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An investigation of particle size effects in ultrafine barium ferrite
Journal of Magnetism and Magnetic Materials 125 (1993) 199-208 North-Holland
An investigation of particle size effects in ultrafine barium ferrite V.K. Sankaranarayanan,
Q.A. Pankhurst, D.P.E. Dickson and C.E. Johnson
Department of Physics, University of Liverpool, Liverpool, UK
Received 23 November 1992; in revised form 29 January 1993
Ultrafine particles of barium ferrite in the size range 5-100 nm have been synthesized by thermal decomposition of a citrate precursor. The precursor decomposed at 425°C is amorphous, but crystalline barium ferrite starts forming at temperatures of 550°C and above. Barium ferrite which shows a monophase X-ray diffraction pattern and well-resolved Mossbauer spectra is obtained at 700°C. The Mossbauer spectra at both liquid helium and room temperature of samples annealed at 700, 750 and 800°C (with average particle sizes in electron micrographs of 60, 80 and 100 nm, respectively) could be satisfactorily resolved into five components corresponding to the five sublattice sites in barium ferrite. The Mijssbauer parameters: magnetic hyperfine field, quadrupole splitting and isomer shift, all show particle size dependence. The magnetic hyperfine field and quadrupole splitting are smaller for smaller particles (compared to bulk barium ferrite) and they increase with increasing particle size and annealing temperature for all the five sublattice sites. Isomer shifts also differ from bulk values but the variation with particle size is also dependent on the sublattice sites.
1. Introduction
Ultrafine particles of barium ferrite, BaFe,, 019 and its substituted derivatives are very promising media for high-density perpendicular magnetic recording. Perpendicular magnetic recording facilitates a higher recording density compared to conventional longitudinal recording because of the reduction of demagnetizing effects in the transition region between bits [l]. The platelet morphology of the barium ferrite particles means that they have the potential for incorporation into tapes for perpendicular recording. Moreover, due to the high anisotropy, the threshold size for superparamagnetic behaviour (caused by thermal fluctuations of the magnetization) is very small in barium ferrite, perhaps less than 10 nm, and therefore, at least theoretically, the bit Correspondence to: Dr D.P.E. Dickson, Department of Physics, University of Liverpool, Oliver Lodge Laboratory, Oxford Street, PO Box 147, Liverpool L69 3BX, UK. Tel: +44 (51) 794 3371; fax: +44 (51) 794 3441.
0304-8853/93/$06.00
size could be brought down considerably, although the anisotropy and coercivity must also be optimized for practical applications. However, it is very difficult to prepare such ultrafine particles, and, when prepared, even the intrinsic properties such as saturation magnetization and magnetic hyperfine field are observed to differ from those of the bulk materials, for reasons which are not yet fully understood [2,3]. Study of these particles therefore has considerable technological and scientific interest. Miissbauer spectroscopy is a useful technique for studying ultrafine ferrimagnetic particles and the effect of finite particle size. This is particularly the case for materials which have more than two magnetic sublattices. Barium ferrite has five distinct magnetic sublattices, as described in table 1. Due to the small measurement time of 10-s s it is often possible to resolve the subspectra corresponding to different sublattices, even for extremely small particles whose behaviour is near the thermal relaxation limit. Although there has been some disagreement in the literature con-
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Storage: Basic and Applied
no
V.K. Sunkarmaruyanur~
Table 1 Crystallographic
and magnetic
characteristics
et ul. / Particle
of Fe3 ’ sites in
BaFe,zO,, Site
Coordination
Spin
Occupancy
12k
Octahedral Tetrahedral Octahedral Octahedral Trigonal bipyramidal
UP Down up Down
I3 1 2 4
~JP
?
df,, 2a 4f,, 2h
(fivefold)
cerning the Mossbauer parameters for the five different iron sublattice sites in barium ferrite, recent measurements made by two groups working in different laboratories, on the same samples, have led to agreement with respect to the room temperature parameters for bulk barium and strontium ferrites [4]. Analysis of the Mijssbauer spectra of ultrafine particles often presents additional problems resulting either from different compositions, different synthesis conditions, or both, because the wet chemical methods employed for their synthesis demand extreme care. One of the major problems in studying ultrafine particles, as mentioned earlier, is associated with their synthesis. Preparation of ultrafine particles requires low-temperature processing to control the particle growth. Conventional ceramic methods involving the mixing, grinding and firing of the constituent oxides and/or carbonates invariably yield large particles in the micrometre size range because the diffusion controlled solid state reaction requires temperatures of 1000°C or above. The glass ceramic method and a variety of wet chemical methods such as coprecipitation, hydrothermal synthesis, the liquid mix technique, and the metallo-organic method have been employed in the preparation of ultrafine particles of barium ferrite [5-g]. These methods do yield fine particles but the processing temperatures at which monophase barium ferrite is obtained are often as high as 900°C [2]. The citrate precursor method, however, has been shown to produce ultrafine particles of a series of rare earth iron garnets at temperatures as low as 700°C [lo]. The atomiclevel blending of the constituent elements in the
size efjrects in ultrafine
huriurn fbritc
required stoichiometric ratio attained in the citrate precursor complex enables the decomposition of the precursor directly into the final ternary oxide at a lower temperature, without the formation of intermediate oxide phases that delays the formation of the final ternary oxide. The aims of the present study have therefore been: (i) to prepare such a citrate precursor complex; (ii) to obtain monophase barium ferrite at a relatively low temperature of 700°C or below by the decomposition of this precursor; and (iii) to investigate the particle size effects by Mossbauer spectroscopy. The lowering of the processing temperature is important from the point of view ot producing the smaller particles useful in perpendicular magnetic recording and studying their properties.
2. Experimental The samples were produced by the citrate precursor method. The citrate precursor was prcpared by a pH-controlled reaction between barium nitrate, iron nitrate and citric acid in an aqueous medium followed by alcohol dehydration. The preparation procedure of the precursor has been described elsewhere [ 1 I]. Thermal analysis of the precursor by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) was carried out by using a DuPont 2000 thermal analyzer. The precursor was decomposed at the temperatures indicated by thermal analysis for a sufficiently long time to ensure complete decomposition. The decomposed precursor was subsequently annealed at 500, 550, 600, 650, 700, 750 and 800°C for 6 h each to obtain ultrafine barium ferrite of different particle sizes. The X-ray diffraction (XRD) was carried out on a Philips PW1710-based diffractometer using CuK, radiation. The magnetization measurements were done on a Princeton Applied Research PAR 4500 vibrating-sample magnetometer. “Fe Mossbauer spectra were recorded using a “Co source embedded in a Rh matrix with a conventional constant acceleration spectrometer.
