J. Cent. South Univ. Technol. (2010) 17: 1129−1132 DOI: 10.1007/s11771−010−0607−0
Mössbauer spectra of MnFe2−2x Al2xO4 (0≤x≤0.4) ferrites BATOO K M1, KUMAR S2, PRAKASH R3, ALIMUDDIN4, SONG J I3, CHUNG H5, JEONG H5, KOO B H2, LEE C G2 1. King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia; 2. School of Nano and Advanced Materials Engineering, Changwon National University, Changwon 641-773, Korea; 3. Department of Mechanical Engineering, Changwon National University, Changwon 641-773, Korea; 4. Department of Applied Physics, Aligarh Muslim University, Aligarh 202002, India; 5. Department of Precision & Mechanical Engineering and BK21 Eco-Friendly Heat & Cooling Energy Mechanical Research Team, Gyeongsang National University, Tongyeong 650-160, Korea © Central South University Press and Springer-Verlag Berlin Heidelberg 2010 Abstract: The effect of non-magnetic Al3+ ion doping on the magnetic properties of MnFe2−2xAl2xO4 (0≤x≤0.4) spinel ferrites was studied using Mössbauer spectroscopy measurements at room temperature. From the Mössbauer study, it is observed that the resolved hyperfine sextets are due to the distribution of Fe ions on the two sublattices of the spinel ferrites. The value of the isomer shift obtained from the fitting of the Mössbauer spectra indicates that Fe ions are in +3 state. A paramagnetic doublet is observed at degree of inversion x=0.4, superimposed on the hyperfine sextets, indicating that the super-exchange interaction A-B decreases due to the dilution of sublattice by Al3+ ions. The hyperfine magnetic field decreases at both interstitial sites of tetrahedral (A) and octahedral (B) with the increase in Al concentration. Key words: ferrites; hyperfine interactions; Mössbauer spectroscopy; isomer shift
1 Introduction Spinel ferrites constitute an important class of magnetic materials. The spinel structure consists of a closed packed FCC cage of oxygen ion with two interstitial sites of tetrahedral (A) and octahedral (B) symmetries. In general, the cation distribution in spinel lattice has the form: (D1−xMx)[DxM2−x]O42−, where D and M are divalent and trivalent ions, respectively, and x is the degree of inversion. The round and square brackets denote the cations located at the center of the tetrahedral lattice of oxygen (A) and those at the octahedral (B) lattice, respectively. The magnetic properties of the spinel ferrites strongly depend on the type of the metal ions that occupy the two sublattice sites (i.e., site A and site B), the relative strengths of the inter-sublattice interactions JAB and intra-sublattice interactions JAA and JBB. Furthermore, the spinel structures offer the possibility of selective magnetic dilution of sites A and B, which results in the variety of magnetic behaviours [1−5]. Moreover, the substitution of non-magnetic ions also alters the magnetic interactions, such as A−A, B−B and
A−B, which gives various types of magnetic ordering ranging, from ferrimagnetism, antiferrimagnetism, local canted spin (LCS) to semi-spin-glass, spin-glass, etc [1]. VILLAIN [6] worked out a quantitative phase diagram that predicted a ferromagnetic to semi-spin-glass transition as a function of the magnetic ion concentration at sites A and B. Mössbauer spectroscopy is a very useful technique for probing the interaction between the atomic nuclei and the electric and magnetic fields in solids. The technique is based on the recoilless resonant emission and absorption of γ-radiation by atomic nuclei bound in solids. The study of Zeeman effect associated with the recoil-free resonance absorption (Mössbauer effect) of 14.4 keV γ-ray of Fe57m in magnetic materials containing iron provides the information about valence state and the magnetic interaction in the these materials [7−10]. In this work, Mössbauer spectroscopy measurements were performed to investigate the effect of Al doping on the magnetic properties of MnFe2−2xAl2xO4 (0≤x≤0.4) ferrites at room temperature without application of any external magnetic field.
