UDK 620.3:66.017:621.318	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 48(5)675(2014)
MORPHOLOGY AND MAGNETIC PROPERTIES OF Fe3O4-ALGINIC ACID NANOCOMPOSITES
MORFOLOGIJA IN MAGNETNE LASTNOSTI NANOKOMPOZITOV Fe3O4-ALGINSKA KISLINA
Malgorzata Kazmierczak1,2, Katarzyna Pogorzelec-Glaser1, Andrzej Hilczer1,
Stefan Jurga2, Lukasz Majchrzycki3, Marek Nowicki3, Ryszard Czajka3, Filip Matelski3, Radoslaw Pankiewicz4, Boguslawa L^ska4, Leszek K^pinski5,
Bartlomiej Andrzejewski1
1Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60179 Poznan, Poland 2NanoBioMedical Centre, Adam Mickiewicz University, Umultowska 85, 61614 Poznan, Poland
3Poznan University of Technology, Nieszawska 13A, 60965 Poznan, Poland 4Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61614 Poznan, Poland 5Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okolna 2, 50422 Wroclaw, Poland
malgorzata.kazmierczak@ifmpan.poznan.pl
Prejem rokopisa - received: 2013-09-27; sprejem za objavo - accepted for publication: 2013-11-19
The morphology, structure and magnetic properties of the nanocomposites of magnetite (Fe3O4) nanoparticles and alginic acid (AA) are studied. Magnetite Fe3O4 nanoparticles and the nanoparticles capped with alginic acid exhibit very distinct properties. The chemical bonding between alginic acid and the surface of magnetite nanoparticles results in the recovery of surface magnetization. On the other hand, it also leads to the enhanced surface spin disorder and unconventional behavior of the magnetization observed in Fe3O4-AA nanocomposites at low temperatures.
Keywords: nanocomposite, magnetite nanoparticles, alginic acid, enhanced magnetization
Preučevali smo morfologijo, strukturo in magnetne lastnosti nanokompozitov na osnovi nanodelcev magnetita (Fe3O4) in alginske kisline (AA). V primerjavi z magnetitnimi nanodelci izkazujejo nanokompoziti Fe3O4-alginska kislina precej drugačne lastnosti. Molekule alginske kisline se kemijsko vežejo na površino magnetitnih nanodelcev in s tem povzročijo vrnitev površinske magnetizacije. Hkrati pa se s tem v nanokompozitih Fe3O4-AA pri nižjih temperaturah poveča površinska neurejenost spinov in nekonvencionalno vedenje magnetizacije.
Ključne besede: nanokompozit, nanodelci magnetita, alginska kislina, povečana magnetizacija
1 INTRODUCTION	Nanostructured magnetite exhibits different magnetic, electronic and optical properties than the bulk
The interest in the composites of polymers with	material. Particularly, a significant reduction in the
magnetic nanoparticles stems from their unique physical	magnetization at the surface of Fe3O4 nanoparticles
properties and potential future applications for mag-	makes them useless for many applications. This obstacle
netic-data storage,1 electronic devices and sensors,2	can be overcome by capping the magnetic nanoparticles
biomedical applications in magnetic resonance imaging,3	with polymers8 or organic acids, which allows a
drug delivery4 and hyperthermia agents.5 From this point	restoration of the surface magnetism.9
of view, one of the most preferred magnetic materials is	One of the best capping material is alginic acid,
magnetite Fe3O4 because it is a biocompatible mineral	which is a cheap, common and nontoxic natural biopoly-
with a low toxicity (for example, the crystals of magne-	mer.1011 The aim of this work is to study the effect of the
tite are magnetoreceptors in the brains of some ani-	alginic-acid capping on the surface magnetization reco-
mals6). It also exhibits a large magnetic moment and a	very in Fe3O4 nanoparticles. spin-polarized electric current - the features highly desired for the applications in spintronics.
