The electron-capture decay of a cobalt-57 atom triggers an Auger event resulting in the loss of several electrons from the molecule in which it is incorporated. The 14.4 keY Miissbauer emission conveys information regarding the chemical forms in which the daughter iron-57 is stabilized within IO~ 7 sec following electron capture. The electronic relaxation occurs very rapidly and several tens of e V electronic excitation energy is deposited on the molecule during charge neutralization. Emission Miissbauer spectra of cobalt-57 labeled complexes of the strong field ligands phenanthroline and ethylenediamine. namely [ 57 Co(III)(phenhl(C10.),.2H,O, [ 57 Co(III)(phen),(en)]Cl,.H,O, ["Co(III)(phen)(en),] (N0 3 h, and["Co(III)(en)3]Cl, were studied. The chelate molecules possess varying degree of conjugation. The presence of strong field ligands in the chelates made it easier to identify the high spin ionic iron species Pe H and Pe3+, which can form only as a result of fragmentation. Molecules which have no conjugation, e.g., ["Co(III)(en),l'+, or which possess small degree of conjugation like ["Co(III)(phen) (en),l' + , fragment in practically 100% of the Auger events. On the other hand, the molecules of ['7Co(III)(phen),(en)1'+ , which possess relatively higher degree of conjugation, escape fragmentation in about 20% of the events; while the highly conjugated molecules of [ 57 Co(III) (phen),]3+ seem to escape fragmentation in almost 100% of the events. One can perhaps conclude that the probability of fragmentation of a molecule as a consequence of the Auger event diminishes with higher degree of conjugation in the molecule. It is speculated that the aftereffects of the Auger event result in collective excitation of the 7T-electrons in a highly conjugated system. The de-excitation of the "plasmon" occurs rapidly, presumably through emission of a single electron or an energetic photon. The mixed ligand chelates, ['7Co(III)(phen),(en)]3+ and ['7Co(III)(phen) (en),l'+ show an interesting phenomenon. The cationic iron species formed as a result of fragmentation recombines with electron(s) to form an electronically excited cation, presumably 57Pe+2 which interacts with neighboring cobalt chelate molecules. The phenanthroline ligands are preferentially transferred onto the electronically excited cation forming ["Pe(III)(phen),l'+ quite efficiently. The transfer of ligands occur in less than 10-7 sec., as is evidenced by the manifestation of the tris-phenanthroline chelate in the emission Miissbauer spectra.

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Giant intrinsic magnetic hardness is found at low temperatures in structurally homogeneous materials SmNi5−xCux. Magnetic hardness peaks at a composition SmNi2.5Cu2.5 where the crystal field disorder due to compositional randomization is at a maximum. By comparison with SmCo5−xNix, where giant intrinsic magnetic hardness originates, in part, from fluctuations from site to site of the magnitude of moment and strength of exchange of the transition metal, magnetic hardness in the SmNi5−xCux is predominantly due to anisotropy and concomitant exchange fluctuations on Sm sites.

The system SmCo5−xCux was studied with respect to constitution and magnetic properties, focusing on the mechanism of magnetization. Different modes of preparation result either in single-phase or in multiphase materials as determined by x-ray diffraction. Pseudobinary materials show giant intrinsic magnetic hardness irrespective of mode of preparation. The temperature dependence of coercive force (Hc) is in accordance with a model based on thermally activated domain-wall propagation. Maximum values of Hc extrapolated to absolute zero are the highest for any material so far reported (of order Hc=300 kOe). These values are slightly greater than the maximum in SmCo5−xNix. It is found that a local moment picture predicts relative magnitudes of both magnetic moment and coercivity as a function of composition. Accordingly coercivity is dependent on the number of weakly coupled transition metal atoms.