Top-down patterning of topological surface and edge states using a focused ion beam.

The conducting boundary states of topological insulators appear at an interface where the characteristic invariant ℤ2 switches from 1 to 0. These states offer prospects for quantum electronics; however, a method is needed to spatially-control ℤ2 to pattern conducting channels. It is shown that modifying Sb2Te3 single-crystal surfaces with an ion beam switches the topological insulator into an amorphous state exhibiting negligible bulk and surface conductivity. This is attributed to a transition from ℤ2 = 1 → ℤ2 = 0 at a threshold disorder strength. This observation is supported by density functional theory and model Hamiltonian calculations. Here we show that this ion-beam treatment allows for inverse lithography to pattern arrays of topological surfaces, edges and corners which are the building blocks of topological electronics.

Small-angle neutron scattering of long-wavelength magnetic modulations in reduced sample dimensions.

Magnetic small-angle neutron scattering (SANS) is ideally suited to providing direct reciprocal-space information on long-wavelength magnetic modulations, such as helicoids, solitons, merons or skyrmions. SANS of such structures in thin films or micro-structured bulk materials is strongly limited by the tiny scattering volume vis a vis the prohibitively high background scattering by the substrate and support structures. Considering near-surface scattering just above the critical angle of reflection, where unwanted signal contributions due to substrate or support structures become very small, it is established that the scattering patterns of the helical, conical, skyrmion lattice and fluctuation-disordered phases in a polished bulk sample of MnSi are equivalent for conventional transmission and near-surface SANS geometries. This motivates the prediction of a complete repository of scattering patterns expected for thin films in the near-surface SANS geometry for each orientation of the magnetic order with respect to the scattering plane.

Network of Topological Nodal Planes, Multifold Degeneracies, and Weyl Points in CoSi.

We showcase the importance of global band topology in a study of the Weyl semimetal CoSi as a representative of chiral space group (SG) 198. We identify a network of band crossings comprising topological nodal planes, multifold degeneracies, and Weyl points consistent with the fermion doubling theorem. To confirm these findings, we combined the general analysis of the band topology of SG 198 with Shubnikov-de Haas oscillations and material-specific calculations of the electronic structure and Berry curvature. The observation of two nearly dispersionless Shubnikov-de Haas frequency branches provides unambiguous evidence of four Fermi surface sheets at the R point that reflect the symmetry-enforced orthogonality of the underlying wave functions at the intersections with the nodal planes. Hence, irrespective of the spin-orbit coupling strength, SG 198 features always six- and fourfold degenerate crossings at R and Γ that are intimately connected to the topological charges distributed across the network.

In Operando Study of the Hydrogen-Induced Switching of Magnetic Anisotropy at the Co/Pd Interface for Magnetic Hydrogen Gas Sensing.

Heterostructures exhibiting perpendicular magnetic anisotropy (PMA) have traditionally served the magnetic recording industry. However, an opportunity exists to expand the applications of PMA heterostructures into the realm of hydrogen sensing using ferromagnetic resonance (FMR) by exploiting the hydrogen-induced modifications to PMA that occur at the interface between Pd and a ferromagnet. Here, we present the first in operando depth-resolved study of the in-plane interfacial magnetization of a Co/Pd film which features tailorable PMA in the presence of hydrogen gas. We combine polarized neutron reflectometry with in situ FMR to explore how the absorption of hydrogen at the Co/Pd interface affects the heterostructures spin-resonance condition during hydrogen cycling. Experimental data and modeling reveal that the Pd layer expands when exposed to hydrogen gas, while the in-plane magnetic moment of the Co/Pd film increases as the interfacial PMA is reduced to affect the FMR frequency. This work highlights a potential route for magnetic hydrogen gas sensing.

Tailoring exchange bias in ferro/antiferromagnetic FePt3 bilayers created by He+ beams.

We report on artificial exchange bias created in a continuous epitaxial FePt3 film by introducing chemical disorder using a He+ beam, which features tailorable exchange bias strength through post-irradiation annealing. By design, the ferromagnetic (FM)/antiferromagnetic (AF) heterostructure exhibits stratified degrees of chemical order; however, the chemical composition and stoichiometry are invariant throughout the film volume. This uniquely allows for a consideration purely of the magnetic exchange across the FM/AF interface without the added hindrance of structural boundary parameters which inherently affect exchange bias quality. Annealing at 840 K results in the strongest exchange biased system, which displays a cross-sectional morphology of fine (<10 nm) domain structure composed of both of chemically ordered and chemically disordered domains. A magnetic model developed from fitting the characteristic polarised neutron reflectometry spectral features reveals that dual interactions can be attributed to the observed exchange bias: magnetic coupling at the FM/AF interface and also between neighbouring FM (chemically disordered) and AF (chemically ordered) domains within the nominally FM layer. Our results indicate that exchange bias is hindered at interfaces which are both chemically and magnetically perfect, while annealing can be used to balance the volume proportions of interfacial FM and AF domains to enhance the magnetic interface roughness for customisable exchange bias in mono-stoichiometric FM/AF heterostructures crafted by ion beams.

Field dependence of the ferromagnetic/superconducting proximity effect in a YBCO/STO/LCMO multilayer.

The interaction between superconductivity and magnetism in spatially confined heterostructures of thin film multilayers is investigated in the ferromagnetic manganite La2/3Ca1/3MnO3 (LCMO) and the high-temperature superconductor YBa2Cu3O7-δ (YBCO) mediated by an intermediate insulating SrTiO3 (STO) layer. The STO layer is used to mediate and tune the range of interactions between the ferromagnet and superconductor. A magnetically depleted layer with zero-magnetisation within the LCMO layer is shown by polarised neutron reflectometry measurements. This zero-magnetisation layer is caused by the onset of superconductivity in YBCO despite being separated by an insulating layer with a thickness much larger than the superconducting coherence length. The magnetic field dependence of this interaction is also explored. We show that the magnetism of the depleted layer can be restored by applying a magnetic field that partially destroys the superconductivity in YBCO, restricting the electronic interaction between the materials.

Direct Measurement of the Intrinsic Sharpness of Magnetic Interfaces Formed by Chemical Disorder Using a He+ Beam.

Using ion beams to locally modify material properties and subsequently drive magnetic phase transitions is rapidly gaining momentum as the technique of choice for the fabrication of magnetic nanoelements. This is because the method provides the capability to engineer in three dimensions on the nanometer length scale. This will be an important consideration for several emerging magnetic technologies (e.g., spintronic devices and racetrack and random-access memories) where device functionality will hinge on the spatial definition of the incorporated magnetic nanoelements. In this work, the fundamental sharpness of a magnetic interface formed by nanomachining FePt3 films using He+ irradiation is investigated. Through careful selection of the irradiating ion energy and fluence, room-temperature ferromagnetism is locally induced into a fractional volume of a paramagnetic (PM) FePt3 film by modifying the chemical order parameter. A combination of transmission electron microscopy, magnetometry, and polarized neutron reflectometry measurements demonstrates that the interface over which the PM-to-ferromagnetic modulation occurs in this model system is confined to a few atomic monolayers only, while the structural boundary transition is less well-defined. Using complementary density functional theory, the mechanism for the ion-beam-induced magnetic transition is elucidated and shown to be caused by an intermixing of Fe and Pt atoms in antisite defects above a threshold density.