Multi-susceptibile Single-Phased Ceramics with Both Considerable Magnetic and Dielectric Properties by Selectively Doping

Multiferroic ceramics with extraordinary susceptibilities coexisting are vitally important for the multi-functionality and integration of electronic devices. However, multiferroic composites, as the most potential candidates, will introduce inevitable interface deficiencies and thus dielectric loss from dissimilar phases. In this study, single-phased ferrite ceramics with considerable magnetic and dielectric performances appearing simultaneously were fabricated by doping target ions in higher valence than that of Fe3+, such as Ti4+, Nb5+ and Zr4+, into BaFe12O19. In terms of charge balance, Fe3+/Fe2+ pair dipoles are produced through the substitution of Fe3+ by high-valenced ions. The electron hopping between Fe3+ and Fe2+ ions results in colossal permittivity. Whilst the single-phased ceramics doped by target ions exhibit low dielectric loss naturally due to the diminishment of interfacial polarization and still maintain typical magnetic properties. This study provides a convenient method to attain practicable materials with both outstanding magnetic and dielectric properties, which may be of interest to integration and multi-functionality of electronic devices.

N owadays, the integration and multi-functionality are important goals in the area of developing high quality electronic devices. To contribute multi susceptibilities such as magnetic and dielectric properties simultaneously, percolative ceramic composites composed of ferroelectric and ferromagnetic phases have been extensively studied in recent decades [1][2][3][4][5][6] . The percolative composites are impressive to contribute both extraordinary dielectric and magnetic performances, because the paradox caused by composite law is solved 7,8 . However, the mismatched interfaces or grain boundaries originating from the inevitable contact between dissimilar lattice structures will be introduced, thus deficiencies as well as space charges will appear in the composite ceramics. The dielectric loss determined by Maxwell Wagner effect will increase considerably sometimes to ,1 9 , which implies that it is of great importance to eliminate the mismatch and diminish imperfect interfaces to decrease the dielectric loss.
Single-phased ceramic materials are naturally thought to be candidates with relatively matched interfaces and low dielectric loss. Herein, single-phased BaFe 12-x Ti(Nb, Zr) x O 19 ceramics were fabricated by a sol-gel process. It is worth noting that BaFe 12 O 19 is a typical ferrite and has excellent magnetic properties. Ti 41 , Nb 51 and Zr 41 are universal doping ions with higher electron valence than that of Fe 31 . In this case, Ti 41 , Nb 51 and Zr 41 ions are found to replace Fe 31 ions due to their close ionic radii and valance state, making part of neighbor Fe 31 ions transfer into Fe 21 ions for charge balance 10 . The electron hopping between Fe 31 and Fe 21 ions form electron pair dipoles(Fe 31 /Fe 21 pairs), which will cause colossal permittivity 11 . Dielectric loss is decreased, which is exactly lower than that of the percolative composites, due to the elimination of the mismatched grain boundaries and thus diminishment of space charges in this case. In this work, the simultaneous advent of colossal permittivity, excellent magnetic properties and low losses suggests that the single-phased ferrite ceramics doped by target ions are potential multifunctional ceramic candidates. structural parameters 'a' and 'c' as well as cell volume of barium ferrite ceramics are listed in Table 1. It is found that lattice constants and cell volume both decrease initially from x 5 0 to x 5 0.4 and then increase slowly with Ti 41 ions increasing. The smallest ''a'' and ''c'' are obtained when x 5 0.4. Fig. 2 shows SEM photographs of the barium ferrite ceramics. It is seen that the ceramics with or without Ti 41 doping all form typical hexagonal plate-like particles. The grain size seems to become larger when Ti 41 content increases from x 5 0 to x 5 0.8 as shown from Fig. 2(a) to Fig. 2(d), which indicates that the formation and grain growth of barium ferrite ceramics are promoted apparently with increasing Ti 41 content.
The lattice constants of the ferrites decrease initially and then increase slowly with addition of Ti 41 . As is known, Ti 41 ions added into the ceramic matrix probably substitute for constituent Fe 31 ions in barium ferrite due to the close ionic radius of Ti 41 (0.605 Å ) and Fe 31 (0.645 Å ) 12 , which has been evidenced by Mössbauer spectroscopy elsewhere 13 . Meanwhile, defect reaction as following will be triggered.
