Neutron diffraction study of magnetism in van der Waals layered MnBi_2n Te_3n+1

Abstract

Two-dimensional van der Waals MnBi2n Te3n+1 (n = 1, 2, 3, 4) compounds have been recently found to be intrinsic magnetic topological insulators rendering quantum anomalous Hall effect and diverse topological states. Here, we summarize and compare the crystal and magnetic structures of this family, and discuss the effects of chemical composition on their magnetism. We found that a considerable fraction of Bi occupies at the Mn sites in MnBi2n Te3n+1 (n = 1, 2, 3, 4) while there is no detectable Mn at the non-magnetic atomic sites within the resolution of neutron diffraction experiments. The occupancy of Mn monotonically decreases with the increase of n. The polarized neutron diffraction on the representative MnBi4Te7 reveals that its magnetization density is exclusively accumulated at the Mn site, in good agreement with the results from the unpolarized neutron diffraction. The defects of Bi at the Mn site naturally explain the continuously reduced saturated magnetic moments from n = 1 to n = 4. The experimentally estimated critical exponents of all the compounds generally suggest a three-dimensional character of magnetism. Our work provides material-specified structural parameters that may be useful for band structure calculations to understand the observed topological surface states and for designing quantum magnetic materials through chemical doping.

Unusual dynamic susceptibility arising from soft ferromagnetic domains in MnBi8Te13 and Sb-doped MnBi 2n Te 3n+1 (n = 2, 3)

“…To reduce the interlayer AFM coupling, one strategy is to add nonmagnetic Bi 2 Te 3 quintuple layers (QL) between SLs to physically separate the magnetic MnBi 2 Te 4 layers to get MnBi 2n Te 3n+1 (MBT). By this design, MnBi 4 Te 7 and MnBi 6 Te 10 are found to be weakly-coupled A-type AFM [23][24][25][26][27][28][29][30][31][32][33], and ferromagnetism is achieved when n ≥ 4 for MnBi 8 Te 13 and higher n members [34,35]. The other strategy is to dope Bi by Sb.…”

Crystal Defect Doping on Antiferromagnetic Topological Insulator Candidate EuMg2Bi2

“…The local disorder in this material has a direct impact on the narrowing of the inverted gap and band broadening. Likewise, the magnetic behavior can be tuned, as with the well-known first intrinsic magnetic topological insulator MnBi 2 n Te 3 n +1 − with Sb-doping leading to the materials Mn­(Bi 1– x Sb x ) 4 Te 7 and Mn­(Bi 1– x Sb x ) 6 Te 10. , As a result of the antisites induced by the Sb-doping, a magnetic transition from an antiferromagnetic ordering to a ferromagnetic-like behavior occurs.…”

Magnetic dilution effect and topological phase transitions in (Mn$_{1-x}$Pb$_x$)Bi$_2$Te$_4$

“…where A is a proportional constant, β is the critical exponent of the order parameter, and B is the background. Unlike the undoped sample whose order parameter can be fitted by the 3D Heisenberg model near the critical temperature [43], from 5 to 20 K, the best fit is shown as the red curve, which yields T N = 14.1 K and the critical exponent β = 0.44 (2), considerably larger than that in MnBi 2 Te 4 (0.36) [43]. We also show the curves with β = 0.36 (3D Heisenberg case) and β = 0.32 (3D Ising case), which clearly deviate from the data.…”