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P72/4 fresh sample. (a) Backscattered electron image of a titanomagnetite grain; (b) central part of a hysteresis loop measured at 295 K in a 7 T maximum field. The measured loop is shown in black, corrected for high-field slope according to the extrapolation method of Starunov et al. ( 2019 ) in red. Inset shows full ± 7 T loops; (c) FORC distribution (smoothing factors, SFc = SFb = 2 and damping factors, λc = λb = 0.04). Contours are drawn every 10 per cent of maximum hysteron density. Bold dashed lines mark the location of the maximum.
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Titanomagnetite containing up to 0.6-0.7 Ti atoms per formula unit is a primary magnetic mineral phase in submarine basalts and in some terrestrial volcanic rocks. On a geological timescale, it often undergoes alteration, forming new magnetic phases that may acquire (thermo)chemical remanent magnetization. The initial stage of this natural process...
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... electron (BSE) imaging of the fresh sample reveals the presence of skeletal titanomagnetite grains typical for rapidly cooled submarine basalts (Fig. 1 a). No exsolution structures within the grains were observ ed. Howev er, the fine size of titanomagnetite grains did not allow one to determine their precise composition from microprobe data. Magnetic hysteresis properties ( Fig. 1 b and Table 1 ) indicate that the sample is in a pseudo-single domain (PSD) magnetic state. Worth noting is ...
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... er, the fine size of titanomagnetite grains did not allow one to determine their precise composition from microprobe data. Magnetic hysteresis properties ( Fig. 1 b and Table 1 ) indicate that the sample is in a pseudo-single domain (PSD) magnetic state. Worth noting is a relati vel y low value of H cr / H c of 1.25, which is sometimes encountered in submarine basalts (Gee & Kent 1999 ). ...
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... noting is a relati vel y low value of H cr / H c of 1.25, which is sometimes encountered in submarine basalts (Gee & Kent 1999 ). The central part of the FORC diagram ( Fig. 1 c) has an arrowhead shape centred at about 15 mT, while the outer contours are clearly onion-like, the 90 per cent density contour extending to about 55 mT along the μ 0 H c axis. Both features are characteristic of small PSD grains (Roberts et al. 2014 ). ...
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... of a ne gativ e mean interaction field. In addition, an increased hysteron density in the vicinity of the μ 0 H b axis may also be due to interactions (Muxworthy et al. 2006 ). Indeed, nonnegligible magnetostatic interactions are expected to act between the close segments of abundant skeletal titanomagnetite grains as re vealed b y BSE images (Fig. 1 ...
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... diagrams for samples annealed at 500 and 550 • C are shown in Fig. 10 . All diagrams extend to nearly 150 mT along the μ 0 H c axis, more than twice compared to those measured for the fresh sample and samples annealed at 355 • C. The spread of the FORC distribution along the μ 0 H b axis is also notably larger, whereas the downward shift of the distribution maximum relative to the μ 0 H c = 0 line ...
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... diagrams extend to nearly 150 mT along the μ 0 H c axis, more than twice compared to those measured for the fresh sample and samples annealed at 355 • C. The spread of the FORC distribution along the μ 0 H b axis is also notably larger, whereas the downward shift of the distribution maximum relative to the μ 0 H c = 0 line increases. Overall, hysteron density distribution appears more single-domain-like in these samples, in line with considerably enhanced coercive force and coercivity of remanence (compare Figs 1 c and 5 , on one hand, and Fig. 10 on the other). Increased downward shift of the distribution maximum with respect to the μ 0 H c = 0 line may indicate a more significant magnetostatic interaction between SD-like regions. ...
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... μ 0 H b axis is also notably larger, whereas the downward shift of the distribution maximum relative to the μ 0 H c = 0 line increases. Overall, hysteron density distribution appears more single-domain-like in these samples, in line with considerably enhanced coercive force and coercivity of remanence (compare Figs 1 c and 5 , on one hand, and Fig. 10 on the other). Increased downward shift of the distribution maximum with respect to the μ 0 H c = 0 line may indicate a more significant magnetostatic interaction between SD-like regions. However, the details vary depending on the annealing temperature. At 500 • C, coercivity increases with annealing time and the M rs / M s ratio ...
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... properties at cryogenic temperatures exhibit pro gressi ve changes with annealing temperature and duration. The magnetic transition shifts to still lower temperatures, 38-39 K for the samples annealed at 500 • C, and 34-36 K for those annealed at 550 • C (Fig. 11 ). The difference between the ZFC and FC curves persists, but the slope of the ZFC curve below the transition is significantly greater than for the samples annealed at 355 • C. The sample annealed at 500 • C for 110 hr also exhibits a broad Verwey transition centred at 95 K, indicating that in this case, the ne wl y formed phase is close ...
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... shape of susceptibility temperature dependencies in the 2-300 K range evolves to develop well-pronounced frequencydependent maxima of the in-phase susceptibility (Figs 12 a, c, e and f). On the other hand, the maxima of the out-of-phase susceptibility (Figs 12 b, d, f and h) decrease in magnitude and shift to considerably lower temperatures ( ca . ...
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... shape of susceptibility temperature dependencies in the 2-300 K range evolves to develop well-pronounced frequencydependent maxima of the in-phase susceptibility (Figs 12 a, c, e and f). On the other hand, the maxima of the out-of-phase susceptibility (Figs 12 b, d, f and h) decrease in magnitude and shift to considerably lower temperatures ( ca . 45-60 K) compared to the samples annealed at 355 • C. ...
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... the same time, the general shape of low-temperature remanence (Fig. 11 ) and AC susceptibility curves (Fig. 12 ) remain similar to those typical for more Ti-rich intermediate titanomagnetites ( Kosterov et al. 2009Kosterov et al. , 2018Almeida et al. 2014 ). The two most prominent features in remanence curves are: (i) a characteristic triangular shape separating FC and ZFC curves, and (ii) cooling and ...
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... the same time, the general shape of low-temperature remanence (Fig. 11 ) and AC susceptibility curves (Fig. 12 ) remain similar to those typical for more Ti-rich intermediate titanomagnetites ( Kosterov et al. 2009Kosterov et al. , 2018Almeida et al. 2014 ). The two most prominent features in remanence curves are: (i) a characteristic triangular shape separating FC and ZFC curves, and (ii) cooling and warming branches of the RT-SIRM zero-field c ...