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Sankaranarayanan
201
et al. / Particle size effects in ultrafine barium ferrite
60 t
I
I
5o0
I
I
Temperature Fig. 1. Thermogravimetric
analysis
(a) and derivative
Thermal analysis of the citrate precursor using DSC and TGA show two stages of decomposition at 219 and 420°C as can be seen in the TG and
7
.. -
1
a
I
-6 Velocity
I
I
0
+6
1
(mm/s)
Fig. 2. Room temperature Miissbauer spectra of samples (a) as-decomposed at 425”C, and following annealing at (b) 5OO”C, (c) 550°C and (d) 600°C.
I 600
I
I 800
(“0
thermogravimetric
3. Results
I
I
400
200
analysis (b) curves for the citrate
precursor.
DTG curves in fig. 1. The precursor was decomposed at these temperatures for 24 and 48 h, respectively, to ensure complete decomposition. It may be noticed that there is no more weight loss beyond this temperature, indicating completion of the decomposition process. The as-decomposed samples and those annealed at 500°C are amorphous in X-ray diffraction, while the electron micrographs show the presence of some extremely small particles. The Mossbauer spectra at room temperature for the as-decomposed sample and the sample annealed at 5OO”C,show only a doublet, characteristic of either the superparamagnetic or amorphous nature of the sample, as shown in figs. 2(a) and (b). On annealing at 550°C (fig. 2c), the Miissbauer spectrum consists of both doublet and sextet components with broad lines. After annealing at 600°C the relative area of the doublet is very small, as shown in fig. 2(d). The 700°C sample gives well-defined and well-resolved Miissbauer spectra at room temperature and 4.2 K, as shown in figs. 3(a) and 4(a). The XRD pattern for the sample annealed at 700°C is shown in fig. 5. It may be noted that all the lines correspond to the BaFe,,O,, phase. The electron micrograph in fig. 6 gives some idea of the particle size and shape for this sample, the
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LCK. Sankaranarayanan
202
0
et al. / Particle size effects in ultrafine barium ferrite
t
4 20 .-5 ‘r $ 94 0
4
t
1
I -6 Velocity
Fig.
1
0
I
1
-1
+6
-6
(mm/s)
3. Room-temperature MGssbauer spectra for samples annealed at (a) 7OO”C, (b) 750°C and (c) 800°C.
average particle size being 60 nm. The magnetization in an applied field of 50 kOe is 61.5 emu/g, which is lower than the bulk value of 72 emu/g.
800
0 Velocity
+6 (mm/s)
Fig. 4. Miissbauer spectra at 4.2 K for samples (a) 700°C. (b) 7So”C and (c) 800°C.
200 100 0 0”
28 pattern
at
The samples annealed at 750 and 800°C understandably have larger average particle sizes in the range 80-100 nm, as observed in the electron
r
Fig. 5. X-ray diffraction
annealed
of the sample
annealed
at 700°C
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%K. Sankuranarayanan
et al. / Particle size effects in ultrafine barium ferrite
micrographs. The Mossbauer spectra for these samples are again well-resolved at both room temperature and 4.2 K as shown in figs. 3(b), 3(c), 4(b) and 4(c). The Mossbauer spectra and the hyperfine parameters are quite similar to those originally reported for barium and strontium ferrite [4]. The Mossbauer parameters isomer shift, quadrupole splitting and magnetic hyperfine field for the samples annealed at 700, 750 and 800°C are given in table 2. The parameters for bulk barium ferrite are included for comparison. These parameters appear to change with annealing temperature. The quadrupole splitting for all the sublattices are smaller for smaller particles and they seem to increase with increasing particle size. The isomer shifts however, depend also on the sublattice sites. For the 12k and 2b sublattice sites, the
Table 2 Mossbauer spectral parameters at room temperature: isomer shifts 6 (mm/s), quadrupole splittings A (mm/s), magnetic hyperfine fields H,, (kOe) and relative areas of subspectra R, (%I for the samples annealed at 700, 750 and 800°C along with the bulk barium ferrite data [4] Site
Hhr
R.4
0.36 0.29 0.38 0.39 0.31
0.38 0.18 0.09 0.14 2.14
408 484 505 513 397
56.0 22.0 10.7 5.0 5.1
Site
Sample
12k 4fi, 2a 4fG 2b
6 0.36 0.27 0.36 0.37 0.32
Site
Sample
at 700°C.
annealed A 0.40 0.22 0.08 0.17 2.12
annealed
at 750°C H,, 410 485 504 514 397
RA
49.7 26.8 7.9 9.4 5.9
at 800°C
6
A
H,,
RA
12k 4C” 2a 4fG 2b
0.35 0.26 0.36 0.39 0.31
0.41 0.23 0.11 0.17 2.16
415 487 507 515 400
48.5 26.2 8.3 11.6 5.8
Site
Bulk sample 6
A
Hts
RA
0.35 0.31 0.45 0.40 0.20
0.76 0.40 0.20 0.44 2.30
414 492 511 517 406
50.0 16.6 8.3 16.6 8.3
4f”i 2b
annealed
at 700°C
12k 4fi, 2a 4fG 2b
2a
of the sample
annealed A
4fi”
micrograph
Sample s
12k
Fig. 6. Electron
203
small particles have relatively larger isomer shifts compared to the bulk, while the 2a, 4f,, and 4f,, sublattice sites have smaller isomer shifts for smaller particles.
