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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.28 No.7 pp.371-375
DOI : https://doi.org/10.3740/MRSK.2018.28.7.371

The Properties of Mn, Ni, and Al Doped Cobalt Ferrites Grown by Sol-Gel Method

Seung Han Choi
Department of Biomedical Engineering, Daegu Haany University, Gyeongsan 38610, Republic of Korea
Corresponding author
E-Mail : ccsshh@dhu.ac.kr (S. H. Choi, Daegu Haany Univ.)
May 15, 2018 June 26, 2018 June 26, 2018

Abstract


The manganese-, nickel-, and aluminum-doped cobalt ferrite powders, Mn0.2Co0.8Fe2O4, Ni0.2Co0.8Fe2O4, and Al0.2CoFe1.8O4, are fabricated by the sol-gel method, and the crystallographic and magnetic properties of the powders are studied in comparison with those of CoFe2O4. All the ferrite powders are nano-sized and have a single spinel structure with the lattice constant increasing in Mn0.2Co0.8Fe2O4 but decreasing in Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4. All the Mössbauer spectra are fitted as a superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the Fe3+ ions. The values of the magnetic hyperfine fields of Ni0.2Co0.8Fe2O4 are somewhat increased in the A and B sites, while those of Mn0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 are decreased. The variation of Mössbauer parameters is explained using the cation distribution equation, superexchange interaction and particle size. The hysteresis curves of the ferrite powders reveal a typical soft ferrite pattern. The variation in the values of saturation magnetization and coercivity are explained in terms of the site distributions, particle sizes and the spin magnetic moments of the doped ions.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1 Introduction

    Ferrites have been used to fabricate components in a wide variety of electromagnetic devices, including microwave and magnetic recorders. Ferrites have a spinel crystal structure(space group Fd3m) in which the O2− ion sublattice forms areas with tetrahedral and octahedral local symmetry, referred to as A and B sites, respectively. Co ferrite, CoFe2O4, is basically an inverse Fe+3 spinel, with which a corrected cation distribution becomes (Co0.1- Fe0.9)[Co0.9Fe1.1]O4. The degree of inversion depends on the thermal history. Cobalt ferrite is a well known hard magnetic material that has been studied in detail because of high coercivity(5.40 kOe) and moderate saturation magnetization(≈ 80 emu/g), as well as its remarkable chemical stability and mechanical hardness.1-2) When manganese ferrite, MnFe2O4, is prepared at high temperatures(> 1,173 K), 20 % of the Mn2+ ions are known to migrate from the A to the B sites, implying that MnFe2O4 can be characterized as a mixture of normal and inverse spinel ferrite. Manganese ferrite has been widely used in microwave and magnetic recording applications.1) Nickel ferrite, NiFe2O4, has an inverse spinel structure in which the A sites are occupied by Fe3+ ions and the B sites by Fe3+ and Ni2+ ions. Ni-Zn ferrite has a mixed spinel structure in which the A sites are occupied by Zn2+ and Fe3+ and the B sites by Ni2+ and Fe3+ ions.3-4) Aluminum substituted cobalt ferrite is a soft ferrite with a low magnetic coercivity and high resistivity, thus it is also an excellent core material for power transformers in electric and telecommunication applications.5-6) Several approaches have been exploited to obtain more usable magnetic materials, including adjusting the substitution ions and ratio through the use of various synthesis methods. The sol-gel method is known as a technique for the low temperature synthesis of glass, ceramics, and other materials, using the dip coating solution or spin coating for thin film.7-8) One of the advantages of using the sol-gel method is the lower annealing temperature that enables smaller grained powders to be grown. These ultrafine cobalt ferrite particles have been intensively investigated. The sol-gel method can provide multi-component oxide with a homogeneous composition, and has been employed to prepare many high purity oxide powders and film, including some products with spinel-type structures.

    To our knowledge, there have been few detailed studies on the Mn, Ni and Al doped cobalt ferrites synthesized by the sol-gel method. In this study, using the sol-gel method, we synthesized Mn, Ni and Al doped cobalt ferrite powders and studied the crystallographic and magnetic properties by means of X-ray diffractometry, field emission scanning electron microscopy(FESEM), Mössbauer spectroscopy, and vibrating sample magnetometry(VSM).

