Electron spin echo and spin relaxation of low-symmetry Mn(2+)-complexes in ammonium oxalate monohydrate single...

Pulse EPR experiments were performed on low concentration Mn(2+) ions in ammonium oxalate monohydrate single crystals at X-band, in the temperature range 4.2-60K at crystal orientation close to the D-tensor z-axis. Hyperfine lines of the resolved spin transitions were selectively excited by short nanosecond pulses. Electron spin echo signal was not observed for the low spin transition (+5/2↔+3/2) suggesting a magnetic field threshold for the echo excitation. Echo appears for higher spin transitions with amplitude, which grows with magnetic field. Opposite behavior displays amplitude of echo decay modulations, which is maximal at low field and negligible for high field spin transitions. Electron spin-lattice relaxation was measured by the pulse saturation method. After the critical analysis of possible relaxation processes it was concluded that the relaxation is governed by Raman T(7)-process. The relaxation is the same for all spin transitions except the lowest temperatures (below 20K) where the high field transitions (-3/2↔-1/2) and (-5/2↔-3/2) have a slower relaxation rate. Electron spin echo dephasing is produced by electron spectral diffusion mainly, with a small contribution from instantaneous diffusion for all spin transitions. For the highest field transition (-5/2↔-3/2) an additional contribution from nuclear spectral diffusion appears with resonance type enhancement at low temperatures.