We report results of a comprehensive study of the phase transition at T N (~643 K) as a function of particle size in multiferroic BiFeO 3 system. We employed electrical, thermal, and temperature dependent X -ray diffraction (XRD) studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime 2-300 K under a magnetic field 100-10000 Oe both on bulk and nano-crystalline systems. While in the bulk system a sharp endothermic peak at T N together with a broad feature, ranging over nearly ~150 K (∆T), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at T N , is found to occur both at T O and T N across ∆T in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metastability increases in the nanoscale BiFeO 3 with divergence between zero-field cooled (ZFC) and field cooled (FC) magnetization below ~100 K and faster magnetic relaxation. Interestingly, in nanoscale BiFeO 3 , one also observes finite coercivity at lower temperature which points out that suitable design of particle size and shape may induce ferromagnetism. The inhomogeneous distribution of Bi/Fe-ions and/or oxygen nonstoichiometry seems to be giving rise to broad features in thermal, magnetic as well as in electrical responses.

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SnbductionAlthough the magnetic properties of commercial SI femte magnets have been impmved mainly by means of process optimization, the further improvement is intensively demanded in automobile and electric appliance industry. In this study, the effect of the partial substitution of F@+ and Srz+ ion for Cd' and Last ion respectively on the coerdve force of Sr ferrite magnets was investigated. The composition formula of ferrite materials which were investigated is as follows.(S++l.Laa+x)O * n((FeJ+~.yCo*+y)zOos}, x=Zny. n=5.4-6.0 where x=Zny is the charge compensation condition which expresses the number of La9 ion ia equal to that of C4+ ion and n is the molar ratio of Fe& to SrO.After FeaOs. SrCOa, L a o s and CosOlpowder were weighted so that materials with x=O.0-0.4 in the above composition formula were obtained after calcination, they were mixed in water, then dried and calcined in air. The calcined powder was wet milled with additives(CaCOs, SiOz) using an attritor. The finely milled slurry was pressed into disk shaped compacta in the magnetic field of lOkOe which was parallel to the pressing direction. The pressed compacts were sintered at 1190-124053 in air. X-ray diEraction, Curie temperature and B-H hysteresis curve measurement were carried out.Bssults and discussip??X-ray diEraction examination showed that the calcined powder of x=0.0-0.4 was composed of aingle phase of Magnetoplumbite crystal structure. Fig.1 shows the cbange of lattice constant a and c of hexagonal structure by the addition of La-Co. It is found that c ads is made shorter by the La-Co addition although c axis remains constant. Fig2 shows the result of Curie temperature measurement. It is found that Curie temperature is decreased by the La-Co addition. Because lattice constant and Curie temperature varies systematically with La-Co content as shown in Fig 1 and 2, it is considered that La-Co substituted material whose composition formula is shown above was substantially formed. Fig.3 shows the effect of La-Co substitution on the magnetic properties of sinteredSr ferrite magnets. The magnetic properties of Sr fernte magnets to which CrzOa was added instead of La-CO are also shown for comparison. It is found in this figure that the coercive force is increased without si&cant decrease in residual flux density by La-Co substitution. Temperature eo&cients of coercive force were found tu be also improved by La-CO substitution a8 shown in Fig.4.The residual orbital magnetic moment of C4* ion[l] is considered to play a main role to increase the coercive force and improve the temperature coefEcient of La-Co substituted Sr femte magnets. ; . : m 23.02 23.00 22'98 0 0.1 0.2 0.3 0.4