SUPERCONDUCTIVITY, HIGH TEMPERATURE SUPERCONDUCTIVITY Magnetization relaxation in superconducting fulleride K3C60 single crystals V. A. Buntar, F. M. Sauerzopf, and H. W. Weber Atominstitut der O¨ sterreichischen Universita¨ten, A-1020 Wien, Austria A. G. Buntar Vinnitsa State Technical University, 286021 Vinnitsa, Ukraine H. Kumani and M. Haluska Institut fu¨r Festko¨rperphysik, Universita¨t Wien, A-1090 Wien, Austria ~Submitted September 10, 1996! Fiz. Nizk. Temp. 23, 365–371 ~April 1997! 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. © 1997 American Institute of Physics. @S1063-777X~97!00104-7# 1. INTRODUCTION also observed in fullerides. However, these two types of su- perconductors exhibit considerable differences. Supercon- The measurements of relaxation magnetization serve as ductivity in HTS compounds is two-dimensional and a universal method for studying irreversible properties of strongly anisotropic, while FS are three-dimensional super- type II superconductors and provide information on pinning conductors with a cubic lattice. of Abrikosov vortices, their dynamic properties, the nature of Thus, an analysis of time relaxation of magnetization in kinetic state, and so on. Relaxation is usually described with FS and a comparison of the obtained data with the results for the help of the flux creep activation energy ~FCAE! U . The 0 HTS materials are of considerable interest. In spite of the Anderson–Kim model1 presumes, for example, the existence fact that superconductivity in metal-doped fullerides has of a homogeneous barrier for magnetic vortex depinning and been discovered several years ago,20 only a few experimental leads to a logarithmic time dependence of the form results on vortex pinning21,22 and magnetic relaxation23–26 were reported. The estimates obtained in Ref. 23 give the M~t!5M 0@12~kT/U0!ln~t/t0!#, ~1! values of activation energy in FS of the order of 1022 eV. It where M 0 is the value of magnetization at the initial instant should be noted, however, that all the previous measure- of time, and t0 the time constant. ments were made in powders, and magnetization relaxation The experimental investigations of magnetization relax- usually was not logarithmic.24 Some measurements ~on ation in HTS materials prove that the logarithmic depen- RbCs2C60! even revealed peaks on the M(t) dependence. dence is observed in most cases,2–9 and the activation energy Such a behavior can be associated with the interaction be- U0 obtained from these measurements increases with tween grains in powder samples and with weak intergranular temperature.3,4,6–8,10 Several theoretical models, including a bonds that can exist in low-quality samples. These factors transition to the vortex glass state,11 violation of intergranu- can stronly affect the magnetization relaxation. lar bonds,6 percolation of vortices in a random pinning po- In this communication, we report on the results of inves- 4 tential distribution, a nonlinear dependence U p( j) of the tigation of magnetic relaxation, obtained on FS crystals. pinning potential, where j is the current density,5,12 and col- These measurements were made on samples of different lective pinning,13 were proposed for explaining this phenom- quality. Our experiments showed that vortex system insta- enon. bilities emerging due to the presence of weak bonds in a Many properties of fullerene-based superconductors sample mask the logarithmic process completely. Magnetic ~FS!, such as a relatively high transition temperature and a relaxation in high-quality samples has a clearly manifested very high value of the Ginzburg–Landau parameter, are logarithmic form, which makes it possible to calculate the similar to the properties of HTS compounds.14–19 In addition normalized relaxation rate S and the flux creep activation to the abovementioned properties, magnetization relaxation energy U0 in a wide range of temperatures and magnetic in a mixed state, which is as strong as in HTS materials, is fields. We found that the temperature dependence of U0 has 267 Low Temp. Phys. 23 (4), April 1997 1063-777X/97/040267-05$10.00 © 1997 American Institute of Physics 267 FIG. 1. Time dependence of magnetic moment in sample S3 at T55 K in different magnetic fields. a gently sloping peak, and the value of activation energy lies x-ray diffraction analysis indicated the presence of a mosaic in the interval from 10 to 80 meV in the temperature range structure in a sample with a 3° disorientation between from 5 K to Tc . blocks. The dc SQUID measurements on the