Structural, electrical and magnetic properties of Zr–Mg cobalt ferrite

Structural, electrical and magnetic properties of Zr–Mg cobalt ferrite

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 320 (2008) 845–850 www.elsevier.com/locate/jmmm Structural, electrical and magnetic pro...

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ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 320 (2008) 845–850 www.elsevier.com/locate/jmmm

Structural, electrical and magnetic properties of Zr–Mg cobalt ferrite Muhammad Javed Iqbal, Mah Rukh Siddiquah Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan Received 12 May 2007; received in revised form 1 September 2007 Available online 25 September 2007

Abstract Spinel cobalt ferrite, CoFe2xMxO4 has been synthesized by substitution of the combination of metallic elements M ¼ Zr–Mg by the microemulsion method using polyethylene glycol as a surfactant. Powder X-ray diffraction analysis reveals that the substitution results in shrinkage of the unit cell of cobalt ferrite due to higher binding energy of the synthesized samples. The energy-dispersive X-ray fluorescence analysis confirms the stoichiometric ratios of the elements present. The thermogravimetric analysis shows that the minimum temperature required for the synthesis of these substituted compounds is 700 1C. A two-point probe method was employed for the measurement of the electrical resistivity in a temperature range of 29375 to 67375 K. It appears that there is a decrease in the number of Fe2+/Fe3+ pairs at the octahedral sites due to the substitution and corresponding migration of some of the Fe3+ ions to tetrahedral sites, consequently increasing the resistivity and the activation energy of hopping of electron at the octahedral sites. The susceptibility data also suggest migration of Fe3+ to tetrahedral site in the initial stage, which results in an increase in A–B interactions leading to large increase in the blocking temperature (TB) as observed in samples having dopant content x ¼ 0.1. r 2007 Elsevier B.V. All rights reserved. Keywords: Cobalt ferrite; Microemulsion method; DC-electrical resistivity; AC-magnetic susceptibility; Blocking temperature; Drift mobility

1. Introduction Spinel cobalt ferrite, CoFe2xMxO4 (M ¼ Mn, Mg, Zn, Ni, Co, etc.), has a face-centered cubic lattice with a large unit cell containing eight formula units. They have two kinds of lattice sites for cation to occupy: A and B sites having tetrahedral and octahedral co-ordinations, respectively. Commonly, the M2+ and Fe3+ cations are distributed 3+ at both the sites. Normally the Fe3+ superexchange A –FeB 2+ interaction is different from the MA –Fe3+ interaction. B The variation of cation distribution over the A and B sites leads to different electrical and magnetic properties even though the chemical composition of the compound does not change [1]. The properties of ferrites are dependent on the distribution of cations on the octahedral and tetrahedral sites in spinel lattice [2]. The nanoparticles are three dimensional inorganic solids with diameter of the order of a nanometer. It has been established [3–5] that the cation Corresponding author. Tel.: +92 51 90642143; fax: +92 51 90642241.

E-mail address: [email protected] (M. Javed Iqbal). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.09.009

distribution in nanocrystalline ferrites is different from that in the bulk ferrites. The unusual properties of the nanoferrites can be explained on the basis of the dependence of cation distribution of these ferrites on the method of preparation, particle size, pH, sintering temperature and rate, etc. The information regarding the lattice structure of spinel ferrite systems provides excellent opportunities for understanding the chemical manipulations of the super-paramagnetic properties of their nanoparticles [6]. For applications of nanomaterials in various diverse fields [7–9], new materials of more predictable properties than what are currently available need to be designed and exploited. Cobalt ferrite, owing to strong ferromagnetism and high Curie temperature, is used in electronic appliances as it causes the materials to stay magnetized even when the applied magnetic field is turned off, leading to a useful way of storing information. Substitution of Al3+ [10], Cr3+ [11], Nd3+ [12] and Ni2+ [13] decreases the saturation magnetization in cobalt ferrite, while Zn2+ [14,15], Zn–Zr [16], Cu2+ [17] and Cr3+ [11] decrease the Curie temperature (TC) and blocking temperature (TB). High electrical

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resistivity is an important property for the materials to be used in data storage devices. Little attention has been given to the study of electrical properties of the substituted cobalt ferrite. The goal of the present work is to synthesize and characterize new materials with expected higher electrical resistivity and blocking temperature and to investigate the corresponding changes in electrical conduction mechanism and magnetic behavior for the creation of new technology. A series of CoZrxMgxFe22xO4 (x ¼ 0.1–0.5) spinel ferrite samples are prepared by the microemulsion method [18] and the consequent changes in a number of parameters, namely, lattice parameter, porosity, X-ray density, bulk density, blocking temperature, DC-electrical resistivity, drift mobility and activation energy of hopping have been reported.

