V superlattices

Physica 135B (1985) 182-184 North-Holland, Amsterdam

SUPERCONDUCTING T c ANOMALIES IN M o / V SUPERLATTICES M.G. K A R K U T , J.-M. T R I S C O N E , D. A R I O S A and ~ . F I S C H E R Universitd de Gen~ve, 24 Quai Ernest Ansermet, CH 1211 Gen~ve 4, Switzerland

We have epitaxially grown by magnetron sputtering Mo/V superlattices in two different crystalline orientations, (001) and (110), on substrates of ((101) MgO and (1120) AI_,O3, respectively. We have measured T~ as a function of chemical modulation wavelength A and find that the behavior cannot be explained by application of proximity effect theories. We also observe an orientational dependence of T which can be understood by effectively rescaling A into the number of atomic planes of vanadium in one wavelength.

Metallic multilayers are beginning to receive a great deal of attention [1]. Some of their fascination lies in the large number of combinations of elements that can be used to build up these artificial structures. A more restrictive class of synthetic materials are the metallic superlattices which display a structural coherence throughout the sample. What restricts the number of superlattice candidates is the difference in lattice parameters between the two constituents: the larger the mismatch the more likely misfit dislocations will appear throughout the film, thus destroying the coherence. But it is just this coherence, when accompanied by a relatively large mismatch, for which the most dramatic effects involving superconductivity and magnetism would be expected to occur. We report here the epitaxial growth of single crystal M o / V superlattices, in two different crystalline orientations. We also report an unexpected behaviour of the superconducting critical temperature T c as a function of the superlattice wavelength A which cannot be explained by proximity effects alone, and a crystalline orientational dependence of this transition temperature. The implications of these results are that the potential number of metallic superlattices is expanded and the characterization of T c for these systems should be revised. We grew M o / V superlattices in two separate crystalline orientations: (001) Mo / V planes parallel to (_001) MgO, and (110) M o / V planes parallel to (1120) A120 3. We believe this is the first time

such structural manipulation of superlattices has been achieved by sputtering. The samples were made by magnetron sputtering in an ultra-high vacuum chamber using targets of 99.9% Mo and 99.8% V. The base pressure of the vacuum system was better than 4 x 10 -9 Torr, and a flowing Ar pressure of 10 mTorr was maintained during sputtering. The sputtering rate was 2.5 ~ / s and the substrates were held at a temperature of - 7 0 0 ° C during deposition. By adjusting the speed with which the substrates passed beneath the targets, we made a series of samples of equal Mo and V thicknesses with wavelengths from 24 ~ to 250 ~ , and with a total sample thickness of 2500 A. The initial layer, as well as the final layer of each sample, was always Mo. We display a typical 0 - 2 0 X-ray diffractogram of a sample in the (001) orientation (solid line) along with an X-ray model fit to this sample (dashed line) in fig. 1. Diffractograms of this type are the characteristic signatures of multilayer structures. They reflect the periodic chemical modulation of the multilayer whose wavelength A is inversely related to the angular distance between two adjacent peaks. The widths of the peaks also give information about the coherence length in the growth direction. In fig. 1 the full-width at half-maximum is 0.45 ° and in our best samples is 0.3 ° which corresponds, after taking into account instrumental broadening, to a coherence length in the growth direction of about 400 A. We have also used Laue back-reflection

0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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M . G . Karkut et al. / T c anomalies in M o / V superlattices --'1

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photography and were able to index [2] the spots (indicative of single crystallinity) from the (001) oriented samples to show that [110] Mo/V is parallel to [100] MgO. The dashed line fit to the curve in fig. 1 is the result of an X-ray model one of us has developed [3] to help us further characterize the superlattices. The model incorporates lattice strain and interdiffusion and fits the data reasonably well. One must note that the widths of the model fit are only cosmetic: the model does not consider this aspect of the superlattice. The model simulates the diffractogram peak positions and the relative intensities of the peaks with a linear strain field and an interdiffusion layer of four lattice planes, thus tending to confirm our initial hope of growth primarily by interfacial strain. We have measured resistivities, residual resistance ratios (RRR's), transition temperatures T~, and the upper critical magnetic fields He2 for an applied field both parallel and perpendicular to the growth direction for these superlattices. The residual resistivities ranged from about 5 to 20 ixl-I cm and RRR's from 1.4 to 3.3. The critical fields in general showed surface superconductivity, i.e. n~211=l.7Hc2 ±. Also - ( d H ~ 2 / d T ) T c was between 2 to 3 k G / K for most samples. Using the above figures and standard formulas, we esti-

