A nanometer-sized superconducting quantum interference device (nanoSQUID) is
fabricated on the apex of a sharp quartz tip and integrated into a scanning
SQUID microscope. A simple self-aligned fabrication method results in
nanoSQUIDs with diameters down to 100 nm with no lithographic processing. An
aluminum nanoSQUID with an effective area of 0.034 $\mu$m$^2$ displays flux
sensitivity of 1.8$\cdot 10^{-6}$ $\Phi_0/\mathrm{Hz}^{1/2} and operates in
fields as high as 0.6 T. With projected spin sensitivity of 65
$\mu_B/\mathrm{Hz}^{1/2}$ and high bandwidth, the SQUID on a tip is a highly
promising probe for nanoscale magnetic imaging and spectroscopy.Comment: 14 manuscript pages, 5 figure

Aslamazov L. G.

Atia W. A.

Cardwell D. A.

Cleuziou J.-P.

Cohen R. W.

Foley C. P.

Gershenson M.

Girit C.

Granata C.

Hao L.

Hasselbach K.

Huber M.

Huber M. E.

Karrai K.

Ketchen M.

Kirtley J. R.

Kleiner R.

Koshnick N. C.

Lam S. K. H.

Rugar D.

Tesche C. D.

Tilbrook D. L.

Troeman A. G. P.

Welty R.

Wu C. H.

English

arXiv

Crossref

Self-Aligned Nanoscale SQUID on a Tip

A nanometer-sized superconducting quantum interference device (nanoSQUID) is fabricated on the apex of a sharp quartz tip and integrated into a scanning SQUID microscope. A simple self-aligned fabrication method results in nanoSQUIDs with diameters down to 100 nm with no lithographic processing. An aluminum nanoSQUID with an effective area of $0.034 \mu m^2$ displays flux sensitivity of $1.8 \times 10^{−6} \phi_0/Hz^{\frac {1}{2}}$ and operates in fields as high as $0.6 T$. With projected spin sensitivity of $65 \mu_B/Hz^{\frac {1}{2}}$ and high bandwidth, the SQUID on a tip is a highly promising probe for nanoscale magnetic imaging and spectroscopy.Physic

