CC BY
50
Study of magnetic and optical transitions in MFe2O4 (M=Co, Zn, Fe, Mn)
with spinel structure
Nitika1*, Anu Rana1, Vinod Kumar2 department of Physics, SRM University, Delhi NCR, Sonepat, 131029, India 2Department of Physics, NSUT, Dwarka, New Delhi, 110078, India *nitika270593@gmail.com
DOI 10.17586/2220-8054-2021-12-4-481-491
Spinel ferrite (MFe2O4) nanoparticles were successfully synthesized by the coprecipitation method. X-ray diffraction technique was employed for structural analysis. Single-phase cubic spinel structure with an average crystallite size ranging from 5-20 nm was obtained for the prepared ferrites. The Fourier transform infrared spectra exhibits an absorption band at 550 cm-1, which is attributed to metal-oxygen bond vibrations at tetrahedral sites. The thermogravimetric analysis revealed the instability of MnFe2O4 and Fe3O4 above 500 °C whereas CoFe2O4 is found to be the most stable ferrite. The hysteresis parameters demonstrate the superparamagnetic nature of the prepared nanoparticles with low coercivity except for CoFe2O4. The direct optical band gap energy derived from UV-visible spectra is calculated to be 2.82, 2.83, 2.81, and 2.44 eV for M=Co, Zn, Fe, and Mn respectively. The magnetic and optical properties show a strong dependence on cation site occupancy. Keywords: Spinel ferrites, Fourier transform infrared spectroscopy, thermo-gravimetric analysis, hysteresis curve, optical properties. Received: 10 May 2021 Revised: 27 July 2021
1. Introduction
Magnetic nanocrystalline bimetallic iron-based oxides are technologically important due to an excellent combination of structural, magnetic, and electrical properties. The transition metal oxides with formula MFe2O4, also termed as magnetic spinels, have attracted research interest in recent years attributable to their significant stability, ease of synthesis, high magnetization, low toxicity, small coercivity, less remanent magnetization, and high electrical resistance. The tunable magnetic and electrical properties of ferrites have turned the prime focus of researchers towards their potential use in biomedical and electronic applications [1-11]. The unique physicochemical properties are dependent on the structure of the crystal. The occupancy of metal cations on both sites is governed by electrostatic, elastic, and crystal field stabilization energy. Following the cation distribution, ferrites can be classified into normal, inverse, and mixed spinel. In normal spinel, the divalent cations occupy tetrahedral sites (A-sites), whereas the trivalent cations occupy octahedral sites (B-sites). On the contrary, in inverse spinel structure, divalent cations occupy B-sites, while trivalent cations occupy the A-sites. In mixed spinels, the site occupancy is intermediate to that of normal and inverse spinel structure [12-15]. Among ferrites, CoFe2O4, ZnFe2O4, Fe3O4, andMnFe2O4 nanoparticles have drawn appreciable attention due to their wide application range taking into consideration their magnificent properties. CoFe2O4 and Fe3O4 possess inverse spinel structures; ZnFe2O4 forms normal spinel ferrite, whereas MnFe2O4 represents mixed spinel ferrite with 80% predominantly inverse structure. Although significant research has been done which reports the physicochemical properties of all the above ferrites, a systematic and thorough investigation is imperative to study and compare the properties of transition metal nano-ferrites [16-22].
Naseri et al. [20] reviewed ZnFe2O4, MnFe2O4, and CoFe2O4 nanoparticles prepared by the thermal treatment method. The average particle size of prepared ferrites ranged from 15 nm to 21 nm with very low saturation magnetization (2-5 emu/g). The saturation magnetization values reported are not sufficient for their use in electronic devices and are not in agreement with previous reports [23-28]. Kolhatkar et al. [29] presented a review of magnetic nanoparticles focusing on magnetic properties. Pandervand et al. [30] studied XFe2O4 (X=Mn, Fe, Ni, Co, and Zn) nanoparticles prepared by coprecipitation and hydrothermal methods. The average particle size varied from 21 -30 nm. The paper lacks any detailed discussion of magnetic behavior and its relation to the structure. Banerjee et al. [31] synthesized polyethylene glycol (PEG) coated transition metal ferrites and compared the observed properties. PEG coating increases saturation magnetization but is undesirable due to its nonbiodegradability and holds several drawbacks in the biomedical field [32, 33]. Moreover, insufficient literature exists highlighting the comparative study of optical and thermal properties. The properties of ferrites at the nanoscale are not well defined due to their dynamic behavior. Replication of experimental study is required to observe the patterns and trends in the behavior of the material and to conclude the findings based on sufficient evidence.
