1-370 GHZ EPR LINEWIDTHS FOR K3CRO8 - A COMPREHENSIVE TEST FOR THE ANDERSON-WEISS MODEL

Electron paramagnetic resonance (EPR) measurements have been carried o ut over the frequency range of 1-370 GHz on single crystals of potassi um peroxychromate (K3CrO8) with the view of examining the current mode ls of exchange narrowing of EPR signals in solids. K3CrO8 has a simple (tetragonal) lattice structure, can be grown as single crystals pure or diluted with an isostructural diamagnetic host K3NbO8, and its para magnetism can be described by a very simple (S = 1/2, I = 0) spin Hami ltonian. The measurements were made at various orientations of single crystals in the Zeeman field, with emphasis on the principal direction s of the g-tensor. For essentially all orientations, the linewidth dec reases monotonically for measurements at resonance frequencies, omega( 0), from 1 to about 100 GHz, and then starts to increase at higher ome ga(0). In order to delineate the spin exchange effects from other sour ces of line broadening, the measurements were repeated with a diluted spin system, K3NbO8 containing congruent to 0.5 mole % of K3CrO8, repr esenting the broadening effect of all the magnetic field dependent ter ms, such as the broadening due to the g-strain and sample holder/waveg uide magnetization at the high field utilized, up to 14 T. Using these data, the K3CrO8 linewidths were analyzed in terms of the current mod els of spin exchange narrowing in three-dimensional systems. A reasona bly good agreement was found with the Anderson-Weiss model, when modif ied for various line broadening effects. The accuracy of the analysis procedure was confirmed by the comparison of the presently determined values of the exchange constant, J, and the dipolar field, H-p, with t heir values obtained by de magnetic susceptibility measurements and th eoretical analysis, respectively; the agreement was within 5% for J (= 1.35 K) and about 25% for H-p (160 G). However, some deviations and un usual splittings were noted in measurements at 370 GHz, whose origin r emains unclear. (C) 1998 Academic Press.