2. Experimental The compounds employed in sample preparation are cobalt acetate (Fluka, 99.0%), iron nitrate nona-hydrated (Merck, 99.0%), polyethylene glycol (PEG) (BDH, 98.0%), hydrated zirconyl oxychloride (BDH, 96.0%), hydrated magnesium nitrate (Merck, 99.9%) and ammonia solution (Fluka, 33.0%). Since these chemicals were of high purity, they are used as received. The samples are synthesized by a microemulsion method [18] following a procedure described as follows: Aqueous solutions of cobalt acetate, iron nitrate nonahydrated, hydrated zirconyl oxychloride and magnesium nitrate in a molar ratio of 1:2 of divalent (Co2+) to trivalent (Fe3++ Zr4++Mg2+) metal cations are homogenously mixed with constant stirring in an aqueous solution of PEG. Two molar ammonia solution is added to adjust solution pH ¼ 9.50 with the help of a pH-meter. The solution is then aged for 3 h in an oven at 90 1C, followed by its centrifugation. The precipitates thus collected are washed repeatedly with doubly distilled water and ethanol in order to remove any un-reacted substances. The precipitates are dried at 120 1C for 7 h and the powdered precursor is sintered in air at a rate of 5 1C/min up to 800 1C using a temperature controlled tube furnace (Carbolite CFT 12/100) and maintained at the top temperature for 3 h. After sintering pellets are made by pressing at 90 kN for electrical resistivity and magnetic susceptibility measurements. The samples are analyzed by X-ray diffraction (XRD) (PANalytical 3040/60) and by energy-dispersive X-ray Fluorescence (ED-XRF) spectroscopy (Horiba, MESA-500). Thermogravimetric analysis (TGA) (Perkin-Elmer, TGA/DTA), differential thermogravimetric analysis (DTA), DC-electrical resistivity measurement by two-point probe method [19] and AC-magnetic susceptibility using a double-coil system operating at 273 Hz with a magnetic field of 0.1 Oe [20], are carried out in the temperature range of 293–673 K.

3. Calculations The lattice constant (a) is calculated from the powder XRD data using the following equation [19]  1=2 a ¼ d 2 ðh2 þ k2 þ l 2 Þ , (1) where d is the value of d-spacing of lines in XRD pattern and hkl are the corresponding indices to each line in the pattern. Crystallite size (D) is calculated using the Debye–Scherrer equation [3] kl , b cos yB



(2)

where b is the broadening of diffraction line measured at half width of its maximum intensity, l is the X-ray wavelength used for analysis (1.542 A˚), yB is Bragg’s angle of diffraction and k the shape constant, which has a value of 0.89 for a cubic system. The X-ray density (dx) of the sample is calculated using the following relation [8]: dx ¼

ZM , V cell N A

(3)

where Z is the number of molecules per formula unit (Z ¼ 8 for spinel cubic system), M is the molar mass, Vcell and NA have their usual meanings. Electrical resistivity is measured by the two-point probe method as described previously [19]. The DC resistivity was calculated by the following equation A , (4) L where L is the height of pellet, R is the resistance and A is the area. A ¼ Pr2 of pellet of the sample having radius r. The activation energy of hopping Ea is calculated from the temperature dependence of resistivity r of the ferrite samples using the Arrhenius-type equation   Ea r ¼ ro , (5) kB T

r¼R

where ro is the resistivity value at 0 K and kB and T have their usual meanings. The activation energy (Ea) for hopping of electron is calculated from the slope of graph between ln r and T1. The drift mobility (m) of the charge carriers in the synthesized samples is calculated using the following equation [21]: m¼

