Direct observation of the flux line lattice in Al-doped YBa2Cu3O7−δ

PHYSICA

Physica C 199 (1992) 73-83 North-Holland

Direct observation of the flux line lattice in Al-doped YBa2Cu3OT_a I.V. Grigorieva 1, K.E. Bagnall, P.A. Midgley, K. Sasaki 2 and J.W. Steeds H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, UK Received 4 April 1992 Revised manuscript received 25 May 1992

The high-resolution Bitter pattern technique has been used to study the flux line lattice (FLL) in Al-doped YBa2Cu307_a (YBCO) single crystals with 4% of Cu atoms substituted for Al. In contrast to undoped YBCO, the FLL was isotropic and no effects of vortex interaction with twin boundaries were found which is consistent with the extremely small size of the twin domains 30 A in these crystals brought about by Al-doping. In addition, the FLL consisted of misoriented domains inside which an undisturbed regular vortex arrangement was seen, whose dimensions rapidly increased with the magnetic field. This behaviour testifies to the FLL being collectively pinned by weak pinning sites such as vacancies, small defect clusters, etc. Proceeding from the dimensions of these domains, we estimated the critical current density for our crystals to be Jc ~ 3 × 102 A/cm 2, two orders of magnitude lower than the typical value for undoped YBCO single crystals, hut of the same order of magnitude as for Fe-doped YBCO.

I. Introduction

Much interest has recently been paid to the effects partial substitution of Cu(1) atoms in YBaECU307_a by impurity M atoms (M = Fe, Al, Co etc). It was found that this doping enables deliberate change of both the crystal structure and various superconducting characteristics of the YBCO crystals: it results in a substantial decrease of the critical temperature Tc (depending on the amount of impurity atoms) [ 1 ] and pinning forces Fp [2 ] and in a rapid decrease of twin domain sizes down to 30-100 A for x>0.025. High-resolution electron microscopy [ 3 ] and electron diffraction studies [4,5 ] have shown that within the domains the structure remains orthorhombic, but due to the random orientation and extremely small size of the domains a quasi-tetragonal structure is produced on a macroscopic scale (as revealed by Xray and neutron diffraction experiments). This type of

1 Permanent address: Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow District, Russia. 2 Permanent address: Department of Material Science and Engineering, Nagoya University, Nagoya, Japan.

of crystal structure is often referred to as a "tweed" structure. In this paper we present the first results of the direct observation of the magnetic flux structure (vortex distribution) in Al-doped single crystals performed with the help of the high-resolution Bitter technique [6 ] which uses fine ferromagnetic particles (e.g. Fe, Co, N.i) to "decorate" vortices at the surface of the superconductor. Although this technique is restricted to low magnetic fields (H
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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L 1I..Grigorieva et al. ~Flux line lattice in Al-doped YBaeCu~Oz_~

tures of the vortex structure in YBa2 (Cu ~_xAlx) 3 0 7 - - 6 crystals with x = 0.04 for several values of the external magnetic field and examined the pinning characteristics of these crystals in relation to their "tweed" structure and typical local surface defects. It was also possible to estimate the volume pinning force Fp and corresponding critical current for crystals with ideally fiat (defect-free) surfaces. The estimated value of F o can be regarded as an intrinsic value for these crystals. A brief report on the microstructure of Al-substituted samples is also provided.

2. Samples and experimental technique Figure 1 shows schematically a chamber which was used for decoration. A sample and a filament covered with the ferromagnetic material (both iron and cobalt were used) were placed in the chamber filled with inert He gas at a pressure of 0.08-0.1 Torr. The samples were cooled from T > Tc down to 4.2 K with the external magnetic field applied (so called fieldcooling mode). In this mode the magnetic flux penetrating into the superconductor near Tc remains trapped due to pinning at lower temperatures and

