FMR investigation of Fe2O3 nanoparticles doped with ZnO

(1)

FMR investigation of Fe O

₂

₃

nanoparticles doped with ZnO

1,2

2

3

3 N. Guskos , G. Zolnierkiewicz , J. Typek , D. Sibera , J. Kaszewski ,

3

4

3

2 D. Moszyñski ,W. £ojkowski , U. Narkiewicz , and A. Guskos

1

Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784, Greece;

2

Institute of Physics, West Pomeranian University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland;

3

Institute of Chemical and Environmental Engineering, West Pomeranian University of Technology, Al. Piastow 17, 70-310 Szczecin, Poland;

4

Institute of High Pressure Physics of the Polish Academy of the Sciences, Soko³owska 29/37, 01-142 Warszawa

20 25 30 35 40 45 50 55 60 65 In te n s it y [a rb . u n it s ] 2 theta [deg] -Fe 2O3 20 25 30 35 40 45 50 55 60 65 In te n s it y [a rb . u n it s ] 2 theta [deg] -Fe₂O₃ 534 532 530 528 526 200 400 600 800 1000 1200 C o u n ts BE [eV] O1s 1026 1025 1024 1023 1022 1021 1020 1019 1018 1017 3200 3400 3600 3800 4000 4200 C o u n ts BE [eV] Zn2p3/2 730 725 720 715 710 705 1200 1400 1600 1800 2000 2200 2400 Fe2p1/2 C o u n ts BE [eV] Fe2p3/2 1000 2000 3000 4000 5000 6000 7000 -80000 -60000 -40000 -20000 0 20000 40000 d c "/ d H [A rb . u n it s ] Magnetic field H [Gs] 4.4 K 140 K 1000 2000 3000 4000 5000 6000 7000 -150000 -100000 -50000 0 50000 100000 d c "/ d H [A rb . u n it s ] Magnetic field H [Gs] 90 K 290 K DT=10 K 0 50 100 150 200 250 300 0 500 1000 1500 2000 2500 3000 3500 0 50 100 150 200 250 0 500 1000 1500 2000 2500 3000 3500 R e s o n a n c e fi e ld Hr [G s ] Temperature T [K] R e s o n a n c e fi e ld H r [G s ] Temperature T [K] line 1 line 2 0 50 100 150 200 250 300 400 600 800 1000 0 50 100 150 200 250 0 50 100 150 200 250 300 A m p li tu d e App [A rb . u n it s ] Temperature T [K] L in e w id th D H 1 /2 p p [G s ] Temperature T [K] line 1 line 2 0 50 100 150 200 250 300 0 50000 100000 150000 200000 0 50 100 150 200 250 0 50 100 150 200 250 300 A m p lit u d e A p p [A rb . u n its ] Temperature T [K] A m p lit u d e Ap p [A rb . u n it s ] Temperature T [K] line 1 line 2 0 50 100 150 200 250 300 0.00E+000 1.00E+010 2.00E+010 3.00E+010 4.00E+010 0 50 100 150 200 250 0,00E+000 5,00E+012 1,00E+013 1,50E+013 In te g ra te d in te n s it ie s b o th lin e s Iinte gr [A rb . u n it s ] Temperature T [K] In te g ra te d in te n s it ie s Iinte g r [A rb . u n it s ] Temperature T [K] line 1 line 2

Abstract

Fine particles of Fe O doped with ZnO (5 wt. %) were prepared

₂ ₃

by wet chemistry method. According to XRD analysis the phase

composition of the sample was dominated by Fe O phase. The

₂ ₃

mean crystalline size of Fe O was about 20 nm. The

₂ ₃

ferromagnetic resonance (FMR) investigations of the obtained

samples have been carried out in the temperature range from

liquid helium to room temperature. The asymmetrical and very

intense magnetic resonance line was recorded in the whole

temperature range. Its resonance field shifted towards low

magnetic fields with decreasing temperature. Very good fitting

by Lorentzian function has been obtained with two lines due to a

strong anisotropic magnetic interaction. Analysis of the

resonance line has shown domination of the blocking processes

in the spin system leading to a preferred ordering in the direction

of the z axis at ~100 K.

