UDK 537.622
Original scientific article/Izvirni znanstveni članek
ISSN 1580-2949 MTAEC9, 47(1)71(2013)
SYNTHESIS OF A Fe3O4/PAA-BASED MAGNETIC FLUID FOR FARADAY-ROTATION MEASUREMENTS
SINTEZA MAGNETNE TEKOČINE NA OSNOVI Fe3O4/PAA ZA MERITVE FARADAYEVE ROTACIJE
Serhat Ku^ukdermenci1,3, Deniz Kutluay1,2, Ibrahim Avgin1
department of Electrical and Electronics Engineering, Ege University, Bornova 35100, Izmir, Turkey 2 Departments of Electronics and Communications Engineering, Izmir University, Uckuyular 35290, Izmir, Turkey 3Department of Electrical and Electronics Engineering, Balikesir University, Campus of Cagis, 10145 Balikesir, Turkey
serhat.kucukdermenci@ege.edu.tr
Prejem rokopisa — received: 2012-07-24; sprejem za objavo - accepted for publication: 2012-08-27
Highly water-soluble Fe3O4/PAA (polyacrylic acid) nanoparticles (NPs) were synthesized with the high-temperature hydrolysis method. We report the first demonstration of Faraday rotation (FR) for a magnetic fluid (MF) synthesized with this novel method. The experiments were performed in the DC regime (0-6 • 10-2 T) at room temperature for 14 concentrations from 1.8 mg/ml to 5 mg/ml. The maximum rotation was recorded as 0.96° cm-1 for 3.33 mg/ml and this is called the critical concentration (Ccritical). It was found that the rotation tends to decrease when the concentration is higher than Ccritical. The MF behavior for FR is discussed with respect to substructure interactions (particle-particle, chain-chain). This work provides a new insight for the FR investigations of MFs including highly water-soluble magnetic NPs.
Keywords: magnetic fluids, Fe3O4 nanoparticles, magneto-optic response, Faraday rotation
V vodi dobro topni nanodelci Fe3O4/PAA (poly acrylic acid) so bili sintetizirani po metodi visokotemperaturne hidrolize. Poročamo o prvi demonstraciji Faradayeve rotacije (FR) za magnetno raztopino (MF), sintetizirano po tej novi metodi. Preizkusi so bili izvršeni v DC-režimu (0-6 • 10-2 T) pri sobni temperature za 14 različnih koncentracij od 1,8 mg/ml do 5 mg/ml. Največja rotacija je bila ugotovljena kot 0,96° cm-1 pri 3,33 mg/ml, kar smo imenovali kritična koncentracija (Ccritical). Ugotovljeno je bilo, da se rotacija zmanjšuje, če je koncentracija višja od Ccritical. Lastnost MF za FR je bila obravnavana z interakcijami podstrukture (delec-delec, veriga-veriga). To delo ponuja nov pogled v študij FR magnetnih raztopin (MFs), vključno z v vodi dobro topnimi nanodelci (NPs).
Ključne besede: magnetna tekočina, nanodelci Fe3O4, magnetnooptični odgovor, Faradeyeva rotacija
1 INTRODUCTION
Magnetic NPs have unique colloidal, magnetic and optical properties that differ from their bulk counterparts.1-8 Core-shell NPs have been a topic of great interest due to their potential use in biology9,10, imaging11, medicine12-14 and DNA separation.15,16 Colloidal suspensions of magnetic NPs can self-assemble into ordered structures. The ability to manipulate this assembly with external tuning parameters such as the field, the temperature and the concentration is essential for developing new stimuli-responsive materials.
MFs, also named ferrofluids, are colloidal suspensions of magnetic NPs that have both characteristics -the fluidity of liquids and the magnetism of solid magnetic materials. Several applications of MFs have recently been introduced, such as a detection system design for glucose concentration in addition to optical-device applications.17 It is suitable for fabricating optical devices such as optical attenuator, light modulator, optical switch, etc., by using the magneto-optic properties of MFs.18-20
In 2007, the Yin group synthesized novel superpara-magnetic, magnetite colloidal NPs that can self-assemble into one-dimensional (1D) particle chains and exhibit
excellent tunable photonic properties.21 A suspension of these NPs displays tunable colors in the visible range of the electromagnetic spectrum. The freedom to tune a diffraction color not only depends on the particle size but also varies with the strength of an applied external magnetic field. Since then there has been a widespread interest in these NPs and their applications. Despite their tremendous potential in various applications, interesting fundamental questions referring to their colloidal crystallization with and without a magnetic field remain unanswered. Therefore, we report on the first demonstration of FR for MFs based on these NPs.
FR has been demonstrated in the visible22 23 NIR24 25 and MIR26 regimes for different kinds of ferrofluids. Experimental investigation on y-Fe2O3 NPs FR was done due to the particle-size dependence. Water-based ferro-fluid samples are synthesized with the coprecipitation method followed by a size-sorting process27. The wavelength and concentration dependence of FR in MFs was studied by Yusuf et al.28 29 Here we demonstrate that long-term, stable MFs including highly water-soluble NPs show FR in a DC regime (0-6 ■ 10-2 T) at room temperature. Water-based ferrofluid samples are synthesized with a novel high-temperature hydrolysis method.
2 MATERIALS AND METHODS
2.1	Materials
Diethylene glycol (DEG, 99.9 %), anhydrous ferric chloride (FeCl3, 97 %), sodium hydroxide (NaOH, 96 %), and poly acrylic acid (PAA, Mw = 1800) were purchased from the Sigma-Aldrich company. Distilled water was used in all the experiments. All the chemicals were used as received without further treatment and/or purification.
2.2	Synthesis of water-dispersible Fe3OJPAA NPs
The polyol method based on the theory that NPs will be yielded upon heating precursors in a high-boiling-point alcohol at elevated temperature. In this method, DEG is chosen as the solvent because it can easily dissolve a variety of polar inorganic materials due to its high permittivity (£ = 32) and high boiling point (246 °C). DEG is not only a solvent but also a reducing agent in an reaction. Hence, FeCl3 can be used as the only precursor for synthesizing Fe3O4. PAA is used as the capping agent, on which the carboxylate groups show a strong coordination with Fe3+ on the Fe3O4 surface and the uncoordinated carboxylate groups extend into the water solution, rendering the particles with high water dispersibility. A strong coordination of carboxylate groups with the surface iron cations and the multiple anchor points for every single polymer chain is an important factor in creating a robust surface coating of PAA on magnetite NPs. Therefore, we used PAA as the capping agent in our synthesis to confer upon the particles high water dispersibility. A mixture of ethanol and water was used to wash the particles and remove the unwanted leftover material from the particles.
For the synthesis of Fe3O4 NPs, a NaOH/DEG solution was prepared by dissolving 100 mmol of NaOH
Stock A NaOH, DEG		' Stock B * FeCt3, PAA, DEG
0		0
" 120 C, lh, r 226,55 min,"^ nitrogen atm.] ^ nitrogen atm J		
0 0
[ 220 C, 200 mini nitrogen atm. J
washing, sedimentation, drying
0
[magnetic particles]
Figure 1: Flow chart of the synthesis Slika 1: Potek sinteze
in 40 ml of DEG at 120 °C under nitrogen for 1h. Then the light-yellow solution was cooled to 70 °C (the stock solution A). In a 100-ml, three-necked flask equipped with a nitrogen inlet, a stirrer and a condenser, 10 mmol of FeCl3 and 20 mmol of PAA were dissolved in 41 ml of DEG under vigorous stirring. The solution was purged with bubbling nitrogen for 1 h and then heated to 220 °C for 50 min (the stock solution B). Subsequently, 20 ml of the NaOH/DEG solution was injected rapidly into the above solution. The reaction was allowed to proceed for
2	h. The black color of the solution confirms the formation of magnetite NPs. The resultant black product was repetitively washed with a mixture of ethanol and water and collected with the help of a magnet. The cycle of washing and magnetic separation was performed five times. A one-pot synthesis was done with the Fe3O4/PAA particles, so no extra process was needed for the surface modification. A flow chart of the synthesis is shown in Figure 1.
