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Effect of high-energy ball milling time on structural and magnetic properties of nanocrystalline cobalt ferrite powders
Journal of Magnetism and Magnetic Materials 341 (2013) 17–24
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of high-energy ball milling time on structural and magnetic properties of nanocrystalline cobalt ferrite powders Yarilyn Cedeño-Mattei a,n, Oscar Perales-Pérez a,b, Oswald N.C. Uwakweh b a b
Department of Chemistry, University of Puerto Rico—Mayagüez Campus, Mayagüez, PR 00681, Puerto Rico Department of Engineering Science & Materials, University of Puerto Rico—Mayagüez Campus, Mayagüez, PR 00681, Puerto Rico
art ic l e i nf o
a b s t r a c t
Article history: Received 5 November 2012 Received in revised form 1 April 2013 Available online 15 April 2013
Cobalt ferrite nanocrystals synthesized by conventional and size-controlled coprecipitation methods were treated by high-energy ball milling, HEBM, in order to study the effect of crystal size reduction and/ or strain on the resulting magnetic properties. Processed nanocrystals were characterized by X-ray diffraction, Brunauer, Emmett, and Teller surface area analysis, transmission electron microscopy (TEM), and vibrating sample magnetometry. The cobalt ferrite nanocrystals exhibited crystal size reduction from initial values (average crystallite sizes of 12 7 1 nm and 18 7 3 nm, respectively) down to 10 nm after HEBM for 10 h. The specific surface area was decreased by milling (from 96.5 to 59.4 m2/g; for the 12 nm cobalt ferrite nanocrystals), due to particles aggregation. TEM analyses corroborated the aggregation of the nanoparticles at such long milling times. The same cobalt ferrite nanocrystals exhibited a rise in coercivity from 394 to 560 Oe after 5 h ball milling which was attributed to the introduction of strain anisotropy, namely point defects, as suggested by the systematic shift of the diffraction peaks towards higher angles. In turn, the magnetic characterization of the starting 18 nm-nanocrystals reported a drop in coercivity from 4506 Oe to 491 Oe that was attributed predominantly to size reduction within the single domain region. A correlation between particle size, cationic distribution, and HEBM processing conditions became evident. & 2013 Elsevier B.V. All rights reserved.
Keywords: Cobalt ferrite Nanocrystal High-energy ball milling Specific surface area Magnetic property
1. Introduction The selection of suitable synthesis and processing conditions of nanometric ferrites will affect their crystal size, morphology, cation distribution and hence, the resulting magnetic properties. Although coercivity is mainly governed by the magnetocrystalline anisotropy energy, contribution from surface anisotropy, strain anisotropy and/or shape anisotropy also affect the magnetic anisotropy, particularly at the nanoscale. High-energy ball milling (HEBM), so-called mechano-chemical processing, has emerged as a technique to produce nanomaterials capable to achieve a remarkable particle size reduction from a previously synthesized materials [1–4], or induce solid-phase transformations [5]. HEBM has also been widely used in the preparation of ferrites from metal oxides [6]. In HEBM, powdered samples are placed inside metallic jars containing suitable proportion of grinding media, usually ceramic balls, and contacted at extremely high rotational speeds to promote the impact and abrasion of the powder with the
n Correspondence to: University of Puerto Rico, Department of Engineering Science and Materials, Call Box 9000, Mayaguez, PR 00681-9000, Puerto Rico. Tel.: +787 832 4040x2086; fax: +787 265 3816. E-mail addresses: [email protected], [email protected] (Y. Cedeño-Mattei).
0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.04.015
grinding media, which in turn will induce the generation of defects and strain along with particle size reduction [7]. The advantage of this processing technique relies on the use of inexpensive and environment-friendly solvents and low temperatures, when compared to wet-chemistry-based routes or conventional solid-state processing (e.g. sintering). The mechano-chemical synthesis or activation of spinel structured oxides by HEBM has been reported with the resulting final outcome depending on the type of material and grinding conditions. For instance, the use of inert grinding materials are reported to only impact structural disorder due to change in degree of inversion as was reported in the case of MgFe2O4 [8]. On the other hand, the formation of NiFe2O4 was observed when solid NiO and α-Fe2O3 solid precursors were ball-milled [9] using stainless steel grinding media. Cobalt ferrite nanocrystals were selected for this study because its magnetic properties are easily tunable when composition, size, shape, and cation distribution are modified. Nanometric cobalt ferrite exhibits a strong size-dependent coercivity; therefore, any change in the ferrite synthesis and/or processing conditions will affect the crystal size and hence, the magnetic properties [10]. Besides, the high-energy impacts inside the milling apparatus should induce strain anisotropy in the crystal lattice, thereby modifying the corresponding coercivity.
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On this basis, the present work is focused on the study of the dependence of specific surface area, structural (average crystallite size and lattice parameter), and magnetic (coercivity and maximum magnetization) with HEBM processing conditions of cobalt ferrite nanocrystals of different average crystallite sizes.
A WEBRES spectrometer operating in the transmission mode, with a 50 mCi 57Co source in a Rh matrix from Ritverc, GmbH was for Mössbauer measurements. The degree of Fe3+ distribution within the tetrahedral and octahedral sites was deduced via the following expression: I Tet =I Oct ¼ ðƒ Tet =ƒ Oct Þ½λ=ð2−λÞ
2. Material and methods 2.1. Materials CoCl2 6H2O (ACS, 98–102%, Alfa Aesar), FeCl3 6H2O (ACS, 97–102%, Alfa Aesar), and NaOH (pellets, 98%, Alfa Aesar) were used without further purification, as precursor for the synthesis of cobalt ferrite nanocrystals. 2.2. Synthesis of cobalt ferrite nanocrystals
ð2Þ
with, ITet and IOct, referring to the relative abundances associated with the tetrahedral and octahedral sites, and ƒ, the recoilless fraction at the corresponding sites for the temperature of measurement (in our case, a room temperature value of 0.94 was used [15]). The inversion parameter, λ, translates to the following structural designation: (M1−λFeλ)Tet[MλFe2−λ]OctO4 and expresses the degree of departure of the cations (“M”¼ Co in our case, and “Fe”) from their expected octahedral and tetrahedral occupations.
