Ni2O3 composite nanoparticles

Materials Chemistry and Physics 134 (2012) 407–411

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Preparation, magnetization, and microstructure of ionic ferrofluids based on ␥-Fe2 O3 /Ni2 O3 composite nanoparticles Lihua Lin, Jian Li ∗ , Jun Fu, Yueqiang Lin, Xiaodong Liu School of Physical Science & Technology, MOE Key Laboratory on Luminescence and Real-Time Analysis, Southwest University, Chongqing 400715, China

a r t i c l e

i n f o

Article history: Received 19 October 2011 Received in revised form 20 February 2012 Accepted 1 March 2012 Keywords: Magnetic materials Chemical synthesis Microstructure Magnetic properties

a b s t r a c t Ionic ferrofluids based on ␥-Fe2 O3 /Ni2 O3 composite nanoparticles are a polydispersed system prepared by the Massart method. The magnetization and optical relaxation behaviors of these ferrofluids show that, in addition to the ring-free micelle aggregates, there are also chainlike aggregates in the ferrofluids. The chainlike aggregation is attributed to so-called “depletion force” in the polydispersed ferrofluids because magnetic interaction between the ferrofluid particles is so weak that these particles cannot form the aggregates just by the magnetic interaction. For the ␥-Fe2 O3 /Ni2 O3 ionic ferrofluids, the “depletion force” stimulates the larger ferrofluid particles, forming short chains in the absence of a magnetic field and their macroscopic properties, e.g., magnetization and optical relaxation, all result from the short chains. Ferrofluids having chainlike aggregates alone could have excellent magneto-optical effects. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Ferrofluids such as magnetic fluids (liquid) and magnetic colloids are stable colloidal dispersions of nanosized ferro- or ferri-magnetic particles suspended in a liquid carrier. The most commonly used, chemically stable ferrites are magnetite (Fe3 O4 ) and maghemite (␥-Fe2 O3 ) [1]. The particles in ferrofluids are about 10 nm in size and may be considered to have a single-domain structure [2]. These ferrofluids are a magnetic dipolar fluid [3] and the volume fraction of the magnetic material is typically only a few percent [4]. Ferrofluids are sensitive to external magnetic fields and can exhibit novel magnetic [4], optical [5], and rheological properties [6], to name a few. Conventionally, a surfactant/polymer is used to prevent the particles from aggregating by steric repulsion and such ferrofluids are called surface ferrofluids. Massart proposed a method of preparation for aqueous ferrofluids based on ferrite nanoparticles by the charged repulsion in 1980 [7], and these ferrofluids are known as ionic ferrofluids [8], electrical double-layered ferrofluids [9], or charge-stabilized ferrofluids [10]. Ionic ferrofluids have attracted considerable attention because of their special behavior and this has led to new cross-disciplinary activities in physics, chemistry, and biomedicine [11–13].

While the interaction between the particles can be neglected, the relation between the magnetization of the ferrofluid Mf and one of the particles Mp is Mf = v Mp ,

(1)

where v is the volume fraction of the particle in the ferrofluid. According to the Langevin paramagnetic theory, Mp as a function of applied magnetic field H is expressed as Mp = Mp,s L(˛), L(˛) = coth ˛ −

(2) 1 , ˛

3

˛=

0 mp 0 Mp,s dv H= H, kB T 6kB T

where L(˛) is called the Langevin function ˛ is the Langevin parameter, Mp,s is the saturation magnetization of the particles, mp (= Mp,s dv3 /6) is the magnetic moment of a particle, dv is the volume averaged diameter of the particles, 0 is the vacuum permeability, kB is the Boltzman constant, and T is the absolute temperature. For ferrofluids, moments of the particles tend to align in the direction of the magnetic field under a high field limit, the saturation magnetization of the ferrofluids Mf,s and that of the particles Mp,s can be written as [3] Mf,s = v Mp,s .

