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Site and distributed hyperfine magnetic fields in synthetic magnetite
International Journal of Inorganic Materials 2 (2000) 371–374
Site and distributed hyperfine magnetic fields in synthetic magnetite Monica Sorescu a , *, D. Tarabasanu b , L. Diamandescu b b
a Duquesne University, Physics Department, Bayer Center, Pittsburgh, PA 15282 -0321, USA Institute of Atomic Physics, National Institute for Materials Physics, R-76900 Bucharest-Magurele, Romania
Abstract We synthesized Fe 32x Co x O 4 (x50–0.6) using the hydrothermal method at 3008C in order to demonstrate substitution effects on the ¨ hyperfine magnetic fields and populations of the tetrahedral (A) and octahedral (B) sites of magnetite. Our Mossbauer spectroscopy results were first analyzed using the site model, which provided direct evidence for the presence of the Co substitution in the (B) sublattice. This was accompanied by a systematic increase of the hyperfine magnetic field at (B) sites. The mechanism we propose relies on the substitution of Fe 21 by Co 21 in the (B) sublattice and is supported by the observed dependence of the populations of the (A) and (B) sites ¨ on the content x of cobalt substitution. Finally, the Mossbauer spectroscopy results were also analyzed using the hyperfine magnetic field distribution model for direct comparison. For the first time, it was demonstrated that the field values provided by the site model fall within the standard deviation obtained from the distribution model. 2000 Elsevier Science Ltd. All rights reserved. ¨ spectroscopy; D. microstructure Keywords: A. magnetic materials; B. chemical synthesis; C. Mossbauer
1. Introduction Magnetite (Fe 3 O 4 ) is an oxide with the inverse spinel structure, which has one Fe 31 ion on the tetrahedral (A) site and two Fe ions, with a total valence of 51, on the octahedral (B) site [1,2]. Although the syntheses and transformations of various iron oxides and oxyhydroxides, such as hematite, maghemite and goethite have been studied in detail [3–11], not much is known about the formation of synthetic magnetites and the incorporation of impurity elements into them. In the present paper we report the preparation of cobaltdoped magnetite by the hydrothermal method, in order to investigate the effect of cobalt substitution on the hyperfine magnetic fields and site populations in the magnetite structure.
2. Experimental Fe 32x Co x O 4 (x50–0.6) was prepared using the method of coprecipitation of the iron and cobalt hydroxides from sulphate solutions, mediated by sodium hydroxide in *Corresponding author. Tel.: 11-412-396-4166; fax: 11-412-3964829. E-mail address: [email protected] (M. Sorescu)
conditions controlled by concentration and pH (7.5). The reactive mixture was treated hydrothermally at 3008C for 30 min. Substituent concentrations in the resulting samples were determined using atomic absorption spectroscopy. ¨ Room temperature transmission Mossbauer spectra were recorded using a constant acceleration spectrometer and a 57 Co (Rh) source. The spectra were analyzed considering two sextets, corresponding to the tetrahedral (A) and octahedral (B) magnetic sublattices present in the samples as well as a hyperfine magnetic field distribution. Leastsquares fitting was performed using the NORMOS SITE and DIST programs and all variables of the fit were free.
3. Results and discussion ¨ Selected transmission Mossbauer spectra are displayed in Fig. 1(a)–(c), corresponding to a cobalt content x50, 0.3 and 0.6, respectively. It can be seen that increasing the amount x of cobalt substitution dramatically affects the (B) sublattice of magnetite, changing the shape of all transmis¨ sion Mossbauer spectra. This demonstrates that Co ions exhibit a clear preference for the octahedral sites. In order to quantify this effect, we extracted from these spectra the values of the hyperfine magnetic fields of the tetrahedral and octahedral sites and plotted them as functions of the amount x of cobalt substitution. Thus, it can be
1466-6049 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 00 )00039-8
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M. Sorescu et al. / International Journal of Inorganic Materials 2 (2000) 371 – 374
Fig. 2. Dependence of the hyperfine magnetic field of the tetrahedral and octahedral sites of magnetite on the content x of cobalt substitution. The errors are considered within the size of the data markers.
