Magnetic, physical and electrical properties of Zr–Ni-substituted co-precipitated strontium hexaferrite nanoparticles

l h M zb a am U ivers ; acce s, SrZ rang ith Z ia. T hts r Keywords: Nanoparticles; Coercivity; DC resistivity; X-ray diffraction; Magnetic susceptibility resistance to corrosion and perfect chemical stability, Ni2+ with Ti4+ requires higher heat treatment for pure phase formation, Zr4+, being isoelectronic with Ti4+, has been chosen for the present study. The aim of the Zr4+–Ni2+ substitution is to reduce the dielectric sized by the co-precipitation method. tium hexaferrite. X-ray diffraction (XRD) analysis was performed to confirm the phase formation of hexafer- rites (Philips X’Pert PRO 3040/60 diffractometer, Cu Ka radiation). The elemental composition was deter- mined by energy-dispersive X-ray fluorescence (ED- XRF) analysis using a Horiba MESA-500 spectrometer. Transmission electron microscopy (TEM) analysis was *Corresponding author. Tel.: +92 51 90642143; fax: +92 51 90642241; e-mail: [email protected] 93–10 good thermal durability, good mechanical hardness and high Curie temperature are other advantages [4,5]. The physical, electrical and magnetic properties of strontium hexaferrite change drastically with composi- tion due to occupancy by the substituted ions at tetrahe- dral, trigonal bipyramidal and octahedral sites. Ti4+– Ni2+-substituted M-type hexaferrites are being investi- gated for magnetic recording applications [6]. The presence of Ni2+ is known to reduce the temperature coefficient of coercivity, and improve the electrical resistivity, magnetic and dielectric properties [7]. The material requires high DC electrical resistivity for appli- cation in microwave devices [8]. As the combination of The chemicals used in the synthesis of samples are Fe(NO3)3Æ9H2O (Sigma-Aldrich, 98%), Sr(NO3)2 (Fluka, P99%), Ni(CH3COO)2Æ4H2O (Merck, 99%), ZrOCl2Æ 4H2O (BDH, 96%) and NaOH (Fluka, P97%). The strontium hexaferrite samples substituted with Zr–Ni, having nominal composition SrZrx- NixFe12�2 xO19 (where x = 0.0–0.8) were prepared by a chemical co-precipitation method, maintaining the molar ratio (Fe/Sr = 11). The details of the process are given elsewhere [9]. A Fourier transform infra-red (FTIR) spectropho- tometer (Bio Rad Merlin FTS3000MX) was used to investigate the formation of Zr–Ni-substituted stron- Being different from their bulk counterparts, nano- sized materials exhibit novel properties [1]. Strontium hexaferrite has been widely investigated due to the advantages it offers in many scientific and technological applications, e.g. telecommunication, magnetic record- ing media and microwave devices [2,3]. In addition, their constant, coercivity and dielectric loss factor and to im- prove the DC electrical resistivity and saturation magnetization. This paper reports the effect of Zr–Ni substitution on the structural, electrical, magnetic and dielectric proper- ties of the strontium hexaferrite nanoparticles synthe- Magnetic, physical and electrica co-precipitated strontium Muhammad Javed Iqbal,a,* Pablo Hernandez-Gome aDepartment of Chemistry, Quaid-i-Az bDepartmento Electricidad y Electro´nica, Un Received 31 July 2007 A series of Zr–Ni-substituted strontium hexaferrite material cipitation method and the crystallite size determined to be in the to 98 kA m�1 while coercivity decreased from 1710 to 428 Oe w makes these materials suitable for applications in recording med can be useful for application in microwave devices. � 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rig Scripta Materialia 57 (2007) 10 1359-6462/$ - see front matter � 2007 Acta Materialia Inc. Published by El doi:10.1016/j.scriptamat.2007.08.017 properties of Zr–Ni-substituted exaferrite nanoparticles uhammad Naeem Ashiq,a nd Jose Maria Munozb niversity, Islamabad-45320, Pakistan idad de Valladolid, 47071 Valladolid, Spain pted 20 August 2007 rxNixFe12�2xO19 (x = 0.0–0.8), was synthesized by the co-pre- e of 30–47 nm. The saturation magnetization increased from 72 r–Ni substitution. This improvement in both these properties he increase in resistivity suggests that the synthesized materials eserved. 