3	CHARACTERIZATION
3.1	Structural and magnetic characterization of
Fe3OJPAA NPs
A powder X-ray diffraction (XRD) analysis was performed on a Phillips EXPERT 1830 diffractometer with Cu K radiation. The XRD data were collected over the range of 10-80° (2 d) with a step interval of 0.02° and a preset time of 1.6 s per step at room temperature. Magnetic measurements were carried out using a Lake-Shore 7400 (Lakeshore Cryotronic) vibration sample magnetometer (VSM) at 300 K. Particle sizes of NPs were measured using a Zetasizer 4 Nano S, dynamic light scattering instrument (Malvern, Worcestershire, UK). Light-scattering measurements were carried out with a laser of the wavelength of 633 nm at the 90° scattering angle. FTIR spectra were recorded on a KBr disc on a Perkin Elmer 100 spectrometer.
For the FR experiments we used a Thorlabs model HGR20 2.0 mW/nm laser source, a GMW Electromagnet-Systems model 5403 electromagnet, a Kepco power-supply model BOP 20-5M, a LakeShore model 455 DSP Gaussmeter, a Stanford research systems model SR830 DSP lock-in amplifier, a new focus model 2051 photo detector, an ILX Lightwave model OMM - 6810B optical multimeter and an OMH - 6703B model silicon power head.
3.2	Experimental setup for a magneto-optical characterization
The magneto-optical-measurement setup is shown in Figure 2. The measurements of FR were made using an optical arrangement consisting of a He-Ne laser, a polarizer, MR3-2 magneto-optical glass (from Xi'an Aofa Optoelectronics Technology Inc., China) placed in the gap between the two poles of the electromagnet for the
Figure 2: Magneto-optic-measurement setup Slika 2: Magnetoopticni merilni sestav
calibration process, an analyzer, collimating lens and a power meter. The electromagnet generates a uniform magnetic field in the sample region. The strength of the
Figure 3: Pictures of the nanofluids containing Fe3O4/PAA NPs with various concentrations kept for different times: a) 1 h, b) 2 weeks, c) 4 weeks
Slika 3: Posnetki nanotekočin z Fe3O4/PAA-nanodelci z različno koncentracijo po zadržanju: a) 1 h, b) 2 tedna, c) 4 tedne
magnetic field can be adjusted by tuning the magnitude of the supply current and is monitored by a gauss meter.
According to Malus' law, as the polarized light Io passes through the transparent magneto-optical material, the light intensity I can be expressed as:
l = 10 cos2 0 = 10 cos2(a-p)
(1)
where a is the angle of the polarization axes of the polarizer and analyzer and < is the rotation angle of the polarized plane of the transmitted light. With respect to MFs, this can be expressed as:
M (B)
((B) = C^VBl(B)
(2)
where l is the chain length at the magnetic field B in an MF sample, C is a constant that can be found at a high field assuming that the saturated chain length has no change, M is the magnetization of the sample at the magnetic field B, Ms is the saturation magnetization of the sample and V is the Verdet constant varying with the wavelength and temperature. Generally, a positive Verdet constant corresponds to the L-rotation (anticlockwise) when the direction of propagation is parallel to the magnetic field and to the R-rotation (clockwise) when the propagation direction is anti-parallel. To obtain the maximum sensitivity of the transmitted power, especially under very small magnetic fields, the FR angle < can be identified as zero and the initial polarization angle of the analyzer is set, with a = 45°, according to equation (1). It can be described as:
d(cos2(0))
" • " - • -	(3)
d(0)
- = 2 sin 0 cos 0 = sin 20
With respect to the FR experiments, the sensitivity is maximum when 0 is 45° and dI/d© = 1 from equation (3). Therefore, the optical axis of the analyzer is aligned at an angle of 45° to the optical axis of the polarizer.