tinction to this, the hyperfine magnetic field of the octahedral sublattice increases steadily as a function of cobalt concentration. This result demonstrates that the presence of cobalt at octahedral iron sites increases the values of the hyperfine magnetic fields at these positions. This behavior is similar to that observed in FeCoBSi metallic glass, which was found to exhibit an increased value of the hyperfine magnetic field at the iron sites, induced by the presence of cobalt atoms [12]. The mechanism by which Co 21 substitutes for Fe 21 in the (B) sublattice is supported by the concentration dependence of the populations of the tetrahedral and octahedral sites, derived from the areal ratios of the two ¨ sublattices in the Mossbauer spectra. In connection with Fig. 2, Fig. 3 provides direct evidence of this phenomenon:
¨ Fig. 1. Room-temperature transmission Mossbauer spectra of Fe 32x Co x O 4 : (a) x50; (b) x50.3; (c) x50.6. The solid line represents the result of the least-squares fitting.
inferred from Fig. 2 that the hyperfine magnetic field of the tetrahedral sites is essentially not altered by the cobalt ions. This result is consistent with the fact that the cobalt impurities are not present at tetrahedral sites. In contradis-
Fig. 3. Dependence of the populations of the tetrahedral and octahedral sites on the amount x of cobalt substitution. The errors are considered within the size of the data markers.
M. Sorescu et al. / International Journal of Inorganic Materials 2 (2000) 371 – 374
373
the population of the octahedral sites linearly decreases with increasing cobalt content, due to cobalt substituting iron. The population of the tetrahedral sites increases correspondingly.
Fig. 5. Average hyperfine magnetic field as a function of cobalt substitution x calculated within the frames of both site and distribution models.
Fig. 4(a)–(c) present the hyperfine magnetic field dis¨ tributions extracted from the Mossbauer spectra corresponding to a cobalt content of x50, 0.3 and 0.6, respectively. It may be noted that the low field component is shifted towards larger field values as the cobalt content increases. This result is in agreement with the previously observed trends in the hyperfine magnetic field of the tetrahedral and octahedral sites. Fig. 5 shows the average values of the magnetic hyperfine field of all cobalt substituted samples calculated with both site and distribution models. Fig. 6 presents the standard deviation in the values of the magnetic hyperfine field, obtained with the distribution model. It can be seen that both models predict a gradual increase in the average hyperfine magnetic field with increasing the amount x of
Fig. 4. , Hyperfine magnetic field distributions extracted from the ¨ transmission Mossbauer spectra of Co-substituted magnetites: (a) x50; (b) x50.3 and (c) x50.6. The arrows indicate the tetrahedral sublattice.
Fig. 6. Standard deviation in the average hyperfine magnetic field values obtained using the distribution model.
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cobalt substitution. Moreover, it may be noted that the discrepancy between the two sets of values falls within the standard deviation plotted in Fig. 6. In turn, the standard deviation is expected to decrease while the two peaks corresponding to the tetrahedral and octahedral sites come closer together.
Acknowledgements The work at Duquesne was supported by the National Science Foundation under grant DMR-9800753.
References
4. Conclusions To summarize, the main effects of the cobalt substitutions on the magnetite structure evidenced directly by our ¨ Mossbauer study are a linear increase in the hyperfine magnetic field and linear decrease in the population of the octahedral sublattice as functions of substituent concentration. For the first time, we obtained a direct comparison of the results obtained with two independent fitting models and demonstrated that the results provided by the site model fall within the standard deviation in the values of the average hyperfine magnetic field obtained with the distribution model.
[1] Sorescu M. J Mater Sci Lett 1998;17:1059. [2] Okamura A, Nakamura S, Tanaka M, Siratori K. J Phys Soc Jpn 1995;64:3484. [3] Sorescu M, Mihaila-Tarabasanu D, Diamandescu L. Appl Phys Lett 1998;72:2047. [4] Sidhu PS, Gilkes RJ, Posner AM. J Inorg Nucl Chem 1978;40:429. [5] Ok HN, Pan LS, Evans BJ. Phys Rev B 1978;17:85. [6] Schwertmann U, Murad E. Clays Clay Miner 1990;38:196. [7] Persoons RM, DeGrave E, Vandenberghe RE. Hyperfine Interact 1990;54:655. [8] Hsu PH. Soil Sci 1972;23:17. [9] Dickof PA, Schurer PJ, Morrish AH. Phys Rev B 1980;22:115. [10] Morup S, Madsen MB, Franck J, Villadsen J, Koch CJW. J Magn Magn Mater 1983;40:163. [11] Giri AK, DeJulian C, Gonzales JM. J Appl Phys 1994;76:6573. [12] Sorescu M, Knobbe ET. Phys Rev B 1995;52:16086.
