A new development in small-angle neutron scattering with polarization analysis allows us to directly extract the average spatial distributions of magnetic moments and their correlations with three-dimensional directional sensitivity in any magnetic field. Applied to a collection of spherical magnetite nanoparticles 9.0 nm in diameter, this enhanced method reveals uniformly canted, magnetically active shells in a nominally saturating field of 1.2 T. The shell thickness depends on temperature, and it disappears altogether when the external field is removed, confirming that these canted nanoparticle shells are magnetic, rather than structural, in origin

Booth, R. A.

Borchers, Julie A.

Chen, W. C.

Dedon, Liv R.

Gentile, T. R.

Hogg, C. R.

Ijiri, Y.

Krycka, Kathryn L.

Laver, M.

Watson, S. M.

Digital Commons at Oberlin (Oberlin College)

OberlinDigital Commons at OberlinFaculty & Staff Scholarship5-21-2010Core-shell magnetic morphology of structurallyuniform magnetite nanoparticlesKathryn L. KryckaR. A. BoothC. R. HoggY. IjiriOberlin College, Yumi.Ijiri@oberlin.eduJulie A. BorchersSee next page for additional authorsFollow this and additional works at: https://digitalcommons.oberlin.edu/faculty_scholPart of the Physics CommonsThis Article is brought to you for free and open access by Digital Commons at Oberlin. It has been accepted for inclusion in Faculty & Staff Scholarshipby an authorized administrator of Digital Commons at Oberlin. For more information, please contact megan.mitchell@oberlin.edu.Repository CitationKrycka, K.L., R.A. Booth, C.R. Hogg, et al. "Core-shell magnetic morphology of structurally uniform magnetite nanoparticles."Physical Review Letters 104: 207203.AuthorsKathryn L. Krycka, R. A. Booth, C. R. Hogg, Y. Ijiri, Julie A. Borchers, W. C. Chen, S. M. Watson, M. Laver, T.R. Gentile, and Liv R. DedonThis article is available at Digital Commons at Oberlin: https://digitalcommons.oberlin.edu/faculty_schol/3304Core-Shell Magnetic Morphology of Structurally Uniform Magnetite NanoparticlesK. L. Krycka,1 R.A. Booth,2 C. R. Hogg,2 Y. Ijiri,3 J. A. Borchers,1 W. C. Chen,1,4 S.M.Watson,1 M. Laver,5 T. R. Gentile,1L. R. Dedon,3 S. Harris,3 J. J. Rhyne,6 and S. A. Majetich21NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA2 Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA3Department of Physics and Astronomy, Oberlin College, Oberlin, Ohio 44074, USA4Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742-2115, USA5Paul Scherrer Institute, Villigen, Switzerland6Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA(Received 25 February 2010; published 19 May 2010)A new development in small-angle neutron scattering with polarization analysis allows us to directlyextract the average spatial distributions of magnetic moments and their correlations with three-dimensional directional sensitivity in any magnetic field. Applied to a collection of spherical magnetitenanoparticles 9.0 nm in diameter, this enhanced method reveals uniformly canted, magnetically activeshells in a nominally saturating field of 1.2 T. The shell thickness depends on temperature, and itdisappears altogether when the external field is removed, confirming that these canted nanoparticle shellsare magnetic, rather than structural, in origin.DOI: 10.1103/PhysRevLett.104.207203 PACS numbers: 75.70.iMonodomain magnetic nanoparticles are an importantprototype for understanding the behavior of magnetic re-cording media, ferrofluids, and beads for magnetic separa-tion of biological species. While it is tempting to assumethat the moments are uniformly aligned within each nano-particle, their spin configurations are typically far morecomplex [1]. When the particles contain both ferromag-netic and antiferromagnetic phases, exchange bias effectsmay occur [2–5]. Surface disorder can lead to a magneti-cally dead layer or a spin glasslike phase [6–9]. Surfacescan also introduce a cubic anisotropy term when the ma-terial otherwise has