Structural and magnetic properties of aluminium and chromium co-substituted cobalt ferrite

Structural and magnetic properties of aluminium and chromium co-substituted cobalt ferrite

June 2000 Materials Letters 44 Ž2000. 91–95 www.elsevier.comrlocatermatlet Structural and magnetic properties of aluminium and chromium co-substitut...

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June 2000

Materials Letters 44 Ž2000. 91–95 www.elsevier.comrlocatermatlet

Structural and magnetic properties of aluminium and chromium co-substituted cobalt ferrite D.R. Mane, U.N. Devatwal, K.M. Jadhav ) Department of Physics, Dr. B.A. Marathwada UniÕersity, Aurangabad 431 004, India Received 24 September 1999; received in revised form 7 January 2000; accepted 10 January 2000

Abstract Polycrystalline cobalt aluminium chromium ferrites ŽCoAl xCr x Fe 2y2 x O4 . with varying Al–Cr substitution Ž0 - x - 0.5. have been prepared in pellet form by standard double sintering technique and studied by X-ray diffraction ŽXRD., magnetization and ac susceptibility measurements. The lattice constants are determined and the applicability of Vegard’s law has been verified. The saturation magnetization Ž ss . decreases with Al–Cr content Ž x ., indicating reduction in ferrimagnetic behaviour. The variation of the saturation magnetic moment per formula unit measured at 300 K with Al–Cr content is satisfactorily explained on the basis of Neel’s colinear spin ordering model for x s 0.0–0.3. Thermal variation of low field ac susceptibility measurements from room temperature to 860 K exhibits almost normal ferrimagnetic behaviour and the Neel temperature ŽT N . decreases with increasing Al–Cr content x, which is consistent with the observed decrease in ss with x. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ferrites; Cation distribution; Magneton number; Neel temperature

1. Introduction Ferrites having high resistivity and low eddy current losses have been found to be the most versatile to be used for technological applications. Cobalt ferrite CoFe 2 O4 possesses an inverse spinel structure, and the observed degree of inversion depends upon the heat treatment w1x. The basic magnetic properties of these materials depend on what kind of metal ion is present at the different sites and how these ions are distributed. The advantage of the

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Corresponding author.

mixed spinel considered here is that all interactions are well defined near neighbour antiferromagnetic with < JAB < 4 < J BB < 4 < JAA < and JAB superexchange interactions rendering the spinel ferrimagnetism. CoCr2 O4 is a normal spinel with a canted ferrimagnetic structure Žcanting of individual moments on B site. w2x having a Curie temperature TC of 97 K w3x. Though magnetic properties of CoAl x Fe 2yxO4 w4,5x and CoCr x Fe 2yxO4 w6x have been studied separately, no attempt has been made to study the combined effect of Al–Cr substitution for Fe in CoFe 2 O4 . The purpose of this paper is to present the results of investigation of the crystal structure, cation distribution and magnetic properties of Al–Cr co-substituted CoAl x Cr x Fe 2y2 xO4 Ž x s 0.0–0.5. mixed

00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 0 8 - 2

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D.R. Mane et al.r Materials Letters 44 (2000) 91–95

spinel by means of X-ray, magnetization and ac susceptibility measurements systematically.

2. Experimental The samples of Al and Cr co-substituted cobalt ferrite CoAl x Cr x Fe 2y2 xO4 with x s 0–0.5 were prepared by the usual ceramic method. The starting materials were CoO, Al 2 O 3 , Cr2 O 3 and Fe 2 O 3 all 99.9% pure supplied by E. Merck. The oxides were mixed thoroughly in stoichiometric proportions to yield the desired composition and wet ground. The mixture was dried and pressed into pellets. These pellets were presintered at 9908C for 12 h and slowly cooled to room temperature. The samples were again powdered, pressed into pellets, refired in air at 11008C for 24 h and cooled slowly to room temperature to obtain ferrite phase. The powder X-ray diffraction ŽXRD. pattern for all the samples were recorded at room temperature with a Philips ŽPW 1710. diffractometer using CuK 0 radiation. X-ray diffractograms showed sharp lines corresponding to single-phase spinel. The magnetization measurements of all the samples were carried out using high field hysteresis loop technique w7x at 300 K. The ac susceptibility measurements on powdered samples were made in the temperature range 300–860 K using a double-coil set up w8x operating at a frequency of 263 Hz and in rms field of 7 Oe.

