57Fe Mössbauer and magnetic studies on the spinel system Co1+xGexFe2−2xO4

Solid State Communications, Vol. 78, No. 6, pp. 539-542, 1991. Printed in Great Britain.

0038-1098/91 $3.00 + .00 Pergamon Press plc

57Fe M O S S B A U E R A N D M A G N E T I C STUDIES ON T H E SPINEL SYSTEM Co, +xGe~Fe2

2xO4

H.H. Joshi, R.B. Jotania and R.G. Kulkarni Department of Physics, Saurashtra University, Rajkot 360 005, India and R.V. Upadhyay Department of Physics, Bhavnagar University, Bhavnagar 364002, India

(Received 15 November 1990 by D. Van Dyck) The magnetic properties of Co j+xGex Fe 2 2xO4system have been investigated by M6ssbauer, Magnetization and low field a.c.-susceptibility techniques. The low field a.c.-susceptibility shows the presence of spin clusters in the samples. The M6ssbauer spectra for x ~< 0.4 suggest the existence of two hyperfine fields due to A and B-sites Fe ions while for x = 0.5 they show a relaxation effect. The results are explained on the basis of A-B and B-B supertransferred hyperfine interactions.

1. I N T R O D U C T I O N E M P I R I C A L L Y it is known that the diamagnetic cations such as Li, Mg, A1, Ti and Nb prefers the octahedral (B) sites and cations such as Zn, Cd, Ga and Si prefer tetrahedral (A) sites in the spinel ferrites. Gordy and Thomas [1] have studied the site preference of various cations with respect to electronegativities and ionic radii. This study has revealed that the Ti 4+ ion has a strong preference for the B-site and the Ge ion has an A-site preference. Recently, we have reported the magnetic properties of the Co~ +xTix Fe2_zxO4 system [2]. Therefore, it is interesting to study the magnetic properties of Ge-substituted CoFe204 In the solid solution of spinel Co~ +xGGFe2 2xO4, the Fe 3÷, Ge 4+, and Co 2÷ are distributed among octahedral (B) and tetrahedral (A) interstitial sites of the f.c.c, lattice. The x = 0.0 i.e. CoFe204 possesses

an inverse spinel structure and the degree of inversion depends upon the heat treatment [3]. While x = 1.0 i.e. Co2GeO4 has been reported to be a normal spinel with all Ge ions on A-sites [4, 5]. The present paper reports the magnetic properties of the Co~+~GG Fe2 zxO4 spinel ferrite system (for x ~< 0.5) using M6ssbauer spectroscopy, low field a.c.-susceptibility and magnetization measurement techniques. 2. E X P E R I M E N T A L The samples were prepared by the usual double sintering ceramic process. The starting materials were analytical reagent grade Fe203 (Thomas Baker), CoO (J.T. Baker) and GeO2 (E. Merck). The oxides were mixed in stoichiometric proportions and presintered at 900°C for 12 h. In the final sintering process the material was held at 1150°C for 24 h and slowly cooled

Table 1. The M6ssbauer parameters at 300K for Co~+~GexFe2 2x04 system: Isomer shift (LS), quadrupole splitting (Q.S) x

0.0 0.1 0.2 0.3 0.4 0.5

I . S ( m m s i),

Q . S ( m m s i)

I.S(B)

I.S(A)

0.32 + 0.02 0.26 + 0.02 0.29 + 0.02 0.25 + 0.02 0.28 + 0.02 RELAXATION

0.26 0.21 0.23 0.20 0.22

+ + + _ +

0.02 0.02 0.02 0.02 0.02

* With respect to iron metal. 539

Q.S(B)

Q.S(A)

0.00 + 0.02 0.00 + 0.03 0.00 + 0.02 0.00 + 0.02 0.00 + 0.02 SPECTRUM

0.00 0.00 0.00 0.00 0.00

+ + + + +

0.03 0.02 0.02 0.02 0.02

540

M O S S B A U E R A N D M A G N E T I C STUDIES ON THE SPINEL SYSTEM Vol. 78, No. 6

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800 ~.-x 8.39 x

700

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600 -

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Z

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--

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Fig. 3. Variation of Ndel temperature with Ge content

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(x).

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Ge Content (X) Fig.

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Co~ +xGe~ Fe2

of

lattice

constant

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to room temperature (2°C rain ]). The X-ray powder patterns for all the ferrite samples were recorded using FeK~ radiation on a Philips X-ray diffractometer. The X-ray diffraction patterns clearly indicated the presence of only the f.c.c, spinel phase with no extra lines corresponding to any other phase. The M6ssbauer spectra were obtained with a constant acceleration transducer and a 512 multi-channel analyser operating in time-mode. A gamma source of Co(Pd) of 10mCi was used. All the spectra were obtained at

300 K in transmission geometry and 14.4 keV gamma rays were detected with a Xenon-Methane filled proportional counter. The a.c.-susceptibility of polycrystalline samples was measured using the double coil set-up described elsewhere [6]. Magnetization measurements were carried out using the high field hysteresis loop technique [7].

3. RESULTS A N D D I S C U S S I O N For all the samples X-ray diffraction showed 100 • )-

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I

300 &00 500 600 700 800 Temperature(K)

Fig. 2. Thermal variation of low field a.c. susceptibility for x ~< 0.5.

