Cobalt ferrite (CoFe2O4) nanoparticles synthesized by chemical co-precipitation method were studied to see the effect of annealing temperatures on the structural parameters and magnetic properties of the sample. X-ray powder diffraction (XRD), transmission electron microscope (TEM) and SAD pattern demonstrated that single face (fcc) spinel structure of CoFe2O4 has been formed showing particles in completely crystalline state which was further confirmed by the lattice parameter and the unmixed hkl values. The particle size is in the range of about 5-10 nm depending on the annealing temperature from 200°C to 400°C. The grain growth occurred monotonically with the increase of annealing temperature. The magnetic properties demonstrated a strong dependence on particle size. The saturation magnetization Ms, remanent ratio Mr/Ms and coercivity Hc increased with the increase of particle size. The hysteresis curves for samples of different grain size showed the ferrimagnetic behavior which is completely analogous to the slow relaxation sextet peaks of Mössbauer spectroscopy. Other parameters such as chemical shift, quadruple splitting and hyperfine field and site occupancy of Fe3+ were determined by Mössbauer spectroscopy. Ferrimagnetic to superparamagnetic transition temperature known as blocking temperature TB was determined from the temperature dependent magnetization curves. With the increase of grain size, Blocking temperature also increases. Maximum entropy changes due to magnetic phase transition were also observed for mentioned annealing temperatures in the context of Magnetocaloric effect.

In recent days, nanocrystalline ferrites become a subject of fascination because of their overwhelming application in human life as well as research areas. Iron-oxide nanoparticles have been used as an important specimen in the field of research and practical life on account of their unusual physical, electronic and magnetic properties compared to their bulk counterparts. However, with the reduction of particle size into nanoscale, some of these properties change drastically.1–5 Complex relationship between nanostructural features and the macroscopic magnetic properties are not same for all combinations. To know the applications of a combination of ferrite, it is mandatory to inspect this relationship properly.6 

Nanoparticles are important scientific tools that have been and are being explored in various biotechnological, pharmacological, industrial and technological uses. In biomedical fields, nano-crystalline ferrites have outstanding applications like magnetically guided drug delivery, magnetic resonance imaging, hyperthermia etc where the conventional drugs may not work.7,8 Moreover, in the field of environmentally friendly magnetic refrigeration technology, ferrite nanoparticles are extensively used as a substitution of rare earth materials due to their reasonable cost and easy procurement. These applications of ferrite are influenced by the physical and chemical properties of the materials including microstructure, combination, particle size etc. These properties are dependent on some factors such as synthesis process, concentration, heating and cooling condition, annealing temperature, annealing time etc.9 Small amount of change in any of these factor may consequence to drastic change in properties of ferrites.

A remarkable example is that of, cobalt ferrite (CoFe2O4) which has drawn attention due to its exquisite physical and chemical stability, reasonable saturation magnetization, high remanence and high magneto-crystalline anisotropy.2,10 The crystal structure of CoFe2O4 is inverse spinel type structure where oxygen atoms constitute a face center cubic (fcc) lattice. In this regard, divalent Co2+ ions are shifted to octahedral “B” sites and trivalent Fe3+ ions are occupied by both tetrahedral “A” and octahedral “B” sublattice sites.11,12 Ferrimagnetism are witnessed in cobalt ferrite due to unequal antiferromagnetic A-B superexchange interaction by the collaboration of the magnetic dipole moments.13 

Now-a-days, Global warming is a crucial threat for world’s environment and ecosystem. One of the principal reasons of global warming is the inflation of ozone depleting chemicals and greenhouse gases such as Chloflurocarbons (CFC), Hydrochloflurocarbons, Hydroflurocarbons etc. These environmentally toxic elements have been used mostly in our refrigeration system. In this concern, magnetic refrigeration is a newly highly efficient and environmental friendly refrigeration system that does not use green house and CFC gas. The basis of magnetic refrigeration is known as magnetocaloric effect. The magnetocaloric effect is a “Magneto Thermodynamic” phenomenon that is acquainted with the heating and cooling of magnetic materials due to the application of the external magnetic field. Typically, a magnetic material with a noticeable magnetocaloric effect (MCE) will rise in temperature upon application of an applied field and will lower in temperature upon relaxation of the field. The mechanism of the change in temperature and the change in entropy arises from the manipulation of the magnetic moments in the materials. If the system is kept under isothermal conditions and constant pressure, the total entropy will decrease when a field is applied and increases when the field is removed. If the system is kept under adiabatic conditions and constant pressure, the total entropy remains constant. In this regard, the temperature increases when the field is applied and decreases when the field is removed.14–16 

