Evidence for Strain Broadening in the ESR Absorption Spectrum of Tetragonally Distorted Cr3+ Ions in MgO

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Abstract
An anisotropic line broadening has been observed in the ESR absorption spectrum of Cr3+ ions located at tetragonally distorted sites of single‐crystal MgO. For an impurity ion whose tetragonal distortion is along a crystal [100] direction. a sin2φ dependence is observed in the linewidth when the Zeeman field is rotated in the plane perpendicular to this direction. The broadening is interpreted by introducing strain induced terms in the conventional spin‐Hamiltonian, where it is assumed that the strain‐coupled coefficients are the same as those measured for Cr3+ at the cubic sites of MgO. This formalism is consistent with both the angular dependence that is observed in the linewidth and the order of magnitude of the line broadening after correcting for contributions from other mechanisms such as variations in the crystalline electric field parameter D. Analysis of the linewidth data gives C44 α/g β = (2.13 ± 0.2) Oe and ΔD/g β = (1.3 ± 0.3) Oe, where a is the parameter that characterizes the distribution of strains within the crystal.