    2 Experimental procedure

    The CoFe2O4, Mn0.2Co0.8Fe2O4, Ni0.2Co0.8Fe2O4, and Al0.2CoFe1.8O4 were synthesized by the sol-gel method. Measured amounts of Co(CH3CO2)2·4H2O, Ni(NO3)2· 6H2O, Mn(NO3)2·H2O, Ni(NO3)2·6H2O, [(CH3)2CHO]3Al and Fe(NO3)·9H2O were first dissolved in 2-Methoxyethanol with an ultrasonic cleaner for 30-50 min. The solution was refluxed at 353 K for 12 h in order to gel, and was dried at 363 K in a dry oven for 24 h. The dried powder samples were ground and annealed at 773 K for 6 h, and all the heat-treatment processes were carried out in an N2 atmosphere to prevent oxidation of the anion ions. A large amount of N2 gas flowed early in the annealing process to remove oxygen inside a quartz tube and emitting the gas from a dry powder. In order to verify the purity, all samples were analyzed using an Xray diffractometer with CuKα (1.54 Å) radiation. The surface microstructure was observed using FESEM at room temperature, and the Mössbauer spectra of the powders were obtained with a 57Co source in a constant acceleration mode to identify the magnetic phase of the ferrite powders. The saturation magnetization and the coercivity were then determined using vibrating sample magnetometry(VSM).

    3 Results and Discussion

    The X-ray diffraction patterns of the CoFe2O4, Mn0.2Co0.8Fe2O4, Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 ferrite powders are shown in Fig. 1. The X-ray diffraction measurements show that all peaks are consistent with those of typical spinel structures of ferrite powders, and no extra peaks corresponding to any secondary phase were observed. As shown in Table 1, the nickel and aluminum doped ferrites show that the lattice constants decrease from 0.835 nm for CoFe2O4 to 0.834 nm for Ni0.2Co0.8Fe2O4, and to 0.832 nm for Al0.2CoFe1.8O4, respectively, but the lattice constant of Mn0.2Co0.8Fe2O4 increases to 0.843 nm. These can be explained using Vegard’s law, which means that the Co2+(0.074 nm) ion is substituted by smaller Ni2+(0.069 nm) and Al3+(0.0511 nm) ions leading to a decrease in the lattice constant, but substituted by larger Mn2+(0.091 nm) ions, leading to an increase in the lattice constant. The size of the particles was then determined from the diffraction peak broadening with the use of the Scherrer equation,9)t = (0.9λ)/(BcosθB), where λ represents the X-ray wavelength, B is the half width of the (311) peak, and θB is the angle of the (311) peak. As shown in Table 1, the particle size for Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 decreased, whereas that of Mn0.2Co0.8Fe2O4 increased. These dopings lead to a broadening of the major peak, that is, a growth of the smaller particle size of the spinel powders as well as to an improved crystallization. The average particle size decreases from 25.9 nm for CoFe2O4 to 25.0 nm for Ni0.2Co0.8Fe2O4 and to 24.5 nm for Al0.2CoFe1.8O4, but for Mn0.2Co0.8Fe2O4, it increases to 26.0 nm. All these suggest that the particle size of the Ni-doped, Mn-doped, and Al-doped cobalt ferrites powder obtained by the solgel method are from 24.0 nm to 26.0 nm, which are smaller in size than those of the powders obtained using ceramic and wet chemical methods.10) As shown in Fig. 2, the shape of the grains of the ferrites can be confirmed by FESEM as 100,000 magnifications. All samples have nano-sized grains of a nearly homogeneous in size, and the grain size decreases by the nickel and aluminum doping, while that of manganese doped ferrite slightly increases.12)

    The Mössbauer absorption spectra that were measured at room temperature for CoFe2O4, Mn0.2Co0.8Fe2O4, Ni0.2- Co0.8Fe2O4 and Al0.2CoFe1.8O4 ferrite powders are shown in Fig. 3(a)~(d) respectively. All the spectra are fitted with two six-line subspectra that are assigned to a tetrahedral A site and octahedral B sites of a typical spinel structure. As shown in Table 2, the values of the quadruple splitting(QS) and isomer shift(IS) are almost unchanged with manganese, nickel, and aluminum doping. The magnetic hyperfine fields(Hhf) of Ni0.2Co0.8Fe2O4 are increased somewhat, in the A and B sites, but those of Mn0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 are decreased at the same sites. The variation in the values of the magnetic hyperfine fields can be explained by using the cation distribution. The cation distribution depends on a number of factors, such as the temperature, pressure, and composition, 11-15) as well as the compound preparation method. The Mössbauer absorption area ratio of the A and B sites, and the occupation preference of Mn, Ni and Al ions in a spinel structure, can both be used to determine the cation distribution as:

    Eq1.gif

    Using these cation distribution equations, we can explain the superexchange interaction being stronger with the nickel doping. The increase in nickel ions results in a higher magnetic moment for the Fe3+ ions(5 μB) at the A sites, so the A-O-B superexchange becomes stronger in Ni0.2Co0.8Fe2O4 even though the lower magnetic moment Co2+(3 μB), Fe3+(5 μB), Mn2+(5 μB), Ni2+(2 μB) and Al3+(0 μB) ion distribution are somewhat changed.