It is seen that some Fe 31 ions will be transferred to be Fe 21 ions in terms of charge balance. In fact, there are many deficiencies such as oxygen vacancies with positive charge exist in barium ferrite ceramics originally 14 (Fig. 3a) and BaFe 11.2 Ti 0.8 O 19 (Fig. 3b) after subtracting baseline, in which C1s peak at 285 eV was used for charge correction and the peaks at ,709.3 eV and ,722.8 eV are belong to  Magnetic properties kept high effectively in the doping ferrite.   16 : where A is the inhomogeneity parameter, B is the anisotropy parameter and x p is the high-field differential susceptibility. B of hexagonal symmetry can be expressed as Eq. (3).
It is shown in Table 2 that H a of BaFe 12-x Ti x O 19 decreases dramatically from 15.43 kOe to 10.43 kOe as x varies from 0 to 0.8. In fact, grain size increases and thus the amount of grain boundaries will decrease with increasing Ti 41 ions in the ferrites as shown in Fig. 2. Fe 31 ions which in general contributes H a in barium ferrite is substituted by non-magnetic Ti 41 ions in this case and hence H a will decrease. The more the Fe 31 ions are replaced, the weaker the H a is Ref. 16. The H a of BaFe 12-x Ti x O 19 decreases therefore with increasing Ti 41 content to x 5 0.8 although the reduction in grain boundaries will increase H a . In addition, coercivity which represents the ability   to resist magnetization reversal process under a reverse magnetic field is mainly controlled by the hindrance for nucleation of reverse domain, domain wall motion and spin rotation in magnetic materials 17 . As H a which impedes spin rotation decreases with increasing Ti 41 ions in the ferrites, the coercivity decreases hence with increasing Ti 41 ions 18 . Moreover, nuclei of reverse domains are easily formed around the deficiencies such as grain boundaries and dislocations, while the deficiencies can also act as pinning centers to hinder domain wall motion. In fact, grain boundaries have a marked influence on pinning domain wall instead of inducing nucleation of reverse domains in barium ferrite 19 . The coercivity of barium ferrite will decrease significantly with decreasing grain boundaries (increasing grain size). Due to the decrease in both H a and grain boundaries, the coercivity decreases eventually from 3.04 kOe to 1.06 kOe with increasing Ti 41 content from x 5 0 to x 5 0.8, which is about 65% lower than that of undoped one. It implies that the Ti 41 doped barium ferrite ceramics will be a good candidate with low energy consumption applied in devices.
Meanwhile, M s and M r still maintain high values in BaFe 11.2 Ti 0.8 O 19 . In fact, Fe 31 ions of BaFe 12 O 19 in 12k, 2a, and 2b are up-spin and 4f 1 and 4f 2 are down-spin 20 . In this case, the net magnetization of the ferrites is contributed by excess of up-spin magnetic moments. That is to say, if Fe 31 ions are substituted by non-magnetic Ti 41 ions in 12k, 2a, and 2b sites, magnetization will decrease. Conversely, magnetization will be improved if Fe 31 ions which are in 4f 1 and 4f 2 sites are substituted. As is reported, Ti 41 substitutes preferably for Fe 31 in 12k, 2b, and 4f 2 sites with both states of up spin and down spin 21 . It leads to only a gentle decline of both saturation and residual magnetization in the ferrites with increasing Ti 41 . Apparently, M s and M r of BaFe 11.2 Ti 0.8 O 19 of about 60 emu/g and 29 emu/g, which are only 20 , 30% lower than that of undoped one, are still high enough in practical use in devices keeping typical magnetic properties. Fig. 6(a) and 6(b) shows the permeability and magnetic loss tangent of the BaFe 12-x Ti x O 19 (x 5 0, 0.4, 0.6 and 0.8) ceramics as a function of frequency respectively. It is seen that permeability of all the samples is almost independent of frequency, except for a little bit decrease in BaFe 11.2 Ti 0.8 O 19 above 70 MHz. Meanwhile, the frequency independent permeability increases rapidly from about 1.5 to 5.1 with x varying from 0 to 0.8. The magnetic loss tangent of the ferrites depends on frequency and Ti 41 content. It decreases from ,0.35 to ,0.07 at low frequency of 1 MHz and increases from ,0.1 to ,0.4 around 100 MHz respectively with increasing content of Ti 41 ions from x 5 0 to x 5 0.8. While it is as low as ,0.1 at moderate frequency.
Obviously, the permeability improves with increasing Ti 41 ions in barium ferrites. As is known, domain wall motion and domain rotation are two dominant magnetization processes for polycrystalline ferrites 22 . For the BaFe 12-x Ti x O 19 polycrystalline ceramics, grain boundaries abating due to larger grain size in high Ti 41 doped samples promotes domain wall motion 23 . Furthermore, anisotropic field and demagnetizing field impeding domain rotation in the ferrites decreases with increasing Ti 41 content 24 . Consequently, controlled by enhancing both domain wall motion and spin rotation, the permeability of BaFe 11.2 Ti 0.8 O 19 ceramic is improved to 5.1, which is 3 , 4 times of BaFe 12 O 19 ceramic.
