Robust A -Type Order and Spin-Flop Transition on the Surface of the Antiferromagnetic Topological Insulator MnBi2Te4

Abstract

Here we present microscopic evidence of the persistence of uniaxial A-type antiferromagnetic order to the surface layers of MnBi2Te4 single crystals using magnetic force microscopy. Our results reveal termination-dependent magnetic contrast across both surface step edges and domain walls, which can be screened by thin layers of soft magnetism. The robust surface A-type order is further corroborated by the observation of termination-dependent surface spin-flop transitions, which have been theoretically proposed decades ago. Our results not only provide key ingredients for understanding the electronic properties of the antiferromagnetic topological insulator MnBi2Te4, but also open a new paradigm for exploring intrinsic surface metamagnetic transitions in natural antiferromagnets.Recent progress in topological quantum materials suggest that antiferromagnets may host interesting topological states [1]. For example, it has been proposed that an axion insulator state with topological magnetoelectric response could be realized in an antiferromagentic topological insulator (TI) phase [2,3], where the Z2 topological states are protected by a combination of timereversal symmetry and primitive-lattice translation. The antiferromagnetic TI state adiabatically connects to a stack of quantum Hall insulators with alternating Chern numbers [4], thus providing a promising route to realizing the quantum anomalous Hall (QAH) effect in a stoichiometric material. The prior observation of the QAH effect in magnetically doped TI thin films is limited to extremely low temperature because of the inherent disorder [5][6][7][8][9], though the disorder effects can be partially alleviated by material engineering [10][11][12]. The MnBi2Te4 (MBT) family was predicted and confirmed to be an antiferromagnetic TI that may host QAH and axion-insulator states in thin films with odd and even numbers of septuple layers (SLs) respectively [13][14][15][16][17]. Recent transport measurements on exfoliated thin flakes provide compelling evidence for these predictions [18,19], suggesting gapped topological surface states. On the other hand, recent highresolution angle-resolved photoemission spectroscopy (ARPES) studies reveal gapless (or smallgap) surface states below the antiferromagnetic ordering temperature, suggesting a surface relaxation of the A-type order and/or the formation of nanometer-sized magnetic domains [20][21][22][23]. The antiferromagnetic domain structure of MnBi2Te4 was revealed by imaging of domain walls using magnetic force microscopy (MFM) [24]. The observed domain size is on the order of 10 µm, excluding the speculated nanometer-size domain scenario [22].

MnBi2Te4-family intrinsic magnetic topological materials

“…Rapid and exciting experimental progress has been made in MnBi 2 Te 4 -family materials in the past years, constructing one of the hottest topics in condensed matter physics recently 8,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] . Single-crystal samples of MnBi 2 Te 4 -family materials have been successfully prepared by several groups [20][21][22][23][24][25][26][27] .…”

“…Single-crystal samples of MnBi 2 Te 4 -family materials have been successfully prepared by several groups [20][21][22][23][24][25][26][27] . The A-type AFM structure has been demonstrated with SQUID measurements and directly observed with magnetic force microscopy 28,29 . A magnetic field of several Tesla is enough to overcome the weak AFM interlayer coupling and make MnBi 2 Te 4 FM, as shown by the stepped hysteresis loops characterizing a van der Waals magnetic material 8 .…”

Controllable quantum point junction on the surface of an antiferromagnetic topological insulator

“…Our proposal aims to highlight a robust way to construct intersecting chiral channels by making use of a material system that is on the verge of discovery. In fact, these junctions were recently observed to appear naturally at the surface of the putative AFM TI MnBi 2 Te 4 37 . Moreover, we show that a QPJ can be controlled by scanning tips of the kind used in scanning tunneling microscopy (STM) and related methods.…”

Влияние Магнитных Доменных Стенок На Электронную Структуру Поверхности Топологических Изоляторов

“…ВЛИЯНИЕ МАГНИТНЫХ ДОМЕННЫХ СТЕНОК НА ЭЛЕКТРОННУЮ СТРУКТУРУ ПОВЕРХНОСТИ ТОПОЛОГИЧЕСКИХ ИЗОЛЯТОРОВ 1 Русинов И.П., 1,2,3 Меньшов В.Н., 2,4,5 Чулков E.В. Формирование магнитного порядка на поверхности топологических изоляторов (ТИ) может приводить к необычным явлениям, таким как квантовый аномальный эффект Холла, состояние аксионного изолятора, фермионы Майораны.…”

“…Обычно при описании этих явлений используется предположение об однородно намагниченной поверхности. Однако, многие экспериментальные факты свидетельствуют о более сложной магнитной текстуре поверхности ТИ, что часто проявляется в формировании доменных стенок [1]. Присутствие магнитных неоднородностей такого вида может заметно модифицировать поверхностную электронную структуру вблизи уровня Ферми и повлиять на спектральные и кинетические систем на основе ТИ [2].…”