Influence of Al3+ doping on structural and magnetic properties of CoFe2-xAlxO4 Ferrite nanoparticles

Accepted Manuscript 3+ Influence of Al doping on structural and magnetic properties of CoFe2-xAlxO4 Ferrite nanoparticles N. Dipesh, L. Wang, H. Adhikari, J. Alam, S.R. Mishra PII:

S0925-8388(16)32062-X

DOI:

10.1016/j.jallcom.2016.07.030

Reference:

JALCOM 38198

To appear in:

Journal of Alloys and Compounds

Received Date: 8 April 2016 Revised Date:

22 June 2016

Accepted Date: 3 July 2016

3+ Please cite this article as: N. Dipesh, L. Wang, H. Adhikari, J. Alam, S.R. Mishra, Influence of Al doping on structural and magnetic properties of CoFe2-xAlxO4 Ferrite nanoparticles, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.07.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

2HM

-3

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20 15 10 5 0 -3

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x = 0.5

20 15 10 5 0

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dM/dH (emu/g.Oe)

30x10 25

-3

20x10

10 5

x = 0.9

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15

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dM/dH (emu/g.Oe)

x = 0.0

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dM/dH (emu/g.Oe)

30x10 25

0

3

-10x10

dM/dH vs. H for CoFe2-xAlxO4.

-5

0 Field (Oe)

5

10

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Influence of Al3+ doping on structural and magnetic properties of CoFe2xAlxO4 Ferrite nanoparticles N. Dipesh, L. Wang, H. Adhikari, J. Alam, and S. R. Mishra

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Department of Physics and Materials Science, The University of Memphis, Memphis, TN 38152

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Abstract

A series of Al3+ substituted CoFe2-xAlxO4 (x = 0.0 to 0.9) ferrite nanoparticles were synthesized

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via atuocombustion method. XRD data show that all the samples exhibit single phase cubic spinel structure where the lattice parameter was observed to decrease linearly with increasing Al3+ content. Structurally Al3+ doping was observed to bring in grain refinement and increase dislocation density in CoFe2-xAlxO4. The magnetic parameters namely saturation magnetization, MS, coercive field, HC, and remanent magnetization, Mr vary significantly with Al3+ doping. The Al3+ doping resulted in reduction in Ms and Hc due to magnetic dilution effect and reduction in

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anisotropy constant K1, respectively. Room temperature Mössbauer spectra were analyzed to extract cation distribution and hyperfine parameters. The hyperfine magnetic field decreases at both tetrahedral (A) and octahedral (B) interstitial sites with the increase in Al3+ content. The

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prefers B sites.

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Mössbauer spectral analysis reveals that Al3+ has preference for the tetrahedral A site and Co2+

Key Words: Key words— Magnetic Materials, oxides, chemical synthesis, x-ray diffraction, Magnetic properties Corresponding Author S. R. Mishra Department of Physics and Materials Science, The University of Memphis, Memphis, TN 38152 [email protected] 1

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Introduction Ferrimagnetic ferrites, cubic spinels, are interesting magnetic oxide materials due to their combined magnetic and insulating properties which endows ferrite with many technological applications. The magnetic and electrical properties of ferrites much depends on the cation

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distribution on sublattices viz. tetrahedral A site and octahedral B site. Cobalt Ferrite, CoFe2O4, possess high cubic magnetocrystalline anisotropy [1], high Curie temperature [2], and reasonable saturation magnetization with moderate coercivity [3] which makes it good a candidate for applications in high frequency devices, memory cores, recording media, and in biomedical field

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[4,5, 6,7]

As per the application need, additional changes in magnetic and electrical properties of

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spinel ferrite can be achieved by doping magnetic and non-magnetic ions. Depending upon the type and valence, the substitution ion distributes on tetrahedral (A) and/or octahedral (B) sites of spinel ferrite which eventually affects ferrite physical properties [8,9,10,11,12]. Among various ions substitution, Al3+ substitution in ferrites decreases magnetic hardness and increases electrical resistivity. These properties make Al3+ doped ferrite desirable for application in power transformers and telecommunication applications [13,14].

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This work presents the result of systematic doping of non-magnetic Al3+ on the structural and magnetic properties of CoFe2-xAlxO4 ferrites synthesized via auto-combustion method. Aluminum ion has been proven to have substantial influence on M-Ferrites (M=Al, Mg, Mn, Li, Co, etc.) [15], especially increasing resistivity and reducing eddy current losses [16,17,18]. In

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addition, the doping of non-magnetic Al3+ in ferrite brings in grain refinement and also alters magnetic properties to a great extent [19,20]. The present work provides a systematic analysis of

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x-ray diffraction data, magnetic, and Mössbauer data to provide a comprehensive view of structural-property relationship. In particular, Mössbauer data analysis establishes Co2+, Al3+, and Fe3+ distribution at A and B sites. In turn the distribution of ions project strong influence on the magnetic properties of the compound [21].

Experimental One pot method was used to synthesize CoFe2-xAlxO4 (x = 0.0 to 0.9) ferrite. Nitrate salts viz. Co(NO3)2. 9H2O, Fe(NO3)3 .9H2O and Al(NO3)3. 9H2O were mixed in stoichiometric amount, as listed in Table I, in 30ml distilled water. The solution was ultrasonicated for 30 minutes. After 2

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ultrasonication the solution was cooled down to the room temperature and then the pH was adjusted around 6.5 using ammonium hydroxide. The solution was then heated at 110 oC to get rid of the extra water. The dried solid product was calcined at 950 oC for 12 hours which resulted in a black powder of CoFe2-xAlxO4.

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The x-ray diffraction (XRD) patterns were collected using Bruker D8 Advance (Bruker Inc.) x-ray diffractometer using Cu Kα radiation (power 40kV, 40mA) in the range of 20 to 70o with 0.05o step size and 0.2sec/step acquisition time. Structural assessment was performed using diffraction data analysis using TOPAS (Bruker Inc.). The infrared spectra, FTIR, were collected 1

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in transmission mode using Thermo IR100 spectrometer in the wave number range 400-4000 cmon a compacted sample-KBr pellet. The magnetic properties of the samples were investigated at

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room temperature (RT) using vibrating sample magnetometer (VSM) at maximum 1.2 kOe field. The Curie temperature (Tc) of samples were measured using modified thermogravemetirc analyzer (TGA, DuPont 951) equipped with a permanent magnet. RT

57

Fe Mössbauer

spectroscopy was used to derive hyperfine parameters. Mössbauer spectrometer (SEE Co) was calibrated against α-Fe foil. The Mössbauer spectra were analyzed using WMoss software from

Results and Discussion

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SEE Co.

