Stress-induced relaxation mechanisms in single-crystalline titanomagnetites

Abstract

The effect of titanium (Ti4+) substitution on slightly oxidized magnetite single crystals, Fe3−xTixO4+δ—with and —is studied by means of magnetic after-effect (MAE) spectroscopy in the temperature range 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 near 480 K and—proportionally with increasing doping—shifted to lower temperatures, reaching 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 re-substitution of Fe2+ ions from B- onto A-sites of the tetrahedral sublattice.

Transport Properties Of Fe[sub 3−x]Ti[sub x]O[sub 4] (x = 0.0 and 0.0206) Epitaxial Thin Films

“…The XRD patterns suggest that all the films [indicating as 'F' in the pattern] are grown with highly orientated in the (111) direction with a cubic structure. However, the Ti-doped Fe 3 O 4 films also exhibit a smooth R-T curve without any discontinuity consistent with the previous data [6]. Fe 3 O 4 exhibits a characteristic order-disorder phase transition known as the Verwey transition at ∼ 120 K [4].…”

Impact of irradiation‐induced point defects on electronically and ionically induced magnetic relaxation mechanisms in titano‐magnetites

“…)no traces of the former system-inherent, electronically induced specific lowtemperature processes could be detected any more [6] 1 . Instead, pronounced, ionically induced MAE processes appeared near 450 K and 200 K, in addition to a peculiar, sign-reversed process near 65 K, as depicted in Fig.…”

“…1a. Systematic analysis of these basic relaxations, observed over the range of increasing doping rates, suggested to associate them with local reorientations of anisotropic Fe 2+ Ti 4+ -pair-vacancy configurations (450 K), Ti 4+ stressinduced 2 interstitials (200 K) and Ti 4+ -modified stressreleasing (sign-reversed) electron hopping [6].…”

“…On the other hand, there exist only a few magnetic after-effect (MAE) measurements [4,5], performed on polycrystals within limited temperature ranges, starting above liquid nitrogen (80 K). This motivated us to extend the experimental knowledge of titano-magnetite [6][7][8] especially within the most interesting low-temperature range 4 K 125 T < < K, up to 500 K by means of a highly sensitive, automated (MAE) technique [9] applied on carefully prepared single crystals.…”

Magnetic susceptibility measurements as a function of temeperature and freqeuncy I: inversion theory

“…This property can be used to characterize samples containing mixtures of particles with different domain states, as they often occur in nature, whereby the signal produced by non‐SP particles is efficiently suppressed. Some limitations occur with large amounts of multidomain particles, since a small out‐of‐phase susceptibility component is produced by electron hopping and/or tunnelling in stoichiometric magnetite below the Verwey temperature (Balanda et al 2005), and by vacancy‐mediated relaxation near room temperature in low‐Ti magnetite (Walz et al 2003). Since these phenomena are observed in large crystals that can be removed from sediments by magnetic extraction, quantitative investigation of SP minerals in natural samples based on susceptibility measurements is possible.…”