Effect of Mg substitution on the magnetic properties of Ni Zn ferrites

Transcription

1 Pramana J. Phys. (2017) 88:88 DOI /s Indian Academy of Sciences Effect of Mg substitution on the magnetic properties of Ni Zn ferrites Y RAMESH BABU Department of Basic Sciences, G Pulla Reddy Engineering College (Autonomous), Kurnool , India yrbgprec@gmail.com MS received 11 July 2016; revised 29 October 2016; accepted 16 December 2016; published online 31 May 2017 Abstract. Nickel zinc ferrites with the general chemical formula, Ni 0.3 Zn 0.7 x Mg x Fe 2 O 4 with x varying from 0.00 to 0.25 in steps of 0.05, have been prepared by conventional solid-state method. Final sintering of the samples was carried out at 1200 C for 6 h in air to investigate their structural and magnetic properties. X-ray diffraction patterns of all the samples confirm the cubic spinel structure. Percent porosity and lattice constants of the samples are similar for all the samples except for the sample with x = 0.05 implying that the changes in magnetic properties could be solely attributed to the effects caused by substitutions only. The saturation magnetization has been observed to increase continuously with the substitution of Mg 2+ ions in the place of Zn 2+ ions. Curie temperature of the system was also found to increase from 261 C(x = 0.00) to 364 C(x = 0.25) with the increase in magnesium content. Smooth coercivity variation suggests better structural homogeneity. The results are discussed in the light of the distribution of the cations among octahedral and tetrahedral sites. Keywords. PACS Nos Ni Zn ferrites; spinels; lattice constant; saturation magnetization; crystal structure Np; Fb; e 1. Introduction In recent times, Ni Zn ferrites have been considered to be one of the widely studied soft magnetic materials due to their high resistivity and very interesting magnetic properties such as high magnetization, low losses and moderate permeability. They are useful as components in many electronics applications such as transformer cores, antenna rods, inductors, deflection yokes, recording heads, etc. Because of the flexibility in making and fine tuning the nickel-to-zinc ratio, the properties of Ni Zn ferrites can be extended to work over a wide frequency range. So, the ferrites are considered to be the most versatile and they are being used widely in electronic industry. The structural and magnetic properties depend on the composition and distribution of cations over the octahedral and tetrahedral interstitial sites. The substitution of any cation in the parent ferrite system might result in the displacement of other cations between the sublattices while leading to changes in the properties of ferrites. These changes depend not only on the site occupancy but also on the charge of the substituted cations. Efforts were made by the researchers to improve the basic properties of Ni Zn ferrites by substituting nonmagnetic cations of different valences. It has been observed that the trivalent substitutions in these ferrites modify the magnetic properties [1]. The tetravalent substitutions have been found to improve the electrical properties while degrading the magnetic performance [2,3]. Recently, additions of pentavalent ions are reported to be effective in minimizing the power loss and thereby making these materials as potential candidates for high-frequency power applications [4]. However, substitutions of divalent ions in place of either Ni or Zn ions in these systems have received little attention. Particularly, no data are available on systematic replacement of diamagnetic zinc ions by Mg ions. There were some studies on the influence of Mg substitutions by replacing either Ni or Fe ions in Ni Zn ferrites [5,6]. In these studies, though the properties got modified, the role of Mg ions in modifying the properties was not established satisfactorily. One study reported the degradation of electromagnetic properties due to the presence of Mg 2+ ions, while the other found enhanced magnetic response. As there is little work reported on the presence

