Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin-orbit interaction and the spin-momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.

The structure, magnetic, and transport properties of thin films of the Heusler ferrimagnet Mn2CoAl have been investigated for properties related to spin gapless semiconductors. Oriented films were grown by molecular beam epitaxy on GaAs substrates and the structure was found to transform from tetragonal to cubic for increasing annealing temperature. The anomalous Hall resistivity is found to be proportional to the square of the longitudinal resistivity and magnetization expected for a topological Berry curvature origin. A delicate balance of the spin-polarized carrier type when coupled with voltage gate-tuning could significantly impact advanced electronic devices.

Magnetic exchange driven proximity effect at a magnetic-insulator-topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. Here we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS=Sb 2−x V x Te 3 hybrid heterostructure, where V doping is used to drive the TI (Sb 2 Te 3 ) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering. The time-reversal symmetry (TRS) breaking and surface band gap opening of a topological insulator (TI) are essential ingredients necessary for towards the observation of novel quantum phases and realization for TI-based devices [1,2]. In general, there are two approaches to break the TRS: transitional-metal (TM) ion doping [3][4][5] and magnetic proximity effect where a magnetic insulator (MI) adlayer induces exchange coupling [3,[6][7][8]. Doping TM impurities into a TI will introduce a perpendicular ferromagnetic (FM) anisotropy and provide a straightforward means to open up the band gap of a TI's surface state, with profound influence to its electronic structure [4,[9][10][11][12][13][14]. In particular, quantum anomalous Hall effect (QAHE), where quantum Hall plateau and dissipationless chiral edge channels emerge at zero external magnetic field, has recently been realized in Cr-doped and V-doped TIs [9,10,[15][16][17][18][19][20]. Ideally, compared to the doping method, proximity effect has a number of advantages, including spatially uniform magnetization, better controllability of surface state, freedom from dopant-induced scattering, as well as preserving TI intrinsic crystalline structure, etc. [21,22]. However, due to the inplane anisotropy and low Curie temperature, such MIs are usually too weak to induce strong proximity magnetism in a TI. In fact, compared to a magnetically doped TI which can induce as large as a 50 meV surface band gap [4], the EuS-TI system has only a 7 meV gap opening due to the strongly localized Eu f orbitals [23]. Therefore, the enhancement of proximity magnetism is highly desirable to make it a valuable approach as doping hence takes full advantage.In this Letter, we report significant enhancement of the proximity effect in MI EuS/magnetic-TI Sb 2−x V x Te 3 hybrid heterostructure. Using polarized neutron reflectometry (PNR), we inferred an increase of proximity magnetization per unit cell (u.c.) in TI, from 1.2μ B =u:c. to 2.7μ B =u:c. at x ¼ 0.1 doping level. High-resolution transmission electron microscopy (HRTEM) identifies the TI-EuS interfacial sharpness and excludes the false positive magnetism signal from interdiffused Eu ions into a TI. Furthermore, the proximity effect enhancement is accompanied by a decrease of the interfacial magnetization of EuS, resulting in an exotic antiferro...

While antiferromagnets have been proposed as components to limit stray magnetic fields, their inability to be spin polarized inhibits their use in spintronic devices. Compensated ferrimagnets are a unique solution to this dilemma since they have zero net moment, but their nonsymmetric density of states allows achievement of full spin polarization. Density functional theory predicts Mn3Al in the D03 structure to be fully compensated and retain half-metallicity at room temperature. In this work, 50 nm Mn3Al thin films were synthesized using molecular beam epitaxy and annealed at various temperatures in order to investigate their magnetic properties. Magnetometry measurements confirmed the high Curie temperature of 605 K. Polarized neutron reflectometry (PNR) indicated a low net magnetic moment, along with depth profiles of the structure and magnetization. From the PNR data, we extract a saturation moment of 0.11 ± 0.04 µB/f.u., confirming the nominal zero moment present in these thin films.