Physiea C 185-189 (1991) 2195-2196 North-Holland
ANISOTROPY OF THE IRREVERSIBILITY FIELD IN YBA2CU307 SINGLE CRYSTALS MA,: ANG,A,D~ Z~X. SHEN and .A.D..CA PLIN, Blackett Laboratory, Imperial College, London SW7 2BZ, UK; • , , ~ M u ~ , enysiKadscnes Institul, uoethe Universitat, 6000 Frankfurt, Germany; L LEONYUK, Geology Dept, Moscow State University, Moscow_ 117234, USSR; V.V. MOSHCHALKOV, Physics Dept, Moscow ~tate university, MOSCOW117234, USSR; G. EMELCHENKO, Institute of Solid State Physics, USSR Academy of Sciences, 142432 Chernogolovka, MosCOw District, USSR. Magnetic measurements have been made of the anisotropy of the irreversibility field in YBa2Cu307 single crystals. Care has been taken to eliminate any contribution from currents circulating in the a-b plane to the magnetisation measured with the applied field closely parallel to these planes. Not only do the results demonstrate that persistent currents can flow in the c-direction, but they show also that at a given temperature the irreversibility field is about a factor of 4 higher for B II ab than B II c. This magnitude of anisotropy is similar to that in Bc2.
The oxide superconductors are strongly anisotropic, so that their magnetic response when screening currents attempt to flow in the c-direction is of great interest. The first question to answer is whether truly persistent currents can flow in this direction, bearing in mind that transport measurements can set only a fairly high upper bound to the resistivity. Secondly, if they do flow, does the magnetic phase diagram for B II ab resemble that for the usual B II c to the extent of including an "irreversibility field" Birr? Because of the large magnetic response to B II c by currents flowing in the ab plane, it is difficult to measure the magnetisation with B II ab. Any field misalignment results in a field component parallel to c, contributing a large m.~gnetic moment in this direction, which in turn has a component along the original field direction. Indeed, as the field is first rotated away from the c-axis, the magnetic response can be welldescribed by such a simple vector model 1. Even with careful alignment, there is still the concern that there may be within the crystal small inclusions olf different orientation. We orient the crystal accurately in the magnetometer with a simple device, which allows the field to be scanned through the ab plane with a setting
accuracy of better than 1o. The correct orientation with B II ab can be identified as an extremum in the magnetic response. Next, we analyse the data to check whether the magnetisation could have arisen from a small miscrientation angle o~: For a given field B, the component parallel to c would be ¢zB,giving rise to a moment mc(ecB) in the c-direction, and having a component ~mc(~B ) along the original field direction. For example, consider the M-H loops in Figure 1. At the maximum field of 8 T II ab, the measured moment is 8x10-6 Am 2. If o~w6re 1°, the field component along c would be 0.14 T, inducing a moment in that direction of 4.4x10-5 Am 2, whose component in the original field direction would be 8x10-7 Am2, a factor of 10 less than observed. A length scale analysis 2 provides a more stringent test: With B II al% the plate-like crystal has small demagnetisation factor, and so has a differential susceptibility Z'= -1 as the field is first reduced from its maximum value. On the other hand, were the observed moment to be merely the component of a moment in the c-direction, Z' = -e~2R/t, where R is the lateral dimension and t the thickness of the crystal. For 1o misorientation, Z' for our crystals would be ~ -3x10 -3. With B II ab, the measured X' is always close to -1, showing not only that the alignment is
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M.A, Angadi et aL / lrreversibilityfield in YBa2CusO7 single crystals
OFRA2 B / / e A
B II ob
FIGURE 1 Magnetic moment of a YBa2Cu307 sing!e crystal (FRA 2) versus field at 75 Kfor the two field orientations. The background diamagnetic slope arises from the sample holder. good, but also that the observed magnetic response is from the entire crystal, and not just from some misoriented inclusion. With this confidence that the measurements are of a genuine magnetisation II ab, it is meaningful to consider the "irreversibility field" Bin., defined here simply as the field at which the measured moment drops to noise level. Figure 2 shows Bin . as a function of temperature for the two symmetry directions, and for two crystals of slightly different Tc'S. It is apparent immediately that Bin . is about a factor of 4 largerwhen the field is parallel to the ab planes than it is when perpendicular to them. We believe that these are the first reliable magnetic measurements of Birr in the II ab configuration. Previously, because of the difficulties associated with magnetic measurements, estimates of Bin. II ab have been obtained from mechanical oscillator experiments3, 4, or from some rather arbitrary criterion on the 'tail' of the resistive transition5. The latter estimate is one to two orders of magnitude larger than the former; our magnetic measurements lie between the two. The generally accepted magnitude of anisotropy of Bc2 in YBa2Cu307 is a factor of 4 or 5, essentially identical to that we have measured in Birr. This congruence lends support to the idea that the "irreversibility field" does have some fundamental significance in the field-temperature phase diagram of these materials.
• FRA2 B / / a b Z~FRA1 B / / c • FRA1 a / / a b
oa" , ....( ~ A 80
FIGURE 2 The irreversibility fields for two YBa2Cu307 crystals for the two field orientations. REFERENCES .
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