(3)

In the low field limit, the Langevin function can be written as L(˛) = ˛/3. Thus, the initial susceptibility of the Langevin theory L is given [14] by ∗ Corresponding author. Tel.: +86 02368252355; fax: +86 02368252356. E-mail address: [email protected] (J. Li). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.009

L =

M  H

H→0

=

0 Mp,s · mp v 3kB T

(4)

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L. Lin et al. / Materials Chemistry and Physics 134 (2012) 407–411

The experiments revealed an essential deviation from the Langevin formula for ferrofluids other than very dilute ferrofluids [15], and the deviation is generally attributed to the magnetic interaction between the particles [16]. In addition, experiments have shown that the saturation magnetization of a ferrofluid Mf,s could be less v Mp,s [17–19], which could be due to the presence of self-assembled aggregates of ring-like micelle structures [19,20]. Also, the interaction between the particles would enhance the initial susceptibility of the ferrofluids [4]. Obviously, the apparent magnetization behaviors of ferrofluids are in relation to their microstructure. Generally, the electron microscope is a direct method for nanostructure observation. But, for investigation of ferrofluids structure, only information about the size of separate particles can be obtained because the structure would undergo uncontrollable change while a sample is prepared for the examination [21]. Superseding, magneto-optical effects are useful tools for probing the aggregate formation in ferrofluids or the properties of nanoparticle interaction [22,23]. In a gradient magnetic field, chainlike aggregates will move under the effects of three forces, i.e., the force towards the center from the magnetic field gradient known as the “magnetic convergent force” (MCF), the repulsive force between near chains known as the “magnetic divergent force” (MDF), and the viscosity drag force (VDF), such that the variation of the light transmitted through a ferrofluid film can behave as a relaxation process from the “geometric shadowing effect” [24,25]. In the presented work, ferrofluids based on ␥-Fe2 O3 /Ni2 O3 composite nanoparticles were prepared and the magnetization curves of the particles and ferrofluids were measured. For revealing further the relation between magnetization behavior and microstructure of the ferrofluids, the optical relaxation was measured also.

2. Experiments and results 2.1. Preparation of the ferrofluids Fe–Ni bioxide composite nanoparticles were prepared by two steps [26]. First, a precursor was synthesized using coprecipitation method. An aqueous mixture of FeCl3 (40 ml, 1 M) and Ni(NO3 )2 (10 ml, 2 M, in HCl 0.05 mol) was added to NaOH solution (500 ml, 0.7 M). The solution was heated to boiling point for 5 min with stirring. After the heating was stopped, the precursor was gradually precipitated. The second step was to add the precursor, which had been washed to pH = 7–8, to FeCl2 solution to obtain 400 ml of mixture solution. Then the mixture solution was heated to boiling for 30 min, and the particles were precipitated gradually after the heating stopped. The analysis of X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) indicated that the finally as-formed particles were ␥-Fe2 O3 /Ni2 O3 composite nanoparticles in which Ni2 O3 was formed outside the ␥-Fe2 O3 core, and the molar ration between ␥-Fe2 O3 and Ni2 O3 was about 9:1 [26]. Thus, the averaged density of the ␥-Fe2 O3 /Ni2 O3 p was estimated to be 4.941 g cm−3 using the equation p = (9 + Ni )/10, in which  is the density of ␥-Fe2 O3 as 4.899 g cm−3 and Ni is that of Ni2 O3 as 5.320 g cm−3 . To synthesize the ionic ferrofluids by the Massart method [7], the raw ␥-Fe2 O3 /Ni2 O3 particles were put into a boiling ferric nitrate solution (0.5 M), boiled for 30 min, then dehydrated and dried into powder (i.e. ferrofluid particles). Transmission electron microscopy (TEM) revealed that the ferrofluid particles were nearly spherical, with diameters ranging from 3.35 nm to 17.53 nm, as shown in Fig. 1. Statistical analysis indicated that diameter (d) of the ferrofluid nanoparticles fit a log-normal distribution, and the median particle diameter (geometric average diameter, dg ) was 10.59 nm, with a standard deviation (ln  g ) of 0.31.

