Magnetic properties of nanostructured MnZn ferrite

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 152–156

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Magnetic properties of nanostructured MnZn ferrite Mohammad Javad Nasr Isfahani a,, Maxym Myndyk b, Dirk Menzel c, Armin Feldhoff d, Jamshid Amighian a, Vladimir Sˇepela´k b,1 a

Physics Department, Faculty of Sciences, The University of Isfahan, Isfahan 81746-73441, Iran Institute of Physical and Theoretical Chemistry, Braunschweig University of Technology, Hans-Sommer-Strasse 10, D-38106 Braunschweig, Germany Institute of Condensed Matter Physics, Braunschweig University of Technology, Mendelssohnstrasse 3, D-38106 Braunschweig, Germany d Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, Callinstrasse 3-3A, D-30167 Hannover, Germany b c

a r t i c l e in f o

a b s t r a c t

Article history: Received 9 June 2008 Received in revised form 29 July 2008 Available online 14 August 2008

Mn0.5Zn0.5Fe2O4 nanoparticles (10–30 nm) have been prepared via mechanochemical processing, using a mixture of two single-phase ferrites, MnFe2O4 and ZnFe2O4. SQUID measurements (field-cooled magnetization curves and hysteresis loops) were performed to follow the mechanically induced evolution of the MnFe2O4/ZnFe2O4 mixture submitted to the high-energy milling process. The resulting single MnZn nanoferrite phase was characterized by SQUID (M–H curve), Faraday balance (M–T curve) and transmission electron microscopy. The magnetic characteristics of the mechanosynthesized material were compared with those of bulk Mn0.5Zn0.5Fe2O4. It was found that the saturation magnetization of nanostructured Mn0.5Zn0.5Fe2O4 (87.2 emu/g) is lower than that of the bulk Mn0.5Zn0.5Fe2O4, but, the Ne´el temperature of the sample (583 K) is higher than that of the bulk Mn0.5Zn0.5Fe2O4. & 2008 Elsevier B.V. All rights reserved.

PACS: 75.50.Gg 74.25.Ha 75.50.Tt 81.20.Ev. Keywords: Ferrimagnetics Magnetic properties Nanocrystalline materials Mechanochemical processing

1. Introduction Nanosized spinel-type ferrites with the general formula MFe2O4 (M is a divalent metal cation) are very important materials because of their interesting magnetic and electrical properties as well as of their chemical and thermal stabilities [1]. These materials have been used in many applications including electronics, magnetic storage, ferrofluid technology, as carriers for magnetically guided drug delivery, and as contrast agents in magnetic resonance imaging [2]. MnZn ferrite, as a soft magnetic material, is an important member of spinel family [3]. Apart from its magnetic and electronic applications [4], MnZn ferrite finds a number of applications, for example, in transformers, choke coils, noise filters, and recording heads [5]. Large-scale applications of nanosized ferrites have prompted the development of several widely used methods, including coprecipitation [6], sol–gel [7], hydrothermal reactions [8], combus-

 Corresponding author. Tel.: +98 3117932419; fax: +98 3117932409.

E-mail addresses: [email protected], [email protected] (M.J. Nasr Isfahani). 1 On leave from the Slovak Academy of Sciences, Kosˇice, Slovakia. 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.08.054

tion synthesis [9] and high-energy milling [10], for the fabrication of stoichiometric and/or nonstoichiometric spinel ferrite nanoparticles. These materials exhibit magnetic properties different from those of bulk samples prepared by standard ceramic method, e.g., enhanced or reduced spontaneous magnetization [11,12] and Ne´el temperature [13,14]. These phenomena can be explained in terms of a modified core–shell model [11], in which a competition between the effects of spin canting and site exchange of cations (both located in the surface shell of nanoparticles) plays a decisive role. In previous paper [15], we reported preparation procedure of nanocrystalline Mn0.5Zn0.5Fe2O4 via high-energy ball milling of the mixture of two single-phase ferrites, MnFe2O4 and ZnFe2O4. In the above-named paper, we discussed the detailed results of Mo¨ssbauer spectroscopic study of the milled MnFe2O4/ZnFe2O4 mixture. The present work represents a continuation of our previous work [15] with the following goals:

(i) to follow the mechanically induced evolution of the MnFe2O4/ ZnFe2O4 mixture by magnetic measurements (the field-cooled (FC) magnetization curves in the temperature range from 5 to

ARTICLE IN PRESS M.J. Nasr Isfahani et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 152–156

20 K, the low-temperature (10 K) hysteresis loops, the magnetization measurements at elevated temperatures); (ii) to reveal the morphology of the mechanosynthesized Mn0.5Zn0.5Fe2O4 nanoparticles.

