- Email: [email protected]
Effect of HEBM on the cation distribution of Mn-ferrite
Physica B 291 (2000) 49}53
E!ect of HEBM on the cation distribution of Mn-ferrite M.H. Mahmoud *, H.H. Hamdeh, A.I. Abdel-Mageed , A.M. Abdallah , M.K. Fayek Physics Department, Faculty of Science, Assiut University, Assiut, Egypt Physics Department, Wichita State University, Wichita, KS, USA Atomic Reactor Division, Atomic Energy Authority, Egypt Received 23 July 1999; received in revised form 4 October 1999
Abstract The e!ect of high-energy ball milling (HEBM) on the cation distribution of manganese ferrite (MnFe O ) was investigated by means of MoK ssbauer spectroscopy in the presence of high external magnetic "eld. The results show that the Fe> ion occupancy to A site increases with increasing the milling time. Particle-size investigation shows that this increase of the inversion parameter could be attributed to a particle-size reduction in the ball-milled samples. 2000 Elsevier Science B.V. All rights reserved. PACS: 75.50.C; 76.80 Keywords: Ball milling; MoK ssbauer spectroscopy; Ferrites; Cation distribution
1. Introduction Spinel ferrites with the general formula M Fe O are a technologically important class of magnetic oxides, where M is a divalent cation. The spinel-type structure can be described in terms of a nearly cubic close-packed arrangement of anions with one-half of the octahedral interstices (B-sites) and one-eighth of the tetrahedral interstices (Asites) "lled with cations. The cation arrangement can vary between two extreme cases. One is the normal spinel, where all divalent cations occupy
* Corresponding author. E-mail address: [email protected] (M.H. Mahmoud).
A-site and all the trivalent cations occupy B-site. The other is the inverse spinel, where all divalent ions occupy B-site, and the trivalent cations equally divided between A and B sites. Spinel with cation distribution intermediate between normal and inverse (i.e. partially inverse) are also very frequent. The interesting physical and chemical properties of spinel ferrites arise from the ability of these compounds to distribute the cations amongst the available A and B sites. Therefore, control of cation provides a means to control magnetic behaviour. This so-called cation distribution is proved to be an equilibrium function of temperature, pressure and composition [1]. It was found that the cation distribution of Mn-ferrite prepared by the high-temperature method (ceramic method) does not depend on
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 3 8 1 - 2
50
M.H. Mahmoud et al. / Physica B 291 (2000) 49 }53
the preparation condition, e.g. slow cooling, quenching, etc. [2]. However, recent investigations have shown that the inversion parameter in Mnferrites nanoparticles, prepared by aqueous phase precipitation method, has a particles size dependence [3]. It was found that the inversion parameter increases with decreasing particle size. On the other hand, recent studies of ball-milled zinc ferrite samples synthesized by ceramic method show that the inversion degree changes from normal to approximately inverse spinel in a relatively short period of milling time (28 min), which was attributed to crystal lattice contraction [4]. In the present work we investigate the e!ectiveness of high-energy ball milling (HEBM) to alter, in a controllable way, the cation distribution in spinel Mn-ferrite powder, synthesized by the ceramic method, by means of MoK ssbauer spectroscopy in the presence of high external magnetic "eld.
2. Experiment MnFe O was prepared in polycrystalline form by the conventional ceramic method. The bulk sample of MnFe O was subject to mechanical grinding using high-energy ball milling. Five grams of the sample were sealed in a tungsten carbide vial 950 cm in volume) with two tungsten carbide balls of 12 mm in diameter and 12 g in weight. The sample was ball milled for the periods of time; 10, 30, 120 and 180 min. Milling experiments were performed in argon atmosphere. X-ray di!raction measurements were carried out after each period of milling.
3. Results and discussion Fig. 1 shows the X-ray di!raction patterns of MnFe O as prepared and ball-milled samples. One can notice that the spinel structure is still observed after the milling process and no phases other than the spinel could be detected in the diffraction patterns. It is clear even with visual inspection, that the width of the X-ray lines increases with increasing the milling time. This gives an evidence for a decrease in the mean particle size of the
Fig. 1. X-ray di!raction patterns of MnFe O samples ball milled for various periods of time (a) as prepared (b) 10 (c) 30 (d) 120 and (e) 180 min.