96 www.elsevier.com/locate/scriptamat sevier Ltd. All rights reserved. carried out with a Jeol JEM-1200EX microscope. The AC magnetic susceptibility was measured using a pri- mary and secondary coil set up operating at a frequency of 273 Hz and with a very low AC field (0.1 Oe) [10]. The DC electrical resistivity was measured by a two- point probe method [11]. The dielectric measurements were carried out at room temperature in a frequency range 80 Hz to 1 MHz using an LCR meter bridge (Wayne Kerr LCR 4275). The hysteresis loops were measured using a standard AC induction method [12], in which both the magnetic field H and the magnetic induction B were measured through two coils placed near the sample and saturation magnetization and coer- civity were calculated. Measurements of magnetic disac- commodation, i.e. the time evolution of magnetic permeability after sample demagnetization, were carried out with a system based on a LCR bridge [13], in the temperature range 80–500 K. The DC electrical resistiv- ity and dielectric properties were measured for pellets of 13 mm in diameter and 2.5 mm thick. The FTIR spectrum of the sample SrZr0.4Ni0.4- substituted samples. The bulk density increases, whereas the porosity decreases up to x = 0.6. The elemental compositions of the synthesized sam- ples were determined by ED-XRF analysis. The calcu- lated elemental composition is in good agreement with the nominal composition as shown in Table 1. The par- ticle size of the sample annealed at 920 �C is in the range of 40–65 nm measured by TEM, which agrees with that calculated by the Scherrer formula; however, for the sample annealed at 1170 �C the size increases to 100– 200 nm. Hence it can be seen that there is a direct rela- tionship between the annealing temperature and the par- ticle size. Various magnetic properties were calculated from hysteresis loops. Figure 1a shows the effect of annealing temperature on saturation magnetization (Ms) and coer- civity (Hc) of SrZr0.8Ni0.8Fe10.4O19. The maximum value of Ms is obtained for the sample annealed at 1120 �C. The coercivity of the samples decreases with increasing temperature. The decrease in coercivity with tempera- ture is due to the increase in particle size as confirmed ), X- ectric x = 46.9 5.88 23.0 690 2.80 5.15 0.46 703 353 0.56 152 139 0.26 0.22 11.5 11.5 1.20 1094 M. J. Iqbal et al. / Scripta Materialia 57 (2007) 1093–1096 Fe11.2O19 annealed at 920 �C for 1 h shows absorption bands at 592, 557 and 439 cm�1 identified as the metal oxygen stretching vibrations of hexaferrite [14]; other peaks representing water or nitrate ions are absent. The indexed XRD pattern of SrZrxNixFe12�2 xO19 (x = 0.0–0.8) shows that all the peaks match with the standard pattern (ICSD 00-051-1879). Various parame- ters such as a, c, cell volume, X-ray density and crystal- lite size are calculated from the XRD data (Table 1). The crystallite size of the different samples is 30– 47 nm. These values are much smaller than those reported previously (4.69 lm) [8]. The value of a remains constant but the value of c and cell volume increases with increase in Zr–Ni content. This is due to the ionic radii of Zr4+ (0.80 A˚) and Ni2+ (0.69 A˚) being larger than that of Fe3+ (0.64 A˚). From Table 1 it is clear that the X-ray density also increases with Zr–Ni concentra- tion, which is due to the larger molar masses of the Table 1. Crystallite size (D), lattice constants (a and c), cell volume (V temperatures (Tc), activation energy (Ea), dielectric loss factor, diel SrZrxNixFe12� 2xO19 Parameters x = 0.0 Crystallite sizes (D) nm 30.00 Lattice constant (a) A˚ 5.87 Lattice constant (c) A˚ 23.06 Cell volume (V) A˚3 688.10 Bulk density(qm) g cm �3 2.36 X-ray density(qX-ray) g cm �3 5.13 Porosity (P)% 0.54 Curie temperature (Tc) K 748 Transition temperature (TM�S) K – Activation energy (Ea) eV 0.491 Dielectric constant (�e) at 600 kHz 221 Dielectric constant (�e) at 1000 kHz 183 Loss factor (tand) at 600 kHz 0.387 Loss factor (tand) at 1000 kHz 0.352 Magnetic moment (NB)lB 11.18 Fe mol l�1 11.93 Sr mol l�1 1.10 Zr mol l�1 – 0.23 Ni mol l�1 – 0.18 by the TEM analysis. Figure 1b shows the effect of Zr–Ni concentration on the saturation magnetization and coercivity for the sam- ples annealed at 1120 �C. It is clear that the saturation magnetization increases up to x = 0.6 while the coerciv- ity decreases continuously with the increase in Zr–Ni content. This can be explained on the basis of the Zr4+ and Ni2+ ions occupying different sites in the Fe3+ sublattices. It has been reported that Zr4+ ions replace Fe3+ ions at the 2b site for small substitutions (x � 0.1) and at 4f1 for higher substitutions, while Ni 2+ ions replace Fe3+ ions at the 4f2 site for x � 0.1, and at the 12k site for larger values of x [15]. When a nonmagnetic Zr4+ ion replaces a Fe3+ ion at the 4f1 site with spin down, then the total number of unpaired electrons with upward spin is increased, causing the saturation magne- tization of the samples to increase. The Ni2+ ions replace Fe3+ at the 12k site with spin up and a magnetic ray density (qx), bulk density (qm), porosity (P), Curie and transition constant (�e), magnetic moment (NB) and nominal composition of 0.2 x = 0.4 x = 0.6 x = 0.8 0 43.28 32.72 29.89 5.89 5.89 5.89 6 23.08 23.12 23.28 .45 693.40 694.60 699.41 2.86 2.90 2.52 5.16 5.18 5.19 0.45 0.44 0.51 685 600 549 368 458 393 4 0.687 0.939 0.679 144 118 189 132 111 168 7 0.223 0.167 0.325 1 0.142 0.133 0.204 2 11.83 14.00 12.51 7 11.10 10.68 10.30 1.16 1.10 1.12 0.45 0.65 0.84 0.38 0.63 0.81 Mater moment of 2 lB, which is also less than that of Fe3+ (5 lB), but the total magnetic moment increases by 2 lB due to replacement of ferric ions by a nonmagnetic Zr4+ ion at the 4f1 site. The decrease in saturation mag- netization above x = 0.6 is due to the larger amount of nonmagnetic ions which are responsible for the weaken- ing of the exchange interactions. The coercivity decreases with increasing Zr–Ni content, which is due to the decrease in the magnetocrystalline anisotropy. There are very few reported examples in which the saturation magnetization increases and at the same time the coercivity decreases with substitutions in M-type hexaferrites. Recording media require high enough coer- civity above 600 Oe and saturation magnetization as high as possible [16]. For the materials we have synthe- sized, the coercivity decreases drastically from 1720 to 428 Oe but the sample with x = 0.6 has coercivity 729 Oe and saturation magnetization increases from 72 to 98 kA m�1 with Zr–Ni, substitution suggesting that the materials are suitable for application in recording media. The magnetic moment, NB, is calculated using the following equation: NB ¼ M �M s 5:585� db ; ð1Þ where M is the molecular weight of the samples, Ms the saturation magnetization and db is the measured density of the samples. The behavior of the magnetic moment with annealing temperature is same as that of saturation magnetization. The effect of substitution of Zr–Ni on 50 60 70 80 90 100 900 1000 1100 1200 Temperature/ºC M s/ KA m -1 400 500 600 700 800 H c/ O e Ms Hc 0 20 40 60 80 100 120 0 0.2 0.4 0.6 0.8 Zr-Ni content M s/ KA m -1 0 300 600 900 1200 1500 1800 H c/ O e Ms Hc Figure 1. Variation of coercivity (Hc) and saturation magnetization (Ms) of SrZr0.8Ni0.8Fe10.4O19 as a function of (a) temperature and (b) Zr–Ni content. M. J. Iqbal et al. / Scripta the magnetic moments is shown in Table 1. Clearly both the NB and Ms of Zr–Ni-substituted samples increase continuously and reach a maximum at x = 0.6. The behavior of the magnetic moment of the samples is in good agreement with the saturation magnetization (Fig. 1b). Figure 2a shows the behavior of DC electrical resis- tivity and drift mobility with Zr–Ni content at room temperature. The electrical resistivity