2. Experimental The mechanochemical processing was used for the preparation of nanosized Mn0.5Zn0.5Fe2O4. MnFe2O4 and ZnFe2O4 were used as starting materials. Details of the mechanosynthesis of Mn0.5Zn0.5Fe2O4 are given in Ref. [15]. Hysteresis loops at 10 K and FC curves at 0.01 T were measured using a SQUID magnetometer. The Ne´el temperature was determined using a Faraday balance equipped with a permanent magnet (0.004 T). The morphology of powders and the sizes of individual particles were studied using a field-emission transmission electron microscope (TEM) of the type Joel JEM-2100F. Prior to TEM investigations, powders were crushed in a mortar, dispersed in 2-propanol, and fixed on a copper-supported carbon grid.

3. Results and discussion The FC magnetization curves of the ZnFe2O4/MnFe2O4 mixtures milled for various times are shown in Fig. 1. As can be seen, the FC curves of the mixture milled for 15 min and 20 h exhibit a

153

single maximum at about 8 K, which can be associated with the Ne´el temperature (TN) of ZnFe2O4 [16,17]. This is in contrast to the magnetic behavior of the sample milled for 30 h, whose magnetization monotonically decreases with increasing temperature (see Fig. 1c). Thus, the presence of the local maximum in the FC curve indicates that the sample milled for 20 h still contains ZnFe2O4. On the contrary, the absence of the local maximum in the FC curve of the ZnFe2O4/MnFe2O4 mixture milled for 30 h (Fig. 1c) confirms that the mechanochemical synthesis of Mn0.5Zn0.5Fe2O4 is complete. Fig. 2 compares the low-temperature (10 K) hysteresis loops of the ZnFe2O4/MnFe2O4 mixtures milled for 15 min and 30 h. As can be seen, the hysteresis loop of the sample milled for 15 min does not saturate even at maximum attainable field (H ¼ 5 T). The point that the magnetization of this sample increases linearly with the applied field in the field range from about 1 to 5 T indicates the presence of paramagnetic phase (ZnFe2O4). Thus, the low-temperature hysteresis loop of the initial sample is a superposition of ferromagnetic and paramagnetic parts, corresponding to MnFe2O4 and ZnFe2O4 phases, respectively. On the contrary, the magnetization of the ZnFe2O4/MnFe2O4 mixture milled for 30 h does not increase linearly in high-field region due to the absence of the paramagnetic phase. Thus, from the results of magnetic measurements mentioned above and results of the Mo¨ssbauer spectroscopy (see Ref. [15]), it is concluded that the complete formation of the Mn0.5Zn0.5Fe2O4 spinel is obtained after 30 h of high-energy milling.

Fig. 1. The FC magnetization curves for the ZnFe2O4/MnFe2O4 mixtures milled for (a) 15 min, (b) 20 h, and (c) 30 h. The curves were taken at H ¼ 0.01 T.

ARTICLE IN PRESS 154

M.J. Nasr Isfahani et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 152–156

Fig. 2. Magnetization hysteresis loops for the ZnFe2O4/MnFe2O4 mixtures milled for (a) 15 min and (b) 30 h measured at 10 K after field cooling with H ¼ 5 T.