ball-milled samples. The average particle size deduced from X-ray line breadth and TEM graphs are listed in Table 1. It can be noticed that a signi"cant di!erence is observed for the size determined by these two methods. This di!erence is attributable to the fact that "ne particles are often composed of smaller crystallites [5]. The width of X-ray di!raction lines provides a measure of the crystallite sizes, which is always smaller than the actual particle size. Fig. 2 compares the milling time dependence of the sizes as measured by XRD and TEM. As indicated, both methods yield almost the same behaviour. However, in Ref. [3] "ne particles of MnFe O were found to be single crystals, as no di!erence between the sizes was obtained with the two techniques. The di!erence between our results and that of Ref. [3] is probably attributed to the method of preparation of the sample. In Ref. [3], they used the coprecipitation method, which provided homogeneous nanosize particles, while in the present
M.H. Mahmoud et al. / Physica B 291 (2000) 49 }53
51
Table 1 Average particles size for MnFe O samples ball-milled for di!erent periods of time Milling time (min)
Average size from XRD (nm)
Average size from TEM (nm)
0 10 30 120 180
128 48 17 12 10
1000 200 120 90 82
X-ray di!raction. Transmission electron microscope. Fig. 3. TEM bright-"eld image of MnFe O samples ball milled for 120 min.
Fig. 2. Milling time dependence of the sizes as measured by XRD (lower curve) and TEM (upper curve).
work the method we used, HEBM, yields heterogeneous particle size and possible aggregations of di!erent particles. Fig. 3 shows a broad particle size distribution observed in a bright-"eld image of the sample ball-milled for 120 min. Information on the cation distribution can be obtained from the area ratio of the A and B sites in MoK ssbauer spectra. Good result is obtained when the spectra are well resolved. As it is known, no clear splitting occurs in the MoK ssbauer spectrum of MnFe O . The implication is that the MoK ssbauer
parameters for A and B sites in the case of manganese ferrite are similar [6]. Therefore, in order to improve the resolution, MoK ssbauer measurements were carried out in the presence of high external magnetic "eld. Mn-ferrite has the collinear structure, so that in the presence of an applied magnetic "eld, the B sites moments are oriented parallel and those of the A sites antiparallel to the "eld direction. Since the internal "eld at the iron nucleus is negative the magnetic hyper"ne splitting at the A site is increased by the applied "eld, while the B site splitting is decreased, which results in a clear splitting of the A and B sites. Fig. 4 shows MoK ssbauer spectra for the bulk and "ne particle samples taken at 10 K in the presence of 5 T magnetic "eld parallel to the incident gamma-ray beam. As the "gure indicates, one can detect a resolved splitting of the outer lines. The calculated MoK ssbauer parameters from the computer "tting are given in Table 2. At low temperature the recoilless free fraction for iron ions on the A and B sites are essentially equal [7] therefore, the occupation numbers of the iron ions can be considered proportional to the corresponding areas of their MoK ssbauer spectra. Consequently, the relative numbers of Fe> ions in A and B sites are determined from the ratio of the areas under the two sublattices. From this, the ratio of the Mn ions on A and B sites could be inferred. The data obtained regarding the cation distribution
52
M.H. Mahmoud et al. / Physica B 291 (2000) 49 }53
Fig. 4. MoK ssbauer spectra at 10 K in a longitudinal external magnetic "eld of MnFe O samples ball-milled for various periods of time (a) as prepared (b) 10 (c) 30 (d) 120 and (e) 180 min.
between A and B sites and consequently the inversion parameter are given in Table 3, where the round brackets refer to the tetrahedral sites and the square brackets refer to the octahedral sites. As it is clear from the table, the cation distributions for ball milled and bulk samples are di!erent. The inversion parameter increases with increasing milling time. The same behavior was observed, for ZnFe O samples, by S[ epelaH k et al. [4] who ob tained a 97% inversion for the normal zinc ferrite after 28 min of milling. They attribute this increase in the inversion parameter to a crystal lattice contraction (decrease of the crystal lattice constant) in the ball-milled samples with increasing the milling time. In the present study, X-ray di!raction investigations show that; the ball-milled samples have the same crystal lattice constant (within the experimental error) as that of the bulk. This rules out the possibility of using changes in crystal lattice to explain the observed increase in the inversion parameter. However, particle-size analysis shows that changes in the inversion parameter of the ball-milled samples could be attributed to particle-size dependence. Fig. 5 shows a comparison between the increase of the inversion parameter and the decrease of the particle size with increasing the milling time. It is clear from this "gure that the sharp increase in inversion parameter followed by slow variation with time corresponds to a sharp drop in
Table 2 MoK ssbauer parameters of MnFe O ball-milled samples in the presence of a magnetic "eld of 5 T. w is the line width, Q is the quadrupole splitting, H is the hyper"ne "eld, S is the relative subspectral area Milling time (min)
Site
H (kOe)
w (mm/s)
Q (mm/s)
S (%)
Zero
A B A B A B A B A B
537$1.5 465$1.5 531$1.5 469$1.5 544$1.5 477$1.5 529$1.5 462$1.5 548$1.5 485$1.5
0.15$0.02 0.53$0.02 0.30$0.05 0.59$0.02 0.25$0.05 0.63G0.02 0.34$0.05 0.68$0.02 0.35$0.05 0.85$0.02
!0.1 $0.05 0.08$0.05 !0.08$0.05 0.03$0.05 !0.20$0.05 0.03$0.05 !0.19$0.05 0.03$0.05 !0.26$0.05 0.001$0.05
23 77 39 61 41 59 43 57 45 55
10 30 120 180
M.H. Mahmoud et al. / Physica B 291 (2000) 49 }53
53
Table 3 Cation distribution of MnFe O samples ball-milled for di!erent periods of time Milling time (min)
0 10 30 120 180
Relative areas of Fe> A
B
23 39 41 43 45
77 61 59 57 55
Cation distribution
Inversion parameter (%)
(Mn Fe ) (Mn Fe ) (Mn Fe ) (Mn Fe ) (Mn Fe )
23 39 41 43 45
[Mn Fe ] [Mn Fe ] [Mn Fe ] [Mn Fe ] [Mn Fe ]
particle size followed by slow variation with time, which explains the inversion parameter dependence upon the milling time in these samples.