Figure 1 shows XRD patterns of CoFe2-xAlxO4 ferrites. All samples exhibit single phase cubic spinel structure with space group Fd3m without presence of any secondary phase. The

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lattice parameters obtained from refinement is listed in Table II. The lattice parameters “a” and “V” show a monotonic decrease with Al3+ content. The lattice parameters “a” and “V” decrease

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at the rate of -0.12 Å and -26.43 Å3 per Al3+ substitution, respectively, thus obeying the Vegard’s law [22]. The observed right shift of diffraction peaks, as shown in inset Fig. 1, with Al3+ substitution indicate lattice contraction. This lattice contraction in CoFe2-xAlxO4 upon Al3+ substitution is due to the smaller ionic radius of Al3+ ion (~0.535 Å) with respect to the ionic radius of Fe3+ ion (~0.645 Å). The average crystallite size of the nanoparticles was deduced from the Halder-Wagner-Langford’s (HWL) plot technique [23] applied to the XRD data. The HWL equation relates the FWHM of peaks, β, with the mean crystallize size, “D”, and the microdeformation of a grain, ε as follow. 3

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2

β∗  1  β∗  ∗  =  ∗2 Dd d 

 ε   +   … eq. (1)  2 2

Where, β* is given by β ∗ = 2

λ

sin (θ ) . The plot of (β*/d*)2 versus β∗/d∗2 is a straight line, for which the intercept and the

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d∗ =

β cos(θ ) , where λ is the x-rays wavelength, and d* is given as λ

slope allow the values of the microstrain (ε) and the crystallite size (D), to be determined. The HW plot has a great advantage that data for reflections at low and intermediates angles are given

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more weight than those at higher diffraction angles, which are often less reliable. Crystallite size, D, and microstrain, ε, derived from the Fig. 2 is listed in Table II. While the dislocation density

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listed in Table II was estimated using the equation δ = 1/D2 (lines/m2) [24]. The dislocation density (δ) indicates the length of dislocation lines per unit volume and measures the amount of defects in a crystal. It is observed from the Table II, that the crystallite size decreases with Al3+ content and reaches a minimum value of 32.24 nm at x = 0.9. The grain refinement upon Al3+ substitution can result (1) from the difference in size between Fe3+ and Al3+ which increases microstrain and defect density with Al3+ content and (2) if Al3+ diffused to the grain boundaries,

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the migration that will restrain the grain growth by lowering down the grain mobility. As lattice constant closely follows the Vegard’s law and with the increased microstrain in with Al3+ substitution it can be inferred that the grain growth arrest most likely result from induced microstrain upon Al3+ substitution in CoFe2-xAlxO4.

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The physical properties of ferrites are sensitive to the cation’s nature, the valance state and their distribution on tetrahedral A and octahedral B sites of the spinel structure. Thus,

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understanding of the cation distribution is essential in understanding the intricate structural and physical property relationship. In this view, ionic radii, site radii and cation distribution is calculated, where later is extracted from the Mössbauer data. The ionic site radii, rA and rB and bond distances R(A-O) and R(B-O) are calculated using following relations and tabulated in Table VI.

rA = [CCo RCo + C Al R Al + C Fe RFe ]

rA =

1 [CCo RCo + C Al R Al + C Fe RFe ] 2

…eq. (2)

…eq. (3)

Where Cx is the fraction of “x” cations obtained from Mössbauer analysis, Table VI, and 4

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[ ) = [r

R( A − O 2− ) = rA + rO 2 − R( B − O 2−

B

+ rO 2 −

] ]

…eq.(4) …eq.(5)

Where, rO 2− ~ 1.32 Ǻ, rCo 2− ~ 0.72 Ǻ, rAl 3− ~ 0.535 Ǻ, rFe3− ~ 0..645 Ǻ.

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It is observed from the Table VI that A-O2- bond distance decreases while B-O2- bond distance remains almost invariant with Al3+ substitution.

The Fourier Transform Infrared Spectroscopy (FTIR) spectrum was recorded in transmission geometry using (KBr) discs in the range of 400–4000cm-1. The IR spectra for the samples as a function of Al3+ doping is shown in Fig. 3. Two major broad absorption bands

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corresponding to metal-oxygen in ferrites in particular are observed between 400-610 cm-1. As per Waldron [25] two bands, υT (590 to 610 cm-1) and υO (400 to 420 cm-1), indicate the

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stretching of cation–anion bonds in the tetrahedral (A) and octahedral (B) sites, respectively, thus further confirming the formation of the spinel structure. The degree of Fe-O covalent bonding determines the band position for the A and B sites. The Fe-O distance (1.89 Å) at A site is smaller than that of B site (1.99 Å) [26], thus the degree of covalent bonding Fe-O at A site is more than that at B site which results in vibration stretching at higher wave numbers. Table III, list the band position υT (tetrahedral site) and υO (octahedral site) as a function of Al3+

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substitution. The variation in band position is expected due to altered Fe3+-O2- interaction upon Al3+ substation. The υT and υO bands shift to higher wavenumber value with Al3+ substation. Furthermore band broadening was observed due to statistical distribution of Fe3+, Co+2, and Al+3

sites.

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ions on their respective sites and distribution of vacancies among the octahedral and tetrahedral

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The force constant kT and kO for the A and B sites for spinel is given as [27]:

kT =7.62 x MT x υT2 x 10-7 N/m

…eq.(6)

kO = 10.62 x MO/2 x υO2 x 10-7 N/m

…eq.(7)

where, MT and MO are the molecular weight of cations on A and B sites, respectively. The Table

III present the variation of force constant with Al3+ content. The force constant is observed to 5

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decrease on tetrahedral site while increase on octahedral site with Al3+ doping. The observed variation in the force constant values reflect changes in the Fe-O covalency with lattice contraction upon Al3+ substitution. The magnetic order in spinel is mainly via super-exchange interaction mechanism

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occurring between the metal ions in the A and B sublattices mediated via oxygen ions. The magnetic properties of ferrimagnetic ferrites are usually explained on the basis of magnetic interactions between sites, A-A, B-B and A-B. Among these interactions A-B interaction is predominant and is responsible for the net magnetization of the material. As a result of A-B

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interaction the magnetic moments of A sites are held antiparallel to those on B sites and the spontaneous magnetization of the domain is therefore due to the difference in moments at A and B sites. Furthermore, the magnetic properties of ferrite are dependent on the kind of metal ion

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situated at tetrahedral A and octahedral B site. The substitution of ion such as Al3+, which occupies A sites according to Mössbauer analysis, results in reduction of exchange interaction between A and B sites. Hence, Al3+ is expected to affect magnetic properties of CoFe2-xAlxO4.