Fig. 1. Typical TEM image of the ferrofluid particles. The size bar is 50 nm.

Finally, the mother ferrofluids with a mass fraction of particles m = 9.17% was synthesized by mixing the powder with HNO3 solution, then the ferrofluid with m = 3.76% was obtained by diluting the mother ferrofluids. 2.2. Measurement of the density Standardizing the magnetization measurement and concentration of colloids against unit mass is more satisfactory than standardizing against unit volume since it is easier to measure mass accurately. The magnetization relationship between per unit mass  and per unit volume M is given by M = ,

(5)

where  is the density of the measured sample. For a ferrofluid, the particle volume fraction v is an important parameter for characterizing the system [4]. The relation between particle mass fraction m and particle volume fraction v is v =

m m + (1 − m )f ·p /c

(6)

where f·p is the density of the ferrofluids particles and c is density of the carrier liquid. By directly measuring the density of both the ferrofluid and the carrier liquid, the density of the ferrofluids particles f·p can be obtained by f ·p =

m f c c − (1 − m )f

,

(7)

where f is the density of the ferrofluid. In the experiment, the densities of both the carrier liquid and the ferrofluids were measured using a density meter (DMA 35). The measured density of the carrier liquid was 0.9988 g cm−3 , and the density of the ferrofluids with m = 9.17% (mother ferrofluids) was 1.0734 g cm−3 , with m = 3.76% was 1.0281 g cm−3 . By substituting the values for the mother ferrofluids m , f and carrier liquid c into formula (7), the density of the ferrofluid particles was calculated. Also, the volume fraction of the particles in ferrofluids v was deduced from formula (6). These data are listed in Table 1. 2.3. Measurement of the magnetization The magnetization curves of  vs. H of both the particles and ferrofluids were measured with a vibrating sample magnetometer (VSM, HH-15) under a series of go-and-return magnetic field cycles at room temperature, as shown in Figs. 2 and 3. It can be

L. Lin et al. / Materials Chemistry and Physics 134 (2012) 407–411 Table 1 The characteristic parameters of the particles and the ferrofluids (f.f.).

1.0

Raw particles

f.f. particles

f.f. with m = 9.17%

f.f. with m = 3.76%

100 4.9411 54.49 269.24 269.24

100 4.1254 39.38 162.46 162.46

2.40 1.0734 3.63 3.90 162.50

0.94 1.0281 1.46 1.50 159.57

0.8

Tr

v (%)  (g cm−3 )  s (emu g−1 ) Ms (kA m−1 ) Ms /v (kA m−1 )

409

(b)

0.6

(a)

0.4

60

(a)

40

0.2

(b)

0.0

σ (emu/g)

20

400

600

800

1000

1200

2.4. The measurement of the optical relaxation

-40

-600

-400

-200

0

200

400

600

800

H (kA/m) Fig. 2. The magnetization curves of (a) the ␥-Fe2 O3 /Ni2 O3 composite particles and (b) the ferrofluid particles.

seen from the measured results that the mother ferrofluids with m = 9.17% exhibited quasi-magnetic-hysteresis behavior, i.e., its demagnetizing curve and magnetizing curve do not coincide in a high magnetic field and that the former lies above the latter. For the ferrofluids with m = 3.76%, the difference of magnetization between the demagnetizing and magnetizing process could be so small that they apparently coincided with each other. The specific saturation magnetization  s is deduced from the relation  vs. 1/H under a high magnetic field [27]. The values for  s and Ms are listed also in Table 1.

4

(a)

3 2

σ (emu/g)

200

Fig. 4. The Tr –t curves of the ferrofluids with (a) m = 9.17% and (b) m = 3.76%. The magnetic field was applied at t = 0.