increases from 2.04 · 108 to 35.5 · 108 X cm, whereas the drift mobility decreases from 19.1 · 10�13 to 1.2 · 10�13 cm2 V�1 s�1 with the Zr–Ni content of x 6 0.6. However, for the samples with x > 0.6, the DC electrical resistivity de- creases and the drift mobility increases. The conduction mechanism operating at these two concentration levels can be explained on the basis of electron hopping be- tween Fe2+ and Fe3+ at octahedral sites. Ni2+ ions occu- py the 12k (octahedral) site and Zr4+ ions occupy the 4f1 (tetrahedral) as well as the 2b (trigonal bipyramidal) sites [15]. When Ni2+ ions replace iron ions at the octa- hedral sites, the number of iron ions decreases at that site and as a result the number of hopping electrons de- creases, causing the DC electrical resistivity to increase and the drift mobility to decrease. It is believed that Ni3+ may have been formed during the annealing pro- cess so that the exchange interaction of the type shown below becomes possible: Fe3þ þNi2þ $ Fe2þ þNi3þ The conductivity is now due to electron hopping between iron ions as well as due to the transfer of holes between Ni2+ and Ni3+. Since electron transfer is easier than the hole exchange, the resistivity of the material is expected to increase and the drift mobility to decrease. The temperature dependence of DC resistivity shows very interesting results (Fig. 2b). The resistivity increases with temperature, showing metallic behavior, and after a specific temperature TM–S, the so-called metal–semicon- ductor transition temperature, the resistivity shows a decreasing trend as expected in semiconductors. The values of the transition temperature (TM�S) for different samples are given in Table 1. It is clear that TM�S in- creases with Zr–Ni content for x 6 0.6 as shown by the compositional dependence of resistivity. The activation energy (Ea) has also been calculated from the temperature dependence of resistivity data and the values are given in Table 1. The activation energy increases with Zr–Ni content of x 6 0.6. The decrease in Ea for electron hopping and decrease in resis- tivity values in the samples with xP 0.6 may be due to migration of iron ions from tetrahedral to octahedral sites, resulting in an increased content of Zr at tetrahe- 0 10 20 30 40 0 0.2 0.4 0.6 0.8 Zr-Ni contents ρ (o hm c m ) 1 08 0 4 8 12 16 20 μd (c m 2 V- 1 se c- 1 ) 1 0- 13μd ρ 0 10 20 30 40 50 60 70 250 350 450 550 650 Temperature (K) ρ (o hm c m ) 1 08 0 0.2 0.4 0.6 0.8 Figure 2. (a) The dependence of DC electrical resistivity (q) and drift mobility (ld) on Zr–Ni content at room temperature. (b) Variation of DC electrical resistivity with temperature. ialia 57 (2007) 1093–1096 1095 dral sites. The data for Ea and the drift mobility, ld are in good agreement with the resistivity data. The dielectric constant, (�e) is calculated from a well- known equation [10]. It is observed that �e and dielectric loss factor, tand, decrease with an increase in frequency. The observed decrease in both the �e and tand by increas- ing the frequency is attributed to the electron exchange between Fe2+ and Fe3+ ions being slow to follow the change in the external applied field beyond a certain fre- quency. The compositional dependence of �e and tand is shown in Table 1 at frequencies of 600 and 1000 kHz. It is observed that the values of both �e and tand also de- crease with the increase in Zr–Ni contents of x 6 0.6. The behavior can be explained by assuming that the mechanism of dielectric polarization is similar to that of the electrical conduction. Replacement of Fe ions from the 12k (octahedral) sites by Ni2+ causes the elec- tron hopping to decrease due to a reduction in the num- tion energy increase but the Curie temperature, dielec- tric constant, dielectric loss factor and drift mobility decrease with the Zr–Ni contents of x 6 0.6. The improvement in DC electrical resistivity makes the syn- thesized material suitable for application in microwave devices. The temperature dependence of resistivity of the substituted samples shows a transition from metallic to semiconductor behavior. The data of activation en- ergy, drift mobility, dielectric constant and dielectric loss factor are in good agreement with the DC electrical resistivity data. Financial support to one of the authors (MNA) for this work from the Higher Education Com- 0.43 0.44 0.45 0.46 0.47 0.48 300 400 500 600 700 Temperature (K) X (a .u ) 0 0.2 0.4 0.6 0.8 Figure 3. Temperature dependence of AC magnetic susceptibility (v) of SrZrx NixFe12�xO19 (x = 0.0–0.8). 