SAMPLE l i e a LIQUID

~

.------~ MAGNET

CAN FII.I.I'I) WITH 0.1 TORR lIE 4 GAS

|~AI:FLE ~

"l't !NGSTI'~N FII.AMENT

Fig. 1. Schematic diagram of the evaporation chamber used in the flux decoration experiments.

allows observation of vortices even in the magnetic fields H < H c ~ ( 1 - N ) . Here N i s the demagnetisation factor; N=0.85-0.9 for our samples. After evaporation, iron or cobalt particles move slowly towards the surface of a superconductor and are attracted to locations of strong magnetic field gradients which correspond to the vortices emerging from the superconducting bulk. The resultant distribution of particles which reveals the FLL was then examined in a scanning electron microscope in the secondary electron mode. The samples were YBae(Cul_xAlx)3OT_~ single crystals with x = 0.04. A detailed procedure of the crystal growth is given in ref. [8 ]. The starting materials were BaCO3, Y203, and CuO powders with 99.9% purity. The powders were milled, dried at 150°C for 5 h and calcinated at 900°C for 8 h in air. The powdered samples were than charged in alumina boats, heated up to 1150°C and kept at this temperature for 8 h. The samples were cooled from 1150°C to 900°C with a cooling rate of approximately 2°C/h. The crystals tended to be lamellar in shape with the short side parallel to the c-axis with typical sizes of 2 × 2 × 0 . 0 5 mm. The external magnetic field was always applied parallel to the c-axis. The homogeneity and microstructure of the samples were verified by transmission electron microscopy and X-ray diffraction. The A1 distribution was found to be homogeneous and no second phases could be detected. TEM has revealed that all our samples had a "tweed" structure, were very homogeneous and relatively defect-free (very low density of dislocations). A typical example of this structure is shown in fig. 2. The scale of the contrast in this dark-field image is about 30 A and its origin lies in the disruption of the C u - O chains in the YBCO structure brought about by the tendency of a dopant atom to increase its oxygen coordination [ 9,10 ]. From high-resolution electron microscopy studies [2 ] it may be concluded that it is still true micro-twinning but the structure of twin boundaries is likely to be more complicated than in the undoped crystals [ 10 ]. Although we did not control the critical temperature of our samples, it can be estimated from a study of the effect of Al-doping on Tc [ 1 ] as Tc.~ 80 K.

L V. Grigorievaet aL ~Flux line latticein Al-doped YBa2CujOz_~

75

Fig. 2. g= 010 dark field image of 4% Al-dopedYBCOshowingcharacteristic tweed structure.

3. Experimental results and discussion Figure 3(a) shows a typical image of the vortex lattice in YBa/(CUl_xAlx)307_d crystals for the average magnetic induction in the sample B = 20 G. It is shown together with an auto-correlation function (ACF) of the whole image in fig. 3(b). To obtain the ACF the digitised image was processed using the SEMPER image processing package. The ACF is defined as

G(Sx, ~y) = ~ I ( x + 5x, y+ ~y)I(x, y ) d ( x ) d ( y ) , where I(x, y) is the image intensity at pixel (x, y). In essence, G(Sx, By) gives the probability of finding a similar degree of intensity in the image if one moves (Sx, 6y) from any point. It may be obtained from the whole image or from selected areas to enable comparisons. The FLL is seen to be relatively regular for this very low value of the magnetic field: the orientational order is preserved for tens of vortex spacings which is seen as the preservation of the directions of closepacked vortex rows in the FLL and as well defined

reflections in the ACF extending out to high orders. This corresponds to a very low level of pinning forces in this material compared to the undoped YBCO and is consistent with the observed high homogeneity of tis microstructure. In addition the vortex lattice is isotropic as revealed from the regular hexagonal arrangement of reflections in the ACF. This is again in contrast to the undoped YBCO which shows a sizeable anisotropy of the FLL in the (a-b) plane of about 20% [ 11 ]. This can be clearly explained by the quasi-tetragonal symmetry of the microstructure of the Al-doped crystals on a scale much larger than the size of microdomains 30-100 /~. Although the orthorhombicity is preserved inside the microdomains, their scale is two orders of magnitude smaller than all scales in the FLL-vortex spacings ~ 1 ~tm and vortex size ~ ~2 (2 is the magnetic field penetration depth). Thus, the random distribution of twin microdomains makes it impossible for vortices to notice this orthorhombicity. ~tl Here and in what follows "vortex size" denotes the vortex magnetic extension as this is its only characteristic which can be revealed by the decoration technique.