Results and discussion

The high temperature in coprecipitation-calcination synthesis of oxides is the main reason of agglomeration of the

o

grains. To avoid this process low calcination temperature is needed. For this reason a temperature of 300 C was used. Density and specific surface area measurements were conducted. The obtained results are shown in Table 1. The real chemical composition of the sample determined by AES ICP technique is almost the same as that calculated on the basis

of initial amount of salts (Table 1). XRD analysis had shown that the main phase in the sample is ã-Fe O (Figure 1), 2 3

however low intensity peaks belonging to other phases are visible. Figure 2 shows 2È positions of ZnO (blue vertical

line) and ZnFe O (red vertical line). Mean crystallite size of Fe O phase calculated by Scherrer method was 20 nm. SEM 2 4 2 3

images (Figure 3) reveals agglomerated structure of obtained material. Agglomerates have size of about 60 nm and are bound to each other creating large structures. The powder exhibits homogeneous structure, with distribution of

agglomerates' sizes in narrow range. Most likely observed structures originate from Fe O phase.2 3

XPS spectra are shown in Figure 4. Charging of sample was determined using C1s line (286,5 eV). O1s spectrum reveals two chemical states of oxygen in the sample. The line around 530,0 eV is responsible for oxygen ions in the oxide lattice. The line located around 532,1 eV corresponds to oxygen ions on surface – OH groups. At the side of lower binding

3 2+

energies ghost peak is seen. Zn2p / spectrum shows two chemical surroundings of Zn ion in the sample, indicating that 2

part of the zinc ions are present in the inverse spinel structure. Higher binding energy line is related to octahedrally

2+ 2+

coordinated Zn ion and lower binding energy line is responsible for tetrahedrally coordinated Zn ion. In the Fe2p spectrum shake up is located at 718,6 eV. For iron three chemical states are suggested. Lines at 710,3 eV and 711,7 eV

3+ 3+

correspond to Fe ions tetrahedrally and octahedrally coordinated, respectively. Tetrahedrally coordinated Fe is present

in the inverse spinel structure as well as in Fe O .2 3

Figure 5 presents the temperature dependence of the FMR spectra in the range of 4-290 K. The FMR spectra is dominated by very intense antisymmetric and broad line which could be satisfactory fitted by two Lorentzian lineshape

lines. The resonance lines are centered at g =2.026(2) (H =334.0(1) mT) with peak-to-peak linewidth Deff r H =67.5(5) mT pp

and g =2.174(2) (H =311.8(1) mT) with peak-to-peak linewidth Deff r H =34.2(5) mT at room temperature. The FMR pp

spectra could be the result of different agglomerated states of irons ions as it was showing from XPS measurements. It forms strongly magnetically anisotropic system.

Figure 6 presents temperature dependence of FMR parameters of two resonance lines. In both cases a strong temperature dependence of the resonance fields, linewidths and integrated intensities are observed. The change of the

resonance fields and linewidths shows almost the same character but the gradient DH /Dr T (ratio vs temperature resonance

field) is greater for the second line in high and low temperatures (Fig. 6a). Three temperature regions with great

differences in values of the gradient DH /Dr Tis are observed: from 290 K to 60 K DH /Dr T=3.7(1) Gs/K and DH /Dr T =8.3(1)

Gs/K, from 60 to 40 is a plateau with DH /Dr T =0 for the both resonance lines, below 40 K DH /Dr T =40.5(1) Gs/K and

DH /Dr T =49.6(1) Gs/K, respectively

The temperature dependence of the integrated intensities shows drastically different behaviour of the two resonance lines (Fig. 6d). Below 270 K a decreasing integrated intensity is a characteristic behaviour for the magnetic nanoparticles in clusters or agglomerates [21]. The intensity of the line 1 is increasing below 220 K with maximum value about 100 K

(Fig. 6d). This is behaviour is connected with superparamagnetic state with blocking temperature T =110 K. b

Figure 1. The XRD patterns of Fe O ₂ ₃

doped with ZnO. Peaks attributed to Fe O ₂ ₃

are marked as .¦

Figure 2. XRD pattern showing most intense peaks' 2È positions of ZnO (blue

line) and ZnFe O (red line).₂ ₄

Concentration Fe2O3 [% wt.] Concentration ZnO [% wt.] Density [g/cm3] Surface area [m2/g] Real concentracion Fe2O3 [% wt.] Real concentracion ZnO [% wt.] 95 5 4,8 85 95,4 4,6

Figure 6. Temperature dependence of the FMR parameters, (a) resonance field H , (b) linewidth ÄH ,_r _pp

2

(c) amplitude A and (d) integrated intensity A ÄH ._pp _pp _pp

Figure 5. Temperature dependence of the

FMR spectra of Fe O doped with ZnO.₂ ₃ _{Figure 3. SEM images of obtained powder.}