-20

1

(b)

0 -1 -2 -3 -4 -800

0

t(s)

0

-60 -800

B=300Gs

-600

-400

-200

0

200

400

600

800

H (kA/m) Fig. 3. The magnetization curves of the ferrofluids with (a) m = 9.17% and (b) m = 3.76%.

The ferrofluids were injected into a glass cell 0.3 mm thick to form the films of each ferrofluid. A He–Ne laser (632.8 nm, 10 mW) was used for the light source and the light beam direction was perpendicular to the film and parallel to the magnetic field generated by the electromagnet. The field gradient was about −30 Gs mm−1 , which resulted in a small hole of optical path in the center of the electromagnet. The details of the experimental device are described in Ref. [28]. In the experiment, the field was controlled by directly taking on electric current. The variation of the relaxation of the relative transmission light intensity Tr (= I /I, in which I and I is the intensity of transmitted light after and before the application of the magnetic field, respectively) over time t is measured at a central magnetic field of 300 Gs. Using a computer, the Tr –t curves were obtained, as shown in Fig. 4. From Fig. 4, it can be seen that the light transmitted by the ␥-Fe2 O3 /Ni2 O3 ferrofluid film exhibits optical relaxation behavior similar to that of other ferrofluids, which shows the chain’s formation and motion [24,28] 3. Discussion From the measured magnetization curves of the particles, we can see that the magnetization of the ferrofluids particles was much less than that of the raw ␥-Fe2 O3 /Ni2 O3 particles. This means that in the ferric nitrate treatment, the Fe(NO3 )3 ·9H2 O could coat the outer surface of the raw particles, similar to the construction of Fe3 O4 nanoparticles treated by ferric nitrate [29]. For the spherical ferrofluid particles, neglecting the magnetization of the Fe(NO3 )3 ·9H2 O, the relationship between the magnetic core (raw ␥-Fe2 O3 /Ni2 O3 particles) diameter dm and the diameter df of the ferrofluid particles can be calculated [30] as dm /df = 0.78. Thus, while df is taken as the arithmetic diameter of the ferrofluid particles da , which is calculated by da = exp[ln  g + 0.5 ln2  g ] [31], the average thickness of the coating layer ı (=0.5df (1 − dm /df )) can be estimated as about 1.2 nm. From the measured specific saturation magnetization  s and the density , the saturation magnetization Ms of the particles and the ferrofluids can be obtained, and are listed in Table 1. Obviously, the ferrofluid saturation magnetization reduced by v is in agreement with that of the ferrofluid particles. This shows that in the ferrofluids, there is not a ring-like micelle aggregation which would result in the saturation magnetization decreasing [19,20]. To further reveal the magnetization behavior of the ferrofluids, the reduced magnetization curves of both the measured curves and

410

L. Lin et al. / Materials Chemistry and Physics 134 (2012) 407–411

200

M/φv (kA/m)

150

100

50

0

0

200

400

H (kA/m)

600

800

Fig. 5. The reduced magnetization curves of both the measurement and Langevin theory for the ferrofluids. The inset is the reduced initial magnetization curves.

the Langevin theory curve (ML / = Mf·p,s L(˛)) are drawn, as shown in Fig. 5. We found that in a low magnetization field, magnetization of the ferrofluids was larger than the Langevin theory and in the high field tended to the Langevin theory. The influence of the particle interactions on magnetic properties was most evident in a weak field, so the investigation of the initial susceptibility i was important [4]. From the initial magnetization curves (see the inset in Fig. 5), the reduced initial susceptibility i /v of the ferrofluids with v = 2.4%, 0.94% were obtained as 11.04 and 13.45, respectively, while the L /v was 2.56. Obviously, the reduced susceptibility of the ferrofluids was much larger than that of the Langevin theory. Also, the difference in the reduced susceptibility of both ferrofluids was in agreement with the conclusion that the reduced initial susceptibility of the two ferrofluids decreased with increasing concentration [32]. The magnetization behaviors and the microstructure characterization of the ferrofluids can therefore be discussed as follows. In a ferrofluid, the magnetic particles interact pairwise with each other via a long-range dipole–dipole interaction potential Um–m , which can be written as Um–m = −0 [3(mi · rij /rij )(mj · rij /rij ) − mi mj ]/4rij3 , where rij is the distance vector between particles i and j. For a polydispersed system, the maximum magnetic interaction of two particles in contact with each other Um–m, max can be written as Um–m,max = −