1 , ner

(6)

where n is the number of charge carriers, e is the charge on electron and r is the resistivity at a given temperature. 4. Results and discussion Fig. 1 shows TGA and DTA curves of the dried CoZr0.1Mg0.1Fe1.8O4 precursor measured with a heating

ARTICLE IN PRESS M. Javed Iqbal, M. Rukh Siddiquah / Journal of Magnetism and Magnetic Materials 320 (2008) 845–850

10

0

8 -10

7 6

-20 (a)

5

-30

4 -40

3

(b)

2

-50

1

Heat Flow Endo Down (mW)--------

10

9

Weight (mg)_ _

847

-60

0 0

200

400

600

800

1000

1200

1400

Temperature (°C) Fig. 1. TG (a) and DSC (b) curves for sample CoZrx0.1Mgx0.1Fe1.8O4.

x = 0.5

x = 0.4

x = 0.3

Counts

D-1 x = 0.2

x = 0.1

(511)

pH9:5

þ 1:8FeðNO3 Þ3 þ NH4 OH ! CoðOHÞ2 þ 1:8FeðOHÞ3

(400)

(220)

0:1MgðNO3 Þ2 þ CoðCH3 COOÞ2 þ 0:1ZrOCl:H2 O

(440)

(311)

rate of 51C/min. The DTA curve shows three exothermic peaks at 195, 230 and 420 1C and also three endothermic peaks at 150, 180 and 405 1C. The TG curve shows that both the endothermic and exothermic peaks are accompanied by weight loss. The endothermic peak at 150 1C represents the loss of absorbed water; the endothermic peak at 180 1C represents decomposition of the metal hydroxides to oxides and the peak at 405 1C is due to the formation of the spinel phase. The TG curve also shows major weight loss in the temperature range 150–200 1C corresponding to the water loss from the precursor. There is a very slow weight loss from 400–700 1C and this region can be attributed to the crystallization of the spinel ferrite. The chemical reaction may occur according to the following equations:

x = 0.0

o

180 C

þ 0:1MgðOHÞ2 þ 0:1ZrðOHÞ4 ! CoO þ 0:9Fe2 O3 þ 0:1ZrO2 þ 0:1MgO þ CoO þ 0:9Fe2 O3 400700o C

þ 0:1ZrO2 þ 0:1MgO ! CoMg0:1 Zr0:1 Fe1:8 O4 : The XRD patterns of CoZrxMgxFe22xO4 (Zr–Mg content, x ¼ 0.1–0.5) show a single spinel phase of cubic symmetry (ICSD 01-076-2496) present in all the samples synthesized in this study (Fig. 2). The lattice parameter ‘a’ of the pure cobalt ferrite has a value of 8.385 A˚, which is close to the value 8.380 A˚ obtained for bulk ferrites by Chikazumi [22]. The value of ‘a’ shows a decreasing trend with an increasing concentration of Zr–Mg (Fig. 3). This indicates that the dopants must have simply replaced the Fe3+ ions without distortion of the cubic symmetry of the host cobalt ferrite. The value of Vcell of the Zr–Mg-doped cobalt ferrite samples, however, decreases, indicating slight shrinkage of the unit cell (Table 1) for the reason that the

20

30

40

50

60

70

Position [°2Theta] Fig. 2. Comparison of XRD patterns of the synthesized CoZrxMgx Fe22xO4 samples of different compositions (x ¼ 0.0–0.5).

binding energy of magnesium oxide (530.9 eV) is slightly larger than that of iron oxide (530.0 eV), while that of zirconium oxide (529.9 eV) is comparable to it [23]. The presence of stoichiometric molar amounts of various elements in the samples is confirmed by the ED-XRF analysis of the samples. The values of X-ray density (dx), the bulk density (db) and the average crystallite size (D) calculated from the XRD data using Eqs. (1–3) are listed in Table 1. Increase in

ARTICLE IN PRESS M. Javed Iqbal, M. Rukh Siddiquah / Journal of Magnetism and Magnetic Materials 320 (2008) 845–850

593

8.410 a

8.400

Vcell

591 589

8.390

587

a / °A

8.380

585 8.370 583 8.360

Vcell / °A3

848

581

8.350

579

8.340

577

8.330 0.00

0.10

0.20

0.30

0.40

575 0.50

Content 'x' Fig. 3. Lattice parameter (a) and cell volume (Vcell) of CoZrxMgxFe22xO4 samples (x ¼ 0.0–0.5) as a function of the content ‘x’ of Zr–Mg.