97 ~

i.I -10.0

' '

"' :'

m -5"0

": ""

"

j 0

"'"':" " " "

I 90

I 10"0

Velocity (m m/s )

Fig. 4. M6ssbauer spectra for x ~< 0.5 at 300 K.

541

Vol. 78, No. 6 MOSSBAUER A N D M A G N E T I C STUDIES ON T H E SPINEL SYSTEM sharp lines corresponding to single phase f.c.c, spinel for x ~< 0.5. The lattice constant decreases linearly with Ge substitution [Fig. 1]. This is because of the fact that smaller Ge 4+ ions replace larger Fe 3+ ions from the A-sites. The analysis of X-ray intensity data suggest the following cation distribution,

A- site

520 ~..--- _

I

-3-

B-site ,

\

4B0

-

\

-

2+ 3+ B 2 (Ge~4 + Fe~3+ x)A [Coj+~Fe~ x] O4 .

\ \\~

O

Plots of normalised low field a.c. susceptibility (Z/ZRT) against temperature are shown in Fig. 2. For x ~< 0.5. In pure CoFe204 and in C o - Z n ferrites [8], a peak was observed near 533K which is referred to as the isotropic peak. This peak could be seen clearly for a magnetic material in a multi-domain state only if the material has a temperature at which the magnetocrystalline anisotropy is zero [9]. Beyond the temperature at which the isotropic peak occurs the shape anisotropy will be dominated as a result the coercive force arises. According to Bean [10] the susceptibility is inversely proportional to the coercive force, therefore the increase in susceptibility after the isotropic peak is attributed to a decrease in coercive force. The existence of a coercive force clearly indicates that the samples contain spin clusters of different size and each spin cluster should be large since a very high blocking temperature is observed. The addition of 10% Ge 4+ and Co 2+ reduces the coercive force and as a result, the peak value of the susceptibility decreases. Therefore, it can be said that the samples of these system contain multi-domain and spin clusters. The N6el temperatures were determined from susceptibility measurements are depicted in Fig. 3, for x ~< 0.5. The M6ssbauer spectra obtained at 300K are displayed in Fig. 4 for x ~< 0.5. For x ~< 0.4 the spectra exhibits Zeeman sextets due to the Fe 3+ ions at B and A-sites. The spectrum for x -- 0.5 is a broad magnetic sextet with no resolution characteristics of two sites. The parameters derived from the leastsquare fits are given in the Table. It is evident from the Table that the isomer shift exhibits very weak dependence on Ge content indicating that s-electron charge distribution is negligibly influenced by Ge substitution. The average weighted isomer shifts of A and B-sites are I.S.(A) = 0.22 +_ 0.02mms ~ and I.S(B) = 0.28 _+ 0.02mms J, respectively. The quadrupole shifts in the Zeeman sextets remain almost zero within the experimental error for A and B-sites which is a general observation for spinel ferrites [I 1, 12]. A general line broadening is observed in M6ssbauer lines for both A and B-sites sextets which is due to the varying chemical environment seen by different iron ions in the same sublattice. The internal magnetic

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Fig. 5. (a) Hyperfine field vs Ge content for Co~+xGe~ Fez 2xO4 system. (b) Variation of relative magnetization with Ge content (x). field value at the Fe nucleus shows a faster decrease for H A and Ha at x > 0.2 (Fig. 5a), which can be explained on the basis of A-B and B-B supertransferred hyperfine interactions. Fig. 5b shows the variation of relative magnetization with x, obtained from M6ssbauer and magnetization data. It is apparent from Fig. 5b that there is a large discrepancy between the two for x /> 0.2 indicating the presence of YK type of spin canting in the system, which has been explained in detail in [13]. The relaxation observed for x = 0.5 at 300 K is related to the localized canting of the spins at the B-site. REFERENCES 1. 2. 3. 4.

W. Gordy & W.J.G. Thomas, J. Chem. Phys. 24, 439 (1956). H.H. Joshi & R.G. Kulkarni, Solid State Commun. 60, 67 (1986). G.A. Sawataky, F. Van der Woude & A.H. Morrish, J. Appl. Phys. 39, 1204 (1962). F,C. Romeijn, Philips. Res. Rep. 8, 304 (1962).

542 5. 6.

.

8.

MOSSBAUER AND MAGNETIC STUDIES ON THE SPINEL SYSTEM Vol. 78, No. 6 G. Blasse & J.F. Fast, Philips. Res. Rep. 18, 393 (1963). C. Radhakrishnamurty, S.D. Likhite & P.W. Sahasrabudhe, Proc. Ind. Accd. Sci. 87A, 245 (1978). C. Radhakrishnamurty, S.D. Likhite & N.P. Sastry, Phil. Mag 23, 503 (1971). R. Satyanarayan & S. Ramana Murthy, J. Mat. Sci. Lett. 4, 241 (1965).

9.

10. 11. 12. 13.

S.A. Plotinnikov & G.M. Prikhodkine. Soy. Phys. Solid State. 7, 241 (1965). C.P. Been, J. Appl. Phys. 26, 1381 (1955). L.K. Leung, B.J. Evans & A.H. Morrish. Phys. Rev. 829, (1973). G.A. Pattit & D.W. Forester, Phys. Rev. 4, 3912, (1971). R.V. Upadhyay, Solid State Commun. 73, 463 (1990).