0 m2 3

2d

=−

2 0 dv6 Mp,s

72da3

,

(8)

where dv and da correspond to the diameter of the average volume and the arithmetic mean diameter, respectively, and can be obtained by dv = exp[ln dg + 1.5 ln2  g ] and da = exp[ln dg + 0.5 ln2  g ] [31]. For a ferrofluid, in addition to the particle volume fraction v , another parameter to characterize the system under H = 0, is the dipole coupling constant , which relates the maximum dipole–dipole interaction potential to the thermal energy =−

2 0 dv6 Mp,s 1 Um–m,max = . 3 2 kB T 144da kB T

(9)

In formula (9), a prefactor of 1/2 is considered since the coupling constant is taken from a possible particle (moment) pair. At room temperature T = 300 K, can be calculated as 0.43, so the particles in the ferrofluids cannot form aggregates from magnetic interaction because the is less than 2 [33,34]. Accordingly, ring-free micelle aggregation in the ferrofluids can be attributed to the magnetic interaction being less than the thermal effect

[35]. Nevertheless, molecular dynamics studies show that for the weak interaction range ≤ 2, i agrees with I = L (1 + L /3) or I = L (1 + L /3 + 2L /144), and while the particles aggregate, the sensitivity of the ferrofluid system to a weak external magnetic field will be amplified to enhance the initial susceptibility [34]. Therefore, it can be judged that the chainlike aggregates can still form under zero field though < 2 for the ␥-Fe2 O3 /Ni2 O3 ferrofluids since the measured i are all much larger than the L . Also, the results of the optical relaxation also give evidence of the chainlike aggregates. Obviously, the number density of the aggregates is in direct proportion to the particle concentration. Therefore, with the increase in the particle concentration, the affection of “magnetic convergent force” (MCF) and “magnetic divergent force” (MDF) enhance, respectively. For the fine chainlike aggregates, the enhancement of the MDF could be less than the MCF, so that during the relaxation process of transmission light, the maximum of the ferrofluid with v = 2.40% (m = 9.17%) is less than the ferrofluid with v = 0.94% (m = 3.76%) (see Fig. 4). Such particle aggregation could originate from a non-magnetic “depletion force” according to the bidispersed model for polydispersed ferrofluids [36]. In the model, real polydispersed ferrofluids are modeled by a bidispersed magnetic colloid composed of “large” and “small” particles, and the small particles could stimulate the large particles to combine into the chain aggregates by the so-called “depletion force.” Also, under the application of a magnetic field, the chain would lengthen by a “structured force” arising from magnetic interaction [37], resulting in higher initial susceptibility [38]. In the high field region, the chain lengthening stops and the magnetization behavior is determined by the single ferrofluid particle orientation, so that the magnetization curve of the ferrofluids approach a saturation value according to the Langevin magnetization law in which Mf = Mf·s (1 − 1/˛)[39]. The quasi-magnetic-hysteresis behavior could also result from the chainlike aggregates under zero field, which will be discussed in detail elsewhere. In addition, the magneto-optical experiment shows that for the ferrofluids with v = 2.40%, the transmitted light is still strong enough to exhibit the optical relaxation behavior, while the particle volume fraction of other ferrofluids does not exceed 1% [25,40–42]. This means that the ferrofluids based on ␥-Fe2 O3 /Ni2 O3 composite nanoparticles have higher transparency than the ferrofluids based on strong magnetic nanoparticles ( > 2), such as Fe3 O4 ferrofluids [25], CoFe2 O4 ferrofluids [40], and CoFe2 O4 -p-MgFe2 O4 binary ferrofluids [41,42]. Such high transparency could come just from the single chainlike structure in the ␥-Fe2 O3 /Ni2 O3 ferrofluids.