Fig. 4 show the room temperature hysteresis loops of CoFe2-xAlxO4 powder samples as a function of Al3+ substitution. As seen in Fig 4, upon increasing the Al3+ content the saturation

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magnetization, Ms, is observed to decrease. The saturation magnetization measured at 1.2 kOe was found to be a maximum at 86.5 emu/g for x = 0.0 and decreased with further increase in Al3+ to 31.7 emu/g for x = 0.9. The decrease in Ms value is at the rate of -56.0 emu/g per Al3+ substitution. This suggests the weakening of Fe3+-O2—Fe3+(Al3+) super-exchange interactions due

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substitution of non-magnetic Al3+. However surface broken symmetry, spin canting, surface magnetic moment non-collinearity and reduced magnetocrystalline anisotropy in nanocrystalline

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particles due to defects and dislocation can also add to the reduction in magnetization [28,29, 30]. Our observations are in good agreement with the findings on other substituted ferrites [20,31,32,]. The room temperature magneton number (nB) per formula unit was calculated using the following formula [33] and are listed in Table IV.

nB =

M s .MW 5585

…eq.(8)

Where, MW is the molecular weight of the composition and Ms is the saturation magnetization. It is observed from Table IV, that the room temperature magneton number also decreases with 6

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the increase in Al3+ content from 3.63 µB for x = 0 to 1.18 µB for x = 0.9 [34]. The observed decrease in magneton number is attributed to the decrease in A-B interaction as magnetic Fe3+ ions of 5µB are replaced with non-magnetic Al3+ ions.

Figure 4 inset shows the coercivity, Hc, of CoFe2-xAlxO4 as a function of Al3+ content. It

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is observed from the Fig. 4 inset and Table IV, that Hc decreases from 2183 at x = 0.0 to ~617 Oe for x = 0.9. The Hc value is observed to decrease at the rate of -647.41 Oe per Al3+ substitution. It is known that the coercivity depends on anisotropy and the particle size [35]. As per the Brown’s relation [36] relation HA = 2K1/µ0MS, the anisotropy field is inversely

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proportional to the saturation magnetization. The decrease in Hc along with Ms is suggestive of the fact that the origin of decrease in coercivity may lie in the reduction in the particle size and

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magneto crystalline anisotropy [30,37,38].

The room temperature anisotropy CoFe2-xAlxO4 samples were obtained from, the ‘‘Law of Approach’’ to saturate magnetization and are listed in Table IV. The “Law of approach” describes the relation between magnetization M on the applied magnetic field for H greater than coercive field Hc. The magnetization near the saturation Ms can be written as [39],

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a b   M = Ms1 − − 2  + κH , where M is the magnetization, H is the applied magnetic field, and H H   MS is the saturation magnetization. The term κH represents the field-induced increase in the spontaneous magnetization of the domains. This term is very small at temperature well below the Curie temperature and may be neglected. The term “a” is generally interpreted as due to

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microstress and ignored in the high field region, and “b” as due to crystal anisotropy. While K1

105b . The numerical coefficient 8

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is the cubic anisotropy constant and is given as, K1 = µ o M s

8/105 applies to cubic anisotropy of random polycrystalline samples. The room temperature b   experimental data M vs. H is fitted to equation, M = Ms1 − 2  . An example of a fitting curve  H  for x = 0.4 M vs. H data is shown in Fig. 5. The values of Ms and b were obtained from fitting and were used in calculating K1. The calculated value of K1 as a function of Al3+ doping is shown in Fig. 5 inset. And is in agreement with the earlier reports on the nanocrystalline ferrites [40,41,42]. From Table IV it is observed that the cubic anisotropy constant decreases in magnitude with the Al3+ content from K~1.09x106 erg/cm3 for x=0 to 0.42 x106 erg/cm3 for 7

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x=0.9. In fact CoFe2O4 has strong multiaxial magnetocrystalline anisotropy, K ~2x106 erg/cc [43] because of the spin-orbit coupling of Co2+ seating in a crystal field (trigonal field) incapable of removing the orbital degeneracy Co ion at the octahedral site. As inferred from the Mössbauer analysis below, the Co2+ occupancy at the B site remains unperturbed upon Al3+ substitution,

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which leads to the inference that anisotropy field is disturbed by the lattice distortion and particle size reduction with the Al3+ substitution. In fact the magnetocrystalline anisotropy constant that describes the preference for spins to align in a particular direction within the particle depends on various anisotropies like shape anisotropy, surface anisotropy, stress anisotropy, and

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unidirectional anisotropy, and is approximately given by K = 30 kB TB/V, where TB is the blocking temperature and “V” is the particle volume. As the particle size decreases the effective

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anisotropy energy must increase. However, in our case, anisotropy is observed to decrease with particle size. This observed reduction in anisotropy can thus be attributed to the negative contribution to anisotropy due to the radial orientation of the surface spins, resulting in an effective magnetocrystalline anisotropy reduction with decreasing nanoparticle size [44]. On the other hand, Kumar et.al attributed the anisotropy reduction to the Co2+ displacement from B site to A site in Al3+ substituted CoFe2-xAlxO4 [41], which is unlikely cause of anisotropy reduction

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in our case as Co2+ ions do not displace out of B site.