Finkler, Amit

Segev, Yehonathan

Myasoedov, Yuri

Rappaport, Michael

Vasyukov, Denis

Zeldov, Eli

Martin, Jens

Huber, Martin

Yacoby, Amir

Ne'eman, Lior

Harvard University - DASH 

 Self-aligned Nanoscale SQUID on a Tip  (Article begins on next page)The Harvard community has made this article openly available.Please share how this access benefits you. Your story matters.Citation Finkler, Amit, Yehonathan Segev, Yuri Myasoedov, Michael L.Rappaport, Lior Ne'eman, Denis Vasyukov, Eli Zeldov, MartinHuber, Jens Martin, and Amir Yacoby. 2010. Self-alignednanoscale SQUID on a tip. Nano Letters 10(3): 1046–1049.Published Version doi://10.1021/nl100009rAccessed February 19, 2015 9:11:55 AM ESTCitable Link http://nrs.harvard.edu/urn-3:HUL.InstRepos:8015280Terms of Use This article was downloaded from Harvard University's DASHrepository, and is made available under the terms and conditionsapplicable to Open Access Policy Articles, as set forth athttp://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#OAParXiv:1002.2921v1  [cond-mat.supr-con]  15 Feb 2010 Self-aligned nanoscale SQUID on a tipAmit Finkler,∗,† Yehonathan Segev,† Yuri Myasoedov,† Michael L. Rappaport,†Lior Ne’eman,† Denis Vasyukov,† Eli Zeldov,† Martin E. Huber,‡ Jens Martin,¶ andAmir Yacoby¶Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel,Departments of Physics and Electrical Engineering, University of Colorado, Denver, CO 80217,USA, and Department of Physics, Harvard University, Cambridge, MA 02138, USAAbstractA nanometer-size superconducting quantum interference device (nanoSQUID) is fabri-cated on the apex of a sharp quartz tip and integrated into a scanning SQUID microscope.A simple self-aligned fabrication method results in nanoSQUIDs with diameters down to 100nm with no lithographic processing. An aluminum nanoSQUID with an effective area of 0.034µm2 displays ﬂux sensitivity of 1.8×10−6 F0/√Hz and operates in ﬁelds as high as 0.6 T.With projected spin sensitivity of 65 mB/√Hz and high bandwidth, the SQUID on a tip is ahighly promising probe for nanoscale magnetic imaging and spectroscopy.Imaging magnetic ﬁelds on a nanoscale is a majorchallenge in nanotechnology, physics, chem-istry, and biology. One of the milestones in this endeavor will be the achievement of sensitivitysufﬁcient for detection of the magnetic moment of a single electron.1 There are three main tech-nological challenges: fabrication of a sensor with a high magnetic ﬂux sensitivity, reduction of†Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel‡Departments of Physics and Electrical Engineering, University of Colorado, Denver, CO 80217, USA¶Department of Physics, Harvard University, Cambridge, MA 02138, USA1the size of the sensor, and the ability to scan the sensor nanometers above the sample. Supercon-ducting quantum interference devices (SQUIDs) have the highest magnetic ﬁeld sensitivity, buttheir loop diameter is usually many microns. Much effort has been devoted recently to the de-velopment of nanoSQUIDs, which have shown very promising ﬂux sensitivity.2–8 Most of thesedevices, however, are based on planar technology using lithographic or focused ion beam (FIB)patterning methods;3–11 the large in-plane size of the devices precludes bringing the SQUID loopinto sufﬁciently close proximity to the sample (due to alignment issues) to scan it with optimalsensitivity. Recently, a terraced SQUID susceptometer was developed that is based on a multilay-ered lithographic process combined with FIB etching. This device includes a 600 nm pickup loopwhich can be scanned 300 nm above the sample surface.12 Here we present a simple method forthe self-aligned fabrication of a DC nanoSQUID on a tip with effective diameter as small as 100nm that can be scanned just a few nm above the sample.We have fabricated several SQUID-on-tip (SOT) devices of various sizes. A quartz tube of 1mm outside diameter is pulled to a sharp tip with apex diameter that can be controllably variedbetween 100 and 400 nm. The fabrication of the SOT consists of three “self-aligned” steps ofthermal evaporation of Al, as shown schematically in Fig. 1a. In the ﬁrst step, 25 nm of Al aredeposited on the tip tilted at an angle of -100◦ with respect to the line to the source. Then the tipis rotated to an angle of 100◦, followed by a second deposition of 25 nm. As a result, two leadson opposite sides of the quartz tube are formed, as shown in Fig. 1b. In the last step 17 nm of Alare evaporated