Therefore, in the present work, we investigate the properties of four different magnetic nanoparticles with varying spinel structures synthesized by the co-precipitation technique, which is a simple and cost-effective method with an eco-friendly route. The structural, magnetic, optical, and thermal properties of the obtained ferrites are studied and compared for a better understanding of structure-property relation from a specific application based-perspective.
2. Materials and methods
2.1. Synthesis method
Cobalt ferrite, zinc ferrite, ferrous ferrite, and manganese ferrite nanoparticles were prepared using the co-precipitation technique. Analytical grade chemical reagents used for preparation were FeSO47H2O, Fe(NO3)3-9H2O, Zn(NO3)2 6H2O MnCl2 4H2O, and CoCl2 6H2O. Separate homogeneous aqueous solutions of two precursors including Fe(NO3)3 9H2O were prepared in distilled water with continuous stirring. The solutions were mixed, keeping the resultant solution at 70 °C. Surfactant (oleic acid) was added to avoid agglomeration. After 30 min, aqueous NH3 was added drop-wise with uninterrupted stirring to attain pH 11. The precipitates so formed were finally dried at 80 °C on a hot plate followed by crushing using agate mortar. Further, the prepared samples were annealed at 300 °C for 2 hrs to improve their crystalline properties. The physicochemical properties of obtained samples were studied using various characterization techniques.
2.2. Characterization tools
Powder X-ray diffraction measurements were performed in the 26 range of 20-70 ° with a scan rate of 5 °/min using Cu-Ka radiation (A = 1.5406 A). Thermogravimetric analysis (TGA) was carried out to study mass loss at a heating rate of 10 °C/min by varying temperature from room temperature to 800 °C in an aerobic atmosphere. FTIR spectra were recorded using a Perkin Elmer Frontier FTIR spectrophotometer in the range of 4000 - 450 cm-1. Magnetic measurements were recorded using a Vibrating sample magnetometer (Lakeshore model 7400) at room temperature within the range of ±1.5 T. The absorption spectra are obtained using an Ultraviolet spectrophotometer by Perkin Elmer (LAMBDA 950) in the range 200 - 1600 nm.
3. Results and Discussion 3.1. Structural analysis
Figure 1 shows the XRD pattern of CoFe2O4 (CNP), ZnFe2O4 (ZNP), Fe3O4 (FNP) and MnFe2O4 (MNP). The formation of a single-phase spinel cubic structure with an Fd3m space group for all the samples is confirmed by the diffraction peaks (JCPDS #22-1086, #89-1010, #19-0629, #73-1964). The average crystallite size is calculated using the most intense peak belonging to the (311) plane by employing the Scherrer formula (1).
(440)
MNP <220> <400> <511>
(311)
g ZNP
20 25 30 35 40 45 50 55 60 65 70 26(deg)
FIG. 1. XRD pattern of CoFe2O4 (CNP), ZnFe2O4 (ZNP), Fe3O4 (FNP), MnFe2O4 (MNP) nanoparticles
Table 1. Average crystallite size, lattice parameter, X-ray density, specific surface area, microstrain and dislocation density for CNP, ZNP, FNP, and MNP
Average Lattice X-ray Specific Micro- Dislocation
Sample crystallite size Parameter Density surface area strain Density pD
D (nm) a(A) Px (g/cc) S (m2/g) e (1015 m-2)
CNP 11.35 8.381 5.29 99.85 0.003 4.82
ZNP 5.89 8.411 5.38 189.28 0.006 17.81
FNP 12.54 8.337 5.31 90.14 0.003 3.97
MNP 19.86 8.334 5.29 57.08 0.002 1.58
table 2. Interionic distances and hopping lengths at A and B sites for CNP, ZNP, FNP and MNP
Sample dAL(A) dBL(A) dAE (A) dBE (A) dBEU (A) La(A) Lb (A)
CNP 1.8871 2.0542 3.0817 2.8446 2.9643 3.6291 2.9631
ZNP 1.8939 2.0616 3.0927 2.8548 2.9749 3.6421 2.9737
FNP 1.8772 2.0434 3.0655 2.8297 2.9488 3.6100 2.9476
MNP 1.8765 2.0427 3.0644 2.8287 2.9477 3.6087 2.9465
D = A, «)
p cos 0
where D is the average crystallite size, K is Scherrer constant (0.9 for spherical crystallites with Cubic symmetry), A is X-ray wavelength (1.5406 A), p is FWHM of XRD peak in radians, 0 is the diffraction angle. The lattice parameter (a) is calculated using (2) [34]:
a = d^h2 + k2 + l2, (2)
where d is interplanar spacing and h, k, l is the Miller indices. All the elementary information like X-ray density px, specific surface area (S), micro-strain (e), and dislocation density pD can be retrieved from the XRD pattern using the below-mentioned formulas (3-6):
