File 2596 (7).qxd Bangladesh J. Sci. Ind. Res. 41(3-4), 171-180, 2006 Introduction Curie temperature is the temperature when the energy of the thermal motion of the atoms is sufficient to overcome the interaction forces between the atomic moments. At this temperature a ferromagnetic substances changes over to paramagnetic substances. This temperature is also a measure of exchange forces in a ferromagnetics. Near Curie temperature, Tc, some of the intrinsic parameter such as the magnetization, the sus- ceptibility, thermal and electrical conductivity are seen to behave differently than predicted by classical molecular field theory. In general ferrite shows an increasing value of permeability with increasing temperature, until they approach their Curie points when the permeability falls sharply. This happens due to magnetic transition from the ferrimag- netic phase to the paramagnetic phase. The temperature variation of resistivity of mag- netic materials shows similar irregularities at Studies on Anomalous Behaviour at Curie Point, Tc of Some Classes of Mixed Ferrites M. Tofazzal Hossain,a Suravi Islam,a Mokbul Hossain Mondala and A. H. Khanb aBCSIR Laboratories, Dhaka and Department of Physics, bJahangirnagar University, Savar, Dhaka. Abstract A comparative studies on anomalous behaviour at Curie point between clay additives Mn-Zn ferrites and other mixed ferrites such as Mn-Zn, Ni-Zn, Mg-Mn and Ni-Mg have been carried out in this investigation. The changes in Tc on compositions, addi- tives and manufacturing processes of such soft mixed ferrites have been determined from the temperature dependent dc resistivity data and the log ρ vs. (1/1000T) graphs. It is observed that the Curie temperature is dependent on compositions, additives, manufacturing process and microstructure. Definite changes of Curie points of clay additives Mn-Zn ferrites with other ferrites were observed. Both the Mn-Zn ferrites and clay additives Mn-Zn ferrites showed some changes in their Tc behavior. The Tc value of Mn-Zn ferrites with clay additives decreases with increasing additives. In the case of clay substituted Mn-Zn ferrites the decrease of Curie point can be understood by a decrease of the number of Fe3+ magnetic ions substituted by nonmagnetic Al3+ ions on O sites of the system. magnetic transition point. The magnetic tran- sition point can often be clearly observed in the anomalous temperature variation of trans- port properties of magnetic material near Tc. The magnetic transition point Tc of pure iron was studied by Lunchbury.1 He studied tem- perature dependent electrical resistivity ρ and observed Tc around 770O C. The expected lin- ear relationship between lnρ and 1/T has often been found for ferrite materials accom- panied by a change in activation energy near the Curie temperature. It has been observed that in mixed ferrites, where the divalent metal ions consist of some magnetic ions such as Ni, Co, Fe and some nonmagnetic ions such as Zn, Ca, Cu inter- esting phenomenon occurs. The diamagnetic Zn2+ ions content of the mixed Zn2+ ferrite has a marked effect on the Curie temperature. The Curie temperature decreases continuous- ly as the Zinc content increased.2 This was experimentally found out by Forestier in Zn2+ ions substituted mixed ferrite.3 On the other hand, the addition of diamagnetic Zn2+ con- tent increase the room temperature perme- ability of Mn ferrite from 250 to 1000 and Ni ferrite from 10 to 700. So, mixed ferrite of appropriate composition can yield conven- ient Tc values for operating at high perme- ability.4,5 The study of the variation of Tc values with the content of a nonmagnetic substance like Zn2+ ions is clearly of impor- tance. The aim of the present experiment is to investigate the value of Tc of some mixed fer- rites like Mn-Zn, Ni-Zn, Mg-Mn and Ni-Mg ferrites with varying the compositions, addi- tives and manufacturing process and to observe the resulting change in their transi- tion temperature, Tc. Besides, a comparative study of Curie points between Mn-Zn ferrites and clay additive Mn-Zn ferrites