Physica B 405 (2010) 4054–4058
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Physica B journal homepage: www.elsevier.com/locate/physb
Magnetic study of Fe2O3/ZnO nanocomposites N. Guskos a,b, S. Glenis a, G. Zolnierkiewicz b, J. Typek b,n, D. Sibera c, J. Kaszewski c, D. Moszyn´ski c, W. Łojkowski d, U. Narkiewicz c a
Solid State Section, Department of Physics, University of Athens, Panepistimiopolis 15-784, Greece Institute of Physics, Pomeranian University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland c Institute of Chemical and Environmental Engineering, Pomeranian University of Technology, Al. Piastow 17, 70-310 Szczecin, Poland d Institute of High Pressure Physics of the Polish Academy of the Sciences, Soko!owska 29/37, 01-142 Warszawa, Poland b
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 May 2010 Accepted 24 June 2010
Fine particles of Fe2O3/ZnO were synthesized by wet chemical method. The morphological and structural properties of the mixed system were investigated by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The major phase was determined to be the cubic spinel phase of g-Fe2O3 maghemite with mean crystalline size of about 20 nm together with small amounts of hexagonal ZnO and ZnFe2O4. The magnetic properties of the material were investigated by ferromagnetic resonance (FMR) in the temperature range from liquid helium to room temperature. An asymmetric and very intense FMR signal was recorded exhibiting strong shift to low magnetic fields with decrease in temperature. Analysis of the FMR spectra in terms of two separate line components indicates the presence of strongly anisotropic interactions. & 2010 Elsevier B.V. All rights reserved.
Keywords: Ferromagnetic resonance Nanopowders
1. Introduction Iron oxides are among the most important compounds very helpful in the study of magnetic interactions in the nanoscale range [1–6]. Nanostructured materials consisting of magnetic nanoparticles embedded in different non-magnetic matrices have been intensively investigated recently [7–11]. Introduction of small amounts of magnetic nanoparticles in polymers has been reported to affect the glass and melting temperatures of the polymer matrix [12]. Ferromagnetic resonance (FMR) is a powerful method to study interparticle interactions, mainly of dipolar and to a lesser extent exchange/superexchange origin, providing information on the dynamic properties of the matrix and nanoparticle assemblies [13]. In this respect, it will be very interesting to study the temperature dependence of FMR spectra for samples with high concentration of magnetic nanoparticles in which magnetic interactions should be very intense. Recently, we have prepared fine magnetic particles of the composition n(Fe2O3)/(1 n)ZnO (0on o1). For high values of index n the composites are dominated by the iron oxide phase [14]. The aim of this work is to study the magnetic properties by means of ferromagnetic resonance of fine magnetic particles of 0.95(Fe2O3)/0.05ZnO prepared by wet chemical method. The morphology and phase composition of the powders have been identified by scanning electron microscopy, X-ray diffraction
n
Corresponding author. E-mail address:
[email protected] (J. Typek).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.06.055
(XRD), inductively coupled plasma atomic emission (ICP-AES) and X-ray photoelectron spectroscopy (XPS).
2. Experimental The mixture of iron and zinc hydroxides was obtained by addition of ammonia solution to 20% solution of proper amount of Zn(NO3)2 6H2O and Fe(NO3)3 4H2O in water. The obtained hydroxides were filtered, dried and calcined at 300 1C for 1 h. The morphology of the obtained samples was investigated by using scanning electron microscopy (SEM—LEO 1530). The phase composition of samples was determined using XRD (CoKa radiation, X’Pert Philips). The mean crystallite size was determined using Scherrer’s formula. The real chemical composition of samples was determined using inductively coupled plasma atomic emission spectroscopy (Yvon-Jobin, France). The specific surface area of the nanopowders was determined by the BET method (nitrogen adsorption) using Gemini 2360 of Micromeritics. The helium pycnometer AccuPyc 1330 of Micromeritics was applied to determine the density of powders. XPS spectra were acquired using Prevac system with Scienta SES 2002 electron analyser. Analyser worked at fixed-analyser transition mode (FAT) with a constant pass energy of 50 eV. As an excitation source a dual anode X-ray lamp was used. MgKa radiation of energy 1253.7 eV was utilized. Charging effect was determined and corrected assuming that O1s line is placed at 530.0 eV.
