We have grown precisely ordered and precisely located arrays of ultra-small magnetic dots. The nanofabrication process is based on the use of a protein crystal etch mask which is used to create a hexagonal lattice of holes in Si substrates. An assembly of (Fe/Pd)(4) dots with the average dot size of 10 nm in diameter, 6.5 nm height, and an average separation between dot centers of 22 nm was grown using molecular-beam epitaxy. The dot locations are determined by the biological mask that is used to create ordered arrays of similar to4 nm deep holes in Si. Fe/Pd multilayers (1 nm thick Fe and 0.4 nm thick Pd layers) were deposited to create dots within these holes. The dots extend similar to2.5 nm above the surface, with a thicker (1.5 nm) final layer of Pd for protection of these structures during measurements. Magneto-optical Kerr effect and magnetometry data showed that these objects are magnetic even at room temperature and are fairly soft with a coercive field of similar to40 Oe. Measurements of the hysteresis loop revealed that magnetization is in plane and that 4piM(eff) is on the order of 15 kG

Camley, R. E.

Celinski, Z.

Douglas, K.

Malkinski, L.

Whipple, S. G.

Winningham, T. A.

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2003 Hexagonal lattice of 10-nm magnetic dots L. Malkinski R. E. Camley Z. Celinski T. A. Winningham University of Central Florida S. G. Whipple See next page for additional authors Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article; Proceedings Paper is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Malkinski, L.; Camley, R. E.; Celinski, Z.; Winningham, T. A.; Whipple, S. G.; and Douglas, K., "Hexagonal lattice of 10-nm magnetic dots" (2003). Faculty Bibliography 2000s. 3910. https://stars.library.ucf.edu/facultybib2000/3910 Authors L. Malkinski, R. E. Camley, Z. Celinski, T. A. Winningham, S. G. Whipple, and K. Douglas This article; proceedings paper is available at STARS: https://stars.library.ucf.edu/facultybib2000/3910 Journal of Applied Physics 93, 7325 (2003); https://doi.org/10.1063/1.1543861 93, 7325© 2003 American Institute of Physics.Hexagonal lattice of 10-nm magnetic dotsCite as: Journal of Applied Physics 93, 7325 (2003); https://doi.org/10.1063/1.1543861Published Online: 09 May 2003L. Malkinski, R. E. Camley, Z. Celinski, T. A. Winningham, S. G. Whipple, and K. DouglasHexagonal lattice of 10-nm magnetic dotsL. Malkinskia)University of New Orleans, AMRI, New Orleans, Louisiana 70148R. E. Camley and Z. CelinskiDepartment of Physics, University of Colorado at Colorado Springs, Colorado 80919T. A. WinninghamDepartment of Physics, University of Central Florida, Orlando, Florida 32816S. G. Whipple and K. DouglasDepartment of Physics, University of Colorado, Boulder, Colorado 80309~Presented on 13 November 2002!We have grown precisely ordered and precisely located arrays of ultra-small magnetic dots. Thenanofabrication process is based on the use of a protein crystal etch mask which is used to create ahexagonal lattice of holes in Si substrates. An assembly of (Fe/Pd)4 dots with the average dot sizeof 10 nm in diameter, 6.5 nm height, and an average separation between dot centers of 22 nm wasgrown using molecular-beam epitaxy. The dot locations are determined by the biological mask thatis used to create ordered arrays of;4 nm deep holes in Si. Fe/Pd multilayers~1 nm thick Fe and0.4 nm thick Pd layers! were deposited to create dots within these holes. The dots extend;2.5 nmabove the surface, with a thicker~1.5 nm! final layer of Pd for protection of these structures duringmeasurements. Magneto-optical Kerr effect and magnetometry data showed that these objects aremagnetic even at room temperature and are fairly soft with a coercive field of;40 Oe.Measurements of the hysteresis loop revealed that magnetization is in plane and that 4pMeff is onthe order of 15 kG. ©2003 American Institute of Physics.@DOI: 10.1063/1.1543861#INTRODUCTIONDuring the last two decades, the use of ultrahigh vacuumtechnology has made possible the creation of ultrathin mag-netic structures with interfaces that are sharp on the atomicscale. This development has brought not only other ways ofengineering artificial magnetic materials but also excitingdiscoveries such as exchange coupling, giant magnetoresis-tance, and magnetic tunneling.1 As a result, the significantinterest by industry in different magnetic materials and de-vices has rejuvenated activities in the field of ultrathin mag-netic structures.One issue of paramount importance is the patterning ofultrathin magnetic structures into objects of nanometer scale2and the study of their static and dynamic properties. Thereare techniques, such as electron-beam or x-ray lithography,which make such patterning possible, although there are sig-nificant restrictions. For example, electron-beam lithographycan make objects on a scale of 10 nm, but its time-intensiveserial nature makes large-area patterning with this techniqueunfeasible. X-ray photolithography requires use of synchro-tron radiation, which