Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi₄Te₇ with alternating septuple [MnBi₂Te₄] and quintuple [Bi₂Te₃] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi₄Te₇ is a Z₂ antiferromagnetic TI with two types of surface states associated with the [MnBi₂Te₄] or [Bi₂Te₃] termination, respectively. Additionally, its superlattice nature may make various heterostructures of [MnBi₂Te₄] and [Bi₂Te₃] layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi₄Te₇ make it an ideal system to investigate rich emergent phenomena.

We present a candidate material, ${\mathrm{NaYbO}}_2$, that realizes the genuine spin-liquid state on the triangular lattice and benchmarks recent theoretical predictions on the relevant spin models. Synchrotron x-ray diffraction and neutron scattering exclude both structural disorder and crystal-electric-field randomness. Our thermodynamic measurements, neutron diffraction, and muon spectroscopy coincidentally prove the absence of magnetic order and persistent spin dynamics down to at least 70 mK. Continuous magnetic excitations first observed by inelastic neutron scattering show a gapless feature and the low-energy spectral weight accumulating at the $K$ point of the Brillouin zone, in agreement with theoretical predictions for the spin-liquid phase of triangular antiferromagnets. Such a gapless spin-liquid phase is further confirmed by our magnetic specific heat analysis that reveals a departure from simple power-law behavior. Our work demonstrates that ${\mathrm{NaYbO}}_2$ practically gives direct experimental access to the spin-liquid physics of triangular antiferromagnets.

The discovery of an intrinsic ferromagnetic axion insulator MnB _i8 Te ₁₃ paves the way for quantized magnetoelectric effect.

Neutron diffraction is used to reveal the origin and control of the thermal expansion properties of the cubic intermetallic compounds Mn3ZnxN and Mn3[Zn-(Ag,Ge)]xN. We show that the introduction of Zn vacancies induces and stabilizes an antiferromagnetic phase with huge spin-lattice coupling that can be tuned to achieve zero thermal expansion (ZTE) over a wide temperature range. We further show that the antiferromagnetic ordering temperature (TN) that controls this ZTE can be tuned by chemical substitution, again on the Zn site, to adjust the span of ZTE temperatures from well above room temperature to well below. This establishes a quantitative relationship and mechanism to precisely control the ZTE of a single material, enabling it to be tailored for specific device applications.

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 near zero temperature coefficient of resistivity (NZ-TCR) in Mn-based antiperovskite Mn₃Ni_1−xCu_xN is reported. The temperature range of NZ-TCR is controllable by changing Cu content. Further, the TCR value of 0.09 ppm K⁻¹ was obtained in Mn₃Ni_0.5Cu_0.5N over a broad temperature range around room temperature. The anomalous resistivity change of Mn₃Ni_1−xCu_xN from metal-like to NZ-TCR behavioris apparently due to a magnetic transition. The possible reason for the formation of NZ-TCR is interpreted on the basis of spin-disorder scattering.