Site and distributed hyperfine magnetic fields in synthetic magnetite Monica Sorescu a , *, D. Tarabasanu b , L. Diamandescu b b
a Duquesne University, Physics Department, Bayer Center, Pittsburgh, PA 15282 -0321, USA Institute of Atomic Physics, National Institute for Materials Physics, R-76900 Bucharest-Magurele, Romania
Abstract We synthesized Fe 32x Co x O 4 (x50–0.6) using the hydrothermal method at 3008C in order to demonstrate substitution effects on the ¨ hyperfine magnetic fields and populations of the tetrahedral (A) and octahedral (B) sites of magnetite. Our Mossbauer spectroscopy results were first analyzed using the site model, which provided direct evidence for the presence of the Co substitution in the (B) sublattice. This was accompanied by a systematic increase of the hyperfine magnetic field at (B) sites. The mechanism we propose relies on the substitution of Fe 21 by Co 21 in the (B) sublattice and is supported by the observed dependence of the populations of the (A) and (B) sites ¨ on the content x of cobalt substitution. Finally, the Mossbauer spectroscopy results were also analyzed using the hyperfine magnetic field distribution model for direct comparison. For the first time, it was demonstrated that the field values provided by the site model fall within the standard deviation obtained from the distribution model. 2000 Elsevier Science Ltd. All rights reserved. ¨ spectroscopy; D. microstructure Keywords: A. magnetic materials; B. chemical synthesis; C. Mossbauer
1. Introduction Magnetite (Fe 3 O 4 ) is an oxide with the inverse spinel structure, which has one Fe 31 ion on the tetrahedral (A) site and two Fe ions, with a total valence of 51, on the octahedral (B) site [1,2]. Although the syntheses and transformations of various iron oxides and oxyhydroxides, such as hematite, maghemite and goethite have been studied in detail [3–11], not much is known about the formation of synthetic magnetites and the incorporation of impurity elements into them. In the present paper we report the preparation of cobaltdoped magnetite by the hydrothermal method, in order to investigate the effect of cobalt substitution on the hyperfine magnetic fields and site populations in the magnetite structure.
2. Experimental Fe 32x Co x O 4 (x50–0.6) was prepared using the method of coprecipitation of the iron and cobalt hydroxides from sulphate solutions, mediated by sodium hydroxide in *Corresponding author. Tel.: 11-412-396-4166; fax: 11-412-3964829. E-mail address: [email protected] (M. Sorescu)
conditions controlled by concentration and pH (7.5). The reactive mixture was treated hydrothermally at 3008C for 30 min. Substituent concentrations in the resulting samples were determined using atomic absorption spectroscopy. ¨ Room temperature transmission Mossbauer spectra were recorded using a constant acceleration spectrometer and a 57 Co (Rh) source. The spectra were analyzed considering two sextets, corresponding to the tetrahedral (A) and octahedral (B) magnetic sublattices present in the samples as well as a hyperfine magnetic field distribution. Leastsquares fitting was performed using the NORMOS SITE and DIST programs and all variables of the fit were free.
3. Results and discussion ¨ Selected transmission Mossbauer spectra are displayed in Fig. 1(a)–(c), corresponding to a cobalt content x50, 0.3 and 0.6, respectively. It can be seen that increasing the amount x of cobalt substitution dramatically affects the (B) sublattice of magnetite, changing the shape of all transmis¨ sion Mossbauer spectra. This demonstrates that Co ions exhibit a clear preference for the octahedral sites. In order to quantify this effect, we extracted from these spectra the values of the hyperfine magnetic fields of the tetrahedral and octahedral sites and plotted them as functions of the amount x of cobalt substitution. Thus, it can be
1466-6049 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 00 )00039-8
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M. Sorescu et al. / International Journal of Inorganic Materials 2 (2000) 371 – 374
Fig. 2. Dependence of the hyperfine magnetic field of the tetrahedral and octahedral sites of magnetite on the content x of cobalt substitution. The errors are considered within the size of the data markers.