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204 4.
rt al. / Particle size effects
Discussion
4.1. Sample preparation The Mijssbauer spectrum of the sample annealed at 550°C shows both doublet and sextet components as shown in fig. 2(c). On comparison with the spectrum of standard barium ferrite [4], the sextet components correspond to the same sublattice sites. It is surprising that the barium ferrite phase starts developing at a temperature as low as 550°C. It may be remembered that ceramic methods require much higher temperatures, often above lOOO-1200°C. In situ neutron powder diffractometry on the synthesis reaction of BaFe,,O,, small particles using the liquid mix technique showed that BaFe,,O,, appears from 680 up to 880°C and the reaction involves intermediate cx-Fe,O, and BaFe,O, phases [12]. Using this method, barium ferrite which shows a monophase XRD pattern and well-resolved Mossbauer spectrum is obtained at temperatures as high as 900°C. In the present precursor method, the very good XRD patterns and well-resolved Mijssbauer spectra are obtained for the sample annealed at 700°C. This shows that, in the present method of preparation, we have succeeded in obtaining a citrate precursor complex, with the Ba and Fe ions in the required stoichiometric ratio of 1: 12, which decomposes to produce the ternary oxide BaFe,,O,,, without any intermediate oxide phases. In earlier work samples obtained by decomposition of the precursor at 470°C and subsequent annealing at 650, 700 and 800°C were observed to contain small amounts of the BaFe,O, phase [ll]. The Mossbauer spectra then required six components to obtain a satisfactory fit. In the present study using the same precursor, when the decomposition was carried out appropriately at the temperatures indicated by thermal analysis, the barium ferrite phase starts developing at 550°C and monophase barium ferrite is obtained at 700°C. The present study therefore demonstrates the importance of proper thermal decomposition. It shows how a change in the decomposition temperature employed can lead to the formation of inhomogeneties in the form of the extra phases even if the correct precursor is
in ultrufinr
barium
ferrite
used. Hence proper thermal decomposition is equally as important as the reaction to produce the correct precursor together with proper solvent removal, which are two further important factors in the precursor method. The success of the present method in producing monophase ternary oxides such as barium ferrite, directly by decomposition of the precursor without intermediate oxide phases, is encouraging in that it establishes the possibility of preparation of ternary oxides directly from complex precursors. It may be possible to produce precursor complexes using organic complexing materials other than citrates, which decompose at still lower temperatures. Lowering of the preparation temperature will be extremely useful for preparing ultrafine particles with the smallest size range (nanocrystals), which have been observed to possess strikingly different properties compared to the bulk [13]. It is also important from the point of view of preparing ultrafine particles with a controlled size distribution for applications in high-density perpendicular magnetic recording media. 4.2. Miissbauer spectra The gradual changes in the Mossbauer spectra in samples annealed at temperatures between 500 and 700°C (fig. 2) show the increasing development of well-crystallized barium ferrite from amorphous barium ferrite The isomer shift (0.30 mm/s) of the sample annealed at 500°C shows that the iron is present as Fe”+ and the large quadrupole splitting (0.70 mm/s) is indicative of the high electric field gradient at the Fe3+ sites resulting from the distorted environment in the amorphous structure. The crystallization sets in between 500 and 550°C and the crystallization process is not completed even at 600°C as is evident from the presence of a weak doublet. The sextet components in the Miissbauer spectra have broad lines indicating the disordered or distributed local environments. Prolonged heat treatment at this temperature may eliminate the amorphous phase completely, even at 600°C. After annealing at 7OO”C, however, a well-resolved Mossbauer spectrum and an XRD pattern indi-
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et al. / Particle size effects in ultrafine barium ferrite
eating well-crystallized material are observed, as shown in figs. 3, 4 and 5. Moreover, the Miissbauer spectra at both room temperature and 4.2 K could be satisfactorily resolved into the five components corresponding to the five sublattice sites in barium ferrite. The spectra and parameters are quite similar to those reported for bulk barium and strontium ferrites [4] and there is no indication of any superparamagnetic component. To our knowledge, barium ferrite as a single phase with well-resolved Mijssbauer spectra has not been prepared earlier at a temperature as low as 700°C by any preparation method, nor has the Mijssbauer spectrum for fine particles been satisfactorily resolved into the five components at both 4.2 K and room temperature. The Miissbauer spectra for 750 and 800°C annealed samples also show well-resolved subspectra as would be expected (figs. 3 and 4). An idea of the particle size and shape for these samples is given by the electron micrographs shown in fig. 6 for the 700°C sample. The samples annealed at 700, 750 and 800°C can be safely assumed to consist of an assembly of single domain particles because the critical single domain size for isolated spherical particles of barium ferrite has been predicted to be 0.5 km [141, which is well above the size range in the present samples. Moreover, a direct observation of the domain structure of barium ferrite particles has indicated that the critical size is around 1 urn [15]. In addition the high value of coercivity observed at room temperature (4000-6000 Oe) is close to the value expected theoretically for a randomly oriented assembly of uniaxial single domain particles undergoing magnetization reversal by coherent rotation [161. In the 4.2 K spectra shown in fig. 4 the components are not as well resolved as in the room-temperature spectra. This is characteristic of barium ferrite. At 4.2 K, the hyperfine field (Hhf) corresponding to the 12k site is very close to that of the 4f, site, (518 and 520 kOe, respectively). The 12k site in barium ferrite has a stronger temperature dependence of the hyperfine field and magnetization [17] and the hyperfine field falls much faster to 408 kOe, while for the 4f, site it reduces to 484 kOe only. Therefore these compo-
205