76

L V. Grigorieva et al. ~Flux line lattice in Al-doped YBa2Cu~07_a

Fig. 3. (a) SEM image of the vortex lattice with B=20 G; (b) auto-correlation function obtained from the image in (a) indicating a well-ordered hexagonal lattice. Figures 4 (a) and 5 (b) show typical images of the FLL for two smaller values of the magnetic induction: 15 G and 10 G. It is clearly seen that the ordering in the FLL decays with a decrease in the magnetic field. For 15 G there are regions of well ordered hexagonal patterns of vortices that extend over 8-10 vortex spacings while for l0 G the vortex lattice is

already disordered and some hexagonal correlations can be found only for nearest neighbours. The central part of the ACF (fig. 5(b) ) shows a rather noisy pattern but two rings can just be discerned showing that there is some regularity in the vortex spacings. An important observation here is that the FLL, where it is regular, consists of misoriented domains

L V. Grigorieva et al. ~Flux line lattice in Al-doped YBaeCu307_6

77

Fig. 4. (a) SEM image of the vortex lattice with B= 15 G; (b) two auto-correlationfunctions showinghexagonal ordering obtained from the areas marked in (a) by dashed lines. Note the misorientation between the two domains.

as can be easily seen for larger fields 15 G and 20 G. The dimensions of these domains rapidly increase with an increase of the external magnetic field (compare the vortex patterns in fig. 4 ( a ) and 3 ( a ) ) . In fig. 4 ( b ) ( B = 15 G), two ACFs are shown from two outlined regions. Both ACFs show a hexagonal pattern indicating good hexagonal ordering and the misorientation between the two patterns shows that the two regions are rotated with respect to each other by about 20 °. Two misoriented domains can be also found in the left part of fig. 3(a). Thus we have observed preservation of the regular vortex lattice inside misoriented domains whose dimensions rapidly increase with an increase in the magnetic induction while the long-range order in the FLL is destroyed. Such behaviour of the FLL is the typical response to collective pinning from weak and random pinning centres. It was carefully studied in our previous work [ 12 ] and also in ref. [ 13 ]. Under the conditions of

collective pinning whole parts of the vortex lattice, rather than individual vortices, adjust themselves to large groups of pinning sites as was predicted by the theory of collective pinning (CP) [ 14 ]. This theory gives the relation between the correlation dimensions (dimensions of the undisturbed areas in the FLL) and the volume pinning force Fp. The volume of the correlation regions decreases for softer FLL (smaller elastic moduli) a n d / o r stronger pinning centres. In ref. [ 12 ] the FLL in N b - M o single crystals was pinned by a random distribution of dislocations and the observed dependence of the average domain size in the FLL on the magnetic field was in good qualitative agreement with that predicted by the CP. In addition the volume pinning force Fp estimated from the average domain size in ref. [ 12 ] according to CP was in good agreement with its value measured independently for the studied crystals. Similar response of the FLL to weak collective pin-

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I. V, Grigorieva et al. / Flux line lattice in Al-doped YBa2CujO7_a

Fig: 5. (a) SEM image of the vortex lattice with B = 10 G; (b) auto-correlation function obtained from the image in (a) showing two diffuse rings indicating a disordered lattice.

