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SiO2 granular nanocomposites
ARTICLE IN PRESS Physica B 404 (2009) 2777–2779
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Structural and magnetic studies of Fe2O3/SiO2 granular nanocomposites G. Pozo-Lo´pez a,d,, S.P. Silvetti a, A.F. Cabrera b,d, A.M. Condo´ c,d a
´tica, Astronomı´a y Fı´sica, Universidad Nacional de Co ´rdoba, 5000 Co ´rdoba, Argentina Facultad de Matema Departamento de Fı´sica, Facultad de Ciencias Exactas, Universidad Nacional de la Plata, 1900 La Plata, Argentina ´mico Bariloche, Comisio ´n Nacional de Energı´a Ato ´mica – Instituto Balseiro, Universidad Nacional de Cuyo, 8400 S.C. de Bariloche, Rı´o Negro, Argentina Centro Ato d Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Argentina b c
a r t i c l e in fo
Keywords: Magnetic nanoparticles Mechanical alloying Magnetic properties Coercivity
abstract Fe2O3/SiO2 nanocomposites were synthesized by mechanical alloying, using Fe and SiO2 powders as precursors. After 340 h milling, the sample essentially consists of hematite and amorphous silica. TEM images show hematite particles embedded in and surrounded by an amorphous silica matrix. A broad size distribution—5–50 nm—of hematite particles is found, and other group of very small—2–3 nm—unidentified particles are observed. Room temperature Mo¨ssbauer spectra show a paramagnetic doublet, which may correspond to a non-crystalline phase in the sample (probably the small unidentified particles), and a sextet corresponding to hematite. Magnetic properties were investigated by measuring hysteresis curves at different temperatures (5–300 K) and by zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves (10 mT). The hysteresis loops were well fitted by a ferromagnetic contribution. No evidence of Morin transition is found down to 5 K. & 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoscale materials have attracted interest in recent years from both academic and technical view [1,2]. In these materials, size and surface effects and inter-particle interactions, strongly dependent on the degree of dispersion of the magnetic particles in the non-magnetic host, play a key role in their hysteretic properties [3]. In this work, we describe the different phases forming during mechanical milling of Fe–SiO2 powders. In particular, the magnetic and structural properties of the powder milled for 340 h, essentially consisting of hematite particles embedded in an amorphous silica matrix, are discussed and correlated.
2. Experimental procedures The starting materials for high energy ball milling (HEBM) were a mixture (50 wt% Fe+50 wt% SiO2) of crystalline powders of analytic purity, with a mean crystal size of about 30 nm. The milling was carried out in a planetary ball mill equipped with hardened steel balls and vials. The initial ball to powder mass ratio was 10:1 and the powder was milled at a speed of 200 rpm, for times up to 340 h, in air atmosphere without any additive
(dry milling). The milling process was interrupted after selected times x and small amounts of powder were sampled (Sx) for characterization. The effect of HEBM on the powder structure was monitored by X-ray diffraction (XRD) in a Philips PW 3830 diffractometer, using Cu Ka radiation (l ¼ 1.5418 A˚). Rietveld analysis was applied to the spectra. A profile fitting was also made for each maximum in the spectra to determine the peak width, after applying the corrections for the instrumental broadening. The average crystallite size D was calculated from the peak broadening, using the Scherrer equation [4]. Samples for magnetic measurements were cylinders of 6.5 mm diameter and typically 2 mm height. Room temperature hysteresis curves were measured in a vibrating sample magnetometer (VSM) Lakeshore 7300, with a maximum field up to 1.5 T. Isothermal hysteresis loops and ZFC–FC curves were measured in a quantum design SQUID magnetometer, in the range between 5 and 300 K. All Mo¨ssbauer effect spectra were recorded in transmission geometry, using a constant acceleration spectrometer with a 10-mCi 57CoRh source, at room temperature. The structure and morphology of the powder milled 340 h (S340) were also examined by high resolution transmission electron microscopy (HRTEM) with a Philips CM 200, 200 kV microscope.