4. Conclusion Ionic ferrofluids based on ␥-Fe2 O3 /Ni2 O3 composite nanoparticles are a polydispersed system with a median particle diameter of 10.59 nm and a standard deviation of 0.31. The particles of the ferrofluids have a coating layer about 1.2 nm thick, which formed during the Fe(NO3 )3 treatment. The magnetic interaction between the particles is very weak so that particle aggregation cannot result from the magnetic interaction due to the influence of the thermal effect. Therefore, the chainlike aggregates, which are revealed by initial susceptibility and optical relaxation, can only be attributed to the “depletion force” and “structured force” in the polydispersed ferrofluids. And, for the same reason (weak magnetic interaction), there is not a ring-like micelle aggregate in the ferrofluids and their saturation magnetization reduced by particle volume fraction agrees with that of the ferrofluid particles. The orientation or aggregation of pre-aggregates, which are already present in the absence of a magnetic field, can produce the large magnetically induced optical anisotropies [43]. So, the ␥-Fe2 O3 /Ni2 O3 ionic ferrofluids could very sensitively respond to a magnetic field and have

L. Lin et al. / Materials Chemistry and Physics 134 (2012) 407–411

excellent magneto-optical effects because alone there is chainlike aggregative structure in such ferrofluids. These will be investigated further. Acknowledgment Financial support for this work was provided by the National Natural Science Foundation Project of PR China (No. 11074205). References [1] S. Odenbach, Ferrofluids, Springer-Verlag, Berlin/Heidelberg, 2002, p. 4. [2] B.M. Berkovsky, V.F. Medvedev, M.S. Krakov, Magnetic Fluids, Oxford University Press Inc., New York, 1993, p. 26. [3] C. Holm, J.-J. Weis, Curr. Opin. Colloid Interface Sci. 10 (2005) 133. [4] B. Huke, M. Lüke, Rep. Prog. Phys. 67 (2004) 1731. [5] P.C. Scholten, Indian J. Eng. Mater. Sci. 11 (2004) 323. [6] A. Yu Zubarev, J. Exp. Theor. Phys. 11 (2001) 323. [7] R. Massart, IEEE Trans. Magn. 17 (1981) 1247. [8] F.A. Tourinho, R. Frank, R. Massart, J. Mater. Sci. 25 (1990) 32. [9] M.H. Sousa, F.A. Tourinho, J. Depeyrot, G.T. de Silva, M.C.F.L. Lara, J. Phys. Chem. B 105 (2001) 1168. [10] G. Mériguet, M. Jardat, P. Turg, J. Chem. Phys. 121 (2004) 6078. [11] J.-C. Baci, R. Perzynski, D. Salin, J. Magn. Magn. Mater. 85 (1990) 27. [12] N. Fauconnier, J.N. Pons, J. Roger, A. Bee, J. Colloid Interface Sci. 194 (1997) 427. [13] A. Halbreich, J. Roger, J.N. Pons, D. Geldwerth, M.F. Dasilva, M. Roudier, J.C. Bacri, Biochimie 80 (1998) 379. [14] B.H. Erne, K. Butter, B.W.M. Kwipens, G.J. Vroege, Langmuir 19 (2003) 8218. [15] S. Taketomi, R.D. Shull, J. Appl. Phys. 91 (2002) 8546. [16] J. Li, Y. Huang, X.-D. Liu, Y.Q. Lin, L. Bai, Q. Li, Sci. Technol. Adv. Mater. 8 (2007) 448. [17] R.W. Chantrell, J. Popplewell, S.W. Charles, IEEE Trans. Magn. 14 (1978) 975. [18] A.E. Berkowitz, J.A. Lahut, C.E. van Buren, IEEE Trans. Magn. 16 (1980) 184.

