RESUMO
The theory of heterodyne/stroboscopic detection of nuclear resonance scattering is developed, starting from the total scattering matrix as a product of the matrix of the reference sample and the sample under study. This general approach holds for all dynamical scattering channels. In the forward channel, which has been discussed in detail in the literature, the electronic scattering manifests itself only in an energy-independent diminution of the scattered intensity. In all other channels, complex resonance line shapes of the heterodyne/stroboscopic spectra are encountered, as a result of the interference of electronic and nuclear scattering. The grazing-incidence case will be evaluated and described in detail. Experimental data of classical X-ray reflectivity and their stroboscopically detected resonant counterpart spectra on the [(nat)Fe/(57)Fe]10 isotope periodic multilayer and antiferromagnetic [(57)Fe/Cr]20 superlattice are fitted simultaneously.
RESUMO
Reciprocity is when the scattering amplitude of wave propagation satisfies a symmetry property, connecting a scattering process with an appropriate reversed one. We report on an experiment using nuclear resonance scattering of synchrotron radiation, which demonstrates that magneto-optical materials do not necessarily violate reciprocity. The setting enables us to switch easily between reciprocity and its violation. In the latter case, the exhibited reciprocity violation is orders of magnitude larger than achieved by previous wave scattering experiments.
RESUMO
The valence states of the nucleogenic (57)Fe arising from the nuclear disintegration of radioactive (57)Co by electron capture decay, (57)Co(EC)(57)Fe, have been studied by Mössbauer emission spectroscopy (MES) in the (57)Co-labeled systems: [(57)Co/Co(terpy)(2)]Cl(2).5H(2)O (1), [(57)Co/Co(terpy)(2)](ClO(4))(2).(1)/(2)H(2)O (2), and [(57)Co/Mn(terpy)(2)](ClO(4))(2). (1)/(2)H(2)O (3) (terpy = 2,2':6',2' '-terpyridine). The compounds 1, 2, and 3 were labeled with ca. 1 mCi of (57)Co and were used as the Mössbauer sources at variable temperatures between 300 K and ca. 4 K. [Fe(terpy)(2)]X(2) is a diamagnetic low-spin (LS) complex, independent of the nature of the anion X, while [Co(terpy)(2)]X(2) complexes show gradual spin transition as the temperature is varied. The Co(II) ion in 1 "feels" a somewhat stronger ligand field than that in 2; as a result, 83% of 1 stays in the LS state at 321 K, while in 2 the high-spin (HS) state dominates at 320 K and converts gradually to the LS state with a transition temperature of T(1/2) approximately 180 K. Variable-temperature Mössbauer emission spectra for 1, 2, and 3 showed only LS-(57)Fe(II) species at 295 K. On lowering the temperature, metastable HS Fe(II) species generated by the (57)Co(EC)(57)Fe process start to grow at ca. 100 K in 1, at ca. 200 K in 2, and at ca. 250 K in 3, reaching maximum values of 0.3 at 20 K in 1, 0.8 at 50 K in 2, and 0.86 at 100 K in 3, respectively. The lifetime of the metastable HS states correlates with the local ligand field strength, and this is in line with the "inverse energy gap law" already successfully applied in LIESST relaxation studies.
RESUMO
The domain structure of an antiferromagnetic superlattice is studied. Synchrotron Mössbauer and polarized neutron reflectometric maps show micrometer-size primary domain formation as the external field decreases from saturation to remanence. A secondary domain state consisting mainly of at least 1 order of magnitude larger domains is created when a small field along the layer magnetizations induces a bulk-spin-flop transition. The domain-size distribution is reproducibly dependent on the magnetic prehistory. The condition for domain coarsening is shown to be the equilibrium of the external field energy with the anisotropy energy.