier for pinning of a single vortex on a pinning center and the effective pinning force are calculated. It is shown that the effective barrier falls almost linearly with the increase of temperature and that the harmonic thermal fluctuations strongly decrease the effective pinning force as ${\mathit{T}}^{3/2}$.

We present the results of magnetic measurements in fields up to 50 kOe for superconducting Rb 3 C 60. The temperature dependence of upper H c 2 critical magnetic field is determined, from which the zero temperature value Hc2(0)=465±50 kOe is evaluated. The magnitudes of penetration depth and coherence length are calculated as ξ(0)=26.7±3 Å; λ L (0)=2150±100 Å at zero temperature. The temperature dependence of ξ(T) and λ L (T) for T≧23 K is in good agreement with the Ginzburg-Landau theory. The critical current densities for different temperatures are calculated, showing a strong decrease of Jc with increasing temperature for T≤7 K . Two regions of fading critical current density on a magnetic field dependence are found.

A review of the superconducting magnetic properties of doped fullerenes is presented. Experimental results on the main superconducting properties, such as the critical fields and the characteristic lengths, are critically discussed. Different methods to evaluate the lower critical field are discussed. Finally, experimental data on properties connected to flux pinning, such as the critical current density, the irreversibility line and the pinning force, are summarized and discussed.

A review of some of the solid state and the magnetic properties of fullerenes is presented. We summarise and discuss experimental results on magnetism of pure C60, C70 and higher fullerenes. The main features of the structure and the magnetic properties of intercalated fullerenes, such as metallofullerenes, TDAE-C60 and polymeric A1C60, are presented. Attention is also paid to the mixed state properties of fullerene superconductors, especially to the evaluation of the critical fields and the characteristic lengths of these materials. Some new experimental results on this subject are also presented.

The lower critical magnetic field Hc1 of superconducting Rb 3 C 60 (Tc=28.5 K ) is estimated by different methods. The zero temperature value is found to be given by Hc1=16.2±1.0 mT, and the penetration depth is λL=215±10 nm . The Bean model analysis leads to threshold fields of 5.3 mT for T=5 K , and 4.0 mT for T=17 K . The big influence of intergranular connections on Hc1 is demonstrated. Good agreement between the low- and the high-field analysis is found.

The penetration depth λ and the coherence length ξ in Rb 3 C 60 fullerene are determined by means of magnetic measurements in fields up to 50 kOe. For the Ginzburg-Landau coherence length the estimate ξ GL (0)=23 Å is obtained, while the magnitude of λ GL (0) extracted from the high-field data turns out to lie between 1200 and 1400 Å, with the upper value being more likely. Non-superconducting diamagnetic susceptibility is also measured and found to be equal to −(0.60±0.02)×10−5 which is an order of magnitude higher than that of undoped fullerene.

Magnetic susceptibility, magnetization curves and flux creep of superconducting Rb3C60 with Tc?=?27.5 K have been studied. The critical current for Rb3C60 was evaluated to be 4???106 A/cm2 at 7 K and 10 kOe, which is considerably higher than for K3C60. The flux creep measurements give the activation energy of U(7 K) ? 3.3???10-2 eV.

We report on detailed magnetic investigations of big ${\mathrm{Rb}}_{3}{\mathrm{C}}_{60}$ single crystals with sizes of $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{mm}$. The ${\mathrm{Rb}}_{3}{\mathrm{C}}_{60}$ crystals were prepared by doping ${\mathrm{C}}_{60}$ crystals in rubidium vapor. Experiments with different annealing times and various annealing temperatures and temperature gradients were made to assess the optimal conditions for homogeneous doping. The crystals were characterized by ac magnetization measurements. Doping at 463 K with 10% excess of rubidium and annealing at 685 K for 27 days was found to produce the best crystals with almost 100% of the pure ${\mathrm{Rb}}_{3}{\mathrm{C}}_{60}$ phase. The temperature dependence of the upper and lower critical fields was obtained, the latter by the trapped magnetic moment method. The coherence length and the penetration depth at $T=0$ and close to the transition temperature were evaluated and the irreversibility line was determined. The results are compared to those obtained on powder and polycrystalline samples.

Rb3C60 crystals with a size of several tenths of a millimetre were prepared by doping C60 crystals in rubidium vapour. Experiments with different annealing times and various annealing temperatures and temperature gradients were performed to assess the optimal conditions for homogeneous doping. The crystals were characterized by ac magnetization measurements. Doping at 463 K with 10% excess of rubidium and annealing at 685 K for 27 days were found to produce the best crystals with almost 100% of the superconducting Rb3C60 phase.

A review of some of the solid state and the magnetic properties of fullerenes is presented. We summarise and discuss experimental results on magnetism of pure C60, C70 and higher fullerenes. The main features of the structure and the magnetic properties of intercalated fullerenes, such as metallofullerenes, TDAE-C60 and polymeric A1C60, are presented. Attention is also paid to the mixed state properties of fullerene superconductors, especially to the evaluation of the critical fields and the characteristic lengths of these materials. Some new experimental results on this subject are also presented.