The flow of the Berry curvature vector field

The concept of Berry phase and Berry curvature has become ubiquitous in solid state physics as it relates to variety of phenomena, such as topological insulators, polarization, and various Hall effects. It is well known that large Berry curvatures arise from close proximity of hybridizing bands, however, the vectorial nature of the Berry curvature is not utilized in current research. On bulk bcc Fe, we demonstrate the flow of the Berry curvature vector field which features not only monopoles but also higher dimensional structures with its own topological features. They can provide a novel unique view on the electronic structure in all three dimensions. This knowledge is also used to quantify particular contributions to the intrinsic anomalous Hall effect in a simple analytical form.

MnBi2Se4-Based Magnetic Modulated Heterostructures

Thin films of magnetic topological insulators (TIs) are expected to exhibit a quantized anomalous Hall effect when the magnetizations on the top and bottom surfaces are parallel and a quantized topological magnetoelectric effect when the magnetizations have opposite orientations. Progress in the observation of these quantum effects was achieved earlier in the films with modulated magnetic doping. On the other hand, the molecular-beam-epitaxy technique allowing the growth of stoichiometric magnetic van der Waals blocks in combination with blocks of topological insulator was developed. This approach should allow the construction of modulated heterostructures with the desired architecture. In the present paper, based on the first-principles calculations, we study the electronic structure of symmetric thin film heterostructures composed of magnetic MnBi2Se4 blocks (septuple layers, SLs) and blocks of Bi2Se3 TI (quintuple layers, QLs) in dependence on the depth of the magnetic SLs relative to the film surface and the TI spacer between them. Among considered heterostructures we have revealed those characterized by nontrivial band topology.

Improvement of the detectivity in an Fe-Sn magnetic-field sensor with a large current injection

A ferromagnetic nanocrystalline Fe-Sn is an excellent platform for magnetic-field sensor based on anomalous Hall effect (AHE) owing to simple fabrication and superior thermal stability. For improvement of the magnetic-field sensitivity, doping impurity and increasing injection current are effective approaches. However, in the light of magnetic-field detectivity, the large current may increase the voltage noise. In this study, a maximum allowable current of was improved by employing the overlayer electrode configuration on a Ta-doped Fe-Sn AHE sensor. In noise measurements, the 1/f noise becomes significant with increasing the current at low frequency, resulting in saturation of the detectivity to 240 nTHz-1/2 at 120 Hz. At high frequency, the detectivity reaches 48 nTHz-1/2 at 3.1 mA showing ten times improvement of the detectivity compared with the non-doped Fe-Sn AHE sensor. Material design and device structure optimization will accelerate further improvement of the sensing properties of the Fe-Sn-based AHE sensor.

The positive exchange bias property with hopping switching behavior in van der Waals magnet FeGeTe

The magnetic exchange bias effect is one of the representative interlayer magnetic coupling phenomena and is widely utilized in numerous technological applications. However, its mechanism is still elusive even in a simple magnetic bilayered system because of the complex interface magnetic orders. Van der Waals layered magnetic materials may provide an essential platform for deeply understanding the detailed mechanism of the exchange bias owing to its ideal interface structure. Here we first observed the positive exchange-biased anomalous Hall effect (AHE) with a hopping switching behavior in the FeGeTe Van der Waals nano-flakes. After systemically studying the cooling field dependence properties of the exchange bias effect, we propose that the coexistence of stable and frustrated surface magnetization of the antiferromagnetic phase will modify the total interface coupling energy density between the ferromagnetic (FM) and antiferromagnetic (AFM) phases. This model could provide a consistent description for such unusual exchange bias effect based on microspin simulation.

Transport Properties of Rare Earth Nitrides

<p>In this thesis we investigate the transport properties of SmN, NdN and GdN, members of the rare earth nitride series of intrinsic ferromagnetic semiconductors. GdN is the central member of the series with seven occupied majority spin 4f states and seven empty minority spin 4f states. Both the filled and unfilled 4f states are some few eV away from the conduction and valence band extrema, resulting in transport properties which are dominated by the extended Gd 5d band. The half filled 4f shell, with zero net orbital angular momentum, furthermore simplifies calculations and as such GdN is the most studied both experimentally and in theory. As one moves to lighter members, the filled 4f states become unfilled states in the conduction band and the 4f shell now has a net orbital angular momentum. Calculations concerning these members are now significantly more complicated, and as such there exists a wide range of predictions concerning the conduction band minima in the lighter rare earth nitrides. To inform the current theoretical and experimental literature we report on three studies concerning the transport properties of SmN, NdN and GdN. To begin we report on the anomalous Hall effect in SmN, NdN and GdN. Under the symmetry of the rock-salt rare earth nitrides the magnitude of the anomalous Hall effect can imply the wave function of the conduction electron (i.e. d or f band). Measurements of the anomalous Hall effect in moderately doped samples are used to show the conduction channel in SmN and NdN is an f band or hybridised f/d band. Furthermore the sign of the anomalous Hall effect can be used to determine the orientation of the spin magnetic moment of the conduction electrons. Optical measurements of SmN, NdN and GdN films are then reported. Optical measurements provide a probe of the band structure of a material via direct transitions between the valence and conduction bands. Measurements of reflectivity and transmission on undoped SmN and NdN films were used to locate the unfilled majority spin 4f bands which form the conduction band minima in each material. Finally a preliminary study of heavily doped SmN, NdN and GdN is discussed. Structural measurements show a reduced lattice parameter while transport results find a significantly enhanced conductivity in heavily doped films. The Curie temperature is found to be enhanced and optical measurements show an increased absorption and red-shifted optical edge in doped films. The superconducting state of SmN is discussed and it is shown only to be present in moderately doped films, i.e. superconductivity is not present in undoped or degenerately doped SmN, within our measurement limits.</p>