nents become better resolved at room temperature. Thus the difference in appearance of the spectra at 4.2 K and room temperature is due to the different temperature dependences of hyperfine fields of Fe3+ ions in different sublattice sites. 4.3. Site occupancies Barium ferrite is a ferrimagnet with five sublattices, as shown in table 1. It follows that the Mossbauer spectrum is built up from five subspectra. The area of each subspectrum is proportional to the number of ferric ions on the corresponding sublattice and to the recoilless fraction. If the recoilless fraction can be assumed to be the same for different sublattice sites, it can be seen that there is a difference in the relative areas of the subspectra compared with the theoretical occupancy of the corresponding sites, as shown in table 2. The 4f, subspectrum appears to have higher relative intensities than expected, while the 2b subspectra show lower relative intensities. A relatively high population of the 4f, sublattice sites can lead to a reduction of magnetization because the 4f, and 4f,, sublattice magnetizations are aligned antiparallel to the resultant magnetization direction. Such a difference in the relative population of sublattice sites could be one of the reasons for the lower magnetization which is frequently observed in fine particle ferrimagnets. The fine particles are normally prepared at relatively low temperature which may not allow the necessary diffusion of Fe3+ ions into the equilibrium Fe3+ sites in the crystal structure. With increasing heat-treatment temperatures and processing times, the parameters will presumably approach the bulk value. The relative area of the 2b site subspectrum for the 700°C annealed sample is lower than the bulk value and it increases with increasing annealing temperature and particle size. A low 2b site population leads to a substantial decrease in the magnetization [l&19], although it should be noted that the low recoil-free fraction previously observed [20] will also contribute to the low relative areas. Vacancies at the 2b site could lead to a magnetic structure where S-blocks along the c-axis
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are separated by nonmagnetic layers within Rblocks. This configuration inhibits the exchange interaction between ions in such blocks, thus decreasing the magnetization. Mossbauer spectra recorded in the presence of high enough applied fields are capable of resolving the subspectra corresponding to the antiparallel sublattices. The relative areas of subspectra in the high-field Mossbauer spectra can therefore be expected to give more evidence on the differences in sublattice site occupancies in these fine particle samples. Our preliminary measurements in applied fields do show higher relative intensities for the 4f, sites and lower values for the 2b sites. 4.4. Miissbauer hyperfine parameters A comparative study of the Miissbauer parameters: magnetic hyperfine field, quadrupole splitting and isomer shift for the samples annealed at 700, 750 and 800°C show (table 2) a number of features. It may be remembered that the average particle sizes for these samples as observed by electron microscopy increase progressively from 60 to 80 to 100 nm. Interestingly, the Mossbauer parameters also show corresponding changes. It may be observed in table 2 that in the case of all the five sublattices the hyperfine field is lowest for the 700°C annealed sample, which has the smallest particle size, and it increases with increasing annealing temperature and particle size to approach the bulk value. Fine particles of a variety of magnetic materials have been reported to show a relatively low hyperfine field when compared to the bulk materials, and various theories have been proposed to account for this [21231. The quadrupole splittings in table 2 indicate, surprisingly, that the fine particles have a lower quadrupole splitting for all the Fe3+ sites than the bulk material, and it increases with increasing particle size. Fine particles of barium ferrite prepared earlier by a hydrothermal method also show lower quadrupole splittings than the bulk 1241. The lattice contribution of the quadrupole splitting depends on the electric field gradient around the Fe nucleus resulting from the deviation of the
Fe-O oxygen polyhedra from cubic symmetry (the core and valence contributions being negligible for Fe”+). Therefore the lower quadrupole splitting in fine particles could be an indication of increased symmetry around the Fe-?+ sites. An increase in symmetry compared with bulk barium ferrite appears possible considering the high quadrupole splitting and considerable deviation from cubic symmetry at all the sublattice sites in the bulk barium ferrite [17] as shown in table 2. However, it is still surprising that fine particles can have less distorted polyhedra. It is possible that there is a change in the specific volume of the lattice in barium ferrite fine particles which alter the Fe-O bond distances such that the symmetry of iron oxygen polyhedra is increased. Such changes in the specific volume of the lattice and resulting changes in the Mossbauer parameters have already been reported in the case of garnets [25]. A number of other metallic and nonmetallic fine particle materials have also been reported to exhibit lattice expansion or contraction [26-291. Changes in specific volume and Fe-O bond distances may also be reflected in changes in the isomer shifts. Larger isomer shifts correspond to a lower s-electron density at the Fe nucleus and hence may indicate a larger Fe-O bond distance. Therefore Fe-O octahedra with larger Fe-O bond distances generally show greater isomer shift values compared with Fe-O tetrahedra in garnets and spine1 ferrites. It may be noted that the three octahedral sites (12k, 4f,, and 2a) do indeed show larger isomer shifts in the case of both bulk and smaller particles, compared to the tetrahedral (4fi,) and trigonal bipyramidal site (2b). Changes in Fe-O bond distances do appear to take place, as indicated by the differences in isomer shifts between fine particles and bulk. For 12k and 2b sites, the isomer shifts are larger for fine particles compared to bulk, whereas for 4f,,, 4f,, and 2a sites they are smaller for fine particles. However, in the case of barium ferrite the general correlation between isomer shift and bond distances should be applied with caution because of the highly distorted nature of the oxygen polyhedra. In fact the 2b and 12k sites with larger Fe-O bond distances (2.04 and 2.03 A, respectively)
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have smaller isomer shifts (0.31 and 0.35 mm/s, respectively) compared to the 4fvi and 2a sites (bond distances 2.02 and 2.00 A; isomer shifts 0.40 and 0.45 mm/s, respectively) in bulk barium ferrite [30,31]. Therefore the differences in isomer shifts between fine particles and bulk materials cannot be directly correlated with the changes in Fe-O bond distances and the resulting changes in lattice volume, as has been done earlier in the case of garnets [25].
207
bridge at the University of Wales, Bangor, for assistance in magnetic measurements.
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Ultrafine particles of well-crystallized, monophase barium ferrite, BaFe,,O,,, have been prepared at temperatures as low as 700°C by the thermal decomposition of a citrate precursor. Following decomposition the material obtained is amorphous but annealing produces crystalline barium ferrite with the smallest crystalline particles forming at only 550°C. Higher annealing temperatures lead to larger particle sizes. Miissbauer spectra obtained at both room temperature and 4.2 K from the samples annealed at 700, 750 and 800°C could be satisfactorily resolved into the five sextet components corresponding to the five Fe3+ sublattice sites in barium ferrite. The Mossbauer hyperfine parameters show a variation with particle size. In the case of the quadrupole splittings, which are generally low in the fine-particle samples, this variation appears to be correlated with a decreasing distortion of the iron-oxygen polyhedra with decreasing particle size.
