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

Abstract

This report deals with the analysis of point‐defect‐induced magnetic after‐effect (MAE) processes occurring in as‐grown, single‐crystalline titano‐magnetite (Fe2.8–D Ti0.2O4), containing B‐site vacancies (D < 0.005). In a foregoing experiment, these processes, situated near 450 K, 200 K and 65 K, have been completely recovered in the course of systematic annealing up to Ta ≤ 1200 K. Analysis of respective annealing kinetics suggested to associate them with vacancy‐ (450 K) and interstitial‐based (200 K) defect reorientations, including modified electron hopping (65 K). The present study was performed in order to check the previously developed relaxation model by low‐temperature (80 K) electron (e–) irradiation of the prerecovered crystal and investigating if thereby the point‐defect relaxations of the initial, as‐grown, state could be reconstituted. MAE analyses, based on systematic sample annealing in the temperature range 80 K ≤ Ta ≤ 1200 K prove, indeed, radiation‐induced reproduction of related processes. Partial modifications of respective process genealogies may be conveniently assigned to the alternative techniques of defect induction, i.e. high‐temperature crystal growth in thermal equilibrium or out‐of‐equilibrium, low‐temperature e–‐irradiation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Point-defect interactions in electron-irradiated titanomagnetites—as analysed by magnetic after-effect spectroscopy on annealing within 80 K <Ta< 1200 K

“…Regarding the annealing behaviour over the full temperature range 80 K T a 1200 K, we recognize three sub-intervals of recovery, cf figures 2, 3 and table 2: (i) 80 K < T a < 500 K, within which most of the radiationinduced extra effects are annealing-thereby reconstituting qualitatively, though with varied amplitude ratios, the original spectrum: cf figures 1(a) and 2(m); (ii) 500 K < T a < 950 K, wherein, with the exception of the 65 K process (figure 3(a)), the relaxation amplitudes undergo only minor modulations; (iii) 950 K < T a < 1200 K, being characterized by the complete annihilation of processes (a) and (c)whereas the radiation-induced component (b) had disappeared already during T a < 500 K. Interestingly, such final process extinction on continued annealing is not observed for relaxation components (d) and (e) whose amplitudes-after crossing a minimum near T a 1050 K-are found to re-grow again on cooling-down from higher annealing temperatures (T a 1050 K), cf figures 2(q), (r) and 3. This astonishing behaviour, already previously observed in related annealing studies [17,18], will be further discussed in section 4.3.…”

“…(ii) In the temperature range 950 K < T a < 1200 K we observe here too, analogous to the previous annealing of the as-grown crystal (figure 1(a)) [17], concomitant recombination of the 200 and 450 K relaxation processes (cf figure 5) with coinciding reaction order γ = 2, closely related activation enthalpies of Q = 2.43 ± 0.05 eV and rate constants K 0 2 10 8.0±0.5 s −1 . (iii) Inset (c) of figure 5 summarizes the 65 K recovery analyses of differently pretreated crystals: (i) asgrown [17]; (ii) as-grown and subsequently e − -irradiated (actual state); (iii) complete annihilation of the as-grown MAE spectrum, followed by e − -irradiation [18]. The determined reaction parameters agree within the limits: Q = 2.45 ± 0.3 eV, K 0 1,2 = 10 8.0±0.3 s −1 , tending to reaction order γ = 2.…”

“…As described in more detail in the preceding study (d), [18], concerned with identically composed titanomagnetites (Fe 3−x− Ti x O 4 , x = 0.2, 0.005), appropriately powdersintered polycrystals were grown to 100 -axis-oriented single crystals, using a floating zone technique [19]. Finally, these crystals were subjected to a phase-controlled tempering (during 24 h at 1473 K) and subsequent cooling-down in adjusted CO-CO 2 -H 2 atmospheres, so as to induce a vacancy content of about 0.005, necessary to evoke the characteristic MAE spectrum of figure 1(a), [16,19].…”

“…4 < T < 20 0.03 ± 0.02 10 −10±1 Incoherent electron tunnelling [ 9, 10, 13] V 1B 30 0.07 ± 0.01 10 −12±1 Intra-ionic excitation (Twofold superposition) 0.08 ± 0.01 V 2 50 < T < 125 0.25 ± 0.10 10 −12±1 Variable range electron hopping V 2R 65 0.17 ± 0.04 10 −14±1 Ti 4+ -modified ('reversed') electron hopping [16][17][18] IV plateau 150 ± 50 0.54 ± 0.26 10 −12±1 Frenkel-pair interstitial (Fe 2+ ) reorientation [18] (e − -irr.) IV 200 0.70 ± 0.05 10 −14±1 Intrinsic interstitial reorientation [10,16,17] III 1 300 0.85 ± 0.05 10 −14±1 B-site vacancy-mediated Fe 2+ -hopping [10,[16][17][18]] III 2 0.93 ± 0.05 II 450 1.24 ± 0.15 10 −14±1 Reorientation of anisotropic Ti 4+ Fe 2+ -vacancy complexes [16][17][18] i.e. electric conductivity, heat capacity, thermoelectric power, Mössbauer effect, etc-frequently on polycrystalline samples or crystals of uncontrolled quality, as compiled in [7][8][9].…”

“…The hopping-induced, stress-assisted 65 K peak undergoes delayed recovery, setting-in only in the course of stress extinction due to combined annihilation of anisotropic defect configurations. (d) Next, we studied the effect of re-introduction of point defects-by means of low-temperature (80 K) e − irradiation-into the completely annealed crystal (c), [18]. On subsequent annealing from 80 K to T a 500 K, most radiation-induced processes are annihilated [10]with the exception of the 65 K peak which, after resurrection near T a 220 K, up to T A > 1000 K, shows comparable annealing behaviour as observed in (c).…”