3. Results and discussion
Cobalt ferrite powders were synthesized using the conventional and the size-controlled coprecipitation methods. The later considered the synthesis under fixed flow-rate (0.71 mL/min) of reactants addition according to the method developed by CedeñoMattei et al. [11]. A micro-peristaltic pump was used for this purpose. Control on flow-rate leads to the formation of nanocrystals with larger size due to modification in oversaturation conditions that promotes heterogeneous nucleation [12]. A 0.315 M NaOH boiling solution was the precipitant agent. The reaction time was set to one hour based on our previous works [11]. 2.3. HEBM processing of cobalt ferrite nanocrystals Cobalt ferrite samples were synthesized by the conventional and size-controlled coprecipitation routes. These two samples were processed by high-energy ball milling in a Fritch Pulverisseette-4 milling apparatus with tungsten carbide (WC) balls and jars. The ballmilling assembly consisted of 2 jars of 45 mL each one bearing 17 balls of 10 mm diameter each. A ball to powder ratio (BPR) of 40:1, a milling speed of 1400 rpm, and milling times in the 20 min–10 h range, were used in all our experiments. The choice of BPR of 40:1 was based on previous studies on bulk cobalt ferrite which showed its effectiveness in particle size reduction [13]. Small amounts of sample were withdrawn at pre-selected milling times to monitor the progress of the size reduction/aggregation process and determine the corresponding structural and magnetic properties.
3.1. XRD analyses XRD patterns from the 12 71 nm and 18 73 nm as-synthesized cobalt ferrite powders before (0 min) and after 30 min, 60 min, and 300 min of milling times are shown in Figs. 1 and 2, respectively. Peak broadening became evident after prolonged milling time that suggested the decrease in crystallite size. The average crystallite size of the 12 nm powders showed a just minor decrease (10 nm) even after 10 h of milling, whereas a more noticeable diminution in the average crystallite size (from 18 nm to 10 nm) was exhibited at the end of the 10 h milling period by the sample synthesized under size-controlled conditions. Evidently, the impact and abrasion forces generated inside the milling jar must have broken up the crystals even at the nanoscale. Figs. 1 and 2 also evidenced the shift of the XRD peaks towards smaller diffraction angles. This shift in diffraction angle is attributed to strain which is also reflected by the increase of the corresponding increase of the interplanar distances [7,16], the interplanar distance between (440) planes was increased from 1.483 Å to 1.486 Å when the milling time was prolonged up to 10 h. An increase in the lattice parameter could be attributed to point defects, e.g. ferric ion occupies normally unoccupied sites [17]. In turn, it will cause a change in the degree of inversion of cobalt ferrite.
2.4. Nanocrystals characterization The structural characterization of the nanocrystal ferrites was carried out by X-ray diffractometry using a Siemens D500 powder diffractometer with Cu Kα radiation. The average crystallite sizes reported were determined by XRD measurements and the Scherrer's equation. The specific surface area of the powders was measured in a Horiba SA-9600 series surface area analyzer. The external specific surface area of spheres, SSA, with diameter ‘t’ (or cubes with edge length ‘t’) and density ρ can be estimated by [14]: SSA ¼ 6=ρt
ð1Þ
This formula does not take into account interparticle overlapping (aggregation) but only reversible agglomeration. Morphological analyses of the samples were done in a JEOL 2011 transmission electron microscope. A LakeShore 7400 series vibrating sample magnetometer was used to determine the magnetic properties of the powdered samples at room temperature.
Fig. 1. XRD patterns of 12 nm-cobalt ferrite nanocrystals ball-milled at different times.
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Table 2 Variation in average crystallite size, ‘t’, lattice parameter, ‘a’, and specific surface area, ‘SSA’, measurements of ball-milled cobalt ferrite nanocrystals with initial average crystallite size of 18 nm. Milling time (min)
t ( 7 3 nm)
a ( 70.01 Å)
Calculated SSA (m2/g)
BET SSA (m2/g)
0 20 30 40 50 60 180 300 600
18 14 12 12 11 10 9 10 10
8.39 8.38 8.39 8.38 8.39 8.39 8.39 8.39 8.41
66.11 92.27 97.18 99.58 102.10 121.51 140.87 121.51 124.04
71.35 62.36 55.72 52.26 54.41 59.38 63.5 58.73 55.11
Fig. 2. XRD patterns of 18 nm-cobalt ferrite nanocrystals ball-milled at different times.
Fig. 4. Variation in average crystallite size, ‘t’, and surface area, ‘SSA’, as a function of milling time of 18 nm-cobalt ferrite particles. The dotted line shown in the figure is a visual guide to show the decreasing trend in both, average crystallite size and SSA.
Fig. 3. Variation of the average crystallite size, ‘t’, and specific surface area, ‘SSA’, with milling time of cobalt ferrite crystals with initial average crystallite size of 12 nm. The dotted line is a visual guide to show the decreasing trend in both, average crystallite size and SSA.
Table 1 Variation in average crystallite size, ‘t’, lattice parameter, ‘a’, and specific surface area, ‘SSA’, measurements of ball-milled cobalt ferrite nanocrystals with initial average crystallite size of 12 nm. Milling time (min)
t ( 7 1 nm)
a ( 7 0.02 Å)
Calculated SSA (m2/g)
BET SSA (m2/g)
0 20 30 40 50 60 180 300 600
12 11 10 10 10 10 10 9 10
8.38 8.38 8.38 8.38 8.39 8.39 8.39 8.40 8.40
99.06 109.40 113.69 116.74 119.08 111.23 121.51 131.23 124.04
96.45 77.21 71.27 69.22 71.25 73.39 65.36 62.15 59.39
3.2. BET-specific surface area of ball-milled powders Tables 1 and 2 show the calculated (using Eq. 1) and experimental (BET) specific surface area values for the 12 nm and 18 nm
ferrite samples, respectively, ball-milled at different times. The density of cobalt ferrite, involved in determining the calculated SSA, was 5.304 g/cm3. As observed, the calculated and the measured BET specific surface area values were very similar in the starting powders, which suggest that individual and nonaggregated nanocrystals were produced. However, the discrepancy between these two values became remarkable after prolonging the milling time. The decreasing trend in specific surface area with milling time can be attributed to strong interparticle aggregation promoted by intensive milling (Fig. 3). A similar aggregation phenomenon has been observed in different materials after prolonged milling [18–20]. The 18 nm-sample exhibited a reduction in SSA during the first 50 min of milling; a fact that can be attributed to some extent of aggregation. The small increase in SSA, observed in the 50 min180 min milling time range, can be mainly attributed to reduction in crystal size; nevertheless, aggregation of ferrite particles becomes the dominant effect of HEBM processing at prolonged milling times. In turn, as Fig. 4 and the data in Table 2 suggest, the crystal size reduction in the 12 nm-ferrite sample was not significant and aggregation predominates from the very beginning of the milling process. The high-energy impact of the grinding media on the ferrite particles should have promoted their binding due to the development of inter-particle strong interacting forces.