In bulk, magnetite crystallizes in the inverse spinel	2 EXPERIMENTAL WORK AB2O4 structure with two nonequivalent Fe sites placed
..... ,, ^ ^ ^ ^ , A , •	2.1 Sample synthesis in the fcc lattice of O2- ions. Tetrahedral A sites contain
Fe2+ ions, whereas octahedral B sites are occupied by	All the chemicals used in the experiments were
Fe2+ and Fe3+ ions. The magnetic sublattices located on A	purchased from SIGMA ALDRICH. To obtain the dis-
and B sites are ferrimagnetically coupled. The mixed	aggregated nanoparticles of magnetite Fe3O4, a portion
valence of Fe ions and fast electron hopping between B	of 9.0 mmol of FeCl3 ■ 6H2O was dissolved in 200 m^ of
sites are responsible for a relatively high electric	ethylene glycol. The solution was vigorously stirred.
conductivity of Fe3O4 above the Verwey transition, Tv ~	After 15 min 131.7 mmol of CH3COONa and
125 K.7	1.575 mmol of polyethylene glycol PEG 400 were added
and the stirring was continued until they completely dissolved. Then, the solution was transferred into 50 mL teflon reactors and heated using microwave radiation (MARS 5, CEM Corporation) at 160 °C for 25 min. The black suspension of the nanoparticles obtained as a result of the reaction was first cooled, isolated by centrifuga-tion and washed with absolute ethanol. The final product was dried in a vacuum oven at 40 °C. A nanocomposite was prepared from the aqueous dispersion of the magnetite nanopowder and alginic acid (AA) that was then air-dried at room temperature. The nanocomposite had the form of flakes with flat surfaces.
2.2 Sample characterization
The crystallographic structures of the samples were studied by means of X-ray powder diffraction (XRD) using an ISO DEBYE ELEX 3000 instrument with a Co lamp (X = 0.17928 nm). The morphology of Ee3O4 nanoparticles was observed using a Philips CM20 SuperTwin transmission electron microscope (TEM). The structures of nanocomposites were studied by means of an atomic force microscope (Dimension Icon®, Bru-ker) using the magnetic-force-microscope (MEM) mode and NANO-SENSORS™ PPP-MEMR probes. The magnetic measurements were performed using a Quantum Design physical property measurement system (PPMS) fitted with a vibrating-sample-magnetometer (VSM) probe.
3 RESULTS AND DISCUSSION
Figure 1 shows the X-ray powder diffraction patterns of the as-obtained Ee3O4 nanoparticles (panel a) and of the Ee3O4-AA nanocomposite with the magnetite content equal to the mass fraction w = 10 % (panel b).
Figure 1: XRD powder pattern and line profile fitting of: a) Ee3O4 nanoparticles and b) Ee3O4-AA nanocomposite Slika 1: XRD-difraktogrami in ujemanje linij za: a) nanodelce Ee3O4 in b) nanokompozit Ee3O4-AA
The solid line corresponds to the best Rietveld profile fit calculated by means of the EULLPROE software for the cubic crystal structure with the Ed-3m space group and X-ray radiation with the wavelength of 0.17928 nm, as used in the experiment. The vertical bars correspond to the Bragg peaks and the line below them is the difference between the experimental data and the fit. XRD studies verified the Ed-3m point group of the Ee3O4 nanopowder with the lattice parameters of a = 0.83641 nm and the mean crystallite size of 20 nm determined with the Scherrer method. Eor the composite, the intensity of diffraction peaks is too low to perform an analysis, even if the content of magnetite is high and equal to w = 10 %.
A TEM image of magnetite nanoparticles is presented in the inset to Figure 2. The magnetic nanoparticles are almost monodisperse and spherical. A histogram of the particle-size distribution of Ee3O4 nanoparticles is presented in Figure 2. The distribution can be fitted with a log-normal function:
f(^) = -
1
:V2
na
r
exp|^-
1
2a2
ln2
1
(x)
(1)
where (x) is the mean size of the nanoparticles and a is the distribution width. The values characterizing the distribution are: (x) = 20.5 nm and a = 0.11, with (x) corresponding well to the XRD data.
Figure 3 shows the topography (panel a), elastic properties (panel b) and magnetic domains (panels c and d) of the Ee3O4-AA composite surface with the Ee3O4 content of 10 %, studied with MEM. The roughness of the surface is below 10 nm for the scanned area of 500 nm X 500 nm. The knobs on the topography image (the white spots) indicate the presence of small agglomerates of Ee3O4 nanoparticles that are also seen as white areas on the phase-contrast image (panel b). The amplitude and phase contrast of the magnetic signal (panels c and d) indicate the presence of magnetic domains with the size close to 100 nm. The actual size of these domains
Figure 2: Histogram for Ee3O4 nanoparticles with a log-normal fitting. A TEM image is shown in the inset.