References [1] J. Smith, H.P.J. Wijn, Ferrites, Wiley, New York, 1959. [2] J.M. Hasting, L.M. Corliss, Phys. Rev. 104 (1956) 328. [3] A.H. Morrish, Z.W. Li, X.Z. Zhou, J. Phys. C1 (1997) 513. [4] V. S[ epelaH l, K. TkaH c\ ovaH , V.V. Boldyerv, S. Wissmann, K.D. Becker, Physica B 234}236 (1997) 617. [5] K. Haneda, H. Kojima, A.H. Morrish, P.J. Picone, K. Wakai, J. Appl. Phys. 53 (1982) 2686. [6] A.H. Morrish, P.E. Clark, Phys. Rev. B 11 (1975) 278. [7] G.A. Sawatzky, F. Van Der Woude, A.H. Morrish, Phys. Rev. 183 (1969) 383.
Fig. 5. Comparison between changes in the inversion parameter and the particle size in MnFe O samples with ball-milling time.
E!ect of HEBM on the cation distribution of Mn-ferrite M.H. Mahmoud *, H.H. Hamdeh, A.I. Abdel-Mageed , A.M. Abdallah , M.K. Fayek Physics Department, Faculty of Science, Assiut University, Assiut, Egypt Physics Department, Wichita State University, Wichita, KS, USA Atomic Reactor Division, Atomic Energy Authority, Egypt Received 23 July 1999; received in revised form 4 October 1999
Abstract The e!ect of high-energy ball milling (HEBM) on the cation distribution of manganese ferrite (MnFe O ) was investigated by means of MoK ssbauer spectroscopy in the presence of high external magnetic "eld. The results show that the Fe> ion occupancy to A site increases with increasing the milling time. Particle-size investigation shows that this increase of the inversion parameter could be attributed to a particle-size reduction in the ball-milled samples. 2000 Elsevier Science B.V. All rights reserved. PACS: 75.50.C; 76.80 Keywords: Ball milling; MoK ssbauer spectroscopy; Ferrites; Cation distribution
1. Introduction Spinel ferrites with the general formula M Fe O are a technologically important class of magnetic oxides, where M is a divalent cation. The spinel-type structure can be described in terms of a nearly cubic close-packed arrangement of anions with one-half of the octahedral interstices (B-sites) and one-eighth of the tetrahedral interstices (Asites) "lled with cations. The cation arrangement can vary between two extreme cases. One is the normal spinel, where all divalent cations occupy
* Corresponding author. E-mail address: [email protected] (M.H. Mahmoud).