at an angle of 0◦, coating the apex ring of the tip. The two areas where the leadscontact the ring form “strong” superconducting regions, whereas the two parts of the ring in thegap between the leads, indicated by arrows in Fig. 1c, constitute two weak links, thus forming theSQUID. The resulting nanoSQUID requires no lithographic processing, its size is controlled by aconventional pulling procedure of a quartz tube, and it is located at the apex of a sharp tip that isideal for scanning probe microscopy.The studies were carried out at 300 mK, well below the critical temperature Tc ≈ 1.6 K ofgranular thin ﬁlms of aluminum in our deposition system. Instead of the commonly used current2bias, the SOT was operated in a voltage bias mode, as shown schematically in the inset to Fig.2. We use a low bias resistance Rb of about 2 W and the SOT current, ISOT, is measured using aSQUID series array ampliﬁer (SSAA) working in a feedback mode.5,13,14 Rs is a parasitic seriesresistance.The resulting I −V characteristics display several interesting features, as shown in Fig. 2.First, the advantage of our SOT and the voltage bias setup is that there is no hysteresis, whichavoids the need for complicated pulsed measurements.15 Second, we observe a large negativedifferential resistance over a wide range of biases. This behavior is consistent with the Aslamazov-Larkin model of a single Josephson junction16 if the voltage bias circuit of Fig. 2 is taken intoaccount. Third, small SQUIDs often have a small modulation of the critical current with ﬁeld.4,6Our SOT, in contrast, shows very pronounced Ic(H) modulation as seen in Fig. 2. Finally, the I−Vcharacteristics show ﬁne structure at high biases, e.g., the 25 mT curve in Fig. 2, which apparentlyresults from resonances, the exact nature of which requires further investigation.Figure 3a shows ISOT(Vin,H) plots displaying very pronounced quantum interference patternswith a period of 60.8 mT, corresponding to an effective SQUID diameter of 208 nm. The modu-lation of the critical current is large, Imaxc /Iminc = 1.67, and in addition a large asymmetry betweennegative and positive biases is observed. Due to this asymmetry, the interference patterns at nega-tiveand positivebias are almost out of phase. From a theoretical ﬁt17 to Ic(H), shown in Fig. 3a bythe dashed curves, we extract the following parameters: the critical currents of the two junctions(1−a)I0 = 0.8 µA and (1+a)I0 = 2.4 µA, where I0 =1.6 µA, the asymmetryparameter a =0.5,and b = 2LI0/F0 = 0.85, where L is the loop inductance and F0 = h/2e is the ﬂux quantum. Theasymmetric interference patterns therefore arise from the difference in the critical currents of thetwo junctions. This asymmetry is in fact very advantageous for scanning probe applications sincehigh sensitivity can be attained essentially at any ﬁeld by an appropriate choice of the SOT biaspolarity and voltage.The almost optimal b = 0.85 of the SOT implies a large inductance L = 549 pH. For compari-son, the calculated geometrical inductance of our loop is Lg = m0R￿log 8Rr −2￿= 0.26 pH, which3is more than three orders of magnitude smaller. Here R = 104 nm is the loop radius and r = 15nm is the radius of the loop wire. Our device is therefore governed by the kinetic inductance18 ofthe loop, Lk = 2pm0l2LR/a, due to its small dimensions. Here a = 510 nm2 is the estimated crosssectional area of the loop, resulting in penetration depth lL = 0.58 µm. This lL is much largerthan the bulk value for Al but is quite plausible for very thin granular Al ﬁlms.19,20Usually SQUIDs are operational only at very low ﬁelds. Remarkably, theSOT can operate overa very wide range of ﬁelds without a signiﬁcant reduction in sensitivity. Figures 3b and 3c showsubstantial quantum oscillations at ﬁelds as high as 0.5 T, which provides a unique advantage forinvestigation of various systems. This special property of SOT apparently arises from the fact thatall the device dimensions are very small and the thin superconducting leads along the quartz tubeare aligned parallel to the applied ﬁeld.Figure 4 shows thespectral density of theﬂux noiseof theSOT at variousapplied ﬁelds. Abovea few tens of Hz the low frequency 1/f -like noise changes into white noise on the level of 3×10−5to 1.8×10−6 F0/√Hz over a wide range of ﬁelds, which translates into a ﬁeld sensitivity of1.1×10−7 T. Our ﬂux sensitivityis comparable to that of state of the art SQUIDs,21 yet the area ofthe SOT loop