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The effect of Zn on the defects in sputter deposited Li–Zn ferrite films
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1636}1637
The e!ect of Zn on the defects in sputter deposited Li}Zn ferrite "lms J. Dash , R.P.R.C. Aiyar, Shiva Prasad *, N. Venkataramani, S.K. Date, S.D. Kulkarni, Pran Kishan, Nitendra Kumar Department of Physics, IIT, Powai, Mumbai 400076, India ACRE, IIT, Powai, Mumbai 400076, India NCL, Pune 8, India SSPL, Delhi 54, India
Abstract In this paper we report the magnetization as a function of applied "eld at 300 K and magnetization as a function of temperature for several Li}Zn ferrite thin "lms with varying Zn content. A molecular "eld model has been used to "t the M vs. ¹ data for both the thin "lm and the bulk. The magnetic properties of these thin "lms are found to be di!erent from that of the bulk of similar composition. The di!erence is attributed to the presence of point defects, which leads to paraprocesses, thereby reducing the magnetization of the "lms. 2001 Elsevier Science B.V. All rights reserved. Keywords: Ferrites; Thin "lms; Magnetization vs. temperature; Molecular "eld model
The magnetic properties of ferrite thin "lms have been observed to be di!erent from the bulk of similar composition [1]. In a recent paper [2] we have reported a detailed study of magnetization, M in thin "lm Li Mn Zn Fe O , x"0.32 deposited by \V V \V sputtering and shown that the presence of defects are responsible to a large extent for this behavior. In this paper we report a similar study carried out on samples with varying Zn concentrations (x"0.0, 0.16, 0.48) mainly to see the e!ect of Zn on these defects. We also report the temperature dependence of magnetization on all these samples including the case x"0.32, to see how the defects manifest themselves in the M vs. ¹ curves. The "lms have been deposited using a Leybold Z400 RF sputtering unit on fused quartz, silicon and polycrystalline alumina. The "lms reported in this work have been deposited on polycrystalline alumina, because fairly thick "lms ('0.70 m) are needed for high-temperature magnetization measurements. Such thick "lms tend to
* Corresponding author. Fax: #091-22-5723480. E-mail address: [email protected] (S. Prasad).
peel o!, when deposited on quartz or Si substrates. The magnetization values of the "lms however, have been found to be independent of the substrate. The "lms have been annealed at temperatures in the range of 400}8503C for 2 h. The magnetization, M has been measured as a function of temperature using a PAR 4500 vibrating sample magnetometer with the "eld applied in the plane of the "lm in a constant magnetic "eld of 0.5 T. Similar to our reported results for x"0.32 "lm, the M vs. H data, for all the samples do not show saturation even at the maximum "eld of 1 T available with the VSM. The M vs. H data have been "tted to an expression of the form M"Q[1!(H /H)L]. The best "t for all the "lms is found when the exponent, n equals 0.5. Table 1 shows the parameters Q and H for the as deposited and the an nealed "lms. The value of Q is found to increase with increase in annealing temperature, ¹ and for ¹ "8503C the value of Q is close to the bulk value of saturation magnetization except for the case of x"0. For all values of x, except x"0, Q is found to be minimum for the as deposited "lm. The value of H is maximum for the as deposited "lm and decreases as ¹ increases. For the "lms annealed as mentioned above the value of H decreases with increasing x. For example,
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 0 4 6 - 5
J. Dash et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1636}1637 Table 1 Q and H values for Li Mn Zn Fe O "lms (as \V V \V deposited and annealed) for various values of x. The bulk value of saturation magnetization is shown for comparison. The values for as deposited "lms is indicated as (dep) and that for annealed "lms is indicated as (ann) x
0.00
0.16
0.32
0.48
Q (Dep) (kG) H (Dep) (kG) Q (Ann) (kG) H (Ann) (kG) Bulk 4M (kG)
3.03 0.94 2.55 0.17 3.60
2.97 0.96 4.86 0.14 4.60
3.14 0.86 5.33 0.11 4.80
0.98 0.74 4.63 0.05 4.00
Fig. 1. The magnetization, M, relative to that at 300 K as a function of temperature, ¹ for Li Mn Zn Fe O \V V \V for (a) x"0 and (b) x"0.48 bulk and thin "lms. The calculated data using Dionne's model is also shown.
