Electron-Paramagnetic-Resonance Line Shape ofNi2+in MgO

The derivative EPR line shape of ${\mathrm{Ni}}^{2+}$ in MgO ($S=1\mathrm{}$) consists of (a) a broad line due to the $\Delta M_s=1\mathrm{}$ transitions, which are inhomogeneously broadened by strains, (b) a sharp $\Delta M_s=2\mathrm{}$ double-quantum transition, and (c) a line of inverse phase, both situated at the center of the $\Delta M_s=1\mathrm{}$ line. This last line is attributed to an "internal cross-relaxation" mechanism which causes a broadening of the $M_s=0\mathrm{}\rightleftarrows -1\mathrm{}$ and $M_s=1\mathrm{}\rightleftarrows 0$ transitions at a particular site when the strain at that site is sufficiently small to allow overlapping of the two transitions. An expression for the over-all line shape is obtained, using a density-matrix approach for the $S=1\mathrm{}$ system, and good agreement with experiment is obtained. Values are obtained for the spinlattice relaxation parameters by fitting the observed line shape to that predicted by the theory as a function of the microwave power.