Modification in Magnetic Properties of CoFe2O4 Ferrite Nanoparticles Induced by 100MeV O+7 Ion Irradiation

In this work CoFe2O4 ferrite nanoparticles were prepared via sol-gel method using citric acid as chelating agent. The prepared nanoparticles was sintered at 700°C and the crystallite size is found 34 nm. In this investigation the effect of irradiation of 100 MeV O+7 beam on these nanoparticles with two fluences 5×1012 and 5×1013 ions/cm2 are studied. After irradiation the crystallite size is found to be reduced which is confirmed by transmission electron micrograph. The retentivity and coercivity changes with a measurable value while the saturation magnetization reduces slightly. The value of remanent magnetization increased from 30 emu/g to 34.6 emu/g, it can be attributed to increase of magnetic hardness. The increase in squareness ratio after irradiation leads to increase in anisotropic energy barrier of CoFe2O4 ferrite nanoparticles. Anisotropy field about doubles after irradiation and corresponding anisotropy constant also doubles with the used fluences.


Introduction
Swift heavy ion irradiation is a tool to produce various types of controlled defects such as point, cluster or columnar defects in material [1].The ions with energy from fraction of keV to GeV are useful in synthesis, modification and characterization of materials.If the velocity and mass of the ion in the beam is such that v > v o Z 2/3 and M >> m o then ion is called swift heavy ion; where M is the mass of the ion v o the Bohar velocity (velocity of orbital electron in target atom) and Z is the atomic number of the target atom [2].The energy loss in the medium is caused mainly by the two processes; one by inelastic collision between ions and electrons (Electronic Stopping) and another by elastic collision with ion and target nuclei (Nuclear Stopping).The electronic energy loss process is dominant at high energies (> 1MeV/nucleon) called threshold energy [3].This threshold energy varies from material to material beyond which the formation of columnar defect or latent track occurs in the material.The electronic energy loss for swift heavy ions is about two orders of magnitude higher than the nuclear energy loss [4].The mass and its energy of ions decides the magnitude of the electronic and nuclear energy losses.A swift heavy ion can modify the properties of material such as change in microstructure, increase in electrical resistance; change in magnetic and optical properties etc, this modification is quite different from that of low energy ions.A local phase change could occur, due to the huge amount of energy deposited by swift heavy ions provided that energy is greater than threshold value of electronic stopping power [5].It has been observed that metallic or insulating materials could be strongly modified by swift heavy ion irradiations [6].Nanomaterials may be the good candidates to observe the irradiation modifications because the energy density stored in the nanomaterials after the passage of an ion must be very high as compared to the density in the same material in its bulk phase.Consequently the electronic threshold value should be lower than the corresponding value in the bulk phase.It has been found that the electronic structure of the pristine material is not the main characteristic that explains its behaviour [7,8,9].
Among the transition metal oxides cobalt ferrite is well known hard magnetic material with large magneto-crystalline anisotropy, high coercivity, moderate saturation magnetization and high Curie temperature (~520 o C).Chemical stability and high coercivity value makes cobalt ferrite a best material for magnetic recording applications such as audio, video tape and high density digital recording discs.Cobalt ferrite nanoparticles are being used for various biomedical applications such as magnetic resonance imaging magnetic hyperthermia and targeted drug delivery.Cobalt ferrite is an inverse spinel structure with ferric ions equally shared among the tetrahedral A site and octahedral B site while cobalt ions occupy the octahedral site only.The magnetic properties of the cobalt ferrite depends upon types of cations (Co+ 2 , Fe +3 ) and their distributions among the sites.In the present study we reported the swif heavy ions irradiation experiments performed on cobalt ferrite nanoparticles with 100 MeV O +7 beams.Investigation on cations distribution between two sites A and B in the crystal lattice and its effect on the magnetic properties of CoFe 2 O 4 nanoparticles are reported in this article.

