We report the controlled integration, via Dip Pen Nanolithography, of
monolayer dots of ferritin-based CoO nanoparticles (12 Bohr magnetons) into the
most sensitive areas of a microSQUID sensor. The nearly optimum flux coupling
between these nanomagnets and the microSQUID improves the achievable
sensitivity by a factor 100, enabling us to measure the linear susceptibility
of the molecular array down to very low temperatures (13 mK). This method opens
the possibility of applying ac susceptibility experiments to characterize
two-dimensional arrays of single molecule magnets within a wide range of
temperatures and frequencies.Comment: 4 pages 3 figure

Bellido, E.

de Miguel, R.

Drung, D.

Gómez-Moreno, C.

Lostao, A.

Luis, F.

Martínez-Pérez, M. J.

Ruiz-Molina, D.

Schurig, T.

Sesé, J.

Applied Physics Letters

English

arXiv

We report the controlled integration, via dip pen nanolithography, of monolayer dots of ferritin-based CoO nanoparticles (12 µB) into the most sensitive areas of a microSQUID sensor. The nearly optimum flux coupling between these nanomagnets and the microSQUID improves the achievable sensitivity by a factor 102, enabling us to measure the linear susceptibility of the molecular array down to very low temperatures (13 mK). This method opens the possibility of applying ac susceptibility experiments to characterize two-dimensional arrays of single molecule magnets within a wide range of temperatures and frequencies

Bellido,E.

De Miguel,R.

Sesé,J.

Lostao,A.

Gómez-Moreno,C.

Drung,D.

Schurig,T.

Ruiz-Molina,D.

Luis,F.

Repositorio Universidad de Zaragoza

Alternating current magnetic susceptibility of a molecular magnet submonolayer directly patterned onto a micro superconducting quantum interference device

M. J. Martínez-Pérez

E. Bellido

R. de Miguel

J. Sesé

A. Lostao

C. Gómez-Moreno

D. Drung

T. Schurig

D. Ruiz-Molina

F. Luis

Crossref

Alternating current magnetic susceptibility of a molecular magnet
          submonolayer directly patterned onto a micro superconducting quantum interference
          device

3 páginas, 3 figuras.-- et al.We report the controlled integration, via dip pen nanolithography, of monolayer dots of ferritin-based CoO nanoparticles (12 μB) into the most sensitive areas of a microSQUID sensor. The nearly optimum flux coupling between these nanomagnets and the microSQUID improves the achievable sensitivity by a factor 102, enabling us to measure the linear susceptibility of the molecular array down to very low temperatures (13 mK). This method opens the possibility of applying ac susceptibility experiments to characterize two-dimensional arrays of single molecule magnets within a wide range of temperatures and frequencies.This work was partly funded by Grant No. MAT2009-
13977-C03 of the Spanish MICINN and the Consolider-
Ingenio project on Molecular Nanoscience. M.J.M.-P. thanks
CSIC for a JAE predoctoral fellowship. E.B. and R.dM.
thank the MICINN for FPI and FPU predoctoral grants. A.L.
thanks ARAID for financial support.Peer reviewe

Martínez Pérez, M. J.

