Using single-crystal neutron diffraction, we present a systematic investigation of the crystal structure and magnetism of the van der Waals topological insulators ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$, where rich topological quantum states have been recently predicted and observed. Structural refinements reveal that considerable Bi atoms occupied on the Mn sites in both materials, distinct from the previously reported antisite disorder. We show unambiguously that ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ orders antiferromagnetically below 24 K, featured by a magnetic symmetry $R_I\text{−}3c$, while ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ is antiferromagnetic below 13 K with a magnetic space group $P_c\text{−}3c1$. They both present antiferromagnetically coupled ferromagnetic layers with spins along the $c$ axis. We put forward a stacking rule for the crystal structure of an infinitely adaptive series ${\mathrm{MnBi}}_{2n}{\mathrm{Te}}_{3n+1}$ ($n\ge 1$) with a building unit of $\left[{\mathrm{Bi}}_2{\mathrm{Te}}_3\right]$. By comparing the magnetic properties between ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$, together with recent density-functional theory calculations, we concluded that a two-dimensional magnetism limit might be realized in the derivatives. Our work may promote theoretical studies of topological magnetic states in the series of ${\mathrm{MnBi}}_{2n}{\mathrm{Te}}_{3n+1}$.

The fine control of magnetism and electronic structure in a magnetic topological insulator is crucial in order to realize various novel magnetic topological states including axion insulators, magnetic Weyl semimetals, Chern insulators, etc. Through crystal growth, transport, thermodynamic, neutron diffraction measurements, we show that under Sb doping the newly discovered intrinsic antiferromagnetic (AFM) topological insulator ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ evolves from AFM to ferromagnetic (FM) and then ferrimagnetic. We attribute this to the formation of ${\mathrm{Mn}}_{(\mathrm{Bi},\mathrm{Sb})}$ antisites upon doping, which results in additional Mn sublattices that modify the delicate interlayer magnetic interactions and cause the dominant Mn sublattice to go from AFM to FM. We further investigate the effect of antisites on the band topology using the first-principles calculations. Without considering antisites, the series evolves from AFM topological insulator ($x=0$) to FM axion insulators. In the exaggerated case of 16.7% of periodic antisites, the band topology is modified and a type-I magnetic Weyl semimetal phase can be realized at intermediate dopings. Therefore, this doping series provides a fruitful platform with continuously tunable magnetism and topology for investigating emergent phenomena, including quantum anomalous Hall effect, Fermi arc states, etc.

Eu‐based compounds often exhibit unusual magnetism, which is critical for nontrivial topological properties seen in materials such as EuCd ₂ As ₂ . The authors investigate the structure and physical properties of EuZn ₂ As ₂ through measurements of the electrical resistivity, Hall effect, magnetization, and neutron diffraction. Their data show that EuZn ₂ As ₂ orders antiferromagnetically with an A‐type spin configuration below T _N = 19 K. Surprisingly, there is strong evidence for dominant ferromagnetic fluctuations above T _N , as reflected by positive Curie–Weiss temperature and extremely large negative magnetoresistance (MR) between T _N and T _fl ≈200 K. Furthermore, the angle dependence of the MR _ab indicates field‐induced spin reorientation from the ab ‐plane to a direction ≈45° from the ab plane. Compared to EuCd ₂ As ₂ , the doubled T _N and T _fl make EuZn ₂ As ₂ a better platform for exploring nontrivial magnetic and electronic properties in both magnetic fluctuation ( T _N < T < T _fl ) and ordered ( T < T _N ) regimes.

Ferrotoroidal order, which represents a spontaneous arrangement of toroidal moments, has recently been found in a few linear magnetoelectric materials. However, tuning toroidal moments in these materials is challenging. Here, we report switching between ferritoroidal and ferrotoroidal phases by a small magnetic field, in a chiral triangular-lattice magnet BaCoSiO₄ with tri-spin vortices. Upon applying a magnetic field, we observe multi-stair metamagnetic transitions, characterized by equidistant steps in the net magnetic and toroidal moments. This highly unusual ferri-ferroic order appears to come as a result of an unusual hierarchy of frustrated isotropic exchange couplings revealed by first principle calculations, and the antisymmetric exchange interactions driven by the structural chirality. In contrast to the previously known toroidal materials identified via a linear magnetoelectric effect, BaCoSiO₄ is a qualitatively new multiferroic with an unusual coupling between several different orders, and opens up new avenues for realizing easily tunable toroidal orders.

The magnetic-field-induced assembly of magnetic nanoparticles (NPs) provides a unique and flexible strategy in the design and fabrication of functional nanostructures and devices. We have investigated the field-induced self-assembly of core–shell iron oxide NPs dispersed in toluene by means of small-angle neutron scattering (SANS). The form factor of the core–shell NPs was characterized and analyzed using SANS with polarized neutrons. Large-scale aggregates of iron oxide NPs formed above 0.02 T as indicated by very-small-angle neutron scattering measurements. A three-dimensional long-range ordered superlattice of iron oxide NPs was revealed under the application of a moderate magnetic field. The crystal structure of the superlattice has been identified to be face-centred cubic.

The magnetic ground state of the ${\mathrm{Eu}}^{2+}$ moments in a series of $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ is established. The ground-state magnetic structure of the ${\mathrm{Eu}}^{2+}$ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic $\mathit{c}$ direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around $\mathit{x}=0.100\left(4\right)$, and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the ${\mathrm{Eu}}^{2+}$ moments, which is mediated by the conduction $\mathit{d}$ electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Fe}}^{2+}$ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ single crystals with 0.$10\le \mathit{x}\le 0.18$, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.