Foundation item: Project supported by the Second Stage of Brain Korea 21 Project; Project(RTI04-01-03) supported by the Regional Technology Innovation Program of the Ministry of Knowledge Economy (MKE), Korea Received date: 2010−06−29; Accepted date: 2010−09−22 Corresponding authors: KUMAR S, PhD, Research Professor; Tel: +82−55−2133701; E-mail:
[email protected]; LEE C G, PhD, Professor; Tel: +82−55−2133701; E-mail:
[email protected]
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2 Experimental Polycrystalline bulk samples of MnFe2−2xAl2xO4 (0≤x≤0.4) were prepared by conventional solid state technique. High purity AR grade oxides, iron oxide (Fe2O3), aluminum oxide (Al2O3) and manganese oxide (MnO2) were mixed together according to their stoichiometric ratios. The mixture of each composition was ground to a very fine powder using alcohol and presintered at 1 000 ℃ for 13 h. The pre-sintered mixture was ground again and pressed into pellets under a mass load of 4 t. Finally, the pellets were sintered at 1 300 ℃ for 5 h and slowly cooled to room temperature at a rate of 1 ℃/min. The single phase nature of the samples was studied using Bruker D8 advanced diffractometer. Mössbauer spectra were recorded at room temperature using a constant acceleration Mössbauer spectrometer with 50 mCi 57Co in Rh source. The spectrometer was calibrated with α-Fe foil spectrum. The measured data were analyzed using a NORMOS program.
3 Results and discussion X-ray diffraction (XRD) measurements were used to study the crystal structure of MnFe2−2xAl2xO4 (0≤x≤ 0.4) ferrites. Fig.1 shows the XRD patterns of MnFe2−2xAl2xO4 (0 ≤ x ≤ 0.4) ferrites. All the peaks observed in the XRD patters belong to the FCC structure of the spinel ferrite and rule out the presence of any secondary phase formation. The experimentally observed XRD pattern was further analyzed and indexed using powder-X software. The result indicates that all compositions exhibit single-phase cubic spinel structure with Fd3m space group. Fig.2 shows Mössbauer spectra of MnFe2−2xAl2xO4 (0≤x≤0.4) ferrites at room temperature. The samples show a well-defined Zeeman pattern consisting of two hyperfine sextets corresponding to the distribution of Fe ions among two sites A and B for all the compositions. The composition, x=0.4, shows the presence of paramagnetic doublet, in addition to the two hyperfine sextets. The hyperfine field parameters: isomer shift (IS), quadrupole splitting (QS) and hyperfine field (Hf) obtained from the fitting Mössbauer spectra are listed in Table 1. It can be seen from Table 1 that the value of isomer shift at site B is larger than that at site A. Isomer shift is also known as the chemical isomer shift or the center shift due to the non-zero volume of the nucleus and the electron charge density due to s-electrons within it, leading to an electric monopole (Coulomb) interaction that alters the nuclear energy levels. The volume of the nucleus in its ground and excited states are different and
Fig.1 XRD patterns of MnFe2−2xAl2xO4 (0≤x≤0.4) ferrites: (a) x=0; (b) x=0.2; (c) x=0.4
the s-electron densities are also affected by the chemical environment. The isomer shift is a good method for probing the valence state of the Mössbauer atom. As the wave functions of the s-electrons penetrate into outer shells, changes in these shells will directly alter the s-electron charge density at the nucleus. For example, Fe2+ and Fe3+ ions have electron configurations of 3d6 and 3d5, respectively. Fe2+ ions have less s-electron density at the nucleus due to the greater screening of the d-electrons. This produces a positive isomer shift greater in ferrous iron than in ferric. Therefore, the higher value of the isomer shift at site B as compared to site A in the studied samples is attributed to the larger bond separations and smaller overlapping of the orbits of Fe3+ ions with oxygen ions at sites B as compared to site A. The Fe3+-O2− inter-nuclear separation at site A is smaller, which results in large overlapping of Fe3+-O2− orbital wave functions at the tetrahedral sites, while the Fe3+O2− inter-nuclear separation at site B is large and hence smaller overlapping in Fe3+-O2− orbital wave functions. The smaller the inter-nuclear separation, the greater the covalency, and hence the higher the isomer shift, as the covalency is strongly related with the isomer shift. For this reason, the isomer shift at site A is observed to be smaller as compared to that at site B [11−14]. The value of isomer shift observed for the samples investigated is in good agreement with the isomer shift values reported in the literature, i.e., for Fe2+ ions its value lies in the range of 0.6−1.7 mm/s, while for Fe3+ ions it lies in the range of 0.1−0.5 mm/s [15]. In reference to the above