ning was also observed in ref. [ 13 ] for BSCCO crystals. Another important feature in common between refs. [ 12 ] and [ 13 ] and the present study is how the FLL is deformed. In smaller fields mostly plastic deformation of the FLL is responsible for the formation of correlation regions and many dislocations, vacancies and clusters of disordered vortices are

present (see fig. 4(a) ). On increasing the magnetic field elastic deformations become more important which are seen as a change in the vortex rows' directions at the domain boundaries (see fig. 3 (a)). Due to the discussed similarities, we believe that collective pinning is also present in our Al-doped YBCO crystals. Proceeding from the average dimen-

L V. Grigorievaet al. ~Flux line lattice in Al-doped YBa2Cu3Oz_~ sions of the observed correlation regions in the FLL we can estimate the pinning force and critical current characteristic of these crystals. The average transverse (i.e. perpendicular to the external field) correlation length, Re, in our case was estimated as an average dimension of the observed domains in the FLL within which the orientational order is preserved. Figure 6 shows the sharp increase of R~ with magnetic field, as observed in the decoration patterns, which can be understood according to CP theory in terms of rapidly increasing elastic moduli of the FLL (the shear modulus is given by C66~exp [--1/(3x2b)], where b=B/B¢2, x is the Ginzburg-Landau parameter, for the magnetic fields H
( 1) '

where rp ~ a is the pinning range, a is the average vortex spacing and C66 is given by [ 15 ] 2 r ( b - 1)-] C66= 8/ZoX2 B22 b ( 1 - O . Z 9 b ) ( 1 - b ) e x p / ~ j

(2)

20 RC (2

10

10

l's

zb

B(5) Fig. 6. The transverse correlation length, Re in units of vortex spacing, a, plotted as a function of magnetic induction.

79

Using R¢(20 G ) = 2 0 ~tm, the pinning force density is found to be F p = 5 X 105 N / m 3, corresponding to a critical current ofj¢=Fp/B~.. 3 X 102 A / c m 2 which is at least two orders of magnitude smaller than j¢(B=0, T = 4 . 2 K ) ~ 105 A / c m / for the undoped YBCO crystals but is comparable to that found in Fe-doped YBCO from magnetisation measurements [21. The value of Be2in the above estimations was taken to be 4.2 T, the same as that derived in ref. [2] for the Fe-doped YBCO crystals with the dopant content x=0.04. As doping of YBCO by either Fe or AI appears to have qualitatively similar effects on both the transition temperature and microstructure, it is expected that Bcz in Al-doped crystals will decrease in a similar fashion to that of the Fe-doped material. An indirect confirmation of a strongly decreased lower critical field Be, for Al-doped YBCO is given by a comparatively large average diameter, D, of the vortex image in our vortex patterns D ~ 1 ~tm (see fig. 7). This is almost twice as large as that for the undoped crystals [ 16 ]. As the size of an aggregation of iron particles is > 22, this implies a smaller value of B¢1 ,~2 -2. Note that in the above discussion we considered only crystals having ideally flat surfaces. However, most of the crystals used in our study were characterised by having large growth steps of irregular shape and/or undulating surface relief. Both were found to have dominant effect on the distribution of vortices. Typical examples of the vortex arrangement in the presence of growth steps are shown in figs. 8 and 9. The step heights measured by two-beam interferometry and from SEM photos taken when the sample stage was tilted at a large angle, varied from 0.1-1 ~tm. Many steps (like those shown in figs. 8 and 9) were thus large enough ( > 2 ) to act on vortices as an additional superconductor-vacuum boundary, similar to the edge of the crystal. As seen from the figures, close packed vortex rows tend to be parallel to the steps being at the same time somewhat repelled from their edges. Long dark bands devoid of vortices can also be seen in fig. 8 and appear to lie approximately parallel to the growth steps. The origin of such vortex-free regions is not known at present but one possibility is that each band corresponds to a "ghost image" or memory of a growth front or step below the crystal surface. Another possibility is that