4 RESULTS
4.1 Stability of Fe3O4/PAA-based ferrofluids
NPs in MFs are usually coated with a surfactant material to prevent agglomeration and provide stability.30-33 The size of a magnetic NP, the material concentration, the carrier liquid and the surfactant are the main variations for MFs.
PAA was selected as a surfactant because of its strong coordination of carboxylate groups with iron cations on a magnetite surface. An additional advantage of PAA is that an extension of the uncoordinated carboxylate groups on polymer chains into an aqueous solution confers on the particles a high degree of dispersibility in water.
A well-dispersed nanofluid was prepared as shown in Figure 3. The particle content is 20 mg for various concentrations with an addition of distilled water from 4 ml to 17 ml. NPs can still be dispersing well after the
nanofluid has been kept standing still for more than 4 weeks and no sedimentation is observed for any of the samples. Due to its long-term stability, this kind of MFs is an ideal candidate for optical devices.
4.2 Physical and magnetic properties of Fe3O4/PAA
NPs analyzed with XRD, DLS, TEM and FTIR
The crystal structure of the sample was confirmed with an X-ray diffraction (XRD) analysis as shown in Figure 4. The (220), (311), (400), (422), (511), and (440) diffraction peaks observed on the curves can be indexed to the cubic spinel structure, and all the peaks were in good agreement with the Fe3O4 phase (JCPDS card 19-0629).
The average diameter of the particles obtained with a dynamic light scattering (DLS) analysis is ~10 nm (Figure 5). As seen in the typical transmission electron microscopy (TEM) images of Fe3O4/PAA in Figure 6a-b, the average diameter of Fe3O4/PAA was about 8.16 nm Figure 6c. The average diameter was obtained by measuring about 100 particles.
The magnetic properties of NPs are shown in Figure 7 as measured at 300 K with a vibrating sample mag-
Figure 4: X-ray powder-diffraction pattern for PAA-coated Fe3O4 NPs. The peak positions and relative intensities recorded in the literature for bulk Fe3O4 samples are indicated by vertical bars. Slika 4: Rentgenska difrakcija prahu za s PAA pokritimi Fe3O4-nano-delci. Pozicija vrhov in v literaturi zapisane relativne intenzitete za osnovne Fe3O4-delce so prikazane z navpičnimi črtami.
Figure 6: Representative TEM images of magnetite CNCs with: a) 20-nm scale bar, b) 50-nm scale bar, c) the average diameter of Fe3O4/PAA
Slika 6: Reprezentativni TEM-posnetki magnetitnih CNCs: a) merilo 20 nm, b) merilo 50 nm, c) povprečni premer Fe3O4/PAA
Figure 5: Size distributions of magnetic NPs obtained with DLS Slika 5: Razporeditev velikosti magnetnih delcev (NPs), vzeto iz DLS
Figure 7: Hysteresis loop of superparamagnetic particles at room temperature
Slika 7: Histerezna zanka superparamagnetnih delcev pri sobni temperaturi
Figure 8: FTIR spectrum of: a) pure PAA, b) PAA with carbo-xylate-capped Fe3O4 NPs
Slika 8: FTIR-spekter: a) cisti PAA, b) PAA s karboksilatom omejeni Fe3O4-nanodelci
netometer (VSM). The saturation magnetization was determined as 38.8 emu/g. The particles showed no remanence or coercivity at 300 K, that is, superparamagnetic behavior.
The stability of PAA capped on Fe3O4 was confirmed by measuring the Fourier transform infrared spectroscopy (FTIR) spectrum on the sample as shown in Figure 8a. There is a very strong band at around 1718 cm-1 of pure PAA, which is characteristic of the C=O stretching mode for protonated carboxylate groups. The three peaks shown in Figure 8b and located at (1566, 1454 and 1406) cm-1 can be assigned to the characteristic bands of the carboxylate (COO ) groups, corresponding to the CH2 bending mode, asymmetric and symmetric C-O stretching modes of the COO- group, respectively.
