尖晶石铁氧体TixNi1-xFe2O4中阳离子分布和Ti离子磁矩的实验研究

采用固相反应法制备了系列样品Ti_xNi_1-xFe₂O₄ (x=0.0, 0.1, 0.2, 0.3, 0.4). 室温下的X射线衍射谱表明样品全部为(A)B]₂O₄型单相立方尖晶石结构, 属于空间群Fd3m. 样品的晶格常数随Ti掺杂量的增加而增大. 样品在10 K温度下的比饱和磁化强度σ_S随着Ti掺杂量x的增加逐渐减小. 研究发现, 当Ti掺杂量x≥ 0.2时, 磁化强度σ随温度T的变化曲线出现两个转变温度T_L和T_N. 当温度低于T_N时, 磁化强度明显减小; 当温度达到T_N时, dσ/dT具有最大值. σ-T曲线的这些特征表明, 由于Ti掺杂在样品中出现了附加的反铁磁结构. 这说明样品中的Ti离子不是无磁性的+4价离子, 而是以+2和+3价态存在, 其离子磁矩的方向与Fe和Ni离子的磁矩方向相反. 利用本课题组提出的量子力学方势垒模型拟合样品在10 K温度下的磁矩, 得到了Ti, Fe和Ni三种阳离子在(A)位和B]位的分布情况, 并发现在所有掺杂样品中, 80%的Ti离子以+2价态占据尖晶石结构的B]位.

Study on cation distribution and magnetic moment of Ti ions in spinel ferrites Ni1-xTixFe2O4

Spinel ferrite samples Ti_xNi_1-xFe₂O₄ (x=0, 0.1, 0.2, 0.3, 0.4) were prepared using conventional solid reaction method. The sample exhibit a single-phase cubic spinel structure with a space group obtained Fd3m. The lattice parameter a increases with the increase of Ti doping level x. But the specific saturation magnetizations, σ_s, gradually decrease with increasing Ti doping level x at 10 K and 300 K. It is interesting that when the doping level x≥0.2, two transition temperatures, T_L and T_N, are found: when the temperature is lower than T_N, the magnetization is obviously decreased, while at the temperature T_L, dσ/d T reaches a maximum value. This phenomenon indicates that an additional antiferromagnetic structure arises in the traditional spinel phase of ferrites which results from Ti doping, that Ti ions will show the form of Ti³⁺ and Ti²⁺ cations which have magnetic moments, and that the magnetic moments of the Ti cations are opposite to those of the Fe and Ni cations. The dependence of the magnetic moments of the samples on the Ti doping level x at 10 K was fitted successfully using the quantum-mechanical potential barrier model proposed earlier by our group. In the fitting process, the distributions of Ti, Ni and Fe cations in the samples are obtained. It is found that 80% of the Ti cations will occupy the B] sites in Ti²⁺ form.