ZM (3)
px = N03 ' (3)
S =A, (4)
PxD
ß cos 0 4 '
(5)
15e (6)
PD = *D' (6)
where Z, M, N, ft, and 0 denote the number of atoms per unit cell, molecular weight, Avogadro's number, full width at half maximum, and diffraction angle respectively. The values obtained are tabulated in Table 1. Considering the preferred spinel structure, site occupancy, crystal field stabilization energy, and coordination number (CN), the ionic radii of all the cations can be chosen from the database of ionic radii [35]. The estimated ionic radii are 0.785 A for Fe3+ ions at B sites, 0.63 A for Fe3+ ions at A sites, 0.88 A for Co2+ ions (as it occupies B sites with CN 6 being in the high spin state), 0.74 A for Zn2+ ions (as it occupies A sites with CN 4 being in the high spin state), 0.92 A for Fe2+ ions (as it occupies B sites with CN 6 being in the high spin state) and 0.80 A for Mn2+ ions at A sites and 0.97 A for Mn2+ ions at B sites [36-38]. The hopping lengths (LA and LB), interionic distances (i.e., M-O bond length at A site (dAL), M-O bond length at B site (dBL), tetrahedral and octahedral shared edge length (dAE and dBE), and unshared octahedral edge length (dBEU) are calculated using the relations 7-13 [39-42].
La = V3a/4, (7)
e
Lb = v^a/4, (8)
dAL = aV3(w - 0.25), (9)
r " 11U 43"
dBL = a^(3u2 - — + -), (10)
dAE = aV2(2u - 0.5), (11)
dBE = aV2(1 - 2u), (12)
dBEU = a^J4u2 - 3u + 66, (13)
where u is the oxygen positional parameter. The calculated values are listed in Table 2. From the variation observed in the interionic distances and hopping lengths, it can be inferred that the lattice parameter is strongly influenced by the distances between cations and anions. In the present study, all the cases possess different spinel structures; hence the lattice parameter cannot be compared. Although a cation distribution can be proposed by correlating the lattice parameter and the magnetic measurements which depend on the cations site occupancy. Former studies reveal that effective magnetism is observed in inverse and mixed spinel structures where A-B interactions are dominant. The average crystallite size, lattice parameter, and magnetic moments of the cations determine the magnetic properties of spinel ferrites [1].
3.2. Thermogravimetric analysis
Mass loss of prepared samples (TGA) and the first derivative of TGA (DTG) as a function of temperature are shown in Fig. 2. The thermal decomposition of CNP and ZNP takes place in two stages, whereas that of FNP and MNP takes place in three stages. The initial weight loss up to 250 °C is due to the loss of adsorbed water. This weight loss is approximately 6%, 2%, 6.6%, and 12% for CNP, ZNP, FNP, and MNP respectively which indicates surface water adsorption is highest for MNP and least for ZNP. The second mass loss up to 550 °C is due to the evaporation of surfactant used in the synthesis process. The boiling point of oleic acid used in the synthesis is 360 ° C. Approximately 40%, 57%, 35%, and 34.8% mass loss is observed in this range corresponding to CNP, ZNP, FNP, and MNP respectively. This major loss indicates that nanoparticles prepared with the coprecipitation technique contained a large amount of organic compound. It can be inferred in the synthesis process, organic content increased in the order MNP<FNP<CNP<ZNP with a corresponding decrease in particle size. This phenomenon can be attributed to the increase in specific surface area in the order MNP<FNP<CNP<ZNP. The same trend has been reported and explained previously [43-45]. The third mass loss of 4% at 600 °C for FNP is due to the formation of secondary phases (a-Fe2O3 and Y-Fe2O3). The formation of these phases at higher temperatures is favorable due to its thermal stability above 570 °C [46]. A similar minor mass loss of 0.5% above 700 °C for MNP indicates the partial decomposition of MnFe2O4 to a-Mn2O3 and a-Fe2O3 [47,48]. This signifies an increase in impurity phases in MNP at a higher temperature. As the temperature reaches 800 °C, the remaining mass% values of the samples were 51.6%, 42.2%, 52.3%, and 49.1% for CNP, ZNP, FNP, and MNP respectively. The analysis of all the TGA/DTG curves suggests thermal stability in the order CNP>ZNP>MNP>FNP.