Acknowledgements
We thank Dr Venugopala Rao at the University of Leeds for doing the thermal analysis, Dr Mike Wood and Miss Nicola Dempster at Liverpool John Moores University for X-ray diffraction, Dr A. Green and Mr John Mailor in the Materials Science Department at the University of Liverpool for assistance in electron microscopy work, and Dr K. O’Grady and Mr J.A. Cam-
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An investigation of particle size effects in ultrafine barium ferrite V.K. Sankaranarayanan,
Q.A. Pankhurst, D.P.E. Dickson and C.E. Johnson
Department of Physics, University of Liverpool, Liverpool, UK
Received 23 November 1992; in revised form 29 January 1993
Ultrafine particles of barium ferrite in the size range 5-100 nm have been synthesized by thermal decomposition of a citrate precursor. The precursor decomposed at 425°C is amorphous, but crystalline barium ferrite starts forming at temperatures of 550°C and above. Barium ferrite which shows a monophase X-ray diffraction pattern and well-resolved Mossbauer spectra is obtained at 700°C. The Mossbauer spectra at both liquid helium and room temperature of samples annealed at 700, 750 and 800°C (with average particle sizes in electron micrographs of 60, 80 and 100 nm, respectively) could be satisfactorily resolved into five components corresponding to the five sublattice sites in barium ferrite. The Mijssbauer parameters: magnetic hyperfine field, quadrupole splitting and isomer shift, all show particle size dependence. The magnetic hyperfine field and quadrupole splitting are smaller for smaller particles (compared to bulk barium ferrite) and they increase with increasing particle size and annealing temperature for all the five sublattice sites. Isomer shifts also differ from bulk values but the variation with particle size is also dependent on the sublattice sites.
1. Introduction
Ultrafine particles of barium ferrite, BaFe,, 019 and its substituted derivatives are very promising media for high-density perpendicular magnetic recording. Perpendicular magnetic recording facilitates a higher recording density compared to conventional longitudinal recording because of the reduction of demagnetizing effects in the transition region between bits [l]. The platelet morphology of the barium ferrite particles means that they have the potential for incorporation into tapes for perpendicular recording. Moreover, due to the high anisotropy, the threshold size for superparamagnetic behaviour (caused by thermal fluctuations of the magnetization) is very small in barium ferrite, perhaps less than 10 nm, and therefore, at least theoretically, the bit Correspondence to: Dr D.P.E. Dickson, Department of Physics, University of Liverpool, Oliver Lodge Laboratory, Oxford Street, PO Box 147, Liverpool L69 3BX, UK. Tel: +44 (51) 794 3371; fax: +44 (51) 794 3441.
0304-8853/93/$06.00
size could be brought down considerably, although the anisotropy and coercivity must also be optimized for practical applications. However, it is very difficult to prepare such ultrafine particles, and, when prepared, even the intrinsic properties such as saturation magnetization and magnetic hyperfine field are observed to differ from those of the bulk materials, for reasons which are not yet fully understood [2,3]. Study of these particles therefore has considerable technological and scientific interest. Miissbauer spectroscopy is a useful technique for studying ultrafine ferrimagnetic particles and the effect of finite particle size. This is particularly the case for materials which have more than two magnetic sublattices. Barium ferrite has five distinct magnetic sublattices, as described in table 1. Due to the small measurement time of 10-s s it is often possible to resolve the subspectra corresponding to different sublattices, even for extremely small particles whose behaviour is near the thermal relaxation limit. Although there has been some disagreement in the literature con-
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Storage: Basic and Applied
no
V.K. Sunkarmaruyanur~
Table 1 Crystallographic
and magnetic
characteristics
et ul. / Particle
of Fe3 ’ sites in
BaFe,zO,, Site
Coordination
Spin
Occupancy
12k
Octahedral Tetrahedral Octahedral Octahedral Trigonal bipyramidal
UP Down up Down
I3 1 2 4
~JP
?
df,, 2a 4f,, 2h
(fivefold)
cerning the Mossbauer parameters for the five different iron sublattice sites in barium ferrite, recent measurements made by two groups working in different laboratories, on the same samples, have led to agreement with respect to the room temperature parameters for bulk barium and strontium ferrites [4]. Analysis of the Mijssbauer spectra of ultrafine particles often presents additional problems resulting either from different compositions, different synthesis conditions, or both, because the wet chemical methods employed for their synthesis demand extreme care. One of the major problems in studying ultrafine particles, as mentioned earlier, is associated with their synthesis. Preparation of ultrafine particles requires low-temperature processing to control the particle growth. Conventional ceramic methods involving the mixing, grinding and firing of the constituent oxides and/or carbonates invariably yield large particles in the micrometre size range because the diffusion controlled solid state reaction requires temperatures of 1000°C or above. The glass ceramic method and a variety of wet chemical methods such as coprecipitation, hydrothermal synthesis, the liquid mix technique, and the metallo-organic method have been employed in the preparation of ultrafine particles of barium ferrite [5-g]. These methods do yield fine particles but the processing temperatures at which monophase barium ferrite is obtained are often as high as 900°C [2]. The citrate precursor method, however, has been shown to produce ultrafine particles of a series of rare earth iron garnets at temperatures as low as 700°C [lo]. The atomiclevel blending of the constituent elements in the
size efjrects in ultrafine
huriurn fbritc
required stoichiometric ratio attained in the citrate precursor complex enables the decomposition of the precursor directly into the final ternary oxide at a lower temperature, without the formation of intermediate oxide phases that delays the formation of the final ternary oxide. The aims of the present study have therefore been: (i) to prepare such a citrate precursor complex; (ii) to obtain monophase barium ferrite at a relatively low temperature of 700°C or below by the decomposition of this precursor; and (iii) to investigate the particle size effects by Mossbauer spectroscopy. The lowering of the processing temperature is important from the point of view ot producing the smaller particles useful in perpendicular magnetic recording and studying their properties.
2. Experimental The samples were produced by the citrate precursor method. The citrate precursor was prcpared by a pH-controlled reaction between barium nitrate, iron nitrate and citric acid in an aqueous medium followed by alcohol dehydration. The preparation procedure of the precursor has been described elsewhere [ 1 I]. Thermal analysis of the precursor by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) was carried out by using a DuPont 2000 thermal analyzer. The precursor was decomposed at the temperatures indicated by thermal analysis for a sufficiently long time to ensure complete decomposition. The decomposed precursor was subsequently annealed at 500, 550, 600, 650, 700, 750 and 800°C for 6 h each to obtain ultrafine barium ferrite of different particle sizes. The X-ray diffraction (XRD) was carried out on a Philips PW1710-based diffractometer using CuK, radiation. The magnetization measurements were done on a Princeton Applied Research PAR 4500 vibrating-sample magnetometer. “Fe Mossbauer spectra were recorded using a “Co source embedded in a Rh matrix with a conventional constant acceleration spectrometer.
