Search

The characteristic microstructure of Sm (Co , Fe , Cu , Zr) z (z = 6.7 – 9.1) alloys with SmCo 5 cell walls in Sm 2 Co 17 cells, all intersected by Zr-rich platelets, makes them some of the best performing high-temperature permanent magnets. Plentiful research has been performed to tailor their microstructure at the nanoscale; but due to its complexity, many questions remain unanswered about the effect of the individual phases on the magnetic performance at different temperatures. Here, we explore this effect for three different Sm (Co , Fe , Cu , Zr) z alloys by deploying high-resolution magnetic imaging via in situ transmission electron microscopy and three-dimensional chemical analysis via atom probe tomography. We show that their microstructures differ in terms of SmCo 5 cell-wall and Z-phase size and density as well as the Cu concentration in the cell walls and demonstrate how these features influence the magnetic domain size and density, thus forming different micromagnetic spin structures. Moreover, we illustrate that the dominant coercivity mechanism at room temperature is domain-wall pinning and show that magnets with a denser cell-wall network, a steeper Cu gradient across the cell-wall boundary, and thinner Z-phase platelets have a higher coercivity. We also show that the coercivity mechanism at high temperatures is domain-wall nucleation at the cell walls. Increasing the Cu concentration inside the cell walls decreases the temperature of transition between pinning and nucleation, significantly decreasing the coercivity with increasing temperature. We, therefore, provide a detailed explanation of how the microstructure on the atomic to nanoscale directly affects the magnetic performance and provide detailed guidelines for an improved design of Sm (Co , Fe , Cu , Zr) z magnets. [ABSTRACT FROM AUTHOR]

The monosulfides of the pyrrhotite omission series ( Fe 1 − x S , 0 < x ≤ 0.125) are important remanence carriers for paleomagnetic reconstruction of the Earth's crust and extraterrestrial materials. The ferrimagnetic Fe 7 S 8 polytypes are the endmembers, and their stacking modulations of full and vacant layers generate different magnetic anisotropy properties due to the cation-vacancy configurations. In this study, intergrown long-range ordered polytypes with four- and threefold modulation, i.e., 4C and 3C pyrrhotite, were prepared in a diffusion-driven process by quenching of a natural pyrrhotite crystal with randomized vacancies. In addition, a third constituent with coherence lengths of a few nanometers, denoted 3C*, was found that exhibits spin-glass behavior at about 10 K due to local magnetic anisotropies arising from vacancy-density variations. The concomitant occurrence of this nano-scale constituent with spin disorder and the long-range ordered polytypes indicate competitive diffusion-driven processes during Fe 7 S 8 formation. Such information provides insight into the provenance and genesis of ferrimagnetic pyrrhotite in Earth and extraterrestrial systems and in a broader sense into vacancy-induced materials. [ABSTRACT FROM AUTHOR]

Ferrimagnetic, monoclinic 4C pyrrhotite (Fe7S8) is the only iron sulphide with high relevance for palaeomagnetism and rock magnetism that can be identified in rock materials by its characteristic low-temperature anomaly. Despite its relevance in natural magnetism and the many magnetic studies over the last decades, the physics and the crystallography behind this anomaly, also denoted Besnus transition, is a matter of debate. In this study, we analyse the static and dynamic magnetization associated with the Besnus transition in conjunction with low-temperature structural data of 4C pyrrhotite reported in the literature. The correlation between the Fe–Fe bonds causing spin-orbit coupling and the dynamic magnetic properties show that the magnetic characteristics of the Besnus transition stem from the interaction of two magnetocrystalline anisotropy systems triggered by thermally induced structural changes on an atomic level in monoclinic 4C pyrrhotite. This refutes the widespread view that the Besnus transition is caused by a crystallographic change from monoclinic to triclinic. [ABSTRACT FROM AUTHOR]