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Nuclear Instruments and Methods in Physics Research A 520 (2004) 274–276

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doi:10.1016

Dilute Al–Mn alloys for superconductor device applications

S.T. Ruggieroa,*, A. Williamsa, W.H. Rippardb, A.M. Clarkb, S.W. Deikerb,B.A. Youngc, L.R. Valeb, J.N. Ullomb

aDepartment of Physics, University of Notre Dame, Notre Dame, IN 46556, USAbNational Institute of Standards and Technology, Boulder, CO 80305, USA

c Department of Physics, Santa Clara University, Santa Clara, CA 95053, USA

Abstract

We discuss results on the superconducting and electron-transport properties of Mn-doped Al produced by sputter

deposition. The critical temperature of Al has been systematically reduced to below 50 mK by doping with 1000–

3000 ppm Mn. Values of the a parameter are in the range of 450–500, indicating sharp normal-to-superconductor

transitions. This material is thus of significant interest for both transition-edge sensors operating in the 100mK regime

and superconductor/insulator/superconductor and superconductor/insulator/normal devices, in the latter case where

appropriately doped Al–Mn replaces the normal metal.

r 2003 Elsevier B.V. All rights reserved.

PACS: 74.62.Dh; 73.40.Gk; 85.25.j

Keywords: Mn doped al; Transition-edge sensors; SIS devices

Transition-edge sensors (TES) require films withsuperconducting transition temperatures below1K. One approach to meeting this need is to useelemental superconductors, including W(Tc ¼ 80 mK) [1,2], Ti (Tc ¼ 370 mK) [3], and Ir(Tc ¼ 90–330 mK) [1,4–7]. Perhaps more widelyused are bi-layer systems such as Ti/Au [8,9], Mo/Cu [10], Mo/Au [11,12], and Ir/Au [13]. However,reproducibility has remained an issue with bothelemental and bilayer systems. Ion implantation ofW films with Co, Fe and Ni [14] has also beensuccessfully employed, although the reproduciblepreparation of W films with low critical tempera-tures is required.

onding author. Tel.: +1-303-497-4319; fax: +1-

2.

ddress: ruggiero@boulder.nist.gov (S.T. Ruggiero).

- see front matter r 2003 Elsevier B.V. All rights reserve

/j.nima.2003.11.236

Our work introduces a new approach toproducing films with superconducting transitiontemperatures in the 100 mK regime—with Al filmsdoped with Mn in the 1000—3000 ppm regime.The films were prepared by simple co-depositionfrom two sputter targets: one with a relatively high(3000 ppm) concentration of Mn and a secondtarget of pure Al. The sputter guns were tilted tointersect at a point equidistant from each gun,where substrates were placed. Sputtering rateswere B0.2 nm/s from each gun. Films wereprepared on oxidized, 3 in. diameter Si wafers,which could be rotated to produce films withuniform Mn doping. Systematic studies of filmuniformity have yet to be conducted.

Shown in Fig. 1 is Tc versus Mn concentration,the latter established with Rutherford backscatter-ing. Results for the low-temperature (4.0 K)

d.

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S.T. Ruggiero et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 274–276 275

resistivity, rð0Þ; of the films (inset) can becompared with room-temperature resistivities forAu, Cu, and Mo of 2.04, 1.56 [15], and 5.7 mO cm[16], respectively (room-temperature to low-tem-perature resistance ratios for these materials aretypically 3–5), and rð0ÞB12 mO cm for Ti [17]. Thisimplies that for a given Tc; the conductivity ofdoped Al–Mn films are generally competitive withother systems for TES applications.

In our own investigations, we have seen thatferromagnetic materials such as Fe and Ni do notproduce a rapid depression of Tc in Al, and thatthe rate of Tc suppression with Mn doping is lowerthan typically observed with Abrikosov–Gor’kov[18] pair breaking with magnetic dopants. Thissuggests that Tc suppression in Al–Mn alloys is

0

0.2

0.4

0.6

0.8

1

0 1000 2000

0

1

2

3

4

0 1000 2000Mn conc. (ppm)T

c/T

co ρ(0)

(µΩ

cm

)

Mn Concentration (ppm)

Fig. 1. Reduced critical temperature versus Mn concentration

for Al–Mn thin films. Inset shows low-temperature (4.0K)

resistivity, rð0Þ; versus Mn concentration. Tco ¼ 1:17K.

1.4

1.2

1.0

0.8

0.6

0.40.2

0

0.460.450.440.430.420.410.40

∆T/Tc = 0.041Tc = 0.437 K

Al-Mn(1000 ppm Mn)

T(K)

Res

ista

nce

(arb

. uni

ts)

Fig. 3. Resistive transitions for Al–Mn films with transition tempe

rather the result of pair scattering from resonantmagnetic impurity sites in the context of theFriedel–Anderson model [19], as quantified bythe Kaiser theory [20].

Fig. 2 further illustrates the correlation of Tc

with transport properties. Shown is Tc versusresistance ratio, the ratio of the room temperatureto 4.0 K resistivity. Present are results for filmsmade at specific Mn concentrations and thoseproduced by a phase spread, where a substrate washeld fixed between pure Al and Al–Mn sputtertargets. The majority of the data is for films200 nm or greater in thickness.

The sharpness of transitions is a key parameterfor TES applications, quantified by a ¼dðln RÞ=dðln TÞ; where larger values represent

0

0.1

0.2

0.3

0.4

0.5

1 1.5 2 2.5

Uniform Dilutions

Phase Spread

Tc

(K)

resistance ratio

Fig. 2. Tc versus resistance ratio, defined as R273K/R4.0 K, of

Al–Mn thin films.