Experimental
Sol gel technique was used for the preparation of cobalt ferrite nanoparticles.The stoichiometric amounts of Fe(NO 3 ) 3 .9H 2 O and Co(NO 3 ) 2 .6H 2 O were dissolved in distilled water.In the aqueous salt solution of ferric nitrate and cobalt nitrate citric acid solution was added with the molar ratio 1:3 of cations to citric acid.This solution was heated at 80 o C under constant magnetic stirring until the solution got converted into a viscous gel, and was allowed to cool at room temperature.The cooled gel was dried in a furnace at 100 o C for overnight to form the precursor material [10,11].The precursor material was sintered at 700 o C for 2 hrs to get the nanoparticles of cobalt ferrite and then grinded to a powder to make particle shape uniform.This fine powder of CoFe 2 O 4 were irradiated by 100MeV O +7 beams with the fluences 5×10 12 and 5×10 13 ions/cm 2 .The ion accelerator at Inter University Accelerator Centre, New Delhi, India was used for irradiation.After irradiation these samples were characterized for structural and magnetic properties.X-ray diffraction data were collected on Bruker D8 Advanced X-ray diffractometer using Cu K α radiation (λ=1.54178Å).High-field magnetization measurements at room temperature were carried out with the help of a Quantum Design vibrating sample magnetometer (VSM) (Model 6000, Physical Property Measurement System).Mössbauer spectroscopic measurements were performed using a conventional constant acceleration spectrometer in transmission geometry with 57 Co source in Rh matrix at room temperature.

Result and discussion
3.1 XRD study XRD pattern of the cobalt ferrite (CoFe 2 O 4 ) nanoparticles irradiated by O +7 beam having energy 100MeV is shown in figure 1. Initially this sample was sintered at 700 o C and its crystallite size was 34±2 nm.The diffraction peaks are corresponding to the single phase of the ferrite nanoparticles and the phase analysis is done with JCPDS card no.221086.The diffraction peaks are sharper in the pattern of pristine sample while broader and scattered for irradiated samples.Small deviation in the diffraction angles may be due to some distortion in the unit cell, which indicates that crystallinity is decreasing after irradiation.However, the non appearance of extra peaks reveals that there is no change in cubic spinel structure of ferrite.Measurement of crystallite size was done by Scherer formula [12] using most intense peak (311) i.e.

=
where λ (1.54Å) is the wavelength of X-ray used, θ is the Bragg's angle, β is the full width at half maximum (FWHM) in radian and 'k' is a constant approximately equal to 0.9.The calculated Crystallite size, x-ray density, lattice parameter and interatomic distances are given table 1. Comparing these results with the results for pristine sample we found that crystallite size is decreasing after irradiation.One cause may be due beam due to which partial amorphization produces in the material [13].The decrease in crystallite size is almost same for both fluencies.SRIM code (Stopping and Range of Ions in Matter) shows that the electronic stopping (dE/dx) O +7 ion in CoFe 2 O 4 are 2.69 keV/nm, 0.0015 keV/nm and 48.74μm respectively.The large projectile range means the ions are implanted much deepe The observe modification is therefore mainly due to electronic stopping induced process.This energy produces different types of defects like cluster/point as well as partial amorphization depending on energy of ion.Clearly electronic energy loss is about three orders of magnitude larger than nuclear energy loss; therefore the dominant process is the electronic energy loss in the present system.However the threshold energy to create columnar defec possibility to create columnar defect, but only point defects or defect clusters.The heavy ion irradiation induces strong local distortion and stresses throughout the irradiated region which are radial and isotropic around the spherical defects [8] It is observed that the basic crystal structure remains the same after irradiation.However the peak intensities decreased and the widths of the peaks increased slightly.the SHI irradiation has generated some defect in the system are determined by the following relations.
where h,k,l are the miller indices of the planes hav calculated by using the formula [15] where M, N and 'a' are the molecular weight, Avogadro's number and lattice parameter respectively.The factor 8 is due to the fact that there are eight molecules of cobalt ferrite per unit cell.The irradiation.One cause may be due to breaking of larger particle after striking with high energy ion beam due to which partial amorphization produces in the material [13].The decrease in crystallite size is almost same for both fluencies.SRIM code (Stopping and Range of Ions in Matter) shows that the electronic stopping (dE/dx) e , Nuclear stopping (dE/dx) n and projected range of 100Mev are 2.69 keV/nm, 0.0015 keV/nm and 48.74μm respectively.The large projectile range means the ions are implanted much deeper in the substrate and nuclear stopping is negligible.The observe modification is therefore mainly due to electronic stopping induced process.This energy produces different types of defects like cluster/point as well as partial amorphization depending on energy of ion.Clearly electronic energy loss is about three orders of magnitude larger than nuclear energy loss; therefore the dominant process is the electronic energy loss in the present system.However the threshold energy to create columnar defects is ~13keV/nm [14] indicates that there is no possibility to create columnar defect, but only point defects or defect clusters.The heavy ion irradiation induces strong local distortion and stresses throughout the irradiated region which are radial isotropic around the spherical defects [8].

