Temperature study of FMR
spectra of
0.30(Fe O )/0.70(ZnO)
2
3
1
1
1
K. Wardal , J. Typek , G. Zolnierkiewicz ,
N. Guskos , U. Narkiewicz
1, 2
3
1
Institute of Physics, Faculty of Mechanical Engineering and Mechatronics,
West Pomeranian University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland
2Solid State Physics, Department of Physics, University of Athens Panepistimiopolis, 15 784 Zografos, Greece
3
Institute of Chemical and Environmental Engineering,
West Pomeranian University of Technology, K. Pulaskiego 10, 70-322 Szczecin, Poland
The study of the FMR signal attributed to ZnFe O nanoparticles in the superparamagnetic phase has shown that:2 4
a) magnetic anisotropy is small in the high temperature range, T>60 K, changes sign at ~50 K and increases drastically with decrease in
temperature below 40 K;
b) in the low temperatures range, T<25 K, magnetic anisotropy calculated from the difference of the parallel and perpendicular resonance
n
fields shows power law dependence on temperature, (H1-H2)~T , with the exponent n=-1.86;
c) in the high temperature range, T>60 K, the linewidths of both components change insignificantly with temperature, but below T~50 K a
big increase of linewidth with temperature decrease is registered. A power law dependence on temperature is also observed for both linewidths, with different values of the exponent (-1.25 and -0.25);
d) the integrated intensity of the FMR signal varies slightly with temperature in the high temperature range (T>60 K), but increases sharply
with temperature decrease below 50 K. Below 30 K it follows the Curie-Weiss law with T =-3.5 K indicating on the prevailing CW
antiferromagnetic interactions in the spin system.
The obtained results indicate that in the high temperature range, T>50 K, the investigating spin system consists of interacting magnetic nanoparticles and in the low temperature range, T<50 K, the spins on the surface of nanograins start to freeze. The freezing temperature is below 4 K, the lowest temperature reached in FMR experiments.
Conclusions
We h a v e m a d e t e m p e r a t u r e F M R s t u d y o f t h e
0.3(Fe O )/0.7(ZnO) nanocomposite containing ZnFe O 2 3 2 4
nanoparticles in order to determine the existence of magnetic anisotropy and reveal the presence of magnetic interactions between nanoparticles. Previous dc and ac magnetization study indicated on the spin-glass state of the surface spins and we will try to find evidences in FMR experiment.
Aim of the work
The investigated sample was synthesized using the wet chemistry method. A mixture of iron and zinc hydroxides was obtained by adding
ammonia solution to 20% solution of a proper amount of Zn(NO ) ·6H O and Fe(NO ) ·4H O in water. The obtained hydroxides were filtered, dried 3 4 2 3 3 2
and calcined at 573 K during 1 hour. The details of synthesis are presented elsewhere.
The obtained sample, designated as 0.3(Fe O )/0.7(ZnO), was characterized by means of X-ray diffraction which revealed the presence of 2 3
only two phases: ZnO (nanoparticles with an average size of 51 nm) and ZnFe O (11 nm).2 4
Magnetic resonance measurements were performed on a conventional X-band (í=9.4 GHz) Bruker E 500 spectrometer with 100 kHz magnetic filed modulation. Samples containing around 20 mg of nanopowder were placed in 4 mm quartz tubes. The temperature measurements of FMR spectra were performed in the temperature range of 4-290 K using an Oxford helium-flow cryostat.
Experimental
Fig. 1. FMR spectra of 0.30(Fe O )/0.70(ZnO) nanocomposite registered at different 2 3 temperatures in the 4-300 K range.
0 50 100 150 200 250 300 -1 0 1 2 3 4 Temperature [K] D if fe re n c e in re s o n a n c e fi e ld s H 1 -H 2 [k G ] 0 50 100 150 200 250 300 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Temperature [K] R e s o n a n c e fi e ld [k G ] H1 H2 50 100 150 200 250 300 0 2 4 6 8 10 12 14 16 DH1+DH2 DH1 DH2 Temperature [K] L in e w id th [k G ] 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.0 2.5 3.0 3.5 4.0 4.5 L o g (D H ) Log T 0 50 100 150 200 250 300 0.0 0.5 1.0 1.5 2.0 2.5 In te g ra te d In te n s it y [a rb . u n it s ] Temperature [K] 5 10 15 20 25 30 35 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1 /I n te g ra te d In te n s it y [a rb . u n it s ] Temperature [K]
Fig. 2. Experimental (black) and fitted (blue) spectra of 0.30(Fe O )/0.70(ZnO) nanocomposite at 6 K (top panel), 2 3
120 K (middle panel), and 250 K (bottom panel).
0
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Magnetic field
[kG]
0 1 2 3 4 5 6 7 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 2 3 4 5 -30 -20 -10 0 10 20 30 2 3 4 5 -30 -20 -10 0 10 20 30 6K A m p lit u d e [a rb . u n it s ] Field [kG] A m p lit u d e [a rb . u n it s ] Field [kG] 120K A m p lit u d e [a rb . u n it s ] Field [kG] 250KFig. 4. Temperature dependence of the resonance fields of two components. The inset shows temperature dependence of the
resonance field difference.
Fig. 6. Temperature dependence of the linewidths of two components and their sum (blue). The inset shows this dependence in double logarithmic scale.
Fig. 7. Temperature dependence of the FMR integrated intensity. The inset shows temperature dependence of the reciprocal integrated
intensity at low temperatures. The straight line is the best least-square fit to the Curie-Weiss law.
1,8 2,1 2,4 2,7 3,0 3,3 5,5 6,0 6,5 7,0 7,5 8,0 8,5 L n ( ( H 1 -H 2 ) [G ]) Ln(T[K]) n=-1.86(7) dH~Tn T=24.6 K T=6 K
Fig. 3. Decomposition of the observed spectrum at T= K on two components LL lines. The lines are shifted in an external magnetic field due to diffrent orientations of
nanoparticle magnetization with respect to magnetic field.
Fig. 5. Double logaritmic plot of the temeperature dependence of the difference of resonance fields of two components.
