Electron and nuclear magnetic properties near ZEFOZ region.

In the vicinity of spin level anti-crossings, electron-nuclear spin systems reveal characteristic features that have been investigated by electron paramagnetic resonance (EPR) methods, including electron spin echo envelope modulation (ESEEM). The spectral properties depend considerably on the difference, ΔB, between the magnetic field and the critical field at which the zero first-order Zeeman shift (ZEFOZ) occurs. To analyze the characteristic features near the ZEFOZ point, analytical expressions for the behavior of EPR spectra and ESEEM traces as a function of ΔB are obtained. It is shown that the influence of hyperfine interactions (HFI) decreases linearly when approaching the ZEFOZ point. The HFI splitting of the EPR lines is essentially independent of ΔB near the ZEFOZ point, while the depth of the ESEEM signal has an approximately quadratic dependence on ΔB with a small cubic asymmetry due to the Zeeman interaction of the nuclear spin.