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Mössbauer spectroscopic evaluation of high-energy ball-milled CdFe2O4
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
. Mossbauer spectroscopic evaluation of high-energy ball-milled CdFe2O4 M.H. Mahmouda, A.M. Abdallasa, H.H. Hamdehb, W.M. Hikalb, S.M. Taherb, J.C. Hob,* b
a Department of Physics, Assiut University, Assiut 71516, Egypt Department of Physics, Wichita State University, Wichita, KS 67260, USA
Received 30 September 2002; received in revised form 9 December 2002
Abstract Structural and magnetic changes in cadmium ferrite, CdFe2O4, as induced by high-energy ball-milling were . investigated by Mossbauer spectroscopy. Upon milling the average crystallite size was gradually reduced to the nanometer range. The size reduction was associated with a significant increase in the inversion parameter of the spinel structure, causing the transition from a long-range antiferromagnetic order at around 10 K to a highly temperaturesensitive disordered magnetic state. r 2003 Elsevier Science B.V. All rights reserved. . Keywords: Mossbauer spectroscopy; Cadmium ferrite; Ball-milling; Inversion; Disordered magnetic state
1. Introduction Spinel ferrites, MFe2O4, are iron-base compounds with M being a divalent metal element or a mixture of, e.g., magnetic Fe, Mn and Ni or nonmagnetic Zn and Mg. They play important roles in various electronic devices because of their favorable magnetic permeability, high electrical resistivity and low fabrication cost [1]. By expressing the chemical formula as (M1cFec)[McFe2c]O4, one-third of the cations occupy A-sites denoted by the parenthesis, having *Corresponding author. Department of Physics, Wichita State University, Wichita, KS 67226, USA. Tel.: +1-316-9783992; fax: +1-316-978-3350. E-mail address: [email protected] (J.C. Ho).
a tetrahedral symmetry with four-fold oxygen coordination. The remaining two-thirds occupy B-sites as depicted by the bracket, having an octahedral symmetry with six-fold oxygen coordination. The amount of trivalent Fe ions at A-sites is represented by the inversion parameter c: For magnetic M, negative exchange interaction JAB between the two sublattices dominates the, also negative but much weaker, intrasublattice exchange interactions JAA and JBB ; resulting in a ferrimagnetic ordering, which forces each of the two magnetic sublattices to be aligned. It has been known that zinc ions have a strong preference for occupying A-sites [2]. This behavior has been used to enhance magnetization by incorporating small amounts of Zn into ferrites such as MnFe2O4 and NiFe2O4 [3,4]. However, a
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(02)01573-1
ARTICLE IN PRESS 270
M.H. Mahmoud et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
substantial addition of Zn reduces magnetization and lowers the Curie temperature, as fewer magnetic cations are available in the A-sublattice to maintain the ferrimagnetic order. In fact, for the extreme case of pure zinc ferrite (c ¼ 0), only weak coupling exists in the B-B sublattice and the magnetic state becomes antiferromagnetic below 10 K. For the intermediate case, the magnetic state is more complex and localized canted magnetic moments may occur on either sublattice. Given the strong site preference of zinc ions for the A-sites, considerable inversion parameters for zinc ferrite can only be obtained by driving the compound far from the equilibrium state. Best results have been achieved by using either the aerogel or the ball-milling method [5,6]. Indeed, mechanically activated processes for ferrite systems have received a great attention in recent years. In particular, high-energy ball-milling (HEBM) provides the greatest modification to the atomic arrangement in the solid [7]. The degree of inversion in zinc ferrite, starting from the normal spinel structure, increases with ball-milling time. This leads to an appreciable change in magnetic properties. Even though zinc ferrite is not technologically important, but the knowledge gained from it provides a valuable understanding to ferrites as a whole. . As a comparative study, Mossbauer spectroscopy was employed in this work to evaluate cadmium ferrite, CdFe2O4, following different periods of ball-milling. Like zinc, nonmagnetic cadmium forms a divalent ion, and also has a strong preference for the A-sites.
2. Experiment Bulk CdFe2O4 was prepared in polycrystalline form by the ceramic method. Stoichometric mixtures of pure iron oxide (a-Fe2O3) and cadmium oxide (CdO) powders were calcined at 9001C in air for 12 h, followed by slow cooling. The resulting material was then dry-pressed inside a steel die into green pellets, which were sintered at 10001C for 20 h before slow cooling. X-ray diffraction was used to confirm the cubic spinel structure in the product.