3. Results and discussion Corresponding author at: Facultad de Matema´tica, Astronomı´a y Fı´sica,
Universidad Nacional de Co´rdoba, 5000 Co´rdoba, Argentina. Tel.: +54 351 433 4051; fax: +54 351 433 4054. E-mail address: [email protected] (G. Pozo-Lo´pez). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.06.115
XRD patterns of the powders after different milling times are shown in Fig. 1. For sample S0 only the characteristic lines of bcc-Fe and a-quartz are observed. After 120 h milling, the peaks
ARTICLE IN PRESS 2778
´pez et al. / Physica B 404 (2009) 2777–2779 G. Pozo-Lo
Fig. 3. TEM micrograph showing the general morphology of the sample. Fig. 1. X-ray diffraction spectra for different milling times showing the principal lines of the phases S ¼ SiO2, Fe ¼ a-Fe, M ¼ maghemite and H ¼ hematite.
Fig. 2. Evolution of the mean crystal size of the major phases during milling.
Fig. 4. HRTEM image showing crystalline particles embedded in amorphous SiO2. Inset: Fourier transform of the central particle, corresponding to the hematite ½1 1 1 zone axis.
are significantly broadened as expected for the large amount of defects introduced. A new phase is first observed in sample S160, but due to the large structure distortions induced by milling, the identification is not conclusive. This new phase is also found in sample S180 and later Mo¨ssbauer spectra revealed that it was maghemite (g-Fe2O3). After 220 h of milling, other new peaks appear. These new peaks are further enhanced in the sample S340 and correspond to a-Fe2O3 (hematite). Between 220 and 340 h of milling, the volume percentage of hematite increases at expenses of maghemite, and at 340 h milling only the hematite lines are detected. The average crystallite size of the bcc-Fe, maghemite, hematite and SiO2 as a function of milling time is shown in Fig. 2. TEM analysis of S340 evidences a broad particle size distribution, ranging from a few nanometers to about 50 nm. Fig. 3 shows the general morphology of the sample, characterized by rounded particles of hematite embedded in an amorphous silica matrix and Fig. 4 illustrates a HRTEM image of the sample and the Fourier transform of the central particle (11 nm diameter) corresponding to the ½1 1 1 zone axis of hematite. Room temperature hysteresis loops up to 1.5 T were also measured at successive stages during the milling process. The
variation of the magnetic properties of the samples with the milling time is plotted in Fig. 5. Saturation magnetization first decreases with time up to 180 h of milling, accompanying the amorphization of the precursor phases. For sample S180, MS begins to rise due to the apparition of ferromagnetic maghemite but at 220 h milling, weakly ferromagnetic hematite phase appears. For sample S340, magnetization presents its minimum value (0.5 Am2/kg) and a coercivity of 21.3 mT. Shown in Fig. 6 is the ZFC and FC magnetization as a function of temperature for S340. It is clear from this figure that the effective blocking temperature is above room temperature. The magnetization increase observed at low temperatures is likely to be due to a paramagnetic unknown phase in the sample, not detected by XRD, but observed in TEM images and by Mo¨ssbauer studies. In fact, the room temperature spectra of S340 show a paramagnetic doublet, corresponding to a non-crystalline phase in the sample (the smallest particles observed by HRTEM), and a sextet which corresponds to hematite. The hysteresis curves of sample S340 measured at different selected temperatures in the range 5–300 K were fitted by the sum of two contributions: a ferromagnetic one [5] and a superparamagnetic one. Satisfactory fits were obtained considering only the ferromagnetic
ARTICLE IN PRESS ´pez et al. / Physica B 404 (2009) 2777–2779 G. Pozo-Lo
2779
Fig. 5. Evolution of coercivity, remanence and saturation magnetization with the milling time.
Fig. 7. Temperature dependence of the coercive field.
Fig. 6. ZFC–FC curves for sample S340 measured with an applied field of 10 mT.
contribution MF described by [5] 2MsF H HC pMr arctan tan M F ðHÞ ¼ p HC 2MsF
(1)
where MsF, HC and Mr are the effective saturation magnetization, the intrinsic coercive field and the remanent magnetization associated to the ferromagnetic contribution. Fig. 7 shows the coercive field vs. temperature for sample S340; the fit plotted corresponds to a mechanism of coherent rotation [6]: " 1=2 # T (2) HC ðTÞ ¼ HC ð0Þ 1 TB where HC(0) is an effective anisotropy field ( ¼ 2K/MS) and TB is an effective blocking temperature ( ¼ 25kB/K/VS), with K the anisotropy constant, kB Boltzmann constant, Ms the saturation magnetization of hematite and /VS an effective particle volume.