    The magnetic properties of the CoFe2O4, Mn0.2Co0.8-Fe2O4, Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 can be determined at room temperature by means of vibrating sample magnetometry(VSM). Fig. 4 shows the hysteresis curve, and Table 3 shows the changes in the saturation magnetization(MS) and coercivity(HC). In Mn0.2Co0.8Fe2O4, the value for MS increased to 98.2 emu/g and HC also increased to 1,521 Oe than pure CoFe2O4. These changes can be explained by considering the differences in the site distributions and in the spin magnetic moments of the substituted ions. The doping of a lesser magnetic moment(3 μB for Co2+) ions by a greater magnetic moments( 5 μB for Mn2+ and Fe3+) ions could be expected to increase the saturation magnetization and coercivity. The XRD and FESEM results also show that the particle size increased with manganese ion doping, and these could have led to an increase in the saturation magnetization and coercivity. Similar reports have been presented for Co0.5Mn0.5Fe2O4 nanoparticles.14) The values of Ms decreased from 85.38 emu/g for CoFe2O4 to 72.21 emu/g for Ni0.2Co0.8Fe2O4, and to 48.5 emu/g for Al0.2CoFe1.8O4. The decrement of them can thus be explained by a smaller magnetic moment for Ni2+(2 μB) ions and Al3+(0 μB) ions, as compared to a larger magnetic moment of Co2+(3 μB) ions. From the cation distribution equation, the Fe3+(5 μB) ions distributed between the A and B sites remain almost unchanged, so the Ms values of the Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 ferrites should decrease, by nickel and aluminum doping. The HC values also rapidly decrease from 1,480.0 Oe for CoFe2O4 to 1,255.0 Oe for Ni0.2- Co0.8Fe2O4, and 910 Oe for Al0.2CoFe1.8O4. The coercivity in polycrystalline ferrites is well known to be strongly dependent on the magneto-crystalline anisotropy constant, and the grain size. In Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 ferrite powders, the grain size does not change abruptly, so the main effect on the coercivity decrease may be a lower magneto-crystalline anisotropy constant for Ni2+ and Al3+ ions, compared to that of Co2+ and Fe3+. These results indicate that the nickel and aluminum doped cobalt ferrites show lower saturation magnetization and coercivity than pure cobalt ferrite powders.

    4 Conclusion

    Manganese, nickel, and aluminum doped nano-sized cobalt ferrites, CoFe2O4, Mn0.2Co0.8Fe2O4, Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4, were fabricated by the sol-gel method, and their crystallographic and magnetic properties were compared. All ferrite powders had a single spinel structure and the lattice constants increased in Mn0.2Co0.8Fe2-O4, but decreased in Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 ferrites, whereas the particle size also increased in Mn0.2-Co0.8Fe2O4 ferrite but decreased in Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 ferrites. For all the CoFe2O4, Mn0.2Co0.8-Fe2O4, Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8Fe2O4 ferrites, the Mössbauer spectra could be fitted as a superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the Fe3+ ions. The values of the quadruple splitting(QS) and isomer shift(IS) are almost unchanged with manganese, nickel, and aluminum doping. The magnetic hyperfine fields(Hhf) of Ni0.2Co0.8Fe2O4 are increased somewhat in the A and B sites, but those of Mn0.2Co0.8-Fe2O4 and Al0.2CoFe1.8O4 are decreased at the same sites. The area ratio of the Mössbauer spectra can be used to decide the cation distribution as

    (Co0.28Fe0.72)[Co0.72Fe1.28]O4 for CoFe2O4, (Mn0.1Co0.22Fe0.68.)[Mn0.1Co0.58Fe1.32]O4 for Mn0.2Co0.8Fe2O4, (Co0.22Fe0.78)[Ni0.20Co0.58Fe1.22]O4 for Ni0.2Co0.8Fe2O4, (Co0.28Fe0.64Al0.08) [Co0.72Fe1.16Al0.12]O4 for Al0.2CoFe1.8O4. We explained the variation of Mössbauer parameters using this cation distribution equation, superexchange interaction and particle size.