4.3 Magneto-optic properties of the Fe3O4/PAA-based
magnetic fluid
Figure 9 shows FR versus the magnetic field and Table 1 indicates the maximum rotations of MFs with different concentrations. The field and concentration dependence of FR in MFs was investigated. A 10-mm-thick cuvette was filled with the liquid of 14 various concentrations from 1.8 mg/ml to 5 mg/ml. It can be seen that the rotation increases rapidly with the field at low fields.
The initial susceptibility % is determined by a linear magnetic response M = % ■ H at the field strength H ^ 0 and it depends on the particle concentration of fluids. At low fields in Figure 9 the initial slope is: C M (B)
m. =-
B M,
+Vl( B)
(4)
Figure 9: Applied magnetic-field dependence and concentration dependence of FR
Slika 9: Odvisnost FR od uporabljenega magnetnega polja in koncentracije
particular field. In other words, the volume fraction of MF and the initial slope are decreased. Black curves relate to samples 8-14 (Table 1) and their slopes are less steep than the ones of the first seven samples, because their magnetizations are lower and the chain lengths are shorter than those of the first seven samples. It needs to be pointed out that though the maximum FR of sample 1 (indicated as the yellow curve) is relatively low, its initial susceptibility is the highest value due to the volume fraction.
Table 1: Maximum FRs of the samples with different concentrations Tabela 1: Maksimalna Faradayeva rotacija (FR) pri vzorcih z različnimi koncentracijami
Sample	Particle	Water	Concentration	Faraday rot.
no	mg	ml	mg/ml	Max. degree
1	20	4	5.00	0.49
2	20	5	4.00	0.87
3	20	6	3.33 (Ccritical)	0.96
4	20	7	2.86	0.83
5	20	8	2.50	0.78
6	20	9	2.22	0.70
7	20	10	2.00	0.65
8	20	11	1.82	0.56
9	20	12	1.67	0.48
10	20	13	1.54	0.44
11	20	14	1.43	0.37
12	20	15	1.33	0.35
13	20	16	1.25	0.30
14	20	17	1.18	0.28
As long as the concentration is decreased, fewer NPs are found in the medium and this situation causes a lower magnetization and a shorter chain length at a
As the volume fraction of NPs increases, dipole interactions between the particles overcome the thermal forces more easily. A graphical representation of the maximum FR can be seen in Figure 10. It is important to mention that most experimental investigations, based on optical observations, of the chain formation in MFs are usually carried out on the samples in the low concentration regime, where the chain-chain interaction is weak.
Figure 10: Graph of the maximum FR of the samples Slika 10: Graf maksimalne Faradayeve rotacije (FR) vzorcev
However, we tried many samples with variable concentrations to see the effect of higher concentrations. When we analyze Figure 10, it can be seen that FR increases with higher concentrations (from 1.18 mg/ml to 2.26 mg/ml) up to Ccritical (3.33 mg/ml). But after the CCRITICAL (sample 3) value, FR tends to decrease with higher concentrations (4 mg/ml and 5 mg/ml). According
Figure 11: a) Transmissivity of Fe3O4 NPs, B = 0 T, b) transmitted power difference of Fe3O4 NPs at particular fields and with different concentrations
Slika 11: a) Transmisivnost Fe3O4-nanodelcev, B = 0 T, b) posredovana razlika v moči Fe3O4-nano delcev pri določenem polju z različno koncentracijo
to Stokes' law, there is a friction between the moving particles and the carrier fluids, which depends on the viscosity of the fluids.34 Thus, the viscosity of MF and the friction in the medium were high for highly concentrated MFs and they affected the activity of the magnetic particles. It is also important to point out that the chain-chain interaction becomes much stronger once the concentration of a sample is increased and may lead to the closure of some chains or even the curling.35,36 Therefore, it can be said that FR tends to decrease after Ccritical due to the substructure (particle-particle, chain-chain) interactions.
Figure 11a shows the transmission of unpolarized light as percentage and Figure 11b shows the transmission difference of unpolarized light under magnetic fields for each concentration in the 1-6 ■ 10-2 T region. The transmitted light difference as percentage in Figure 12b is calculated from the difference between the intensity of the light transmitted in a particular (1-6 ■ 10-2 T) magnetic field and the intensity in the zero field for the samples with different concentrations.