Except for the permeability, magnetic loss is also related naturally to the doping content in the ferrites. Considering the high electrical resistivity of barium ferrite, eddy loss can be neglected 10 . Magnetic loss of BaFe 12-x Ti x O 19 over the frequency range between 1 MHz and 100 MHz is contributed dominantly by hysteresis loss and residual loss 25 . Actually, hysteresis loss is predominant over lower frequency range, while residual loss takes charge in higher frequency range 26 . As can be seen in Fig. 5, area of hysteresis loops reduces with increasing Ti 41 ions content. The magnetic loss decreasing with Ti 41 content at low frequency is thus controlled by hysteresis loss, which is ,0.35 of BaFe 12 O 19 to ,0.07 of BaFe 11.2 Ti 0.8 O 19 at ,1 MHz. However, the residual loss is contributed in general by domain wall resonance which occurs at frequency above 100 MHz in BaFe 12 O 19 27 . As shown in Fig. 6(b), it seems that the resonance peak moves toward lower frequency range with doping Ti 41 in the ferrites. It implies that the magnetic loss of the BaFe 12-x Ti x O 19 ceramics is importantly controlled by the residual loss, which increases a little from ,0.1 to ,0.4 with doping Ti ions from x 5 0 to 0.8 at ,100 MHz. At moderate frequency, magnetic loss is as low as ,0.1, which is a little bit decrease with doping Ti in the ferrites due to a little reduction of hysteresis loss. Obviously, the magnetic loss of BaFe 12-x Ti x O 19 ceramics is diminished to be lower than ,0.1 with increasing Ti 41 content to x 5 0.8, especially at frequencies below 70 MHz.
Colossal permittivity and low dielectric loss of the doped singlephased ferrites. Fig. 7(a) and 7(b) shows the permittivity and dielectric loss tangent of the BaFe 12-x Ti x O 19 (x 5 0, 0.4, 0.6 and 0.8) ceramics as a function of frequency respectively. The permittivity of the ceramics increases significantly at low frequency range with doping Ti 41 . Unlike the permittivity of barium ferrite without doping which is almost independent of frequency, the permittivity of the Ti 41 ions doped barium ferrites decreases rapidly with increasing frequency and the decreasing speed becomes slow as the content of Ti 41 is high from x 5 0.4 , 0.6 to x 5 0.8. A steplike shoulder appears typically in permittivity at moderate frequency with Ti content of x $ 0.6. Colossal  It is known that space charge polarization contributes most probably high permittivity especially at low applied frequencies 28 . Free electric charges may easily increase with doping and move freely in ceramics without localization. It will contribute the permittivity to the ceramics due to the charge response and the permittivity thus decreases with frequency. Such as in the ceramics with Ti 41 content of x 5 0.4 , 0.6, the permittivity is apparently higher than that of the undoping one and decreases rapidly with increasing frequency at low frequency range, because the un-localized charges form in the barium ferrites with initially doping Ti 41 ions. Higher permittivity and more rapid decline at low frequency are exhibited with Ti 41 content increasing from x 5 0.4 to x 5 0.6. However, Fe 31 will most probably transform into Fe 21 to keep charge balance in the ferrites with high doping of Ti 41 ions. It implies that the electric charges generated will be localized between the two ions to form Fe 31 and Fe 21 pairs. Thus, as revealed in Fig. 6(a), the decline of permittivity at low frequency range becomes smoother with increasing Ti 41 content from x 5 0.4 , 0.6 to x 5 0.8.
In fact, the step-like shoulder at middle frequency range in BaFe 12-x Ti x O 19 with x $ 0.6 is based on Fe 31 /Fe 21 pair dipoles. As analyzed above, Fe 21 ions are supposed to be abundantly produced in the ferrites since x reaches 0.6. Fe 31 and Fe 21 pairs make most probably electron pair dipoles in the ferrites. The higher the Ti 41 content is in the ceramics, the more the amount of pair dipoles is. So the steplike shoulder which is generated generally by relaxation dipoles such as Fe 31 and Fe 21 pairs appears initially in BaFe 12-x Ti x O 19 with x $ 0.6 and becomes more apparent in BaFe 11.2 Ti 0.8 O 19 ceramic reasonably. Hence, the high permittivity is probably dominantly contributed by the Fe 31 /Fe 21 pair dipoles in BaFe 12-x Ti x O 19 with x $ 0.6 instead of by charge response in the ferrites with x , 0.6. Moreover, considering polycrystalline ceramics in this case, conductivity inhomogeneity will appear in the ferrites due to the different electron hopping styles or hopping species in grains compared with those in grain boundaries. According to the Koop's opinions, the conductivity inhomogeneity contributes importantly the high permittivity 11 . The colossal permittivity which is about 100 k below 100 KHz and 20 k above 1 MHz appears hence in high Ti 41 doped ferrite ceramics of BaFe 11.2 Ti 0.8 O 19 based on these two important contributions.