Even when the fit is acceptable, the resulting values of HC(0) ¼ 550 Oe, TB ¼ 480 K, K ¼ 58 J/m3 and the effective size of the reversing particles /DS ¼ 176 nm are not consistent with isolated hematite particles of 20 nm in size undergoing coherent rotation. These results suggest that hematite particles are magnetically coupled by inter-particle phases. It has been demonstrated that the Morin temperature diminishes as the particle size decreases or strain defects appear [7]. Moreover, for particles smaller than 20 nm the transition might be suppressed [8]. In the present case no evidence of Morin transition is found even though TEM analysis evidences hematite particles ranging from a few nanometers to about 50 nm. These values are insufficient to explain why the Morin transition is suppressed, so further studies are necessary to understand the observed behavior. Summarizing, a ferromagnetic composite containing hematite crystals dispersed in a major silica matrix is obtained after air milling for 340 h Fe and silica precursor powders. The magnetic behavior of the composite is consistent with a large inter-particle interaction, mediated by the matrix. References [1] X. He, Q. Zhang, Z. Ling, Mater. Lett. 57 (2003) 3031. [2] C. Caiser, M. Popovici, C. Savii, Acta Mater. 51 (2003) 3607. [3] M. Alonso-San˜udo, J.J. Blackwell, K.O. O0 Grady, J.M. Gonza´lez, F. Cebollada, M.P. Morales, J. Magn. Magn. Mater. 221 (2000) 207. [4] H. Klug, L. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, 1974. [5] M.B. Stearns, Y. Cheng, J. Appl. Phys. 75 (1994) 6894. [6] P. Gaunt, J. Appl. Phys. 59 (12) (1986) 4129–4132. [7] N. Amin, S. Arajs, Phys. Rev. B (1987) 34810. [8] R.E. Vandenberghe, Mo¨ssbauer Spectroscopy and Applications in Geology, International Training Centre for Post-Graduate Soil Scientists, Belgium, 1991.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Structural and magnetic studies of Fe2O3/SiO2 granular nanocomposites G. Pozo-Lo´pez a,d,, S.P. Silvetti a, A.F. Cabrera b,d, A.M. Condo´ c,d a
´tica, Astronomı´a y Fı´sica, Universidad Nacional de Co ´rdoba, 5000 Co ´rdoba, Argentina Facultad de Matema Departamento de Fı´sica, Facultad de Ciencias Exactas, Universidad Nacional de la Plata, 1900 La Plata, Argentina ´mico Bariloche, Comisio ´n Nacional de Energı´a Ato ´mica – Instituto Balseiro, Universidad Nacional de Cuyo, 8400 S.C. de Bariloche, Rı´o Negro, Argentina Centro Ato d Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Argentina b c
a r t i c l e in fo
Keywords: Magnetic nanoparticles Mechanical alloying Magnetic properties Coercivity
abstract Fe2O3/SiO2 nanocomposites were synthesized by mechanical alloying, using Fe and SiO2 powders as precursors. After 340 h milling, the sample essentially consists of hematite and amorphous silica. TEM images show hematite particles embedded in and surrounded by an amorphous silica matrix. A broad size distribution—5–50 nm—of hematite particles is found, and other group of very small—2–3 nm—unidentified particles are observed. Room temperature Mo¨ssbauer spectra show a paramagnetic doublet, which may correspond to a non-crystalline phase in the sample (probably the small unidentified particles), and a sextet corresponding to hematite. Magnetic properties were investigated by measuring hysteresis curves at different temperatures (5–300 K) and by zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves (10 mT). The hysteresis loops were well fitted by a ferromagnetic contribution. No evidence of Morin transition is found down to 5 K. & 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoscale materials have attracted interest in recent years from both academic and technical view [1,2]. In these materials, size and surface effects and inter-particle interactions, strongly dependent on the degree of dispersion of the magnetic particles in the non-magnetic host, play a key role in their hysteretic properties [3]. In this work, we describe the different phases forming during mechanical milling of Fe–SiO2 powders. In particular, the magnetic and structural properties of the powder milled for 340 h, essentially consisting of hematite particles embedded in an amorphous silica matrix, are discussed and correlated.
