Department of Earth Sciences, University of Cambridge, Cambridge, UK.
J Phys Condens Matter. 2013 Mar 20;25(11):116006. doi: 10.1088/0953-8984/25/11/116006. Epub 2013 Feb 19.
Hematite, Fe(2)O(3), provides in principle a model system for multiferroic (ferromagnetic/ferroelastic) behavior at low levels of strain coupling. The elastic and anelastic behavior associated with magnetic phase transitions in a natural polycrystalline sample have therefore been studied by resonant ultrasound spectroscopy (RUS) in the temperature range from 11 to 1072 K. Small changes in softening and attenuation are interpreted in terms of weak but significant coupling of symmetry-breaking and non-symmetry-breaking strains with magnetic order parameters in the structural sequence R3(¯)c1'-->C2/c-->R3(¯)c. The R3(¯)c1'-->C2/c transition at T(N) = 946 ± 1 K is an example of a multiferroic transition which has both ferromagnetic (from canting of antiferromagnetically ordered spin moments) and ferroelastic (rhombohedral → monoclinic) character. By analogy with the improper ferroelastic transition in Pb(3)(PO(4))(2), W and W' ferroelastic twin walls which are also 60° and 120° magnetic domain walls should develop. These have been tentatively identified from microstructures reported in the literature. The very low attenuation in the stability field of the C2/c structure in the polycrystalline sample used in the present study, in comparison with the strong acoustic dissipation reported for single crystal samples, implies, however, that the individual grains each consist of a single ferroelastic domain or that the twin walls are strongly pinned by grain boundaries. This absence of attenuation allows an intrinsic loss mechanism associated with the transition point to be seen and interpreted in terms of local coupling of shear strains with fluctuations which have relaxation times in the vicinity of ~10(-8) s. The first order C2/c-->R3(¯)c (Morin) transition occurs through a temperature interval of coexisting phases but the absence of an acoustic loss peak suggests that the relaxation time for interface motion is short in comparison with the time scale of the applied stress (at ~0.1-1 MHz). Below the Morin transition a pattern of attenuation which resembles that seen below ferroelastic transitions has been found, even though the ideal low temperature structure cannot contain ferroelastic twins. This loss behavior is tentatively ascribed to the presence of local ferromagnetically ordered defect regions which are coupled locally to shear strains.
赤铁矿，Fe(2)O(3)，在低应变耦合水平下为多铁性（铁磁/铁弹）行为提供了一个理想的模型系统。因此，通过共振超声光谱（RUS）在 11 至 1072 K 的温度范围内研究了天然多晶样品中与磁相变相关的弹性和非弹性行为。软化和衰减的微小变化可以用对称性破缺和非对称性破缺应变与结构序列 R3(¯)c1'->C2/c->R3(¯)c 中的磁序参数的弱但显著的耦合来解释。在 T(N) = 946 ± 1 K 处的 R3(¯)c1'->C2/c 转变是一个多铁转变的例子，它具有铁磁性（来自反铁磁有序自旋矩的倾斜）和铁弹（菱面体→单斜）特性。通过与 Pb(3)(PO(4))(2)中的不正当铁弹转变类似，W 和 W'铁弹孪晶壁也应该是 60°和 120°的磁畴壁。这些已经从文献中报道的微观结构中被初步识别出来。与单晶样品报道的强烈声衰减相比，在本研究中使用的多晶样品的 C2/c 结构稳定场中非常低的衰减，意味着每个晶粒都由单个铁弹畴组成，或者孪晶壁被晶界强烈固定。这种没有衰减的情况允许看到与转变点相关的固有损耗机制，并根据剪切应变与弛豫时间在10(-8) s 附近的波动的局部耦合来解释。第一类 C2/c->R3(¯)c（Morin）转变发生在共存相的温度间隔内，但没有声学损耗峰表明，界面运动的弛豫时间比施加的应力的时间尺度（在0.1-1 MHz 左右）短。在 Morin 转变以下，即使理想的低温结构不能包含铁弹孪晶，也发现了类似于铁弹转变以下的衰减模式。这种损耗行为被暂时归因于局部铁磁有序缺陷区域的存在，这些区域局部耦合到剪切应变。
