Millimeter-sized MnBi2Te4 single crystals are grown out of a BiTe flux and characterized using magnetic, transport, scanning tunneling microscopy, and spectroscopy measurements. The magnetic structure of MnBi2Te4 below TN is determined by powder and single-crystal neutron diffraction measurements. Below TN = 24 K, Mn2+ moments order ferromagnetically in the ab plane but antiferromagnetically along the crystallographic c axis. The ordered moment is 4.04(13)μB/Mn at 10 K and aligned along the crystallographic c axis in an A-type antiferromagnetic order. Below TN, the electrical resistivity drops upon cooling or when going across the metamagnetic transition in increasing magnetic fields. A critical scattering effect is observed in the vicinity of TN in the temperature dependence of thermal conductivity, indicating strong spin-lattice coupling in this compound. However, no anomaly is observed in the temperature dependence of thermopower around TN. Fine tuning of the magnetism and/or electronic band structure is needed for the proposed topological properties of this compound. The growth protocol reported in this work might be applied to grow high-quality crystals where the electronic band structure and magnetism can be finely tuned by chemical substitutions.

Neutron and x-ray diffraction measurements are presented for powders and single crystals of CaCo 2 As 2 . The crystal structure is a collapsed-tetragonal ThCr 2 Si 2 -type structure as previously reported, but with 7(1)% vacancies on the Co sites corresponding to the composition CaCo 1.86(2) As 2 . The thermal expansion coefficients for both the a and c axes are positive from 10 to 300 K. Neutron diffraction measurements on single crystals demonstrate the onset of A-type collinear antiferromagnetic order below the Néel temperature T N = 52(1) K with the ordered moments directed along the tetragonal c axis, aligned ferromagnetically in the ab plane and antiferromagnetically stacked along the c axis.

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Néel temperature TN = 625 K and a large ordered moment µ = 3.9 µB/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K, while retaining the same crystal and antiferromagnetic structures together with nearly the same high TN and large µ. Ba1−xKxMn2As2 is thus the first metallic ThCr2Si2-type M As-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba1−xKxMn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high Tc superconductivity.Superconducting transition temperatures T c > 50 K have been observed for only two classes of materialslayered cuprates and iron arsenides [1,2]. Both classes contain stacked square lattice layers of the transition metal atoms. However, the parent compounds of the two families exhibit divergent physical properties. For example, La 2 CuO 4 is a local magnetic moment antiferromagnetic (AF) insulator [1] while BaFe 2 As 2 is metallic and its AF ordering is widely considered to be best characterized as a spin-density wave arising from conduction carriers [2]. These differences create barriers for a general and comprehensive understanding of the underlying mechanisms of high-T c superconductivity and related phenomena in a broad spectrum of materials. Thus, it is desirable to create a material that can bridge the gap between the cuprates and iron arsenides. Herein we report the synthesis and properties of such a material, Ba 1−x K x Mn 2 As 2 (x = 0.016, 0.05), which shares properties with both classes.The undoped parent compound BaMn 2 As 2 crystallizes in the same body-centered-tetragonal (bct) ThCr 2 Si 2 -type structure as the M Fe 2 As 2 (M = Ca, Sr, Ba) iron arsenide parent compounds do at room temperature [2][3][4]. It is a semiconductor with an activation energy of ∼ 30 meV determined from electrical resistivity ρ(T ) measurements [4,5], consistent with electronic structure calculations that indicate a band gap of ∼ 100-150 meV [5]. Heat capacity C p measurements at low-T yield an electronic linear heat capacity coefficient γ = 0 which is consistent with an insulating ground state [4]. BaMn 2 As 2 orders into a G-type (Néel-or checkerboard-type) AF structure below a Néel temperature T N = 625(1) K with an ordered moment at 10 K of µ = 3.88(4) µ B /Mn aligned along the crystallographic c axis [2, 4, 6]. Since BaMn 2 As 2 is an insulator at low temperatures, these results demonstrate that the antiferromagnetism arises from ordering of local Mn magnetic moments instead of from itinerant current carriers. Both the static and dynamic magnetic properties for T = 4-1000 K are welldescribed by the AF J 1 -J 2 -J c local moment Heisenberg model, with a Mn spin S = 5/2 as expected from the 3d 5 e...

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Nuclear magnetic resonance (NMR), neutron diffraction (ND), x-ray diffraction, magnetic susceptibility χ and specific heat measurements on the frustrated A-site spinel CoAl2O4 compound reveal a collinear antiferromagnetic ordering below TN = 9.8(2) K. A high quality powder sample characterized by x-ray diffraction that indicates a relatively low Co-Al inversion parameter x = 0.057 (20) in (Co1−xAlx)[Al2−xCox]O4, shows a broad maximum around 15 K in χ(T ) and a sharp peak at TN in heat capacity Cp. The average ordered magnetic moment of Co 2+ (S = 3/2) ions at the A-site is estimated to be 2.4(1) µB from NMR and 1.9(5) µB from ND which are smaller than the expected value of 3 µB for S = 3/2 and g = 2. Antiferromagnetic spin fluctuations and correlations in the paramagnetic state are revealed from the χ, NMR and ND measurements, which are due to spin frustration and site inversion effects in the system. The ND data also show short-range dynamic magnetic ordering that persists to a temperature that is almost twice TN.

Knowledge of magnetic symmetry is vital for exploiting nontrivial surface states of magnetic topological materials. EuIn2As2 is an excellent example, as it is predicted to have collinear antiferromagnetic order where the magnetic moment direction determines either a topological-crystalline-insulator phase supporting axion electrodynamics or a higher-order-topological-insulator phase with chiral hinge states. Here, we use neutron diffraction, symmetry analysis, and density functional theory results to demonstrate that EuIn2As2 actually exhibits low-symmetry helical antiferromagnetic order which makes it a stoichiometric magnetic topological-crystalline axion insulator protected by the combination of a 180∘ rotation and time-reversal symmetries: $${C}_{2}\times {\mathcal{T}}={2}^{\prime}$$ C 2 × T = 2 ′ . Surfaces protected by $${2}^{\prime}$$ 2 ′ are expected to have an exotic gapless Dirac cone which is unpinned to specific crystal momenta. All other surfaces have gapped Dirac cones and exhibit half-integer quantum anomalous Hall conductivity. We predict that the direction of a modest applied magnetic field of μ0H ≈ 1 to 2 T can tune between gapless and gapped surface states.

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The results of crystallographic analysis, magnetic characterization, and theoretical assessment of β-Mn-type Co-Zn intermetallics prepared using high-temperature methods are presented. These β-Mn Co-Zn phases crystallize in the space group P4(1)32 [Pearson symbol cP20; a = 6.3555(7)-6.3220(7)], and their stoichiometry may be expressed as Co(8+x)Zn(12-x) [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of ∼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the spontaneous magnetization of this phase arises exclusively from local moments at the Co atoms. Inspection of the atomic arrangements of Co(8+x)Zn(12-x) reveals that the β-Mn aristotype may be derived from an ordered defect, cubic Laves phase (MgCu2-type) structure. Structural optimization procedures using the Vienna ab initio simulation package (VASP) and starting from the undistorted, defect Laves phase structure achieved energy minimization at the observed β-Mn structure type, a result that offers greater insight into the β-Mn structure type and establishes a closer relationship with the corresponding α-Mn structure (cI58).