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3.3. TEM analyses Fig. 5 shows high resolution TEM images of the starting 12 nm and 18 nm cobalt ferrite nanocrystals. TEM images of the 12 nm cobalt ferrite particles, before and after 5 h of milling, (shown in Fig. 6) evidenced the strong interparticle aggregation after
prolonged milling times. The severe particle aggregation at longer milling times also became evident for the 18 nm ferrite samples (Fig. 7). These images clearly explained the discrepancy between the calculated and measured SSA values: the longer the milling, the more pronounced the particle aggregation and, consequently, the less the corresponding SSA.
Fig. 5. High-resolution TEM images of cobalt ferrite with an average crystallite size of 12 nm (left) and 18 nm (right), before milling.
Fig. 6. TEM images of cobalt ferrite nanocrystals before, (left), and after 5 h of high-intensity ball milling, (right). The average crystallite in the starting sample was 12 nm.
Fig. 7. TEM images of cobalt ferrite nanocrystals before, (left), and after 5 h of high-intensity ball milling (right). The average crystallite in the starting samples was 18 nm.
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3.4. Room temperature M–H measurements Room temperature M–H measurements of the 12 nm-cobalt ferrite nanocrystals treated by high-energy ball milling are shown in Figs. 8 and the corresponding magnetic properties summarized in Table 3. A careful analysis of the data in Table 3 evidenced that prolonging of the milling time promoted the diminution in crystallite size and increase in coercivity up to 42% (from 394 Oe to 560 Oe) after 5 h of milling (Figs. 9). This increase in coercivity can be attributed to the strain induced in the ferrite structure, as suggested by XRD measurements, caused by the high-energy impacts of the grinding media with the ferrite nanocrystals. Williamson–Hall plots and the corresponding estimated strain values are shown in Figs. 12 and 14. As observed, the maximum strain was observed after 5 h of milling time, which corresponds to the maximum coercivity value achieved for this sample (560 Oe). The reduction in the crystallite size for the 5 h-ground sample was negligible. Accordingly, the observed correspondence between maximum strain and coercivity values confirm that strain anisotropy would be the main factor responsible for such increase in the magnetic property. This interpretation contrasts with Liu and Ding's one in the sense that residual strain would not affect
Fig. 10. Room temperature M–H measurements of cobalt ferrite nanocrystals with an initial average crystallite size of 18 nm ball-milled at different times. The inset shows the M–H data around the origin.
Table 3 Coercivity, ‘Hc’, and maximum magnetization, ‘Mmax’, of cobalt ferrite ball-milled at different times. The initial average crystallite size of the ferrite was 12 nm.
Fig. 8. Room temperature M–H measurements of cobalt ferrite nanocrystals with an initial average crystallite size of 12 nm ball-milled at different times. The inset shows the loops around the origin.
Milling time (min)
Hc (Oe)
Mmax (emu/g)
0 20 30 40 50 60 180 300 600
394 470 483 480 466 462 495 560 491
61 58 58 57 58 59 59 55 56
significantly the magnetic properties of nanostructured cobalt ferrite when compared to bulk [7]. The corresponding M–H loops and magnetic properties for the 18 nm ferrite samples are shown in Fig. 10 and Table 4, respectively. As seen, the 18 nm-sample exhibited a continuous reduction in coercivity with prolonged milling time (Fig. 11). This trend was in agreement with the decrease in crystal size evidenced by XRD estimations. The corresponding Williamson-Hall plots and strain values are shown in Figs. 13 and 14. The maximum strain was attained after 3 h of milling; however, no enhancement in coercivity was detected, on the contrary, it decreased from 4506 Oe to 708 Oe. The predominant effect of size reduction over strain anisotropy could explain these results. After 10 h of milling, the coercivity reached 491 Oe. The maximum magnetization was not greatly affected by the milling process and varied in the 55 emu/g–61 emu/g and 52 emu/g–55 emu/g range for the 12 nm- and 18 nm-samples, respectively. 3.5. Mössbauer spectroscopy measurements
Fig. 9. Variation in coercivity and specific surface area with milling time of cobalt ferrite with initial average crystallite size of 12 nm.
The room temperature Mössbauer spectra for the 12 nm- and 18 nm-samples ball-milled at 3 h, 5 h and/or 10 h are shown in Figs. 15 and 16, respectively. The spectra clearly show sextet peaks typical of a magnetic ordering. The spectra were resolved in terms of four sites, comprising of one tetrahedral site and a distribution within the octahedral sites consisting of three sites. The three octahedral sites distribution assertion made here is based on the fact that nanostructured materials exhibit large surface-to-volume
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Fig. 11. Variation in coercivity and specific surface area with milling time of cobalt ferrite with initial average crystallite size of 18 nm. Fig. 14. Strain, Δd/d, induced in 12- and 18 nm-cobalt ferrite nanocrystals milled at different times.
Table 4 Coercivity, Hc, and maximum magnetization, Mmax, of cobalt ferrite submitted to high-energy ball milling. The initial average crystallite size was 18 nm. Milling time (min)
Hc (Oe)
Mmax (emu/g)
0 20 30 40 50 60 180 300 600
4506 4011 3539 2974 2748 2290 708 555 491
53 55 53 54 55 55 55 52 53
Fig. 12. Williamson–Hall plot for 12 nm-cobalt ferrite nanocrystals before, (0 h), and after 10 h of milling.
Fig. 15. Fitted Mössbauer spectra corresponding to 12 nm-cobalt ferrite nanocrystals milled at different times.
Fig. 13. Williamson–Hall plot for 18 nm-cobalt ferrite nanocrystals before, (0 h), and after 10 h of milling.
ratio, and as such, equivalent sites within grain interiors and on the grain surfaces are not going to have the same level of atomic bonding. The higher value internal magnetic field values are ascribed to Fe-sites at the grain interiors, while the remaining two octahedral sites are assigned to Fe-sites, associated with inter-
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Fig. 16. Fitted Mössbauer spectra corresponding to 18 nm-cobalt ferrite nanocrystals milled at different times.
Table 5 Mössbauer parameters corresponding to 12 nm-cobalt ferrite nanocrystals milled at different times.