uniaxial anisotropy [10]. Surface an-isotropy has led to predictions of a ‘‘hedgehog’’configuration where the spins spike outward normal tothe surface [11–14], an ‘‘artichoke’’ confirmation wherethey are parallel to the surface and directed from one poleto the other [13], and a ‘‘throttled’’ configuration where thespins have been tilted slightly inward at the south pole andoutward at the north pole [13]. Experimental verification ofthese theoretical predictions is extremely challenging be-cause the magnetization from the distorted surface spinstypically averages out for macroscopic measurements.Here we extend the capability of small-angle neutronscattering (SANS) with polarization analysis in order todirectly measure the ordered magnetic structure perpen-dicular to an applied field, thus allowing us to ‘‘see’’ for thefirst time the magnetically distinctive core-shell morphol-ogy. We demonstrate that magnetic shells 1.0 to 1.5 nmthick, oriented 90 perpendicular to their ferrimagneticcores, form between 160 and 320 K under conditions ofnominal saturation at 1.2 T, yet, vanish in the absenceeither of an applied field or interparticle interactions.To minimize structural disorder, we have focused ourSANS investigation with polarization analysis on 9.0 nmmagnetite nanoparticles prepared by high temperaturechemical methods [15]. These particles are monodisperseand can be self-assembled into face-centered cubic (FCC)superlattices or nanoparticle crystals [16]. The use of nano-particle crystals leads to enhanced scattering at Braggpeaks, and the overall structural uniformity of the sampleenables precise determination of the magnetic lengthscales.Polarization analyzed SANS is ideal for discriminatingamong proposed models because it can detect the distribu-tion and orientation of magnetic structures, even buriedfeatures, with subnanometer resolution. With the neutronpolarization spin state as þ or , measurement of all fourneutron spin cross-sections (þþ, þ , þ , and  )allows for the unique separation of nuclear scattering (N2)from magnetic scattering, irrespective of whether the sam-ple is magnetically saturated. Application of polarizationanalysis to SANS [17–19] has been very limited since itrequires a polarization analyzer capable of capturing adivergent, 2D, scattered beam, such as a polarized 3Hespin filter [20]. Additionally, we expand beyond the for-malism of previous work [18] to directly measure themagnetic structure from moments oriented perpendicularto the applied field (M2Y þM2Z ¼ 2M2PERP), as well as themoments parallel to the applied magnetic field (M2X ¼MPARL). This provides a true 3D measurement of magneticstructure. Building upon our initial investigation of mag-netite nanoparticles in zero field [21] where M2PARL andM2PARP were equivalent, here we examine the magneticresponse within a high applied field where M2PERP is dis-tinctly different from M2PARL.Our setup is shown in Fig. 1(a) with the incoming beamalong Z, the applied field along X, and the detector in theX-Y plane. The measured scattering intensity, I, is propor-PRL 104, 207203 (2010) P HY S I CA L R EV I EW LE T T E R Sweek ending21 MAY 20100031-9007=10=104(20)=207203(4) 207203-1  2010 The American Physical Societytional to the squared sum of the spatial nuclear and mag-netic Fourier transforms defined asN;MJðQÞ ¼XKN;MJ ðKÞei ~Q ~RK ; (1)where J is any Cartesian coordinate, N;M is the nuclear ormagnetic scattering length density, and ~RK is the relativeposition of the Kth scatterer. ~Q is the scattering wavevector. Typical, efficiency-corrected 2D scattering patternsfrom the magnetite nanoparticles in high field are shown inFig. 