ESR line broadening in Cr³⁺/MgO at liquid helium temperatures

Abstract

Line broadening has been observed at 9 GHz in the ESR absorption spectrum of Cr³⁺ in MgO in the liquid helium temperature range, for a range of Cr³⁺ concentrations from 800 7400 ppm. The broadened linewidths at 4.2 K are about two times larger than those at 77 K and depend on polar angle. The lineshapes are Gaussian, in contrast to the Lorentzian lineshape between 293 and 77 K. The broadening is interpreted by the combined effects of strain, due to the charge misfit of Cr³⁺ and the host cation, and temperature which causes both exchange striction and departure from the cubic symmetry to a lower symmetry. Analysis of the linewidth data gives ΔD=(5.25±0.40)×10⁻⁴ cm⁻¹ and confirms that the total linewidth, ΔH _t, is given by ΔH _t=ΔH ₀+∥(2ΔD/gβ) cos θ _H∥ where ΔH ₀ is the linewidth independent of concentration, temperature and polar angle, θ _H.