For HEBM, 5 g of the sample were sealed under a pure argon atmosphere in a 50-cm3 tungsten carbide vial. Inside the vial were two 12-g tungsten carbide balls of 12 mm in diameter. Milling process was performed for the time periods of 10, 20, 60, and 180 min, respectively. . Mossbauer absorbers were prepared by evenly dispersing powder samples between two layers of tape. All measurements were made in a transmission mode, using a 57Co source mounted in a constant acceleration drive unit. For low temperature measurements, the absorbers were placed inside a Cryo-Industries closed cycle refrigerator. Measurements in a 5-T external field were conducted by positioning the absorbers in the bore of a superconducting magnet, with the gamma-beam directed along the direction of the external field.
3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns for the as-prepared powder and the 60and 180-min ball-milled samples. The as-prepared powder displays sharp peaks expected for a wellcrystallized cubic spinel. The decrease in amplitude and the increase in width of the peaks with increasing HEBM time reflect the increased structural disorder and reduced particle dimensions caused by the ballistic nature of the milling process. In the XRD pattern for the prolonged, 180-min ball-milled sample, an enhanced background and a broad peak at low angles indicate the presence of a substantial amorphous component. The mean particle size obtained from the Scherrer formula is 226, 14 and 8 nm for the samples milled for the periods of 0, 60 and 180 min, respectively. In the absence of any magnetic order, . Mossbauer spectra from Fe ions on either A- or B-sites of the spinel would show only one or a set of doublets with particular values of isomer shift and quadrupole splitting. On the other hand, magnetically ordered Fe ions would yield sextets of six absorption lines each, reflecting hyperfine magnetic splittings. The line widths are often sensitive to sample crystallinity and temperature.
ARTICLE IN PRESS M.H. Mahmoud et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
Fig. 1. X-ray diffraction patterns of CdFe2 O4 samples ballmilled for various periods of time.
The thermal effect can be most pronounced in fine particles. This is indeed the case for the room. temperature Mossbauer spectra in Fig. 2. The asprepared and the 10-, 20-, and 60-min ball-milled samples are clearly all paramagnetic, showing a doublet each. In contrast, the 180-min ball-milled sample exhibits two magnetic components along with a paramagnetic doublet, reflecting a severe disorder caused by the lengthy period of HEBM. One of the magnetic signals is a sextet typical of those associated with a good magnetic order, even though thermal effects at room temperature are
271
apparent from the considerable broadening of the spectral lines. Even more so, the second magnetic spectrum is totally collapsed to result in simply a broad peak. Such a superparamagnetic behavior is expected for the nano-scaled particles. In principle, . in Mossbauer measurements, superparamagnetic effects prevail at temperatures where the fluctuation relaxation time becomes less than the Larmor precession time (tL ) of the nuclear magnetic moment, with the effect being strongly dependent on the volume of single-domain magnetic clusters. The mean quadrupole splitting, isomer shift and line width were deduced from a least-squares fitting routine. These values are listed in Table 1. The isomer shift values for all samples are typical of the high-spin Fe3+ ions. The observed trend of . the Mossbauer line width and the average quadrupole splitting point to a gradual increase in structural disorder with milling time as corroborated by XRD. In the extreme case of milling for 180 min, the amount of Fe has a ratio of 3:2 between the doublet and the two magnetic subspectra in Fig. 2 as deduced from areal data fitting. . Mossbauer data in Fig. 3 reveal the milling-time dependence of magnetic order in the samples at 20 K. The number of magnetically ordered Fe ions increases within few minutes of ball milling. This trend can be explained by an induced chemical disorder, which results in Fe ions occupying both sublattices of the spinel, and thus the formation of localized magnetic clusters. The size and shape of these magnetic clusters are influenced by the strong JAB interaction between Fe ions on the Asites and Fe ions on the B-sites. A random arrangement of Fe ions on both sites will yield many clusters with different volumes and magnetic order. Since the relaxation time is sensitive to . volume [8], the Mossbauer spectrum is the sum of spectra with different relaxation times. Accordingly, we fit the experimental data to a superposition of three types of sub-spectra. A broadened quadrupole doublet from Fe with a short relaxation time (totL ), a collapsing spectrum form Fe with an intermediate relaxation time (tBtL ) and a broadened sextet from Fe with a long relaxation time (t > tL ). The resulting percentages of Fe in the sextet components at 20 K are listed in Table 2.