Low transmission responses to magnetic fields were observed for samples 1 and 2. Except for these samples, remarkable changes were observed for all the other samples because of the weakening of the viscosity effect. it can be said that samples 1 and 2 have a blocking property and other samples have a channeling property due to their concentrations. A decreasing percentage of NPs in a unit volume indicates a lower magnetization and a smaller chain length, which play important roles in the Faraday effect.37
5 DISCUSSION
5.1 Model for Explaining the Experimental Results
There are two physical phenomena for the magneto-optical effect in MFs in the presence of an external magnetic field. One is the orientation theory (magnetic orientation or physical orientation) based on the optical anisotropy of magnetic particles or its aggregation, and the other is the formation theory based on the chain formation of the magnetic particles (Figure 12). In the zero magnetic field the particles are distributed randomly with
Figure 12: Magnetic NP alignment and the chain formation in the direction of the external magnetic field
Slika 12: Magnetna ureditev nanodelcev in nastajanje verig v smeri zunanjega magnetnega polja
no coercivity and remanence forming an isotropic material. Particles start to coagulate and form chain-like structures in the direction of the field with the help of an external magnetic field.38,39
Magnetic field induces NPs to line up or to behave asymmetrically, introducing anisotropy and resulting in birefringence. If lattice atoms of a crystal were not completely symmetrically arrayed, the binding forces on the electrons would be anisotropic, causing a material to be circularly birefringent with different indices of refrac-tion.40 When the applied magnetic field's direction is parallel to the light beam, the anisotropy is circular in a longitudinal configuration. However, the rotation of polarization does not seem to be linked to the particles' anisotropy-axis orientation but to the orientation of the magnetic moments of the particles in the applied field's (H) direction.41
When an external magnetic field is applied parallel to the plane of MF, magnetic particles in the fluid agglomerate to form chain-like structures. As the field strength is further increased, more particles contribute to agglomeration and the chains become longer under a higher field. It has been found that the chain length varies with the applied magnetic field and with the concentration of the MF.42 In some experimental conditions, in which the wavelength of the electromagnetic waves passed through the sample is very small in comparison with the chain length, FR is not only governed by magnetization of the fluid but also affected by the chain formation.43
Additionally, MFs can be diluted magnetically by passive liquid carriers such as glycerol, ethylene glycol, diester, isopar M, ethanol or simply distilled water and it was seen that FR was affected by a change in the concentration. Different concentrations of MFs can help us describe various friction forces among the magnetic NPs. Carrier fluids significantly influence the response of the Faraday effect. Consequently, the chain formation is a crucial parameter of the optical properties of MFs. The chain lengths in MFs composed of magnetic NPs vary with respect to concentration.44-46
The positional and magnetic field dependence of the colloidal assembly of magnetite NPs arise from a sensitive interplay between the local concentrations of the particles, causing the effect of three types of forces between the colloidal NPs. These forces are (1) the hard-sphere repulsion between the particles in contact; (2) a combination of electrostatic repulsion due to the presence of the charges on the surface of NPs and Van-der-Waals attraction; (3) the magnetic dipolar attraction/repulsion due to the magnetite cores of the particles. The interaction potential of the third force is given by:
Uab(r) = ^(Ma ■ £b)	(5)
The dipolar interaction between two magnetic particles, a and b, depends on the magnitude and direc-
tion of their magnetic moments ^i as well as on their relative position rab. Depending on the particles configuration, the dipolar energy may be repulsive or attractive. Heinrich and coworkers showed that these forces play a role in the assembly of magnetic nano-particles.47 When a magnetic field is applied these particles initially form chain-like structures. These chains are then arranged into two-dimensional hexagonally packed sheets. This occurs by shifting a neighboring chain by a distance of r corresponding to the radius of NP. The chains are formed along the direction of an external magnetic field.