Table 1 Lattice parameter (a) unit cell volume (Vcell), crystallite size (D), X-ray density (dx), bulk density (db), porosity (p) and blocking temperature (TB) of CoZrxMgxFe22xO4 (x ¼ 0.00.5) x

a (A˚)

Vcell (A˚3)

D (nm)

dx (70.01 gcm3)

db (70.04 gcm3)

p

TB(71 K)

0.0 0.1 0.2 0.3 0.4 0.5

8.385 8.387 8.364 8.350 8.343 8.339

589.58 589.87 585.12 582.16 580.68 579.84

19.88 52.43 52.66 46.62 46.65 34.82

5.12 5.12 5.17 5.21 5.23 5.24

3.45 2.75 3.04 2.81 2.83 3.18

0.33 0.46 0.41 0.46 0.46 0.39

545 670 613 604 576 539

350 ρ (293 K)

Ea

0.48

300

0.44

200 0.40

150

Ea / eV

ρ / (Ωcm) 107

250

100 0.36 50 0

0.32 0.0

0.1

0.3

0.2

0.4

0.5

Content 'x' Fig. 4. Plot of room temperature resistivity (r) and activation energy of hopping (Ea) of CoZrxMgxFe22xO4 samples of different compositions (x ¼ 0.0–0.5) versus Zr–Mg content ‘x’.

the values of dx and db with an increment in dopant concentration x is noticeable. The average value of D in various samples is found to be in the range 30–5574 nm (Table 1). The DC-electrical resistivity of the synthesized samples measured at 293 K increases with increase in the dopant concentration (Fig. 4). The conduction in ferrites is believed to be due to the exchange of electrons between Fe2+ and Fe3+ ions that results in local displacement of

charges causing polarization of the lattice. The magnitude of this exchange depends on the Fe3+/Fe2+ ion pairs present on octahedral (B) sites [23]. As the distance between the two ions (Fe3+ and Fe2+) present at the octahedral site is minimal, hopping mechanism could be responsible for the electrical conduction. On doping cobalt ferrite with Zr–Mg, the Fe3+ ions are partially replaced by Mg2+ ions, which also have a strong octahedral (B) site preference [24], while Zr4+ has tetrahedral site preference

ARTICLE IN PRESS M. Javed Iqbal, M. Rukh Siddiquah / Journal of Magnetism and Magnetic Materials 320 (2008) 845–850

300

ρ/(Ωcm) 107

250 200 150

x = 0.0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5

100 50 0 275

325

375

425

475

525

575

625

675

T/K Fig. 5. Electrical resistivity (r) of CoZrxMgxFe22xO4 samples of different compositions (x ¼ 0.00.5) as a function of temperature (T).

225

μ/(cm2 V-1 sec-1) 1010

200 175 150 125 100 75 50 25 0 275

x = 0.0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5 325

375

425

475

525

575

625

675

T/K Fig. 6. Drift mobility (m) of CoZrxMgxFe22xO4 samples with different composition (x ¼ 0.0–0.5) as a function of temperature (T).

2.30 2.25 2.20 1/X

[25]. Thus, the substitution of Zr–Mg would decrease the number of Fe3+/Fe2+ ion pairs present at the octahedral (B) sites, consequently increasing the room temperature DC-electrical resistivity with increasing dopants concentration. The observed decrease in the value of DC-electrical resistivity (r) with the increase in temperature indicates that all synthesized samples are semiconductors in nature (Fig. 5) and that it follows Arrhenius-type temperature dependence. The value of Ea for a pure cobalt ferrite sample is 0.3470.02 eV which increases with an increase in the value of ‘x’ of the samples and reaches a maximum value of 0.4870.02 eV at x ¼ 0.5 as shown in Fig. 4. The substitution of Fe3+ by a combination of Zr4+ and Mg2+ ions causes decreased migration of Fe3+ ions to the tetrahedral site. This decrease in Fe3+ at the octahedral site reduces the frequency of electron exchange by hopping of the electron at the octahedral site, between Fe2+ and Fe3+ [26]. The drift mobility (m) increases with temperature (Fig. 6), and it is believed that this behavior represents