Figure 1. x-ray diffraction pattern
It is observed that the basic crystal structure remains the same after irradiation.However the peak intensities decreased and the widths of the peaks increased slightly.These observations exhibits that the SHI irradiation has generated some defect in the system.The lattice constant and determined by the following relations.

= (ℎ + + ) /
where h,k,l are the miller indices of the planes having interplaner spacing 'd hkl calculated by using the formula [15] = 8 /( ) g/cm 3   M, N and 'a' are the molecular weight, Avogadro's number and lattice parameter respectively.The factor 8 is due to the fact that there are eight molecules of cobalt ferrite per unit cell.The to breaking of larger particle after striking with high energy ion beam due to which partial amorphization produces in the material [13].The decrease in crystallite size is almost same for both fluencies.SRIM code (Stopping and Range of Ions in Matter) calculation and projected range of 100Mev are 2.69 keV/nm, 0.0015 keV/nm and 48.74μm respectively.The large projectile r in the substrate and nuclear stopping is negligible.The observe modification is therefore mainly due to electronic stopping induced process.This energy produces different types of defects like cluster/point as well as partial amorphization depending on the energy of ion.Clearly electronic energy loss is about three orders of magnitude larger than nuclear energy loss; therefore the dominant process is the electronic energy loss in the present system.
~13keV/nm [14] indicates that there is no possibility to create columnar defect, but only point defects or defect clusters.The heavy ion irradiation induces strong local distortion and stresses throughout the irradiated region which are radial It is observed that the basic crystal structure remains the same after irradiation.However the peak These observations exhibits that lattice constant and X-ray density hkl '. x-ray density ρ is M, N and 'a' are the molecular weight, Avogadro's number and lattice parameter respectively.The factor 8 is due to the fact that there are eight molecules of cobalt ferrite per unit cell.The interatomic distance (hopping length octahedral B (L B ) sites was calculated by using the following relations [16] It is clear that after oxygen ion irradiation the hopping length remains almost constant.The broadening of peaks after irradiation may be due to the effect of increased temperature in the material medium during the passes of oxygen ion beam.increase in the volume of the unit cell.clearly indicated by the decrease in line intensity and the increase in background.This result thus supports the applicability of thermal spike model of ion matter interaction

TEM Study
It is clear that after oxygen ion irradiation the hopping length remains almost constant.The broadening of peaks after irradiation may be due to the effect of increased temperature in the material medium during the passes of oxygen ion beam.The decrease in x-ray density is attributed to increase in the volume of the unit cell.The SHI irradiation of the pristine sample amorphize clearly indicated by the decrease in line intensity and the increase in background.This result thus supports the applicability of thermal spike model of ion matter interaction.

magnetic ions at tetrahedral A (L A ) and
It is clear that after oxygen ion irradiation the hopping length remains almost constant.
L B (Å) 3.63 2.96 3.63 2.96 3.64 2.97 The broadening of peaks after irradiation may be due to the effect of increased temperature in the ray density is attributed to f the pristine sample amorphized it, as is clearly indicated by the decrease in line intensity and the increase in background.This result thus of irradiated cobalt ferrite (a) (b) Surface morphology and particle size distribution of CoFe 2 O 4 magnetic nanoparticles was investigated by transmission electron microscopy (TEM).Figure 2 shows the TEM image of cobalt ferrite irradiated with 5×10 12 ions/cm 2 and 5×10 13 ions/cm 2 respectively.Their corresponding particle size distribution curves are shown on the right side of TEM images.Measurement of average particle size is done by Lorentzian fit of these histograms and particle sizes were found ~26±1nm.

