Nanocrystalline ZnO Doped with Fe2O3 — Magnetic and Structural ...

690 I. Kuryliszyn-Kudelska et al.The advantage of micro-Raman technique is the veryaccurate chemical phases determination. The Ramanspectroscopy has already proven to be a unique tool forprobing nanophases dispersed in a matrix (see eg. [1]).The selected Raman spectra are shown in Fig. 1.The main characteristics of experimental Raman spectrumin 200 to 1600 cm −1 spectral region are sharppeak at 436 cm −1 and broad two-phonon structure at≈ 1150 cm −1 , typical for ZnO; broad structure below700 cm −1 is attributed to ZnFe 2 O 4 nanoparticles.Fig. 2. The temperature dependence of the in-phasecomponent Re(χ) of the magnetic susceptibility for severalZnO: Fe 2O 3 nanocrystalline samples in the widerange of magnetic dopant. The data were taken inexternal magnetic field H AC = 2 Oe and frequencyf = 625 Hz.TABLEThe positions of maxima T f derived at f = 625 Hz anddetermined values of parameter Φ for ZnO:Fe 2 O 3 .Fig. 1. The selected Raman spectra for nanocrystallinesamples ZnO:Fe 2O 3.The Raman spectroscopy measurements allowed to determinethe detailed structural characterization. In particular,XRD analysis did not reveal the presence of ZnOphase in samples doped with high content of magneticdopants. Despite of fact that XRD does not evident ZnO,sharp peaks at 436 cm −1 is clearly E (2)2 mode of ZnO.This peak is typical for undoped ZnO nanoparticles [6].The Raman spectroscopy studies revealed the presenceof two phases: ZnO and ZnFe 2 O 4 in the entire contentrange.2.2. Magnetic propertiesThe dynamic magnetic properties were studied bymeans of AC susceptibility χ. The real, Re(χ), as wellas imaginary, Im(χ) parts of magnetic susceptibility weremeasured using a mutual inductance method in an ACmagnetic field of frequency range 7–10 kHz and amplitudenot exceeding 5 Oe. Figure 2 shows the temperaturedependence of the real part of the AC susceptibility forselected ZnO:Fe 2 O 3 samples in the wide range of magneticdopant. The Re(χ) curves show pronounced maxima(the positions of maxima T f derived at f = 625 Hzare summarized in Table).We have observed the pronounced changes in the positionsof temperature maxima T f with the nominal con-NominalconcentrationT f [K](625 Hz)5 wt% 30.2 0.0630 wt% 33 0.0340 wt% 31.8 0.0450 wt% 34 0.0460 wt% 43.3 0.0470 wt% 34 0.04centration of Fe 2 O 3 . The measurements of AC susceptibilitywere performed at the small AC magnetic fieldwith amplitude not exceeding 5 Oe and different frequencyvalues (from 7 Hz to 9970 Hz). We observedthat the positions of the maxima shift towards highervalues with increasing driving frequency (Fig. 3). Theabove-described experimental features are expected inthe superparamagnetic system [7]. A useful criterion forclassifying the observed freezing/blocking process is theempirical parameter Φ [8, 9] that is a quantitative measureof the frequency shift and is given by the relativeshift of the peak temperature per decade shift in frequencyΦ = ∆T f /T f ∆ log 10 (f), (1)where ∆T f is the difference between the peak temperaturemeasured in the ∆ log 10 (f), and f is the AC magneticfield frequency.We obtained values in the range from 0.04 for the samplewith 70 wt% of Fe 2 O 3 up to 0.06 for the sample with5 wt% of Fe 2 O 3 (the determined values of parameter Φare gathered in Table). For the superparamagnetic sys-Φ

Nanocrystalline ZnO Doped with Fe 2 O 3 . . . 691AC susceptibility curve using the empirical parameter Φ[8, 9]. For ZnO doped with 5 wt% of Fe 2 O 3 , the results oflow-field AC susceptibility are satisfactorily explained bysuperparamagnetic model including inter-particle interactions.For higher content of magnetic Fe 2 O 3 , we haveobserved the spin-glass-like behavior.ReferencesFig. 3. The frequency dependence of the in-phase componentRe(χ) of the magnetic susceptibility for selectedZnO:Fe 2O 3 nanocrystalline sample with x = 30 wt%.tem of noninteracting nanoparticles, the frequency dependentpeaks in linear AC susceptibilities are characterizedby the so-called blocking temperature (T b ), andthe relative variation of T f = T b with frequency is between0.10 and 0.13 [7]. For interacting nanoparticles,superparamagnetic systems exhibit values of Φ between0.05 and 0.10, in spin-glass systems Φ < 0.05 [7, 8].3. ConclusionsWe have studied the dynamic magnetic properties ofZnO:Fe 2 O 3 by carrying out the frequency-dependent ACsusceptibility measurements. We analyzed the observedfrequency dependence of the peak temperature in the[1] M. Knobel, W.C. Nunes, L.M. Socolovsky, E. De Niasi,J.M. Vargas, J.C. Denardin, J. Nanosci. Nanotechnol.8, 2836 (2008).[2] An-Hui Lu, E.L. Salabas, F. Schüth, Angew. Chem.Int. Ed. 46, 1222 (2007).[3] P. Wust, U. Gneveckow, M. Johannsen, D. Böhmer,T. Henkel, F. Kahmann, J. Sehouli, R. Felix, J. Ricke,A. Jordan, J. Nanosci. Nanotechnol. 22, 673 (2006).[4] K.R. Kittilstved, W.K. Liu, D.R. Gamelin, NatureMater. 5, 291 (2006).[5] U. Narkiewicz, D. Sibera, I. Kuryliszyn-Kudelska,L. Kilański, W. Dobrowolski, N. Romcevic, ActaPhys. Pol. A 113, 1695 (2008).[6] R.Y. Sato-Berru, A. Vázquez-Olmos, A.L. Fernández--Osorio, S. Sortes-Martinez, J. Raman Spectrosc. 38,1073 (2007).[7] J.L. Dorman, D. Fiorani, E. Tronc, Adv. Chem. Phys.98, 283 (1997).[8] G.F. Goya, T.S. Berquo, F.C. Fonseca, M.P. Morales,J. Appl. Phys. 94, 3520 (2003).[9] J.A. Mydosh, Spin Glasses: An Experimental Introduction,Taylor and Francis, London 1993.