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spectra of investigated samples (see Table 1). When chemical disorder is present in a material, a non-zero quadrupole splitting is generated, which will produce an electric field gradient of varying magnitudes, directions, sign and symmetry, and hence the distribution in the quadrupole shift can be seen. The composition, x=0.4, has a complex hyperfine structure in which a quadrupole doublet is superimposed on magnetically split sextets, which is assumed possibly due to the interaction of electric field gradient (EFG) with the quadruple moment of Fe57 nucleus and the reduction in the magnetic interactions between Fe ions due to Al3+ dilution. The interaction between a nucleus and its surrounding environment is known as a hyper fine interaction. This interaction is very small compared with the energy levels of the nucleus itself but the extreme energy resolution of the Mössbauer effect enables this interaction to be observed. Fig.3 shows the magnetic hyperfine field (MHF) at sites A and B for all the compositions of Al doped Mn ferrite. For x=0, the average maximum amplitudes of MHF for tetrahedral and octahedral are 41.42 and 43.52 T, respectively. When Al is added to the system, the MHF value decreases and reaches up to 38.53 and 39.71 T, respectively. The MHF for site B is generally larger than that for site A, which is attributed to the dipolar field due to deviation from cubic symmetry and covalent nature of tetrahedral bond [11]. The observed variation of the A- and B-site hyperfine fields with the Al concentration is shown in Fig.3. Similar variations were reported in the literatures for Ni-Zn and Mn-Zn ferrites [16−18]. The decreasing value of the hyperfine fields at the two sites with compositions can be explained on the basis of supertransferred hyperfine interaction (STHI). The STHI for Fe3+ compounds has previously been discussed for linear chain by some groups [19−20]. The STHI involves the use of s orbit of Fe3+ (2), pg of the ligand O2− and t2g of Fe3+ (1). Construction of Π bonding molecular orbit takes place, when a certain fraction of unpaired spin density is transferred from t2g of Fe3+ (1) to 2pg orbit on the ligand. This unpaired spin density on the O 2−
Fig.2 Mössbauer spectra of MnFe2−2xAl2xO4 (0 ≤ x ≤ 0.4) ferrites at room temperature: (a) x=0; (b) x=0.1; (c) x=0.2; (d) x=0.3; (d) x=0.4
values the values of isomer shift listed in the table suggest that the resolved hyperfine sextets in our samples are due to Fe3+ state of the iron. The existence of quaderupole interaction is one of the most useful features in the Mössbauer spectroscopy. Any nucleus with spin quantum number greater than 1/2 has a non-spherical charge distribution that is characterized by quaderupole moment. When the nuclear quadrupole moment experiences an asymmetric electric field, produced by an asymmetric electronic charge distribution or ligand arrangement and characterized by a tensor quantity called the electric field gradient (EFG), an electric quaderupole interaction occurs, which gives rise to the splitting of the nuclear energy levels. A nonzero quadruple splitting is observed in the Mössbauer
Table 1 Calculated values of isomer shift (IS), quadrupole splitting (QS) and hyperfine magnetic field (Hf) for MnFe2−2xAl2xO4 (0≤x ≤0.4) ferrites at room temperature x
IS/(mm·s−1)
QS/(mm·s−1)
Hf/T
Paramagnetic doublet
A
B
A
B
A
B
A
B
0
0.372
0.526
−0.083
0.382
41.42
43.52
—
—
0.1
0.468
0.556
0.124
−0.980
40.82
42.41
—
—
0.2
0.428
0.545
0.121
−0.478
40.32
42.15
—
—
0.3
0.212
0.601
0.117
−0.450
39.76
40.75
—
—
0.4
0.275
0.541
0.406
−0.345
38.53
39.71
0.31
1.89
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[4]
[5]
[6]
Fig.3 Hyperfine field variation with Al composition x
orbit can be partly transferred to the unoccupied s orbit on Fe3+ (2) through orthogonlization, giving an unpaired s-electron spin density with the same spin direction as t2g orbit of Fe3+ (1) [21]. Thus, this covalent transfer process leads to a decrease in the absolute value of the hyperfine field. The decrease in the hyperfine field with the composition can also be explained on the basis of Neel’s model [22]. According to this model, the intra-sub lattice interactions are weaker than the inter-sub lattice interactions, as a result of unsatisfied bonding between them, i.e., JAB＞JBB＞JAA. Also, Mn ferrite is thought to be 80% normal, 20% inverse when 20% of Mn migrates from site A to site B [22], which means that Mn and Fe are both present at sites A and B. Hence, the following interactions are mainly considered: Fe 3A+ − O − Fe3B+ , Fe 3A+ − O − Fe 3A+ , Fe 3B+ − O − Fe 3B+ , Mn 2B+ − O − Fe3B+ . When non-magnetic Al3+ ion is doped in the investigated samples, it occupies both sites A and B, which results in weakening of A-A, B-B and A-B superexchange interaction. Hence, the magnetic hyperfine field at the two sites decreases.
4 Conclusions (1) The XRD results show that all the samples of MnFe2−2xAl2xO4 (0≤x≤0.4) ferrites have single phase nature. (2) The value of isomer shift observed from the fitting of the Mössbauer spectra shows that Fe is in +3 state. (3) The decreasing behavior of magnetic hyperfine field indicates that Al doping dilutes the interaction.
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