9.6 GHz and 34 GHz electron paramagnetic resonance studies of chromium-doped forsterite

Abstract

Chromium‐doped forsterite single crystals grown under conditions that produce a high Cr⁴⁺/Cr³⁺ ratio were examined by electron paramagnetic resonance (EPR) at 9.6 and 34 GHz. The crystals were grown in 2–3 atm of oxygen by the floating‐zone method starting from polycrystalline chromium‐doped forsterite powder synthesized via a sol–gel method. Three crystals with chromium concentrations of 110, 300, and 390 ppm were studied. At 34 GHz, transitions are observed for the laser‐active tetrahedral Cr⁴⁺ species that are not observable at 9.6 GHz, which improve the resolution and accuracy with which the magnetic parameters can be measured by EPR. In addition, peaks for a non‐Kramers species appear at 34 GHz that were not observed at 9.6 GHz. These peaks are not analyzed in detail, but are tentatively ascribed to Cr⁴⁺ in the octahedral substitution sites of the crystal. At the highest chromium concentration, the Cr³⁺ spectra show evidence of direct interaction with Cr⁴⁺. A global least‐squares fit of the combined 9.6 and 34 GHz data for the 300 ppm crystal gives D=64.26±0.18 GHz, E=−4.619±0.009 GHz, g_x=1.955±0.009, g_y=2.005±0.040, g_z=1.965±0.006, and places the magnetic z axis in the ab plane at an angle of 43.8±0.3° from the b crystallographic axis (in P_bnm). A method for accurately measuring the Cr⁴⁺/Cr⁴⁺ ratio using EPR line intensities is given. The EPR linewidth of the Cr⁴⁺ center exhibits a strong orientation dependence that is well‐modeled by including site variations in the D and E zero‐field splittings and in the orientation of the z magnetic axis. The linewidth analysis reveals a high degree of correlation between the distributions in D and E, and a somewhat weaker correlation between E and the z axis orientation. These results are interpreted to suggest that the tetrahedral Cr⁴⁺ sites vary mainly in the degree of compression of the tetrahedral cage along the a crystallographic axis. The Cr⁴⁺ EPR linewidths increase significantly at higher chromium concentration, but maintain the same qualitative orientation dependence. The EPR data indicate that the major contribution to inhomogeneity in the tetrahedral site, which may be related to the tunable range of the Cr⁴⁺ laser center, is distortion induced by chromium substitution into the crystal lattice rather than direct chromium–chromium interactions.