tinction to this, the hyperfine magnetic field of the octahedral sublattice increases steadily as a function of cobalt concentration. This result demonstrates that the presence of cobalt at octahedral iron sites increases the values of the hyperfine magnetic fields at these positions. This behavior is similar to that observed in FeCoBSi metallic glass, which was found to exhibit an increased value of the hyperfine magnetic field at the iron sites, induced by the presence of cobalt atoms [12]. The mechanism by which Co 21 substitutes for Fe 21 in the (B) sublattice is supported by the concentration dependence of the populations of the tetrahedral and octahedral sites, derived from the areal ratios of the two ¨ sublattices in the Mossbauer spectra. In connection with Fig. 2, Fig. 3 provides direct evidence of this phenomenon:
¨ Fig. 1. Room-temperature transmission Mossbauer spectra of Fe 32x Co x O 4 : (a) x50; (b) x50.3; (c) x50.6. The solid line represents the result of the least-squares fitting.
inferred from Fig. 2 that the hyperfine magnetic field of the tetrahedral sites is essentially not altered by the cobalt ions. This result is consistent with the fact that the cobalt impurities are not present at tetrahedral sites. In contradis-
Fig. 3. Dependence of the populations of the tetrahedral and octahedral sites on the amount x of cobalt substitution. The errors are considered within the size of the data markers.
M. Sorescu et al. / International Journal of Inorganic Materials 2 (2000) 371 – 374
373
the population of the octahedral sites linearly decreases with increasing cobalt content, due to cobalt substituting iron. The population of the tetrahedral sites increases correspondingly.
Fig. 5. Average hyperfine magnetic field as a function of cobalt substitution x calculated within the frames of both site and distribution models.
Fig. 4(a)–(c) present the hyperfine magnetic field dis¨ tributions extracted from the Mossbauer spectra corresponding to a cobalt content of x50, 0.3 and 0.6, respectively. It may be noted that the low field component is shifted towards larger field values as the cobalt content increases. This result is in agreement with the previously observed trends in the hyperfine magnetic field of the tetrahedral and octahedral sites. Fig. 5 shows the average values of the magnetic hyperfine field of all cobalt substituted samples calculated with both site and distribution models. Fig. 6 presents the standard deviation in the values of the magnetic hyperfine field, obtained with the distribution model. It can be seen that both models predict a gradual increase in the average hyperfine magnetic field with increasing the amount x of
Fig. 4. , Hyperfine magnetic field distributions extracted from the ¨ transmission Mossbauer spectra of Co-substituted magnetites: (a) x50; (b) x50.3 and (c) x50.6. The arrows indicate the tetrahedral sublattice.
Fig. 6. Standard deviation in the average hyperfine magnetic field values obtained using the distribution model.
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M. Sorescu et al. / International Journal of Inorganic Materials 2 (2000) 371 – 374
cobalt substitution. Moreover, it may be noted that the discrepancy between the two sets of values falls within the standard deviation plotted in Fig. 6. In turn, the standard deviation is expected to decrease while the two peaks corresponding to the tetrahedral and octahedral sites come closer together.
Acknowledgements The work at Duquesne was supported by the National Science Foundation under grant DMR-9800753.
References
4. Conclusions To summarize, the main effects of the cobalt substitutions on the magnetite structure evidenced directly by our ¨ Mossbauer study are a linear increase in the hyperfine magnetic field and linear decrease in the population of the octahedral sublattice as functions of substituent concentration. For the first time, we obtained a direct comparison of the results obtained with two independent fitting models and demonstrated that the results provided by the site model fall within the standard deviation in the values of the average hyperfine magnetic field obtained with the distribution model.
[1] Sorescu M. J Mater Sci Lett 1998;17:1059. [2] Okamura A, Nakamura S, Tanaka M, Siratori K. J Phys Soc Jpn 1995;64:3484. [3] Sorescu M, Mihaila-Tarabasanu D, Diamandescu L. Appl Phys Lett 1998;72:2047. [4] Sidhu PS, Gilkes RJ, Posner AM. J Inorg Nucl Chem 1978;40:429. [5] Ok HN, Pan LS, Evans BJ. Phys Rev B 1978;17:85. [6] Schwertmann U, Murad E. Clays Clay Miner 1990;38:196. [7] Persoons RM, DeGrave E, Vandenberghe RE. Hyperfine Interact 1990;54:655. [8] Hsu PH. Soil Sci 1972;23:17. [9] Dickof PA, Schurer PJ, Morrish AH. Phys Rev B 1980;22:115. [10] Morup S, Madsen MB, Franck J, Villadsen J, Koch CJW. J Magn Magn Mater 1983;40:163. [11] Giri AK, DeJulian C, Gonzales JM. J Appl Phys 1994;76:6573. [12] Sorescu M, Knobbe ET. Phys Rev B 1995;52:16086.