3.3. Fourier transform infrared spectroscopy analysis
Figure 3 shows FTIR spectra of CNP, ZNP, FNP, and MNP respectively. The absorption bands between 3600 -3300,1700- 1500 and 1000- 1200 cm-1 correspond to stretching and bending of H2O molecules [49]. Low-intensity peaks are observed in the above-mentioned regions corresponding to ZNP indicating less surface water adsorption by the particles. Although the peaks for ZNP belonging to range 2800 - 2900 cm-1 are more intense, indicating the presence of organic groups more than others. Oleic acid inhibits grain growth by binding the surface molecules which in turn induces strain [50]. The same has been proved in XRD and TGA analysis for ZNP. The peaks belonging to region 1300 - 1500 cm-1 are attributed to the stretching and bending of the COO- group [51]. The presence of absorption band at frequency ^550 cm-1 for all the samples is attributed to the intrinsic vibrations of the M-O bonds (M=Co, Zn, Fe, Mn) at tetrahedral sites which is the characteristic feature confirming the formation of spinel ferrite structures [52]. It is also observed that wavenumber corresponding to tetrahedral vibrations increases in the order FNP<ZNP<MNP<CNP respectively. The same trend has been observed in various reports [53]. This phenomenon particularly depends on the cation distribution, cation mass, and the ionic radii of the divalent cations.
FIG. 2. TGA-DTG curve for (a) CNP (b) ZNP (c) FNP (d) MNP TABLE 3. Magnetic parameters for CNP, ZNP, FNP, and MNP
Samples MS (emu/g) He (Oe) Mfl(emu/g) R K(erg/Oe) 'IB (pb )
CNP 35.55 595.34 8.98 0.253 21593.22 1.493
ZNP 15.29 51.43 0.41 0.027 802.34 0.660
FNP 26.04 15.62 0.44 0.017 415.10 1.079
MNP 14.94 97.88 1.65 0.111 1492.59 0.617
3.4. Magnetic properties
Fig. 4 shows the magnetization curves for CNP, ZNP, FNP, and MNP recorded at room temperature. The values of saturation magnetization (MS), coercivity (HC), retentivity (MR), squareness ratio (R), magnetic anisotropy constant (K), and magnetic moment (nB) are tabulated in Table 3 [54]. The relation between magnetic anisotropy constant, squareness ratio, and magnetic moment is given below:
HC MS
K = C S, (14)
0.98 ' v y
R = Mr (15)
R = MS, (15)
(M x Ms)
nB = 5585 . (16)
The variation of MS follows the order CNP>FNP>ZNP>MNP. However, a very small difference exists between MS of MNP and ZNP. Saturation magnetization is mainly governed by cation distribution and average crystallite size. Magnetic moments corresponding to different cations belonging to high spin state follows the order of Mn2+=Fe3+ (5.92 pb )>Fe2+(4.90 pb )>Co2+(3.87 pb )>Zn2+(0 pb ) [55,56]. The variation of Ms can be explained
0-1-1-1-1-1-1-1-^ o -|-,-,-,-,-,-,-fj
4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm"1) Wavenumber (cm1)
FIG. 3. FTIR Spectra for (a) CNP (b) ZNP (c) FNP (d) MNP
-1--1-
•16000 -8000 0 8000 16000 Applied field (Oe)
FIG. 4. Magnetic hysteresis curves for CNP, ZNP, FNP, and MNP
efficiently by taking into consideration their respective spinel structure and net magnetic moment. The net magnetization per formula unit is calculated using Neel's model where nnet is the difference between net magnetization at B-sites and net magnetization at A-sites [57]. CNP belongs to the inverse spinel structure, with a majority of Co2+ ions and Fe3+ ions occupying B-sites. This increases the net magnetic moment which is clear from the data in the table. FNP also belongs to inverse spinel structure, due to which net magnetization of B-sites is greater than that of A-sites, thereby yielding a high magnetic moment. A slight variation of the net magnetic moment for both the ferrites is due to the inversion degree. The fraction of Co2+ ions occupying B-sites in CNP is greater than that of Fe2+ ions in FNP. ZNP belongs to a normal spinel structure in which a maximum of non-magnetic Zn2+ ions occupy the A-sites. A small magnetic moment of 0.6 agrees well with previous reports, which is correlated with a low fraction of Fe3+ ions in A-sites [12]. MNP belongs to a mixed spinel structure in which Mn2+ ions get distributed between both sites. Moreover, the magnetic moments of Mn2+ and Fe3+ are equal, resulting in very small magnetic moments. Small values of coercivity and retentivity for all the prepared ferrites except CNP indicate superparamagnetic nature. However, CNP exhibits ferromagnetic behavior. CNP possessed the highest coercivity with a highly anisotropic structure. The values of MR, HC, and K follow the same variation as MS. The squareness ratio termed as Stoner-Wohlfarth value is less than 0.5 signifying uniaxial anisotropy influenced by lattice strain [58].
