Espinosa-Faller Francisco J, Howell Rafael C, Garcia-Adeva Angel J, Conradson Steven D, Ignatov Alexander Y, Tyson Trevor A, Farrow Robin F C, Toney Michael F
Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
J Phys Chem B. 2005 May 26;109(20):10406-18. doi: 10.1021/jp037449+.
The local atomic structure of the Mn in NiMn/NiFe exchange coupled films was investigated using Mn K-edge extended X-ray absorption fine structure (EXAFS) measurements to elucidate the possible correlation between the coercivity that can occur even in samples that display no signs of NiMn L1(0) ordering in diffraction patterns and such ordering on a length scale below the diffraction limit. Raising the substrate growth temperature from 3 to 200 degrees C increases the extent of L1(0) ordering in the NiMn pinning layer and the associated coercivity. A short-range order parameter (S(SRO)) was derived from EXAFS data for comparison with the long-range order parameter (S(LRO)) obtained from the X-ray diffraction measurements. Analogous to S(LRO), S(SRO) increases in tandem with the pinning layer coercivity, implying the presence of nanometer-scale ordered clusters at the beginning stages of macroscopic L1(0) phase formation that apparently foster antiferromagnetism despite their small size. The behavior of the EXAFS, especially the contributions of the more distant shells, also suggests that the overall structure in materials that are not fully L1(0)-ordered is more accurately described as locally ordered, magnetically ordered, incoherent nanodomains of the L1(0) phase separated by locally disordered, strained, interdomain regions that globally average to the fcc lattice with little or no local fcc structure present. The constraints on the sizes and other characteristics of these domains were explored by examining the diffraction patterns calculated for several two-dimensional analogue structures. These demonstrated that one of the most important structural features in the development of a two-phase diffraction pattern was the presence of dislocations in response to the elastic strain at the interfaces between domains where the accumulated expitaxial mismatch was greater than half of the bond length that rendered the domains incoherent with respect to each other.
利用锰K边扩展X射线吸收精细结构（EXAFS）测量研究了NiMn/NiFe交换耦合薄膜中锰的局部原子结构，以阐明即使在衍射图案中没有显示出NiMn L1(0)有序迹象的样品中出现的矫顽力与低于衍射极限长度尺度上的这种有序之间可能存在的相关性。将衬底生长温度从3℃提高到200℃会增加NiMn钉扎层中L1(0)有序的程度以及相关的矫顽力。从EXAFS数据中导出了一个短程有序参数（S(SRO)），用于与从X射线衍射测量中获得的长程有序参数（S(LRO)）进行比较。与S(LRO)类似，S(SRO)与钉扎层矫顽力同步增加，这意味着在宏观L1(0)相形成的初始阶段存在纳米级有序团簇，尽管它们尺寸小，但显然促进了反铁磁性。EXAFS的行为，特别是更远壳层的贡献，还表明未完全L1(0)有序的材料中的整体结构更准确地描述为局部有序、磁有序、由局部无序、应变的畴间区域分隔的L1(0)相的非相干纳米畴，这些区域在全局上平均为fcc晶格，几乎没有或不存在局部fcc结构。通过检查为几种二维类似结构计算的衍射图案，探索了对这些畴的尺寸和其他特征的限制。这些结果表明，在两相衍射图案发展过程中最重要的结构特征之一是，在畴间界面处由于弹性应变而存在位错，在这些界面处累积的外延失配大于键长的一半，使得畴彼此不相干。