second ~S2! crystal revealed 100% screening and 10% Meissner effect. The su- 2. EXPERIMENT perconducting transition temperature Tc of this sample was This research mainly aims at experimental study of mag- 19.2 K. The ac magnetic measurements on this made sample netization relaxation in high-quality samples, which makes it did not reveal any features of granular structure for super- 27 possible to avoid the effect of intergranular currents. For this conducting currents. purpose, measurements were made on crystalline K3C60 Sample S3 samples with 100% of superconducting phase. In addition, The dc SQUID measurements on this sample proved that we also made measurements on crystalline samples with it had 25% screening. Neutron diffraction experiments re- various volumes of superconducting phases ~from 25% to vealed the presence of a mosaic structure in the sample with 100%! in order to study the effect of nonsuperconducting a 5°-disorientation between blocks. Also, the intensity ratio inhomogeneities on the relaxation process. I220 /I311 was 1.3, which lies between the expected values 1.8 We used KN3 as a source of potassium which can be and 0.64 for pure C60 and K3C60 respectively. Thus, this weighed in small amounts in air. crystal is undoubtedly a mixture of the K3C60 phase and a Samples S1 and S2 phase containing a smaller amount of potassium. The S1 sample was a set of four K3C60 crystals soldered Sample S4 in a quartz capsule. The total mass of these crystals was This crystal had a mass of 4 mg. The x-ray diffraction 13.2 mg. Preliminary dc magnetic measurements revealed pattern displayed highintensity lines belonging to C60 as well 100% of magnetic field screening and 9% of the Meissner as lower peaks corresponding to the compound K3C60 . This effect. sample was also characterized by a 5°-disorientation of Subsequently, these two crystals were soldered in sepa- blocks. The dc SQUID measurements revealed that rate quartz capsules. One of them ~S2 sample! was used for Tc519.5 K, the magnetic field screening was 65%, and the magnetic measurements. X-ray studies on the other crystal Meissner effect amounted to 7%. showed the presence of clearly distinguishable @111#, @222#, The dc magnetic measurements were carried out on a and @333# reflexes corresponding to the composition commercial SQUID magnetometer ‘‘Quantum Design’’ in K3C60 . No traces of pure C60 were detected. In addition, the temperature range from 5 K to Tc;19 K. The external 268 Low Temp. Phys. 23 (4), April 1997 Buntar et al. 268 FIG. 2. Time dependence of magnetic moment in sample S3 in a magnetic field m0Hex50.5 T at different temperatures. magnetic field was varied in the limits m H !0.1 T<m H<1T!m H , i.e., the measurements 0 c1 0 0 irr were made under the conditions when the field penetrated the sample. In all the cases, the rate of increase in the magnetic field to a certain value was 28 mT/s. The sensitivity of our 28 2 magnetometer was not worse than 10 A•m . Magnetic re- 4 laxation was measured during 5•10 s. The measurements were made as follows. The sample was cooled from T530 K.Tc to a preset temperature T,Tc in zero magnetic field. After temperature stabilization, the external magnetic field Hex was created, and the magnetic moment m of the sample was measured for fixed values of T and Hex . The first measurement of the magnetic field was completed after t0580 s after the field stabilization. Subsequent measure- ments were made after each 60–63 s. The data obtained on samples S1 and S2 were virtually identical, and hence we mainly present here the results obtained for S1 as well as S3 and S4. 3. DISCUSSION OF RESULTS The results of magnetic relaxation measurements ob- tained for samples S3 and S4 with incomplete field screening ~nonideal stoichiometry! are shown in Figs. 1 and 2 for dif- ferent temperatures and magnetic fields. It can be seen that relaxation is clearly nonlogarithmic; the more so, the m(t) curves exhibit magnetization jumps. In order to verify pos- sible instabilities of our experimental measuring system, the same measurements were made on another, noncommercial FIG. 3. Time dependence of magnetic moment in sample S3 at the instants SQUID magnetometer. Although most of small jumps on the corresponding to jumps shown by arrows in Figs. 1 and 2. 269 Low Temp. Phys. 23 (4), April 1997 Buntar et al. 269 The amplitude of the magnetization jumps observed by us increases with the magnetic field ~see Fig. 1!. At the same time, the m(t) curves become smoother with increasing tem- perature ~see Fig.