Bhati, Kapil

Joshi, Urmi M

Pandya, H N

IACS Institutional Repository

Indian J- Phys. 72A(4), 301-306 (1998)IJP Aan intcmationaT journalStudy of bismuth substitution in cobalt ferriteUrmi M Joshi, Kapil Bhatt and H N Pandya Department of Electronics, Suurashtra University, Rajkot-360 005, IndiaRei eived 18 Dei ember 1997, accepted 28 April 1998Abstract : The bismuth substituted cobalt ferrite, that is. CoBi2;rFe2_2»04 has been prepared by ceramic method. The single phase has been confirmed by X-ray diffractograms The electrical behaviour of the system is studied by the measurements of electrical resistivity and dielectric constant The magnetic behuviour is studied through low  field ac susceptibility. The electrical and magnetic behaviours arc explained on the basis of single domain (SD) to superparamagnclic (SP) transitionKeywords : bismuth substituted cobalt, resistivity, susceptibility PACS No. : 75.50 Gg1. IntroductionThe pure cobalt ferrite has been well studied by many researchers [1-3]. The substituted C o F c: 0 4 has also been studied by various researchers [4-6]. Some people have also studied the mixed cobalt ferrite [7,8]. As far our knowledge goes, there have been no studies on the bismuth substituted cobalt ferrite. In this paper, we represent the effect of bismuth substitution in place of Fe in cobalt ferrite. The system CoBi2vFe2-2*04 has been characterized by X-ray diffraction and found single phase. The resistivity and dielectric constants have been measured as a function of temperature. The resistivity against temperature curves exhibit prominent rise after the critical temperatures of the samples. This is attributed to superparamagnetic (SP) to paramagnetic transition. The susceptibility measurements indicate the presence of single domain particles.2. Experimental techniquesAll the samples of the system C o B i^ F e ^ ^  with x = 0.0 to x = 0.1 in steps of 0.025 were prepared using the standard ceramic method. The stoichiometric proportion of CoO, Bi20, and Fe20^ of high purity were thoroughly mixed, pelletized and sintered at 950°C© 1998 IACS302 l/rmi M Joshi, Kapil Bhatt and H N Pandyafor 12 hours. These samples were reground and refired at 950°C for 12 hours. The X-ray diffractograms were obtained with the help of Philips (PM 9220) diffractometer using FeKrt radiation.The resistivity arid dielectric constant measurements were performed on all the samples of thickness 4 mm and diameter 10 mm. The Aplab made microprocessor based LCR bridge was used for above measurements. Before measurements, the faces of the pellets were carefully polished and rubbed with graphite. The low field ac susceptibility of powdered samples for all the samples were measured using double coil apparatus 19], from room temperature to 800 K.3. Results and discussionThe X-ray diffractograms exhibited a well-defined pattern of lines. When these lines were indexed they indicated a single phase. This is shown in Figure 1. The extra lines for the—---1j . l 1* ’ 0000 l l , .*-0-02 5ii ,—Ll— Ll*■0-050_Ll —*j0-0?5---A"'*■0100 E*lru  peak0 026 006 0076 AtCoHccatratloa ( v )Figure 1. X-ray diffractograms of the Figure 2. Lattice parameter a versussystem CoBi2AFc2-.£r0 4 . concentration x.sample x = 0.100 which is shown with arrow in Figure 1 indicates that for x > 0.100 the cobalt ferrite does not accommodate bismuth in its cubic phase. The variation of latticeStudy of bismuth substitution in cobalt ferrite 303constants against concentration is shown in Figure 2. From Figure 2, it is clear that substitution of bismuth ions slightly but steadily increases the lattice constant. Of course, this rise is not noteworthy because of the very small concentration of bismuth. The steady rise can be attributed to the replacement of Fe ions of smaller radius by bismuth ions of larger radius.The low field ac susceptibility for all the samples as a function of temperature is shown in Figure 3. The sample* = 0.000, that is, cobalt ferrite exhibits a constant rise upto,-0.000 Tb-7UK C*,.T, -  803 K -*Xi * 0.079Tk-(U KTc-TS«K------  ■ ^  • ■; a - « . I M.... 