Electron Spin Resonance of a Quasi-One Exchange-Coupled Chromium Tris-Phosphinate Polymer.

In this investigation, magnetic properties of the chromium-tris-phosphinate polymer {Cr(OPMe(phi)O)(,2)(OPOct(,2)O) }(,N), also known as poly-CrTP, were studied using conventional electron spin resonance (ESR) techniques. The chromium spins are coupled by antiferromagnetic Heisenberg exchange, with the nearest-neighbor exchange parameter J/k = 5.0 K, obtained from an independent study of static magnetic susceptibility. The ESR results showed that the spins comprise a quasi-one-dimensional magnetic system with single-site crystal field anisotropy of the easy-plane variety, characterized by the anisotropy parameter D/k = 0.21 (+OR-) .01 K. Linear polymer chain segments appear to form a branched network, and magnetic interactions between chains were shown to significantly influence the spin dynamics. A traditional theoretical formalism was utilized in analyzing the temperature dependent ESR absorption lineshape and linewidth. The theoretical results show overall general agreement with the experimental data, but there are certain obvious discrepancies. In order to understand the underlying collective spin dynamics of exchange-coupled systems in a more intuitive, physical framework, three simple classical models were constructed: The Two-Spin Model, the Spin Cluster Model, and the Random Walk Model. In addition to reproducing the results of the more formal, but less transparent theory, these models also provide important insight into possible origins of the disagreement between the theory and experiments. Part of the disagreement, combined with the results of the susceptibility study, strongly suggests that the Cr spin sites along Poly -CrTP chains are inequivalent with respect to the orientation of the principal axes of the crystal field anisotropy. The remainder of the disagreement between the theory and data, indicates that there are inherent inadequacies in the traditional theoretical approach. The classical models could easily be extended to predict at least qualitatively the expected collective spin behavior outside the range of validity of the traditional formal theory. Thus, they proved extremely useful in suggesting appropriate modifications for the theory when its inadequacies became apparent.