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Register a new userMagnetic proximity effect of a topological insulator and a ferromagnet in thin film bilayers of $BiSbTe_3$ and $SrRuO_3$
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Authors
Gad Koren
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Magnetic proximity effect of a topological insulator in contact with a ferromagnet is reported in thin film bilayers of 15 nm thick $BiSbTe_3$ on either 15 or 40 nm thick $SrRuO_3$ on (100) $SrTiO_3$ wafers. Magneto transport results of the bilayers were compared with those of reference films of 15 nm $BiSbTe_3$ and 15 or 40 nm $SrRuO_3$. Comparison of the temperature coefficient of resistance [(1/R)$times$dR/dT which is qualitatively proportional to the magnetization] of the bilayer and reference ferromagnetic film normalized above $T_c$, shows a clear suppression in the bilayer by about 50% just below $T_c$, indicating a weaker proximity magnetization in the bilayer. Resistance hysteresis loops versus field at 1.85$pm$0.05 K in the bilayer and reference films show a clear magnetic proximity effect, where the peak resistance of the bilayer at the coercive field shifts to lower fields by $sim$30% compared to a hypothetical bilayer of two resistors connected in parallel with no interaction between the layers. Narrowing of the coercive peaks of the bilayers as compared to those of the reference ferromagnetic films by 25-35% was also observed, which represents another signature of the magnetic proximity effect.
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In a search for a simple proximity system of a topological insulator and a superconductor for studying the role of surface versus bulk effects by gating, we report here on a first step toward this goal, namely the choice of such a system and its characterization. We chose to work with thin film bilayers of grainy 5 nm thick NbN films as the superconductor, overlayed with 20 nm thick topological layer of $rm Bi_2Se_3$ and compare the transport results to those obtained on a 5 nm thick reference NbN film on the same wafer. Bilayers with ex-situ and in-situ prepared $rm NbN-Bi_2Se_3$ interfaces were studied and two kinds of proximity effects were found. At high temperatures just below the superconducting transition, all bilayers showed a conventional proximity effect where the topological $rm Bi_2Se_3$ suppresses the onset or mid-transition $T_c$ of the superconducting NbN films by about 1 K. At low temperatures, a cross-over of the resistance versus temperature curves of the bilayer and reference NbN film occurs, where the bilayers show enhancement of $T_c(R=0)$, $I_c$ (the supercurrent) and the Andreev conductance, as compared to the bare NbN films. This indicates that superconductivity is induced in the $rm Bi_2Se_3$ layer at the interface region in between the NbN grains. Thus an inverse proximity effect in the topological material is demonstrated.
We report on a study of an ultrathin topological insulator film with hybridization between the top and bottom surfaces, placed in a quantizing perpendicular magnetic field. We calculate the full Landau level spectrum of the film as a function of the applied magnetic field and the magnitude of the hybridization matrix element, taking into account both the orbital and the Zeeman spin splitting effects of the field. For an undoped film, we find a quantum phase transition between a state with a zero Hall conductivity and a state with a quantized Hall conductivity equal to $e^2/h$, as a function of the magnitude of the applied field. The transition is driven by the competition between the Zeeman and the hybridization energies.
Topological crystalline insulators represent a new state of matter, in which the electronic transport is governed by mirror-symmetry protected Dirac surface states. Due to the helical spin-polarization of these surface states, the proximity of topological crystalline matter to a nearby superconductor is predicted to induce unconventional superconductivity and thus to host Majorana physics. We report on the preparation and characterization of Nb-based superconducting quantum interference devices patterned on top of topological crystalline insulator SnTe thin films. The SnTe films show weak antilocalization and the weak links of the SQUID fully-gapped proximity induced superconductivity. Both properties give a coinciding coherence length of 120 nm. The SQUID oscillations induced by a magnetic field show 2$pi$ periodicity, possibly dominated by the bulk conductivity.
The possible realization of dissipationless chiral edge current in a topological insulator / magnetic insulator heterostructure is based on the condition that the magnetic proximity exchange coupling at the interface is dominated by the Dirac surface states of the topological insulator. Here we report a polarized neutron reflectometry observation of Dirac electrons mediated magnetic proximity effect in a bulk-insulating topological insulator (Bi$_{0.2}$Sb$_{0.8}$)$_{2}$Te$_{3}$ / magnetic insulator EuS heterostructure. We are able to maximize the proximity induced magnetism by applying an electrical back gate to tune the Fermi level of topological insulator to be close to the charge neutral point. A phenomenological model based on diamagnetic screening is developed to explain the suppressed proximity induced magnetism at high carrier density. Our work paves the way to utilize the magnetic proximity effect at the topological insulator/magnetic insulator hetero-interface for low-power spintronic applications.
The magnetic proximity effect is a fundamental feature of heterostructures composed of layers of topological insulators and magnetic materials since it underlies many potential applications in devices with novel quantum functionality. Within density functional theory we study magnetic proximity effect at the 3D topological insulator/magnetic insulator (TI/MI) interface in Bi$_2$Se$_3$/MnSe(111) system as an example. We demonstrate that a gapped ordinary bound state which spectrum depends on the interface potential arises in the immediate region of the interface. The gapped topological Dirac state also arises in the system owing to relocation to deeper atomic layers of topological insulator. The gap in the Dirac cone is originated from an overlapping of the topological and ordinary interfacial states. This result being also corroborated by the analytic model, is a key aspect of the magnetic proximity effect mechanism in the TI/MI structures.
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