Stress-induced relaxation mechanisms in single-crystalline titanomagnetites

The effect of titanium (Ti4+) substitution on slightly oxidized magnetite single crystals, Fe3-xTixO4+delta-with 0. 1 < x less than or equal to 1.0 and delta < 0.005-is studied by means of magnetic after-effect (MAE) spectroscopy in the temperature range 4 K < T less than or equal to 500 K. The MAE spectra corresponding to these, relatively high, substitution rates are characterized by the following items: (i) suppression of low-temperature (4 K < T < 35 K) electron (e(-))-tunnelling; (ii) severely damaged variable-range e(-)-hopping (50 K < T < 125 K), culminating in a pronounced 'negative' (sign-reversed) Debye-type peak near 65 K; (iii) the occurrence of a remarkable MAE near 210 K, being identified as resulting from reorientations of intrinsic interstitials which had formed under the action of Ti4+- induced lattice distortions; (iv) the nearly complete suppression of all 'type III' related, vacancy-mediated relaxations near 300 K; (v) the occurrence of a huge, 'type II' relaxation maximum being situated initially (x similar or equal to 0.1) near 480 K and-proportionally with increasing doping-shifted to lower temperatures, reaching T-max similar or equal to 380 K for x = 0.6. These processes are shown to interfere with internal stresses induced into the orthogonal (B) sublattice upon Ti-doping due to the transformation of Fe3+ into more extended Fe2+ ions which have a greater volume. This stress induction is found to reach its maximum near x = 0.2; its decay on further doping is explained by the beginning of a resubstitution of Fe2+ ions from B- onto A-sites of the tetrahedral sublattice.