Site-1 Site-2 Site-3 Site-4
As-synthesized λ¼ 0.68
3 h HEBM λ¼ 0.57
10 h HEBM λ¼ 0.60
H (kOe)
RA (%)
H (kOe)
RA (%)
H (kOe)
RA (%)
482.8 467.9 167.6 424.2
27.7 35.5 6.5 30.4
479.5 453.6 161.4 394.7
36.0 29.8 7.6 26.7
480.1 454.4 159.6 385.7
35.7 31.2 11.6 21.5
H: Internal magnetic field in kOe; RA: Relative abundance in percent.
Table 6 Mössbauer parameters corresponding to 18 nm-cobalt ferrite nanocrystals milled at different times.
Site-1 Site-2 Site-3 Site-4
As-synthesized λ¼ 0.82
5 h HEBM λ ¼0.59
10 h HEBM λ ¼0.70
H (kOe)
RA (%)
H (kOe)
RA (%)
H (kOe)
RA (%)
471.7 449.7 162.7 402.2
28.4 42.6 5.0 24.1
489.0 451.4 145.5 391.8
38.7 30.9 8.28 22.1
480.1 463.6 162.7 412.8
18.5 36.5 8.0 37.0
H: Internal magnetic field in kOe; RA: Relative abundance in percent.
grain areas, and the exposed surfaces of the nanosized particles. Thus, while the coordination of the octahedral sites is the same, distinctions arise as whether the octahedral site is at the interior, between grains, and at the exposed surface of the particles. The second case corresponding to inter-grain areas is applicable to agglomerated particles, whereas the case for the exposed surfaces is clearly distinct; thermodynamically, the intra-grain octahedral sites are more stable that the inter-grain ones and the one terminating at the surface of the particles. In bulk spinel ferrites, they are known as inverse when λ¼ 0 whereas they are referred to as a normal ferrite for a λ value of one. However, given the preponderance of surface atoms in comparison to entire volume in nanometric crystals, some deviations can occur in the value of λ. This change in the atomic distribution can also be occasioned by intensive processing techniques such as HEBM, which can render a “normal” ferrite into an “inverted” one.
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Table 5 shows the Mössbauer parameters corresponding to the 12 nm-cobalt ferrite nanocrystals milled at different times. The designated site-1, site-3, and site-4 refer to the octahedral sites in the ferrite lattice and are due to the distribution promoted by the nanometer size of the ferrite particles, which yield a high proportion of surface atoms in comparison to bulk, as mentioned earlier. In turn, the tetrahedral site is identified as site-2. Substantively, the relative changes associated with the internal magnetic fields of the octahedral sites are rather insignificant. The previous observations were in clear contrast to those for the size-controlled nanoparticles with an average crystallite size of 18 nm (data shown in Table 6). In this case, the internal magnetic field for the octahedral Fe3+ cations was deduced to be 471.7 kOe in the non-milled sample, while the other corresponding octahedral sites displayed values of 402.2 kOe and 162.7 kOe. The tetrahedral site internal magnetic field values showed increments from 449.7 kOe in the non-milled sample to 451.4 kOe and 463.6 kOe for the 5 h and 10 h HEBM treatments, respectively. While overall, the data displayed in Tables 5 and 6 suggested a spin-canting effect based on the departure of the relative intensities of the sextets peaks from the randomly oriented 3:2:1:1:2:3 values for the α-Fe case, the inversion parameter, (λ), showed a trend that correlated with both particle size and coercivity changes associated with the HEBM treatment time. First, for the non-milled 12 nm- sample the λ value was 0.68. Following HEBM for 3 h and 10 h, the Fe3+ tetrahedral site populations for the 12 nm-sized materials changed to 0.57 and 0.60, respectively. These values represented a decrease in the λ values of 16.1% for the 3 h-milled sample while a marginal increment of 5.3% (from 0.57 to 0.60) was attained with further milling for 10 h. The corresponding changes for the 18 nm-sample was deduced to be a decrease from 0.82 (non-milled sample) to 0.59 and 0.70 for the 5 h and 10 h-HEBM treated samples, respectively. Proportionately, this corresponds to 28% initial decrease with respect to the starting sample and an increase of 18.6% (from 0.59 to 0.70) at the end of the 10 h of HEBM treatment. These changes are in good agreement with the trend observed for the corresponding variation in the coercivity. Accordingly, the correlation between particle size and cationic distribution, promoted by HEBM, becomes evident and can be rationalized in terms of the increased surfaces, reduction in particle size and thereby, increase of surface of octahedral planes instead of the tetrahedral ones. It has been established both on theoretical calculation [21], and via EELS studies by Beaufils and Barbaux [22,23], that low index planes, e.g. (111) and (110), are preferred termination surface planes of the spinels with absence of occupied tetrahedral sites at the surfaces as a general property of the spinels are consistent with the outcome of the present study, based on the determined cationic distributions. The Mössbauer measurements of the present study clearly show that cationic redistribution occurred to different degrees in the 12 nm and 18 nm-CoFe2O4 nanoparticles, while their overall surface morphologies evolutions are similar.
4. Conclusions The strong dependence of structural and magnetic properties of cobalt ferrite with milling time under intensive HEBM conditions became evident even at the nanoscale. Ball milled cobalt ferrite powders exhibited crystal size reduction from initial 12 nm and 18 nm down to 10 nm in both cases. After prolonged milling times, the BET specific surface area was less than expected due to interparticle aggregation, a fact that was evidenced by TEM analyses. A tuning in magnetic properties was achieved thanks to the prevalence of different effects when changing crystal size.
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The cobalt ferrite nanocrystals with initial 12 nm diameter experienced an increase in coercivity (42%) attributed to the introduction of strain anisotropy, by point defects, as suggested from the analysis of the corresponding Williamson–Hall plots and also indicated by the shift of the diffraction peaks. Regarding the 18 nm-nanocrystals, their coercivity decreased from 4506 Oe down to 491 Oe after 10 h milling time. This drop in coercivity was mainly attributed to predominant size reduction from starting 18 nm down to 10 nm at the end of the 10 h milling. These results are consistent with the cationic distribution when the inversion parameters obtained from the Mössbauer spectroscopic measurements are matched with the variation in coercivity. The connection between the increase of materials surfaces as a function of time during HEBM treatments and the cationic distribution are made. However, the intricate effects of each variable on coercivity could be further explored with materials having a range of particles sizes to help decisively separate particle size induced superparamagnetic phenomenon, and contributions thereof to magnetic properties.
Acknowledgments This material is based upon work supported by the NSF-EPSCoR Institute for Functional Nanomaterials (IFN). TEM analyses were carried out at National High Magnetic Field Laboratory supported by the NSF Cooperative Agreement no. DMR-0084173 by the State of Florida.