in Fig. 2b. This suggests the existence of canted spins and/or a nonequilibrium cation distribution in the surface shell of the ferrite nanoparticles [18]. By extrapolating the highfield region (HX3 T) of the M(H) curve to infinitive field, we estimated the value of the saturation magnetization of mechanosynthesized Mn0.5Zn0.5Fe2O4 to be approximately 82.7 emu/g, which is about 41% lower than the value reported for the bulk Mn0.5Zn0.5Fe2O4 (140 emu/g) [19]. It should be noted that the reduced saturation magnetization (Ms ¼ 92 emu/g at 5 K) has already been reported for Mn0.5Zn0.5Fe2O4 nanoparticles [20]. In analogy with the other mechanosynthesized spinel ferrites, the reduced magnetization observed in the present case can be attributed to the effect of spin canting that dominates over the effect of site exchange of cations in the surface shell of Mn0.5Zn0.5Fe2O4 nanoparticles [11,21]. Fig. 3 shows the magnetization variation of the ferrite sample with temperature, measured in a magnetic field of 0.004 T. As expected, the magnetization decreases with increasing temperature; at 543 K there is an inflection point on the M(T) curve and finally, the magnetization falls to zero at around 583 K, forming a cusp. This type of unusual variation of magnetization with temperature has already been reported for magnetic nanopowders [22,23]. Thus, from the M–T curve the Ne´el temperature of Mn0.5Zn0.5Fe2O4 nanoparticles is estimated to be 583 K. This value is higher than that reported for the bulk Mn0.5Zn0.5Fe2O4 (413 K) [19]. The increased Ne´el temperature can be attributed to the strengthening of the A–O–B superexchange interaction [24]. The shape, size and morphology of mechanosynthesized Mn0.5Zn0.5Fe2O4 particles were examined by direct observation via a TEM. Bright-field TEM images (Fig. 4a and b) illustrate the nanoscale nature of ferrite particles. As shown in Fig. 4a, the nanoparticles tend to agglomerate because they experience a permanent magnetic moment proportional to their volume [25]. Hence, each particle is permanently magnetized and gets agglomerated. The micrographs also show that the mechanosynthesized ferrite consists of particles mostly in the 10–30 nm size range. The shape of the majority of the nanoparticles appears spherical. TEM micrographs of nanocrystalline mechanosynthesized Mn0.5Zn0.5Fe2O4 at low and high magnifications are shown in Fig. 4c and d. The crystalline nature of nanosized particles is also visible (see the lattice planes in Fig. 4d). Corresponding selected area electron diffraction (SAED) pattern, displayed in Fig. 4e, shows both the discrete diffraction spots (originated from the well-crystalline regions) and the Debye–Scherrer rings (indicating the presence of structurally disordered regions) in the mechanosynthesized Mn0.5Zn0.5Fe2O4.

4. Conclusions

Fig. 3. The magnetization variation of the mechanosynthesized Mn0.5Zn0.5Fe2O4 with temperature. The magnetization data taken at H ¼ 0.004 T.

It is interesting to mention that the magnetization of the mechanosynthesized Mn0.5Zn0.5Fe2O4 does not saturate even at the maximum attainable field (H ¼ 5 T); see the M–H loop shown

The present study demonstrates that nanosized Mn0.5Zn0.5 Fe2O4 consisting of particles mostly in the 10–30 nm size range, can be synthesized from the mixture of two ternary oxides in a one-step mechanochemical processing. The M–H curve reveals that the saturation magnetization of mechanosynthesized Mn0.5Zn0.5Fe2O4 takes a value of Ms ¼ 82.7 emu/g, which is about 41% lower than the value reported for bulk Mn0.5Zn0.5Fe2O4. This reduced saturation magnetization can be attributed to the prevailing effect of spin canting. The M–T curve of nanoscale Mn0.5Zn0.5Fe2O4 gives evidence that the mechanosynthesized material exhibits higher Ne´el temperature than the bulk sample. The enhanced Ne´el temperature can be attributed to the effect of strengthening of the A–O–B superexchange interaction in the mechanosynthesized spinel phase.

ARTICLE IN PRESS M.J. Nasr Isfahani et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 152–156

155

Fig. 4. (a) TEM bright-field image of nanoscale mechanosynthesized Zn0.5Mn0.5Fe2O4. (b) Nanosized Zn0.5Mn0.5Fe2O4 particles exhibit a strong tendency for agglomeration. TEM micrographs of Mn0.5Zn0.5Fe2O4 nanocrystals at (c) low and (d) high magnifications. (e) SAED pattern of Mn0.5Zn0.5Fe2O4 nanoparticles.

