EPR LINE-WIDTH ANISOTROPY IN CsMnCl3[middle dot]2H2O

Abstract

The unusual anisotropy1 of the EPR line width in the linear chain antiferromagnet (CH3)4NMnCl3, TMMC, appears to be direct evidence of one‐dimensional spin coupling. We have examined the temperature dependence and anisotropy of the EPR line width in CsMnCl3·2H2O. This linear chain antiferromagnet2, has a smaller ratio of intra‐ to interchain exchange (TN=4.9°K) and‐ larger interchain dipolar interactions than TMMC. The results, while similar at high temperatures to those for TMMC, reveal a number of features not reported for that substance. Thus, at 296° and 77°K the line width AH passes through a minimum when θ, the angle between the chain axis and the applied field, is ∼54° as for TMMC and presumably reflects the long time‐persistence of spin correlations characteristic of one‐dimensional systems. The magnitude of AH, while much larger than that predicted by conventional exchange narrowing theory, is less for θ=0 than is obtained by assuming the spin time‐correlation function to have the form suitable to an ideal linear chain model in the spin diffusion limit, namely ψ(τ)=A/|τ|1/2. The line width increases and its anisotropy changes as the temperature is lowered. At 20.2°K we find ΔH∝(1+cosθ) suggesting that the non‐secular part of the dipole interaction is becoming important. Between 296° and 15°K for H| | chain axis, ΔH‖ =ΔH∞ exp(δH/kT) where ΔH∞=200 oe and δH/k=34°K͌l0 J/k. δH appears to be related to an activation energy for spin diffusion. For H⊥chain axis, ΔH⊥ is only weakly T‐dependent down to ∼10 TN, below which its T‐dependence is similar to that of δH| . Following Soos3, we have treated the problem of the linear chain subject to small randomizing interactions by assuming that ψ(τ)=Ae−nt/τ1/2 where A=(4πD)−1/2 and the diffusion constant D(T)=D∞ exp(−δD/kT. We further assume that η(T)=η∞ exp(δD/kT), With D∞10.9×04 oe, η∞=900 oe, and δD/k=34°K, a spin time‐correlation function of this form yields a semi‐quantitative fit of the magnitude and anisotropy of the line width as well as its temperature dependence.