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of high-energy ball milling time on structural and magnetic properties of nanocrystalline cobalt ferrite powders Yarilyn Cedeño-Mattei a,n, Oscar Perales-Pérez a,b, Oswald N.C. Uwakweh b a b
Department of Chemistry, University of Puerto Rico—Mayagüez Campus, Mayagüez, PR 00681, Puerto Rico Department of Engineering Science & Materials, University of Puerto Rico—Mayagüez Campus, Mayagüez, PR 00681, Puerto Rico
art ic l e i nf o
a b s t r a c t
Article history: Received 5 November 2012 Received in revised form 1 April 2013 Available online 15 April 2013
Cobalt ferrite nanocrystals synthesized by conventional and size-controlled coprecipitation methods were treated by high-energy ball milling, HEBM, in order to study the effect of crystal size reduction and/ or strain on the resulting magnetic properties. Processed nanocrystals were characterized by X-ray diffraction, Brunauer, Emmett, and Teller surface area analysis, transmission electron microscopy (TEM), and vibrating sample magnetometry. The cobalt ferrite nanocrystals exhibited crystal size reduction from initial values (average crystallite sizes of 12 7 1 nm and 18 7 3 nm, respectively) down to 10 nm after HEBM for 10 h. The specific surface area was decreased by milling (from 96.5 to 59.4 m2/g; for the 12 nm cobalt ferrite nanocrystals), due to particles aggregation. TEM analyses corroborated the aggregation of the nanoparticles at such long milling times. The same cobalt ferrite nanocrystals exhibited a rise in coercivity from 394 to 560 Oe after 5 h ball milling which was attributed to the introduction of strain anisotropy, namely point defects, as suggested by the systematic shift of the diffraction peaks towards higher angles. In turn, the magnetic characterization of the starting 18 nm-nanocrystals reported a drop in coercivity from 4506 Oe to 491 Oe that was attributed predominantly to size reduction within the single domain region. A correlation between particle size, cationic distribution, and HEBM processing conditions became evident. & 2013 Elsevier B.V. All rights reserved.
Keywords: Cobalt ferrite Nanocrystal High-energy ball milling Specific surface area Magnetic property
1. Introduction The selection of suitable synthesis and processing conditions of nanometric ferrites will affect their crystal size, morphology, cation distribution and hence, the resulting magnetic properties. Although coercivity is mainly governed by the magnetocrystalline anisotropy energy, contribution from surface anisotropy, strain anisotropy and/or shape anisotropy also affect the magnetic anisotropy, particularly at the nanoscale. High-energy ball milling (HEBM), so-called mechano-chemical processing, has emerged as a technique to produce nanomaterials capable to achieve a remarkable particle size reduction from a previously synthesized materials [1–4], or induce solid-phase transformations [5]. HEBM has also been widely used in the preparation of ferrites from metal oxides [6]. In HEBM, powdered samples are placed inside metallic jars containing suitable proportion of grinding media, usually ceramic balls, and contacted at extremely high rotational speeds to promote the impact and abrasion of the powder with the
n Correspondence to: University of Puerto Rico, Department of Engineering Science and Materials, Call Box 9000, Mayaguez, PR 00681-9000, Puerto Rico. Tel.: +787 832 4040x2086; fax: +787 265 3816. E-mail addresses: [email protected], [email protected] (Y. Cedeño-Mattei).
0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.04.015
grinding media, which in turn will induce the generation of defects and strain along with particle size reduction [7]. The advantage of this processing technique relies on the use of inexpensive and environment-friendly solvents and low temperatures, when compared to wet-chemistry-based routes or conventional solid-state processing (e.g. sintering). The mechano-chemical synthesis or activation of spinel structured oxides by HEBM has been reported with the resulting final outcome depending on the type of material and grinding conditions. For instance, the use of inert grinding materials are reported to only impact structural disorder due to change in degree of inversion as was reported in the case of MgFe2O4 [8]. On the other hand, the formation of NiFe2O4 was observed when solid NiO and α-Fe2O3 solid precursors were ball-milled [9] using stainless steel grinding media. Cobalt ferrite nanocrystals were selected for this study because its magnetic properties are easily tunable when composition, size, shape, and cation distribution are modified. Nanometric cobalt ferrite exhibits a strong size-dependent coercivity; therefore, any change in the ferrite synthesis and/or processing conditions will affect the crystal size and hence, the magnetic properties [10]. Besides, the high-energy impacts inside the milling apparatus should induce strain anisotropy in the crystal lattice, thereby modifying the corresponding coercivity.
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On this basis, the present work is focused on the study of the dependence of specific surface area, structural (average crystallite size and lattice parameter), and magnetic (coercivity and maximum magnetization) with HEBM processing conditions of cobalt ferrite nanocrystals of different average crystallite sizes.
A WEBRES spectrometer operating in the transmission mode, with a 50 mCi 57Co source in a Rh matrix from Ritverc, GmbH was for Mössbauer measurements. The degree of Fe3+ distribution within the tetrahedral and octahedral sites was deduced via the following expression: I Tet =I Oct ¼ ðƒ Tet =ƒ Oct Þ½λ=ð2−λÞ
2. Material and methods 2.1. Materials CoCl2 6H2O (ACS, 98–102%, Alfa Aesar), FeCl3 6H2O (ACS, 97–102%, Alfa Aesar), and NaOH (pellets, 98%, Alfa Aesar) were used without further purification, as precursor for the synthesis of cobalt ferrite nanocrystals. 2.2. Synthesis of cobalt ferrite nanocrystals
ð2Þ
with, ITet and IOct, referring to the relative abundances associated with the tetrahedral and octahedral sites, and ƒ, the recoilless fraction at the corresponding sites for the temperature of measurement (in our case, a room temperature value of 0.94 was used [15]). The inversion parameter, λ, translates to the following structural designation: (M1−λFeλ)Tet[MλFe2−λ]OctO4 and expresses the degree of departure of the cations (“M”¼ Co in our case, and “Fe”) from their expected octahedral and tetrahedral occupations.