is only 0.034 µm2, which is the smallest reported to date.4,9 The small size of SOTis highly advantageous for spin detection since spin sensitivity in units of mB/√Hz is given bySn = FnRre￿1+h2R2￿3/2,where R is theradius of theloop, h is theheight ofthe loopabovethespindipole, re =2.82×10−15m, and Fn is the ﬂux sensitivity in F0/√Hz.12,22 For h < R we obtain spin sensitivity of 65mB/√Hz for spins located in the center of the loop with an on-axis magnetic moment. In the SOTgeometry, however, enhanced sensitivitycould be achieved by imaging thespins near theperimeterof the loop.23 In this case the sensitivity is mainly determined by the width of the weak link ofabout 30 nm rather than the diameter of the loop of 208 nm, leading to an estimated sensitivity ofabout 33 mB/√Hz. Such sensitivity should allow imaging of the spin state of a single molecule,244for example Mn12-acetate (m = 20mB), with integration of a few seconds. Moreover, our smallestoperating SOT has an effective diameter of 130 nm. Assuming that it has the same ﬂux noise asthe larger SOT, the estimated spin sensitivity could be increased to below 20 mB/√Hz.We have integrated the SOT into a scanning probe microscope operating at 300 mK in whichthe tip is glued to one tineof a quartz tuning fork. The frequency shift or the reduction in amplitudeof the resonance peak of the tuning fork are used as a feedback mechanism for tip proximity tothe sample surface.25,26 This method provides the possibility of simultaneous imaging of sampletopography and of the local magnetic ﬁeld. The amplitude of the tip oscillation is typically lessthan 1 nm, and hence does not degrade the spatial resolution of the magnetic imaging. As a testsample we used a 200 nm thick ﬁlm of Al patterned into a serpentine structure. Figure 5c shows atopographical scan across two adjacent strips of the seprpentine using the tip and the tuning fork.A transport current of 2 mA at 510 Hz was applied to the sample and the self-induced magneticﬁeld was measured by the SOT at various heights above the surface. The results are shown in Fig.5a. The data are in good agreement with the theoretically calculated ﬁeld proﬁles shown in Fig.5b. A ﬁeld as low as 1 µT is readily measurable, which allows detection of the seprpentine signalfrom a distance of 10 µm above the surface.In summary, we have developed a simple method for fabrication of sensitive nanoSQUIDson the apices of sharp tips and have incorporated them into a scanning SQUID microscope. AnanoSQUID with effective area of 0.034 µm2 operated at ﬁelds as high as 0.6 T and displayed ﬂuxsensitivityof1.8×10−6 F0/√Hz, whichtranslates intoon-axisspin sensitivityof65 mB/√Hz. Byoptimizing the SOT parameters, a further reduction in the noise can be expected, which, combinedwith a smaller loop diameter, could lead to a signiﬁcant improvement in spin sensitivity. Com-pared to other SQUID technologies, the ability to image magnetic ﬁelds just a few nm above thesample surface renders the SQUID on tip a highly promising tool for study of quantum magneticphenomena on a nanoscale.5AcknowledgementWe thank D.E. Prober, M.R. Beasley, I.M. Babich, and G.P. Mikitik for fruitful discussions. Thiswork was supported by the European Research Council (ERC) Advanced Grant, by the IsraelScience Foundation (ISF) Bikura grant, and by the Minerva Foundation.References(1) Rugar, D.; Budakian, R.; Mamin, H. J.; Chui, B. W. Nature 2004, 430, 329(2) Kirtley, J. R. Supercond. Sci. Technol. 2009, 22, 064008(3) Huber, M. E.; Koshnick, N. C.; Bluhm, H.; Archuleta, L. J.; Azua, T.; Björnsson, P. G.;Gardner, B. W.; Halloran, S. T.; Lucero, E. A.; Moler, K. A. Rev. Sci. Instrum. 2008, 79,053704(4) Troeman, A. G. P.; Derking, H.; Borger, B.; Pleikies, J.; Veldhuis, D.; Hilgenkamp, H. NanoLett. 2007, 7, 2152(5) Hao, L.; Macfarlane, J. C.; Gallop, J. C.; Cox, D.; Beyer, J.; Drung, D.; Schurig, T. Appl.Phys. Lett. 2008, 92, 192507(6) Lam, S. K. H. Supercond. Sci. Technol. 2006, 19, 963(7) Foley, C. P.; Hilgenkamp, H. Supercond. Sci. Technol. 2009, 22, 064001(8) Cleuziou, J.-P.; Wernsdorfer, W.; Bouchiat, V.; Ondarcuhu, T.; Monthioux, M. Nature Nan-otech. 