A-site and all the trivalent cations occupy B-site. The other is the inverse spinel, where all divalent ions occupy B-site, and the trivalent cations equally divided between A and B sites. Spinel with cation distribution intermediate between normal and inverse (i.e. partially inverse) are also very frequent. The interesting physical and chemical properties of spinel ferrites arise from the ability of these compounds to distribute the cations amongst the available A and B sites. Therefore, control of cation provides a means to control magnetic behaviour. This so-called cation distribution is proved to be an equilibrium function of temperature, pressure and composition [1]. It was found that the cation distribution of Mn-ferrite prepared by the high-temperature method (ceramic method) does not depend on
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 3 8 1 - 2
50
M.H. Mahmoud et al. / Physica B 291 (2000) 49 }53
the preparation condition, e.g. slow cooling, quenching, etc. [2]. However, recent investigations have shown that the inversion parameter in Mnferrites nanoparticles, prepared by aqueous phase precipitation method, has a particles size dependence [3]. It was found that the inversion parameter increases with decreasing particle size. On the other hand, recent studies of ball-milled zinc ferrite samples synthesized by ceramic method show that the inversion degree changes from normal to approximately inverse spinel in a relatively short period of milling time (28 min), which was attributed to crystal lattice contraction [4]. In the present work we investigate the e!ectiveness of high-energy ball milling (HEBM) to alter, in a controllable way, the cation distribution in spinel Mn-ferrite powder, synthesized by the ceramic method, by means of MoK ssbauer spectroscopy in the presence of high external magnetic "eld.
2. Experiment MnFe O was prepared in polycrystalline form by the conventional ceramic method. The bulk sample of MnFe O was subject to mechanical grinding using high-energy ball milling. Five grams of the sample were sealed in a tungsten carbide vial 950 cm in volume) with two tungsten carbide balls of 12 mm in diameter and 12 g in weight. The sample was ball milled for the periods of time; 10, 30, 120 and 180 min. Milling experiments were performed in argon atmosphere. X-ray di!raction measurements were carried out after each period of milling.
3. Results and discussion Fig. 1 shows the X-ray di!raction patterns of MnFe O as prepared and ball-milled samples. One can notice that the spinel structure is still observed after the milling process and no phases other than the spinel could be detected in the diffraction patterns. It is clear even with visual inspection, that the width of the X-ray lines increases with increasing the milling time. This gives an evidence for a decrease in the mean particle size of the
Fig. 1. X-ray di!raction patterns of MnFe O samples ball milled for various periods of time (a) as prepared (b) 10 (c) 30 (d) 120 and (e) 180 min.
ball-milled samples. The average particle size deduced from X-ray line breadth and TEM graphs are listed in Table 1. It can be noticed that a signi"cant di!erence is observed for the size determined by these two methods. This di!erence is attributable to the fact that "ne particles are often composed of smaller crystallites [5]. The width of X-ray di!raction lines provides a measure of the crystallite sizes, which is always smaller than the actual particle size. Fig. 2 compares the milling time dependence of the sizes as measured by XRD and TEM. As indicated, both methods yield almost the same behaviour. However, in Ref. [3] "ne particles of MnFe O were found to be single crystals, as no di!erence between the sizes was obtained with the two techniques. The di!erence between our results and that of Ref. [3] is probably attributed to the method of preparation of the sample. In Ref. [3], they used the coprecipitation method, which provided homogeneous nanosize particles, while in the present
M.H. Mahmoud et al. / Physica B 291 (2000) 49 }53
51
Table 1 Average particles size for MnFe O samples ball-milled for di!erent periods of time Milling time (min)
Average size from XRD (nm)
Average size from TEM (nm)
0 10 30 120 180
128 48 17 12 10
1000 200 120 90 82
X-ray di!raction. Transmission electron microscope. Fig. 3. TEM bright-"eld image of MnFe O samples ball milled for 120 min.
Fig. 2. Milling time dependence of the sizes as measured by XRD (lower curve) and TEM (upper curve).
work the method we used, HEBM, yields heterogeneous particle size and possible aggregations of di!erent particles. Fig. 3 shows a broad particle size distribution observed in a bright-"eld image of the sample ball-milled for 120 min. Information on the cation distribution can be obtained from the area ratio of the A and B sites in MoK ssbauer spectra. Good result is obtained when the spectra are well resolved. As it is known, no clear splitting occurs in the MoK ssbauer spectrum of MnFe O . The implication is that the MoK ssbauer
parameters for A and B sites in the case of manganese ferrite are similar [6]. Therefore, in order to improve the resolution, MoK ssbauer measurements were carried out in the presence of high external magnetic "eld. Mn-ferrite has the collinear structure, so that in the presence of an applied magnetic "eld, the B sites moments are oriented parallel and those of the A sites antiparallel to the "eld direction. Since the internal "eld at the iron nucleus is negative the magnetic hyper"ne splitting at the A site is increased by the applied "eld, while the B site splitting is decreased, which results in a clear splitting of the A and B sites. Fig. 4 shows MoK ssbauer spectra for the bulk and "ne particle samples taken at 10 K in the presence of 5 T magnetic "eld parallel to the incident gamma-ray beam. As the "gure indicates, one can detect a resolved splitting of the outer lines. The calculated MoK ssbauer parameters from the computer "tting are given in Table 2. At low temperature the recoilless free fraction for iron ions on the A and B sites are essentially equal [7] therefore, the occupation numbers of the iron ions can be considered proportional to the corresponding areas of their MoK ssbauer spectra. Consequently, the relative numbers of Fe> ions in A and B sites are determined from the ratio of the areas under the two sublattices. From this, the ratio of the Mn ions on A and B sites could be inferred. The data obtained regarding the cation distribution
52
M.H. Mahmoud et al. / Physica B 291 (2000) 49 }53
Fig. 4. MoK ssbauer spectra at 10 K in a longitudinal external magnetic "eld of MnFe O samples ball-milled for various periods of time (a) as prepared (b) 10 (c) 30 (d) 120 and (e) 180 min.