80

L V. Grigorieva et aL ~Flux line lattice in Al-doped YBaeCu3Oz_a

Fig. 7. High magnificationimage of the FLL at B= l0 G showing the size of the vortices to be D~ ! ~tm. there is a decrease in the AI content at or near the surface of the crystal. Another example of strong pinning from the surface relief is shown in figs. 9 and 10. Two beam interferometry applied to such portions of the crystal surface showed oscillatory fringes indicating that the surface is undulated against a background of increasing height rather than being just composed of simple steps. In other words, these regions of the crystal exhibited a natural modulation of their thickness, about 30 nm, as measured from the interference patterns. Due to a change of vortex self-energy proportional to the vortex length, vortices appeared to be strongly pinned at the places of larger thickness. (Evidently, they were pinned during the re-arrangement of the vortex structure on cooling the sample before decoration. ) Thus, these crystals represent a natural system which may demonstrate matching effects in pinning, i.e. an enhancement of the volume pinning force when an average vortex spacing is a multiple of the distance between peaks in the sample thickness. This type of pinning has been previously studied in artificially modulated A1 films (40 A modulation of 2000 A thick films) [17].

4. Conclusions The

flux

line

lattice

(FLL)

in

YBa2(Cul_xAlx)307_ 6 single crystals with x = 0 . 0 4

has been directly observed using a technique of decorating the superconductor with fine ferromagnetic particles, for several values of the external magnetic field. Al-doping was found to strongly affect the disorder in the FLL in comparison with undoped twinned YBCO crystals. No influence of twin boundaries on the vortex distribution was found, consistent with the extremely small size of twin domains in 4% Al-doped crystals, ,~ 30 A, i.e. much smaller than the vortex size and intervortex spacing. Vortices were distributed homogeneously and the extent of their ordering rapidly increased with an increase in the external magnetic field. In fields as small as 15 G and 20 G, a regular and hexagonal FLL was observed but it consisted of misoriented domains thus testifying to the FLL being collectively pinned by weak and random pinning centres such as small defect clusters, vacancies, etc. The volume pinning force and critical current estimated from the average size of correlation domains were Fp~ 5 X 105 N / m 3 and j c ~ 3 X 1 0 2 A / c m 2, that is two orders of magnitude less than undoped twinned YBCO but in ap-

L V. Grigorieva et al. ~Flux line lattice in Al-doped YBa2CusOz_6

81

Fig. 8. SEM image of the flux line lattice near growth steps showinglattice adjustment to the steps.

proximate agreement with the values derived from bulk magnetic measurements for Fe-doped YBCO. Apart from these "intrinsic" characteristics of the FLL, a strong pinning of vortices by different elements of surface relief was observed. The influence of large growth steps, ~ 1 Ixm in height, was similar to that from the edge of the crystal while the undulation of the crystal surface ( ~ 30 nm natural modulation of the sample thickness) acted as strong

linear pinning centres concentrating vortices along or between lines of greatest thickness. Thus, it can be concluded that due to a very low level of intrinsic pinning, YBCO crystals with a high level of Al-doping provide an ideal object for investigations of phase transitions in the FLL and other unusual properties of the vortex system in high-To superconductors.

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L V. Grigorieva et al. / F l u x line lattice in Al-doped YBaeCu3Oz_

Fig. 9. SEM image of the flux line lattice in a region containing simultaneously growth steps and surface undulation. The micrograph was taken with the sample tilted 45 ° so that the height of the steps can be seen, The difference between the two types of pinning is clearly demonstrated (see text),

Fig. 10. Vortex pinning by surface undulation shown at high magnification.

I. K Grigorieva et al. ~Flux line lattice in Al-doped YBa2Cu~07_~

Acknowledgements T h e a u t h o r s w i s h to a c k n o w l e d g e the Science a n d E n g i n e e r i n g R e s e a r c h C o u n c i l for s u p p o r t o f this work. K E B a n d I V G t h a n k t h e W o l f s o n F o u n d a t i o n for f i n a n c i a l s u p p o r t ; P A M t h a n k s the R o y a l C o m m i s s i o n for the E x h i b i t i o n o f 1851 for the a w a r d o f a Research Fellowship.

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