is not commonly accessible. The aim ofour work was to grow patterned magnetic structures using abiologically derived parallel fabrication technique and tostudy the magnetic properties of the structures. This ap-proach allowed us to create large areas of patterned magneticstructures (;cm2). In this article, we describe in detail thepreparation of Si wafers, the growth process, and the resultsof superconducting quantum interference device magnetome-ter ~SQUID! and magneto-optical Kerr effect~MOKE! mea-surements on a regular hexagonal array of 10 nm in diameter(Fe/Pd)4 magnetic dots.SAMPLE PREPARATIONThe growth process is based on the use of two-dimensional protein crystals as an etch mask. The prepara-tion of the protein crystals and their use as masks have beendescribed in previous publications.3–6 They are native two-dimensional crystals that form the surface layer (S layer! ofthe archaebacteriasulfolobus acidocaldarius. They possess ahexagonal array of pores with a 22 nm lattice constant andpore diameters of 10 nm. The size of the native protein crys-tal fragments is on the order of 1 to 2mm in diameter.The protein crystals were applied to the Si wafer in anaqueous suspension. The density of the surface coverage iscontrolled by regulating the protein concentration in theaqueous suspension. The Si substrates used in our studieswere approximately 50% covered withS layers, which in-sured that we avoided overlapping protein crystal fragments.After a short drying process, the Si wafers were placed in anelectron-beam deposition system where a layer of Cr wasdeposited under an angle of approximately 60°. The periodictopography of the protein crystals produced a shadow at eachpore site, effectively creating an ordered array of holes in thedeposited Cr film. Inductively coupled plasma was used totransfer the pattern of the protein crystals into the Si sub-strate in the form of an ordered array of holes, verified byatomic force microscopy~AFM!. Areas not covered by Cra!Electronic mail: lmalkins@brain.uccs.eduJOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 10 15 MAY 200373250021-8979/2003/93(10)/7325/3/$20.00 © 2003 American Institute of Physicsare etched while those with Cr are protected. The size of theholes was adjusted by varying the etch parameters and time.These Si substrates, with the Cr/protein crystal mask still inplace, were introduced into the ultrahigh vacuum depositionsystem.We grew the arrays of magnetic dots at room tempera-ture in a molecular-beam epitaxy system.7 Using Knudsencells, we deposited four 1 nm thick Fe layers separated by0.4 nm thick Pd layers. The final Pd layer was 1.5 nm thickto prevent oxidization during measurements in ambient con-dition. Figure 1 shows a schematic diagram of our individualmagnetic dot. The thicknesses of the individual layers andthe rate of growth (;0.1 nm/min) were monitored by athickness monitor. In addition to the patterned Si substrate,we deposited a control sample directly on bare unpatternedSi. Despite the fact that the Si substrates were introduced tothe molecular-beam epitaxy system with the protein mask onthem, we did not observe an increase of the base pressure,indicating that outgassing was minimal. After deposition, weremoved the protein mask leaving arrays of (Fe/Pd)4 dots onthe Si substrates.AFM measurements were made in air using a DigitalInstruments Nanoscope III operating in tapping mode~Fig.2!. Dot diameters were on the order of 10 nm. Because wehave not yet optimized our mask removal technique, defectsand missing dots can be found in the array, most likely cre-ated during the removal or liftoff process. Magnetic forcemicroscopy measurements were attempted, but preliminaryefforts have been unsuccessful. We have had difficulty get-ting the resolution below 30 nm, and thus have not been ableto image individual dots with magnetic contrast.MAGNETIC MEASUREMENTSBefore presenting the magnetic results we would like tocomment on the question of superparamagnetic behavior inthese structures. The prepared magnetic dots are very small~10 nm in diameter and 6.5 nm in height!. Pure iron objectswith such small dimensions are superparamagnetic above ap-proximately 50 K since thermal fluctuations are much largerthan the anisotropy energy. To increase the blocking tem-perature one needs to introduce an additional anisotropy tothe structures. By layering the Fe with Pd, nine Pd/Fe inter-faces were created which significantly increased the effectiveanisotropy.8 Furthermore, the 0.4 nm thick Pd interlayer be-tween the Fe layers produces a strong ferromagnetic cou-pling between the Fe films, and each dot thus responds as asingle magnetic unit.9We conducted a series of magnetic measurements usinga MOKE system and a SQUID magnetometer. Both tech-niques revealed that the system is ferromagnetic even atroom temperature because a coercive field was observed.The shape of the hysteresis loops was very different for thepatterned samples than that observed for samples with a con-tinuous film ~see Fig. 3!. For the continuous film structureswe