ARTICLE IN PRESS M.H. Mahmoud et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
272
0 min.
10 min.
Intensity (a.u.)
Intensity (a.u.)
180 min.
20 min.
60 min.
-9
-6
-3
0
3
6
Velocity (mm/s)
(a)
-9
9
(b)
-6
-3
0
3
6
9
Velocity (mm/s)
. Fig. 2. Room temperature Mossbauer spectra of CdFe2O4 samples ball-milled for (a) 0–60 min and (b) 180 min. The solid lines represent the fitted components and their sum.
Table 1 Milling-time dependence of isomer shift, quadrupole splitting, . and line width of doublets in room-temperature Mossbauer spectra Time of milling (min)
IS (mm/s)a
QS (mm/s)
HWHM (mm/s)
0 10 20 60 180b
0.38 0.38 0.38 0.37 0.38
0.7870.02 0.8070.02 0.8270.02 0.8570.02 0.8670.02
0.1770.01 0.1870.01 0.2070.01 0.2470.01 0.2670.01
a b
Relative to IS of a-Fe. The doublet component of the spectrum.
The impact of ball-milling on the chemical order and the magnetic state can be more directly elucidated through measurements at low temperatures and in a strong magnetic field. Indeed, the data in Fig. 4 for the 10-, 60- and 180-min HEBM
samples, which were taken at 10 K and in a 5 T magnetic field, display distinct hyperfine sextets following the suppression of thermal fluctuations. Two significant observations can be made from the application of such a field. The first is the splitting of the two outermost (first and sixth) peaks of a given spectrum into two distinct subpeaks. This is expected from the tendency of the applied field to align the majority moments (minority moments) parallel (antiparallel) to its direction. It is well known that the internal hyperfine magnetic field at the Fe nucleus is antiparallel to the atom’s magnetic moment. Consequently, Fe ions with antiparallel moments contribute to the sextet corresponding to the external peak, and Fe ions with parallel moments contribute to the sextet corresponding the other peak. However, the latter sextet is usually divided into a pair of sextets in order to incorporate Fe ions having a broad range of local environments [9]. The three sextets are
ARTICLE IN PRESS M.H. Mahmoud et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
273
10 Min.
0 min.
20 min.
60 Min.
Intensity (a.u.)
Intensity (a.u.)
10 min.
180 Min.
60 min.
180 min.
B2
A B1
-12
-9
-6
-3
0
3
6
9
12
Velocity (mm/s) -9
-6
-3
0
3
6
9
Velocity (mm/s) . Fig. 3. Mossbauer spectra at 20 K of CdFe2O4 samples ballmilled for various periods of time. The solid lines represent the fitted components and their sum.
Table 2 Milling-time dependence of percentage of Fe being magnetic at 20 K Milling time (min)
Percentage of magnetic Fe
0 10 20 60 180
9 17 38 69 100
labeled A, B1 and B2 in Fig. 4. The second observation is the reduction in the ratio, R, between the areas of the second (fifth) and the first (sixth) peaks, as anticipated from the rotation of the magnetic moments toward the common direction of the applied field and the gammabeam. Actually, R is zero when all moments are either parallel or antiparallel to the applied field.
. Fig. 4. Mossbauer spectra at 10 K in a longitudinal external magnetic field of CdFe2O4 samples ball-milled for various periods of time. The solid lines represent the fitted components and their sum.
