A Neutron Diffraction Investigation of High Valent Doped Barium Ferrite With Super-wide Tunable Microwave Absorption


 The Barium ferrite BaTixFe12-xO19 (x = 0.2, 0.4, 0.6, 0.8) ceramics doped by Ti4+ (BFTO-x) were synthesized by a modified sol-gel method. The crystal structure and magnetic structure of the samples were determined by neutron diffraction, and confirm that the BFTO-x ceramics were high quality single phase with sheet micro-structure. With x increasing from 0.2 to 0.8, the Ms decreases gradually but the change of Hc is complex under the synergy of the changed grain size and the magnetic crystal anisotropy field. Relying on the high valence of Ti4+, double resonance peaks are obtained in the curves of μ′′ and the resonance peaks could move towards the low frequency with the increase of x, which facilitate the samples perform an excellent wide-band modulation microwave absorption property. In the x = 0.2 sample, the maximum reflection loss can reach –44.9 dB at the thickness of only 1.8 mm, and the bandwidth could reach to 5.28 GHz at 2 mm when the RL is less than –10 dB. All the BFTO-x ceramics show excellent frequency modulation ability varying from 18 GHz (x = 0.8) to 4 GHz (x = 0.4), which covers 81% of the investigated frequency in microwave absorption field.


1 Effects of titanium substitution on crystal structure and microstructure
The neutron diffraction patterns of BFTO-x (x = 0.2, 0.4, 0.6, 0.8) ceramics are shown in Figure 1(a), which contains the complete neutron diffraction pattern before and after refinement using Rietveld method. Since the neutron diffraction patterns include both structural and magnetic information, the process of BFTO-x refinement using FullProf can be divided into two parts: structural and magnetic phase [17]. The detailed results of refinement contained structural and magnetic parameters also appear in the Figure 1(a), and the corresponding refinement factors of each components were also shown in the figure. Compared with the neutron diffraction Bragg positions of pure barium ferrite, no other impurity peaks were found, indicating that the four components were single phase with space group P63/mmc. Moreover, the corresponding magnetic space group of BFTO-x is R-3m:H, which was determined by utilizing the Sarah Analysis to calculate the possible magnetic space groups [18]. The result of structural refinement shows that the lattice parameters changes little with the increasing of Ti   Table 1. The result of fitting shows that Ti tends to occupy the position 2a, 2b, and 12k, in where Fe is spin-up.  The surface morphologies of BFTO-x ceramics are shown in Figure 2. It can be observed that all components perform typical hexagonal plates, and the grain boundary is distinct. The particle size of BFTO-x ceramics varies with the increase of x, where the grain size of BFTO-0.4 about 1 µm is larger than those of other three components.
Compared with the solid state sintering method, the grain size of BFTO ceramics prepared by the sol-gel process is smaller, which has an important effect on coercive force [20].

2 Effects of titanium substitution on magnetic properties
The magnetic properties of BFTO-x (x = 0.2, 0.4, 0.6, 0.8) ceramics are derived from magnetic hysteresis loops at room temperature in Figure 3. It can be seen that all samples are approaching saturation as the applied field is increased up to 15 kOe. With x increasing from 0.2 to 0.8, the Ms decreases gradually from 31.87 emu/g to 22.13 emu/g, while the change trend of Hc is complicated. The change trend of Ms and Hc is related to the magnetic moment of Fe 3+ and the occupied position during doping [21].
The replacement from Ti 4+ ion to the magnetic Fe 3+ ion (5 µB) would reduce the Ms of barium ferrite ceramics since the Ti 4+ ion is non-magnetic, and Ms exhibits a downward trend with the increase of Ti 4+ ion doping [22]. It is known that Hc is mainly  Moreover, refined magnetic structure shows that non-collinear magnetic structure exists in BFTO-x ceramics [17,24]. The magnetic moment of primitive cell varies with the doping content of Ti, which is same with experimental data, and the corresponding results was listed in Table 2. The experimental magnetic moment of primitive cell (Mpc) was calculated by the following formula 1:

3 Effects of titanium substitution on electromagnetic and microwave absorption properties
The electromagnetic properties of BFTO-x ceramics were shown in Figure 4  According to the formula fr = 1.4gHa, the natural resonant frequency is correlated with the magnetic anisotropy field (Ha) and the ceramic sample and the Hollande factor (g) [25]. Due to the low anisotropy field of BFTO-0.4 ceramic, the natural resonant frequency of the x = 0.4 sample is the lowest and appears near 8 GHz, while double resonance peaks were obtained in x = 0.6 and x = 0.8 components. The high-valence ionic Ti 4+ doping, which destroys the valence equilibrium of barium ferrite, will produce a new g factor, and the appearance of double resonance peaks shows that there are two g factors in these samples. With the increase of Ti 4+ doping, the anisotropy field decreases, and the natural resonance frequency moves to the lower frequency range.
Therefore, the appearance of double resonance peak is advantageous to obtain the larger bandwidth. More evidence can be found in Figure S2, which shows the dielectric and magnetic loss of PVA mixed BFTO-x ceramics. The large dielectric and magnetic loss was obtained in x = 0.2 sample, which indicates a large reflection loss. The loss curves of x = 0.6 and 0.8 have evident fluctuation, and, could get large bandwidth.
The reflection loss (RL) of PVA mixed BFTO-x ceramics can be calculated from the measured electromagnetic parameters [26], as shown in Figure 5.     x samples, and the maximum reflection loss and bandwidth are listed in Table 3. For the x = 0.2, the greatest loss can be obtained at the thickness of 1.8 mm, and the bandwidth can reach to 5.28 GHz at 2 mm when the reflection loss is less than -10 dB, while the bandwidth of other three components is about 3 GHz.  In the ion-doped barium ferrite ceramic, the strong absorbing ability originates from the high attenuation constant [29]. The attenuation constant is expressed by