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corresponding to hexagonal ZnO (blue vertical line) and ZnFe2O4 (red vertical line) are also traced (Fig. 1b). Peaks corresponding to ZnFe2O4 are not clearly visible because of a very low content of this phase, but their presence was shown in our former paper [14] where the content of ZnO was 50–95 wt%. The mean crystallite size of the Fe2O3 phase, calculated by the Scherrer formula, was found to be 20 nm. SEM images reveal the agglomerated structure of the synthesized material, as shown in Fig. 2. The agglomerates have a mean size of about 60 nm and are bound to each other creating large structures. The powder exhibits homogeneous structure, with a distribution of the agglomerates’ sizes in a narrow range. Most likely the observed structures originate from the Fe2O3 phase. XPS spectra were acquired for pure ZnO sample and the ZnO/ Fe2O3 material with 5% of ZnO. In Fig. 3 XPS Zn2p3/2 spectra for those samples are shown. The Zn2p3/2 line for pure ZnO (Fig. 3a) has a maximum at 1021.0 eV, which agrees well with previous reports [15–17]. Its FWHM (full width at half maximum) is only 1.56 eV. The Zn2p3/2 line for mixed oxides is shifted to higher energies with a maximum at 1021.66 eV. Moreover the peak is much broader with FWHM amounting to 2.28 eV. The shift and
The measurements of magnetic resonance spectra were performed on a conventional X-band (n ¼9.4 GHz) Bruker E500 spectrometer with 100 kHz magnetic field modulation. Samples containing around 20 mg sample powder were placed in 4 mm diameter quartz tubes. The FMR thermal studies were performed in the temperature range 4–300 K using an Oxford helium-flow cryostat.
3. Results and discussion The application of high temperatures in the co-precipitation/ calcination synthesis of oxides is the main reason for grain agglomeration. To avoid this process a low calcination temperature of 300 1C was used. Table 1 summarizes the results of the density and specific surface area measurements together with sample chemical composition determined by ICP-AES measurements, in perfect agreement with the nominal one calculated from the initial concentration of the precursor salts. XRD analysis has shown that the main phase is the cubic spinel g-Fe2O3 (Fig. 1a). However, low intensity diffraction peaks
Table 1 Characteristic parameters of 0.95(Fe2O3)/0.05ZnO sample. Nominal concentration Fe2O3 (%wt)
Nominal concentration ZnO (%wt)
Density (g/cm3)
Surface area (m2/g)
Measured concentration Fe2O3 (%wt)
Measured concentration ZnO (%wt)
95
5
4.8
85
95.4
4.6
-Fe2O3 Intensity [arb. units]
Intensity [arb. units]
-Fe2O3
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 2 theta [deg]
31
32
33
34 35 36 2 theta [deg]
37
38
Fig. 1. XRD pattern showing the most intense peaks’ 2Y positions of ZnO (blue line) and ZnFe2O4 (red line). Red squares mark Fe2O3 peaks. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. SEM images of the 0.95(Fe2O3)/0.05ZnO nanopowder.
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20000 18000
Intensity [CPS]
16000 14000 12000 10000 8000 6000 4000 2000 1030
1028
1026
1024
1022
1020
1018
1016
1020
1018
1016
BE [eV]
4200
Intensity [CPS]
4000 3800 3600 3400 3200 1030
1028
1026
1024 1022 BE [eV]
Fig. 3. XPS Zn2p3/2 spectra of: (a)pure ZnO and (b)0.95(Fe2O3)/0.05ZnO.