3. Results and discussion
Cobalt ferrite powders were synthesized using the conventional and the size-controlled coprecipitation methods. The later considered the synthesis under fixed flow-rate (0.71 mL/min) of reactants addition according to the method developed by CedeñoMattei et al. [11]. A micro-peristaltic pump was used for this purpose. Control on flow-rate leads to the formation of nanocrystals with larger size due to modification in oversaturation conditions that promotes heterogeneous nucleation [12]. A 0.315 M NaOH boiling solution was the precipitant agent. The reaction time was set to one hour based on our previous works [11]. 2.3. HEBM processing of cobalt ferrite nanocrystals Cobalt ferrite samples were synthesized by the conventional and size-controlled coprecipitation routes. These two samples were processed by high-energy ball milling in a Fritch Pulverisseette-4 milling apparatus with tungsten carbide (WC) balls and jars. The ballmilling assembly consisted of 2 jars of 45 mL each one bearing 17 balls of 10 mm diameter each. A ball to powder ratio (BPR) of 40:1, a milling speed of 1400 rpm, and milling times in the 20 min–10 h range, were used in all our experiments. The choice of BPR of 40:1 was based on previous studies on bulk cobalt ferrite which showed its effectiveness in particle size reduction [13]. Small amounts of sample were withdrawn at pre-selected milling times to monitor the progress of the size reduction/aggregation process and determine the corresponding structural and magnetic properties.
3.1. XRD analyses XRD patterns from the 12 71 nm and 18 73 nm as-synthesized cobalt ferrite powders before (0 min) and after 30 min, 60 min, and 300 min of milling times are shown in Figs. 1 and 2, respectively. Peak broadening became evident after prolonged milling time that suggested the decrease in crystallite size. The average crystallite size of the 12 nm powders showed a just minor decrease (10 nm) even after 10 h of milling, whereas a more noticeable diminution in the average crystallite size (from 18 nm to 10 nm) was exhibited at the end of the 10 h milling period by the sample synthesized under size-controlled conditions. Evidently, the impact and abrasion forces generated inside the milling jar must have broken up the crystals even at the nanoscale. Figs. 1 and 2 also evidenced the shift of the XRD peaks towards smaller diffraction angles. This shift in diffraction angle is attributed to strain which is also reflected by the increase of the corresponding increase of the interplanar distances [7,16], the interplanar distance between (440) planes was increased from 1.483 Å to 1.486 Å when the milling time was prolonged up to 10 h. An increase in the lattice parameter could be attributed to point defects, e.g. ferric ion occupies normally unoccupied sites [17]. In turn, it will cause a change in the degree of inversion of cobalt ferrite.
2.4. Nanocrystals characterization The structural characterization of the nanocrystal ferrites was carried out by X-ray diffractometry using a Siemens D500 powder diffractometer with Cu Kα radiation. The average crystallite sizes reported were determined by XRD measurements and the Scherrer's equation. The specific surface area of the powders was measured in a Horiba SA-9600 series surface area analyzer. The external specific surface area of spheres, SSA, with diameter ‘t’ (or cubes with edge length ‘t’) and density ρ can be estimated by [14]: SSA ¼ 6=ρt
ð1Þ
This formula does not take into account interparticle overlapping (aggregation) but only reversible agglomeration. Morphological analyses of the samples were done in a JEOL 2011 transmission electron microscope. A LakeShore 7400 series vibrating sample magnetometer was used to determine the magnetic properties of the powdered samples at room temperature.
Fig. 1. XRD patterns of 12 nm-cobalt ferrite nanocrystals ball-milled at different times.
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Table 2 Variation in average crystallite size, ‘t’, lattice parameter, ‘a’, and specific surface area, ‘SSA’, measurements of ball-milled cobalt ferrite nanocrystals with initial average crystallite size of 18 nm. Milling time (min)
t ( 7 3 nm)
a ( 70.01 Å)
Calculated SSA (m2/g)
BET SSA (m2/g)
0 20 30 40 50 60 180 300 600
18 14 12 12 11 10 9 10 10
8.39 8.38 8.39 8.38 8.39 8.39 8.39 8.39 8.41
66.11 92.27 97.18 99.58 102.10 121.51 140.87 121.51 124.04
71.35 62.36 55.72 52.26 54.41 59.38 63.5 58.73 55.11
Fig. 2. XRD patterns of 18 nm-cobalt ferrite nanocrystals ball-milled at different times.
Fig. 4. Variation in average crystallite size, ‘t’, and surface area, ‘SSA’, as a function of milling time of 18 nm-cobalt ferrite particles. The dotted line shown in the figure is a visual guide to show the decreasing trend in both, average crystallite size and SSA.
Fig. 3. Variation of the average crystallite size, ‘t’, and specific surface area, ‘SSA’, with milling time of cobalt ferrite crystals with initial average crystallite size of 12 nm. The dotted line is a visual guide to show the decreasing trend in both, average crystallite size and SSA.
Table 1 Variation in average crystallite size, ‘t’, lattice parameter, ‘a’, and specific surface area, ‘SSA’, measurements of ball-milled cobalt ferrite nanocrystals with initial average crystallite size of 12 nm. Milling time (min)
t ( 7 1 nm)
a ( 7 0.02 Å)
Calculated SSA (m2/g)
BET SSA (m2/g)
0 20 30 40 50 60 180 300 600
12 11 10 10 10 10 10 9 10
8.38 8.38 8.38 8.38 8.39 8.39 8.39 8.40 8.40
99.06 109.40 113.69 116.74 119.08 111.23 121.51 131.23 124.04
96.45 77.21 71.27 69.22 71.25 73.39 65.36 62.15 59.39
3.2. BET-specific surface area of ball-milled powders Tables 1 and 2 show the calculated (using Eq. 1) and experimental (BET) specific surface area values for the 12 nm and 18 nm
ferrite samples, respectively, ball-milled at different times. The density of cobalt ferrite, involved in determining the calculated SSA, was 5.304 g/cm3. As observed, the calculated and the measured BET specific surface area values were very similar in the starting powders, which suggest that individual and nonaggregated nanocrystals were produced. However, the discrepancy between these two values became remarkable after prolonging the milling time. The decreasing trend in specific surface area with milling time can be attributed to strong interparticle aggregation promoted by intensive milling (Fig. 3). A similar aggregation phenomenon has been observed in different materials after prolonged milling [18–20]. The 18 nm-sample exhibited a reduction in SSA during the first 50 min of milling; a fact that can be attributed to some extent of aggregation. The small increase in SSA, observed in the 50 min180 min milling time range, can be mainly attributed to reduction in crystal size; nevertheless, aggregation of ferrite particles becomes the dominant effect of HEBM processing at prolonged milling times. In turn, as Fig. 4 and the data in Table 2 suggest, the crystal size reduction in the 12 nm-ferrite sample was not significant and aggregation predominates from the very beginning of the milling process. The high-energy impact of the grinding media on the ferrite particles should have promoted their binding due to the development of inter-particle strong interacting forces.