3. Results and discussion Analysis of XRD revealed that all samples have a single phase spinel structure. The values of lattice parameter ‘a’ determined from X-ray data with an ˚ for all the samples are shown accuracy of "0.002 A in Fig. 1Ža. as a function of Al–Cr content x. The lattice constant a decreases with Al–Cr concentration x. The observed linear decrease in a with x is due to the replacement of larger ionic crystal radius ˚ . by the smaller Al 3q Ž0.51 A˚ . and of Fe 3q Ž0.64 A 3q Ž ˚ . Cr 0.63 A simultaneously, and it obeys Vegard’s law w9x. Usually, in a solid solution of spinel within the miscibility range, a linear change in the lattice constant with concentration of the components is observed w9x. The densities of CoAl x Cr x Fe 2 O4 have

Fig. 1. Variation of lattice parameter with Al–Cr content x.

been calculated from the molecular weight and the volume of the unit cell. The variation of X-ray density with Al–Cr concentration x is shown in Fig. 1Žb.. The X-ray density decreases slowly with increase of Al–Cr concentration this is because, the decrease in mass overtakes the decrease in volume of the unit cell. Since cobalt aluminate is a partially inverted spinel, Al 3q can occupy both A and B sites, on the other hand cobalt chromite is a normal spinel, Cr 3q occupies wBx site only as Cr 3q has strong preference for octahedral wBx site w10x and Fe 3q ions occupy both ŽA. and wBx sites w11x. Comparing the site preference energies of the constituent ions w11x and from the earlier studies w4x, the cation distribution of CoFe 2 O4 has been accepted as A B Ž Co 0.05 Fe 0.95 . w Co 0.95 Fe1.05 x

Ž 1.

In order to determine the cation distribution, XRD intensities were calculated using the formula suggested by Buerger w12x Ih k l s < Fh k l < 2 PL p

Ž 2.

where Ih k l is the relative integrated intensity, Fh k l the structure factor, P the multiplicity factor, and L p the Lorentz polarization factor ws Ž1 q cos 2 2 u .r Žsin2u cos u .x.

D.R. Mane et al.r Materials Letters 44 (2000) 91–95

Fig. 2. X-ray intensity ratio Ž I220 r I440 . and I422 r I400 . vs. Al–Cr content x.

The formulae for the structure factors for the planes Ž hkl . given by Furuhashi et al. w13x have been used. The formulae for the multiplicity factor and Lorentz polarization factors are taken from literature w9x. The absorption and temperature factors are not taken into account in our calculations because these do not affect the relative intensity calculations for spinels at room temperature w14x. Table 1 shows the best-found cation distribution among available tetrahedral ŽA. and octahedral wBx sites obtained from XRD and magnetization data.

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Fig. 3. The distance between magnetic ions in both octahedral and tetrahedral sites as a function of x.

The distribution of divalent and trivalent cations among octahedral and tetrahedral sites in the CoAl x Cr x Fe 2y2 xO4 was determined from the ratio of XRD lines I220rI440 and I422rI400 . The intensity ratios have been calculated taking into consideration the possible combinations of cations, and were compared with the observed intensity ratios. The results of X-ray intensity calculations for various possible models have been tried for samples x s 0.0–0.5 and those which agree with the experimental intensity ratios are shown in Fig. 2. The calculated I220rI440 and I422rI400 ratios for x s 0.0–0.3 are in good

Table 1 Cation distribution data for CoAl x Cr x Fe 2y2 x O4 system x

0.0 0.1 0.2 0.3 0.4 0.5

Fe 3q tetrahedral

Cation distribution A site

B site

Fe 3q octahedral

ŽCo 0.05 Fe 0.95 . ŽCo 0.1 Fe 0.88 Al 0.02 . ŽCo 0.12 Fe 0.84 Al 0.04 . ŽCo 0.15 Fe 0.79 Al 0.06 . ŽCo 0.2 Fe 0.72 Al 0.08 . ŽCo 0.25 Fe 0.65 Al 0.1 .

wCo 0.95 Fe1.05 x wCo 0.9 Fe 0.92 Cr0.1 Al 0.08 x wCo 0.88 Fe 0.76 Al 0.16 Cr0.2 x wCo 0.85 Fe 0.61 Al 0.24 Cr0.3 x wCo 0.80 Fe 0.48 Al 0.32 Cr0.4 x wCo 0.75 Fe 0.35 Al 0.4 Cr0.5 x