the value of H for annealed x"0 "lm is about four times that of annealed x"0.48 "lm. The M vs. ¹ data for x"0 and 0.48 are shown in Fig. 1, both for the bulk and the thin "lm case. We clearly
1637
see that the M vs. ¹ data for thin "lm and bulk of the same composition is di!erent, the di!erences becoming less evident with increasing x. The data indicate that the curie temperature, ¹ is the same for bulk as well as thin "lm of the same composition, if looked from the tail of the M vs. ¹ curve. A molecular "eld model based on a localised canting, as proposed by Dionne [3], has been used to "t the M vs. ¹ data. The composition studied by Dionne does not have Mn. Dionne has mentioned that the presence of Mn is likely to reduce the molecular "eld coe$cients between A}B and B}B sublattices. We assume that both these coe$cients reduce by the same factor () so that the canting angle remains the same. The bulk data "ts well when the molecular "eld model is used with "0.90, for all the compositions. The molecular "eld model does not give a good "t, for the case of the thin "lms. The result of the "tting for x"0 and 0.48 is shown in Fig. 1. The "gure shows that M(¹)/M(300) is lower for the case of thin "lms at higher temperatures. This can be explained by assuming that the molecular "eld coe$cients are lower than that of the bulk. Moreover, the paraprocesses contribute signi"cantly to M(¹)/M(0), lengthening the tail. The behavior of the M vs. H data for the thin "lms can also be explained on the basis of paraprocesses present in the thin "lm due to the presence of point defects [2]. The presence of defects is also reported in FMR studies on these "lms. With increase in x, the lowering of Q and H values, as well as the reduction in the di!erences between bulk and thin "lm M vs. ¹ data indicate that the presence of Zn reduces the inhomogenities [2]. The paraprocesses cannot be included quantitatively in the molecular "eld model. However, the observed lower magnetization of thin "lms shows that the paraprocesses due to defects are responsible for the behaviour of magnetization of thin "lm ferrites.
References [1] J. Dash, N. Venkataramani, R. Krishnan, S.K. Date, S.D. Kulkarni, Shiva Prasad, S.N. Shringi, Pran Kishan, N. Kumar, J. Phys. France (Suppl.) IV (1997) C1}477. [2] J. Dash, Shiva Prasad, N. Venkataramani, R. Krishnan, Pran Kishan, N. Kumar, S.D. Kulkarni, S.K. Date, J. Appl. Phys. 86 (1999) 3303. [3] G.F. Dionne, J. Appl. Phys. 45 (1964) 3621.
The e!ect of Zn on the defects in sputter deposited Li}Zn ferrite "lms J. Dash , R.P.R.C. Aiyar, Shiva Prasad *, N. Venkataramani, S.K. Date, S.D. Kulkarni, Pran Kishan, Nitendra Kumar Department of Physics, IIT, Powai, Mumbai 400076, India ACRE, IIT, Powai, Mumbai 400076, India NCL, Pune 8, India SSPL, Delhi 54, India
Abstract In this paper we report the magnetization as a function of applied "eld at 300 K and magnetization as a function of temperature for several Li}Zn ferrite thin "lms with varying Zn content. A molecular "eld model has been used to "t the M vs. ¹ data for both the thin "lm and the bulk. The magnetic properties of these thin "lms are found to be di!erent from that of the bulk of similar composition. The di!erence is attributed to the presence of point defects, which leads to paraprocesses, thereby reducing the magnetization of the "lms. 2001 Elsevier Science B.V. All rights reserved. Keywords: Ferrites; Thin "lms; Magnetization vs. temperature; Molecular "eld model
The magnetic properties of ferrite thin "lms have been observed to be di!erent from the bulk of similar composition [1]. In a recent paper [2] we have reported a detailed study of magnetization, M in thin "lm Li Mn Zn Fe O , x"0.32 deposited by \V V \V sputtering and shown that the presence of defects are responsible to a large extent for this behavior. In this paper we report a similar study carried out on samples with varying Zn concentrations (x"0.0, 0.16, 0.48) mainly to see the e!ect of Zn on these defects. We also report the temperature dependence of magnetization on all these samples including the case x"0.32, to see how the defects manifest themselves in the M vs. ¹ curves. The "lms have been deposited using a Leybold Z400 RF sputtering unit on fused quartz, silicon and polycrystalline alumina. The "lms reported in this work have been deposited on polycrystalline alumina, because fairly thick "lms ('0.70 m) are needed for high-temperature magnetization measurements. Such thick "lms tend to
* Corresponding author. Fax: #091-22-5723480. E-mail address: [email protected] (S. Prasad).