1(b). From these, area-normalized sector slices of10 are taken about specific  angles of interest in orderto extract angular information, where  is the angle be-tween the X axis and the projection of ~Q onto the X-Yplane. The complete, angle-dependent polarization selec-tion rules [22] simplify at several key angles and enable theunambiguous separation of N2, M2PARL, M2PERP as follows:N2ðQÞ ¼ 12ðIþþ¼0 þ I¼0Þ; (2)M2PARLðQÞ ¼ðI¼90  Iþþ¼90Þ216N2; (3)M2PERPðQÞ ¼ 16ðIþ¼0;90 þ Iþ¼0;90Þ; (4)M2PARLðQÞ ¼ ðIþ¼45;135 þ Iþ¼45;135Þ  5M2PERP: (5)Application of these equations produces the magnetic andnuclear nanoparticle scattering profiles (Fig. 2). Note thatthe M2PARL profiles extracted using Eq. (3) or (5) areequivalent [Fig. 2(a)], indicating that the polarization effi-ciency of each element has been accurately accounted for.N2 and M2PARL at 1.2 T, 200 K have remarkably similarpeak profiles centered at 0:080 A1 [Fig. 2(a)], thoughM2PARL is considerably smaller as expected from the knownrelative magnitude of N ¼ 6:97 106 A2 and M ¼1:46 106 A2 (corresponding to a magnetization of5:12 105 A=M). These scattering profiles are the productof nanoparticle packing (structure factor) with the internalnanoparticle morphology (form factor). Regarding the for-mer, the nanoparticles are assembled into face-centeredcubic (FCC) crystallites [16] of lattice length 13.6 nmand retain long-range FCC ordering up to the submicronlevel, as indicated by transmission electron microscopyimaging. Within the measured Q range of 0:030 A1 to0:115 A1, there are two FCC Bragg reflections, (111) and(200), which overlap and merge into a single discernablepeak that becomes heavily weighted toward the (111)reflection (at 0:080 A1) with the introduction of structuralstacking faults. The convolution of the FCC structurefactor [black-pink triangles Fig. 3(a)] with a 9.0 nm spheri-cal form factor [black circles Fig. 3(b)], broadened to0:020 A1 (full width half maximum) to account for in-strumental resolution, results in the fit (solid blue line) inFig. 2(a). In large magnetic fields,M2PARL exhibits the sameFIG. 2 (color online). Data at 1.2 T, 200 K scaled so that N2Bragg peak height is 1000. (a) Note the intensity differencebetween N2 (left axis) and M2PARL (right axis) peaksat 0:080 A1, and the equivalence of M2PARL obtained fromEq. (5), which assumes an isotropic structure, and Eq. (3).(b) Note the prominent M2PERP dip at 0:075 A1. Solid linesshow model of Fig. 3.FIG. 1 (color online). (a) Experimental setup [20,21] includesa polarizing supermirror (i.e., an FeSi multilayer diffractiongrating), an electromagnetic precession coil flipper, sampleholder with cryostat and variable magnetic field, 3He analyzer,and a position sensitive gas detector. Arrows indicate the neutronpolarization direction. (b) 2D SANS images (shown here at1.2 T, 200 K) are corrected for polarization efficiency [21].PRL 104, 207203 (2010) P HY S I CA L R EV I EW LE T T E R Sweek ending21 MAY 2010207203-2long-range structure factor of N2 [Fig. 2(a)], but its formfactor corresponds to a 7.4 nm sphere when N and M aretaken into account. The product of the M2PARL structurefactor [black-pink triangles Fig. 3(a)] and form factor [pinktriangles Fig. 3(b)] results in the fit (solid purple line) inFig. 2(a).In stark contrast with N2 and M2PARL, M2PERP exhibits aprominent dip at 0:075 A1 [Fig. 2(b)]. It is clearly domi-nated by contributions from the form factor rather than thestructure factor. For the structure factor, we use a hardsphere model [23] which approximates the FCC latticestructure with coherence of one to several units in extent[green squares Fig. 3(a)] in order to account for the high Qscattering in Fig. 2(b). For the form factor, the M2PERP dip[Fig. 2(b)] is classic for scattering from a spherical object[23]. However, the dip location cannot result from solidsphere scattering of diameter less than 12.0 nm which isunphysically large. Instead, interpreting the M2PERP dip interms of a shell model produces a physically reasonableaverage shell diameter of 8:22 0:02 nm. A form factorshell [green squares Fig. 3(b)] of inner and outer diametersof 7.44 and 9.0 nm, respectively, combined with a short-range structure factor [green squares Fig. 3(a)] results inthe fit (solid green line) in Fig. 2(b). To refine the shellthickness further, we take into account the relative intensityof N2 compared with M2PERP and apply the additionalknown constraints that (i) the shell resides within a13.6 nm FCC lattice of nearest-neighbor spacing of9.6 nm, (ii) there must be at least 0.1 nm of surfactantbeyond the nanoparticle, and (iii) M, potentially increasedby uncompensated surface spins, is not allowed to deviateabove bulk by more than a factor of 2. In combination,these constraints dictate that the magnetic shell thicknessmust be 1:0 nm 0:2 nm.The combined model, depicted in Fig. 3(c), is comprisedof a 9.0 nm diameter particle with a 7.4 nm diameterferrimagnetic core and a 7.44 to 9.0 nm perpendicular shell[Fig. 3(b)] coupled with their respective structure factors[Fig. 3(a)]. To avoid overdetermination, we have employedthe simplest model that captures the essential scatteringfeatures. While we cannot conclusively rule out a largerferrimagnetic core of reduced M, this is far less plausiblesince the canted shell already contains an enhanced mo-ment. Similarly, we cannot quantify boundary fuzziness ordegree of overlap between core and shell, but we canconclude from the M2PERP scattering that magnetic shellsof nearly uniform magnitude and direction exist. Thus,models involving a disordered outermost shell [6–9], acanted moment that extends throughout the entire volumeof the particle, and ‘‘hedgehoglike’’ [12] or ‘‘throttle’’ [13]symmetry do not fitM2PERP and are excluded. The isolationofM2PERP demonstrated here is only possible using diffrac-tion or other spatially sensitive techniques since momentsquickly average to zero across the sample, as evidenced byan equivalence of M2Y and M2Z [22].The magnetic shell evolution was investigated for aseries of temperatures at 1.2 Tesla [Fig. 4(a)]. Zero-fieldcooling to 10 K before application of the applied fieldcauses all M2PERP scattering from the ordered magneticshell to vanish (though this does not preclude the possibil-ity of a disordered shell). Increasing the temperature to160 K produces a M2PERP dip position of 0:075 A1,which is essentially the same as at 200 K. Increasing thetemperature further to 300 or 320 K reproducibly shifts thisdip to a higher Q of 0:082 A1. Assuming that the shellextends to the outer edge of the nanoparticle, as is the caseat 200 K, the M2PERP dips at 160 K and 300–320 K corre-spond to shells of thickness 1.0 and 1.5 nm [solid curves inFig. 4(a)], respectively.In a remanent field of 0.005 T at 300 K virtually nonearest-neighbor interparticle correlations persist [21],which makes this an ideal condition to probe magneticmorphology with negligible influence from nearest-neighbor nanoparticles. Here the M2PERP ¼ M2PARL scatter-ing [Fig. 4(b)] is consistent with a ferrimagnetic sphere ofdiameter 9:0 nm 0:2 nm, indicating that neither a cantedFIG. 3 (color online). (a) N2, M2PARL (black-pink curve) and M2PERP (green curve) structure factors include instrumental resolutionbroadening for clarity, though in model broadening is included after convolution with respective form factors. (b) Spherical formfactors produce prominent dips at specificQ’s that are highly sensitive to diameter, and convoluted with curves in (a) produce the fits inFig. 2. (c) Composite model involving a ferrimagnetic core of diameter  7:4 nm with a 90 canted magnetic shell of thicknessbetween 0.8 nm and 1.2 nm is depicted.PRL 104, 207203 (2010) P HY S I CA L R EV I EW LE T T E R Sweek ending21 MAY 2010207203-3nor disordered magnetic shell of thickness more than0.1 nm exists. (We may not be able to distinguish betweena fully ferrimagnetic structure and a nearly uniform onewith coherent edge effects, such as an artichoke structure[13].) These remanent and temperature dependent dataclearly demonstrate that the formation of a canted shell ismagnetic, rather than chemical, in origin.In conclusion, we have expanded the capability of po-larization analyzed SANS to directly probe the 3D mag-netic morphology of nanoparticles in any field. Applied to9.0 nm magnetite nanoparticles, this resulted in the discov-ery of magnetic shells 1.0 to 1.5 nm thick, canted 90 totheir ferrimagnetic cores under conditions of nominal satu-ration at temperatures of 160 K or more. The concept of achemically uniform, but magnetically distinct, core andcanted shell is not without precedent [14,24], though notpreviously reported experimentally. We speculate that thelack of radial symmetry arises primarily from the competi-tion between interparticle coupling within the nanoparticlearray [21] (that sets a net direction) and nanoparticlesurface anisotropy [12,13] (that promotes outward spincanting). For example, experiment [3] has shown thatintraparticle exchange biasing can be altered by varyingnanoparticle packing distance. Thus, dipolar interparticlecoupling may play a significant role in determining themagnetic core-shell morphology and could explain theselection of a locally preferred orientation of the magneticshells within small regions. This unique observation ofordered, canted magnetic shells within chemically uniformparticles underscores the complex magnetic interactionspresent within these nanoparticle systems.This work utilized facilities supported in part byNational Science Foundation grants DMR-0454672,DMR-0704178, and DMR-0804779 and Department ofEnergy grant DE-FG02-08ER40481. Development of the3He spin filters was supported in part by the Department ofEnergy. We would like to thank Cedric Gagnon of theNIST Center for Neutron Research for his efforts in mak-ing this experiment successful.[1] R. A. Kodama and A. E. Berkowitz, Phys. Rev. B 59, 6321(1999).[2] Y. Ijiri et al., Appl. Phys. Lett. 86, 243102 (2005).[3] J. Noguès et al., Phys. Rev. Lett. 97, 157203 (2006).[4] S. E. Inderhees et al., Phys. Rev. Lett. 101, 117202 (2008).[5] I. V. Golosovsky et al., Phys. Rev. Lett. 102, 247201(2009).[6] P. Dutta et al., J. Appl. Phys. 105, 07B501 (2009).[7] J. Curiale et al., Appl. Phys. Lett. 95, 043106 (2009).[8] A. Kovacs et al., Phys. Rev. Lett. 103, 115703 (2009).[9] C. Westman et al., J. Phys. D 41, 225003 (2008).[10] R. Yanes et al., Phys. Rev. B 76, 064416 (2007).[11] J. Mazo-Zuluaga, J. Restrepo, and J. Majı́a-López,Physica (Amsterdam) 398B, 187 (2007).[12] J. Mazo-Zuluaga et al., J. Appl. Phys. 105, 123907 (2009).[13] L. Berger et al., Phys. Rev. B 77, 104431 (2008).[14] Y. Hu A and Du, J. Nanosci. Nanotechnol. 9, 5829 (2009).[15] S. Sun et al., J. Am. Chem. Soc. 126, 273 (2004).[16] D. A. Farrell et al., J. Magn. Magn. Mater. 303, 318(2006).[17] T. R. Gentile et al., J. Appl. Crystallogr. 33, 771 (2000).[18] A. Wiedenmann, Physica (Amsterdam) 356B, 246 (2005).[19] A.M. Gaspar et al., Biochim. Biophys. Acta 1804, 76(2010).[20] W.C. Chen et al., Physica (Amsterdam) 404B, 2663(2009).[21] K. L. Krycka et al., Physica (Amsterdam) 404B, 2561(2009).[22] See supplementary material at http://link.aps.org/supplemental/10.1103/PhysRevLett.104.207203.[23] A. Guinier, X-Ray Diffraction in Crystals, ImperfectCrystals, and Amorphous Bodies (Dover, New York,1994).[24] N. N. Phuoc et al., Phys. Status Solidi B 244, 4518 (2007).FIG. 4 (color online). Field and temperature dependence ofM2PERP. (a) Varying the temperature in an applied field of 1.2 Tindicates the presence of canted shells at 160, 300, and 320 K(the latter two similar enough to be averaged), but no orderedshell when zero-field cooled to 10 K before application of thefield. (b) At 300 K in a remanent field of 0.005 T the magneticscattering is well modeled by a uniform sphere of diameter9.0 nm lacking shell features.PRL 104, 207203 (2010) P HY S I CA L R EV I EW LE T T E R Sweek ending21 MAY 2010207203-4

Core-shell magnetic morphology of structurally uniform magnetite nanoparticles

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Core-shell magnetic morphology of structurally uniform magnetite nanoparticles

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