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 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.     

Coherent versus uncorrelated nanoscale heterogeneities in L1(0) solid solutions and their signatures from local and extended probes

The structural properties of the phase coexistence of chemically ordered L1(0) and chemically disordered structures within binary alloys are investigated, using the NiMn system as an example. Theoretical and numerical predictions of the signatures that one might expect in data from local and extended probes are presented in an attempt to explain the presence of antiferromagnetism in NiMn when no L1(0) signatures appear in diffraction data. Two scenarios are considered. In the first scenario, the tetragonal L1(0) structure and fcc chemically disordered structure are distributed evenly into uncorrelated domains of specified average diameter. The diffraction limit, below which the two structures can only be distinguished using a local probe, is quantified with respect to the domain diameter by applying straightforward diffraction analysis. In the second scenario, domains with chemical ordering oriented in different directions are required to maintain their atomic coherence with each other. A numerical treatment is used to illustrate the long-range strain that results from elastic energy considerations, and the effects on the structure factor (extended probe) and pair distribution function (local probe) are investigated.