Hence, a nonzero R provides a direct measure of the degree of collinearity of the magnetic moments relative to the applied field. Based on the above observations, we fitted each spectrum to the summation of three sextets. All hyperfine parameters, along with R, were adjusted freely by least-squares fitting routine. The calculated areal fractions under the A-sextet and average R of the B1- and B2-sextet are listed in Table 3. A value of R ¼ 0:67 is for random orientation of magnetization, and it does not change by applied fields for ideal antiferromagnetic materials. In contrast, R would be zero for normal ferrimagnetic materials in a sufficiently strong field such as 5 T being applied here. As seen from the average value of R for B1- and B2-sextets in Table 3, the 10-min HEBM sample is nearly antiferromagnetic and the other two samples are neither antiferromagnetic nor ferrimagnetic. For HEBM at times longer than 10 min, the average magnetization tends to rotate further into the direction of the applied field. One notable result of the fitting is the
ARTICLE IN PRESS 274
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Table 3 Milling-time dependence, at 10 K and 5 T, of areal fraction of A-sextet and average area ratio (R) of second and first peaks of B-sextets Milling time (min)
Areal fraction of A-sextet
Average area ratio of second and first peaks of B-sextets
10 60 180
0.05 0.09 0.16
0.67 0.56 0.37
negligible value of R for the A-sextet, suggesting that the moments of this sextet are practically collinear with the applied field. The observed canting can thus be attributed to the magnetic moments on the B-sites due to the A-sites being sparsely populated by Fe ions. A similar behavior was also observed in Zn-ferrites [10]. In a ferrimagnetic order, all Fe with antiparrallel moments reside on the A-sites. Hence, the areal fractions of the A-sextet provide reasonable estimates for the inversion parameter c: In this system, however, these values may overestimate c because of the possibility of magnetization reversal on the B-sites.
4. Conclusion The as-prepared CdFe2O4 powders are not an entirely normal ferrite, but exhibit a certain
magnetic order at 20 K. By high-energy ballmilling at progressively longer times, the reduction in particle size is associated with structural and chemical disorder. There are concurrent changes to the magnetic state and greater tendency of magnetization to stabilize at higher temperatures. The evolution from an antiferromagnetic order to a complex magnetic state, which is characterized by local canted moments, is due primarily to small inversion parameters. This behavior is consistent with a diluted exchange coupling between magnetic Fe ions on the A- and B-sites.
References [1] E.C. Snelling, C. Fiee (Ed.), Soft Ferrites Propeties and Applications, Butterwoths, London, 1988. [2] J.B. Evans, S.S. Hafner, H.P. Weber, J. Chem. Phys. 55 (1971) 5282. [3] A.H. Morrish, P.E. Clark, Phys. Rev. B 11 (1975) 278. [4] L.K. Leung, B.J. Evans, A.H. Morrish, Phys. Rev. B 8 (1973) 29. [5] H. Hamdeh, J.C. Ho, S.A. Oliver, R.J. Willey, G. Oliveri, G. Busca, J. Appl. Phys. 81 (1997) 1851. $ [6] V. Sepelak, K. Tk!aov!a, V.V. Boldyrev, S. Wissmann, K.D. Becker, Physica B 234–236 (1997) 617. [7] M.H. Mahmoud, H.H. Hamdeh, J.C. Ho, M.J. O’Shea, J.C. Walker, J. Magn. Magn. Mater. 220 (2000) 139. [8] S. M^rup, H. Tops^e, J. Appl. Phys. 11 (1976) 63. [9] G.A. Sawatzky, F. Van Der Woude, A.H. Morrish, J. Appl. Phys. 39 (1968) 1204. [10] S.A. Oliver, H.H. Hamdeh, J.C. Ho, Phys. Rev. B (1999) 3400.
Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
. Mossbauer spectroscopic evaluation of high-energy ball-milled CdFe2O4 M.H. Mahmouda, A.M. Abdallasa, H.H. Hamdehb, W.M. Hikalb, S.M. Taherb, J.C. Hob,* b
a Department of Physics, Assiut University, Assiut 71516, Egypt Department of Physics, Wichita State University, Wichita, KS 67260, USA
Received 30 September 2002; received in revised form 9 December 2002
Abstract Structural and magnetic changes in cadmium ferrite, CdFe2O4, as induced by high-energy ball-milling were . investigated by Mossbauer spectroscopy. Upon milling the average crystallite size was gradually reduced to the nanometer range. The size reduction was associated with a significant increase in the inversion parameter of the spinel structure, causing the transition from a long-range antiferromagnetic order at around 10 K to a highly temperaturesensitive disordered magnetic state. r 2003 Elsevier Science B.V. All rights reserved. . Keywords: Mossbauer spectroscopy; Cadmium ferrite; Ball-milling; Inversion; Disordered magnetic state
1. Introduction Spinel ferrites, MFe2O4, are iron-base compounds with M being a divalent metal element or a mixture of, e.g., magnetic Fe, Mn and Ni or nonmagnetic Zn and Mg. They play important roles in various electronic devices because of their favorable magnetic permeability, high electrical resistivity and low fabrication cost [1]. By expressing the chemical formula as (M1cFec)[McFe2c]O4, one-third of the cations occupy A-sites denoted by the parenthesis, having *Corresponding author. Department of Physics, Wichita State University, Wichita, KS 67226, USA. Tel.: +1-316-9783992; fax: +1-316-978-3350. E-mail address: [email protected] (J.C. Ho).
