Systemic consequences of disorder in magnetically self-organized topological MnBi₂Te₄/(Bi₂Te₃) _n superlattices

Abstract

MnBi2Te4/(Bi2Te3)n materials system has recently generated strong interest as a natural platform for realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic MnBi2Te4/(Bi2Te3)n it is found that migration of Mn between MnBi2Te4 septuple layers (SLs) and otherwise non-magnetic Bi2Te3 quintuple layers (QLs) has systemic consequences - it induces ferromagnetic coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL separation (n ≳ 4 QLs) the structure cannot be considered as a stack of uncoupled two-dimensional layers. Angle-resolved photoemission spectroscopy and density functional theory studies show that Mn disorder within an SL causes delocalization of electron wave functions and a change of the surface band structure as compared to the ideal MnBi2Te4/(Bi2Te3)n. These findings highlight the critical importance of inter- and intra-SL disorder towards achieving new QAH platforms as well as exploring novel axion physics in intrinsic topological magnets.

SOLARIS National Synchrotron Radiation Centre in Krakow, Poland

“…The cryogenic 5-axis manipulator enables rotation in large ranges of angles, and the temperature stabilization on the sample is in the range from 6.5 to 500 K. In particular, from several possible studies, magnetic topological insulators (MTIs) were intensively researched because they represent a possible route towards manipulating quantum information, coherent spin transport, and high-efficiency catalysis [14]. In the last years, URANOS research groups have tried to understand the phenomena in magnetic MTI, concentrated on studying both the magnetism of topological insulator crystals with ferromagnetic dopants in bulk [15,16], surface magnetism on a topological insulator realized by ferromagnetic monolayer [17], the magnetism of self-organizing ferromagnetic layers separated by layers of a topological insulator which can generate FM or AFM order [18,19] and axion physics [14]. Particularly important are studies on the organization of FM layers that generate intrinsic antiferromagnetic topological insulators in bulk in the compounds MnBi 2 Te 4 /(Bi 2 Te 3 ) n .…”

Delicate Ferromagnetism in MnBi6Te10

“…We emphasize that this disorder tuning leads to a change of the "global" magnetic ground state, which is to be distinguished from the more trivial disorder effect resulting in local changes of magnetism. 36 Our new insight of quantitatively considering the ratio of n m /n o is a substantial advance compared to previous studies which qualitatively pointed out the importance of disorder 36,56 and the theoretical modeling whose predictions sensitively depend on numerical details. 55 This insight, importantly, is obtained by the multimodal measurements combining magnetization, laser-based μARPES, and atomic-resolution TEM on the same batch of samples.…”

Role of Magnetic Defects and Defect-engineering of Magnetic Topological Insulators

“…Antisite Mn defects that occupy Bi or Sb sites (creating magnetic vacancies in the Mn layer) introduce new AFM couplings within a septuple block (intrablock) that lead to defect-driven ferrimagnetism [20,21]. The exchange coupling between antisite defects and surface electronic bands has been shown to be a pivotal factor that controls the surface magnetism and consequently the Chern gap [20,[22][23][24][25]. These studies reveal the important role of Mn defects in MBT and MST, and the prospect that defect-engineering can control and tailor the global magnetic ground state that affect topological surface states.…”