Bellido, Elena

Sesé Monclús, Javier

Ruiz Molina, Daniel

Luis, Fernando

Digital.CSIC

Alternating current magnetic susceptibility of a molecular magnetsubmonolayer directly patterned onto a micro superconducting quantuminterference deviceM. J. Martínez-Pérez, E. Bellido, R. de Miguel, J. Sesé, A. Lostao et al.  Citation: Appl. Phys. Lett. 99, 032504 (2011); doi: 10.1063/1.3609859 View online: http://dx.doi.org/10.1063/1.3609859 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i3 Published by the American Institute of Physics.  Related ArticlesHalf-metallicity, magnetic moments, and gap states in oxygen-deficient magnetite for spintronic applications Appl. Phys. Lett. 100, 032403 (2012) Note on de Haas-van Alphen diamagnetism in thin, free-electron films AIP Advances 2, 012105 (2012) Anomalous magnetic properties of 7nm single-crystal Co3O4 nanowires J. Appl. Phys. 111, 013910 (2012) Uncompensated antiferromagnetic moments in Mn-Ir/FM (FM=Ni-Co, Co-Fe, Fe-Ni) bilayers: Compositionaldependence and its origin J. Appl. Phys. 110, 123920 (2011) Griffiths phase-like behavior and spin-phonon coupling in double perovskite Tb2NiMnO6 J. Appl. Phys. 110, 123919 (2011)  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 20 Jan 2012 to 161.111.180.191. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsAlternating current magnetic susceptibility of a molecular magnetsubmonolayer directly patterned onto a micro superconductingquantum interference deviceM. J. Martı´nez-Pe´rez,1,2 E. Bellido,3 R. de Miguel,4 J. Sese´,2,4 A. Lostao,4,5C. Go´mez-Moreno,4,6 D. Drung,7 T. Schurig,7 D. Ruiz-Molina,3,a) and F. Luis1,2,b)1Instituto de Ciencia de Materiales de Arago´n (ICMA), CSIC-Universidad de Zaragoza,Pedro Cerbuna 12, E-50009 Zaragoza, Spain2Dpto. de Fı´sica de la Materia Condensada, Universidad de Zaragoza,Pedro Cerbuna 12, E-50009 Zaragoza, Spain3Centro de Investigacio´n en Nanociencia y Nanotecnologı´a (CIN2, CSIC-ICN) Edificio CM7,Esfera UAB, Campus UAB, E-08193 Cerdanyola del Valle´s, Spain4Laboratorio de Microscopı´as Avanzadas (LMA)—Instituto de Nanociencia de Arago´n (INA),Universidad de Zaragoza, 50018 Zaragoza, Spain5Fundacio´n ARAID, E-50004, Arago´n, Spain6Dpto. de Bioquı´mica, Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain7Physikalisch-Technische Bundesanstalt (PTB) Abbestraße 2-12, D-10587 Berlin, Germany(Received 14 March 2011; accepted 16 June 2011; published online 20 July 2011)We report the controlled integration, via dip pen nanolithography, of monolayer dots offerritin-based CoO nanoparticles (12 lB) into the most sensitive areas of a microSQUID sensor.The nearly optimum flux coupling between these nanomagnets and the microSQUID improves theachievable sensitivity by a factor 102, enabling us to measure the linear susceptibility of themolecular array down to very low temperatures (13 mK). This method opens the possibility ofapplying ac susceptibility experiments to characterize two-dimensional arrays of single moleculemagnets within a wide range of temperatures and frequencies. VC 2011 American Institute ofPhysics. [doi:10.1063/1.3609859]The ac magnetic susceptibility of magnetic nanopar-ticles and single molecule magnets (SMMs) provides usefulinformation on their spin and magnetic anisotropy,1 as wellas on the magnetic relaxation mechanisms.2–4 Miniaturizedsuperconducting quantum interference devices (SQUIDs)5–9should eventually become capable8,10 of measuring the mag-netization reversal of a SMM (li  20 lB for the archetypalMn12 molecule). However, detecting the linear response setseven more stringent conditions: at T¼ 1 K, a magnetic fieldH¼ 24 A/m (0.3 Oe) induces a magnetic polarizationlh i ’ 0:007lB on the same Mn12 cluster. Measuring the sus-ceptibility of even a molecular monolayer represents there-fore a considerable challenge, which requires to take thesensitivity of magnetic susceptometry beyond its actual lim-its.5 To maximize the magnetic coupling between