have been made in this investigation. Materials and Methods In the present investigation, eight series of samples with various compositions and additives were prepared in our laboratory by using a ceramic method. The samples were classified into eight groups such as A, B, C, D, E, F and G. Group A-E are Mn-Zn ferrites with different compositions and sintering temperature, group F is Ni-Zn ferrites, group G is Mg-Mn ferrites and group H is Ni-Mg ferrites. The main emphasis of this study has been given on group E in which clay has been employed as an additive. More details about the preparation of the ferrite from oxides are given by Snelling and Ahsanullah.4,5 There are some methods available to deter- mine Curie temperature, Tc. The Curie temperature is usually determined from mag- netization or magnetic susceptibility data. The Tc can also be determined by different techniques; which include specific heat, ther- mal expansion and electrical resistivity as a function of temperature. In our present experiment the Curie temperature measure- ment involved measuring the resistivity ρ, at 172 Studies on Anomalous Behaviour 41(3-4) 2006 varing temperature. The same method has been treated by M. A. Hossain, S. R. Swant and R. N. Patil and V. R. Kulkarni et.al.6-9 To characterize the clay additive samples, the X-ray powder pattern were recorded on a Phillips X-ray diffractometer using Cu Kα radiation. Results and Discussion Structural Characterization Figs. 1.1 to 1.3 show the X-ray diffraction patterns for clay additive Mn-Zn ferrites (group E samples). The patterns were ana- lyzed by Phillips X-ray diffractometer and confirm formation of cubic system with space group Fd3m. The structural parameters are shown in Table I. Composition dependent Curie Temperature, Tc values of Mn-Zn, Mg-Mn and Ni-Mg ferrites To determine Tc the temperature dependent dc resistivity data plotted as lnρ Vs (1000/T) have been used. In group A, the Zn2+ ions content is constant and the weight % of the value of Fe2O3 and MnO are changed simul- taneously. The values of Tc on compositions of Mn-Zn ferrites (A-group) are shown in Fig. 2.1 Hossain, Islam, Mondal and Khan 173 Fig. 1.1. X-ray diffraction patterns for Rc 174 Studies on Anomalous Behaviour 41(3-4) 2006 Fig. 1.2. X-ray diffraction patterns for Bc Fig. 1.3. X-ray diffraction pattern for Yc. It is seen from Fig. 2.1 that the Curie temper- ature increases with the increase of weight % of Fe2O3. It is also observed that the slight variation in the composition ratio of Fe2O3 and MnO have a remarkable effect on Tc. It is also well known that the Tc values are dependent on the exchange interaction of magnetic ions of ferrites between octahedral and tetrahedral sites. In the present study, the increase of Tc values in the case of iron rich Mn-Zn ferrites is due to the increase of exchange interaction of AB or BB between octahedral and tetrahedral sites. In this study we have also investigated the composition dependent anomalous behaviour that is Tc of Ni-Zn ferrites. In Ni-Zn ferrites the weight % of Fe2O3 remains constant and the ratio of the value of the ZnO and NiO are changed simultaneously. The Curie tempera- ture of this system is shown in Fig. 2.2. It is seen from the figure that the Curie tempera- ture decreases with the increase of ZnO or increases with increase of NiO. This type of behaviour has also been reported by Forestier in Zn2+ ions substituted Ni-Zn ferrites.3 As the ZnO in Ni-Zn ferrite increases exchange interaction of AB and BB between octahedral and tetrahedral sites decreases and thus decreases Tc. Reverse is in the case of NiO addition. In Mg-Mn and Ni-Mg we have seen that the Mg ions have remarkable effect on Curie temperature as in Figs. 2.3 and 2.4. It is seen from Fig. 2.3 that the Curie temperature Hossain, Islam, Mondal and Khan 175 Samples Structure Space group Lattice parameter unit AO Density unit gm/cc No of formula unit with unit cell E1 Cubic Fd 3m 8.5136 5.010 8 E2 Cubic Fd 3m 8.4975 5.084 8 E3 Cubic Fd 3m 8.4794 5.162 8 Table I. The crystal system, space group