The aim of this study is to synthesis CoFe2O4 nanoparticles by chemical co-precipitation method and to observe the effect of annealing temperature on the structural properties and to characterize various magnetic properties including hysteresis loop, Saturation magnetization, remanent ratio, coercivity, Mössbauer analysis, superparamagnetic behavior at different annealing temperatures and magnetocaloric effect observation in the vicinity of superparamagnetic blocking temperature.

To synthesize CoFe2O4 nanoparticles, analytical grade powder of CoCl2.6H2O salt and FeCl3 were mixed in the molar ratio of 1: 2 and 8M of NaOH solution was added under continuous stirring. NaOH was added to control the pH of the solution. The mixture was heated to 80oC for 1h. Then it was followed by centrifugation operation at 13500 rpm for 20 min and was washed by filtration. The centrifugation and filtration were done for 10 times to remove NaOH from the precipitate. Then the final precipitate was dried at 80oC for 72h. After drying, pellets were prepared from the as-dried powder. Then the pellets of the samples were annealed in the temperatures 200oC and 400oC.17 

For structural characterization of CoFe2O4 nanoparticles, X-ray diffraction patterns for the samples in different annealed conditions were studied that were recorded on a Philips X’Pert Pro powder X-ray diffractometer (PW 3040) by using Cu-Kα (λ= 1.54 Å) radiation. For all the measurements of CoFe2O4, powder specimens of about 15mg were exposed to Cu – Kα radiation with a primary beam power of 40kV and 30mA with a sampling pitch of 0.02o and time for each step data collection was 1 second. TEM bright field (BF) image and SAD pattern was acquired for further structural characterization of as-dried powder by F30 transmission electron microscopy (TEM) technique. The image was acquired by drop drying the diluted nanoparticle solution on the Cu grid. Hysteresis curves, temperature dependent magnetization curves and isothermal magnetization curves were carried out by Vibration sample magnetometer (Model: EV-9, Microsence LLC, USA). Moreover, magnetic phase identification and magnetic ordering were studied by Mössbauer spectrometer (SEE Co. Model VT 400).

The X-ray diffraction patterns for CoFe2O4 nanoparticles for as-dried particles and annealed at 200oC and 400oC are presented in the Fig. 1. The comparison between the XRD pattern and the standard data (JCPDS PDF card no. 22-1086) confirms the formation of CoFe2O4 nanoparticles. The single phase cubic spinel structure (fcc) with reflections from planes of (220), (311), (400), (422), (511) and (440) was identified.18,19 The broadenings of the XRD line indicated that the CoFe2O4 nanoparticles was of nano dimension. Whereas annealing brings about increase in grain growth giving rise to narrow and sharp well defined peaks. The crystallite size of all the heat treated samples of CoFe2O4 nanoparticles was determined using Scherrer’s formula.10 

D=Kλβcosθ
(1)

The crystallite size for the CoFe2O4 nanoparticles has been demonstrated in the range about 5-10 nm for the samples in as-dried condition and annealed at 200oC and 400oC. It can be observed that with the increase of annealing temperature, the grain growth occurred monotonically.

FIG. 1.

X-ray diffraction patterns of CoFe2O4 nanoparticles for as-dried samples and for Ta= 200oC and 400oC.

FIG. 1.

X-ray diffraction patterns of CoFe2O4 nanoparticles for as-dried samples and for Ta= 200oC and 400oC.

Close modal

The lattice parameter ‘a’ of the samples with different annealing conditions were determined by using following formula,

a=dh2+k2+l2
(2)

The variations of crystallite size and lattice parameter with respect to temperature are shown in Table I. The difference in the values of lattice parameter of the as-dried samples and the annealed samples at different temperatures ranging from 200oC to 400oC is a result of the change in surface to volume ratio. This is because the lattice parameter of the spinel lattice depends on the complex interplay between the size of the ions and their distribution.20 

TABLE I.