broadening suggests the existence of an internal structure. In Fig. 3b the result of this line deconvolution as two components is shown: a low binding energy component (max. 1021.06 eV, FWHM¼1.56 eV) and a high binding energy component (max. 1022.02 eV, FWHM¼1.56 eV). The existence of two electronic forms of Zn2 + ion can be justified assuming two different coordinations in the crystalline lattice. Previous report [17] indicates that the low binding energy component corresponds to the tetrahedrally coordinated Zn2 + ion and the high binding energy component—to the octahedrally coordinated Zn2 + ion. Zinc ferrite can be a normal or an inverse spinel, and the difference is in coordination of the zinc ion. In Fe2O3/ZnO system the presence of octahedrally coordinated zinc ions proves the existence of an inverse spinel structure. Therefore we suggest that our samples contain inverse spinel structure and normal spinel structure of zinc oxide. The area ratio for Zn2p3/2 components equals to 0.8 (tetrahedral to octahedral zinc ion). Fig. 4 presents the FMR spectra at different temperatures in the 4–290 K range. The FMR spectra are dominated by a very intense asymmetrical and broad line, which could be fitted effectively by two Lorentzian lines (line 1 and line 2). As an example, the fitting at T¼90 K is presented in Fig. 4c. At room temperature the resonance lines are centered at g1 ¼2.026(2) (Hr ¼3340(1) G) with peak-to-peak linewidth DHpp ¼342(5) G and g2 ¼2.174(2)
(Hr ¼3118(1) G) with peak-to-peak linewidth DHpp ¼675(5) G for lines 1 and 2, respectively. The FMR spectra could arise from different agglomerate states of the iron oxide system as shown by the XPS measurements. The FMR resonance lines may be accordingly attributed to g-Fe2O3 magnetic nanoparticles, with their orientation parallel and perpendicular to the applied magnetic field. The ratio of integrated intensities for these two components I1/I2 is approximately 2:3 at room temperature (Fig. 5d). Fig. 5 presents the temperature dependence of FMR parameters for two resonance lines. In both cases, a strong temperature dependence of the resonance line position, linewidth and integrated intensity is observed. The temperature variation of the resonance lines and linewidths is similar, but the temperature gradient DHr/DT (ratio of resonance field change vs. temperature) is larger for the line 2 at high and low temperature regions (Fig. 5a). Three temperature ranges with large differences are observed for the DHr/DT gradient: from 290 to 60 K, DHr/DT¼3.7(1) and DHr/DT¼8.3(1) G/K, from 60 to 40 K plateau is observed, DHr/DT¼0 for both resonance lines, and below 40 K, DHr/DT¼40.5(1) and DHr/DT¼49.6(1) G/K for lines 1 and 2, respectively. The reorientation processes of magnetic moments for the resonance line 2 are more intense, especially at high temperatures (DHr/DT(line 2)/DHr/DT(line 1) ¼2.3 above 60 K), while at low temperatures (DHr/DT(line 2)/DHr/DT(line 1) ¼1.2 below 40 K). With decrease in temperature the magnetic anisotropy increases essentially because of intense reorientation processes at high temperatures. Previous FMR results on g-Fe2O3 nanoparticles embedded in a polymer matrix have shown that the above gradients are smaller than 1 G/K at high temperatures and smaller than 6.5 G/K at low temperatures, depending on the particle concentration and the degree of agglomeration [18,19]. This indicates that the polymer matrix effectively possess higher ‘‘viscosity’’ for the reorientation processes of magnetic nanoparticles. On the other hand, the FMR lines for a-Fe and Fe3C agglomerates have been reported to shift more rapidly to low magnetic fields wih the decrease of temperature, the largest shift of the high field FMR line being observed below 60 K [20]. The observed temperature dependence of the FMR spectra for 0.95(Fe2O3)/0.05ZnO implies that the spin system enters into a strongly correlated state at low temperatures. This behaviour is further supported by the temperature dependence of the FMR linewidth (Fig. 5b). Specifically, the FMR linewidth increases with decrease in temperature, whereas below 42 K begins to decrease drastically correlating with the sharp variation of the resonance field temperature gradient DHr/DT. In this temperature range, linewidth broadening by dipole–dipole interactions is likely to be small, while exchange interactions could play an important role below 42 K where magnetic ordering processes may take place. The temperature dependence of the amplitude and integrated intensity differs considerably between the two FMR lines (Fig. 5c and d). The resonance line 2 exhibits more intense temperature variation than line 1 (Fig. 5d), which suggested stronger magnetic interaction processes. The decrease of the integrated intensity with temperature decrease in high temperature range (above 270 K) is a characteristic behaviour for low concentration of magnetic nanoparticles (or clusters) in the form of agglomerates embedded in a non-magnetic matrix [18]. The intensity of line 2 is increasing drastically with temperature decrease below 220 K and reaches a maximum value at about 100 K (Fig. 5d) followed by a drastic decrease at the lowest temperatures. The line 1 exhibits a smoother variation of integrated intensity with temperature. High concentration of magnetic nanoparticles in 0.95(Fe2O3)/0.05ZnO sample allows for the possibility of different reorientation processes of the magnetic moments that could be responsible for the strongly anisotropic magnetic interactions.