6	CONCLUSION
In conclusion, high-quality Fe3O4/PAA-based nano-structures for MF formation were synthesized successfully and FR investigations were made for many samples with variable concentrations. We report the first demonstration of FR for MF synthesized with this novel method. We have demonstrated the FR of a highly water-soluble MF that was measured to be in the 0-6 ■ 10-2 T range in the DC regime. The effects of both viscosity and chain formation were observed on FR. We found the maximum FR to be 0.96°/(10 mm) at room temperature for 3.33 mg/ml. FR was on an increase with the higher concentrations up to Ccritical. It was found that the rotation begins to decrease again when the concentration is higher than Ccritical. The reason for this might be the blocking effect that arises from the particle-particle and chain-chain interactions. The experiment results shed some light on the role of agglomeration and chain formation in FR. Taking into account the flexibility of the liquid form (including the long-term stability and no-sedimentation property) in order to predict its optical behavior correctly, and the low-magnetic-field requirements, these fluids can be exploited for the fabrication of a wide range of applications in magneto-optics.
Acknowledgements
The authors wish to thank the Dokuz Eylul University, the Center for Fabrication and Application of Electronic Materials (EMUM) for providing technical support. We are grateful for the many helpful discussions with Dr. Erdal £elik and Dr. Omer Mermer.
7	REFERENCES
1	J. Ge, Y. Hu, Y. Yin, Angew. Chem. Int. Ed., 46 (2007), 7428-7432
2	P. H. C. Camargo, Z. Y. Li, Y. Xia, Soft Matter, 3 (2007), 1215-1222
3	K. Butter, P. H. H. Bomans, P. M. Frederik, G. J. Vroege, A. P. Phi-lipse, Nature Materials, 2 (2003), 88-91
4	A. Yethiraj, A. van Blaaderen, Nature, 421 (2003), 513-517
5	M. Klokkenburg, C. Vonk, E. M. Claesson, J. D. Meeldijk, B. H. Erne, A. P. Philipse, J. Am. Chem. Soc., 51 (2004), 16706-16707
6	A. P. Hynninen, M. Dijkstra, Phys. Rev. Lett., 94 (2005), 138303
7	M. Klokkenburg, B. H. Erne, J. D. Meeldijk, A. Widenmann, A. V. Petukhov, R. P. A. Dullens, A. P. Philipse, Phys. Rev. Lett., 9 (2006), 185702
8	A. K. Agarwal, A. Yethiraj,. Phys. Rev. Lett., 102 (2000), 198301
9	T. J. Yoon, K. N. Yu, E. Kim, J. S. Kim, B. G. Kim, S. H. Yun, B. H. Sohn, M. H. Cho, J. K. Lee, S. B. Park, Small 2 (2006) 2, 209-215
10	J. Lee, Y. Lee, J. K. Youn, H. B. Na, T. Yu, H. Kim, S. M. Lee, Y. M. Koo, J. H. Kwak, H. G. Park, H. N. Chang, M. Hwang, J. G. Park, J. Kim, T. Hyeon, Small 4 (2008) 1, 143-152
11	J. H. Lee, Y. M. Huh, Y. W. Jun, J. W. Seo, J. T. Jang, H. T. Song, S. Kim, E. J. Cho, H. G. Yoon, J. S. Suh, J. Cheon, Nature Medicine, 13 (2007) 1, 95-99