mated a Ginzburg-Landau coherence length of about 200 ,~. We measured the superconducting transition temperatures using a four-point ac resistance method. The Tc's of individual 2500 ~ films of V on MgO and A120 3, and of Mo on AI20 3 were 5.0, 4.8, and 0.62 K, respectively. We present Tc as a function of A in fig. 2. The two main features to be noted here are the sharp drop" in Tc beginning at about A = 70,~ and 60 ~ for (110) and (001) samples, respectively, and the orientational dependence of T~. The first reaction to the sharp drop in Tc is simply that, because of interdiffusion, there is an alloy-like interfacial layer whose low T~ [4, 5] becomes more dominant as A decreases. We attempted to fit the data using a trilayer proximity effect calculation similar to that used by Lowe and Geballe [6] to analyze the Tc behaviour of Nb/Zr multilayers. We extended [2] their analysis by assuming a compositional gradient, determined in part by the X-ray fit over the interfacial layer, to take into account the variation of T¢ across this region. This trilayer calculation, while producing a drop in T~ at short A, cannot explain the character of the data. We have also attempted to apply the Werthamer theory of bilayers [7], which has been successfully used to analyze the Tc behaviour of Nb/Cu multilayers [8], to this system by first applying it to the "bilayer" of Mo and interface, and then combining this with the vanadium layer. The results were not appreciably different than the i

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M . G . Karkut et al. / T anomalies in M o / V superlattices

trilayer calculation. The main point is that any proximity calculation is bound to give a smooth variation of T~ and cannot reproduce the sudden drop of T c around A = 70 A. We assume there is an additional mechanism supressing superconductivity in these materials. One of the results of the X-ray fit is that the lattice is distorted: the interfacial layer is not really an alloy with isotropic lattice constants but one where the lattice is stretched perpendicular, and compressed parallel, to the growth direction. This may, for instance, enhance the magnetic susceptibility of the system and thus affect T~ via spin fluctuations. The behaviour of the resistivity ratio as a function of wavelength also indicates something unusual is happening in the short wavelength region. As the wavelength decreases, the R R R slowly increases until about 70 and 60/~ for the (110) and (001) orientations, respectively, and then sharply drops by a factor of two, thus approximately mimicking the T c vs. wavelength behaviour. We cannot rule out the possibility of a misfit dislocation to a coherence transition occurring at these critical wavelengths. We have been able to eliminate [2] the curious orientational dependence of Tc by considering not A but the n u m b e r of interracial plus vanadium planes in one wavelength while determining T c. A (001) sample will have X/2 times the n u m b e r of planes in one multilayer period than a (110) sample of equal wavelength. Since we have assumed a constant interdiffusion profile for all our

samples (consistent with our evaporation conditions and the results of the X-ray model fit) the n u m b e r of V planes is greater and thus the T c is correspondingly higher for the (001) sample. The low wavelength character of the superconducting transition t e m p e r a t u r e , as well as its epitaxially-induced orientational dependence, are new and exciting features of this M o / V superlattice system. To further investigate the Tc anomaly we are in the process of performing magnetic susceptibility m e a s u r e m e n t s on this system.

References [1] See for example, Synthetic Modulated Structures, L. Chang and B.C. Giessen, eds. (Academic Press, New York, 1984). [2] M.G. Karkut, D. Ariosa, J.-M. Triscone and ~. Fischer, to be published in Phys. Rev. B. [3] D. Ariosa, to be published. [4] K. Andres, E. Bucher, J.P. Maita and R.C. Sherwood Phys. Rev. 178 (1969) 702. [5] H.R. Kahn, K. Lfiders, V. Muller, P. Oelhafen, H. Reisemeier and W. Scheidt, in: LT-17, 1984 (Contributed Papers), U. Eckern, A. Schmid, W. Weber and H. Wuhl, eds. (North-Holland, Amsterdam, 1984), p. FH4. [6] W.P. Lowe and T.H. Geballe, Phys. Rev. B29 (1984) 4961. [7] N. Werthamer, Phys. Rev. 132 (1963) 2440. [8] I.K. Schuller and C.M. Falco, in: Layered Ultrathin Coherent structures, Proc, of the Intern. Conf. on Inhomogeneous Superconductors, D.U. Gubser, T.L. Francavilla, J.R. Leibowiz and S.A, Wolf, eds. (ALP, New York, 1979), pp. 197-202.