ESR of LaS: Er and the origin of random strains in dilute rare earth alloys

ESR of LaS : Er indicates a g{cyrillic}₇ ground state for the Er ion. The data exhibit an angular dependent linewidth upon rotating the magnetic field H in the (011) plane with a minimum of the linewidth for H along the [111] direction. This feature is similar to that obtained for LaSb : Dy (although the effect is significantly smaller in the present case) but it is different from Ag : Dy and Au : Er which exhibit a clear maximum linewidth for H along the [111] direction upon rotating the magnetic field in the (011) plane. Using moment calculation, we demonstrate that this different behaviour is due to the dominance of g{cyrillic}₃ type of strains in LaSb : Dy and LaS : Er but g{cyrillic}₅ type of strains in the case of Ag : Dy and Au : Er. For the case of LaSb : Dy, this idea is supported by direct lineshape analysis. Knowledge of the magnetoelastic constants enables us to extract the width of the strain distributions. Free energy calculations together with the large magnetoelastic coupling constant measured by others for LaSb : Dy, might indicate that the large strain effects are associated with softening of the elastic constants C(g{cyrillic}₃) at low temperatures.