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Sankaranarayanan
201
et al. / Particle size effects in ultrafine barium ferrite
60 t
I
I
5o0
I
I
Temperature Fig. 1. Thermogravimetric
analysis
(a) and derivative
Thermal analysis of the citrate precursor using DSC and TGA show two stages of decomposition at 219 and 420°C as can be seen in the TG and
7
.. -
1
a
I
-6 Velocity
I
I
0
+6
1
(mm/s)
Fig. 2. Room temperature Miissbauer spectra of samples (a) as-decomposed at 425”C, and following annealing at (b) 5OO”C, (c) 550°C and (d) 600°C.
I 600
I
I 800
(“0
thermogravimetric
3. Results
I
I
400
200
analysis (b) curves for the citrate
precursor.
DTG curves in fig. 1. The precursor was decomposed at these temperatures for 24 and 48 h, respectively, to ensure complete decomposition. It may be noticed that there is no more weight loss beyond this temperature, indicating completion of the decomposition process. The as-decomposed samples and those annealed at 500°C are amorphous in X-ray diffraction, while the electron micrographs show the presence of some extremely small particles. The Mossbauer spectra at room temperature for the as-decomposed sample and the sample annealed at 5OO”C,show only a doublet, characteristic of either the superparamagnetic or amorphous nature of the sample, as shown in figs. 2(a) and (b). On annealing at 550°C (fig. 2c), the Miissbauer spectrum consists of both doublet and sextet components with broad lines. After annealing at 600°C the relative area of the doublet is very small, as shown in fig. 2(d). The 700°C sample gives well-defined and well-resolved Miissbauer spectra at room temperature and 4.2 K, as shown in figs. 3(a) and 4(a). The XRD pattern for the sample annealed at 700°C is shown in fig. 5. It may be noted that all the lines correspond to the BaFe,,O,, phase. The electron micrograph in fig. 6 gives some idea of the particle size and shape for this sample, the
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LCK. Sankaranarayanan
202
0
et al. / Particle size effects in ultrafine barium ferrite
t
4 20 .-5 ‘r $ 94 0
4
t
1
I -6 Velocity
Fig.
1
0
I
1
-1
+6
-6
(mm/s)
3. Room-temperature MGssbauer spectra for samples annealed at (a) 7OO”C, (b) 750°C and (c) 800°C.
average particle size being 60 nm. The magnetization in an applied field of 50 kOe is 61.5 emu/g, which is lower than the bulk value of 72 emu/g.
800
0 Velocity
+6 (mm/s)
Fig. 4. Miissbauer spectra at 4.2 K for samples (a) 700°C. (b) 7So”C and (c) 800°C.
200 100 0 0”
28 pattern
at
The samples annealed at 750 and 800°C understandably have larger average particle sizes in the range 80-100 nm, as observed in the electron
r
Fig. 5. X-ray diffraction
annealed
of the sample
annealed
at 700°C
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%K. Sankuranarayanan
et al. / Particle size effects in ultrafine barium ferrite
micrographs. The Mossbauer spectra for these samples are again well-resolved at both room temperature and 4.2 K as shown in figs. 3(b), 3(c), 4(b) and 4(c). The Mossbauer spectra and the hyperfine parameters are quite similar to those originally reported for barium and strontium ferrite [4]. The Mossbauer parameters isomer shift, quadrupole splitting and magnetic hyperfine field for the samples annealed at 700, 750 and 800°C are given in table 2. The parameters for bulk barium ferrite are included for comparison. These parameters appear to change with annealing temperature. The quadrupole splitting for all the sublattices are smaller for smaller particles and they seem to increase with increasing particle size. The isomer shifts however, depend also on the sublattice sites. For the 12k and 2b sublattice sites, the
Table 2 Mossbauer spectral parameters at room temperature: isomer shifts 6 (mm/s), quadrupole splittings A (mm/s), magnetic hyperfine fields H,, (kOe) and relative areas of subspectra R, (%I for the samples annealed at 700, 750 and 800°C along with the bulk barium ferrite data [4] Site
Hhr
R.4
0.36 0.29 0.38 0.39 0.31
0.38 0.18 0.09 0.14 2.14
408 484 505 513 397
56.0 22.0 10.7 5.0 5.1
Site
Sample
12k 4fi, 2a 4fG 2b
6 0.36 0.27 0.36 0.37 0.32
Site
Sample
at 700°C.
annealed A 0.40 0.22 0.08 0.17 2.12
annealed
at 750°C H,, 410 485 504 514 397
RA
49.7 26.8 7.9 9.4 5.9
at 800°C
6
A
H,,
RA
12k 4C” 2a 4fG 2b
0.35 0.26 0.36 0.39 0.31
0.41 0.23 0.11 0.17 2.16
415 487 507 515 400
48.5 26.2 8.3 11.6 5.8
Site
Bulk sample 6
A
Hts
RA
0.35 0.31 0.45 0.40 0.20
0.76 0.40 0.20 0.44 2.30
414 492 511 517 406
50.0 16.6 8.3 16.6 8.3
4f”i 2b
annealed
at 700°C
12k 4fi, 2a 4fG 2b
2a
of the sample
annealed A
4fi”
micrograph
Sample s
12k
Fig. 6. Electron
203
small particles have relatively larger isomer shifts compared to the bulk, while the 2a, 4f,, and 4f,, sublattice sites have smaller isomer shifts for smaller particles.