411

[19] Y.-Q. Lin, J. Li, X.-D. Liu, T.-Z. Zhang, B.-C. Wen, Q.-M. Zhang, H. Miao, Chin. J. Chem. Phys. 23 (2010) 325. [20] S. Taketomi, R.V. Drew, R.D. Shull, J. Magn. Magn. Mater. 307 (2006) 77. [21] V.M. Buzmakov, A.F. Pshenichnikov, J. Colloid Interface Sci. 182 (1996) 64. [22] M. Rasa, J. Magn. Magn. Mater. 201 (1999) 170. [23] D. Jamon, F. Dontini, J. Monin, M. Rasa, V. Rasa, V. Socoline, O. Fillip, D. Bica, V. Sofonea, J. Magn. Magn. Mater. 201 (2009) 174. [24] J. Li, Y. Huang, X.D. Liu, Y.Q. Lin, Q. Li, R.L. Gao, Phys. Lett. A 32 (2008) 6952. [25] J. Li, X.-D. Liu, Y.-Q. Lin, L. Bai, Q. Li, X.-M. Chen, Appl. Phys. Lett. 91 (2007) 253108. [26] Q.-M. Zhang, J. Li, Y.-Q. Lin, X.-D. Liu, H. Miao, J. Alloys Compd. 508 (2010) 396. [27] R. Arulmurugan, G. Vaidyolanthan, S. Sendhilnattan, B. Jeyadevan, Physica B 363 (2005) 225. [28] J. Li, X.D. Liu, Y.Q. Lin, Y. Huang, L. Bai, Appl. Phys. B: Laser Opt. 82 (2006) 81. [29] J. Li, X.-Y. Qiu, Y.-Q. Lin, X.-D. Liu, R.-L. Gao, A.-R. Wang, Appl. Surf. Sci. 256 (2010) 6977. [30] J. Li, Y.-Q. Lin, X.-D. Liu, R.-L. Gao, S.-N. Han, Int. J. Nanosci. 4 and 5 (2008) 269. [31] C.G. Granqvist, R.A. Buhrman, J. Appl. Phys. 47 (1976) 2200. [32] M. Holmes, K. O’Grady, R.W. Chantrell, A. Bradbury, IEEE Trans. Magn. 24 (1988) 1659. [33] J.M. Taveres, J.J. Weis, M.M. Telo da Gama, Phys. Rev. E 59 (1999) 4388. [34] Z. Wang, C. Holm, H.W. Müller, Phys. Rev. E 66 (2002) 021405. [35] M. Blanco-Mantecón, K. O’Grody, J. Magn. Magn. Mater. 296 (2006) 124. [36] A. Yu Zubarev, L. Yu Iskakova, Colloid J. 6 (2003) 711. [37] A.-R. Wang, J. Li, R.-L. Gao, Appl. Phys. Lett. 94 (2009) 212501. [38] A.O. Ivanov, S.S. Kantorovich, V.S. Mendelev, E.S. Pyanzina, J. Magn. Magn. Mater. 300 (2006) e206. [39] V. Mendelev, A. Ivanov, J. Magn. Magn. Mater. 289 (2005) 211. [40] J. Li, X.-D. Liu, Y.-Q. Lin, X.-Y. Qiu, X.-J. Ma, Y. Huang, J. Phys. D: Appl. Phys. 37 (2004) 3357. [41] T.-Z. Zhang, J. Li, H. Miao, Q.-M. Zhang, J. Fu, B.-C. Wen, Phys. Rev. E 82 (2010) 021403. [42] J. Li, X.-Y. Qiu, Y.-Q. Lin, X.-D. Liu, J. Fu, H. Miao, Q.-M. Zhang, T.-Z. Zhang, Appl. Opt. 50 (2011) 5780. [43] P.C. Scholten, IEEE Trans. Magn. 16 (1980) 221.