The Lattice parameters and Crystallite sizes of CoFe2O4 at different annealing temperatures.

Annealing Temperature (Ta) oCLattice Parameter (a) ÅGrain Size (D) nm
as-dried 8.312 5.21 
200 8.345 6.40 
400 8.586 9.29 
Annealing Temperature (Ta) oCLattice Parameter (a) ÅGrain Size (D) nm
as-dried 8.312 5.21 
200 8.345 6.40 
400 8.586 9.29 

To observe the morphological properties i.e. physical size and shape of the CoFe2O4 nanoparticle TEM (Transmission Electron Microscope) bright field image for the sample annealed at 200oC is presented in Fig. 2(a). In the TEM image most of the particles appear spherical in shape. Particle size of CoFe2O4 that is annealed at 200oC by XRD seems to be in fair agreement with the grain size as observed from the TEM bright field image. The inter-planar spacing obtained from SAD (Selected Area Diffraction) patterns is shown in Fig. 2(b). SAD patterns were induced by comparing the miller indices with the standard pattern.

FIG. 2.

(a) TEM bright field image of CoFe2O4 nanoparticles annealed at 200oC. (b) Selected Area Diffraction (SAD) pattern for CoFe2O4 nanoparticles.

FIG. 2.

(a) TEM bright field image of CoFe2O4 nanoparticles annealed at 200oC. (b) Selected Area Diffraction (SAD) pattern for CoFe2O4 nanoparticles.

Close modal

Magnetization as a function of the applied magnetic field up to 15 kOe for the samples in the as-dried condition and in different annealed conditions ranging from 200oC to 400oC are shown in Fig. 3(a), (b), (c) and (d). Various magnetic parameters such as coercivity (Hc), saturation magnetization (Ms) and remanent magnetization (Mr) were measured from the Hysteresis loops given in Table II.

FIG. 3.

(a) Hysterisis loops of CoFe2O4 nanoparticles for as-dried samples. (b) Hysterisis loops of CoFe2O4 nanoparticles for Ta= 200oC. (c) Hysterisis loops of CoFe2O4 nanoparticles for Ta= 300oC. (d) Hysterisis loops of CoFe2O4 nanoparticles for Ta= 400oC.

FIG. 3.

(a) Hysterisis loops of CoFe2O4 nanoparticles for as-dried samples. (b) Hysterisis loops of CoFe2O4 nanoparticles for Ta= 200oC. (c) Hysterisis loops of CoFe2O4 nanoparticles for Ta= 300oC. (d) Hysterisis loops of CoFe2O4 nanoparticles for Ta= 400oC.

Close modal
TABLE II.

Maximum magnetization (Mmax), Remanent magnetization (Mr) and Coercive field (Hc) after annealing at different temperatures.

AnnealingSaturation Magnetization,Remanent Magnetization,Coercive field,
Temperature (Ta) oCMs emu/gmMr emu/gmMr/MsHc Oersted
as-dried 40.073 5.432 0.135 120.369 
200 43.295 5.531 0.127 139.843 
300 52.687 8.793 0.167 184.624 
400 50.051 10.551 0.210 302.897 
AnnealingSaturation Magnetization,Remanent Magnetization,Coercive field,
Temperature (Ta) oCMs emu/gmMr emu/gmMr/MsHc Oersted
as-dried 40.073 5.432 0.135 120.369 
200 43.295 5.531 0.127 139.843 
300 52.687 8.793 0.167 184.624 
400 50.051 10.551 0.210 302.897 

The remanent ratio Mr/Ms was found to increase with increasing of annealing temperature. Basically, the magnetization is the function of annealing temperature as well as particle size. For nanoparticles the smaller the particle size, the greater the surface to volume ratio. For this reason, with decreasing nanoparticle size, more atoms are captured on the surface of the nanoparticle rather than core. So, for smaller nanoparticles, the value of normalized magnetization is small because of the greater surface disorder than bigger nanoparticles. The magnetization also rises by the increase of annealing temperature due to the reduction of the surface disorder.21,22

The coercivity Hc increases with the increasing of annealing temperature form the as-dried conditions i.e. with the increasing of grain size. It has been observed that, for nanoparticles, coercivity decreases with the rise of the particle size which can be expressed as Hc ∼ 1/d, where d is the diameter of the particle. With the decrease of particle size, the role of surface in determining the anisotropy i.e., anisotropy energy increases and hence coercivity increases.23,24 Present study shows that coercivity increases with the increase of grain size. So, the inverse relation of grain size dependent of coercivity is clearly observed, that is the break-down of conventional size law that is common for conventional bulk materials.