SMMsand the SQUID, molecular nanomagnets need to be depos-ited onto specific areas of the sensor.8,10 Even though diversetechniques have been developed for structuring moleculesand nanoparticles on sensors,9,11,12 such a controlled integra-tion remains extremely challenging.In the present work, we apply dip pen nanolithography(DPN)13 to deposit monolayer dots of ferritin-based nano-magnets on the most sensitive areas of a microSQUID acsusceptometer. With its direct write capabilities, DPN is anattractive tool for the nanostructuration on surfaces and forcontrolling the number of units deposited.14–17 The sampleconsisted of cobalt oxide nanoparticles, ’ 2 nm in diameter,whose magnetic moment ’ 12 lB is close to that of typicalSMMs.4 These particles (CoO@Apoferritin) are synthesizedinside the protein nanocavity of horse spleen apoferritin18and can be patterned and immobilized over different sub-strates.19 The bulk magnetic susceptibility of this materialwas characterized using 109 Kg of CoO@Apoferritin.Further details of this and other experimental aspects aregiven in the Supplementary material (see Ref. 20).The microSQUID susceptometer used for these studieshas been described elsewhere.20–22 The pick-up coil mostsensitive (“active”) areas were identified by calculating (seeFig. 1(c) and Ref. 20) the magnetic flux Ucoupled generatedby a sample located at a particular position. The couplingcan be quantified by a flux coupling factora ¼ UcoupledliBpip; (1)where li is the magnetic moment induced by the excitationmagnetic field Bp and ip is the electrical current circulatingvia the primary coil. We find that a can be enhanced by morethan three orders of magnitude by simply placing the nano-magnets sufficiently close to the coil wire edges, where themagnetic field lines concentrate.The rational deposition of CoO@Apoferritin on theseactive areas is depicted in Fig. 2. Three rows of CoO@Apo-ferritin dots separated by 4 lm were fabricated on the pick-up coils labeled 3 and 4 in Fig. 1(a) by traversing the tipsoaked with the ferritin-based nanoparticles over the specificareas. The first row was deposited on the primary Nb coil,and the other two were deposited on the SiO2 layer. Thea)Electronic mail: druiz@cin2.es.b)Electronic mail: fluis@unizar.es.0003-6951/2011/99(3)/032504/3/$30.00 VC 2011 American Institute of Physics99, 032504-1APPLIED PHYSICS LETTERS 99, 032504 (2011)Downloaded 20 Jan 2012 to 161.111.180.191. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsSEM images (Fig. 2(b)) reveal the high precision achieved inpositioning the dots at the positions of maximum a. The dotsdimensions were measured by AFM (see Figs. 2) on arraysdeposited on bare SiO2 and Nb substrates under identicalconditions. We find average diameters of 1.3 lm6 0.1 lmand 1.8 lm6 0.1 lm for SiO2 and Nb substrates, respec-tively. The average dot height was 11 nm6 1 nm in both,close to the size of a single protein (ca. 12 nm), thus showingthat each dot is a monolayer. According to these values, theaverage number of CoO@Apoferritin units per dot is 104and 2 104 for SiO2 and Nb, respectively. The number ofCoO@Apoferritin units deposited over the pick-up coils isn 107.The large coupling between the CoO@Apoferritin dotsand the SQUID enabled us to measure their magnetic suscep-tibility down to T¼ 13 mK (Fig. 3). Below 400 mK, a tem-perature dependent signal shows up above the backgroundsignal of the bare sensor that was previously characterized.22Furthermore this signal shows the same qualitative depend-ence on temperature as the susceptibility v0 of a bulk-likesample of CoO@Apoferritin measured with the same sensorunder the same conditions.20 The magnetic polarization ofthe array can be estimated as lh i ¼ nv0Bp. Its maximumvalue, at T ’ 50 mK, amounts to only 2.3 105 lB.Below approximately 100 mK, v0 depends on frequency.This shows the existence of a thermally activated spin rever-sal with characteristic timescale s¼ s0 