'O’T ,- « T J K  •T« "74* VT»M K)Figure 3. Low field ac susceptibility versus temperature for x = 0.000, 0.025, 0.050, 0.075 and 0 .100.X-ooooT - MJ KX —. Tr-7HKTk - 693 K/C Z */ * Tt - 773 K- uTk - 613 K X - 0 075f  Tr - 753 K-Tk - 671 KX-0 100 '..... -*TC - 743 KFigure 4, Log p versus temperature for the system CoBi2JrFe2- i rC>4 ia peak and then decreases. Such behaviour is also observed by Baldhae; al [7], The constant rise in susceptibility for * = 0.000 is indicative of single domain (SD) particles. As the temperature increases, single domain particles become superparamagnetic particles having increased susceptibility. Thus, with rise in temperature, more and more SD particles304 Urmi M Jos hi, Kapil Bhatt and H N Pandyabecome SP particles. Before Tn at blocking temperature Th, all the SD particles become SP particles. This is known as SD to SP transition. These transition temperatures Th for all samples are shown in Table 1. The absence of anisotropy peak for x = 0.000 suggests the absence of multidomain (MD) particles. As the concentration increases, the steady rise in susceptibility decreases. This shows that addition of bismuth in cobalt ferrite forms a mixture of SD and MD particles. The critical temperature Tc also decreases as the concentration increases. This can be attributed to decrease in Fe ions with the bismuth addition. The transition temperatures 7\ are also shown in Table 1.Table 1. Temperatures al dip and peak of resistivity, blocking temperatures and critical temperatures for the system of CoBi^T^-^tVConcentrationXSecond dip inresistivity Tc( K)BlockingtemperatureTh( K)CriticaltemperatureTC(K)0 0 0 0 - 713 8030 025 - 703 7880.050 773 693 7730 075 758 683 7580.100 743 673 743The resistivity for all the samples as function of temperature is shown in Figure \  The samples x = 0.000 and x = 0.025 exhibit normal behaviour but the samples x = 0.050, x -  0.075 and x = 0.100 show two discontinutes at higher temperatures. This suggests that substitution of bismuth upto certain amount into cobalt ferrite makes the resistivity sensitive to some kind of transition.The comparison of these discontinuity temperatures (sec Table 1) with the blocking temperature Th and the transition temperature Tt of susceptibility suggests that first discontinuity occurring at lower temperature may be due to SD-SP transition while the second discontinuity may be due to superparamagnetic (SP) to paramagnetic transition. The general nature of resistivity is a decrease in resistivity as temperature increases.Figure 5 shows the dielectric constant as a function of temperature for all the samples. The dielectric constant for all the samples initially increases quite negligibly; but after certain higher temperature, it shows prominent rise. It also reveals that dielectric constant does not get affected due to the SD-SP transition. The critical lemperatures Tc of respective samples are indicated in Figure 5. This suggests that superparamegnetic (SP) to paramagnetic transition causes a remarkable rise in dielectric constant. It is alsoStudy of bismuth substitution in cobalt ferrite 305interesting to note that dielectric constant versus temperature has almost inverse behaviour compared to resistivity versus temperature behaviour. From this relation, rise ina •  0.000T( -  MJ K ■VFigure 5. Dielectric constant F. versus temperature For the system CoB 12^ 2-2*04d i e l e c t r i c  c o n s t a n t  c a n  a l s o  b e  a t t r i b u t e d  t o  r a p i d  d e c r e a s e  in  r e s i s t i v i t y  a f t e r  c r i t i c a lt e m p e r a t u r e .4. ConclusionThe present study of the system CoBi2*Fe2_2t04 shows that bismuth can be added only upto 10% in CoFe204. The resistivity becomes sensitive to magnetic transition due to addition of certain amount of bismuth as the temperature is varied. Dielectric constant is affected by rapid decrease in resistivity and superparamagnetic (SP) to paramagnetic transition.AcknowledgmentsThe authors are thankful to RSIC, Nagpur for providing XRD facilities, Urmi Joshi is also thankful to Government of Gujarat for providing financial help in the form of scholarship.306 Urmi M Joshi, Kapil Bhatt and H N PandyaReferences(1 ] G H Jonker J. Phys. Chem. Solids 9165 (1959)[2] G A Sawatzky. F Van der woude and A H Morrish J. Appl. Phys. 39 1204 (1968)[3] G D Reik and J J M Thijssen Acta Cryst. V24 982 (1968)[4] J G Na, T D Lee and S J Park IEEE Trans. Magn. 28(5) 2433 (1992)[5] C M Yagnik and H B Mathur Indian J. Pure Appl Phys. 6 211 (1968)[6] B S Trivedi and R G Kulkami J. Mater. Sci Lett. 12 1401 (1993)[7] G J Baldha, R V Upadhyay and R G Kulkami Mater. Res. Bull 21 1051 (1986)[8] R Satyanarayana and S Ramana Murthy J. Mater. Sa. Lett. 4 606 (1985)19) C Radtaknshnamurthy, S D Likhite and P W Sahastrabudhe Proc. Indian Acad. Sci. 87A 245 (1978)

Study of Bismuth Substitution in Cobalt Ferrite

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Study of Bismuth Substitution in Cobalt Ferrite

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