Does the Jahn-Teller effect influence electron spin relaxation? Electron paramagnetic resonance and electron spin echo studies of the Mn²⁺ doped (NH₄)₂Mg(SO₄) · 6H₂O single crystal and comparison with Cu²⁺ data

Abstract

Continuous wave EPR and pulsed EPR measurements for Mn²⁺ in (NH₄Mg(SO₄)₂·6H₂O are presented and compared with previously published data for Cu²⁺. These data are analysed together, to extract an effect from Jahn-Teller dynamics on the electron spin relaxation of the Cu²⁺ ion. It is possible to estimate the Debye temperature of the crystal as Θ_D = 240 K both for Mn²⁺ and Cu²⁺, and to prove that the vibronic dynamics do not influence T ₁ for Cu²⁺ up to about 100 K, and only an enhancement of the phase relaxation rate 1/T _M by excitations to the higher vibronic level above 20 K is observed. The spin-lattice relaxation rate is higher for Cu²⁺ than for Mn²⁺ over the whole temperature range, indicating that the g factor phonon modulation mechanism of Cu²⁺ relaxation is more effective than the D tensor modulation mechanism of Mn²⁺ relaxation. From a comparison with other relaxation data for Jahn-Teller systems, it is concluded that an enhancement of the relaxation rate is not as strong as suggested previously and can be different depending on the shape of the adiabatic potential surface.