exp(U/kBT), where s0is an attempt time and U is the activation energy of the rever-sal process.23,24 When s becomes comparable to 1/x, thespins cannot follow in phase the oscillations of the excitationmagnetic field. The maximum of v0 vs T that we observe forthe array (see Fig. 3(a) defines the “blocking” temperature,characteristic of a SMM, which occurs when s& 1=x.2,4Curiously enough, the bulk v0 shows no clear maxima above13 mK. At first, this might suggest that the blocking tempera-ture, thus also U, is enhanced in the array by the interactionof the molecules with the substrate. Alternatively, the tem-perature shift can be ascribed to a different thermalization ofboth samples. In the array, with its larger contact-area to vol-ume ratio, the molecules can properly thermalize with thesurrounding He bath. In contrast, the actual temperature ofthe bulk sample can stay above that of the He bath (and ther-mometer), therefore not reaching the blocking temperature.This interpretation is supported by the fact that the bulk sus-ceptibility is shifted with respect to that of the array alreadyat T. 200 mK, when v0 is nearly independent of frequencyand therefore relaxation mechanisms should not influence itstemperature dependence.20Using these data it is possible to determine the averagecoupling factor a. For this, we replace in Eq. (1) li by the netpolarization of the molecular array lh i, defined above. TheFIG. 1. (Color online) (a) SEM image of the SQUID showing the four rec-tangular shaped pick-up coils with effective areas of 63 lm 250 lm. (b)Finite element calculation of the excitation magnetic field (Bp) created by aip¼ 500 lA current flowing through the primary coil, approximated by a cir-cular spire. (c) Numerical calculations of a as a function of the distancefrom the center of the pick-up coil wire towards the center of the coil,approximated also by a circle. The inset shows a 3-D cross section of thepick-up and primary coil wires, where the a profile has been superimposed.FIG. 2. (Color online) (a) Schematic representation of the nanoparticle dep-osition, by DPN, on the most active areas of the sensor. (b) SEM images of asensor right after depositing three rows of CoO@Apoferritin dots. (c) and(d) AFM images and topographic profiles of CoO@Apoferritin dots depos-ited onto SiO2 and Nb substrates, respectively.FIG. 3. (Color online) Top: in-phase ac magnetic susceptibility of 107CoO@Apoferritin molecules arranged as a (sub)monolayer. The out-of-phase component lies below the sensitivity limits of detection and it istherefore not shown. Bottom: in-phase susceptibility of 109 Kg ofCoO@Apoferritin (1012 units).032504-2 Martı´nez-Pe´rez et al. Appl. Phys. Lett. 99, 032504 (2011)Downloaded 20 Jan 2012 to 161.111.180.191. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsexperimental Ucoupled can be determined from the SQUID’soutput voltage, since they are related trough fabricationparameters. Inserting real values in Eq. (1) givesa¼ 28.6(60.1)lU0/lB m, of the same order of magnitude,albeit more than three times larger, as the averagea¼ 8.0(60.1)lU0/lB m extracted from the numerical calcu-lations shown in Fig. 1.20 The discrepancy can be ascribed tothe approximations made to simplify these calculations, inparticular the use of circular primary and pick-up coils. Thisparameter gives a spin sensitivity ’ 300 lBHz1=2 at 13 mK,which represents an enhancement of two orders of magnitudewith respect to the previous calibration performed with a45 lm thick Pb sphere.22Summarizing, we have fabricated submonolayer arraysof ferritin-based nanomagnets (12 lB) on those regions thathave a maximum flux coupling with a microSQUID loop.This controlled integration enhances the sensitivity by a fac-tor 102. Furthermore, the molecular deposition is carried outunder ambient temperature and pressure conditions andimplies no chemical functionalization of the sensor neitherof the sample. The enhanced sensitivity has enabled