2 88 Page 2 of 5 Pramana J. Phys. (2017) 88:88 of Mg ions in ferrites, it is felt necessary to conduct a fresh study on the substitutions of Mg ions particularly in a Ni Zn ferrite composition, Ni 0.3 Zn 0.7 Fe 2 O 4, which is believed to be a high permeability composition because appropriate chemical modifications to this composition would help to increase the magnetization and also make it suitable for high-frequency applications. Hence, this paper reports and analyses the influence of Mg substitution on porosity, lattice constant, saturation magnetization and Curie temperature of Ni Zn ferrites. 2. Experimental details Polycrystalline Ni Zn ferrites with the formula Ni 0.3 Zn 0.7 x Mg x Fe 2 O 4, where x takes the values ranging from 0.00 to 0.25 in steps of 0.05, have been prepared by conventional ceramic technique. Reagent-grade NiO, ZnO, Fe 2 O 3 and MgO powders in stoichiometric proportions are wet grounded homogeneously in methanol medium and calcined at 950 C for 2 h to obtain the spinel phase. The calcined powders are again wet grounded and dried for granulation using 8% polyvinyl alcohol as the binder. The powders are pressed at 150 MPa using a hydraulic press to form cylindrical-shaped pellets of about 10 mm diameter and 3 4 mm thickness and sintered at 1200 C for 6 h in air. After the sintering, the furnace was switched off and the samples were allowed to cool naturally. X-ray diffraction patterns were obtained for all the samples to get information related to the phase composition. They showed single phase cubic spinel structures in all the samples, as evident from the peaks displayed for a typical Ni Zn ferrite (basic composition, x = 0.00) in figure 1. Intensity (A.U.) Ni 0.3 Zn 0.7 Fe 2 O θ Figure 1. Typical X-ray diffraction pattern of Ni 0.3 Zn 0.7 x Mg x Fe 2 O 4 (x = 0.00). The sintered sample densities were approximately 90% to their theoretical limits. Using the theoretical and experimental density data, porosity and lattice constant of the basic composition were estimated. The density (both experimental and X-ray) was measured only for the basic composition. Owing to the fact that the preparation conditions for all the samples remain the same, it can be inferred that the sample densities are approximately of the above order only. Curie temperature measurements were made using a laboratory built-in arrangement described by Soohoo [7]. Saturation magnetization was measured using vibration sample magnetometer. All the measurements were carried out at room temperature only. 3. Results and discussion Variations of porosity and lattice constant of Ni 0.3 Zn 0.7 x Mg x Fe 2 O 4 with Mg concentration are shown in figure 2. The initial concentration of Mg has contributed to obtain less porosity and thereafter the porosity varies little with the Mg concentration. It is thought that the Mg initially entered the lattice completely without creating any oxygen vacancies, thus increases the density. For subsequent concentrations, a slight change in the stoichiometric balance could not be detected through XRD patterns and this may be the reason for not showing such improvements in density. However, all the Mg-substituted samples exhibited lesser porosities compared to the basic Ni Zn ferrite sample. These phenomena seemed to be related to the improved homogeneity when Mg is substituted. In general, excess content of metal oxides in the form of insoluble secondary phases and liquid phases contribute to either slow or rapid densification which in turn generate increased pore volumes and demagnetizing fields [8]. Besides, inhomogeneities in the constituent metal oxide particles in the synthesis increase the defects and pore volumes in the final products. The lesser porosities in the present study indicate that the Mg ions are completely dissolved and entered the spinel lattice. The low porosity of the sample at x = 0.05 may be attributed to extremely high degree of structural homogeneity of that sample. The curve in figure 2, depicting the variation of lattice constant with Mg concentration, also resembles that of the porosity variation. However, except for the sample at x = 0.05, the lattice constant values of all other samples are larger than that of the basic Ni Zn ferrite composition. In the present study, if the Mg ions with ionic radius of 0.72 Å were to enter the lattice in B-sites as Mg 2+ only in place of replaced Zn ions with smaller ionic radius of 0.60 Å. Therefore the lattice can be

3 Pramana J. Phys. (2017) 88:88 Page 3 of 5 88 porosity,% porosity lattice constant lattice constant, A Curie temperature, o C concentration, x Figure 2. Variation of % porosity and lattice constant with Mg concentration of Ni 0.3 Zn 0.7 x Mg x Fe 2 O Figure 3. Variation of Curie temperature with Mg concentration of Ni 0.3 Zn 0.7 x Mg x Fe 2 O 4. continuously expected to enlarge as more and more Mg is introduced in the system provided there is no change in the valence states of the cations involved. The reason for such increase could be understood as occupation of Mg ions with ionic radii of 0.72 Å in B-sites would only force Fe ions with 0.55 Å there (net difference of 0.17 Å) to migrate to A-sites (Fe ions with ionic radius of 0.49 Å in A-sites) only to be occupied in place of replaced Zn ions there with ionic radii of 0.60 Å (net difference of 0.11 Å) for each ion substituted. A previous study under similar preparatory conditions has, however, reported appreciable zinc loss and thereby a change in valence states of the cations present in order to maintain the charge neutrality [9]. The observed lower value of lattice constant for the sample at x = 0.05 is thus attributed to the zinc loss and also to the presence of Mg 3+ ions with smaller ligands apart from Mg 2+ ions. The high sensitivity of Mg to the sintering temperature also favours it to react with the furnace atmosphere and exist simultaneously in different valence states [5]. Once the charge is balanced, Mg would subsequently go to the lattice mostly as Mg 2+ ions. Variation of Curie temperature with Mg concentration is shown in figure 3. It is observed that there is a continuous increase in Curie temperature with the concentration. Obviously, the exchange interactions between magnetic ions would increase with both the density and magnetic moment of the magnetic ions. As the zinc ions are replaced by Mg ions in the present system, the total number of magnetic ions involved in exchange interactions remains unchanged with each step of substitution but because of different cationic preferences of the ions it strengthens the A B exchange interaction and leads to an increase in Curie temperature. Variation of saturation magnetization (emu/g) with Mg concentration is shown in figure 4. The magnetization is observed to increase continuously throughout Magnetization, emu/g Figure 4. Variation of saturation magnetization with Mg concentration of Ni 0.3 Zn 0.7 x Mg x Fe 2 O 4. the full range of substitutions. Before explaining the observed variation with Mg concentration, the possible cationic distribution scenario of the system prior to substitution can be described as follows: Based on the Mössbauer study [10] and with the basic assumption that all the iron in the system is in its trivalent state, the ionic distribution of the basic Ni Zn ferrite composition, Ni 0.3 Zn 0.7 Fe 2 O 4 (for x = 0), in the present study can be expressed as (Zn Fe )[Ni Fe ]O 4 2. The cations within the parenthesis occupy tetrahedral (A-) sites whereas those in square brackets occupy octahedral (B-) sites. Since iron content in A-sites is too small to hold all the iron spins in B-sites aligning antiparallel to the A-sublattice, there exists a possibility of B B spin canting in B-sublattice for this composition. Further, according to the neutron diffraction studies conducted on ferrites prepared by ceramic method [11]and wet chemical method [12], the occupation of cations in the A-sites is differed from a large percentage in the ceramic method to a small percentage in the wet

