Local atomic structure of partially ordered NiMn in NiMn/NiFe exchange coupled layers: 1. XAFS measurements and structural refinement

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 °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 fee 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.