2. Experimental procedures The starting materials for high energy ball milling (HEBM) were a mixture (50 wt% Fe+50 wt% SiO2) of crystalline powders of analytic purity, with a mean crystal size of about 30 nm. The milling was carried out in a planetary ball mill equipped with hardened steel balls and vials. The initial ball to powder mass ratio was 10:1 and the powder was milled at a speed of 200 rpm, for times up to 340 h, in air atmosphere without any additive
(dry milling). The milling process was interrupted after selected times x and small amounts of powder were sampled (Sx) for characterization. The effect of HEBM on the powder structure was monitored by X-ray diffraction (XRD) in a Philips PW 3830 diffractometer, using Cu Ka radiation (l ¼ 1.5418 A˚). Rietveld analysis was applied to the spectra. A profile fitting was also made for each maximum in the spectra to determine the peak width, after applying the corrections for the instrumental broadening. The average crystallite size D was calculated from the peak broadening, using the Scherrer equation [4]. Samples for magnetic measurements were cylinders of 6.5 mm diameter and typically 2 mm height. Room temperature hysteresis curves were measured in a vibrating sample magnetometer (VSM) Lakeshore 7300, with a maximum field up to 1.5 T. Isothermal hysteresis loops and ZFC–FC curves were measured in a quantum design SQUID magnetometer, in the range between 5 and 300 K. All Mo¨ssbauer effect spectra were recorded in transmission geometry, using a constant acceleration spectrometer with a 10-mCi 57CoRh source, at room temperature. The structure and morphology of the powder milled 340 h (S340) were also examined by high resolution transmission electron microscopy (HRTEM) with a Philips CM 200, 200 kV microscope.
3. Results and discussion Corresponding author at: Facultad de Matema´tica, Astronomı´a y Fı´sica,
Universidad Nacional de Co´rdoba, 5000 Co´rdoba, Argentina. Tel.: +54 351 433 4051; fax: +54 351 433 4054. E-mail address: [email protected] (G. Pozo-Lo´pez). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.06.115
XRD patterns of the powders after different milling times are shown in Fig. 1. For sample S0 only the characteristic lines of bcc-Fe and a-quartz are observed. After 120 h milling, the peaks
ARTICLE IN PRESS 2778
´pez et al. / Physica B 404 (2009) 2777–2779 G. Pozo-Lo
Fig. 3. TEM micrograph showing the general morphology of the sample. Fig. 1. X-ray diffraction spectra for different milling times showing the principal lines of the phases S ¼ SiO2, Fe ¼ a-Fe, M ¼ maghemite and H ¼ hematite.
Fig. 2. Evolution of the mean crystal size of the major phases during milling.
Fig. 4. HRTEM image showing crystalline particles embedded in amorphous SiO2. Inset: Fourier transform of the central particle, corresponding to the hematite ½1 1 1 zone axis.
