The magnetic structure and phase diagram of the layered ferromagnetic compound ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ has been investigated by a combination of synthesis, x-ray and neutron diffraction, high-resolution microscopy, and magnetization measurements. Single crystals were synthesized by self-flux reactions, and single-crystal neutron diffraction finds ferromagnetic order with moments of $1.11\left(5\right){\mu }_B/\mathrm{Fe}$ aligned along the $c$ axis at 4 K. These flux-grown crystals have a lower Curie temperature $T_{\text{c}}\approx 150$ K compared to crystals previously grown by vapor transport ($T_{\text{c}}=220$ K). The difference is a reduced Fe content in the flux-grown crystals, as illustrated by the behavior observed in a series of polycrystalline samples. As Fe content decreases, so does the Curie temperature, magnetic anisotropy, and net magnetization. In addition, Hall effect and thermoelectric measurements on flux-grown crystals suggest multiple carrier types contribute to electrical transport in ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ and structurally similar ${\mathrm{Ni}}_{3-x}{\mathrm{GeTe}}_2$.

We synthesized polycrystalline ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7-\delta }$ ($\delta \approx 0.07$) samples by using the sol-gel method without postannealing under high oxygen pressure, and then measured temperature-dependent resistivity under various hydrostatic pressures up to 18 GPa by using the cubic anvil and two-stage multianvil apparatus. We find that the density-wave-like anomaly in resistivity is progressively suppressed with increasing pressure and the resistivity drop corresponding to the onset of superconductivity emerges at pressure as low as $\sim 6\mathrm{GPa}$. Zero resistivity is achieved at 9 GPa below $T_{\mathrm{c}}^{\mathrm{zero}}\approx 6.6\mathrm{K}$, which increases quickly with pressure to 41 K at 18 GPa. However, the diamagnetic response was not detected in the ac magnetic susceptibility measurements up to 15 GPa, indicating a filamentary nature of the observed superconductivity in the studied pressure range. The constructed $T\text{−}P$ phase diagram reveals an intimate relationship between superconductivity, density-wave-like order, and the strange-metal-like behaviors. The observation of zero-resistance state in the polycrystalline ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7-\delta }$ samples under high pressures not only corroborates the recent report of superconductivity in the pressurized ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ crystals but also facilitates further studies on this emerging family of nickelate high-$T_{\mathrm{c}}$ superconductors.

Fe ₃ GeTe ₂ is known as an air-stable layered metal with itinerant ferromagnetism with a transition temperature of about 220 K. From our extensive dc and ac magnetic measurements, we have determined that the ferromagnetic layers of Fe ₃ GeTe ₂ actually order antiferromagnetically along the c -axis below 152 K. The antiferromagnetic state was further substantiated by theoretical calculation to be the ground state. A magnetic structure model was proposed to describe the antiferromagnetic ground state as well as competition between antiferromagnetic and ferromagnetic states. Fe ₃ GeTe ₂ shares many common features with pnictide superconductors and may be a promising system in which to search for unconventional superconductivity.

The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials. Various mechanisms producing MITs have been extensively considered, including the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder), and Peierls (localization via distortion of a periodic one-dimensional lattice) mechanisms. One additional route to a MIT proposed by Slater, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention. Using neutron and x-ray scattering we show that the MIT in ${\mathrm{NaOsO}}_3$ is coincident with the onset of long-range commensurate three dimensional magnetic order. While candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT.