Article pubs.acs.org/JPCC
Magnetic Properties of Small Magnetite Nanocrystals G. Muscas,†,§ G. Concas,§ C . Cannas,‡ A. Musinu,‡ A. Ardu,‡ F. Orrù,‡ D. Fiorani,† S. Laureti,† D. Rinaldi,†,∥,⊥ G. Piccaluga,‡ and D. Peddis*,† †
Istituto di Struttura della Materia-CNR, 00016 Monterotondo Scalo (RM), Italy Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Cittadella Universitaria di Monserrato, bivio per Sestu, 09042 Monserrato, Italy, and INSTM § Dipartimento di Fisica, Università di Cagliari, S.P. Monserrato-Sestu km 0.700, 09042 Monserrato, Italy, and INSTM ∥ Dipartimento di Fisica e Ingegneria dei Materiali e del Territorio, Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy ⊥ Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia (CNISM), 84-00146 Rome, Italy ‡
ABSTRACT: This paper focuses on the study of the magnetic properties of 9 nm magnetite nanocrystals. XRD and TEM measurements indicate the presence of crystalline particles, with a fraction of them only partially crystallized or highly defective. The analysis of the temperature dependence of the zero-field-cooled/field-cooled magnetization and of the thermoremanent magnetization provides evidence of the existence of three magnetic regimes: a high temperature regime (300−100 K), an intermediate regime (100−20 K), and a low temperature regime (below 20 K). The characteristics of such regimes are discussed.
1. INTRODUCTION On entering the nanometer scale regime, the magnetic properties of materials show substantial differences with respect to the bulk state, leading to new physics1 and applications.2 Among nanostructured magnetic materials, nanoparticles are unique complex physical objects. In fact, at the nanoscale magnetic single-domain particles are formed, leading to new supermagnetic behaviors3 depending on nature and strength of interparticle interactions. Generally speaking, strong interparticle interactions can lead to ferromagnetic-like state, called superferromagnetism. Random distribution of particle moments correlated on a long-range scale leads to magnetic properties similar to spin glass systems (superspin glass). Finally, in a system consisting of noninteracting single domain particle a paramagnetic-like behavior, called superparamagnetism, is observed. In addition, the physics of nanoparticles is influenced by the modification of the structural and electronic properties at the particle surface.4,5 The breaking of lattice symmetry and the presence of broken bonds at the surface give rise to site-specific surface anisotropy, weakened exchange coupling, magnetic frustration, and noncollinear spin structure (i.e., spin canting).6 Nanoparticles of spinel ferrites are of great interest, not only for their technological applications but also from the point of view of fundamental science. In particular, magnetite (Fe3O4) has been one of the most widely studied and utilized magnetic materials in the life of humankind. It is characterized by high Curie temperature (TC = 850 K in bulk magnetite) and nearly © 2013 American Chemical Society
full spin polarization at room temperature, which make it a potential spintronic material.7 The transport and magnetic properties of magnetite are very particular. It undergoes a first-order phase transition at 120 K (TV), known as the Verwey transition,8−10 due to an ordering of the random distribution of ferrous and ferric ions in the interstitial sites with octahedral, (Oh), and tetrahedral, (Td), symmetry.8 Below TV, an abrupt increase of resistivity is observed, explained as the result of the localization of interacting electrons at particular position (T > TV(Fe3+)Td[Fe2.5+]Oh; T < TV(Fe3+)Td[Fe3+Fe2+]Oh). This electronic charge ordering is related to a change in the structure symmetry: Fe3+ and Fe2+ located in interstitial sites with octahedral symmetry arrange themselves on alternate (001) planes, leading to an orthorhombic distortion. Verwey transition can be detected through resistance measurements but also by magnetization measurements. The zero-field-cooled (ZFC) magnetization curve allows to gather this phenomenon, showing a quick decay of magnetization and susceptibility values below 120 K.7,10 Fe3O4 has also a peculiar anisotropy profile around Vervey transition and a large increase of the coercivity is observed below TV.11 Received: August 6, 2013 Revised: September 25, 2013 Published: October 18, 2013 23378
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Figure 1. (a) XRD pattern of the powder. (b) Mössbauer spectrum of the sample as prepared. The experimental points are reported as empty dots. The fitting curve is shown as a continuous black line; the two components are shown as dashed and dotted lines.
370 MBq. The calibration was performed using a 6 μm thick natural α-Fe foil; the measured full width at half-maximum of the absorption peak is 0.29(1) mm/s. The isomer shift values were referred to α-Fe. The measurements were carried out at 293 K on powder samples contained in a Plexiglas holder. The surface density of the absorber was 19 mg/cm2; the measurements were performed with an amount of iron in the samples corresponding to an effective thickness t ∼ 5 in order to avoid an excessive peak broadening. The analysis of Mössbauer spectra was performed by fitting the data by peaks of Lorentzian shape and applying a least-squares method. For the TEM analysis one drop of the hexane dispersion was deposited on a carbon-coated copper grid and observed by JEM 2010 UHR equipped with a Gatan imaging filter (GIF) and a 794 slow scan CCD camera. Particle size distribution was calculated on about 200 nanoparticles. Different images, obtained in bright field mode, were analyzed with ImageJ software.15 The contours for each particle was manually defined and thanks to the automated measurement suite of the software the exact particles area has been calculated. Then, assuming a spherical particle shape and knowing the area values, the diameter has been calculated for each particle. Diameters have been finally fitted with a log-normal function:
At the nanoscale, the high surface to volume ratio can lead to surface oxidation of Fe2+, hindering the Verwey transition that becomes difficult to be observed. Indeed, it is often reported that the transition occurs at lower temperature when the nanoparticles diameter is reduced below 50 nm,12 and for particles below 10 nm it is really difficult to be observed.13,14 In addition, the presence of supermagnetic regimes and surface effects complicates the landscape, making extremely difficult to get an evidence of TV by magnetization measurements. This paper focuses on magnetic characterization of a selected sample of sub-10 nm magnetite nanocrystals with morphostructural features suitable to observe the complex behavior of nanoscaled magnetite due to the coexistence of several magnetic effects (i.e., Verwey transition, supermagnetism, surface effects).
2. EXPERIMENTAL SECTION 2.1. Synthesis. To prepare Fe3O4 nanoparticles, iron(III) acetylacetonate (Janssen Chimica, 99%, 3 mmol), oleylamine (Aldrich, 0, the unexpected increase of coercivity is observed at Tonset = 99 K, slightly higher than T2. The field independence of T2 and the larger increase of HC below T2 would suggest an identification of this temperature with the Verwey transition,11 TV, characterized by a large increase of HC. In our samples TV results to be lower than bulk value (TV = 120 K) as expected for nanostructured systems in which the high surface/volume ratio can induce a partial oxidation of Fe2+, modifying the optimal ratio Fe2+/Fe3+. The width of the transition (ΔTM), determined from fwhm of the dMTRM/dT curve is about 30 K, and it is very weakly field dependent (Figure 4). ΔTM is higher with respect to the polycrystalline system (ΔTM,poly ∼ 15 K)8 due to to relatively
⎡
HC(T ) =
HC0⎢⎢1
(6)
H0C
where (14.6 mT) is the coercivity extrapolated at T = 0 K and Tsup is the temperature above which the system is in the superparamagnetic state. In the inset of Figure 5b the coercivity is plotted as a function of T3/4. A linear behavior is observed, 23382
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(4) Papaefthymiou, G. C. Nanoparticle Magnetism. Nano Today 2009, 4, 438−447. (5) Surface Effects in Magnetic Nanoparticles; Fiorani, D., Ed.; Wiley: New York, 2005. (6) Kodama, R. H.; Berkowitz, A. E. Atomic-Scale Magnetic Modeling of Oxide Nanoparticles. Phys. Rev. B 1999, 59, 6321−6336. (7) Bohra, M.; Prasad, S.; Venketaramani, N.; Kumar, N.; Sahoo, S. C.; Krishnan, R. Magnetic Properties of Magnetite Thin Films Close to the Verwey Transition. J. Magn. Magn. Mater. 2009, 321, 3738− 3741. (8) Dediu, V.; Arisi, E.; Bergenti, I.; Riminucci, A.; Solzi, M.; Pernechele, C.; Natali, M. Squid Measurement of the Verwey Transition on Epitaxial (100) Magnetite Thin Films. J. Magn. Magn. Mater. 2007, 316, e721−e723. (9) Poddar, P.; Fried, T.; Markovich, G.; Sharoni, A.; Katz, D.; Wizansky, T.; Millo, O. Manifestation of the Verwey Transition in the Tunneling Spectra of Magnetite Nanocrystals. Europhys. Lett. (EPL) 2003, 64, 98−103. (10) Verwey, E. J. W. Electronic Conduction of Magnetite (Fe3O4) and Its Transition Point at Low Temperatures. Nature 1939, 144, 327−328. (11) Bataille, A. M.; Vincent, E.; Gota, S.; Gautier-Soyer, M. Finite Size Effects in the Verwey Transition of Magnetite Thin Films. arXiv preprint cond-mat/, 2006; 0610291v1 1−5. (12) Goya, G. F.; Berquó, T. S.; Fonseca, F. C.; Morales, M. P.; Berquo, T. S.; Introduction, I.; Fe, M. Static and Dynamic Magnetic Properties of Spherical Magnetite Nanoparticles. J. Appl. Phys. 2003, 94, 3520−3528. (13) Pineider, F.; Fernández, C. de J.; de Julián Fernández, C.; Videtta, V.; Carlino, E.; al Hourani, A.; Wilhelm, F.; Rogalev, A.; Cozzoli, P. D.; Ghigna, P.; et al. Spin-Polarization Transfer in Colloidal Magnetic-Plasmonic Au/Iron Oxide Hetero-Nanocrystals. ACS Nano 2013, 7, 857−866. (14) Santoyo Salazar, J.; Perez, L.; de Abril, O.; Truong Phuoc, L.; Ihiawakrim, D.; Vazquez, M.; Greneche, J.-M.; Begin-Colin, S.; Pourroy, G.; Salazar, J. S.; et al. Magnetic Iron Oxide Nanoparticles in 10−40 nm Range: Composition in Terms of Magnetite/Maghemite Ratio and Effect on the Magnetic Properties. Chem. Mater. 2011, 23, 1379−1386. (15) Rasband, W. S. U. S. N. I. of H. ImageJ, 2012. (16) Morrish, A. H. The Physical Principles of Magnetism; Wiley-IEEE Press: New York, 2001. (17) Gütlich, P.; Link, R.; Trautwein, A. Mössbauer Spectroscopy and Transition Metal Chemistry; Springer-Verlag: Berlin, 1978. (18) Vandenberghe, R. E.; De Grave, E. Mossbauer Spectroscopy Applied to Inorganic Chemistry; Long, G., Grandjean, F., Eds.; Plenum Press: New York, 1989; Vol. 3, p 59. (19) Street, R.; Brown, S. D. Magnetic Viscosity, Fluctuation Fields, and Activation Energies (invited). J. Appl. Phys. 1994, 76, 6386−6390. (20) Karanasos, V.; Panagiotopoulos, I.; Niarchos, D.; Okumura, H.; Hadjipanayis, G. C. Magnetic Properties and Granular Structure of CoPt/B Films. J. Appl. Phys. 2000, 88, 2740−2744. (21) O’Grady, K.; Laidler, H. The Limits to Magnetic Recording Media Considerations. J. Magn. Magn. Mater. 1999, 200, 616−633. (22) Cannas, C.; Musinu, A.; Piccaluga, G.; Fiorani, D.; Peddis, D.; Rasmussen, H. K.; Mørup, S. Magnetic Properties of Cobalt FerriteSilica Nanocomposites Prepared by a Sol-Gel Autocombustion Technique. J. Chem. Phys. 2006, 125, 164714 1−11. (23) O’Grady, K.; Chantrell, R. Remanence Curves of Fine Particle Systems I: Experimental Studies. In Magnetic Properties of Fine Particles; Dormann, J. L., Fiorani, D., Eds.; North-Holland Delta Series: North-Holland: Amsterdam, 1992; pp 93−102. (24) Roca, A. G.; Morales, M. P.; O’Grady, K.; Serna, C. J.; O’Grady, K. Structural and Magnetic Properties of Uniform Magnetite Nanoparticles Prepared by High Temperature Decomposition of Organic Precursors. Nanotechnology 2006, 17, 2783−2788. (25) Coey, J. M. D. Magnetism and Magnetic Materials; Cambridge University Press: New York, 2010.
wide polydispersity of the sample.To clarify the nature of the magnetic transition located at T3, a careful analysis of the high field part of M vs H curves at different temperatures has been performed, using the law of approaching to saturation:29 ⎛ A B⎞ M = M 0 ⎜1 − − 2 ⎟ + χd H ⎝ H H ⎠
(7)
where M0 is the zero-field saturation magnetization and χd is the high-field differential susceptibility.29 The last term accounts for the nonsaturating high field linear component of the magnetization. As shown in Figure 6b, χd rapidly increases below ∼25 K, close to T3, indicating an increase of anisotropy. χd is strongly related to the presence of noncollinear spin structure due to competing interactions between sublattices and to the symmetry breaking at the particle surface.30 The increase of anisotropy can be ascribed to the presence of a noncollinear spin structure mainly located at the nanoparticle surface, undergoing a freezing process in random directions.29,31,32 This is also reflected by the temperature dependence of coercivity that increases much more rapidly below 50 K (a 50% increase).
4. SUMMARY AND CONCLUSIONS The structural and magnetic properties of ∼9 nm magnetite nanocrystals have been investigated. XRD and TEM measurements indicate that the particles consist of a crystalline core in contact with an amorphous fraction. The determination of the activation volume, coherent with the average diameter determined by XRD, confirms that the particle consists of a ferrimagnetically ordered core in contact with an amorphous fraction that does not take part to the coherent reversal process of the magnetization. The temperature dependence of the ZFC/FC magnetization and of TRM indicates the existence of three regimes: a regime above a temperature that could be identified as the Verwey transition temperature TV (around 90 K), a regime below TV, and a low temperature regime (below ∼20 K), characterized by a randomly frozen magnetic state at the nanoparticle surface. Such a low temperature state is characterized by the temperature independence of the FC magnetization, a large increase of coercivity, and a nonsaturating hysteresis loop. The fundamental nature of this study can open many technological perspectives. In fact, sub-10 nm magnetite nanoparticles have important applications in biomedicine (e.g., MRI, drug delivery) and, due to nearly full spin polarization at room temperature, represent interesting potential materials for spintronic devices.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (D.P.). Notes
The authors declare no competing financial interest.
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REFERENCES
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