us todirectly measure the linear susceptibility of the moleculararray, which shows that each molecule preserves its mag-netic properties. The present technology opens the possibilityof using the ac susceptibility to characterize two-dimensionalarrays of single-molecule magnets. The same approach canbe also applied to optimize the flux coupling of magneticmolecules to any other superconducting circuit, such as pla-nar resonators, therefore contributing to the realization ofhybrid architectures for quantum computation.25This work was partly funded by Grant No. MAT2009-13977-C03 of the Spanish MICINN and the Consolider-Ingenio project on Molecular Nanoscience. M.J.M.-P. thanksCSIC for a JAE predoctoral fellowship. E.B. and R.dM.thank the MICINN for FPI and FPU predoctoral grants. A.L.thanks ARAID for financial support. We acknowledge help-ful discussions with C. Carbonera and D. Maspoch. M.J.M.-P.and E.B. contributed equally to the present work.1F. Luis, F. Bartolome´, F. Petroff, J. Bartolome´, L. M. Garcı´a, C. Deranlot,H. Jaffre`s, M. J. Martı´nez, P. Bencok, F. Wilhelm, A. Rogalev, and N. B.Brookes, Europhys. Lett. 76, 142 (2006).2J. M. Herna´ndez, X. X. Zhang, F. Luis, J. Bartolome´, J. Tejada, andR. Ziolo, Europhys. Lett. 35, 301 (1996).3E. M. Chudnovsky and J. Tejada, Macroscopic Quantum Tunneling of theMagnetic Moment, Cambridge Studies in Magnetism (Cambridge Univer-sity Press, Cambridge, 1998).4D. Gatteschi, R. Sessoli, and J. Villain, Molecular Nanomagnets (OxfordUniversity Press, Oxford, 2006).5D. D. Awschalom, J. R. Rozen, M. B. Ketchen, W. J. Gallagher, A. W.Kleinsasser, R. L. Sandstrom, and B. Bumble, Appl. Phys. Lett. 53, 2108(1988).6W. Wernsdorfer, K. Hasselbach, D. Mailly, B. Barbara, A. Benoit,L. Thomas, and G. Suran, J. Magn. Magn. Mater. 145, 33 (1995).7S. K. H. Lam and D. L. Tilbrook, Appl. Phys. Lett. 82, 1078 (2003).8J. P. Cleuziou, W. Wernsdorfer, V. Bouchiat, T. Ondarcuhu, andM. Monthioux, Nat. Nanotech. 1, 53 (2006).9L. Hao, C. Amann, J. C. Gallop, D. Cox, F. Ruede, O. Kazakova,P. Josephs-Franks, D. Drung, and Th. Schurig, Appl. Phys. Lett. 98,092504 (2011).10V. Bouchiat, Supercond. Sci. Technol. 22, 064002 (2009).11S. K. H. Lam, W. Yang, H. T. R. Wiogo, and C. P. Foley, Nanotechnology19, 285303 (2008).12L. Bogani, C. Danieli, E. Biavardi, N. Bendiab, A. L. Barra, E. Dalcanale,W. Wernsdorfer, and A. Cornia, Angew. Chem. Int. Ed. 48, 746 (2009).13R. D. Piner, J. Zhu, F. Xu, S. Hong, C. A. Mirkin, Science 283, 661 (1999).14S. Sekula, J. Fuchs, S. Weg-Remers, P. Nagel, S. Schuppler, J. Fragala,N. Theilacker, M. Franzreb, C. Wingren, P. Ellmark, C. A. K. Borrebaeck,C. A. Mirkin, H. Fuchs, and S. Lenhert, Small 4, 1785 (2008).15K. Salaita, Y. Wang, and C. A. Mirkin, Nat. Nanotech. 2, 145 (2007).16B. Basnar and I. Willner, Small 5, 28 (2009).17E. Bellido, R. de Miguel, D. Ruiz-Molina, A. Lostao, and D. Maspoch,Adv. Mater. 22, 352 (2010).18M. Uchida, S. Kang, C. Reichhardt, K. Harlen, and T. Douglas, Biochim.Biophys. Acta 1800(8), 834 (2010).19E. Bellido, R. de Miguel, J. Sese´, D. Ruiz-Molina, A. Lostao, and D. Mas-poch, Scanning 32, 35 (2010).20See supplementary material at http://dx.doi.org/10.1063/1.3609859 for adetailed description of the calculation of the SQUID-sample flux couplingfactor and the sample characterization.21M. J. Martı´nez-Pe´rez, J. Sese´, F. Luis, D. Drung, and T. Schurig, Rev. Sci.Instrum. 81, 016108 (2010).22M. J. Martı´nez-Pe´rez, J. Sese´, F. Luis, R. Co´rdoba, D. Drung, T. Schurig,E. Bellido, R. de Miguel, C. Go´mez-Moreno, A. Lostao, and D. Ruiz-Molina, IEEE Trans. Appl. Supercond. 21, 345 (2011).23L. Ne´el, Ann. Geophys. 5, 99 (1949).24W. F. Brown, Phys. Rev. B 130, 1677 (1963).25A. Imamoglu, Phys. Rev. Lett. 102, 083602 (2009).032504-3 Martı´nez-Pe´rez et al. Appl. Phys. Lett. 99, 032504 (2011)Downloaded 20 Jan 2012 to 161.111.180.191. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

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