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43296-B10
Magnetism and Phase Transition of Mixed and Dilute Magnetic Systems
Prompted by our recent work on the metamagnetic behavior in the mixed system Fe/Ni dichloride dihydrate (previous report), major reanalysis and interpretation of existing data on single crystal cobalt dithiocyanate dimethanol was pursued. Pronounced anisotropy in the magnetic susceptibility is present throughout the 1.6-300 K range. Susceptibility maxima occur along each of three orthogonal crystal directions at 4.45 K, with data along monoclinic a* having the appearance of an antiferromagnetic easy axis and a magnetic ordering temperature of 4.15 K. There are no indications of lower dimensional behavior, and three dimensional Ising model series expansion fits, both high and low temperature, to the a* susceptibility are very plausible. These imply a dominant ferromagnetic exchange interaction despite the overall antiferromagnetism, a situation often yielding metamagnetism. Such is confirmed in this material via the appearance and analysis of magnetization vs field isotherms along a*. Associated field induced transitions appear in the several kG range. An interpretation emerges in which, somewhat unexpectedly, the exchange within metal thiocyanate layers is ferromagnetic and that between layers is antiferromagnetic. Complementary to the strong a* transitions are much weaker field induced transitions along the orthogonal axes b and c, with rather remarkably two such along c. These are believed to be spin reorientation in nature, and associated with a multisublattice structure involving hidden spin canting. A publication has appeared detailing the above and other observations and analytical results. Work has continued on other materials: the dilute ferromagnet bis(diethyldiselenocarbamato)iron(III) chloride, mixed magnetic Mn/Ni dichloride monohydrate, and mixed magnetic Co/Ni dichloride monohydrate. The composition dependence of the ferromagnetic ordering temperature in the first system is very unusual. In the Mn/Ni mixture there is the very surprising finding of the absence of an ordering temprature, across virtually the entire composition range, in the expected region of 2-6 K. In the Co/Ni mixture some unexpectedly strong irreversibility effects, and the composition dependence of these, have been uncovered. Analysis of all of these data is ongoing. The impact of support via this grant has been major. It has enabled the PI's research to continue in a time of stringent general funding. Regarding students, it has enabled me to provide summer and academic year research opportunities to several talented undergraduate students in the chemistry curriculum here. These research experiences should provide a valuable basis for their future scientific endeavors irrespective of the precise type.