4 88 Page 4 of 5 Pramana J. Phys. (2017) 88:88 Coercivity, Oe Figure 5. Variation of coercivity with Mg concentration of Ni 0.3 Zn 0.7 x Mg x Fe 2 O 4. chemical method. It implies that the cations are highly sensitive to the preparation methodology, particularly when the size scales are reduced in their preference for a particular lattice site. If Mg is substituted for Zn under the given scenario, the Mg ions need not necessarily occupy the A-sites only, but would occupy both the A- and B-sites according to their preference. The percentage of Mg occupied in B-sites would however force equal amount of iron to migrate into A-sites as there exist vacant sites due to the decreased zinc content. In such cases, there are two possibilities under which the magnetization processes can be modified: (i) Since the amount of iron present in B-sites decreases while the amount of iron present in A-sites increases with each step of Mg substitution, the iron ions in A-sublattice becomes comparable to the iron ions in B-sublatice and it would be possible to maintain the antiparallel alignment between the sublattices, which decreases the B B spin canting and enhances the B-sublattice magnetization with every step of Mg substitution and (ii) though the magnetic moment of Mg 2+ ions is the same as that of Fe 3+ ions, the strength of exchange interaction between Mg 2+ Fe 3+ is small and therefore there will be a canting of spins of Mg 2+ and of Fe 3+ ions in the A-sublattice, thereby a decrease of A- sublattice magnetization [13]. The process (i) is directly effective in enhancing the net magnetization of the system for lower concentrations of Mg by reducing the B B spin canting and process (ii) is indirectly helpful in increasing the net magnetization by self-weakening of A-sublattice magnetization at higher Mg concentrations, wherein both Mg and Fe are in sufficient amounts to trigger canting within themselves. The net result is an increase in magnetization as the Mg concentration is increased and the observed variation is in agreement with the arguments made above. Nevertheless, for the initial Mg concentration, as per ref. [13], 19% of 0.05 wt% Mg, which is expected to be in B-sites, is too small to reduce the B B spin canting and to record increase in magnetization. But, the possible zinc loss at the sintering temperature due to volatilization causes Mg to form Mg 1+ to that extent and these ions occupy B-sites. This might have forced equal amount of iron to migrate to A-sites, thus reducing the spin canting in B-sublattice further and contributing to an increase in the net magnetization at this concentration. Variation of coercivity with Mg concentration is shown in figure 5. The variation is smooth for the whole range of Mg concentration. This implies that the motion of domain walls is not hindered by defects or secondary phase pinning points, which could be possible with the complete dissolution of Mg in the system and also with the improved structural homogeneity as discussed earlier. 4. Conclusions In summary, the substitutions of Mg for zinc in Mgsubstituted Ni Zn ferrites have displayed better structural homogeneity and improved magnetic performance. However, the variations of lattice constant and porosity are understood to point towards the simultaneous presence of Mg in two or more oxidation states. Increase in Curie temperature is explained by the increased density of magnetic cations with the addition of Mg 2+ ions with higher magnetic momentum. Increase in saturation magnetization is explained based on the ionic distribution among lattice sites and different degrees of spin canting in different sublattices. The coercivity variation is believed to be another experimental evidence in support of the structural homogeneity of this system. Further study using SEM and chemical analysis would help to supplement the arguments made in this study. References [1] G F Dionne and R G West, J. Appl. Phys. 61, 3868 (1987) [2] B V Bhise, M B Dongare, S A Patil and S R Sawant, J. Mater. Sci. Lett. 10, 922 (1991) [3] B Parvatheeswara Rao, K H Rao, K Asokan and O F Caltun, J. Optoelectr. Adv. Mater. 6, 959 (2004) [4] S Otobe, Y Yachi, T Hashimoto, T Tanimori, T Shigenaga, H Takei and K Hontani, IEEE Trans. Magn. 35, 3409 (1999) [5] M A Amir and M El Hiti, J. Magn. Magn. Mater. 234, 118 (2001) [6] E Rezlescu, L Sachelarie, P D Popa and N Rezlescu, IEEE Trans. Magn. 36, 3962 (2000) [7] R F Soohoo, Theory and applications of ferrites (Prentice-Hall, Englewood Cliffs, NJ, 1960) [8] J G M de Lau and A L Stuijts, Philips Res. Rep. 21, 104 (1966)

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