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3.3. TEM analyses Fig. 5 shows high resolution TEM images of the starting 12 nm and 18 nm cobalt ferrite nanocrystals. TEM images of the 12 nm cobalt ferrite particles, before and after 5 h of milling, (shown in Fig. 6) evidenced the strong interparticle aggregation after
prolonged milling times. The severe particle aggregation at longer milling times also became evident for the 18 nm ferrite samples (Fig. 7). These images clearly explained the discrepancy between the calculated and measured SSA values: the longer the milling, the more pronounced the particle aggregation and, consequently, the less the corresponding SSA.
Fig. 5. High-resolution TEM images of cobalt ferrite with an average crystallite size of 12 nm (left) and 18 nm (right), before milling.
Fig. 6. TEM images of cobalt ferrite nanocrystals before, (left), and after 5 h of high-intensity ball milling, (right). The average crystallite in the starting sample was 12 nm.
Fig. 7. TEM images of cobalt ferrite nanocrystals before, (left), and after 5 h of high-intensity ball milling (right). The average crystallite in the starting samples was 18 nm.
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3.4. Room temperature M–H measurements Room temperature M–H measurements of the 12 nm-cobalt ferrite nanocrystals treated by high-energy ball milling are shown in Figs. 8 and the corresponding magnetic properties summarized in Table 3. A careful analysis of the data in Table 3 evidenced that prolonging of the milling time promoted the diminution in crystallite size and increase in coercivity up to 42% (from 394 Oe to 560 Oe) after 5 h of milling (Figs. 9). This increase in coercivity can be attributed to the strain induced in the ferrite structure, as suggested by XRD measurements, caused by the high-energy impacts of the grinding media with the ferrite nanocrystals. Williamson–Hall plots and the corresponding estimated strain values are shown in Figs. 12 and 14. As observed, the maximum strain was observed after 5 h of milling time, which corresponds to the maximum coercivity value achieved for this sample (560 Oe). The reduction in the crystallite size for the 5 h-ground sample was negligible. Accordingly, the observed correspondence between maximum strain and coercivity values confirm that strain anisotropy would be the main factor responsible for such increase in the magnetic property. This interpretation contrasts with Liu and Ding's one in the sense that residual strain would not affect
Fig. 10. Room temperature M–H measurements of cobalt ferrite nanocrystals with an initial average crystallite size of 18 nm ball-milled at different times. The inset shows the M–H data around the origin.
Table 3 Coercivity, ‘Hc’, and maximum magnetization, ‘Mmax’, of cobalt ferrite ball-milled at different times. The initial average crystallite size of the ferrite was 12 nm.
Fig. 8. Room temperature M–H measurements of cobalt ferrite nanocrystals with an initial average crystallite size of 12 nm ball-milled at different times. The inset shows the loops around the origin.
Milling time (min)
Hc (Oe)
Mmax (emu/g)
0 20 30 40 50 60 180 300 600
394 470 483 480 466 462 495 560 491
61 58 58 57 58 59 59 55 56
significantly the magnetic properties of nanostructured cobalt ferrite when compared to bulk [7]. The corresponding M–H loops and magnetic properties for the 18 nm ferrite samples are shown in Fig. 10 and Table 4, respectively. As seen, the 18 nm-sample exhibited a continuous reduction in coercivity with prolonged milling time (Fig. 11). This trend was in agreement with the decrease in crystal size evidenced by XRD estimations. The corresponding Williamson-Hall plots and strain values are shown in Figs. 13 and 14. The maximum strain was attained after 3 h of milling; however, no enhancement in coercivity was detected, on the contrary, it decreased from 4506 Oe to 708 Oe. The predominant effect of size reduction over strain anisotropy could explain these results. After 10 h of milling, the coercivity reached 491 Oe. The maximum magnetization was not greatly affected by the milling process and varied in the 55 emu/g–61 emu/g and 52 emu/g–55 emu/g range for the 12 nm- and 18 nm-samples, respectively. 3.5. Mössbauer spectroscopy measurements
Fig. 9. Variation in coercivity and specific surface area with milling time of cobalt ferrite with initial average crystallite size of 12 nm.
The room temperature Mössbauer spectra for the 12 nm- and 18 nm-samples ball-milled at 3 h, 5 h and/or 10 h are shown in Figs. 15 and 16, respectively. The spectra clearly show sextet peaks typical of a magnetic ordering. The spectra were resolved in terms of four sites, comprising of one tetrahedral site and a distribution within the octahedral sites consisting of three sites. The three octahedral sites distribution assertion made here is based on the fact that nanostructured materials exhibit large surface-to-volume
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Fig. 11. Variation in coercivity and specific surface area with milling time of cobalt ferrite with initial average crystallite size of 18 nm. Fig. 14. Strain, Δd/d, induced in 12- and 18 nm-cobalt ferrite nanocrystals milled at different times.
Table 4 Coercivity, Hc, and maximum magnetization, Mmax, of cobalt ferrite submitted to high-energy ball milling. The initial average crystallite size was 18 nm. Milling time (min)
Hc (Oe)
Mmax (emu/g)
0 20 30 40 50 60 180 300 600
4506 4011 3539 2974 2748 2290 708 555 491
53 55 53 54 55 55 55 52 53
Fig. 12. Williamson–Hall plot for 12 nm-cobalt ferrite nanocrystals before, (0 h), and after 10 h of milling.
Fig. 15. Fitted Mössbauer spectra corresponding to 12 nm-cobalt ferrite nanocrystals milled at different times.
Fig. 13. Williamson–Hall plot for 18 nm-cobalt ferrite nanocrystals before, (0 h), and after 10 h of milling.
ratio, and as such, equivalent sites within grain interiors and on the grain surfaces are not going to have the same level of atomic bonding. The higher value internal magnetic field values are ascribed to Fe-sites at the grain interiors, while the remaining two octahedral sites are assigned to Fe-sites, associated with inter-
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Fig. 16. Fitted Mössbauer spectra corresponding to 18 nm-cobalt ferrite nanocrystals milled at different times.
Table 5 Mössbauer parameters corresponding to 12 nm-cobalt ferrite nanocrystals milled at different times.