As discussed above, the Al3+ substitution leads to grain refinement in CoFe2-xAlxO4 particles. The critical size of forming single domain grain in CoFe2O4 is in the range 40–70 nm [45]. Statistically there exist distribution of mono and multi-domain particles in as synthesized

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CoFe2-xAlxO4 sample. The effect of mono and multi-domain particles on magnetic properties can be understood from dM/dH curves derived from hysteresis loops of Fig. 4. Magnetic

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susceptibility χ = dM/dH is often computed to evaluate the magnetic interaction in different systems, since this magnitude is related to the switching or inversion field distribution. The maximum in dM/dH coincides with coercivity HC in systems with a single magnetic phase. As an example, dM/dH vs. H curves for x=0.0, 0.5, and 0.9 CoFe2-xAlxO4 is shown in Fig. 6. The dM/dH vs. H curves show symmetric peaks about H ~ 0 with separation of 2HM. The peaks are separated by magnetic field 2HM as shown. The HM, dM/dH at Hc and H=0, obtained from dM/dH vs. H curves and are listed in Table IV. An ideal single domain particle exhibit square shaped M-H loop with infinite susceptibility (dM/dH) at HC and zero at H→0. The finite value of dM/dH at H~0 confirms the presence of pseudo-single and multi-domain particles. As evident 8

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from Table IV, the values of HM are consistently lower than Hc which implies absence of switching field distribution which is mainly observed in nanoparticles with disordered shell structure. Furthermore the peak height dM/dH at HM is higher than dM/dH value at H = 0 for samples up to x =0.5. Later the peak height dM/dH decreases with further addition of Al3+. The

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peak separation HM also decreases linearly with Al3+ substitution. The decrease in HM with increasing Al3+ content indicates increased presence of large number of unstable superparamagnetic particles in sample. Narrowing of the susceptibility peaks for Co2FeO4 spinel oxide prepared by mechanical alloying and subsequent annealing was attributed to a single phase

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structure of Co2FeO4 and to the homogeneous structure of ferrimagnetic domains [46].

The Curie temperature, Tc, of all the samples was determined using thermogravimetric

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(TGA) method and the values are listed in Table IV. TC ~ 802 K for x = 0.0 sample suggests that long range ferromagnetic interactions dominate in the samples. Like saturation magnetization, Curie temperature also decreases with increase in Al3+ content in CoFe2-xAlxO4. The Tc value decreases monotonically at the rate of -394.12 K per Al3+ substitution. The decrease in Curie temperature is directly related to decrease in number of super-exchange interactions and to the strength of interaction due to crystal shrinkage which affects Fe3+-O2—Fe3+ bond angles for

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optimum super-exchange interaction [47,48].

Mössbauer spectroscopy is extremely sensitive to small variations in electron density at the iron nucleus, due to different electronic and structural environments. The room temperature fitted Mössbauer spectra as a function of Al3+ substitution are shown in the Fig. 7. The hyperfine

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parameters extracted from the refinement are listed in Table V and values are typical for cobalt ferrite samples [49]. The Mössbauer spectra were fitted with overlapping Lorentizan sextets. The

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number of sextets were chosen according to the probability distribution of iron, cobalt, and aluminum ions surrounding A and B sites. In a normal spinel, A site is surrounded by 12 B site Fe3+ ions and B site is surrounded by 6 A Fe3+ ions. In a spinel, hyperfine magnetic fields at Bsite 57Fe nuclei are a function of the occupation of the six nearest tetrahedral A sites [20,50]. The probability distribution of Fe3+ around B site is given as follow: PB (n) =

6! (1 − x) 6− n x n …eq. (9) n!(6 − n)!

The Mössbauer spectra were fitted with probability values greater than 5% only. All other values were ignored in the fitting process. The CoFe2-xAlxO4 Mössbauer spectra were fitted 9

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with 5 subspectra, one for A site (AO) and four for B sites (BO, B1, B2, B3). The intensities of the four different B-sites are proportional to the probabilities that an Fe3+ at B-site ion has 6Fe, 5Fe, 4Fe and 3Fe nearest neighbors. Paramagnetic doublet, C1, representing some minor impurity in samples was added too. The fact that superposition of more than one sextet for B-sites are

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required to arrive at an acceptable fit to the data reflects the complex magnetic interactions involved in CoFe2-xAlxO4.

The area under the Mössbauer subspectrum were used to calculate the percent site occupancy of ions, as listed in Table V. The cation site occupancy of CoFe2-xAlxO4 was

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calculated with the following two assumptions; (1) CoFe2O4 is a classical inverse spinel structure with Co2+ ions and half of the Fe3+ located on the octahedral B site while the rest of the Fe3+ is on

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the tetrahedral A sites, and (2) Al3+ has the preference for tetrahedral A sites [51]. The iron distribution is calculated through the Mössbauer fitting data using the formula the following area ratio [52]:

Area ( A) Γ( A) N ( A) =R Area ( B ) Γ( B ) N ( B )

…eq. (10)

where, Area (A/B) is the total weighted average area of A/B sites, R is a scaling factor, Γ

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is the line width at full-width-at-half-maximum (FWHM), and N(A) and N(B) are the number of Fe3+ on A and B sites, respectively. The ratio of N(A)/N(B) is one as per the assumption 1. The area ratio and Area(A)/Area(B) and line-width ration Γ(A)/Γ(B) can be obtained from Table V. The R value is determined for x=0 (R=1.57757) which is used for the rest of the calculation. For

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the remaining CoFe2-xAlxO4 samples, N(A)/N(B) ratio was calculated from the corresponding area ratio and line-width ratio, and the R value as calculated before. For example, the area ratio

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Area(A)/Area(B) for x = 0.5 sample is 0.8660. Then the number of Fe3+ in A and B sites can be calculated using simple algebra as follow. N ( A)  Area ( A) Γ( B)  1  = .   N ( B )  Area ( B) Γ( A)  R 

…eq. (11)

 Area( A) Γ( B)  1  N ( A) =    N ( B) …eq. (12)  Area( B) Γ( A)  R 

And N ( A) + N ( B) = 2 − x

…eq. (13)

Where (2-x) iron atoms per unit formula are not occupied by Al3+. 10

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Values for Fe3+ content, N(A) and N(B), were obtained by solving above two simultaneous equations (4)and(5) and with the assumption that x = 0 sample is fully inverse spinel i.e. Co2+ at B site only. Thus, for x = 0.5 samples, N(A) and N(B) equals to 0.5523 and 0.94776 for A and B sites, respectively. In B site there are 0.94776 Fe3+ ions, the remaining 1.0523 of B site of which

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1 is occupied by Co2+ and 0.0523 is occupied by Al3+. While A site holds 0.5523 Fe3+ and 0.4476 Al3+. The experimentally calculated ion content in CoFe2-xAlxO4 is listed in Table VI. From Table VI, it is evident that (1) Al3+ increasingly prefers A site, (2) Co2+ prefers only B site, and

(3) Fe3+ content decreases on A site with Al3+ substitution. Reported cation distribution result is

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in agreement with earlier results on Al3+ substituted ferrites [53,54 ].

