Dynamic magnetic behavior of cluster-glass ZnFe2O4 nanosystem

ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 320 (2008) e324–e326 www.elsevier.com/locate/jmmm

Dynamic magnetic behavior of cluster-glass ZnFe2O4 nanosystem H.M. Widatallaha, I.A. Al-Omaria, F. Sivesb,c, M.B. Sturlab,c, S.J. Stewartb,c, a

Physics Department, Sultan Qaboos University, P.O. Box 36, 123 Muscat, Sultanate of Oman b IFLP-CONICET, Argentina c Departamento de Fı´sica, Fac. Cs. Exactas, C.C. 67, Universidad Nacional de La Plata, 1900 La Plata, Argentina Available online 4 March 2008

Abstract We present an investigation on the dynamic magnetic behavior of nanosized zinc ferrites with different grain sizes (6–13 nm) and degree of inversion (0.2–0.4). We show that the observed behavior can be interpreted through the progressive blocking and freezing of interacting magnetic clusters formed due to the random non-equilibrium distribution of cations. Furthermore, the glassy character is more pronounced as the ferrite is more inverted, and this coincides with the grain/particle size increment. r 2008 Elsevier B.V. All rights reserved. PACS: 75.50.Tt; 81.20.Wk; 75.75.+a Keywords: Zinc ferrite; Cluster glass; Nanoparticles; ac Susceptibility

The non-equilibrium cationic distribution that occurs in nanosized ZnFe2O4 spinel ferrite modifies its long-range ordering and enhances its magnetization [1–5]. Magnetic ions located at tetrahedral A and octahedral B-sites of the ferrite spinel structure switch on antiferromagnetic JAB couplings that compete with antiferromagnetic JBB interactions. Therefore, depending on the fraction of iron ions at A sites and the randomness of the cationic distribution, diverse and complex magnetic structures can be displayed by zinc ferrite nanoparticles (NPs) systems [1,2]. In previous works [3,4], we showed that chemically synthesized nanocrystalline ZnFe2O4 with an average grain size (D) of 6 nm is partially inverted with an inversion parameter c0.2 (the relative fraction of Fe at A sites). Several authors have proposed that within a core–shell model, the non-equilibrium cation distribution in nanosized ferrites mainly takes place at the particle surface layer [5]. As the surface/volume ratio increases when grain/ particle size diminishes, an enhancement of c was always reported to occur in ZnFe2O4 NPs [1,2]. However, we Corresponding author at: Departamento de Fı´ sica, Fac. Cs. Exactas, C.C. 67, Universidad Nacional de La Plata, 1900 La Plata, Argentina. Tel.: +54 221 4246062; fax: +54 221 4236335. E-mail address: stewart@fisica.unlp.edu.ar (S.J. Stewart).

0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.02.118

found that after milling nanocrystalline zinc ferrite, a partial crystallization takes place. We observed an increment of crystallite and particle sizes and, simultaneously, a cationic swap, i.e., transferences of ions of types Zn2+(A)-Zn2+[B] and Fe3+[B]-Fe3+(A), keeping the initial structural order [3,4]. Here we analyze the dynamic magnetic behavior of chemically prepared ZnFe2O4 NPs (D=6 nm) and that of samples obtained after high-energy milling NPs (MNPs). Milling during 1, 4 and 10 h brings about grain sizes D of 7, 9 and 13 nm, respectively. The degree of inversion, determined by extended X-ray absorption spectroscopy, is 0.2 and 0.4 for NPs and after milling for 10 h, respectively. Details of sample preparation and characterization of these ZnFe2O4 nanoferrites were published elsewhere [3,4]. Magnetic measurements were carried out using both commercial vibrating sample magnetometer and ac-susceptometer. The ac-susceptibility was measured in the 12–310 K range with frequencies n between 0.005 and 10 kHz and field amplitude 1 Oe. The resulting in-w0 and out-of-phase w00 ac-susceptibility components of ZnFe2O4 NPs (Fig. 1) evidence that at high temperatures the system is in a superparamagnetic regime (i.e., the components are frequency independent), while at low temperatures the magnetic moments are blocked in

ARTICLE IN PRESS H.M. Widatallah et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e324–e326

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NPs (6 nm) T0 = 22 ± 2 K MNPs (7 nm); T0 = 28 ± 4 K MNPs (9 nm); T0 = 115 ± 3 K MNPs (13 nm); T0 = 185 ± 5 K

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a state of minimum energy. At intermediate temperatures, where the moment relaxation time is near the experimental time window, a dynamic behavior is displayed. We observed that the blocking temperature TB, taken as the maximum of the out-of-phase component (or the inflexion point of the in-phase component), shifts to higher values with the ac-field frequency (Fig. 2). These ac-susceptibility measurements show that TB progressively increases after subjecting nanocrystalline ZnFe2O4 to mechanical milling, mainly due to the previously demonstrated grain and particle size growth (see Figs. 2 and 3) [3,4]. To analyze the type of dynamic behavior, we examined the w0 component around the cusp region and studied its response when varying the exciting frequency [6]. The relative change of the freezing or blocking temperature (DTB/TB) per decade of frequency allows us to distinguish a spin-glass from a typical superparamagnet [6]. From Fig. 3 we observe that DTB/TB per frequency decade decreases as the grain size (D) increases. Moreover, the reduced value of 0.01 obtained for the NPs milled for 10 h (13 nm-NPs) is almost within the range usually reported for spin-glass or clusterglass systems [6]. Fig. 2 shows the variation of TB as a function of the measuring time t from ac-susceptibility. In all samples, TB shifts towards larger values as t decreases, this variation being less marked for the more-inverted ferrites. To describe this behavior we found that a simple Ne´el– Arrhenius law (t ¼ t0 exp[Ea/k(TB)]), which considers isolated magnetic entities, provides unphysical values for the anisotropy energy Ea and the time constant t0. Instead, a reasonable description can be obtained using a Vogel– Fulcher law (t ¼ t0 exp[Ea/k(TBT0)]) that takes into account a magnetic interaction throughout the parameter T0 [7]. The T0 values obtained are shown in Fig. 2. These results indicate that the interactions are more intense as the grain size and inversion increases by the milling.