    The hysteresis curves of the CoFe2O4, Mn0.2Co0.8Fe2O4, Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 ferrites revealed a typical soft ferrite pattern. In Mn0.2Co0.8Fe2O4 ferrite, for MS increases to 98.2 emu/g and HC also increases to 1,521 Oe compared to pure CoFe2O4. The value of MS decreased from 85.38 emu/g for CoFe2O4 to 72.21 emu/g for Ni0.2Co0.8Fe2O4, and to 48.5 emu/g for Al0.2CoFe1.8O4. The HC value also rapidly decreases from 1,480 Oe for CoFe2O4 to 1,255 Oe for Ni0.2Co0.8Fe2O4 ferrite and 910 Oe for Al0.2CoFe1.8O4 ferrite. These variations could be explained using the site distributions, particle sizes and the spin magnetic moments of the doped ions.

    Figure

    MRSK-28-371_F1.gif

    X-ray diffraction patterns of ferrite powders annealed at 773 K: (a) CoFe2O4 (b) Mn0.2Co0.8Fe2O4 (c) Ni0.2Co0.8Fe2O4 (d) Al0.2Co- Fe1.8O4 (JCPDS No. 22-1086).

    MRSK-28-371_F2.gif

    FESEM images (100,000x) of ferrite powders annealed at 773 K: (a) CoFe2O4 (b) Ni0.2Co0.8Fe2O4.

    MRSK-28-371_F3.gif

    Room temperature Mössbauer spectra of ferrite powders annealed at 773 K: (a) CoFe2O4 (b) Mn0.2Co0.8Fe2O4 (c) Ni0.2Co0.8Fe2-O4 (d) Al0.2CoFe1.8O4.

    MRSK-28-371_F4.gif

    Hysterisis curves of ferrite powders annealed at 773 K: (a) CoFe2O4 (b) Mn0.2Co0.8Fe2O4 (c) Ni0.2Co0.8Fe2O4.

    Table

    Lattice constants and particle size of CoFe2O4, Mn0.2Co0.8- Fe2O4, Ni0.2Co0.8Fe2O4, and Al0.2CoFe1.8O4 ferrite powders annealed at 773 K.

    Room temperature Mössbauer parameters of CoFe2O4, Mn0.2Co0.8Fe2O4, Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 ferrite powders. Hhf is the magnetic hyperfine field, QS is the quadrupole splitting and IS represent the isomer shift relative to metallic iron at room temperature.

    Saturation magnetization(MS) and coercivity(HC) of CoFe2O4, Mn0.2Co0.8Fe2O4, Ni0.2Co0.8Fe2O4 and Al0.2CoFe1.8O4 ferrite powders annealed at 773 K.

    Reference

    1. N.N. Greenwood , T.C. Gibb (1971) Mössbauer spectroscopy., Chapman and Hall Ltd., ; pp.261
    2. V. Blasko , V. Petkov , V. Rusanov , Ll.M. Martinez , B. Martinez , J.S. MuA oz , M. Mikhove (1996) J. Magn. Magn. Mater., Vol.162 ; pp.331
    3. A. Goldman (1990) Modern Ferrite Technology., Van Nostrand Reinhold, ; pp.217
    4. A.S. Albaguergye , J.D. Ardisson , W.A.A. Macedo (2000) J. Appl. Phys., Vol.87 ; pp.4352
    5. T. Abraham (1994) Am. Ceram. Soc. Bull, Vol.73 ; pp.62
    6. P. I. Slick (1980) in: E. P. Wohlfrath(Ed.), Ferromagnetic materials, Vol. 2, p.196, North-Holland, Amsterdam,,
    7. V.K. Sankaranarayana , Q.A. Pankhurst , D.P.E. Dickson , C.E. Johson (1993) J. Magn. Magn. Mater., Vol.125 ; pp.199
    8. J.G. Lee , J.Y. Park , C.S. Kim (1998) J. Mater. Sci., Vol.53 ; pp.3965
    9. B.D. Cullity (1978) Elements of X-Ray Diffraction., Addition Wesley Co., ; pp.102
    10. K. Maaz , S.K. Arif Mumtaz (2007) Hasanain and Abdullah Ceylan., J. Magn. Magn. Mater., Vol.308 ; pp.289
    11. K.P. Chae , W.O. Choi , J.K. Lee , B.S. Kang , S.H. Choi (2013) J. Magn., Vol.18 ; pp.21
    12. W.O. Choi , W.H. Kim , K.P. Chae , Y.B. Lee (2016) J. Magn., Vol.21 ; pp.40
    13. M.Z. Schmalzrifd (1961) J. Phys. Chem., Vol.28 ; pp.203
    14. R. K. Datta , B. Roy (1967) J. Am. Ceram. Soc., Vol.50 ; pp.578
    15. M.K. Shobana , S. Sankar , V. Rayendran (2009) Mater. Chem. Phys., Vol.113 ; pp.10