between A and B sites and consequently the inversion parameter are given in Table 3, where the round brackets refer to the tetrahedral sites and the square brackets refer to the octahedral sites. As it is clear from the table, the cation distributions for ball milled and bulk samples are di!erent. The inversion parameter increases with increasing milling time. The same behavior was observed, for ZnFe O samples, by S[ epelaH k et al. [4] who ob tained a 97% inversion for the normal zinc ferrite after 28 min of milling. They attribute this increase in the inversion parameter to a crystal lattice contraction (decrease of the crystal lattice constant) in the ball-milled samples with increasing the milling time. In the present study, X-ray di!raction investigations show that; the ball-milled samples have the same crystal lattice constant (within the experimental error) as that of the bulk. This rules out the possibility of using changes in crystal lattice to explain the observed increase in the inversion parameter. However, particle-size analysis shows that changes in the inversion parameter of the ball-milled samples could be attributed to particle-size dependence. Fig. 5 shows a comparison between the increase of the inversion parameter and the decrease of the particle size with increasing the milling time. It is clear from this "gure that the sharp increase in inversion parameter followed by slow variation with time corresponds to a sharp drop in
Table 2 MoK ssbauer parameters of MnFe O ball-milled samples in the presence of a magnetic "eld of 5 T. w is the line width, Q is the quadrupole splitting, H is the hyper"ne "eld, S is the relative subspectral area Milling time (min)
Site
H (kOe)
w (mm/s)
Q (mm/s)
S (%)
Zero
A B A B A B A B A B
537$1.5 465$1.5 531$1.5 469$1.5 544$1.5 477$1.5 529$1.5 462$1.5 548$1.5 485$1.5
0.15$0.02 0.53$0.02 0.30$0.05 0.59$0.02 0.25$0.05 0.63G0.02 0.34$0.05 0.68$0.02 0.35$0.05 0.85$0.02
!0.1 $0.05 0.08$0.05 !0.08$0.05 0.03$0.05 !0.20$0.05 0.03$0.05 !0.19$0.05 0.03$0.05 !0.26$0.05 0.001$0.05
23 77 39 61 41 59 43 57 45 55
10 30 120 180
M.H. Mahmoud et al. / Physica B 291 (2000) 49 }53
53
Table 3 Cation distribution of MnFe O samples ball-milled for di!erent periods of time Milling time (min)
0 10 30 120 180
Relative areas of Fe> A
B
23 39 41 43 45
77 61 59 57 55
Cation distribution
Inversion parameter (%)
(Mn Fe ) (Mn Fe ) (Mn Fe ) (Mn Fe ) (Mn Fe )
23 39 41 43 45
[Mn Fe ] [Mn Fe ] [Mn Fe ] [Mn Fe ] [Mn Fe ]
particle size followed by slow variation with time, which explains the inversion parameter dependence upon the milling time in these samples.
References [1] J. Smith, H.P.J. Wijn, Ferrites, Wiley, New York, 1959. [2] J.M. Hasting, L.M. Corliss, Phys. Rev. 104 (1956) 328. [3] A.H. Morrish, Z.W. Li, X.Z. Zhou, J. Phys. C1 (1997) 513. [4] V. S[ epelaH l, K. TkaH c\ ovaH , V.V. Boldyerv, S. Wissmann, K.D. Becker, Physica B 234}236 (1997) 617. [5] K. Haneda, H. Kojima, A.H. Morrish, P.J. Picone, K. Wakai, J. Appl. Phys. 53 (1982) 2686. [6] A.H. Morrish, P.E. Clark, Phys. Rev. B 11 (1975) 278. [7] G.A. Sawatzky, F. Van Der Woude, A.H. Morrish, Phys. Rev. 183 (1969) 383.
Fig. 5. Comparison between changes in the inversion parameter and the particle size in MnFe O samples with ball-milling time.