observed a square hysteresis loop with a coercive field of10 Oe; the patterned structures exhibitedS-shape hysteresisloops with larger coercive fields~on the order of 35 Oe!.FIG. 1. A schematic diagram of the individual magnetic dot.FIG. 2. Atomic force microscopy image of (Fe/Pd)4 magnetic dots grownon a Si substrate. The thickness of an individual Fe layer within a dot was 1nm, the thickness of Pd between Fe layers was 0.4 nm, and the dots werecapped with a 1.5 nm thick Pd layer.FIG. 3. Hysteresis loop for the magnetic dot array measured at 295 K withthe magnetic field applied in the plane of the structure. Inset shows data foridentical continuous film structure. Note the squareness of the hysteresisl op for the continuous film structure.7326 J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Malkinski et al.Moreover, the patterned structure requires a field of 4 kOe toreach saturation. The saturation field decreases with decreas-ing temperature to 3 kOe at 5 K. The coercivity increasedwith decreasing temperature, reaching a value of 90 Oe at 5K. Since the samples do not exhibit translational invarianceover the range larger than 2mm, the measured response is anaverage for all dots in different patterned regions.The out-of-plane measurements showed that it was nec-essary to apply a magnetic field larger than 15 kOe in orderto saturate the sample~see Fig. 4!. This indicates that themagnetization is in the plane of the sample. Moreover, theshape the hysteresis loop in this configuration is very differ-ent than that found in continuous films and also different forthat expected for an array of independent dots. In Fig. 4 thesolid lines represent theoretical calculations of the magneti-zation versus field for the continuous film structure and foran array of independent dots with demagnetizing factorsNx5Ny50.24 and Nz50.52 ~where z is the axis of thecylinder!.10 The calculation forM (H) for the independentdot is based on a minimization of the magnetostatic energyand Zeeman energy and assuming that the interior fields canbe appropriately approximated by the demagnetizing factors.The fact that the measurements lie in between these twolimits suggests that there is substantial interaction betweenthe dots.Figure 5 presents data on the temperature dependence ofthe coercivity for applied field both perpendicular and inplane. The coercivity decreases slowly as temperature in-creases for both configurations. This suggests that the super-paramagnetic limit is well above room temperature. It is in-teresting to note that the coercivity is larger for the out-of-plane geometry.In conclusion, we have grown series of ultra small(Fe/Pd)4 dot arrays. The magnetic properties of the dot ar-rays are very different from those of the equivalent continu-ous films. The coercive field for the dot array is increased bya factor of three and the saturation field is increased by afactor of over 100. The out-of-plane results showed that thebehavior of our system lies between that of a continuous filmand an array of independent dots, indicating that the interac-tion between these dots is important. The presence of coer-civity and the shape of the hysteresis loops indicate thatthese dots are magnetic even at room temperature.Work at UCCS was supported by the National ScienceFoundation~DMR-9970789 and DMR-0114189! and ArmyResearch Office~DAAG19-00-1-0146 and DAAD19-02-1-0174! while research at UC-Boulder was supported by theAFOSR ~Grant No. F49620-99-1-0105!.1B. Heinrich and J. F. Cochran, Adv. Phys.42, 523 ~1993!.2See, for example, J. I. Martı´n, J. Nogue´s, Kai Liu, J. L. Vicent, and IvanK. Schuller, J. Magn. Magn. Mater.,256, 449 ~2003! and the referencestherein.3K. Douglas, N. Clark, and K. Rothschild, Appl. Phys. Lett.48, 676~1986!.4K. Douglas, G. Devaud, and N. Clark, Science257, 642 ~1992!.5T. A. Winningham, H. P. Gillis, D. A. Choutov, K. P. Martin, J. T. Moore,and K. Douglas, Surf. Sci.406, 221 ~1998!.6T. A. Winningham, S. G. Whipple, and K. Douglas, J. Vac. Sci. Technol. B19~5!, 1796~2001!.7Z. Celinski, J. Vac. Sci. Technol. A19, 383 ~2001!.8B. Heinrich, Z. Celinski, J. F. Cochran, A. S. Arrott, and K. Myrtle, J.Appl. Phys.70, 5769~1991!.9Z. Celinski and B. Heinrich, J. Magn. Magn. Mater.99, L25 ~1991!.10Du Xing Chen, James A. Brug, and Ronald B. Goldfarb, IEEE Trans.Magn.27, 3601~1991!.FIG. 4. Hysteresis loop measured at 5 K with the magnetic field appliednormal to the sample plane. The solid lines represent theoretical calculationsof the magnetization vs field for the continuous film structure and for anarray of independent dots with the appropriate demagnetizing factors.FIG. 5. The coercive field as a function of the temperature measured withthe magnetic field applied in plane~open symbols! and normal~solid sym-bols! to the structure.7327J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Malkinski et al.

Hexagonal lattice of 10-nm magnetic dots

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Hexagonal lattice of 10-nm magnetic dots

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