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Pyrite structure transition-metal disulfides exhibit diverse ground states vs d-band filling, spanning diamagnetic semiconducting, ferromagnetic metallic, antiferromagnetic Mott insulating, and superconducting in FeS₂, CoS₂, NiS₂, and CuS₂. NiS is particularly interesting and poorly understood as its Mott insulating behavior is accompanied by complex antiferromagnetic ordering below ~38 K and perplexing weak ferromagnetism below ~30 K. Temperature-, pressure-, and composition-dependent insulator-metal transitions also occur, particularly in bandwidth-controlled NiS_2–xSe_x, hole-doped Ni_1–xCo_xS₂, etc. Here, we use high-quality chemical-vapor-transport-grown NiS₂ single crystals characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetometry, and extensive transport and magnetotransport measurements, to generate new insight into this system. Inmore » particular, resistivity, magnetoresistance, and Hall effect analyses vs temperature, thickness, and surface preparation, provide unequivocal evidence of surface conduction, where the more conductive surface shunts essentially all current at low temperatures. The surface transport changes from two dimensional and insulating to three dimensional and metallic as the surface preparation is varied (also displaying intriguing sensitivity to magnetic ordering), significantly clarifying literature ambiguities with respect to the electronic ground state. These results have immediate implications. First, the temperature-, pressure-, and composition-dependent insulator-metal transitions deduced in the extensive prior work on NiS_2-xSe_x, Ni_1–xCo_xS₂, etc., must clearly be reexamined in light of rife metallic surface conduction, not previously taken into account. Second, NiS₂ now joins FeS₂ and CoS₂ as systems in which bulk and surface electronic behaviors are strikingly different, suggesting that metallic surface states could be a universal feature of pyrite structure transition-metal disulfides.« less

Pyrite FeS₂ is an outstanding candidate for a low-cost, nontoxic, sustainable photovoltaic material, but efficient pyrite-based solar cells are yet to materialize. Recent studies of single crystals have shed much light on this by uncovering a p-type surface inversion layer on n-type (S-vacancy doped) crystals, and the resulting internal p-n junction. This leaky internal junction likely plays a key role in limiting efficiency in pyrite-based photovoltaic devices, also obscuring the true bulk semiconducting transport properties of pyrite crystals. Here, we demonstrate complete mitigation of the internal p-n junction in FeS₂ crystals by fabricating metallic CoS₂ contacts via a process thatmore » simultaneously diffuses Co (a shallow donor) into the crystal, the resulting heavy n doping yielding direct Ohmic contact to the interior. Low-temperature bulk transport studies of controllably Co- and S-vacancy doped semiconducting crystals then enable a host of previously inaccessible observations and measurements, including determination of donor activation energies (which are as low as 5 meV for Co), observation of an unexpected second activated transport regime, realization of electron mobility up to 2100 cm² V^–1 s^–1, elucidation of very different mobilities in Co- and S-vacancy-doped cases, and observation of an abrupt temperaturedependent crossover to bulk Efros-Shklovskii variable-range hopping, accompanied by an unusual form of nonlinear Hall effect. Aspects of the results are interpreted with the aid of first-principles electronic structure calculations on both Co- and S-vacancy-doped FeS₂. Furthermore, this work thus demonstrates unequivocal mitigation of the internal p-n junction in pyrite single crystals, with important implications for both future fundamental studies and photovoltaic devices« less

Here, we report the fabrication of a set of new rare-earth-free magnetic compounds, which form the Fe₃Co₃Ti₂-type hexagonal structure with P-6m2 symmetry. Neutron powder diffraction shows a significant Fe/Co anti-site mixing in the Fe₃Co₃Ti₂ structure, which has a strong effect on the magnetocrystalline anisotropy as revealed by first-principle calculations. Increasing substitution of Fe atoms for Co in the Fe₃Co₃Ti₂ lattice leads to the formation of Fe₄Co₂Ti₂, Fe₅CoTi, and Fe₆Ti₂ with significantly improved permanent-magnet properties. A high magnetic anisotropy (13.0 Mergs/cm³) and saturation magnetic polarization (11.4 kG) are achieved at 10 K by altering the atomic arrangements and decreasing Fe/Co occupancymore » disorder.« less

Here, we report the fabrication of a rare-earth-free permanent-magnet material Co₃Si in the form of nanoparticles and investigate its magnetic properties by experiments and density-functional theory (DFT). The DFT calculations show that bulk Co₃Si has an easy-plane anisotropy with a high K₁ ≈ -64 Merg/cm³ (-6.4 MJ/m³) and magnetic polarization of 9.2 kG (0.92 T). In spite of having a negative anisotropy that generally leads to negligibly low coercivities in bulk crystals, Co₃Si nanoparticles exhibit high coercivities (17.4 kOe at 10K and 4.3 kOe at 300 K). This result is a consequence of the unique nanostructure made possible by anmore » effective easy-axis alignment in the cluster-deposition method and explained using micromagnetic analysis as a nanoscale phenomenon involving quantum-mechanical exchange interactions.« less