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Fig. 2. Logarithm of the experimental time (log t) vs. the inverse of the blocking temperature (T1 B ). T0 is a fitting parameter obtained assuming a Vogel–Fulcher law.

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Fig. 1. Thermal dependence of the in-phase (w0 ) and out-of-phase (w00 ) components of the ac-susceptibility at n ¼ 0.9 kHz for as-prepared ZnFe2O4 nanoparticles (NPs).

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Fig. 3. Blocking temperatures TB (left) and relative change DTB/TB (right) per frequency decade as a function of grain size (D).

In spite of the relatively low T0 value for unmilled NPs, we observe that its magnetization vs. H/T curves at temperatures well above its TB are not coincident. This would indicate that interactions are operative (Fig. 4). Several studies have shown that magnetic properties of nanosized ferrites can be well described considering a core–shell model (see, for example, Refs. [5,8]). The spinglass-like magnetic behavior displayed by nanoferrites is commonly assigned to the contributions from magnetic ions placed at the particle shell. Surface spin is supposed to be in a disordered state due to the existence of unsatisfied bonds and accumulation of defects. As the surface/volume ratio decreases, the contribution from surface spins with a certain degree of disorder is less important. Then, any indication of a disordered or spin-glass behavior should diminish. In the present case we observe the contrary effect,

ARTICLE IN PRESS H.M. Widatallah et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e324–e326

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Fig. 4. Magnetization M vs. H/T curves of ZnFe2O4 NPs plotted at different temperatures T4TB.

i.e., that the disordered state is more pronounced as the ferrite grain/particle size grows. The non-equilibrium distribution of iron ions originates superexchange interactions Fe3+[B]–O2–Fe3+(A) that, depending on the degree of inversion, confers ZnFe2O4 a cluster glass or ferrimagnetic magnetic behavior [1]. The presence of non-magnetic ions (Zn2+) at B sites probably interrupts these superexchange paths that, in addition to an inhomogenously distributed inversion, brings about magnetic clusters of reduced size and are loosely connected. A larger magnetic moment per particle due to c increment increases the strength of dipolar interaction between particles or clusters. Moreover, the induced crystallization might also cause a particle welding and a reduction of the surface disorder. Thus, different particle orientations and random anisotropy directions introduce frustration and disorder giving rise to a spin-glass-like behavior. The magnetic state is more disordered as the interactions are

more pronounced. This result agrees with previous studies on nanostructured ZnFe2O4 diluted in a non-magnetic host [9]. In that case, the collective magnetic state achieved below the superparamagnetic to super-spin-glass transition becomes more disordered when the inter-particle interactions are more intense. Therefore, in our case, while TbTB zinc ferrite particles are in a superparamagnetic state. As the temperature is lowered, cluster regions grow and evolve towards a collective freezing at TB with a characteristic of spin-glass. The collective state is more disordered as the interactions are stronger and the limit size of cluster regions increases. The glassy behavior can be interpreted through the progressive blocking and freezing of the interacting magnetic clusters formed due to the random non-equilibrium distribution of cations. We appreciate financial support by CONICET, Argentina (PIP 6524). dc Magnetic measurements were performed using the RN3M (Argentina) facilities. S.J.S. thanks A.G. Grunfeld for useful comments. References [1] M. Hofmann, S.J. Campbell, H. Ehrhardt, R. Feyerherm, J. Mater. Sci. 39 (2004) 5057. [2] S.A. Oliver, H.H. Hamdeh, J.C. Ho, Phys. Rev. B 60 (1999) 3400. [3] S.J. Stewart, S.J.A. Figueroa, J.M. Ramallo-Lo´pez, S.G. Marchetti, J.F. Bengoa, R.J. Prado, F.G. Requejo, Phys. Rev. B 75 (2007) 073408. [4] S.J. Stewart, S.J.A. Figueroa, M.B. Sturla, R.B. Scorzelli, F. Garcı´ a, F.G. Requejo, Physica B 389 (1) (2007) 155. [5] J.F. Hochepied, P. Bonville, M.P. Pileni, J. Phys. Chem. B 104 (2000) 905. [6] J.A. Mydosh, Spin Glasses: An Experimental Introduction, Taylor & Francis, London, 1993. [7] S. Shtrikman, E.P. Wohlfarth, Phys. Lett. 85A (1981) 467. [8] V. Sepela´k, A. Feldhoff, P. Heitjans, F. Krumeich, D. Menzel, F.J. Litterst, I. Bergmann, K.D. Becker, Chem. Mater. 18 (2006) 3057. [9] O. Cador, F. Grasset, H. Haneda, J. Etourneau, J. Magn. Magn. Mater. 268 (2004) 232.