1096 M. J. Iqbal et al. / Scripta Materialia 57 (2007) 1093–1096 ber of iron ions at octahedral sites. The dielectric constant and dielectric loss factor data are in good agreement with the DC resistivity data (Table 1). The variation in AC magnetic susceptibility, v, with temperature is shown in Figure 3. The value of v in- creases regularly with temperature, reaching a maximum value, but at a specific temperature it suddenly drops. This behavior confirms the presence of a pure phase in the synthesized samples, which is in good agreement with the XRD analysis as already discussed. The value of Curie temperature, Tc, decreases by increasing Zr– Ni concentration because the 4f1 sites are being occupied by nonmagnetic Zr4+ ions the interactions between 4f1– 12k and 2a–4f1 sites most probably decrease. The syn- thesized samples do not show magnetic relaxation even for the sample annealed at 1170 �C. The magnetic relax- ation process depends upon the interactions between lattice vacancies, Fe2+ ions and domain walls. Since the crystallite size of the samples is small, as discussed above, and the particles are in a single domain hence they do not undergo the relaxation process; it is also ex- pected that the materials would have low magnetic losses. Co-precipitation is a reliable and economic method to synthesize single-phase, homogeneous and nanocrys- tallite strontium hexaferrite. Substitution of Zr–Ni at iron sites in strontium hexaferrite produces quite signif- icant and remarkable changes in the structural, electrical and magnetic properties. The saturation magnetization increases while the coercivity decreases. The improve- ment in saturation magnetization and the decrease in coercivity make these materials suitable for applications in recording media. The electrical resistivity and activa- mission (HEC) of Pakistan under indigenous and IRSIP schemes is gratefully acknowleged. The authors (PHG) and (JMM) are thankful to MEC, Junta de Castilla y Leon, for financial support under project no. MAT2004-04688-C02-02. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.scriptamat.2007.08.017. [1] G. Xiong, G. Wei, X. Yang, L. Lu, X. Wang, J. Mater. Sci. 35 (2000) 931. [2] N. Koga, T. Tsutaoka, J. Magn. Magn. Mater. 313 (2007) 168. [3] Q.Q. Fang, W. Zhong, Z. Jin, Y. Du, J. Appl. Phys. 85 (1999) 1667. [4] H. Yu, H. Lin, J. Magn. Magn. Mater. 283 (2004) 190. [5] Q.Q. Fang, H.W. Bao, D.M. Fang, J.Z. Wang, X.G. Li, J. Magn. Magn. Mater. 278 (2004) 122. [6] N. Sugita, M. Maekawa, Y. Ohata, K. Okinaka, N. Nagai, IEEE Trans. Magn. 31 (1995) 285. [7] I.H. Gul, F. Amin, A. Abbasi, M.A. Rehman, A. Maqsood, Scripta Mater. 56 (2007) 497. [8] P. Shepherd, K.K. Mallick, R.J. Green, J. Magn. Magn. Mater. 311 (2007) 683. [9] M.J. Iqbal, M.N. Ashiq, Chem. Eng. J., in press. [10] M.J. Iqbal, M.N. Ashiq, Scripta Mater. 56 (2007) 145. [11] M.J. Iqbal, S. Farooq, Mater. Sci. Engg. B 136 (2007) 140. [12] R.B.G. Kramer, V.S. Egorov, A.G.M. Jansen, W. Joss, J. Magn. Magn. Mater. 310 (2007) 1675. [13] C.D. Francisco, J.I. Iniguez, J.M. Munoz, J. Ayala, IEEE Trans. Magn. 23 (1987) 1866. [14] A. Mali, A. Ataie, Scripta Mater. 53 (2005) 1065. [15] M.V. Rane, D. Bahadur, A.K. Nigam, C.M. Srivastava, J. Magn. Magn. Mater. 192 (1999) 288. [16] Y. Li, R. Liu, Z. Zhang, C. Xiong, Mater. Chem. Phys. 64 (2000) 256. Magnetic, physical and electrical properties of Zr - Ni-substituted co-precipitated strontium hexaferrite nanoparticles References