N. Guskos et al. / Physica B 405 (2010) 4054–4058
90 K
dχ"/dH [Arb. units]
1 0 -1 -2 -3 -4
4.4 K
-5 0
10 8 6 4 2 0 -2 -4 -6 -8 -10 -12 -14
dχ"/dH [Arb. units]
4.4K 5K 6K 8K 10K 12K 16K 20K 24.7K 30.9K 35.9K 40K 45K 50K 55K 60K 65K 70K 75K 80K 86K 90K
2
290 K
ΔT = 10 K
90 K
1000 2000 3000 4000 5000 6000 7000 Magnetic field H [G]
0
1000 2000 3000 4000 5000 6000 7000 8000 Magnetic field H [G]
4057
3 2
Exp Line1 Line2 Line 1+2
dχ "/dH [Arb. units]
1 0 -1 -2 -3 -4 -5 0
1000
2000
3000
4000
5000
6000
7000
Magnetic field H [Gs] Fig. 4. FMR spectra of 0.95(Fe2O3)/0.05ZnO sample at different temperatures: (a) low temperature range (To 90 K), (b) high temperature range (T490 K) and (c) fitting of experimental spectrum at T¼ 90 K by two lines.
line 1
3000
Linewidth ΔH1/2pp [G]
Resonance field Hr [G]
3500
2500 line 2
2000 1500 1000 500 0 0
50
100 150 200 250 Temperature T [K]
1100 1000 900 800 700 600 500 400 300
300
Integrated intensities Iintegr [Arb. units]
Amplitude App [Arb. units]
line 1 10 5 line 2
0 0
50
100 150 200 250 Temperature T [K]
300
line 1
0
20 15
line 2
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
50
100 150 200 250 Temperature T [K]
300
line 2
line 1
0
50
100 150 200 250 300 Temperature T [K]
Fig. 5. Temperature dependence of the FMR parameters for two components (line 1 and line 2) of the 0.95(Fe2O3)/0.05ZnO spectra: (a) resonance field Hr, (b) linewidth 2 DHpp, (c) amplitude App and (d) integrated intensity App DHpp .
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4. Conclusions Fine particles of Fe2O3/ZnO were prepared by a wet chemical method and characterized by SEM, ICP-AES, XRD and XPS. The magnetic properties of the material were investigated by ferromagnetic resonance (FMR) over a wide temperature range. An asymmetric and very intense FMR signal was recorded that shifts rapidly to low magnetic fields with decrease in temperature. Analysis of the FMR spectra in terms of two separate line components reveals a strong though distinct temperature dependence of the FMR parameters for the two lines. This behaviour indicates the appearance of intense reorientation processes at low temperatures that could be connected with the anisotropy in the interparticle interactions and the emergence of a magnetically ordered state. References [1] [2] [3] [4]
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