0.6

0.4

0.2

0

0.0960.0920.0880.084

∆T/Tc = 0.052Tc = 0.0898 K

Al-Mn(1500 ppm Mn)

T(K)

Res

ista

nce

(arb

. uni

ts)

ratures of 437 and 89.8mK. Arrows define transition region.

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S.T. Ruggiero et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 274–276276

sharper transitions. Resistive transitions for Al–Mn films with critical temperatures of 437 and89.8 mK are shown in Fig. 3. They have a values of500 and 450, respectively, compared with reportedvalues for single and bi-layer systems ranging fromB90 to1000 [7–9,12].

Acknowledgements

The authors acknowledge useful discussionswith S. Nam. Work supported by the NationalInstitute of Standards and Technology, Depart-ment of Energy Grant DE FG02-88ER45373, andDARPA SpinS.

References

[1] B.A. Young, S.W. Nam, P.L. Brink, B. Cabrera, B. Chugg,

R.M. Clark, A.K. Davies, K.D. Irwin, IEEE Trans. Appl.

Supercond. 7 (1997) 3367.

[2] D. Fukuda, Y. Noguchi, M. Ohno, H. Takahashi, M.

Ataka, H. Pressler, F. Hirayama, M. Ukibe, M. Ohkubo,

H.M. Shimizu, M. Nakazawa, Low temperature detectors,

Ninth International Workshop on Low Temperature

Detectors, Madison, WI, in: F.S. Porter, D. McCammon,

M. Galeazzi, C.K. Stahle (Eds.), AIP Conference Proceed-

ings, Vol. 605, AIP, Melville, New York, 2002,

pp. 263–266.

[3] A.T. Lee, Shih-Fu Lee, J.M. Gildemeister, P.L. Richards,

Proc. LTD7 (1997) 123.

[4] H. Pressler, M. Koike, M. Ohkubo, D. Fukuda, Y.

Noguchi, M. Ohno, H. Takahashi, M. Nakazawa, Appl.

Phys. Lett. 18 (2002) 331.

[5] M. Tinkham, Introduction to Superconductivity,

McGraw-Hill, New York, 1996.

[6] M. Ohno, Y. Noguchi, D. Fukuda, H. Takahashi, M.

Nakazawa, M. Ataka, M. Ukibe, F. Hirayama, M.

Ohkubo, H.M. Shimizu, Low temperature detectors, AIP

Conference Proceedings, Vol. 605, AIP, Melville, New

York, 2002, pp. 259–262.

[7] S. Trowell, A.D. Holland, G.W. Fraser, D. Goldie, E. Gu,

Low temperature detectors, AIP Conference Proceedings,

Vol. 605, AIP, Melville, New York, 2002, pp. 267–270, and

reference therein.

[8] G. Ventura, M. Barucci, E. Pasca, E. Monticone, M.

Rajteri, Proc. ICATPP7 World Scientific, Singapore, 2002,

p. 677.

[9] M. Ukibe, T. Kimura, T. Nagaoka, H. Pressler, M.

Ohkubo, Low temperature detectors, AIP Conference

Proceedings, Vol. 605, AIP, Melville, New York, 2002,

pp. 207–210.

[10] G.C. Hilton, J.M. Martinis, K.D. Irwin, N.F. Bergren,

D.A. Wollman, M.E. Huber, S. Deiker, S.W. Nam, IEEE

Trans. Appl. Supercond. 11 (2001) 739.

[11] N. Tralashawala, Nucl. Instr. and Meth. A 444 (1999)

1888.

[12] E.F. Feliciano, J. Chervenak, F.M. Fikbeiner, M. Li, M.A.

Lindeman, C.K. Stahle, C.M. Stahle, Low temperature

detectors, AIP Conference Proceedings, Vol. 605, AIP,

Melville, New York, 2002, pp. 239–242.

[13] J. Schnagl, G. Angloher, F.v. Feilitzsch, M. Huber, J.

Jochum, J. Lanfranchi, M.L. Sarsa, S. Wanninger, Nucl.

Instr. and Meth. Phys. Res. A 444 (2000) 245.

[14] B.A. Young, T. Saab, B. Cabrera, J.J. Cross, R.M. Clarke,

R.A. Abusaidi, J. Appl. Phys. 86 (1999) 6975.

[15] N.W. Ashcroft, N.D. Mermin, Solid State Physics, Holt,

Rinehart and Winston, Philadelphia, PA, 1976, p. 8.

[16] http://www.goodfellow.com/csp/active/static/A/MO00.

HTML.

[17] E. Monticone, M. Rajteri, M.L. Rastello, V. Lacquaniti,

C. Gandini, E. Pasca, G. Ventura, Low temperature

detectors, AIP Conference Proceedings, Vol. 605, AIP,

Melville, New York, 2002, pp. 181–184.

[18] A. Abrikosov, L.P. Gor’kov, Soviet Phys. –JETP 12 (1961)

1243.

[19] N.W. Ashcroft, N.D. Mermin, Solid State Physics,

Saunders, 1976, p. 687.

[20] M.B. Maple, in: D.H. Douglas (Ed.), Superconductivity

in d- and f- Band Metals, AIP Conference Proceedings,

Vol. 4, American Institute of Physics, New York, 1972,

pp. 175–203.