The effect of ion distribution is also reflected in hyperfine values as listed in Table VI. The room temperature weighted average hyperfine field for both A (HFA) and B (HFB) sites as a

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function of Al3+ content is plotted in Fig. 8. The weighted average HF is calculated by taking sum of the product of individual site HF values and their corresponding area and dividing the sum by the total spectral area. The observed weighted average HFA = 490 kOe and HFB = 498 kOe for undoped CoFe2O4. Both A-site and B-site hyperfine fields are observed to decrease with increase in Al3+ content. It is evident that there is decrease in the internal hyperfine field values

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with increasing Al3+ substitution. The replacement of Fe3+ by Al3+ influences the internal hyperfine field of the nearest Fe3+ sites through super-transferred hyperfine fields. The HF value of A site is consistently higher than that of B site as the transferred field at B site reduces with Al3+ increasingly occupying A site. The trend in HFA decrease is rather parabolic than linear with

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Al3+ substitution; presumably due to presence of magnetic Co2+ at and B sites. The expected range of isomer shift (IS, δ) for Fe3+ with oxygen coordination is given to

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be between 0.20 – 0.30 mm/s, with an average of about 0.22 mm/s for tetrahedral oxygen coordination and 0.30-0.46 mm/s, with an average of about 0.35 mm/s for the octahedral coordination [55]. This is interpreted as being due to the greater bond separation of Fe3+-O2- for the octahedral ions compared with that for the tetrahedral ions, Table VI. As the orbitals of the Fe3+ and O2- - ions overlap less, the covalency effect is smaller, and hence the isomer shift is large at the octahedral site. The IS values, Table V, for A and B sites are consistent with the above isomer shift values for spinel ferrites [56,57,58]. Figure 9 shows significant change in weighted average isomer shift of CoFe2-xAlxO4 with progressive Al3+ doping, which indicate the effect of Al3+ substitution on s-electron charge density at Fe3+ nucleus. It is observed that the 11

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weighted average isomer shift decreases linearly at the rate of -0.157 mm/s and -0.127 mm/s per Al3+ substitution for A and B site respectively. The isomer shift value is proportional to the total s-electron charge density at the iron nucleus, which is affected by the site volume and the complex nature of bonding i.e. electronic environment [59,60]. The overall decrease in lattice

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volume with Al3+ substitution increases s-electron charge density at the iron nucleus which in turn decreases the isomer shift values. The decrease of the isomer shift with decrease in lattice volume is generally due to the delocalization of the 3d orbitals, and the consequent reduction of the shielding of the 3s orbitals, which increases the 3s charge density at the nucleus [58].

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As evident from Table V, the magnitude of quadrupole shift values for A site increases while that of B site largely remains positive. The quadrupole splitting is a measure of asymmetry

M AN U

of charge distribution around the iron nucleus [61]. Generally, in the ideal spinel structure, the cation at the A site do not experience any electric field gradient due to cubic Td symmetry of A site. But the B-site has trigonal point symmetry and hence large electric field gradient is expected. Also, the chemical disorder in the substituted compound produces distribution of electric field gradient of varying magnitude, direction, sign, and symmetry. The observed changes in QS values of A site reflects considerable distortion in A-site symmetry as evident

KT (Table II & VI). Conclusion

TE D

from the atomic site radii of A site, rA, site-O2- distance R(A-O2-), and change in spring constant

EP

The substitution of Al3+ ions in CoFe2-xAlxO4 system leads to grain refinement and the formation of nanosized mixed spinel ferrites. The unit cell parameters of CoFe2-xAlxO4 decreased linearly with the increase in Al3+ concentration. The effect of decrease in unit cell volume was reflected

AC C

in the decreased isomer shift values with Al3+ substitution. The magnetization value of CoFe2xAlxO4

linearly decreased with the substitution of Al3+ due to magnetic dilution effect. The

coercivity value was also observed to decrease with Al3+ substitution mainly due to reduction in magnetocrystalline anisotropy. Room temperature Mössbauer results establishes Al3+ preference for the A site. These results clearly show that the grain refinement and magnetic softness in ferrites is achieved with non-magnetic ion doping Al3+ [Error! Bookmark not defined.,20,62,63]. This study highlights the structure–property relationship in ferrites and allow

tuning of magnetic properties of CoFe2-xAlxO4 via controlled Al3+ substitution. 12

ACCEPTED MANUSCRIPT

ACKNOWLEDGMENT

S. R. Mishra acknowledges the support from NSF-CMMI (1029780) and TN-SCORE (EPS

AC C

EP

TE D

M AN U

SC

RI PT

1004083).

13

ACCEPTED MANUSCRIPT

Table I:

Stoichiometry of chemicals used in the synthesis of CoFe2-xAlxO4.

Table II: Structural parameters derived from x-ray diffraction pattern analysis.

RI PT

Table III: FTIR Band position and force constants for CoFe2-xAlxO4.

Table IV: Room temperature magnetic parameters of CoFe2-xAlxO4 extracted from hysteresis loops.

SC

Table V: The room temperature Mössbauer parameters of CoFe2-xAlxO4.

AC C

EP

TE D

M AN U

Table VI: The cation distributions, site radii rA and rB, and site-O2- distances for tetrahedral A and octahedral B sites for CoFe2-xAlxO4 obtained from Mössbauer spectral area analysis.

14

ACCEPTED MANUSCRIPT

Figure Caption

RI PT

Figure 1: The x-ray diffraction patterns of the CoFe2-xAlxO4 as function of Al3+ content. The insets show expanded view of XRD patterns between 2θ ~ 35-37o values and lattice constant “a” as a function of Al3+ content for CoFe2-xAlxO4 [20]. Figure 2: HWL plot as a function of Al3+ content for CoFe2-xAlxO4.

FTIR plots of CoFe2-xAlxO4 as a function of Al3+ content, x.

SC

Figure 3:

Figure 4: Room temperature M vs. H curves of CoFe2-xAlxO4. Magnetic parameters, saturation

is presented in insets [20].

Figure 5:

M AN U

magnetization, Ms, and coercivity, Hc, as function of Al3+ content for CoFe2-xAlxO4

“Law of approach” fit to M vs. H data for CoFe2-xAlxO4, x = 0.4. Inset shows magnetic anisotropy energy as a function of Al3+ content for CoFe2-xAlxO4.