a tetrahedral symmetry with four-fold oxygen coordination. The remaining two-thirds occupy B-sites as depicted by the bracket, having an octahedral symmetry with six-fold oxygen coordination. The amount of trivalent Fe ions at A-sites is represented by the inversion parameter c: For magnetic M, negative exchange interaction JAB between the two sublattices dominates the, also negative but much weaker, intrasublattice exchange interactions JAA and JBB ; resulting in a ferrimagnetic ordering, which forces each of the two magnetic sublattices to be aligned. It has been known that zinc ions have a strong preference for occupying A-sites [2]. This behavior has been used to enhance magnetization by incorporating small amounts of Zn into ferrites such as MnFe2O4 and NiFe2O4 [3,4]. However, a
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(02)01573-1
ARTICLE IN PRESS 270
M.H. Mahmoud et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
substantial addition of Zn reduces magnetization and lowers the Curie temperature, as fewer magnetic cations are available in the A-sublattice to maintain the ferrimagnetic order. In fact, for the extreme case of pure zinc ferrite (c ¼ 0), only weak coupling exists in the B-B sublattice and the magnetic state becomes antiferromagnetic below 10 K. For the intermediate case, the magnetic state is more complex and localized canted magnetic moments may occur on either sublattice. Given the strong site preference of zinc ions for the A-sites, considerable inversion parameters for zinc ferrite can only be obtained by driving the compound far from the equilibrium state. Best results have been achieved by using either the aerogel or the ball-milling method [5,6]. Indeed, mechanically activated processes for ferrite systems have received a great attention in recent years. In particular, high-energy ball-milling (HEBM) provides the greatest modification to the atomic arrangement in the solid [7]. The degree of inversion in zinc ferrite, starting from the normal spinel structure, increases with ball-milling time. This leads to an appreciable change in magnetic properties. Even though zinc ferrite is not technologically important, but the knowledge gained from it provides a valuable understanding to ferrites as a whole. . As a comparative study, Mossbauer spectroscopy was employed in this work to evaluate cadmium ferrite, CdFe2O4, following different periods of ball-milling. Like zinc, nonmagnetic cadmium forms a divalent ion, and also has a strong preference for the A-sites.
2. Experiment Bulk CdFe2O4 was prepared in polycrystalline form by the ceramic method. Stoichometric mixtures of pure iron oxide (a-Fe2O3) and cadmium oxide (CdO) powders were calcined at 9001C in air for 12 h, followed by slow cooling. The resulting material was then dry-pressed inside a steel die into green pellets, which were sintered at 10001C for 20 h before slow cooling. X-ray diffraction was used to confirm the cubic spinel structure in the product.
For HEBM, 5 g of the sample were sealed under a pure argon atmosphere in a 50-cm3 tungsten carbide vial. Inside the vial were two 12-g tungsten carbide balls of 12 mm in diameter. Milling process was performed for the time periods of 10, 20, 60, and 180 min, respectively. . Mossbauer absorbers were prepared by evenly dispersing powder samples between two layers of tape. All measurements were made in a transmission mode, using a 57Co source mounted in a constant acceleration drive unit. For low temperature measurements, the absorbers were placed inside a Cryo-Industries closed cycle refrigerator. Measurements in a 5-T external field were conducted by positioning the absorbers in the bore of a superconducting magnet, with the gamma-beam directed along the direction of the external field.
3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns for the as-prepared powder and the 60and 180-min ball-milled samples. The as-prepared powder displays sharp peaks expected for a wellcrystallized cubic spinel. The decrease in amplitude and the increase in width of the peaks with increasing HEBM time reflect the increased structural disorder and reduced particle dimensions caused by the ballistic nature of the milling process. In the XRD pattern for the prolonged, 180-min ball-milled sample, an enhanced background and a broad peak at low angles indicate the presence of a substantial amorphous component. The mean particle size obtained from the Scherrer formula is 226, 14 and 8 nm for the samples milled for the periods of 0, 60 and 180 min, respectively. In the absence of any magnetic order, . Mossbauer spectra from Fe ions on either A- or B-sites of the spinel would show only one or a set of doublets with particular values of isomer shift and quadrupole splitting. On the other hand, magnetically ordered Fe ions would yield sextets of six absorption lines each, reflecting hyperfine magnetic splittings. The line widths are often sensitive to sample crystallinity and temperature.
