Coincidence of superparamagnetism and perfect quantization in the quantum anomalous Hall state

Abstract

Topological insulators doped with transition metals have recently been found to host a strong ferromagnetic state with perpendicular to plane anisotropy as well as support a quantum Hall state with edge channel transport, even in the absence of an external magnetic field. It remains unclear however why a robust magnetic state should emerge in materials of relatively low crystalline quality and dilute magnetic doping. Indeed, recent experiments suggest that the ferromagnetism exhibits at least some superparamagnetic character. We report on transport measurements in a sample that shows perfect quantum anomalous Hall quantization, while at the same time exhibits traits in its transport data which suggest inhomogeneities. We speculate that this may be evidence that the percolation path interpretation used to explain the transport during the magnetic reversal may actually have relevance over the entire field range. 73.43.Fj, 75.45.+j, 75.50.Pp The recent report on the experimental observation of a quantum anomalous Hall effect (QAHE) in Cr-doped (Bi,Sb) 2 Te 3 [1] generated significant interest in this material system for its potential as a magnetic topological insulator and as a test bed for the study of the Quantum Hall effect without the need for an external magnetic field [2][3][4][5][6]. This original report showed that the anomalous Hall contribution [1] appeared to saturate to a value of one conduction quantum as the sample was cooled to mK temperatures, but did not yet provide evidence that the transport takes place in edge states. In order to convincingly verify that the transport takes place in quantum Hall like edge states, non-local geometries are required. Such measurements were first reported in [3], albeit in configurations where the signals were very small, and where their interpretation required invoking some loss mechanism in the edge channels, and subsequently in [4], where convincing evidence of edge state transport was reported. This last paper also observed some unusual temperature and sweep rate related features in their data, which were at least in part interpreted as additional cooling through adiabatic demagnetization mechanisms. Shortly after the first reports on Cr-doped layers, it was discovered by the Moodera group [5-7] that using V instead of Cr appears to lead to more reproducible samples with a more robust magnetic and quantum anomalous Hall state. Using this material system, the authors were able to reproduce both precise quantization of the Quantum Hall state [5], and unequivocal evidence of edge state transport [6]. While the described quantum anomalous Hall phenomenology is now well established, its microscopic origin remains much less clear. The proposed mechanism for the QAHE is the breaking of time reversal symmetry by a perpendicular to plane internal magnetic field which leads to the reversal of the band inversion of one of the two spin species in a ferromagnetic two dimensional topological insulator [1,8]. The origin of the ferromagnetic state in the original paper [1] w...

Observation of superparamagnetism in coexistence with quantum anomalous Hall C = ±1 and C = 0 Chern states

“…2,13,16 Indeed, a number of studies using global magnetization measurements 14,17,18 and magnetic force microscopy 19,20 have observed FM-like response in magnetically doped TI thin films. Surprisingly, however, recent scanning SQUID magnetic imaging 21 and transport studies, 22,23 as well as analysis of Kerr microscopy imaging, 24 have suggested the presence of a superparamagnetic (SPM) state in Cr-doped (Bi, Sb) 2 Te 3 films in which instead of a long-range FM order the magnetism is comprised of single-domain magnetic islands. These islands are of characteristic size of few tens of nanometers and are only weakly coupled to each other.…”

Impurity states in the magnetic topological insulator V:(Bi,Sb)2Te3

“…At low temperatures, small variations of the 3d impurity band affecting the d-shell filling and the bandwidth at E F , e.g. localization of the 3d impurity band, may strongly favor the ferromagnetic superexchange, and influence the onset of ferromagnetism and QAHS recently reported in these systems 2,8,9,14 .…”

“…Therefore it is of great importance to know in detail the electronic structure at the valence band, and in particular to identify the nature of the electronic states near E F in these systems. In this Rapid Communication we exploit the element-specificity of resonant photoemission (resPES), X-ray photoemission The MBE growth has been described elsewhere 9 . Undoped samples were used as reference in the experiments.…”

“…The QAHS in MTI offers promising prospects towards the advent of low energy-consumption electronic devices 1,[4][5][6] , and therefore has been attracting wide attention in condensed matter physics. In fact, the QAHS has been recently reported on two MTI systems, namely V-and Cr-doped (Bi x Sb 1−x ) 2 Te 3 epitaxial thin films, in ultra-low temperature (< 100 mK) magnetotransport studies 3,[7][8][9] .…”

Effect of chiral selective tunneling on quantum transport in magnetic topological-insulator thin films

“…Beside transport properties, the mechanism behind strong ferromagnetic ground state in QAH insulators is, however still under debate 26 . One of the proposals is the existence of magnetic impurities on the surface of TI thin film which can enhance local density of state in the band gap 27 and as a consequence, the RKKY interaction in these materials 28 is intensified.…”