Electron spin resonance in a two-dimensional compound with appreciable interplane coupling: NaCrS2

Measurements of the electron-spin-resonance (ESR) linewidth ΔH in NaCrS2 are presented and compared with the theory of Richards and Salamon for ESR in two dimensions (2d). NaCrS2 was chosen because it is a layer compound with a ferromagnetic intraplane interaction J and a sizable antiferromagnetic interplane interaction J′ (|J′J|=16). Previously the most detailed study of ESR which paid specific attention to the 2d feature of the spin dynamics was for K2MnF4, which has negligible J′ and an antiferromagnetic J; so it is of interest to compare the two systems. ΔH in NaCrS2 does not have a minimum at 3cos2θ=1 (θ=angle of field with respect to c axis), in contrast to the situation in K2MnF4. This feature is explained by the effect of interplane coupling. The theory, modified to account for interplane exchange, also correctly gives the high-temperature ratio ΔH(θ=0)ΔH(θ=90°)=1.6. The measured peak-to-peak linewidth at θ=0 and room temperature, ΔH=80 Oe, is smaller than the calculated 125 Oe. ΔH increases continuously as the temperature T is lowered in a manner described reasonably well by theory. Here agreement is much better than in some other planar ferromagnets which possibly have more nondipolar broadening mechanisms than NaCrS2. The anisotropy ΔH(θ=0)ΔH(θ=90°) decreases as the temperature is lowered, which is in contradiction to theory for a fixed J′J and to results on other quasi-2d compounds and may be indicative of J′J being temperature dependent. © 1974 The American Physical Society.