ARTICLE IN PRESS M.H. Mahmoud et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
Fig. 1. X-ray diffraction patterns of CdFe2 O4 samples ballmilled for various periods of time.
The thermal effect can be most pronounced in fine particles. This is indeed the case for the room. temperature Mossbauer spectra in Fig. 2. The asprepared and the 10-, 20-, and 60-min ball-milled samples are clearly all paramagnetic, showing a doublet each. In contrast, the 180-min ball-milled sample exhibits two magnetic components along with a paramagnetic doublet, reflecting a severe disorder caused by the lengthy period of HEBM. One of the magnetic signals is a sextet typical of those associated with a good magnetic order, even though thermal effects at room temperature are
271
apparent from the considerable broadening of the spectral lines. Even more so, the second magnetic spectrum is totally collapsed to result in simply a broad peak. Such a superparamagnetic behavior is expected for the nano-scaled particles. In principle, . in Mossbauer measurements, superparamagnetic effects prevail at temperatures where the fluctuation relaxation time becomes less than the Larmor precession time (tL ) of the nuclear magnetic moment, with the effect being strongly dependent on the volume of single-domain magnetic clusters. The mean quadrupole splitting, isomer shift and line width were deduced from a least-squares fitting routine. These values are listed in Table 1. The isomer shift values for all samples are typical of the high-spin Fe3+ ions. The observed trend of . the Mossbauer line width and the average quadrupole splitting point to a gradual increase in structural disorder with milling time as corroborated by XRD. In the extreme case of milling for 180 min, the amount of Fe has a ratio of 3:2 between the doublet and the two magnetic subspectra in Fig. 2 as deduced from areal data fitting. . Mossbauer data in Fig. 3 reveal the milling-time dependence of magnetic order in the samples at 20 K. The number of magnetically ordered Fe ions increases within few minutes of ball milling. This trend can be explained by an induced chemical disorder, which results in Fe ions occupying both sublattices of the spinel, and thus the formation of localized magnetic clusters. The size and shape of these magnetic clusters are influenced by the strong JAB interaction between Fe ions on the Asites and Fe ions on the B-sites. A random arrangement of Fe ions on both sites will yield many clusters with different volumes and magnetic order. Since the relaxation time is sensitive to . volume [8], the Mossbauer spectrum is the sum of spectra with different relaxation times. Accordingly, we fit the experimental data to a superposition of three types of sub-spectra. A broadened quadrupole doublet from Fe with a short relaxation time (totL ), a collapsing spectrum form Fe with an intermediate relaxation time (tBtL ) and a broadened sextet from Fe with a long relaxation time (t > tL ). The resulting percentages of Fe in the sextet components at 20 K are listed in Table 2.
ARTICLE IN PRESS M.H. Mahmoud et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
272
0 min.
10 min.
Intensity (a.u.)
Intensity (a.u.)
180 min.
20 min.
60 min.
-9
-6
-3
0
3
6
Velocity (mm/s)
(a)
-9
9
(b)
-6
-3
0
3
6
9
Velocity (mm/s)
. Fig. 2. Room temperature Mossbauer spectra of CdFe2O4 samples ball-milled for (a) 0–60 min and (b) 180 min. The solid lines represent the fitted components and their sum.
Table 1 Milling-time dependence of isomer shift, quadrupole splitting, . and line width of doublets in room-temperature Mossbauer spectra Time of milling (min)
IS (mm/s)a
QS (mm/s)
HWHM (mm/s)
0 10 20 60 180b
0.38 0.38 0.38 0.37 0.38
0.7870.02 0.8070.02 0.8270.02 0.8570.02 0.8670.02
0.1770.01 0.1870.01 0.2070.01 0.2470.01 0.2670.01
a b
Relative to IS of a-Fe. The doublet component of the spectrum.