To estimate the distribution of the iron (Fe3+) in the ferrite spinel for the different annealing temperatures, 57Fe-Mössbauer spectra were recorded. Fig. 4 shows the Mössbauer spectra for as-dried samples and for two annealing temperatures, Ta= 200oC and 400oC at room temperatures. Spectral fits were performed assuming Lorentzian absorption line shapes using the Mössbauer spectral fitting software WMOSS 4R which includes also the slow relaxation. It can also be observed from the figures that the spectra exhibits sextet pattern that is the characteristics of slow relaxation which demonstrates that the material is in the ferrimagnetic state.25–28 

FIG. 4.

(a) Mössbauer spectra of CoFe2O4 nanoparticle for as-dried samples at room temperatures. (b) Mössbauer spectra of CoFe2O4 nanoparticle for Ta= 200oC at room temperatures. (c) Mössbauer spectra of CoFe2O4 nanoparticle for Ta= 400oC at room temperatures.

FIG. 4.

(a) Mössbauer spectra of CoFe2O4 nanoparticle for as-dried samples at room temperatures. (b) Mössbauer spectra of CoFe2O4 nanoparticle for Ta= 200oC at room temperatures. (c) Mössbauer spectra of CoFe2O4 nanoparticle for Ta= 400oC at room temperatures.

Close modal

An acceptable data fitting was obtained when the A and B-site pattern is assumed to be a superposition of more than one sextet. Consequently, the statistical distribution of Co2+ ions at the A-site will result from the distribution of Fe3+ ions in the both A-site and B-site. The various electronic and magnetic parameters such as chemical shift, quadruple splitting, hyperfine field and relative area are presented in Table III as obtained from the Mössbauer spectra. The χ2 (goodness of fitting) values are also reported in the Table III. Based on the relative area of each species cation distribution has been estimated. It is found from the literature that, the outer subspecies represent octahedral sub site and inner subspecies represent tetrahedral subsites. It has also been assumed that the iron species exist only in trivalent form. The higher values of quadruple splitting and lower values of isomer shift and are marked in the tetrahedral position and the lower values of quadruple splitting and higher values of isomer shift marked in the octahedral position of iron groups.29 

TABLE III.

Hyperfine parameters of Mössbauer spectroscopy at room temperature without any applied field.

Annealing Temp oCSiteChemical ShiftQuadrupole splittingHyperfine fieldRel. Area
And χ2Occupancy(δ) mm/sec(dEq) mm/sec(Hint) KG(%)
as-dried 0.303 0.0237 -463.373 53.5 
χ2 = 1.606 0.334 0.116 - 407.325 46.5 
200 0.252 0.235 -400.000 35 
χ2 = 1.286 0.200 0.178 - 471.496 65 
400 0.271 0.305 -418.178 32.5 
χ2 = 1.428 0.283 0.0452 - 481.693 67.5 
Annealing Temp oCSiteChemical ShiftQuadrupole splittingHyperfine fieldRel. Area
And χ2Occupancy(δ) mm/sec(dEq) mm/sec(Hint) KG(%)
as-dried 0.303 0.0237 -463.373 53.5 
χ2 = 1.606 0.334 0.116 - 407.325 46.5 
200 0.252 0.235 -400.000 35 
χ2 = 1.286 0.200 0.178 - 471.496 65 
400 0.271 0.305 -418.178 32.5 
χ2 = 1.428 0.283 0.0452 - 481.693 67.5 

The occupancy of Fe3+ ions in the site A and site B are shown in Table III for different annealing temperatures. The corresponding Isomer shift, Quadrupole splitting and hyperfine fields provides the probable cation distribution. For as-dried samples, 53.5% of the Fe3+ are occupied by B-site and 46.5% of Fe3+ are altered to the A-site. For Ta= 200oC, 65% of the Fe3+ are occupied by B-site and 35% of Fe3+ are shifted to the A-site. For Ta = 400oC, 67.5% of the Fe3+ are occupied by B-site and 32.5% of Fe3+ are shifted to A-site.

