Magnetic Characterization of Synthetic SD-like Pyrrhotite and Its Demagnetization under Hydrostatic Pressure up to 1.8 GPa

Abstract:

Monoclinic pyrrhotite (Fe₇S₈) is a common ferrimagnetic mineral in both terrestrial rocks and meteorites (e.g., SNC, Rumuruti chondrites…). It is also recognized as a candidate magnetic mineral for Martian magnetic anomalies. We synthesized SD-like monoclinic pyrrhotite at 245°C using the molten-salt synthesis method [1]. Here we characterize its magnetic properties including its behavior under hydrostatic pressure up to 1.8 GPa. Data were collected in the 10K to 360°C temperature range and include measurements of low-field magnetic susceptibility (χ₀), thermomagnetic curves χ₀(T), major hysteresis loops, back-field remanence demagnetization curves, first-order reversal curve (FORC), alternating field and pressure demagnetization of saturation isothermal remanent magnetization (SIRM or M_rs), low temperature MPMS datasets (field and frequency dependencies of χ₀, field-cooled and zero-field-cooled remanence FC-ZFC and room temperature SIRM heating-cooling cycles) as well as X-ray diffraction (XRD) spectra. χ₀(T) indicates a single Curie point at 320°C characteristic of monoclinic pyrrhotite.

For application of hydrostatic pressure up to 1.8 GPa we used a nonmagnetic high-pressure cell of piston-cylinder type entirely made of “Russian alloy” (Ni₅₇Cr₄₀Al₃) with 8 mm of inner diameter similar to the cell, described in [2]. Application of 1.8 GPa resulted in demagnetization (decrease in SIRM) of the sample by 38%. Repeated cycling from 1.8 GPa to atmospheric pressure and back resulted in further decrease in remanence by 44% (for 3 cycles). The characteristic Besnus transition of pyrrhotite is observed at ~34 K. The observed hysteresis parameters (M_rs/M_s=0.53, B_cr/B_c=1.17, B_cr=41 mT, where M_s is saturation magnetization, B_c and B_cr are coercivity and remanent coercivity, respectively) are consistent with pseudo-single-domain range, previously established in literature [3]. Superparamagnetic (SP) grains are not present in the sample as no frequency dependence of χ₀ is observed. References: [1] Chareev D. A., Voronin M. V., Osadchii E. G. 2014. Amer. Mineral., doi:10.2138/am-2014-4753 (accepted). [2] Sadykov R.A. et al. 2008. Rev. Sci. Instr. 79: 115102. [3] Dekkers M.J. 1988. Phys. Earth Planet. Int. 52, 376-393.