Measurement of the Gd-Gd exchange and dipolar interactions in Gd0.01Y0.99Ba2Cu3O6

The microscopic parameters of the Gd-Gd pair interaction are measured in si ngle crystals of the Gd0.01Y0.99Ba2Cu3O6 cuprate using high-frequency (225 GHz) Gd3+ electron-spin resonance. In addition to the fine-structure spectr um of single Gd3+ ions, a series of low intensity resonance transitions are resolved at high magnetic fields and low temperatures which arise from Gd- Gd pairs. From the resonance line positions of nearest-neighbor Gd-Gd pairs we determine an isotropic exchange energy of Je 156 mK and an anisotropic (mainly dipolar) interaction energy of D=51.7 mK. The anisotropic interacti on parameters of second, third, and fourth neighbors correspond to the valu es expected for dipolar interaction only. [S0163-1829(99)02618-1].