12	J. H. Lee, K. Lee, S. H. Moon, Y. Lee, T. Gwan Park, J. Cheon, All-in-One Target-Cell-Specific, Angew. Chem. Int. Ed., 48 (2009), 4174-4179
13	C. Xu, K. Xu, H. Gu, X. Zhong, Z. Guo, R. Zheng, X. Zhang, B. J. Xu, Am. Chem. Soc., 126 (2004), 3392-3393
14	A. H. Lu, E. L. Salabas, F. Schuth, Angew. Chem. Int. Ed., 46 (2007), 1222-1244
15	X. Zhao, R. Tapec-Dytioco, K. Wang, W. Tan, Anal. Chem., 75 (2003), 3476
16	S. V. Sonti, A. Bose, Colloids and Surfaces B-Biointerfaces, 8 (1997) 4-5, 199-204
17	S. Liu, E. Li, Q. Zhou, IEEE 3rd Int. Conf. on Bio. Eng. and Inf., (2010), 1532-1535
18	S. Pu, M. Dai, G. Sun, Opt. Comm., 283 (2010), 4012-4016
19	C. Y. Hong, J. Mag. Mag. Mat., 201 (1999), 178-181
20L. Martinez, F. Cecelja, R. Rakowski, Sens. Act. A, 123-124 (2005), 438-443
21	J. Ge, Y. Hu, M. Biasini, W. P. Beyermann, Y. Yin, Angew. Chem. Int. Ed., 46 (2007) 23, 4342-4345
22	N. A. Yusuf, J. Appl. Phys., 64 (1988), 5
23	Z. D. Xianfeng, Appl. Phys. Lett., 89 (2006), 211106
24	Y. T. Pan, J. Appl. Phys., 73 (1993), 10
25	X. Fang, Nanoscale Research Letters, 6 (2011), 237
26	M. M. Maiorov, J. Magnetism and Magnetic materials, 252 (2002), 111-113
27	F. Royera, D. Jamona, J. J. Rousseaua, V. Cabuila, D. Zinsa, H. Rouxa, C. Boviera, The European Physical Journal Applied Physic, 22 (2003) 2, 83-87
28	N. A. Yusuf, A. A. Rousan, H. M. El-Ghanem, J. Appl. Phys., 64 (1988), 2781
29	N. A. Yusuf, I. Abu-Aljarayesh, A. A. Rousan, H. M. El-Ghanem, IEEE Trans. on Magn., 26 (1990), 5
30	R. E. Rosensweig, Nature, 210 (1966), 613
31	M. I. Shliomis, Zh. Eksp. Teor. Fiz., 61 (1971), 2411
32	D. H. Lee, R. A. Condrate, J. S. Reed, J. Mater. Sci., 31 (1996), 471
33	H. Li, C. P. Tripp, Langmuir, 21 (2005), 2585
34	D. Lin, W. Shibin, L. Sen, IEEE Ann. Rep. Conf. on El. Ins. and Diel. Pheno., 2010
35	P. C. Scholten, IEEE Trans. Magn., 16 (1980), 221
36	P. G. De Gennes, P. A. Pincus, Phys. Condens. Matter, 11 (1990), 189
37	R. E. Rosensweig, Magnetic Fluids, Annual Reviews
38	M. Wu, Y. Xiong, Y. Jia, H. Niu, H. Qi, J. Ye, Q. Chen, Chem. Phy. Lett., 401 (2005), 374-379
39	H. E. Horng, C. Y. Hong, S. Y. Yang, H. C. Yang, J. Phy. and Chem. of Sol., 62 (2001), 1749-1764
40	E. Hecht, Optics, 4th ed., Addison Wesley
41	F. Royer, D. Jamon, J. J. Rousseau, V. Cabuil, D. Zins, H. Roux, C. Bovier, Eur. Phys. J. AP, 22 (2003), 83-87
42	N. A. Yusuf, J. Phys. D: Appl. Phys., 22 (1989), 1916-1919
43	A. A. Rousan, H. M. El-Ghanem, N. A. Yusuf, IEEE Trans. on Mag., 25 (1989) 4, 3121-3124
44	N. A. Yusuf, I. Abu-Aljarayesh, A. A. Rousan, H. M. El-Ghanem, IEEE Trans. on Mag., 26 (1990) 5, 2852-2855
45	G. A. Jones, H. Niedoba, J. of Mag. and Mag. Mat., 73 (1988), 33 46W. Syed, D. A. Hammer, M. Lipson, Opt. Let., 34 (2009) 7,
1009-1011
47 D. Heinrich, A. R. Goni, A. Smessaert, S. H. L. Klapp, L. M. C. Cerioni, T. M. Osan, D. J. Pusiol, C. Thomsen, Phys. Rev. Lett., 106 (2011), 208301