are significantly broadened as expected for the large amount of defects introduced. A new phase is first observed in sample S160, but due to the large structure distortions induced by milling, the identification is not conclusive. This new phase is also found in sample S180 and later Mo¨ssbauer spectra revealed that it was maghemite (g-Fe2O3). After 220 h of milling, other new peaks appear. These new peaks are further enhanced in the sample S340 and correspond to a-Fe2O3 (hematite). Between 220 and 340 h of milling, the volume percentage of hematite increases at expenses of maghemite, and at 340 h milling only the hematite lines are detected. The average crystallite size of the bcc-Fe, maghemite, hematite and SiO2 as a function of milling time is shown in Fig. 2. TEM analysis of S340 evidences a broad particle size distribution, ranging from a few nanometers to about 50 nm. Fig. 3 shows the general morphology of the sample, characterized by rounded particles of hematite embedded in an amorphous silica matrix and Fig. 4 illustrates a HRTEM image of the sample and the Fourier transform of the central particle (11 nm diameter) corresponding to the ½1 1 1 zone axis of hematite. Room temperature hysteresis loops up to 1.5 T were also measured at successive stages during the milling process. The
variation of the magnetic properties of the samples with the milling time is plotted in Fig. 5. Saturation magnetization first decreases with time up to 180 h of milling, accompanying the amorphization of the precursor phases. For sample S180, MS begins to rise due to the apparition of ferromagnetic maghemite but at 220 h milling, weakly ferromagnetic hematite phase appears. For sample S340, magnetization presents its minimum value (0.5 Am2/kg) and a coercivity of 21.3 mT. Shown in Fig. 6 is the ZFC and FC magnetization as a function of temperature for S340. It is clear from this figure that the effective blocking temperature is above room temperature. The magnetization increase observed at low temperatures is likely to be due to a paramagnetic unknown phase in the sample, not detected by XRD, but observed in TEM images and by Mo¨ssbauer studies. In fact, the room temperature spectra of S340 show a paramagnetic doublet, corresponding to a non-crystalline phase in the sample (the smallest particles observed by HRTEM), and a sextet which corresponds to hematite. The hysteresis curves of sample S340 measured at different selected temperatures in the range 5–300 K were fitted by the sum of two contributions: a ferromagnetic one [5] and a superparamagnetic one. Satisfactory fits were obtained considering only the ferromagnetic
ARTICLE IN PRESS ´pez et al. / Physica B 404 (2009) 2777–2779 G. Pozo-Lo
2779
Fig. 5. Evolution of coercivity, remanence and saturation magnetization with the milling time.
Fig. 7. Temperature dependence of the coercive field.
Fig. 6. ZFC–FC curves for sample S340 measured with an applied field of 10 mT.
contribution MF described by [5] 2MsF H HC pMr arctan tan M F ðHÞ ¼ p HC 2MsF
(1)
where MsF, HC and Mr are the effective saturation magnetization, the intrinsic coercive field and the remanent magnetization associated to the ferromagnetic contribution. Fig. 7 shows the coercive field vs. temperature for sample S340; the fit plotted corresponds to a mechanism of coherent rotation [6]: " 1=2 # T (2) HC ðTÞ ¼ HC ð0Þ 1 TB where HC(0) is an effective anisotropy field ( ¼ 2K/MS) and TB is an effective blocking temperature ( ¼ 25kB/K/VS), with K the anisotropy constant, kB Boltzmann constant, Ms the saturation magnetization of hematite and /VS an effective particle volume.
Even when the fit is acceptable, the resulting values of HC(0) ¼ 550 Oe, TB ¼ 480 K, K ¼ 58 J/m3 and the effective size of the reversing particles /DS ¼ 176 nm are not consistent with isolated hematite particles of 20 nm in size undergoing coherent rotation. These results suggest that hematite particles are magnetically coupled by inter-particle phases. It has been demonstrated that the Morin temperature diminishes as the particle size decreases or strain defects appear [7]. Moreover, for particles smaller than 20 nm the transition might be suppressed [8]. In the present case no evidence of Morin transition is found even though TEM analysis evidences hematite particles ranging from a few nanometers to about 50 nm. These values are insufficient to explain why the Morin transition is suppressed, so further studies are necessary to understand the observed behavior. Summarizing, a ferromagnetic composite containing hematite crystals dispersed in a major silica matrix is obtained after air milling for 340 h Fe and silica precursor powders. The magnetic behavior of the composite is consistent with a large inter-particle interaction, mediated by the matrix. References [1] X. He, Q. Zhang, Z. Ling, Mater. Lett. 57 (2003) 3031. [2] C. Caiser, M. Popovici, C. Savii, Acta Mater. 51 (2003) 3607. [3] M. Alonso-San˜udo, J.J. Blackwell, K.O. O0 Grady, J.M. Gonza´lez, F. Cebollada, M.P. Morales, J. Magn. Magn. Mater. 221 (2000) 207. [4] H. Klug, L. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, 1974. [5] M.B. Stearns, Y. Cheng, J. Appl. Phys. 75 (1994) 6894. [6] P. Gaunt, J. Appl. Phys. 59 (12) (1986) 4129–4132. [7] N. Amin, S. Arajs, Phys. Rev. B (1987) 34810. [8] R.E. Vandenberghe, Mo¨ssbauer Spectroscopy and Applications in Geology, International Training Centre for Post-Graduate Soil Scientists, Belgium, 1991.