3.5. Optical properties
Figure 5 shows the absorption spectra of CNP, ZNP, FNP, and MNP demonstrating the absorption in the visible and near-infrared region. The absorption band of MNP shows a shoulder-like structure attributed to the absorption taking place due to surface states [59].
FIG. 5. Absorption spectra for CNP, ZNP, FNP, and MNP
The absorption spectrum is obtained due to the transition of the electrons from the valence band of O-2p to the conduction band of Fe-3d. The absorption in the visible region is highest in the case of MNP whereas it is least in the case of CNP. The optical band gap for direct transitions can be calculated using the Tauc plots by employing (17) (Fig. 6) [60]: _
ahv = A^(hv - Eg), (17)
where a is the absorption coefficient, hv is the energy of the photon, Eg is the optical band gap and A is the proportionality constant.
The obtained values of optical band gaps are tabulated in Table 4. The optical bandgaps of prepared spinel ferrites varied from 2.44 - 2.83 eV. The tuning of bandgap signifies the potential of bandgap engineering in spinel ferrites. The variation observed in the optical band gap is explained by correlating it with average crystallite size. A decrease in average crystallite size increases the binding energy of valence electrons and parent atoms, which increases the bandgap energy [61]. The trend verifies Brass's effective mass model. The optical band gap is also influenced by the site occupancy of transition metal cation [56]. The spinel ferrites possess good absorption properties in the visible region which makes them suitable for optoelectronic devices. The study of optical properties is quite useful for photocatalytic reactions.
Fig. 6. Plot of (ahv)2 as a function of photon energy (eV) for CNP, ZNP, FNP, and MNP TABLE 4. Optical bandgaps for CNP, ZNP, FNP, and MNP
Sample Optical band gap (eV) Refractive index (n) Real dielectric constant (e') Imaginary dielectric constant (e'')
CNP 2.82 2.40 3.97 6.47
ZNP 2.83 2.40 3.66 6.96
FNP 2.81 2.41 3.96 6.52
MNP 2.44 2.53 3.41 8.80
The Ravindra empirical relation can be employed to calculate optical parameters such as the refractive index (n), real and imaginary part of dielectric constant (e' and e'') [62,63]. The values obtained using the below relations (1821) are shown in Table 4:
n = A + (B x Eg), (18)
k = t (19)
e' = n2 - k2, (20)
e'' = 2 x n x k, (21)
where A = 4.05 eV-1, B = -0.62 eV-1, k is the extinction coefficient, a is the absorption coefficient and A is the wavelength calculated from bandgap energy. The high value of the refractive index for MNP makes it suitable for antireflection coatings. It can be inferred that optical conductivity strongly depends on the absorption coefficient [64].
4. Conclusion
The physicochemical properties of spinel ferrites entirely depend on the crystal structure and cation distribution. In the present study, four different spinel ferrites belonging to normal, inverse, and mixed spinel structure were successfully synthesized using the chemical co-precipitation technique. XRD confirmed the formation of single-phase cubic spinel structures with average crystallite sizes of 11 nm, 6 nm, 13 nm, and 20 nm for CoFe2O4, ZnFe2O4, Fe3O4, and MnFe2O4 respectively. The lattice parameter ranged from 8.334 - 8.411 A. The specific surface area was highest for ZnFe2O4 and least for MnFe2O4 nanoparticles. TGA/DTG curves illustrated thermal stability in the order CoFe2O4 >ZnFe2O4 >MnFe2O4 >Fe3O4. The fundamental peak attributed to tetrahedral M-O bond vibrations was obtained around 550 cm-1 confirming the formation of spinel ferrites. All the ferrites exhibited superparamagnetic behavior except CoFe2O4. CoFe2O4 showed ferromagnetic behavior with high magnetic coercivity and anisotropy. The saturation magnetization and coercivity ranged from 14.94 emu/g to 35.55 emu/g and 15.62 - 595.34 Oe respectively. A strong dependence of optical properties on average crystallite size was observed. MnFe2O4 has the highest refractive index and imaginary dielectric constant but the least real dielectric constant. Optical conductivity depends on the absorption coefficient. The study of optical and magnetic properties is useful in determining the applications of the prepared ferrites in electrical and magnetic devices.
Acknowledgement
The author(s) received no financial support for the research, authorship, and/or publication of this article. References
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