and values of lattice parameters, density, number of formula units of clay additive Mn-Zn ferrites. Fig. 2.1. Composition dependent Tc in Mn-Zn Ferrites (Group A samples) Fig. 2.2. Composition dependent Tc in Ni-Zn ferrites decreases with the increase of Mg ions. The decrease of Curie point with increase of Mg ions is due to the fact that Mg occupies O- sites, which weaken the AB interaction and hence decreases Tc. In case of Ni-Mg ferrite addition of NiO enhances AB and BB interaction resulting increased Tc values which is evident in Fig. 2.4. Effect of sintering temperature on Curie temperature, Tc The objective of the study of group C was to observe and gather a general knowledge about the Tc values of Mn-Zn ferrites with different sintering temperature. This group of Mn-Zn ferrites was prepared by sintering at 1273 K, 1373 K, 1473 K and 1573 K Calculated values of Curie temperature, Tc of Mn-Zn ferrites sintered at 1273 K, 1373 K, 1473 K and 1573 K are shown in Fig. 2.5 against sintering temperature. It is evident from Fig. 2.5 that the Tc increases with increasing sintering temperature from 1000 to 1300O C. From the experimental findings it is observed that sintering temperature of the Mn-Zn ferrite samples bore some relations with their Tc. The Tc of the experimental sam- ples increases with the increase of the sinter- ing temperature. Because at higher sintering temperature, the density and average grain of the magnetic ions of ferrites are increased. The magnetic ions of Mn-Zn ferrites not only increases magnetic moments but also increases the Tc on sintering temperature. 176 Studies on Anomalous Behaviour 41(3-4) 2006 Fig. 2.3. Composition dependent Tc in Mg-MnFerrites Fig. 2.4. Composition dependent Tc on Ni-Mg ferrite system Fig. 2.5. Variation of Tc on sintering temper- ature (Group C samples) -- -- > C ur ie te m pe ra tu re ,T c( k) Hossain, Islam, Mondal and Khan 177 Effects of Zn-variables on Tc values in Mn-Zn ferrite The purpose of the study of group B samples was to observe the change in Curie tempera- ture due to change of non magnetic Zn2+ ion in Mn-Zn ferrite. In this investigation we have studied and have compared the Tc of some mixed ferrites which containing Zn2+ ions (sample no. B2, B3 and B4) with other ferrite that does not contain Zn2+ ions (sample no B1). It is observed that ferrite samples B1 which does not contain Zn2+ ions has higher Tc value than the other samples of this group. But if we gradually increase the Zn2+ ions in mixed ferrite samples B2 to B4, Tc value decreases. This result is in good agreement with the result of J. Roberts and Galleo.2,10 This type of behavior was experimentally found out by Forester and Robert.3,2 The decrease of Curie point is due to the tetrahe- dral site preference of the Zn2+ ions. Zn2+ ions always prefer A sites i.e. tetrahedral site. The exchange integral for A site i.e. JAA is almost negligible. So substitution of Zn2+ ions only weaken the AB interaction effec- tively. If the number of substituted ions are not too high the overall ferrimagnetic arrangement is not destroyed even though some loosely bound spins may become cant- ed or destroyed, at temperature T< Tc. For large substitution, the AB interaction became comparable to or even weaker than BB inter- action. In this way the collinear ferrimagnet- ic arrangement often changes to canted one. Owing to this reduced A-B exchange interac- tion with increasing Zn2+ ion content the Curie temperature will drop. So the study of the variation of Tc values with the content of nonmagnetic substance like Zn2+ ions is clearly important. Effects of replacement of ZnO by Al2O3, CdO and CaO We have already seen in group B that non magnetic material like Zn2+ ions in mixed ferrites play an important role in relation to their Tc. It was found that with gradual increase of Zn2+ content in mixed ferrites the Curie temperature decreases. Behavior of the substitution of non magnetic materials like Al, Cd, Ca, etc. in Mn-Zn ferrites have been also studied in the mixed ferrites (group D samples) to observe the similar