Figure 7:

TE D

Figure 6: dM/dH vs. H for CoFe2-xAlxO4.

Room temperature Mössbauer fitted data for CoFe2-xAlxO4.

as a function of Al3+ content, x.

AC C

xAlxO4

EP

Figure 8: Room temperature weighted average hyperfine field (HF) parameters for CoFe2-

Figure 9: Room temperature weighted average isomer shift (IS) parameters for CoFe2-xAlxO4

as a function of Al3+ content, x.

15

ACCEPTED MANUSCRIPT

Table I: Stoichiometry of chemicals used in the synthesis of CoFe2-xAlxO4.

Weight in gm.

x

1.42893 1.44947 1.47062 1.49239 1.51482 1.53793 1.56175 1.58633 1.61168 1.63788

3.96724 3.82303 3.67466 3.52189 3.36452 3.20236 3.03518 2.86273 2.68477 2.50103

3.09534 3.13981 3.18561 3.23277 3.28135 3.33141 3.38303 3.43636 3.49127 3.54792

SC

3.96724 3.82303 3.67466 3.52189 3.36452 3.20236 3.03518 2.86273 2.68477

Citric Acid

M AN U

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Al(NO3)3.9H2O Fe(NO3)3.9H2O

RI PT

Co(NO3)2.6H2O

“a” (Å)

“V” (Å )

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

8.382 8.370 8.356 8.343 8.332 8.321 8.305 8.291 8.282 8.267

589.018 586.385 583.596 580.875 578.613 576.159 572.863 570.067 568.225 565.142

Crystallite size D (nm)

Microstrain (-ε)

Dislocation Density, δ (x1014lines/m2)

43.96 42.68 42.72 40.40 39.29 38.91 38.68 35.54 34.45 32.24

0.002 0.002 0.002 0.003 0.003 0.003 0.003 0.003 0.002 0.004

5.17 5.49 5.48 6.13 6.48 6.61 6.68 7.92 8.43 9.62

AC C

EP

x

TE D

Table II: Structural parameters derived from x-ray diffraction pattern analysis.

16

ACCEPTED MANUSCRIPT

Table III: FTIR Band position and force constants for CoFe2-xAlxO4.

υT (cm-1)

υO(cm-1)

MT(g/mol)

MO(g/mol)

KT(N/m)

KO(N/m)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

584.33 584.33 586.25 593.97 590.11 597.82 597.82 607.47 617.11 613.25

414.62 422.33 420.21 428.12 486.69 474.4 476.33 485.97

55.84 54.00 53.23 50.22 45.72 42.92 38.78 36.04 34.62 29.83

114.77 113.73 111.61 111.73 113.34 113.26 114.52 114.37. 112.91 114.81

14.52 14.05 13.94 13.50 12.13 11.69 10.56 10.14 10.04 8.55

10.19 10.58 10.63 11.02 14.40 13.67 13.60 14.40

AC C

EP

TE D

M AN U

SC

RI PT

x

17

ACCEPTED MANUSCRIPT

Table IV: Room temperature magnetic parameters of CoFe2-xAlxO4 extracted from hysteresis

loops. K1 (106) nB (Bohr (erg/cm3) Magneton)/ f.u

802 754 701 672 630 582 554 518 476 442

3.63 3.22 3.13 2.84 2.52 2.38 1.89 1.76 1.59 1.18

18

HM (Oe)

dM/dH (emu/g.Oe) x10-2

RI PT

2183 2179 1775 1496 1468 1300 931 972 672 617

Tc (K)

1.09 0.87 0.88 0.76 0.69 0.50 0.55 0.51 0.46 0.42

1888 1906 1316 1129 1132 1136 916 1083 613 564

SC

0.61 0.59 0.58 0.59 0.51 0.54 0.53 0.50 0.42 0.39

Hc (Oe)

M AN U

53.4 46.5 44.5 41.6 32.3 33.0 26.1 23.3 17.6 12.3

Mr/Ms

TE D

86.5 77.8 76.6 70.4 63.3 60.4 48.8 46.0 42.0 31.7

Mr (emu/g)

EP

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Ms (emu/g)

AC C

x

H→0 →0

H→ →HM

1.32 1.17 1.14 1.17 1.45 1.37 1.49 1.40 1.60 1.19

3.39 2.56 3.28 3.83 2.58 3.04 3.22 2.89 2.67 2.23

ACCEPTED MANUSCRIPT

Table V: The room temperature Mössbauer parameters of CoFe2-xAlxO4. Iron

HF

Wt.

IS, δ

Wt.

∆ EQ

Wt.

Line

Wt.

Site

(kOe)

Avg.

(mm/s)

Avg.

(mm/s)

Avg.

Width,

Avg. Γ,

Area

Γ,

FWHM

(Normalized)

FWHM

(mm/s)

HF

IS

∆ EQ

(kOe)

(mm/s)

(mm/s)

Area

RI PT

x

Wt. Avg.

(mm/s)

0.3

0.4

0.5

486.84 463.67

479.44 444.19

474.79 436.37

74.16 452.96 449.32 399.04

0.2382 0.4834

467.03 426.42

452.96 407.71

-0.0410 0.2244 0.0932 -0.4299 -0.1132 0.6340 -0.0442 0.2850 0.5339 -0.4258 -0.0928 0.6442 -0.0661 0.5355 0.1746 -0.3786 -0.2638 0.6470 -0.0565 0.5626 0.2006 -0.3914 -0.2426 0.6309 -0.0556 0.4757 0.2321 -0.4722 -0.2581

0.0074 0.2314 0.4912 0.5114

-0.0410 0.1377

0.5002 0.3665 0.3665 0.3665 0.3665 0.1449

0.2503 0.4823

0.2770 0.4314

0.2718 0.4216

0.2500 0.4200

0.2314 0.4048

0.6263 -0.0979 0.4011 0.1680 19

0.5002 0.3665

SC

0.2382 0.4753 0.4338 0.6458 0.4941 0.0298 0.2503 0.4924 0.5657 0.6241 0.3365 0.0398 0.2770 0.4841 0.5253 0.6125 0.1665 0.0157 0.2718 0.5268 0.5283 0.3964 0.1659 0.0218 0.2500 0.5257 0.5685 0.6214 0.1848