VSM Study
Saturation magnetization values are estimated by fitting the high field part of the magnetization curve using the relation [17,18] where H is the field strength, a and b are two parameters called fitting parameters.For high field strength the saturation magnetization varies linearly with 1/H.The plot of M vs 1/H is a straight line whose intercept on M axis gives the value of M s [19].The fitted curve is shown in figure 3. It is clear from the figure 4 that a measurable change in magnetic parameters saturation magnetization (M s ), remanent magnetization (M r ) and coercivity (H c ) after the irradiation 100 MeV O +7 beam are found.The sample shows a large value of coercivity and retentivity at room temperature which is the characteristic of multi domain particles.These observations confirm that both pristine and irradiated nanoparticles are multi domain.VSM parameters for the pristine and irradiated samples are given in table 2. The Saturation magnetization values decreased slightly with fluence and also the remanent magnetization is increased with a substantial amount but theses changes in values of saturation magnetization and remanent magnetization are not unique characteristic of fluence.These are closely related to the microstructure, particle size and residual strain etc.The value of magnetic moment per formula unit (magneton number n B ) was estimated by the following relation [18] = Saturation Magnetization × Molecular weight 5585 μ In ideal situation when cobalt ferrite sample grows into a pure inverse type spinal structure with all Co +2 ions located in the octahedral sub-lattice and all Fe +3 ions in the tetrahedral sub-lattice.The magnetic moment of Fe +3 cations is fixed to 5µ and that of Co +2 cations fixed to 3.8µ which is corresponds to the saturation magnetization at 0K of bulk cobalt ferrite [20].A theoretical expression for the net magnetic moment per formula unit can be written as = | − | = [5 + 3.8] − 5 = 3.8µ where M A and M B represents the total magnetic moment of tetrahedral and octahedral sites.Thus saturation magnetization depends upon cation distribution (degree of inversion) which are related to surface spin canting and fine particle size.In present study the observed magnetic moment per formula unit for pristine sample is 3.28µ cobalt ferrite crystallites is not pure inverse, but contains some mixed structure.The va magnetic moment per formula unit reduces as the sample irradiated by oxygen beam and the value of saturation magnetization also shows reduction in irradiated samples.This reduction in magneton number and saturation magnetization is larger for disorder produced in the samples due to irradiation.As the critical single domain particle size for cobalt ferrite is ~14 nm [21], and in this study the x measurement shows that here particle size is much larger than this single domain limit.In multi domain reason the coercivity of samples is given by the fitting parameters and D is particle size.Thus as the particle size decreases correspondi coercivity increased.The reduction in saturation magnetization is smaller than the increase in remanence and coercivity after the irradiation of 100MeV oxygen ions.The squareness ratio for pristine sample, M r /M s = 0.38 at room temperature, is smaller than the expected value of randomly oriented isolated particles with uniaxial anisotropy (0.5) and particle respectively [23], which implies a tendency towards uniaxial anisotropy.The enhancement in squareness ratio after irradiation leads to increase in energy barrier of cobalt ferrite nanoparticles.Anisotropic field demagnetization curves are bifurcates.The anisotropy constant K calculated using relation = field [24].The anisotropy constants for pristine and irradiated samples are given in table 2. Anisotropy energy barrier is defined as K 1 anisotropy constant is increasing and particle vo µ which is less than theoretical value.This signifies the growth of cobalt ferrite crystallites is not pure inverse, but contains some mixed structure.The va magnetic moment per formula unit reduces as the sample irradiated by oxygen beam and the value of saturation magnetization also shows reduction in irradiated samples.This reduction in magneton number and saturation magnetization is larger for larger fluence which may be the consequence of disorder produced in the samples due to irradiation.
H curves for pristine and irradiated cobalt ferrite at different fluencies (±0.As the critical single domain particle size for cobalt ferrite is ~14 nm [21], and in this study the x measurement shows that here particle size is much larger than this single domain limit.In multi domain reason the coercivity of samples is given by a relation = + / [22], where the fitting parameters and D is particle size.Thus as the particle size decreases correspondi The reduction in saturation magnetization is smaller than the increase in e and coercivity after the irradiation of 100MeV oxygen ions.The squareness ratio for = 0.38 at room temperature, is smaller than the expected value of randomly oriented isolated particles with uniaxial anisotropy (0.5) and particles with cubic anisotropy (0.8) respectively [23], which implies a tendency towards uniaxial anisotropy.The enhancement in squareness ratio after irradiation leads to increase in anisotropic energy barrier of cobalt ferrite nanoparticles.Anisotropic field is corresponding to the value of field where magnetization and demagnetization curves are bifurcates.The anisotropy constant K 1 of the assembly of particles was /2 where M s is saturation magnetization and H field [24].The anisotropy constants for pristine and irradiated samples are given in table 2. Anisotropy 1 V, where V is particle volume.After irradiation of oxygen beam anisotropy constant is increasing and particle volume is decreasing but degree of increment of K1 is which is less than theoretical value.This signifies the growth of cobalt ferrite crystallites is not pure inverse, but contains some mixed structure.The value of this magnetic moment per formula unit reduces as the sample irradiated by oxygen beam and the value of saturation magnetization also shows reduction in irradiated samples.This reduction in magneton larger fluence which may be the consequence of H curves for pristine and irradiated cobalt ferrite at different fluencies 2.02×10 5 3.28 3.94×10 5 3.26 3.99×10 5 3.23   As the critical single domain particle size for cobalt ferrite is ~14 nm [21], and in this study the x-ray measurement shows that here particle size is much larger than this single domain limit.In multi [22], where a and b are the fitting parameters and D is particle size.Thus as the particle size decreases corresponding value of The reduction in saturation magnetization is smaller than the increase in e and coercivity after the irradiation of 100MeV oxygen ions.The squareness ratio for = 0.38 at room temperature, is smaller than the expected value of randomly s with cubic anisotropy (0.8) respectively [23], which implies a tendency towards uniaxial anisotropy.The enhancement in squareness ratio after irradiation leads to increase in anisotropic energy barrier of cobalt ferrite is corresponding to the value of field where magnetization and of the assembly of particles was is saturation magnetization and H k is anisotropic field [24].The anisotropy constants for pristine and irradiated samples are given in table 2. Anisotropy V, where V is particle volume.After irradiation of oxygen beam lume is decreasing but degree of increment of K1 is intensities of lines 2 and 5.A hyperfine field parallel to zero intensity for spectral lines 2 and 5 [22]. in all irradiated as well as pristine gamma ray is not parallel to the hyperfine field.Under ion irradiation ferric ions undergo displacement from octahedral to tetrahedral site simultaneously same amount of Co +2 ions moves from tetrahedral to octahedral site.Taking into account the low value of magnetisation measured in the irradiated samples, and it is obvious that these displacements are stable and involve large concentration of iron atoms at tetrahedral site.This outcome of Mössbauer measurement verifies the results of our above VSM study.