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V.K. Sankarunaruyanan
204 4.
rt al. / Particle size effects
Discussion
4.1. Sample preparation The Mijssbauer spectrum of the sample annealed at 550°C shows both doublet and sextet components as shown in fig. 2(c). On comparison with the spectrum of standard barium ferrite [4], the sextet components correspond to the same sublattice sites. It is surprising that the barium ferrite phase starts developing at a temperature as low as 550°C. It may be remembered that ceramic methods require much higher temperatures, often above lOOO-1200°C. In situ neutron powder diffractometry on the synthesis reaction of BaFe,,O,, small particles using the liquid mix technique showed that BaFe,,O,, appears from 680 up to 880°C and the reaction involves intermediate cx-Fe,O, and BaFe,O, phases [12]. Using this method, barium ferrite which shows a monophase XRD pattern and well-resolved Mossbauer spectrum is obtained at temperatures as high as 900°C. In the present precursor method, the very good XRD patterns and well-resolved Mijssbauer spectra are obtained for the sample annealed at 700°C. This shows that, in the present method of preparation, we have succeeded in obtaining a citrate precursor complex, with the Ba and Fe ions in the required stoichiometric ratio of 1: 12, which decomposes to produce the ternary oxide BaFe,,O,,, without any intermediate oxide phases. In earlier work samples obtained by decomposition of the precursor at 470°C and subsequent annealing at 650, 700 and 800°C were observed to contain small amounts of the BaFe,O, phase [ll]. The Mossbauer spectra then required six components to obtain a satisfactory fit. In the present study using the same precursor, when the decomposition was carried out appropriately at the temperatures indicated by thermal analysis, the barium ferrite phase starts developing at 550°C and monophase barium ferrite is obtained at 700°C. The present study therefore demonstrates the importance of proper thermal decomposition. It shows how a change in the decomposition temperature employed can lead to the formation of inhomogeneties in the form of the extra phases even if the correct precursor is
in ultrufinr
barium
ferrite
used. Hence proper thermal decomposition is equally as important as the reaction to produce the correct precursor together with proper solvent removal, which are two further important factors in the precursor method. The success of the present method in producing monophase ternary oxides such as barium ferrite, directly by decomposition of the precursor without intermediate oxide phases, is encouraging in that it establishes the possibility of preparation of ternary oxides directly from complex precursors. It may be possible to produce precursor complexes using organic complexing materials other than citrates, which decompose at still lower temperatures. Lowering of the preparation temperature will be extremely useful for preparing ultrafine particles with the smallest size range (nanocrystals), which have been observed to possess strikingly different properties compared to the bulk [13]. It is also important from the point of view of preparing ultrafine particles with a controlled size distribution for applications in high-density perpendicular magnetic recording media. 4.2. Miissbauer spectra The gradual changes in the Mossbauer spectra in samples annealed at temperatures between 500 and 700°C (fig. 2) show the increasing development of well-crystallized barium ferrite from amorphous barium ferrite The isomer shift (0.30 mm/s) of the sample annealed at 500°C shows that the iron is present as Fe”+ and the large quadrupole splitting (0.70 mm/s) is indicative of the high electric field gradient at the Fe3+ sites resulting from the distorted environment in the amorphous structure. The crystallization sets in between 500 and 550°C and the crystallization process is not completed even at 600°C as is evident from the presence of a weak doublet. The sextet components in the Miissbauer spectra have broad lines indicating the disordered or distributed local environments. Prolonged heat treatment at this temperature may eliminate the amorphous phase completely, even at 600°C. After annealing at 7OO”C, however, a well-resolved Mossbauer spectrum and an XRD pattern indi-
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et al. / Particle size effects in ultrafine barium ferrite
eating well-crystallized material are observed, as shown in figs. 3, 4 and 5. Moreover, the Miissbauer spectra at both room temperature and 4.2 K could be satisfactorily resolved into the five components corresponding to the five sublattice sites in barium ferrite. The spectra and parameters are quite similar to those reported for bulk barium and strontium ferrites [4] and there is no indication of any superparamagnetic component. To our knowledge, barium ferrite as a single phase with well-resolved Mijssbauer spectra has not been prepared earlier at a temperature as low as 700°C by any preparation method, nor has the Mijssbauer spectrum for fine particles been satisfactorily resolved into the five components at both 4.2 K and room temperature. The Miissbauer spectra for 750 and 800°C annealed samples also show well-resolved subspectra as would be expected (figs. 3 and 4). An idea of the particle size and shape for these samples is given by the electron micrographs shown in fig. 6 for the 700°C sample. The samples annealed at 700, 750 and 800°C can be safely assumed to consist of an assembly of single domain particles because the critical single domain size for isolated spherical particles of barium ferrite has been predicted to be 0.5 km [141, which is well above the size range in the present samples. Moreover, a direct observation of the domain structure of barium ferrite particles has indicated that the critical size is around 1 urn [15]. In addition the high value of coercivity observed at room temperature (4000-6000 Oe) is close to the value expected theoretically for a randomly oriented assembly of uniaxial single domain particles undergoing magnetization reversal by coherent rotation [161. In the 4.2 K spectra shown in fig. 4 the components are not as well resolved as in the room-temperature spectra. This is characteristic of barium ferrite. At 4.2 K, the hyperfine field (Hhf) corresponding to the 12k site is very close to that of the 4f, site, (518 and 520 kOe, respectively). The 12k site in barium ferrite has a stronger temperature dependence of the hyperfine field and magnetization [17] and the hyperfine field falls much faster to 408 kOe, while for the 4f, site it reduces to 484 kOe only. Therefore these compo-
205
