Highly Tunable Intrinsic Exchange Bias from Interfacial Reconstruction in Epitaxial NixCoyFe3-x-yO4(111)/{\alpha}-Al2O3(0001) Thin Films

Intrinsic exchange bias up to 12.6 kOe is observed in NixCoyFe3-x-yO4(111)/{\alpha}-Al2O3(0001) (0<=x+y<=3) epitaxial thin films where 0.15<=y<=2. An interfacial layer of rock-salt structure emerges between NixCoyFe3-x-yO4 thin films and {\alpha}-Al2O3 substrates and is proposed as the antiferromagnetic layer unidirectionally coupled with ferrimagnetic NixCoyFe3-x-yO4. In NiCo2O4(111)/{\alpha}-Al2O3(0001) films, results of reflection high energy electron diffraction, X-ray photoelectron spectroscopy, X-ray reflectometry, and polarized neutron reflectometry support that the interfacial layer is antiferromagnetic NixCo1-xO (0.32<=x<=0.49) of rock-salt structure; the interfacial layer and exchange bias can be controlled by growth oxygen pressure revealing the key role of oxygen in the mechanism of the interfacial reconstruction. This work establishes a family of intrinsic exchange bias materials with great tunability by stoichiometry and growth parameters and emphasizes the strategy of interface engineering in controlling material functionalities.Comment: Main Text: 14 pages, 5 figures; Supplemental Materials: 12 pages, 11 figure

Discovery of a high-temperature antiferromagnetic state and transport signatures of exchange interactions in a Bi2Se3/EuSe heterostructure

Spatial confinement of electronic topological surface states (TSS) in topological insulators poses a formidable challenge because TSS are protected by time-reversal symmetry. In previous works formation of a gap in the electronic spectrum of TSS has been successfully demonstrated in topological insulator/magnetic material heterostructures, where ferromagnetic exchange interactions locally lifts the time-reversal symmetry. Here we report an experimental evidence of exchange interaction between a topological insulator Bi2Se3 and a magnetic insulator EuSe. Spin-polarized neutron reflectometry reveals a reduction of the in-plane magnetic susceptibility within a 2 nm interfacial layer of EuSe, and the combination of SQUID magnetometry and Hall measurements points to the formation of an antiferromagnetic layer with at least five-fold enhancement of N\'eel's temperature. Abrupt resistance changes in high magnetic fields indicate interfacial exchange coupling that affects transport in a TSS. High temperature local control of TSS with zero net magnetization unlocks new opportunities for the design of electronic, spintronic and quantum computation devices, ranging from quantization of Hall conductance in zero fields to spatial localization of non-Abelian excitations in superconducting topological qubits

Magnetic proximity-induced energy gap of topological surface states

Topological crystalline insulator surface states can acquire an energy gap when time reversal symmetry is broken by interfacing with a magnetic insulator. Such hybrid topological-magnetic insulator structures can be used to generate novel anomalous Hall effects and to control the magnetic state of the insulator in a spintronic device. In this work, the energy gap of topological surface states in proximity with a magnetic insulator is measured using Landau level spectroscopy. The measurements are carried out on Pb1-xSnxSe/EuSe heterostructures grown by molecular beam epitaxy exhibiting record mobility and a low Fermi energy enabling this measurement. We find an energy gap that does not exceed 20meV and we show that is due to the combined effect of quantum confinement and magnetic proximity. The presence of magnetism at the interface is confirmed by magnetometry and neutron reflectivity. The recovered energy gap sets an upper limit for the Fermi level needed to observe the quantized anomalous Hall effect using magnetic proximity heterostructures

pH-Promoted Exponential Layer-by-Layer Assembly of Bicomponent Polyelectrolyte/Nanoparticle Multilayers

Exponential growth of layer-by-layer (LbL) assembled films is desirable because this method considerably increases the growth rate, resulting in much thicker films in a shorter period of time than is the case with normally linearly grown LbL thin films. For the first time, we demonstrate the exponential LbL (e-LbL) growth of poly(ethyleneimine)/SiO₂ nanoparticles (PEI/SiO₂) bicomponent thin films that consist mostly of SiO₂ nanoparticles (over 90 wt % obtained by thermogravimetric analysis). These results are in contrast to earlier e-LbL studies, where the film thickness was made up mostly of the polyelectrolyte, with a very small percentage coming from the inorganic nanoparticles. Here, we show that the LbL growth of the PEI/SiO₂ system significantly depends on the pH of the PEI and the SiO₂ solutions. The e-LbL growth will only occur when the film is deposited with PEI at a high pH and SiO₂ at a low pH. The exponential growth was characterized using a quartz crystal microbalance, atomic force microscopy and scanning electron microscopy imaging, and neutron reflectometry. It is demonstrated that e-LbL films can grow to thicknesses as large as 2–3 μm within just 10 bilayers. The findings reported in this article emphasize new opportunities for the e-LbL growth of organic/inorganic bicomponent composite thin films that may have applications as electrically conducting films, hydrophobic films, and brick-and-mortar biomimetic films

Voltage-Dependent Profile Structures of a Kv-Channel via Time-Resolved Neutron Interferometry

Available experimental techniques cannot determine high-resolution three-dimensional structures of membrane proteins under a transmembrane voltage. Hence, the mechanism by which voltage-gated cation channels couple conformational changes within the four voltage sensor domains, in response to either depolarizing or polarizing transmembrane voltages, to opening or closing of the pore domain's ion channel remains unresolved. Single-membrane specimens, composed of a phospholipid bilayer containing a vectorially oriented voltage-gated K+ channel protein at high in-plane density tethered to the surface of an inorganic multilayer substrate, were developed to allow the application of transmembrane voltages in an electrochemical cell. Time-resolved neutron reflectivity experiments, enhanced by interferometry enabled by the multilayer substrate, were employed to provide directly the low-resolution profile structures of the membrane containing the vectorially oriented voltage-gated K+ channel for the activated, open and deactivated, closed states of the channel under depolarizing and hyperpolarizing transmembrane voltages applied cyclically. The profile structures of these single membranes were dominated by the voltage-gated K+ channel protein because of the high in-plane density. Importantly, the use of neutrons allowed the determination of the voltage-dependent changes in both the profile structure of the membrane and the distribution of water within the profile structure. These two key experimental results were then compared to those predicted by three computational modeling approaches for the activated, open and deactivated, closed states of three different voltage-gated K+ channels in hydrated phospholipid bilayer membrane environments. Of the three modeling approaches investigated, only one state-of-the-art molecular dynamics simulation that directly predicted the response of a voltage-gated K+ channel within a phospholipid bilayer membrane to applied transmembrane voltages by utilizing very long trajectories was found to be in agreement with the two key experimental results provided by the time-resolved neutron interferometry experiments

Strain-tunable Berry curvature in quasi-two-dimensional chromium telluride

Abstract Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr2Te3 thin films manifests a unique temperature-dependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr2Te3 epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr2Te3 near the coercive field during the magnetization switching process, owing to the presence of strain-modulated magnetic layers/domains. The versatile interface tunability of Berry curvature in Cr2Te3 thin films offers new opportunities for topological electronics