Electron spin relaxation of exchange coupled pairs of transition metal ions in solids. Ti2+-Ti2+ pairs and single Ti2+ ions in SrF2 crystals

EPR (X- and Q-band) and electron spin relaxation measured by electron spin echo method (X-band) were studied for Ti2+(S=1) and Ti2+-Ti2+ pairs in SrF2 crystal at room temperature and in the temperature range 4.2-115 K. EPR spectrum consists of a strong line from Ti2+ and quartets 2:3:3:2 from titanium pairs (S=2). Spin-Hamiltonian parameters of the pairs are g∥=1.883, g⊥=1.975 and D=0.036cm-1. Temperature behavior of the dimer spectrum indicates ferromagnetic coupling between Ti2+. Spin-lattice relaxation of individuals Ti2+ is dominated by the ordinary two-phonon Raman process involving the whole phonon spectrum up to the Debye temperature ΘD=380K with spin-phonon coupling parameter equal to 215cm-1. Important contribution to the relaxation arises from local mode vibrations of energy 133cm-1. The pair relaxation is faster due to the exchange coupling modulation mechanism with the relaxation rate characteristic for ferromagnetic ground state of the pairs 1/T1∝[exp(2J/kT)-1]-1 which allowed to estimate the exchange coupling J=36cm-1. The theories of electron-lattice relaxation governed by exchange interaction are outlined for extended spin systems, for clusters and for individual dimers. Electron spin echo decay is strongly modulated by coupling with surrounding 19F nuclei. FT-spectrum of the modulations shows a dipolar splitting of the fluorine lines, which allows the evaluation of the off-center shift of Ti2+ in pair as 0.132 nm. The electron spin echo dephasing is dominated by an instantaneous diffusion at low temperatures and by the spin-lattice relaxation processes above 18 K.