M AN U

492.84 478.37

TE D

0.2

492.84 504.14 469.14 301.66 463.49 82.74 486.84 500.30 473.74 299.83 463.51 83.78 479.44 482.89 438.32 293.89 453.83 79.56 474.49 476.89 431.62 291.36 447.30 78.07 467.03 464.28 420.04 285.42 438.32

EP

0.1

A B0 B1 B2 B3 C1 A B0 B1 B2 B3 C1 A B0 B1 B2 B3 C1 A B0 B1 B2 B3 C1 A B0 B1 B2 B3 B4 C1 A B0 B1

AC C

0.0

-0.0442 0.2658

-0.0661 0.3803

-0.0565 0.4035

-0.0556 0.3398

-0.0979 0.2815

0.5279 0.3291 0.3291 0.3291 0.3291 0.1451 0.5614 0.3770 0.3770 0.3770 0.3770 0.1646 0.5935 0.4365 0.4365 0.4365 0.4365 0.1745 0.5935 0.4365 0.4365 0.4365 0.4365 0.1648 0.6279 0.6032 0.6032

0.5279 0.3291

0.5614 0.3770

0.5935 0.4365

0.5935 0.4365

0.6279 0.6032

0.7341 0.2435 0.0653 0.0321 0.0000 0.0263

0.6829 0.2265 0.0608 0.0298 0.0000 ---

0.7350 0.1348 0.0564 0.0335 0.0743 0.0203 0.7472 0.1360 0.0631 0.0374 0.0750 0.0244 0.6918 0.1314 0.0709 0.0429 0.1131 0.0243 0.5242 0.1781 0.0898 0.0596 0.1878

0.7109 0.1303 0.0545 0.0324 0.0718

0.00263 0.4761 0.2242 0.1313

--0.7058 0.1285 0.0596 0.0353 0.0708

--0.6588 0.1252 0.0675 0.0409 0.1077

--0.5043 0.1714 0.0864 0.0574 0.1806

--0.4641 0.2186 0.1280

ACCEPTED MANUSCRIPT

386.58 323.26

0.1673 0.3660

-0.1990 0.3606

0.1051 0.4009

20

-0.3163 -0.0106

0.5624 0.7408

0.0684 0.1258 0.0338 0.2688 0.3903 0.1913 0.0877 0.0964 0.0373 0.3075 0.2892 0.1866 0.1179 0.1030 0.0414 0.3122 0.2522 0.1282 0.1621 0.1432 0.0543 0.0903 0.2988 0.1527 0.2312 0.1680 0.1126

RI PT

-0.1647 0.2244

SC

406.79 343.14

0.1853 0.3638

-0.1793 0.1415

0.6032 0.6032 0.1805 0.5624 0.7408 0.7408 0.7408 0.7408 0.1910 0.6788 0.7728 0.7728 0.7728 0.7728 0.2173 0.8162 0.8001 0.8001 0.8001 0.8001 0.2867 0.5955 0.8729 0.8729 0.8729 0.8729 0.5013

M AN U

0.9

428.74 375.55

0.1596 0.3558

-0.3952 -0.4496 0.6042 -0.1793 0.2213 0.0604 -0.2991 -0.6608 0.6118 -0.1647 0.2915 0.1886 -0.2119 -0.6778 0.5921 -0.1990 0.4222 0.3487 -0.0829 -0.6138 0.5355 -0.3163 -0.0624 0.3512 -0.3539 -0.3589 0.0513

TE D

0.8

443.42 403.71

0.5463 0.0626 0.0176 0.1596 0.4114 0.4010 0.4759 -0.0682 0.0413 0.1853 0.4636 0.4198 0.4019 -0.0610 0.0387 0.1673 0.5242 0.5015 0.3419 -0.0066 0.0411 0.1051 0.5004 0.4921 0.3276 0.1945 0.2382

EP

0.7

276.51 413.88 71.06 443.42 440.39 389.68 270.02 400.36 67.66 428.74 424.13 367.81 262.99 382.04 64.55 406.79 397.24 335.43 250.64 359.47 62.55 386.58 386.53 319.00 222.92 323.04 58.85

AC C

0.6

B2 B3 C1 A B0 B1 B2 B3 C1 A B0 B1 B2 B3 C1 A B0 B1 B2 B3 C1 A B0 B1 B2 B3 C1

0.6788 0.7728

0.8162 0.8001

0.5955 0.8729

0.0666 0.1226

--0.2599 0.3773 0.1849 0.0848 0.0932

--0.3209 0.2880 0.1858 0.1174 0.1026

--0.3128 0.2527 0.1285 0.1624 0.1435

--0.0959 0.3176 0.1622 0.2457 0.1786

---

ACCEPTED MANUSCRIPT

Co2+

Al3+

Fe3+

Co2+

Al3+

Fe3+

(Ǻ)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.08 -

0.06 0.09 0.19 0.35 0.45 0.60 0.69 0.74 0.90

1.00 0.94 0.91 0.81 0.65 0.55 0.32 0.31 0.26 0.10

1.00 1.00 1.00 1.00 1.00 1.00 0.92 1.00 1.00 1.00

0.04 0.11 0.11 0.05 0.05 0.01 0.06 -

1.00 0.96 0.89 0.89 0.95 0.95 1.08 0.99 0.94 1.00

0.640 0.633 0.630 0.620 0.604 0.595 0.586 0.571 0.566 0.549

M AN U

TE D EP AC C 21

(Ǻ)

0.680 0.678 0.674 0.674 0.677 0.677 0.676 0.679 0.676 0.680

SC

x

RI PT

Table VI: The cation distributions, site radii rA and rB, and site-O2- distances for tetrahedral A and octahedral B sites for CoFe2-xAlxO4 obtained from Mössbauer spectral area analysis. Site A Site B rA rB R(A-O2-) R(B-O2-)

(Ǻ)

1.960 1.953 1.950 1.940 1.924 1.915 1.906 1.891 1.886 1.869

(Ǻ) 2.0 1.998 1.994 1.994 1.997 1.997 1.996 1.999 1.996 2.000

ACCEPTED MANUSCRIPT

8.38 8.34 8.32 8.30

35.0

35.5 36.0 36.5 2Θ Θ (degree)

x = 0.1 x = 0.2 x = 0.3 x = 0.4

EP

x = 0.5 x = 0.6

AC C 20

30

37.0

x = 0.0

TE D

Intensity (a.u)

