Determination of the mechanism of cation ordering in magnesioferrite (MgFe2O4) from the time- and temperature-dependence of magnetic susceptibility
Determination of the mechanism of cation ordering in magnesioferrite (MgFe2O4) from the time- and temperature-dependence of magnetic susceptibility
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DOI：
10.1007/s002690050192
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发表时间：
1999-04
影响因子：
1.4
通讯作者：
R. Harrison;A. Putnis
中科院分区：
文献类型：
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作者：
R. Harrison;A. Putnis
通过测量合成样品的居里温度(T_C)随等温退火时间的变化，研究了MgFe_2O_4的非收敛阳离子有序化动力学。起始材料是近化学计量比的镁铁氧体的合成样品，由空气中的氧化物合成，在900℃的水中淬火。利用这种材料的小芯片进行了有序实验，并在450°C到600°C的温度下对它们进行了热处理。这些芯片定期从炉中取出，通过测量作为温度(T)到400°C的交变磁场磁化率(χ)来确定它们的居里温度。镁铁氧体的居里温度对尖晶石晶体结构的四面体和八面体位置之间的Fe~(3+)和Mg离子在晶内的分布非常敏感，因此提供了一个非常灵敏的阳离子有序化过程的探针。在等温退火过程中，χ-T曲线发生了两个明显的磁转变，第一个温度对应于无序起始材料的T_c，第二个温度高于平衡有序相的T_c。来自有序相的磁信号的大小作为时间的函数平滑地增加，直到接近平衡，并且χ-T曲线的形状对应于均匀有序相的单个尖锐的磁相变。这些观察表明，镁铁氧体中的阳离子有序化是通过一种非均相机制进行的，包括有序相在无序材料基质中的微尺度磁区的形核和生长。对在558℃下平衡的材料进行了无序实验，并在695℃下对其进行了热处理。等温无序的机制包括有序基质中无序区的成核和长大，以及有序基质的连续无序。这种混合的无序机制可以解释用X射线衍射法观察到的有序化和无序化速率的不同。根据Ginzburg-Landau速率定律讨论了非均相有序/无序机制的起源。在远离平衡的动力学实验中，可能会出现非均相机制，而在缓慢的平衡冷却下，可能会出现均一机制。讨论了这些观测结果对地球测速的影响。
The kinetics of non-convergent cation ordering in MgFe2O4have been studied by measuring the Curie temperature (Tc) of synthetic samples as a function of isothermal annealing time. The starting material was a synthetic sample of near-stoichiometric MgFe2O4, synthesised from the oxides in air and quenched from 900 °C in water. Ordering experiments were performed using small chips of this material and annealing them at temperatures between 450 °C and 600 °C. The chips were periodically removed from the furnace, and their Curie temperatures were determined from measurements of alternating-field magnetic susceptibility (χ) as a function of temperature (T) to 400 °C. The Curie temperature of MgFe2O4is very sensitive to the intracrystalline distribution of Fe3+and Mg cations between tetrahedral and octahedral sites of the spinel crystal structure, and hence provides a very sensitive probe of the cation ordering process. The χ-Tcurve for the starting material displays a single sharp magnetic transition at a temperature of 303 °C. During isothermal annealing, the χ-Tcurve develops two distinct magnetic transitions; the first at a temperature corresponding toTcfor the disordered starting material and the second at a higher temperature corresponding toTcfor the equilibrium ordered phase. The size of the magnetic signal from the ordered phase increases smoothly as a function of time, until equilibrium is approached and the shape of the χ-Tcurve corresponds to a single sharp magnetic transition for the homogeneous ordered phase. These observations demonstrate that cation ordering in MgFe2O4proceeds via a heterogeneous mechanism, involving the nucleation and growth of fine-scale domains of the ordered phase within a matrix of disordered material. Disordering experiments were performed by taking material equilibrated at 558 °C and annealing it at 695 °C. The mechanism of isothermal disordering is shown to involve nucleation and growth of disordered domains within an ordered matrix, combined with continuous disordering of the ordered matrix. This mixed mechanism of disordering may provide an explanation for the difference between the rates of ordering and disordering observed in MgFe2O4using X-ray diffraction. The origin of the heterogeneous ordering/disordering mechanism is discussed in terms of the Ginzburg-Landau rate law. It is argued that heterogeneous mechanisms are likely to occur in kinetic experiments performed far from equilibrium, whereas a homogeneous mechanism may operate under slow equilibrium cooling. The implications of these observations for geospeedometry are discussed.