Slika 2: Histogram nanodelcev Ee3O4. TEM-posnetek je prikazan v vstavku.
Figure 3: a) Surface topography, b) phase contrast, c) magnetic phase and d) magnetic amplitude images for the Ee3O4-AA composite with the Ee3O4 mass fraction of 10 %
Slika 3: a) Površinska topografija, b) fazni kontrast, c) magnetna faza in d) magnetna amplituda kompozitov Ee3O4-AA z masnim deležem Ee3O4 10 %
can be smaller than that presented in the figures because of the insufficient spatial resolution of the MEM method (about 50 nm) which causes a smearing of the images.
The results of the magnetic study are presented in Figures 4 and 5. The magnetization is normalized with respect to the content of magnetite in the samples. Eor the nanoparticles of Ee3O4, the temperature dependence of magnetization M ~ T19 deviates from the Bloch law M ~ T15 valid for the capped nanoparticles of magnetite (Figure 4). The deviation from the Bloch law for the uncapped Ee3O4 nanoparticles can be related to a degraded magnetic ordering at the surface. Moreover, the magnetization of the Ee3O4 nanoparticles at room temperature is only 51 A m2/kg, i.e., much below the saturation value for the bulk magnetite (» 90 A m2/kg), and also lower than the magnetization of the capped particles, equal to 60 A m2/kg. The enhancement of the magnetization and the Bloch-like behavior of the capped nanoparticles can be explained in terms of the recovery of surface magnetism due to the chemical bonding between the AA and Ee3O4 nanoparticles. This bonding between the O atoms in the carboxylic groups and two of the four Ee atoms in the Ee-O surface unit cell makes the coordinations and distances close to those in the bulk.9 The remaining two Ee atoms still exhibit a reduced magnetization because they are closer to the in-plane oxygens, which results in partially empty dx2 - y2 orbitals. The inhomogeneity with respect to the Ee coordination can be responsible for the increased spin disorder or unconventional magnetism at the Ee3O4 surface. This unconventional behavior is manifested as a rapid increase in the magnetization at a low temperature observed for
Figure 4: Magnetization M(T) of the Ee3O4 nanoparticles and Ee3O4-AA composites containing mass fractions 5 % and 10 % of magnetite
Slika 4: Magnetizacija M(T) nanodelcev Ee3O4 in kompozitov Ee3O4-AA z masnim deležem magnetita 5 % in 10 %
Ee3O4-AA composites (Figure 4). The alternative explanation of this magnetization upturn assumes a quantization of the spin-wave spectrum due to the finite size of the particles that occurs at low temperatures and is responsible for the deviation from the Bloch law.12
The magnetization loops M(H) for the Ee3O4 nano-particles and Ee3O4-AA composites are shown in Figure 5.
Both the nanoparticles and composites exhibit ferromagnetic (ferrimagnetic) hysteresis loops, which saturate above about 0.3 T. The magnetization of the composites is enhanced as compared to that of the uncapped Ee3O4 nanoparticles. At low temperatures the magnetization loops for the composites are the superpositions of the ferromagnetic and linear contribution from an unconventional magnetism. This unconventional behavior cannot be simply related to the paramagnetism at the degraded
Figure 5: Magnetization loops M(H) for the Ee3O4 nanoparticles and Ee3O4-AA composites with mass fractions 5 % and 10 % of the magnetite content
Slika 5: Histerezna zanka M(H) nanodelcev Ee3O4 in kompozitov Ee3O4-AA z masnim deležem magnetita 5 % in 10 %
Fe3O4 surface because it is absent in the uncapped nano-particles of magnetite.
4 CONCLUSIONS
The capping of Fe3O4 nanoparticles with alginic acid leads to a partial recovery of the surface magnetization. On the other hand, the bonding between alginic acid and Fe3O4 nanoparticles by means of O atoms results in an unconventional magnetism observed at low temperatures.
Acknowledgments
This project was supported by the National Science Centre through project No. N N507 229040. M.K. was supported through the European Union - European Social Fund and Human Capital - National Cohesion Strategy. We thank dr. Alja Kupec and dr. Brigita Rožič for the Slovenian translation.
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