Domain-wall dynamics in 4C pyrrhotite at low temperature

Abstract

Monoclinic 4C pyrrhotite (Fe 7 S 8 ) is ferrimagnetic due to an ordered defect structure with alternating vacancy and vacancy-free sublattices. Its low-temperature magnetic transition near 35 K is characterized by the distinct increase in coercivity and remanent magnetization. The increase of these parameters has been attributed to changes in the domain wall structure. We present static and dynamic magnetization data of a powder sample to study the domain-wall dynamics across the low-temperature transition. The amplitude-dependent ac susceptibility and the ferromagnetic resonance spectroscopy indicate that the hardening of the domain-wall pinning at the transition occurs simultaneously with the decrease in initial saturation remanent magnetization. These two effects are explained by the enhanced inhomogeneity of the bulk material caused by the persistency of the ordered vacancies and by newly formed defects due to localized distortion of Fe(II) sites in the vacancy-free sublattice. The generated localized defects are the link between the domain wall dynamics and the low-temperature transition in 4C pyrrhotite.

The Besnus Transition in Single‐Domain 4C Pyrrhotite

“…The shift of T crit suggests that an external 3 T field affects the Fe 2+ spin rotation and thus the Besnus transition. The IRM 10K after FC is about 3% higher than the ZFC_IRM 10K , but both have nearly the same decay upon heating with T crit = 31 K. Furthermore, overall losses of both IRM 10K curves are markedly less pronounced to those obtained from MD pyrrhotite (Kind et al, ; Koulialias, Schäublin, et al, ). With this in mind we conclude that the rotational behavior of Fe 2+ spins, that is, change in multiaxial magnetocrystalline anisotropy, associated with the Besnus transition (Koulialias, Charilaou, et al 2018), has a less distinct effect on the remanence properties of 4C pyrrhotite in the absence of domain walls.…”

“…Comparison of hysteresis parameters of the differently mechanically treated powder samples indicates that ball milling generates particles in the SD state. Direct insight into the magnetic domain state can be provided by amplitude‐dependent ac susceptibility measurements (e.g., Kind et al, ; Worm et al, ). Before ball milling, the sample has amplitude‐dependent in‐phase ( χ ′) and out‐of‐phase ( χ ′′) susceptibilities typical of MD 4C pyrrhotite (Figures a and b).…”

“…The χ ′ drops at about T = 35 K to form a local minimum at T ≈ 32 K followed by a local maximum at T = 31 K. The χ ″ peaks at 33 K, which indicates the critical temperature ( T crit ) of the Besnus transition (Koulialias, Schäublin, et al, ). Below this temperature the amplitude dependence ceases, that is, domain walls in the 4C pyrrhotite become strongly pinned (Kind et al, ). In contrast, the ball‐milled sample has no amplitude dependence for χ ′ and nearly no out‐of‐phase χ ″ response over the entire temperature range (Figures c and d), which suggests that the crystallites are in a SD magnetic state.…”

On the Magnetism Behind the Besnus Transition in Monoclinic Pyrrhotite

“…It is worth noting that the B c behavior associated with the Besnus transition is different from that of a single‐crystal magnetite at the Verwey transition where a sharp increase in B c points to a crystallographic change (Özdemir, ). Continuous hardening, however, was reported for other 4C pyrrhotite samples and was interpreted by annihilation of domains in the bulk material (e.g., Dekkers et al, ) or by strong pinning that impedes DW motions (Kind et al, ). In our crystal, B c at T ≤ 35 K can be fitted by B c 1/2 ∝ T 2/3 with a quality factor R 2 = 0.994 (Figure e).…”

“…Despite the fact that the low‐temperature transition has been successfully used to identify 4C pyrrhotite in rock materials, the physics behind this phenomenon is still a matter of debate. The main controversy is whether the transition is of crystallographic or magnetic origin (e.g., Charilaou et al, ; Fillion & Rochette, ; Kind et al, ; Volk et al, ; Wolfers et al, ). A reason for the lack of a generally accepted model is probably the richness of physical features associated with the transition.…”