nents become better resolved at room temperature. Thus the difference in appearance of the spectra at 4.2 K and room temperature is due to the different temperature dependences of hyperfine fields of Fe3+ ions in different sublattice sites. 4.3. Site occupancies Barium ferrite is a ferrimagnet with five sublattices, as shown in table 1. It follows that the Mossbauer spectrum is built up from five subspectra. The area of each subspectrum is proportional to the number of ferric ions on the corresponding sublattice and to the recoilless fraction. If the recoilless fraction can be assumed to be the same for different sublattice sites, it can be seen that there is a difference in the relative areas of the subspectra compared with the theoretical occupancy of the corresponding sites, as shown in table 2. The 4f, subspectrum appears to have higher relative intensities than expected, while the 2b subspectra show lower relative intensities. A relatively high population of the 4f, sublattice sites can lead to a reduction of magnetization because the 4f, and 4f,, sublattice magnetizations are aligned antiparallel to the resultant magnetization direction. Such a difference in the relative population of sublattice sites could be one of the reasons for the lower magnetization which is frequently observed in fine particle ferrimagnets. The fine particles are normally prepared at relatively low temperature which may not allow the necessary diffusion of Fe3+ ions into the equilibrium Fe3+ sites in the crystal structure. With increasing heat-treatment temperatures and processing times, the parameters will presumably approach the bulk value. The relative area of the 2b site subspectrum for the 700°C annealed sample is lower than the bulk value and it increases with increasing annealing temperature and particle size. A low 2b site population leads to a substantial decrease in the magnetization [l&19], although it should be noted that the low recoil-free fraction previously observed [20] will also contribute to the low relative areas. Vacancies at the 2b site could lead to a magnetic structure where S-blocks along the c-axis
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are separated by nonmagnetic layers within Rblocks. This configuration inhibits the exchange interaction between ions in such blocks, thus decreasing the magnetization. Mossbauer spectra recorded in the presence of high enough applied fields are capable of resolving the subspectra corresponding to the antiparallel sublattices. The relative areas of subspectra in the high-field Mossbauer spectra can therefore be expected to give more evidence on the differences in sublattice site occupancies in these fine particle samples. Our preliminary measurements in applied fields do show higher relative intensities for the 4f, sites and lower values for the 2b sites. 4.4. Miissbauer hyperfine parameters A comparative study of the Miissbauer parameters: magnetic hyperfine field, quadrupole splitting and isomer shift for the samples annealed at 700, 750 and 800°C show (table 2) a number of features. It may be remembered that the average particle sizes for these samples as observed by electron microscopy increase progressively from 60 to 80 to 100 nm. Interestingly, the Mossbauer parameters also show corresponding changes. It may be observed in table 2 that in the case of all the five sublattices the hyperfine field is lowest for the 700°C annealed sample, which has the smallest particle size, and it increases with increasing annealing temperature and particle size to approach the bulk value. Fine particles of a variety of magnetic materials have been reported to show a relatively low hyperfine field when compared to the bulk materials, and various theories have been proposed to account for this [21231. The quadrupole splittings in table 2 indicate, surprisingly, that the fine particles have a lower quadrupole splitting for all the Fe3+ sites than the bulk material, and it increases with increasing particle size. Fine particles of barium ferrite prepared earlier by a hydrothermal method also show lower quadrupole splittings than the bulk 1241. The lattice contribution of the quadrupole splitting depends on the electric field gradient around the Fe nucleus resulting from the deviation of the
Fe-O oxygen polyhedra from cubic symmetry (the core and valence contributions being negligible for Fe”+). Therefore the lower quadrupole splitting in fine particles could be an indication of increased symmetry around the Fe-?+ sites. An increase in symmetry compared with bulk barium ferrite appears possible considering the high quadrupole splitting and considerable deviation from cubic symmetry at all the sublattice sites in the bulk barium ferrite [17] as shown in table 2. However, it is still surprising that fine particles can have less distorted polyhedra. It is possible that there is a change in the specific volume of the lattice in barium ferrite fine particles which alter the Fe-O bond distances such that the symmetry of iron oxygen polyhedra is increased. Such changes in the specific volume of the lattice and resulting changes in the Mossbauer parameters have already been reported in the case of garnets [25]. A number of other metallic and nonmetallic fine particle materials have also been reported to exhibit lattice expansion or contraction [26-291. Changes in specific volume and Fe-O bond distances may also be reflected in changes in the isomer shifts. Larger isomer shifts correspond to a lower s-electron density at the Fe nucleus and hence may indicate a larger Fe-O bond distance. Therefore Fe-O octahedra with larger Fe-O bond distances generally show greater isomer shift values compared with Fe-O tetrahedra in garnets and spine1 ferrites. It may be noted that the three octahedral sites (12k, 4f,, and 2a) do indeed show larger isomer shifts in the case of both bulk and smaller particles, compared to the tetrahedral (4fi,) and trigonal bipyramidal site (2b). Changes in Fe-O bond distances do appear to take place, as indicated by the differences in isomer shifts between fine particles and bulk. For 12k and 2b sites, the isomer shifts are larger for fine particles compared to bulk, whereas for 4f,,, 4f,, and 2a sites they are smaller for fine particles. However, in the case of barium ferrite the general correlation between isomer shift and bond distances should be applied with caution because of the highly distorted nature of the oxygen polyhedra. In fact the 2b and 12k sites with larger Fe-O bond distances (2.04 and 2.03 A, respectively)
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have smaller isomer shifts (0.31 and 0.35 mm/s, respectively) compared to the 4fvi and 2a sites (bond distances 2.02 and 2.00 A; isomer shifts 0.40 and 0.45 mm/s, respectively) in bulk barium ferrite [30,31]. Therefore the differences in isomer shifts between fine particles and bulk materials cannot be directly correlated with the changes in Fe-O bond distances and the resulting changes in lattice volume, as has been done earlier in the case of garnets [25].
207
bridge at the University of Wales, Bangor, for assistance in magnetic measurements.
References
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Ultrafine particles of well-crystallized, monophase barium ferrite, BaFe,,O,,, have been prepared at temperatures as low as 700°C by the thermal decomposition of a citrate precursor. Following decomposition the material obtained is amorphous but annealing produces crystalline barium ferrite with the smallest crystalline particles forming at only 550°C. Higher annealing temperatures lead to larger particle sizes. Miissbauer spectra obtained at both room temperature and 4.2 K from the samples annealed at 700, 750 and 800°C could be satisfactorily resolved into the five sextet components corresponding to the five Fe3+ sublattice sites in barium ferrite. The Mossbauer hyperfine parameters show a variation with particle size. In the case of the quadrupole splittings, which are generally low in the fine-particle samples, this variation appears to be correlated with a decreasing distortion of the iron-oxygen polyhedra with decreasing particle size.
Acknowledgements
We thank Dr Venugopala Rao at the University of Leeds for doing the thermal analysis, Dr Mike Wood and Miss Nicola Dempster at Liverpool John Moores University for X-ray diffraction, Dr A. Green and Mr John Mailor in the Materials Science Department at the University of Liverpool for assistance in electron microscopy work, and Dr K. O’Grady and Mr J.A. Cam-
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