effect. The calculated values of Tc on substituted ions are shown in Fig. 2.7. From the experimental finding it was found that molecular weight of different samples bore some relations with Fig. 2.6. Composition dependent Tc in Mn-Zn ferrite (Group B samples) 178 Studies on Anomalous Behaviour 41(3-4) 2006 their Curie temperature. The Curie tempera- ture decreases with the increase of molecular weight. Effects of clay additives in Mn-Zn ferrite on Tc In this investigation three types of clay addi- tive Mn-Zn ferrites have been studied. These types of clay additives are denoted by Rc, Bc and Yc and found in Bijoypur and Savar region of Bangladesh. The main cations of clay material are Si, Al, Mn and Fe. The composition variations of Tc values of clay adittives of ferrite are shown in Fig. 2.8. It is evident, as in Fig. 2.8, that sample Yc has the highest Tc value. The observed variation of Tc values can be explained on the basis of AA, AB and BB interactions. In clay additive Mn-Zn ferrites, the cations are Fe, Mn and Si, which con- tribute to Tc values. It is well known that the cation Fe is magnetic and occupy both the tetrahedral and octahedral sites. In the Fig. 2.7. Variation of Tc on various substitute ions (Group D samples) Table II. Composition variation of Tc values of clay additive ferrites 480 490 500 510 520 530 280 305 330 355 380 Molecular weight Fig. 2.8. Variation of Tc on Fe content in var- ious additives Mn-Zn ferrites. 480 490 500 510 520 530 280 305 330 355 380 Molecular weight Sl. No Composition of ferrites Cations Compound formula Concentration % Curie temperature Tc (K) 1. Rc (Red clay substitute Mn -Zn ferrite) Si Mn Fe SiO MnO Fe2O3 25.79 20.50 33.70 561 2. Bc (Black clay substitute Mn -Zn ferrite) Si Mn Fe SiO MnO Fe2O3 27.75 20.37 35.01 571 3. Yc (Yellow clay substitute Mn -Zn ferrite) Si Mn Fe SiO MnO Fe2O3 31.80 18.38 44.50 577 Hossain, Islam, Mondal and Khan 179 present study the increase of Tc value in the case of Fe rich clay additive Mn-Zn sample is due to the increase of exchange interaction of AB and BB between octahedral and tetrahe- dral sites. Conclusions The Curie temperature Tc, of the various fer- rites is strongly dependent on composition, sintering temperature and various additives. It is seen that the slight variation of the com- position i.e ratio of major constituents have a remarkable effect on Tc. It is also observed that the Tc values depend on the exchange interaction of magnetic ions of ferrites between octahedral and tetrahedral sites and the observed variation may be explained on the basis of AA, BA or BB interactions of various cations and their site location between octahedral and tetrahedral sites. It is evident that sintering temperature of the Mn-Zn ferrite sample bears some relations with their curie temperature. At higher sinter- ing temperature the density and average grain of the magnetic ions of ferrites are increased and the magnetic moments are increased. This enhances the magnetic interaction and hence increases Tc values. In clay additive Mn-Zn ferrites, the cations are Fe, Mn and Si which contribute to Tc val- ues. It is well known that the cations Fe are magnetic and occupy both the tetrahedral and octahedral sites. In the present study the increase of Tc value in case of Fe rich clay additive Mn-Zn sample is due to the increase of exchange interaction of AB and BB between octahedral and tetrahedral sites. Acknowledgements The authors are grateful to the Director, BCSIR laboratories, Dhaka for his encour- agement and permission to publish the work. We are grateful to Mr. Anwar Hossain, ex Scientist-in-charge, Industrial Research Division, BCSIR laboratories, Dhaka who made valuable suggestions regarding experi- ments and project development. We are also indebted to Physical Instrumentation Division of the same laboratory for the use of their computer facilities. The authors wish to thank the Director, IGCRT,BCSIR, Dhaka, for providing the facilities for carrying out this work. References 1. M. D. 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