849

2.15 2.10 2.05 2.00 250

x = 0.0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5 350

450

550

650

750

T/(K) Fig. 7. AC-susceptibility (w) of CoZrxMgxFe22xO4 samples with different composition (x ¼ 0.0–0.5) as a function of temperature (T).

enhanced mobility of the charge carriers due to thermal activation by increase of temperature and not due to generation of the charge carriers. The charge carrier concentration is reported to remain constant throughout the temperature range studied here [19]. Fig. 7 shows the variation of the real part of ACmagnetic susceptibility (w) with temperature. The value of w first increases and reaches a maximum at a temperature called blocking temperature (TB) and then it suddenly drops. This behavior is characteristic of small magnetic particles due to blocking of the individual particle moments along their anisotropy direction at blocking temperature (TB). The sudden decrease in magnetic susceptibility after TB confirms the absence of impurities even in trace amount, leading to the conclusion that a single spinel phase is formed as already predicted on the basis of the XRD patterns of these samples (Fig. 2). Since the samples with x ¼ 0.1–0.3 are found to be stable in the temperature range of 350–600 K, they may be suitable for applications where temperature stability is required. At TB, the anisotropy energy KV (K is the anisotropy constant and V is the volume of the particle) for the particle becomes larger than the thermal energy (kBT), which appears effectively blocked [27]. The TB value for pure cobalt ferrite (543 K), increases with the addition of Zr–Mg in the system. The CoZr0.1Mg0.1Fe1.8O4 sample has the highest value of TB, which continues to decrease with the addition of Zr–Mg, but the value remains larger than the pure cobalt ferrite up to x ¼ 0.4. The value for the transition temperature (TB) found in this study for pure cobalt ferrite is 545 K, which is less than that calculated by Rajendran et al (773 K) [28] and Gul and Maqsood (678 K) [16]. The difference in these values is due to the difference in crystallite size of cobalt ferrite in the three studies. However, only the sample of composition CoZr0.5Mg0.5 FeO4 has a value of TB lower than that of the pure cobalt ferrite. The reason for the initial increase in TB may be the migration of some of the Fe3+ ions to tetrahedral sites with

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the substitution of Zr–Mg in cobalt ferrite, as depicted by the resistivity data, consequently enhancing the A–B exchange interactions in the samples. While for x40.2, non-magnetic Zr–Mg replaces only the Fe3+ ions on the octahedral sites, leading to reduction in the A–B exchange interactions. 5. Summary Thermogravimetric analysis shows that the spinel phase in Zr–Mg doped cobalt ferrite can be obtained by sintering at a temperature as low as 700 1C. Doping Zr–Mg in cobalt ferrite results in a slight shrinkage of the unit cell because of the higher binding energy of zirconium and magnesium oxides. The electrical resistivity, the activation energy of hopping and the magnitude of A–B interactions increase with doping Zr–Mg in cobalt ferrite due to the change in distribution of Fe3+ and Fe2+ ions in the crystal lattice. The number of charge carriers remains constant as the increase in drift mobility of the charge carriers at higher temperatures is due to their thermal activation. Acknowledgment Financial support of the Higher Education Commission of Pakistan under the Indigenous Scholarship Scheme is gratefully acknowledged. References [1] C. Liu, B. Zou, A.J. Rondinone, Z.J. Zhang, J. Am. Chem. Soc. 122 (2000) 6263. [2] L. Zhao, H. Yang, L. Yu, Y. Cui, X. Zhao, B. Zou, S. Feng, J. Magn. Magn. Mater. 301 (2006) 445. [3] J.F. Hochepied, M.P. Pileni, J. Appl. Phys. 87 (2000) 2472.

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