Site-1 Site-2 Site-3 Site-4
As-synthesized λ¼ 0.68
3 h HEBM λ¼ 0.57
10 h HEBM λ¼ 0.60
H (kOe)
RA (%)
H (kOe)
RA (%)
H (kOe)
RA (%)
482.8 467.9 167.6 424.2
27.7 35.5 6.5 30.4
479.5 453.6 161.4 394.7
36.0 29.8 7.6 26.7
480.1 454.4 159.6 385.7
35.7 31.2 11.6 21.5
H: Internal magnetic field in kOe; RA: Relative abundance in percent.
Table 6 Mössbauer parameters corresponding to 18 nm-cobalt ferrite nanocrystals milled at different times.
Site-1 Site-2 Site-3 Site-4
As-synthesized λ¼ 0.82
5 h HEBM λ ¼0.59
10 h HEBM λ ¼0.70
H (kOe)
RA (%)
H (kOe)
RA (%)
H (kOe)
RA (%)
471.7 449.7 162.7 402.2
28.4 42.6 5.0 24.1
489.0 451.4 145.5 391.8
38.7 30.9 8.28 22.1
480.1 463.6 162.7 412.8
18.5 36.5 8.0 37.0
H: Internal magnetic field in kOe; RA: Relative abundance in percent.
grain areas, and the exposed surfaces of the nanosized particles. Thus, while the coordination of the octahedral sites is the same, distinctions arise as whether the octahedral site is at the interior, between grains, and at the exposed surface of the particles. The second case corresponding to inter-grain areas is applicable to agglomerated particles, whereas the case for the exposed surfaces is clearly distinct; thermodynamically, the intra-grain octahedral sites are more stable that the inter-grain ones and the one terminating at the surface of the particles. In bulk spinel ferrites, they are known as inverse when λ¼ 0 whereas they are referred to as a normal ferrite for a λ value of one. However, given the preponderance of surface atoms in comparison to entire volume in nanometric crystals, some deviations can occur in the value of λ. This change in the atomic distribution can also be occasioned by intensive processing techniques such as HEBM, which can render a “normal” ferrite into an “inverted” one.
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Table 5 shows the Mössbauer parameters corresponding to the 12 nm-cobalt ferrite nanocrystals milled at different times. The designated site-1, site-3, and site-4 refer to the octahedral sites in the ferrite lattice and are due to the distribution promoted by the nanometer size of the ferrite particles, which yield a high proportion of surface atoms in comparison to bulk, as mentioned earlier. In turn, the tetrahedral site is identified as site-2. Substantively, the relative changes associated with the internal magnetic fields of the octahedral sites are rather insignificant. The previous observations were in clear contrast to those for the size-controlled nanoparticles with an average crystallite size of 18 nm (data shown in Table 6). In this case, the internal magnetic field for the octahedral Fe3+ cations was deduced to be 471.7 kOe in the non-milled sample, while the other corresponding octahedral sites displayed values of 402.2 kOe and 162.7 kOe. The tetrahedral site internal magnetic field values showed increments from 449.7 kOe in the non-milled sample to 451.4 kOe and 463.6 kOe for the 5 h and 10 h HEBM treatments, respectively. While overall, the data displayed in Tables 5 and 6 suggested a spin-canting effect based on the departure of the relative intensities of the sextets peaks from the randomly oriented 3:2:1:1:2:3 values for the α-Fe case, the inversion parameter, (λ), showed a trend that correlated with both particle size and coercivity changes associated with the HEBM treatment time. First, for the non-milled 12 nm- sample the λ value was 0.68. Following HEBM for 3 h and 10 h, the Fe3+ tetrahedral site populations for the 12 nm-sized materials changed to 0.57 and 0.60, respectively. These values represented a decrease in the λ values of 16.1% for the 3 h-milled sample while a marginal increment of 5.3% (from 0.57 to 0.60) was attained with further milling for 10 h. The corresponding changes for the 18 nm-sample was deduced to be a decrease from 0.82 (non-milled sample) to 0.59 and 0.70 for the 5 h and 10 h-HEBM treated samples, respectively. Proportionately, this corresponds to 28% initial decrease with respect to the starting sample and an increase of 18.6% (from 0.59 to 0.70) at the end of the 10 h of HEBM treatment. These changes are in good agreement with the trend observed for the corresponding variation in the coercivity. Accordingly, the correlation between particle size and cationic distribution, promoted by HEBM, becomes evident and can be rationalized in terms of the increased surfaces, reduction in particle size and thereby, increase of surface of octahedral planes instead of the tetrahedral ones. It has been established both on theoretical calculation [21], and via EELS studies by Beaufils and Barbaux [22,23], that low index planes, e.g. (111) and (110), are preferred termination surface planes of the spinels with absence of occupied tetrahedral sites at the surfaces as a general property of the spinels are consistent with the outcome of the present study, based on the determined cationic distributions. The Mössbauer measurements of the present study clearly show that cationic redistribution occurred to different degrees in the 12 nm and 18 nm-CoFe2O4 nanoparticles, while their overall surface morphologies evolutions are similar.
4. Conclusions The strong dependence of structural and magnetic properties of cobalt ferrite with milling time under intensive HEBM conditions became evident even at the nanoscale. Ball milled cobalt ferrite powders exhibited crystal size reduction from initial 12 nm and 18 nm down to 10 nm in both cases. After prolonged milling times, the BET specific surface area was less than expected due to interparticle aggregation, a fact that was evidenced by TEM analyses. A tuning in magnetic properties was achieved thanks to the prevalence of different effects when changing crystal size.
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The cobalt ferrite nanocrystals with initial 12 nm diameter experienced an increase in coercivity (42%) attributed to the introduction of strain anisotropy, by point defects, as suggested from the analysis of the corresponding Williamson–Hall plots and also indicated by the shift of the diffraction peaks. Regarding the 18 nm-nanocrystals, their coercivity decreased from 4506 Oe down to 491 Oe after 10 h milling time. This drop in coercivity was mainly attributed to predominant size reduction from starting 18 nm down to 10 nm at the end of the 10 h milling. These results are consistent with the cationic distribution when the inversion parameters obtained from the Mössbauer spectroscopic measurements are matched with the variation in coercivity. The connection between the increase of materials surfaces as a function of time during HEBM treatments and the cationic distribution are made. However, the intricate effects of each variable on coercivity could be further explored with materials having a range of particles sizes to help decisively separate particle size induced superparamagnetic phenomenon, and contributions thereof to magnetic properties.
Acknowledgments This material is based upon work supported by the NSF-EPSCoR Institute for Functional Nanomaterials (IFN). TEM analyses were carried out at National High Magnetic Field Laboratory supported by the NSF Cooperative Agreement no. DMR-0084173 by the State of Florida.
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