M AN U

0.2 0.4 0.6 0.8 x, Al Content

SC

8.28 0.0

x = 0.0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5 x = 0.6 x = 0.7 x = 0.8 x = 0.9

RI PT

Intensity (a.u)

a, (Å)

8.36

x = 0.7 x = 0.8 x = 0.9 40

50

2Θ Θ (degree) Figure 1 22

60

70

ACCEPTED MANUSCRIPT

SC M AN U

10

2

3

AC C

EP

1

TE D

5

Figure 2

RI PT

-6

15x10

∗ ∗ 2 (β β /d )

y=0.002275x-1.05E-6 y=0.002343x-1.10E-6 y=0.002341x-1.03E-6 y=0.002475x-1.34E-6 y=0.002545x-1.35E-6 y=0.002570x-1.38E-6 y=0.002585x-1.29E-6 y=0.002814x-1.62E-6 y=0.002903x-1.63E-6 y=0.003102x-2.01E-6 y=0.003079x-1.81E-6

x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6 x=0.7 x=0.8 x=0.9 x=1.0

23

4 ∗ ∗2 β /d

5

-3

6x10

ACCEPTED MANUSCRIPT

584.33

584.33

422.33 414.62

SC

x = 0.3

x = 0.2

RI PT

x = 0.1

x = 0.0

593.97

586.25

M AN U

x = 0.5

x = 0.4

428.12

597.82

420.41 590.11

x = 0.7

TE D

x = 0.6

486.69

474.4 607.47

EP

597.82

AC C

x = 0.8

x = 0.9

485.97

476.33

400

613.25

617.11

500 600 700 800 400 500 600 700 800 -1 -1 Wavenumbers (cm ) Wavenumbers (cm )

Figure 3 24

ACCEPTED MANUSCRIPT

x = 0.0

80

Ms, (emu/g)

x = 0.1 x = 0.2 x = 0.3

70 60

RI PT

x = 0.4 x = 0.5

50 40 0.8

x = 0.9

M AN U

0.2 0.4 0.6 x, Al Content

0

TE D

Magnetization (emu/g)

0.0

x = 0.6 x = 0.7 x = 0.8

SC

50

Hc, (Oe)

EP

1600 1200

AC C

-50

2000

3

-15x10

-10

800 0.0 0.2 0.4 0.6 0.8 x, Al Content -5

0 Field (Oe)

Figure 4 25

5

10

15

ACCEPTED MANUSCRIPT

60

x = 0.4

RI PT

40

30

6

SC

1.0x10

0.9

20

0.8 0.7 0.6

10

0.4

0

TE D

0.5

M AN U

3

K1 (erg/cm )

Magnetization, (emu/g)

50

0.0

0.4 0.6 x, Al Constant

6 8 Field (Oe)

AC C

Figure 5

4

EP

2

0.2

26

10

0.8 12

1.0 3

14x10

ACCEPTED MANUSCRIPT

2HM

-3

20 15 10 5 0

SC

-3

x = 0.5

20

M AN U

dM/dH (emu/g.Oe)

30x10 25 15 10 5 0 -3

15

AC C

0

EP

10 5

x = 0.9

TE D

20x10

dM/dH (emu/g.Oe)

x = 0.0

RI PT

dM/dH (emu/g.Oe)

30x10 25

3

-10x10

-5

0 Field (Oe)

Figure 6

27

5

10

RI PT

ACCEPTED MANUSCRIPT

-5

0 v (mm/s)

5

10

-5

0 v (mm/s)

x = 0.50 5

10

AC C

EP

-10

TE D

a.u

M AN U

-10

SC

x = 0.00

-10

x = 0.90 -5

0 v (mm/s)

Figure 7 28

5

10

ACCEPTED MANUSCRIPT

RI PT

Wt. Avg. HFA Wt. Avg. HFB

SC

400

M AN U

Wt. Avg. HF (kOe)

450

300

0.2

0.4 0.6 x, CoFe2-xAlxO4

AC C

EP

0.0

TE D

350

Figure 8

29

0.8

ACCEPTED MANUSCRIPT

0.6

Wt. Avg. ISA Wt. Avg. ISB

RI PT

0.4

SC

0.3 0.2

M AN U

Wt. Avg. IS (mm/s)

0.5

0.1

0.0

TE D

0.0 0.2

0.4

AC C

Figure 9

EP

x, CoFe2-xAlxO4

30

0.6

0.8

ACCEPTED MANUSCRIPT

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1.

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2.

M. Rajendran, R. C. Pullar, A. K. Bhattacharya, D. Das, S. N. Chintalapudi, C. K.

3.

RI PT

Majumdar, J. Magn. Magn. Mater. 232, 71 (2001).

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5.

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SC

4.

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7.

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8.

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9.

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M AN U

6.

Gajbhiye, J. Alloys and Compd. 485, 798 (2009). 10.

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11.

TE D

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EP

12.

T. Abraham, Am. Ceram. Soc. Bull. 73, 62 (1994).

14.

P. I. Slick, in: E.P. Wohlfrath (Ed.), Ferromagnetic Materials, Vol. 2, North-Holland,

AC C

13.

Amsterdam, 1980, p. 196.

15.

S. Chikazumi and S. H. Charap, Physics of Magnetism (New York: John Wiley & Sons) (1964) p. 296

16.

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17.

M. El-Shabasy, J. Magn. Magn. Mater. 172, 188 (1997).

18.

P. Raj and S. K. Kulshreshtha, J. Phys. Chem. Solids, 31, 9 (1070).

31

ACCEPTED MANUSCRIPT

19.

T. Raghavender, D. Pajic, K. Zadro, T. Milekovic, P. Venkateshwar Rao, K.M. Jadhav, D. Ravinde, J. Magn. Magn. Mater. 316, 1 (2007). L. Wang, B.K. Rai, S.R. Mishra, Mater. Res, Bull. 65, 183 (2015).

21

R. S. Tebble and D. J. Craik, Magnetic Materials (London: John Wiley and Sons) (1969) p. 252.

RI PT

20

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(1) Grain refinement with Al3+ doping (2) Increased magnetic softness with Al3+ content (3) Reduction in Curie temperature with Al3+ content (4) Detailed Mossbauer analysis to determine Al3+ site occupancy (5) Al3+ shows preference for tetrahedral site.

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Influence of Al3+ doping on structural and magnetic properties of CoFe2-xAlxO4 Ferrite nanoparticles