We studied the crystal structure and magnetic properties of the rare-earth-free intermetallic compound Fe3Co3Nb2, which has recently been demonstrated to have potentially high magnetic anisotropy, using temperature-dependent neutron powder diffraction. The temperature dependence of the diffraction spectra reveals a magnetic transition between 300 and 400 K, in agreement with the magnetometry measurements. According to the structural refinement of the paramagnetic state and the substantial magnetic contribution to the diffuse scattering in the ferromagnetic state, the Fe/Co anti-site mixing is so strong that the site occupation for Fe and Co is almost random. The projection of the magnetic moments turned out to be non-zero along the c axis and in the a–b plane of Fe3Co3Nb2, most likely because of the exchange interactions between the randomly orientated nanograins in the samples. These findings suggest that future studies on the magnetism of Fe3Co3Nb2 need to take the Fe/Co anti-site mixing into account, and the exchange interactions need to be suppressed to obtain large remanence and coercivity.
We explore the effect of ionic-liquid gating in the antiferromagnetic Mott insulator ${\mathrm{NiS}}_{2}$. Through temperature- and gate-voltage-dependent electronic transport measurements, a gating-induced three-dimensional metallic state is observed at positive gate bias on single-crystal surfaces. Based on transport, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an $\mathit{electrochemical}$ gating mechanism involving a substantial decrease in the S:Ni ratio over hundreds of nanometers, which is both nonvolatile and irreversible. Such findings are in striking contrast to the reversible, volatile, two-dimensional $\mathit{electrostatic}$ gate effect previously seen in pyrite ${\mathrm{FeS}}_{2}$. We attribute this stark difference in electrochemical vs electrostatic gating response in ${\mathrm{NiS}}_{2}$ and ${\mathrm{FeS}}_{2}$ to the much larger S diffusion coefficient in ${\mathrm{NiS}}_{2}$. The gating irreversibility, on the other hand, is associated with the lack of atmospheric S, in contrast to the better understood oxide case, where electrolysis of atmospheric ${\mathrm{H}}_{2}\mathrm{O}$ provides an O reservoir. The present study of ${\mathrm{NiS}}_{2}$ thus provides insight into electrolyte gating mechanisms in functional materials, in a relatively unexplored limit.
The development of new magnetic materials is of paramount importance to surmount significant worldwide challenges in energy and environmental security. On page 6090, David J. Sellmyer, Balamurugan Balasubramanian, and co-workers report a novel hybrid nanomagnetic material having the highest energy product for permanentmagnet materials free of critical rare-earth elements or expensive Pt. This is achieved in a single-step process by assembling the easy-axis aligned high-anisotropy nanoparticles with a high-magnetization soft Fe-Co phase.
Abstract Nanoscience has been one of the outstanding driving forces in technology recently, arguably more so in magnetism than in any other branch of science and technology. Due to nanoscale bit size, a single computer hard disk is now able to store the text of 3,000,000 average-size books and today's high-performance permanent magnets—found in hybrid cars, wind turbines and disk drives—are nanostructured to a large degree. The nanostructures ideally are designed from Co- and Fe-rich building blocks without critical rare-earth elements and often are required to exhibit high coercivity and magnetization at elevated temperatures of typically up to 180 °C for many important permanent-magnet applications. Here we achieve this goal in exchange-coupled hard-soft composite films by effective nanostructuring of high-anisotropy HfCo 7 nanoparticles with a high-magnetization Fe 65 Co 35 phase. An analysis based on a model structure shows that the soft-phase addition improves the performance of the hard-magnetic material by mitigating Brown's paradox in magnetism, a substantial reduction of coercivity from the anisotropy field. The nanostructures exhibit a high room-temperature energy product of about 20.3 MGOe (161.5 kJ/m 3 ), which is a record for a rare earth- or Pt-free magnetic material and retain values as high as 17.1 MGOe (136.1 kJ/m 3 ) at 180°C.
The pyrite-structure transition-metal disulfide $\mathrm{Ni}{\mathrm{S}}_{2}$ is in principle a model cubic antiferromagnetic Mott insulator that can be doped through insulator-metal transitions with both electrons and holes (in ${\mathrm{Ni}}_{1--x}{\mathrm{Cu}}_{x}{\mathrm{S}}_{2}$ and ${\mathrm{Ni}}_{1--x}{\mathrm{Co}}_{x}{\mathrm{S}}_{2}$), eventually inducing superconductivity and ferromagnetism, respectively. Magnetism and transport have proven challenging to understand in $\mathrm{Ni}{\mathrm{S}}_{2}$, however. The antiferromagnetic spin structure below $\ensuremath{\sim}39\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is complex due to frustration, while unexplained weak ferromagnetism emerges below $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Surface conduction is also now understood to dominate in $\mathrm{Ni}{\mathrm{S}}_{2}$ at low temperatures, raising questions about the interpretation of decades of prior data. Here, we present a complete study of the surface magnetotransport phenomena that emerge at low temperatures in high-quality single-crystal $\mathrm{Ni}{\mathrm{S}}_{2}$, which turn out to be strikingly rich. On cooling, isotropic magnetoresistance due to a field-induced shift of the first-order weak ferromagnetic ordering transition is first uncovered, i.e., metamagnetic magnetoresistance. At lower temperatures, larger, anisotropic magnetoresistance effects arise due to distinct switching events associated with the weak ferromagnetism. Strong evidence is presented that this is due to a field-driven in-plane to out-of-plane reorientation of surface spins, likely correlated with surface steps and terraces. In-plane exchange bias accompanies these effects, further supporting this interpretation. At the lowest temperatures, the spin reorientation field eventually exceeds the 9-T measurement window, generating strongly field-asymmetric magnetoresistance. Some of these unusual phenomena also manifest in the Hall channel, culminating in a sizable anomalous Hall effect at low temperatures. These results significantly demystify recent magnetoresistance and magnetic microscopy observations in $\mathrm{Ni}{\mathrm{S}}_{2}$ crystals and nanoflakes, and constitute an important step in elucidating the complex electronic and magnetic properties of this pivotal antiferromagnetic Mott insulator.
Pyrite ${\mathrm{FeS}}_{2}$ 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 ${\mathrm{FeS}}_{2}$ crystals by fabricating metallic ${\mathrm{CoS}}_{2}$ contacts via a process that 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 $2100\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$, elucidation of very different mobilities in Co- and S-vacancy-doped cases, and observation of an abrupt temperature-dependent 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 ${\mathrm{FeS}}_{2}$. 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.
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