Temperature dependent magnetization (M-T curve) has been analyzed for the as-dried CoFe2O4 nanoparticle samples and the samples annealed at 200oC and 400oC in Fig. 5(a). Blocking temperature (TB) can be found which indicates the superparamagnetic to ferrimagnetic transition of the samples. Below blocking temperature, the specimen exhibits ferrimagnetic nature and above it shows superparamagnetic behavior.30 For ferrimagnetic materials, the internal energy on the direction of spontaneous magnetization is related to magnetic anisotropy. In the single domain uniaxial nanoparticles, spontaneous magnetization is produced when the collective magnetic moments tend to align along certain preferred crystallographic direction. Collective magnetic moments form and fluctuate together due to interaction between moments within the particles. If thermal energy is high enough, the collective magnetic moments flip their direction many times and will no longer stay in the easy direction; the anisotropy energy is overcome by the thermal energy.27 Superparamagnetism occurs when the magnetocrystalline anisotropy energy is much less than that required to reorient (anti-align) the individual spins; as for the volume size, it is also much less than thermal energy per unit volume. For fine magnetic particles, the actual magnetic behavior depends on the value of the measuring time (τm) with respect to the relaxation time (τ) associated with the overcoming of the energy barrier. The relaxation time for a magnetic moment changes with varying temperature. In superparamagnetic materials at T >TB, the direction of collectively coupled magnetic moments alter for several times on account of thermal energy, which is familiar as superparamagnetic state. In this state, the relaxation time is much shorter than measured time. At certain temperature T = TB, the relaxation time of the moments is equal to τm and collective moments will not flip any longer and a net magnetization could be observed. In this regard, the relaxation time is much longer than the measured time, so the collective magnetic moments are fixed in one direction causing an increase in the net magnetization which consequences to a blocked state.31,32 If T < TB, the magnetocrystalline energy is higher enough rather than thermal energy, so that magnetic moment of each particle lies along the easy axis, i.e., the directions are energetically degenerate and separated by an energy barrier. This state is known as ferrimagnetic state.31 

FIG. 5.

(a) Temperature dependent magnetization (M-T) curves of CoFe2O4 nanoparticles for as-dried samples, Ta= 200oC and Ta= 400oC. (b) Change of Blocking temperature with respect to Grain size.

FIG. 5.

(a) Temperature dependent magnetization (M-T) curves of CoFe2O4 nanoparticles for as-dried samples, Ta= 200oC and Ta= 400oC. (b) Change of Blocking temperature with respect to Grain size.

Close modal

The blocking temperatures for as-dried and the samples annealed at the temperatures of 200oC and 400oC were obtained as 30°C, 120°C and 140°C respectively. It is noticeable that, the field applied for as-dried CoFe2O4 and the samples annealed at 200oC and 400oC are 60 Oe, 160 Oe and 320 Oe respectively. It can be seen from Fig. 5(b) that, blocking temperature increases with the increase of grain size. This indicates that for the case of the nanostructured samples, the blocking effect is present and becomes dominant as the grain size is reduced. The increase in TB is linked to the increase of Co2+ ions in the octahedral sites. Besides particle size, the blocking temperature can also be influenced by several intrinsic factors, which mainly include magnetocrystalline surface and shape anisotropy and extrinsic factors, generally related to interactions between particles.30 

Magnetocaloric effect (MCE) is the heating and cooling of a magnetic material due to the application of magnetic field. In order to explain the origin of magnetocaloric effect, thermodynamics is used which relates the magnetic variables (magnetization and magnetic field) to entropy and temperature. The physical origin of the MCE is the coupling of the magnetic sublattice to the applied magnetic field H, which changes the magnetic contribution to the entropy of the solid. The term MCE usually designates the much stronger effect, observed when a phase transition in the materials involves.33,34 The transitions are identified as first or second order, structural or magnetic states of the material. In this regard, magnetic nanoparticles have distinctive advantages. Because, a flexible order-disorder transition associated with superparamagnetic blocked state referred to as the blocking transition (TB) is revealed in nanoparticles.