Furthermore, besides of high permittivity, the dielectric loss tangent of the ferrites decreases attractively with doping Ti 41 ions. The smallest dielectric loss of 0.2 is obtained in BaFe 11.2 Ti 0.8 O 19 at ,10 kHz. It is much lower than that of percolative ferroelectric/ ferromagnetic composite ceramics with both extraordinary dielectric and magnetic properties. In fact, in percolative ferroelectric/ferromagnetic composite ceramics, a great deal of space charges and other deficiencies will be produced in the interfaces between ferrite phases and ferroelectric phases due to the two different lattice structures. However, these deficiencies can be effectively eliminated in the single-phased ceramics as the grain boundaries are relatively perfectly matched among the identical lattice structures. Thus, the part of dielectric loss contributed by interfacial polarization is significantly decreased and low dielectric loss of only 0.2 appears in the single-phased barium ferrite ceramics. Apparently, single-phased ferrite ceramics doped with Ti 41 ions are potential multifunctional ceramics with both impressive magnetic and dielectric properties.
Dual properties appearing universally in the single-phased ferrites with Fe 31 /Fe 21 pairs. As a matter of fact, colossal permittivity as well as excellent magnetic properties appearing simultaneously in the single-phased ferrite ceramics is not due to Ti element itself but due to its higher valence state than that of Fe 31 in the ferrites. As is shown in Fig. 8  It indicates that different kinds of doping ions can be actually used to form the ferrites with dual susceptibilities and low losses, in which the only requirement is that the doping ions, such as Ti 41 , Nb 51 and Zr 41 , must have higher valence state than that of Fe 31 in the ferrites. It is worth noting that the ferrite and high valence ions used in this work are universal ones. That is to say, the single-phased ceramics with dual susceptibilities and low losses can be successfully and broadly obtained, which will benefit the area of developing electronic devices for integration and multi-functionality.

Conclusions
In summary, single-phased ceramics of BaFe 12-x Ti(Nb, Zr) x O 19 with both considerable magnetic and dielectric properties were synthesized successfully by a sol-gel process. The Fe 31 and Fe 21 pair dipoles are produced by the substitution of high valence ions, such as Ti 41 , Nb 51 and Zr 41 , for Fe 31 based on charge balance in the ferrites. As Ti 41 substitutes preferably for Fe 31 in the sites with two compensated spin directions in the barium ferrite, the saturation magnetization and residual magnetization of the Ti 41 doped ferrites still keep high values to be practically used. Controlled by hysteresis loss, the magnetic loss of the Ti 41 doped ferrite ceramics diminishes effectively. Following the electron hopping between Fe 31 and Fe 21 ions and conductivity inhomogeneity between grains and grain boundaries in the ferrites, giant permittivity appears. Eliminating completely the interfaces between dissimilar phase structures, the dielectric loss tangent of the single-phased ferrites reduces significantly compared with that of the extensively interested percolative ferroelectric/ferromagnetic composite ceramics. Obviously, the single-phased ferrite ceramics doped by target ions in higher valence than that of Fe 31 reveal both extraordinary magnetic and dielectric properties simultaneously, which are even more competitive compared with the known systems such as multiferroic composites because of lower dielectric loss and thus become the most promising multifunctional materials in application of electronic devices for integration and multi-functionality. proportion, the solutions A and B are mixed to get solutions C, ammonia was used to adjust the PH value to about 7. The solutions C were dried at 120uC in oven for 2 , 3 days to form fluffy dry gels, the gels were then further calcined at 800uC for 3 h and red-brown powders were achieved. Finally, the powders mixed with appropriate amount of 5% PVA were molded into a ring shape under a pressure of 10 Mpa and then sintered at 1200uC to form BaFe 12-x Ti x O 19 ceramics.

Methods
The phase structure and morphology of the ceramics were determined and observed by X-ray diffraction (XRD) (SHIMADZU XRD-6000, Cu Ka radiation) and scanning electron microscopy (SEM) (Hitachi SU-70 FESEM) respectively. The magnetic and dielectric properties were measured by magnetic property measurement system (MPMS-XL-5) and impedance analyzer (Agilent 4294A).