0.904 0.956 1.105 1.295 1.500 1.857

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Table 2 Saturation magnetization Ž ss ., magneton number Ž n B . and Neel temperature data for CoAl x Cr x Fe 2y2 x O4 system x

ss nB Ž m B . TN ŽK. Žemurg. Observed Calculated Susceptibility Laoria

0.0 0.1 0.2 0.3 0.4 0.5

74.50 70.00 59.01 51.17 40.33 29.09

3.13 2.90 2.41 2.06 1.60 1.12

3.20 2.90 2.48 2.08 1.80 1.48

855 820 750 653 630 600

835 797 725 630 610 585

agreement with the experimental determined I220rI440 and I422rI400 ratios, confirming the colinear spin ordering, while for x ) 0.3, these ratios differ from the experimental ratios ŽFig. 2., indicating that significant canting exist on B sites suggesting that the magnetic structure is non-colinear. The distance between magnetic ions hopping lengths in octahedral sites is given by Ž1r4. a2 , whereas for the tetrahedral site, it is given by Ž1r4. a 3 w15x. Fig. 3 shows the relation between hopping length for octahedral and tetrahedral sites as a function of Al–Cr content Ž x .. The distance between the magnetic ions decreases as the Al–Cr content increases. This may be explained on the basis of the ˚ . and Cr 3q Ž0.63 A˚ . smaller radius of Al 3q Ž0.51 A 3q Ž ˚ . than that of Fe 0.64 A , which makes the magnetic ions close to each other and the hopping length decrease. The values of saturation magnetization and magneton number n B Žthe saturation magnetization per formula unit in Bohr magneton. at 300 K were

Fig. 4. Variation of n B with Al–Cr content x.

Fig. 5. Thermal variation of ac susceptibility with Al–Cr content.

obtained from hysteresis loop technique w7x for x s 0.0–0.5. The values of ss and n B are presented in Table 2. From field dependence of magnetization and observed magnetic moments ŽTable 2., it is clear that the samples with x s 0–0.5 show ferrimagnetic behaviour, which decreases with increasing Al–Cr content values. Fig. 4 shows the variation of n B with x at 300 K for x s 0.0–0.5, and it is evident from Fig. 4 that n B gradually decreases with increasing x.

Fig. 6. Variation of Neel temperature ŽTN . with Al–Cr content x.

D.R. Mane et al.r Materials Letters 44 (2000) 91–95

According to Neel’s two-sublattice model of ferrimagnetism w16x, the Neel’s magnetic moment per formula unit in m B , n NB is expressed as n NB s M B

Ž x . y MA Ž x .

Ž 3.

where M B and MA are the B and A sublattice magnetic moments in m B , respectively. The n NB Ž m B . values for CoAl x Cr x Fe 2y2 xO4 were calculated using the ionic magnetic moments of Fe 3q, Co 2q, Cr 3q and Al 3q with their respective values 5m B , 3 m B , 3 m B and 0 m B . The calculated n NB Ž m B . values for x s 0–0.5 using Eq. Ž3. are also shown in Fig. 4 The calculated n NB values for x s 0–0.3 are in good agreement with the experimentally determined values, confirming the colinear spin ordering, while for x ) 0.3 they clearly differ from the observed values ŽFig. 4., indicating that significant canting exists on B-sites suggesting that magnetic structure is non-colinear. The plots of relative low-field ac susceptibility x Trx RT ŽRT s room temperature. against temperature T, for all the samples are shown in Fig. 5, which exhibit normal ferrimagnetic behaviour. The Neel temperature ŽTN . obtained from ac susceptibility data is shown in Fig. 6 as a function of Al–Cr content x. It is evident from Fig. 6 that Neel temperature decreases with increase in x indicating reduction in ferrimagnetic behaviour, which is in conformity with magnetization results. Neel temperature obtained from ac susceptibility data are in agreement with those obtained by Laoria technique w17x ŽTable 2..

4. Conclusions From the above experimental results and discussions, it can be concluded that, magnetization measurements exhibit non-colinear ferrimagnetic structure for x ) 0.3, which is very well supported by X-ray intensity calculations. AC susceptibility results suggest normal ferrimagnetic behaviour. The Neel temperature decreases with increasing x, which is in consistent with magnetization data.

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Acknowledgements The authors are thankful to Dr. G.K. Bichile, Department of Physics, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, India for his valuable suggestions and the fruitful discussion. The authors are also thankful to RSIC, Nagour for providing X-ray diffractometry charts. One of the authors ŽU.N. Devatwal. is thankful to the UGC, New Delhi, for providing financial support in the form of a fellowship.

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