In this study, MCE of CoFe2O4 nanoparticle was measured nearly blocking temperature for the as-dried sample and the samples annealed at 200oC and 400oC. To measure MCE, the calculation of change in entropy ΔS (T, ΔH) was carried out. To evaluate the MCE of the samples, the isothermal magnetization curves were collected at different fixed temperatures (Fig. 6). The measurement of M/T was determined with change of magnetic field. The magnetic entropy change was determined by using the numerical integration called Maxwell’s thermodynamic relation.35–38 

SHT=MTH
(3)

Integrating over the field,

ΔS=0HMTHdH
(4)

The trend of noticeable entropy change was tried to observe here. Typical values of the entropy change range in 0.1 to 1.0 J/kg/oC over the field of 1T for all the specimens. The temperature dependent entropy change ΔS for as-dried and samples at different annealing conditioned were shown in the Fig. 7. It is distinguishable that, ΔS increases uniformly with the increase of temperature for all specimens. In case of as-dried samples, there is no exhibition of following the shape of blocking peak that is centered around 30°C. Measurement of isothermal curves at much lower temperature might be the layout of the scope of finding expected peak for this as-dried sample. For the samples annealed at 200°C, similar trend of entropy change is visible. There is no noticeable broad peak on or near TB = 130°C. For the samples annealed at 400°C, a peak of maximum entropy change with the value of 0.93J/kg/°C can be observed at 120°C although TB of the sample centered around 140°C.

FIG. 6.

(a) Isothermal magnetization curves for as-dried samples around Blocking temperature TB. The red curve is the data for TB. (b) Isothermal magnetization curves for Ta= 200oC around Blocking temperature TB. The red curve is the data for TB. (c) Isothermal magnetization curves for Ta= 400oC around Blocking temperature TB. The red curve is the data for TB.

FIG. 6.

(a) Isothermal magnetization curves for as-dried samples around Blocking temperature TB. The red curve is the data for TB. (b) Isothermal magnetization curves for Ta= 200oC around Blocking temperature TB. The red curve is the data for TB. (c) Isothermal magnetization curves for Ta= 400oC around Blocking temperature TB. The red curve is the data for TB.

Close modal
FIG. 7.

(a) Entropy change vs Temperature curves for as-dried samples. (b) Entropy change vs Temperature curves for Ta= 200oC. (c) Entropy change vs Temperature curves for Ta= 400oC.

FIG. 7.

(a) Entropy change vs Temperature curves for as-dried samples. (b) Entropy change vs Temperature curves for Ta= 200oC. (c) Entropy change vs Temperature curves for Ta= 400oC.

Close modal

The structural, magnetic and magnetocaloric effect at various annealing conditions is studied here for Cobalt ferrite nanoparticles synthesized by Chemical co-precipitation method. The XRD study confirmed the formation of the single phase cubic spinel structure and the particle size are 5-10nm at different annealing temperature. Hysteresis loops demonstrates that the materials are in ferrimagnetic state which is in excellent agreement sextet pattern of Mössbauer spectra. The annealing operation causes an increase in saturation magnetization, remanent ratio, coercivity and superparamagnetic blocking temperature. Cation distribution and subsequent annealing operation contribute to these significant effects on magnetic properties. The magnetic entropy change (ΔS) of all specimens is showed around their respective blocking temperatures TB. The trend of increasing ΔS as a function of temperature was observed for as-dried samples and Ta= 200oC and 400oC. Moreover, ΔS does not follow the shape of blocking peak which are centered at 30°C, 120°C and 140°C respectively.

Materials Science Division, Atomic Energy Center, Dhaka, and Laboratory facilities of Centre for Advanced Research in Science (CARS), University of Dhaka are greatly acknowledged. Also the support of Ministry of Science and Technology, Bangladesh and International Science Program, Uppsala University, Sweden are acknowledged with gratitude.

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