Conclusion
In this study we discussed the heavy ion irradiation modifications on the nanoparticles of cobalt ferrite prepared using sol gel method.These nanoparticles were sintered at 700 o C for two hours and cooled slowly to room temperature.The pristine sample was irradiated by 100 MeV O +7 beam with two different fluencies 5×10 12 and 5×10 13 ions/cm 2 .X-ray diffraction pattern shows that all three samples are purely crystalline but irradiated sample has scattered background and broader peaks, which is because of reduction in particle size after irradiation.Particle size directly measured by TEM is closed to the size calculated using Scherer's formula.After irradiation the saturation magnetization and magneton numbers reduced very slightly while the coercivity and remanence increased significantly.Mössbauer spectra shows that after irradiation the migration of Fe +3 ions undergoes from octahedral to tetrahedral sites which reduces the magnetization of octahedral site consequently the difference of magnetization of both the sites becomes smaller.The non-zero intensity of spectral lines 2 and 5 exhibits the possibility that hyperfine field at B site is not parallel to the gamma ray.

Figure 2 .
Figure 2. TEM micrograph and Particle size distribution curve with fluence

Figure 3 .
Figure 3. Determination of saturation magnetization using law of approach method.

Figure 4 .
Figure 4. M-H curves for pristine and irradiated cobalt ferrite at different fluencies

Table 1 .
Structural parameters of samples

Table 3 .
[22]bauer parameters Mössbauer spectra of pristine and irradiated cobalt ferrite intensities of lines 2 and 5.A hyperfine field parallel to the direction of gamma ray corresponds to lines 2 and 5[22].In this study the lines 2 and 5 exhibit sufficient intensit pristine samples which may be in agreement with a canted sub lattice and parallel to the hyperfine field.Quadrupole Splitting; LW -Line Width; B hf -Hyperfine Field Mössbauer spectra of pristine and irradiated cobalt ferrite nanoparticles.gamma ray corresponds to ines 2 and 5 exhibit sufficient intensity samples which may be in agreement with a canted sub lattice and