Very strong flux motion at millikelvin temperatures in the heavy fermion superconductor UPt3

Physica B 165&166 North-Holland

(1990)

365-366

VERY STRONG FLUX MOTION AT MILLIKELVIN TEMPERATURES IN THE HEAVY FERMION SUPERCONDUCTOR lJpt3

A. POLLINI’, A.C. MOTA”, P. VISANI’, G. JURI’, and J.J.M. FRANSE+ *Laboratorium fur Festkorperphysik, ETH Hiinggerberg, CH-8093 Zurich, Switzerland +Natuurkundig Laboratorium der Universiteit van Amsterdam, 1018 XE Amsterdam, The Netherlands Extremely strong decays of the remanent magnetization are observed in a single crystal of Upts at temperatures as low as 7 mK. The decays deviate strongly from a logarithmic law. They are described well by stretched exponentials for 10-l < t c 10s set with parameters that change only slightly with temperature. The total decay of Mremfrom the beginning of the measured relaxation to equilibrium is 50% to 70% of its initial value with 30% occurring in the first 10000 sec. This novel behaviour points to strong motion of flux which is not thermally activated. Strong flux motion at millikelvin temperatures was observed for the first time in the high-T su erconducting oxides Sr-La-Cu-0 and Ba-La-&-01. At T = 20 mK, the low-field magnetization of a polycrystalline specimen of Ba-La-Cu-0 decayed following a lo anthmic law with a normalized decay rate M-1 918alogt = 1.3% per decade of time. A comparison with decay rates measured in classical tvoe II suoerconductors at T = 4.2 K showed that the magnetization in HTSC materials relaxes faster by a factor of 40, even at a 200 times lower temperature. Strong flux motion has been observed now in all the HTSC. It has been explained* by using extensions of the AndersonKim and Bean models that include the fingerprints of HTSC, i.e. the high temperatures and the unusually small pinning energies due to short coherence lengths. However, such models do not account for the relaxation rates at temperatures below 1 K, where thermal activation becomes less important. It seems clear to us that a new and until now undetermined mechanism is present in the HTSC in addition to the thermally activated flux creep as observed in conventional type II superconductors. This mechanism might also be the cause of the very hi h decay rates of the magnetization observe dgin other unconventional superconductors, i.e. the heavy fermionss and the organic compounds4. Here we present relaxation data on the heavy fermion compound UPts. This superconductor shows extremely large, almost temperature-independent, non-logarithmic time decays of the remanent magnetization. The specimen we investigated is a single crystal with a transition temperature Tc = 0.43 K. It has the dimensions 4 x 2.3 x 0.5 mms with the c-axis perpendicular to the plane of the specimen. The magnetic fiefd was applied perpendicular to the c-axis. The experimental arrangement and measuring methods have been described elsewheres. 0921-4526/90/$03.50

@ 1990 - El sevier Science

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time (set) Fig. 1 - Remanent magnetization as a function of time for two different cycling fields. Each curve consists of typically 500 points which lie within the thickness of the drawn line. In Fig. 1 we show decays of the remanent magnetization taken at T = 31 mK after cycling the zero field cooled specimen in a field Hi = 33 Oe upper curve) and 8.8 Oe (lower curve). The same a 6 itrary units have been used for both decays with the lower’ curve expanded by a factor of 10. The initial values of the remanent magnetization M-(O) were obtained independently from magnetizatron loops up to the given fields. We notice that after 104 set Mram(O)is reduced by about 31% for Hi = 6.6 Oe B.V. (North-Holland)

366

A. Pollini, A.C. Mota, P. Visani, G. Juri, J. J.M. Franse

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Fig. 2 - Remanent magnetization as a function of time for six different temperatures.

and by about 20% for Hi = 33 Oe. In Fig. 2 we show similar decays at different tern eratures after cycling the specimen to Hi = 6.6 ae .,It is interesting to notice that the percentage decay In the first 104 set is about 30% f 5% and practical1 independent of temperature for 7 c T c 350 mK. E learly, this extremely strong decay cannot be due to thermal activation. A reasonably good fit to the data shown in Figs. 1 and 2 is given by stretched exponentials of the form:

where Mrem(w), zp and 8 are taken as adjustable parameters. As an example, the relaxation curve M&t) at T = 31 mK, after cycling the specimen in a field lit = 6.6 Oe, has been fitted with the following parameters: Mrem(o) = 10.48 arbitrary units, zp = 10900 set, 8 2: 0.56. The result of the fit is shown,in Fig. S,.where we have plotted Mren$ M&m) In a loganthmrc scale as a functron oft . The uantity b/M&O) = [&em(O) Mrem 9 a)yMrem(O) gives the relative decay of the remanent magnetrzation from t = 0.1 set to e uiiibrium at t + 00.For the decay in Fig. 3, b/IIt rem(O)amounts to 0.52. Similar fits for the decays shown in Fig. 2 give values of 8 between 0.5 and 0.6, values of zp between 8000 set and 30060 set and of b&m(O) from 0.5 to 0.7 as the temperature is increased from 7mKto35OmK.

Fig. 3 - Relaxation data at T = 31 mK taken from Fig. 2 plotted as M(t) - M(t + -) vs ts,where M(t + -) and 8 are parameters obtained from the fit using the stretched exponential function given above. In conclusion, we have found that UPts shows much stronger decays of the remanent magnetization than the HTSC. For small magnetii mductions and temperatures as iow as T = 7 mK, about 30% of Mrem decays in the first 104 sec. The decay law is very close to a stretched exponential in the observation time of this experfment: 10-t < t c 105 sec. This novel and extremely fast relaxation seems to be independent of temperature. ACKNOWLEDGEMENTS We are grateful to H.-U. Nissen and R. Wessicken for the determination of the UFQ single crystal orientation by electron channelling patterns. This work was partially supported by the Schweizerixher Nationalfonds zur Forderung der wissenschaftlichen Forschung. REFERENCES 1. A.C. Mota, A. Pollini, P. Visani, K.A. Mtiller, and J.G. Bednorz, Phys. Scripta 37 (1988) 823 2. Y. Yeshurun and A.P. Malozemoff, Phys. Rev. Lett. 60 (1988) 2202 3. A.C. Mota, P. Visani, and A. Pollini, Physica C 153-l 55 (1988) 441, and Phys. Rev. B 37 (1988) 9830 4. A.C. Mota, P. Visani, A. Pollini, G. Juri, and D. Jerome, Physica C 153-l 55 (1988) 1153 5. A.C. Mota, G. Juri, P. Visa& and A. Pollini, Physica C 162-l 64 (1989) 1152