2006, 1, 53(9) Granata, C.; Esposito, E.; Vettoliere, A.; Petti, L.; Russo, M. Nanotechnology 2008, 19,275501(10) Wu, C. H.; Chou, Y. T.; Kuo, W. C.; Chen, J. H.; Wang, L. M.; Chen, J. C.; Chen, K. L.;Sou, U. C.; Yang, H. C.; Jeng, J. T. Nanotechnology 2008, 19, 3153046(11) Girit, C.; Bouchiat, V.; Naaman, O.; Zhang, Y.; Crommie, M. F.; Zettl, A.; Siddiqi, I. NanoLetters 2009, 9, 198(12) Koshnick, N. C.; Huber, M. E.; Bert, J. A.; Hicks, C. W.; Large, J.; Edwards, H.; Moler, K. A.Appl. Phys. Lett. 2008, 93, 243101(13) Welty, R.; Martinis, J. IEEE Trans. Magn. 1991, 27, 2924(14) Huber, M.; Neil, P.; Benson, R.; Burns, D.; Corey, A.; Flynn, C.; Kitaygorodskaya, Y.; Mas-sihzadeh, O.; Martinis, J.; Hilton, G. IEEE Trans. Appl. Supercond. 2001, 11, 4048(15) Hasselbach, K.; Veauvy, C.; Mailly, D. Physica C 2000, 332, 140(16) Aslamazov, L. G.; Larkin, A. I. JETP Lett. 1969, 9, 87(17) Tesche, C. D.; Clarke, J. J. Low Temp. Phys. 1977, 29, 301(18) Handbook of superconducting materials; Cardwell, D. A.; Ginley, D. S.; IOP Publishing:Cornwall, 2002(19) Cohen, R. W.; Abeles, B. Phys. Rev. 1968, 168, 444(20) Gershenson, M.; McLean, W. L. J. Low Temp. Phys. 1982, 47, 123(21) Kleiner, R.; Koelle, D.; Ludwig, F.; Clarke, J. Proceedings of the IEEE 2004, 92, 1534(22) Ketchen,M.; Awschalom,D.; Gallagher,W.; Kleinsasser,A.; Sandstrom,R.; Rozen, J.; Bum-ble, B. IEEE Trans. Magn. 1989, 25, 1212(23) Tilbrook, D. L. Supercond. Sci. Technol. 2009, 22, 064003(24) Lam, S. K. H.; Yang, W.; Wiogo, H. T. R.; Foley, C. P. Nanotechnology 2008, 19, 285303(25) Atia, W. A.; Davis, C. C. Appl. Phys. Lett. 1997, 70, 405(26) Karrai, K.; Grober, R. D. Appl. Phys. Lett. 1995, 66, 18427Figure captionsFig. 1 (a) Schematic description of three self-aligned deposition steps for fabrication of SOT ona hollow quartz tube pulled to a sharp tip (not to scale). In the ﬁrst two steps, aluminum isevaporatedonto oppositesidesof thetubeformingtwo superconductingleads thatare visibleas bright regions separated by a bare quartz gap of darker color in the SEM image (b). In athird evaporation step, Al is evaporated onto the apex ring that forms the nanoSQUID loopshown in the SEM image (c). The two regions of the ring between the leads, marked by thearrows in (c), form weak links acting as two Josephson junctions in the SQUID loop. Theschematic electrical circuit of the SQUID is shown in the inset of (c).Fig. 2 I−V characteristics of the SOT at 300 mK at different applied ﬁelds. The inset shows theschematic measurement circuit. The SOT is voltage biased using a small bias resistor Rb andthe current ISOT is measured using a SQUID series array ampliﬁer (SSAA) with a feedbackloop.Fig. 3 (a) Quantuminterference patterns of the SOT current ISOT(Vin,H) at 300 mK at positiveandnegative voltage bias. The patterns are asymmetric both in ﬁeld and bias and are almost outofphaseforthetwobiaspolarities. Thedashedlineshowsatheoreticalﬁttakingintoaccountthe difference in critical currents of the two weak links. (b) Quantum interference patternsat high ﬁelds up to 0.5 T. (c) Current oscillations ISOT(H) at a constant biasVin = 103.5 mVover a wide ﬁeld range.Fig. 4 Spectral density of the ﬂux noise of the SOT at 300 mK and different applied ﬁelds. Theinset shows ISOT(H) at a constant bias Vin = 100 mV with the ﬁelds, indicated by coloredcircles, for which the noise spectra are presented. The lowest white noise level is 1.8×10−6 F0/√Hz. The mismatch at 104 Hz is an instrumental artifact.Fig. 5 (a) Scanning SOT microscope measurement of a superconducting serpentine. Shown arethe magnetic ﬁeld proﬁles at various heights above the serpentine carrying a 2 mA current8at 510 Hz. (b) Theoretically calculated ﬁeld proﬁles at comparable indicated heights. (c)Topography across two strips of the serpentine based on feedback from a quartz tuning forkoperating in constant height mode. (d) Schematic cross section of the serpentine showingthe direction of the current in each strip.9200 nm Al Al quartz200  m AlAlbare quartzSQUID loopSQUID loop weak linksAluminumAl Al132ab cFigure 11010 20 40 60 80 100 1200.00.51.01.52.02.53.0Vin [mV]ISOT [mA]50 mT100 mT0 mT75 mT25 mTVin5 kΩ RsRbSOTSSAAFigure 211Figure 31210−110010110210310410510−310−410−510−6f [Hz]Noise[Φ0/√Hz]  14.0 mT6.6 mT39.3 mT29.3 mT0 20 40123H [mT]ISOT [µA]Figure 413−5 0 5 10 15 20−80−60−40−20020406080B [mT]−5 0 5 10 15 20x [mm]  1.0 mm1.5 mm2.0 mm4.5 mm6.5 mm8.5 mm−5 0 5 10 15 20X [mm]−5 0 5 10 15 200100200X [mm]Topography [nm]a bd−I +IAl AlcSi/SiO2Figure 514

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Self-aligned Nanoscale SQUID on a Tip

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Self-aligned nanoscale SQUID on a tip

Self-aligned nanoscale SQUID on a tip

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