Acknowledgements The authors (M.J.N.I. and J.A.) would like to thank the Office of Graduate Studies of the University of Isfahan. V.Sˇ. gratefully acknowledges the support by the DFG, APVV (Project 0728-07) and VEGA (Grant 2/0065/08). References [1] S. Maensiri, Ch. Masingboon, B. Boonchom, S. Seraphin, Scr. Mater. 56 (2007) 797. [2] V. Sˇepela´k, P. Heitjans, K.D. Becker, J. Therm. Anal. Calorim. 90 (2007) 93.

[3] M. Sugimoto, J. Am. Ceram. Soc. 82 (1999) 269. [4] H. Inaba, J. Am. Ceram. Soc. 78 (1995) 2907. [5] Q. Yan, R.J. Gambio, S. Sampath, L.H. Lewis, L. Li, E. Baumberger, A. Vaidya, H. Xiong, Acta Mater. 52 (2004) 3347. [6] S.D. Shenoy, A.P. Joy, M.R. Anantharaman, J. Magn. Magn. Mater. 269 (2004) 217. [7] D.H. Chen, X.R. He, Mater. Res. Bull. 36 (2001) 1369. [8] H.W. Wang, S.C. Kung, J. Magn. Magn. Mater. 270 (2004) 230. [9] C.C. Hwange, J.S. Tasi, T.H. Huang, C.H. Peng, S.Y. Chen, J. Solid State Chem. 178 (2005) 382. [10] M. Mouroi, R. Street, P.G. McCormick, J. Amighian, Phys. Rev. B 63 (2001) 184414. [11] V. Sˇepela´k, I. Bergmann, D. Menzel, A. Feldhoff, P. Heitjans, F.J. Litterst, K.D. Becker, J. Magn. Magn. Mater. 316 (2007) e764. [12] S. Dasgupta, K.B. Kim, J. Ellrich, J. Eckert, I. Manna, J. Alloy. Compd. 424 (2006) 13.

ARTICLE IN PRESS 156

M.J. Nasr Isfahani et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 152–156

[13] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.G. Hadjipanayis, Phys. Rev. Lett. 67 (1991) 3602. [14] J.P. Chen, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, E. Devlin, A. Kostikas, Phys. Rev. B 54 (1996) 9288. [15] M.J. Nasr Isfahani, M. Myndyk, V. Sˇepela´k, J. Amighian, J. Alloy. Compd. (2008) (in press). [16] F.S. Li, L. Wang, J.B. Wang, Q.G. Zhou, X.Z. Zhou, H.P. Kunkel, G. Williams, J. Magn. Magn. Mater. 268 (2004) 332. [17] V. Sˇepela´k, Ann. Chim.Sci. Mater. 27 (2002) 61. [18] V. Sˇepela´k, A. Feldhoff, P. Heitjans, F. Krumeich, D. Menzel, F.J. Litterst, I. Bergmann, K.D. Becker, Chem. Mater. 18 (2006) 3057. [19] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, 1959.

[20] K. Parekh, R.V. Upadhyay, L. Belova, K.V. Rao, Nanotechnology 17 (2006) 5970. [21] V. Sˇepela´k, I. Bergmann, A. Feldhoff, P. Heitjans, F. Krumeich, D. Menzel, F.J. Litterst, S.J. Campbell, K.D. Becker, J. Phys. Chem. C 111 (2007) 5026. [22] N.S. Gajbhiye, Mater. Process. 10 (1998) 247. [23] W. Wolski, E. Wolska, J. Kaczmarek, P. Piszora, Phys. Status Solidi A 152 (1995) K19. [24] N. Ponpandian, A. Narayanasamy, C.N. Chinnasamy, N. Sivakumar, J.M. Greneche, K. Chattopadhyay, K. Shinoda, B. Jeyadevan, K. Tohji, Appl. Phys. Lett. 86 (2005) 192510. [25] E. Manova, B. Kunev, D. Paneva, I. Mitov, L. Petrov, C. Estourne`s, C. D’Orle´ans, J.L. Rehspringer, M. Kurmoo, Chem. Mater. 16 (2004) 5689.