peel o!, when deposited on quartz or Si substrates. The magnetization values of the "lms however, have been found to be independent of the substrate. The "lms have been annealed at temperatures in the range of 400}8503C for 2 h. The magnetization, M has been measured as a function of temperature using a PAR 4500 vibrating sample magnetometer with the "eld applied in the plane of the "lm in a constant magnetic "eld of 0.5 T. Similar to our reported results for x"0.32 "lm, the M vs. H data, for all the samples do not show saturation even at the maximum "eld of 1 T available with the VSM. The M vs. H data have been "tted to an expression of the form M"Q[1!(H /H)L]. The best "t for all the "lms is found when the exponent, n equals 0.5. Table 1 shows the parameters Q and H for the as deposited and the an nealed "lms. The value of Q is found to increase with increase in annealing temperature, ¹ and for ¹ "8503C the value of Q is close to the bulk value of saturation magnetization except for the case of x"0. For all values of x, except x"0, Q is found to be minimum for the as deposited "lm. The value of H is maximum for the as deposited "lm and decreases as ¹ increases. For the "lms annealed as mentioned above the value of H decreases with increasing x. For example,
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 0 4 6 - 5
J. Dash et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1636}1637 Table 1 Q and H values for Li Mn Zn Fe O "lms (as \V V \V deposited and annealed) for various values of x. The bulk value of saturation magnetization is shown for comparison. The values for as deposited "lms is indicated as (dep) and that for annealed "lms is indicated as (ann) x
0.00
0.16
0.32
0.48
Q (Dep) (kG) H (Dep) (kG) Q (Ann) (kG) H (Ann) (kG) Bulk 4M (kG)
3.03 0.94 2.55 0.17 3.60
2.97 0.96 4.86 0.14 4.60
3.14 0.86 5.33 0.11 4.80
0.98 0.74 4.63 0.05 4.00
Fig. 1. The magnetization, M, relative to that at 300 K as a function of temperature, ¹ for Li Mn Zn Fe O \V V \V for (a) x"0 and (b) x"0.48 bulk and thin "lms. The calculated data using Dionne's model is also shown.
the value of H for annealed x"0 "lm is about four times that of annealed x"0.48 "lm. The M vs. ¹ data for x"0 and 0.48 are shown in Fig. 1, both for the bulk and the thin "lm case. We clearly
1637
see that the M vs. ¹ data for thin "lm and bulk of the same composition is di!erent, the di!erences becoming less evident with increasing x. The data indicate that the curie temperature, ¹ is the same for bulk as well as thin "lm of the same composition, if looked from the tail of the M vs. ¹ curve. A molecular "eld model based on a localised canting, as proposed by Dionne [3], has been used to "t the M vs. ¹ data. The composition studied by Dionne does not have Mn. Dionne has mentioned that the presence of Mn is likely to reduce the molecular "eld coe$cients between A}B and B}B sublattices. We assume that both these coe$cients reduce by the same factor () so that the canting angle remains the same. The bulk data "ts well when the molecular "eld model is used with "0.90, for all the compositions. The molecular "eld model does not give a good "t, for the case of the thin "lms. The result of the "tting for x"0 and 0.48 is shown in Fig. 1. The "gure shows that M(¹)/M(300) is lower for the case of thin "lms at higher temperatures. This can be explained by assuming that the molecular "eld coe$cients are lower than that of the bulk. Moreover, the paraprocesses contribute signi"cantly to M(¹)/M(0), lengthening the tail. The behavior of the M vs. H data for the thin "lms can also be explained on the basis of paraprocesses present in the thin "lm due to the presence of point defects [2]. The presence of defects is also reported in FMR studies on these "lms. With increase in x, the lowering of Q and H values, as well as the reduction in the di!erences between bulk and thin "lm M vs. ¹ data indicate that the presence of Zn reduces the inhomogenities [2]. The paraprocesses cannot be included quantitatively in the molecular "eld model. However, the observed lower magnetization of thin "lms shows that the paraprocesses due to defects are responsible for the behaviour of magnetization of thin "lm ferrites.
References [1] J. Dash, N. Venkataramani, R. Krishnan, S.K. Date, S.D. Kulkarni, Shiva Prasad, S.N. Shringi, Pran Kishan, N. Kumar, J. Phys. France (Suppl.) IV (1997) C1}477. [2] J. Dash, Shiva Prasad, N. Venkataramani, R. Krishnan, Pran Kishan, N. Kumar, S.D. Kulkarni, S.K. Date, J. Appl. Phys. 86 (1999) 3303. [3] G.F. Dionne, J. Appl. Phys. 45 (1964) 3621.