Raman spin-lattice relaxation, Debye temperature and disorder effects studied with electron spin echo of Cu²⁺in Tutton salt crystals

Abstract

Spin-lattice relaxation time T1 was determined by the electron spin echo (ESE) method in the temperature range 4-60 K in a series of Tutton salt crystals MI2MII(SO4)2·6X2O (MI = NH4, K; MII = Zn, Mg; X = H, D) weakly doped (⩽1018 ions cm-3) with the 63Cu2+ isotope. The ESE signal was undetectable at higher temperatures. The relaxation rate increases over the six decades in the studied temperature range with T1 equal to 1 s at 4 K and 0.5 µs at 50 K. Various possible relaxation mechanisms are discussed with the conclusion that the relaxation is governed by two-phonon Raman processes without a noticeable contribution from the reorientations of Cu(H2O)6 octahedra between Jahn-Teller distorted configurations. Deuteration of the crystal has no effect in spin-lattice relaxation. For a few crystals, having the largest Cu2+ concentration among the studied crystals, a strong and linear in temperature contribution to the relaxation rate was found below 15 K. Possible explanations are discussed with the final conclusion that this effect is due to a non-uniform Cu2+ distribution in the host lattice producing effective relaxation via pairs and triads of the Cu2+ ions. From the T1(T) dependence the Debye temperature ΘD was determined for the all crystals studied. This varies from ΘD = 166 K for K2Zn(SO4)2·6H2O to ΘD = 238 K for (NH4)2Mg(SO4)2·6H2O. The ΘD values are discussed and used for calculation of the sound velocity which was found to be similar in all crystals and equal to ν = 4150(±150) m s-1.

Effects of Vibronic Dynamics of Cu(H2O)6Complexes and Electron Spin Relaxation in Temperature Dependence of EPR Linewidth in Diamagnetic Tutton Salt Single Crystals

“…Single crystals of KZnH and KZnD were grown and Cu 2+ doped as described previously [5,9]. The full deuteration was achieved by fivefold recrystallization from a heavy water solution and the degree of the deuteration was controlled by ESEEM spectra as described in [5].…”

“…The full deuteration was achieved by fivefold recrystallization from a heavy water solution and the degree of the deuteration was controlled by ESEEM spectra as described in [5]. EPR measurements were performed on single crystals along the local z-axis (elongation axis) of the Cu(H 2 O) 6 octahedra using a Radiopan SE/X-2547 spectrometer with 100 kHz magnetic modulation and Oxford ESR900 flow helium cryostat.…”

“…The spin-lattice relaxation rate 1/T 1 of Cu 2+ in the Tutton salt crystals [5] is governed by two-phonon Raman relaxation processes with temperature dependence described by the following equation:…”

“…The ∆B 0 pp is considerably larger for NH 4 Mg because an additional coupling with protons of ammonium group is clearly lower for deuterated compound KZnD (see Table). The inhomogeneity of the lines allows us to induce ESE signals which we have used for the spin relaxation measurements [5,6,9]. The measurements show that the spin packets width continuously increases with temperature and above 60 K approaches the total linewidth.…”

“…In series of papers we have measured T 1 time in various Tutton salts by ESE methods as reviewed in paper [5]. However, the ESE signal was detectable only below 60 K. For higher temperatures the echo dephasing rate was so fast that the ESE signal was hidden in the dead time (about 80 ns) of the pulsed spectrometer.…”