The impact of ball-milling on the chemical order and the magnetic state can be more directly elucidated through measurements at low temperatures and in a strong magnetic field. Indeed, the data in Fig. 4 for the 10-, 60- and 180-min HEBM
samples, which were taken at 10 K and in a 5 T magnetic field, display distinct hyperfine sextets following the suppression of thermal fluctuations. Two significant observations can be made from the application of such a field. The first is the splitting of the two outermost (first and sixth) peaks of a given spectrum into two distinct subpeaks. This is expected from the tendency of the applied field to align the majority moments (minority moments) parallel (antiparallel) to its direction. It is well known that the internal hyperfine magnetic field at the Fe nucleus is antiparallel to the atom’s magnetic moment. Consequently, Fe ions with antiparallel moments contribute to the sextet corresponding to the external peak, and Fe ions with parallel moments contribute to the sextet corresponding the other peak. However, the latter sextet is usually divided into a pair of sextets in order to incorporate Fe ions having a broad range of local environments [9]. The three sextets are
ARTICLE IN PRESS M.H. Mahmoud et al. / Journal of Magnetism and Magnetic Materials 263 (2003) 269–274
273
10 Min.
0 min.
20 min.
60 Min.
Intensity (a.u.)
Intensity (a.u.)
10 min.
180 Min.
60 min.
180 min.
B2
A B1
-12
-9
-6
-3
0
3
6
9
12
Velocity (mm/s) -9
-6
-3
0
3
6
9
Velocity (mm/s) . Fig. 3. Mossbauer spectra at 20 K of CdFe2O4 samples ballmilled for various periods of time. The solid lines represent the fitted components and their sum.
Table 2 Milling-time dependence of percentage of Fe being magnetic at 20 K Milling time (min)
Percentage of magnetic Fe
0 10 20 60 180
9 17 38 69 100
labeled A, B1 and B2 in Fig. 4. The second observation is the reduction in the ratio, R, between the areas of the second (fifth) and the first (sixth) peaks, as anticipated from the rotation of the magnetic moments toward the common direction of the applied field and the gammabeam. Actually, R is zero when all moments are either parallel or antiparallel to the applied field.
. Fig. 4. Mossbauer spectra at 10 K in a longitudinal external magnetic field of CdFe2O4 samples ball-milled for various periods of time. The solid lines represent the fitted components and their sum.
Hence, a nonzero R provides a direct measure of the degree of collinearity of the magnetic moments relative to the applied field. Based on the above observations, we fitted each spectrum to the summation of three sextets. All hyperfine parameters, along with R, were adjusted freely by least-squares fitting routine. The calculated areal fractions under the A-sextet and average R of the B1- and B2-sextet are listed in Table 3. A value of R ¼ 0:67 is for random orientation of magnetization, and it does not change by applied fields for ideal antiferromagnetic materials. In contrast, R would be zero for normal ferrimagnetic materials in a sufficiently strong field such as 5 T being applied here. As seen from the average value of R for B1- and B2-sextets in Table 3, the 10-min HEBM sample is nearly antiferromagnetic and the other two samples are neither antiferromagnetic nor ferrimagnetic. For HEBM at times longer than 10 min, the average magnetization tends to rotate further into the direction of the applied field. One notable result of the fitting is the
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Table 3 Milling-time dependence, at 10 K and 5 T, of areal fraction of A-sextet and average area ratio (R) of second and first peaks of B-sextets Milling time (min)
Areal fraction of A-sextet
Average area ratio of second and first peaks of B-sextets
10 60 180
0.05 0.09 0.16
0.67 0.56 0.37
negligible value of R for the A-sextet, suggesting that the moments of this sextet are practically collinear with the applied field. The observed canting can thus be attributed to the magnetic moments on the B-sites due to the A-sites being sparsely populated by Fe ions. A similar behavior was also observed in Zn-ferrites [10]. In a ferrimagnetic order, all Fe with antiparrallel moments reside on the A-sites. Hence, the areal fractions of the A-sextet provide reasonable estimates for the inversion parameter c: In this system, however, these values may overestimate c because of the possibility of magnetization reversal on the B-sites.
4. Conclusion The as-prepared CdFe2O4 powders are not an entirely normal ferrite, but exhibit a certain
magnetic order at 20 K. By high-energy ballmilling at progressively longer times, the reduction in particle size is associated with structural and chemical disorder. There are concurrent changes to the magnetic state and greater tendency of magnetization to stabilize at higher temperatures. The evolution from an antiferromagnetic order to a complex magnetic state, which is characterized by local canted moments, is due primarily to small inversion parameters. This behavior is consistent with a diluted exchange coupling between magnetic Fe ions on the A- and B-sites.
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