Network of topological nodal planes, multifold degeneracies, and Weyl points in CoSi

We report the identification of symmetry-enforced nodal planes (NPs) in CoSi providing the missing topological charges in an entire network of band-crossings comprising in addition multifold degeneracies and Weyl points, such that the fermion doubling theorem is satisfied. In our study we have combined measurements of Shubnikov-de Haas (SdH) oscillations in CoSi with material-specific calculations of the electronic structure and Berry curvature, as well as a general analysis of the band topology of space group (SG) 198. The observation of two nearly dispersionless SdH 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 NPs. Hence, irrespective of the spin-orbit coupling strength, SG198 features always six- and fourfold degenerate crossings at R and $\Gamma$ that are intimately connected to the topological charges distributed across the network

Functional Magnetic Interface Phenomena in Nano-Architectures

The work embodied in this thesis aims to investigate the occurrence of magnetic interface phenomena in low-dimensional thin-film systems which have conceivable utility in future condensed-matter technologies. Namely, the magnetic interface quality of an FePt3 nano-magnet formed via ion-induced chemical disorder will be critically analysed, in addition to a Co/Pd bilayer which features modifiable magnetic surface anisotropy upon exposure to hydrogen gas. The studies are enabled chiefly through advanced X-ray and neutron scattering techniques specifically chosen to probe interface structure as well as chemical and magnetic orders, and supplemented by traditional lab-based characterisation tools. To begin, a much-anticipated experimental confirmation of the intrinsic sharpness of magnetic interfaces formed by locally driving magnetic phase transitions in materials using ion beams is presented. This is achieved through a unique experimental design whereby a room-temperature ferromagnetic nano-layer is encoded with depth-control onto a paramagnetic FePt3 film by inducing chemical disorder using energy-specific He+ ions. The magnetic transition is investigated through theoretical modelling, whereby the first density functional theory results for the entire suite of potential long-range magnetically ordered states of FePt3 are presented. In doing so, the energetically favourable ground-state spin structure is identified. By analysing several localised defect structures which may form in FePt3 under ion irradiation, the fundamental mechanism of the disorder-driven magnetic transition is revealed and shown to be caused by an intermixing of Fe and Pt atoms in anti-site defects above a threshold density. In a second study, hydrogen-induced modifications to the layer-averaged static magnetisation and macroscopic magneto-dynamic behaviours of a Co/Pd heterostructure are investigated. The modifications are observed and examined in detail through simultaneously probing the magnetic anisotropy energy and studying the changing chemical and magnetic depth-profiles across the entire bilayer during primary hydrogengas absorption. It is revealed that the in-plane interfacial magnetisation of the Co/Pd bilayer irreversibly increases after primary hydrogen-gas absorption, indicating a weakening of the perpendicular magnetic anisotropy energy. To aid in conducting this analysis, an original experimental method is first developed which innovatively combines neutron scattering and microwave spectroscopy; equipment is then commissioned, and feasibility studies are performed

Two-Dimensional Magnets: Forgotten History and Recent Progress towards Spintronic Applications

The recent discovery of 2D magnetic order in van der Waals materials has stimulated a renaissance in the field of atomically thin magnets. This has led to promising demonstrations of spintronic functionality such as tunneling magnetoresistance. The frantic pace of this emerging research, however, has also led to some confusion surrounding the underlying phenomena of phase transitions in 2D magnets. In fact, there is a rich history of experimental precedents beginning in the 1960s with quasi-2D bulk magnets and progressing to the 1980s using atomically thin sheets of elemental metals. This review provides a holistic discussion of the current state of knowledge on the three distinct families of low-dimensional magnets: quasi-2D, ultrathin films, and van der Waals crystals. It highlights the unique opportunities presented by the latest implementation in van der Waals materials. By revisiting the fundamental insights from the field of low-dimensional magnetism, this review highlights factors that can be used to enhance material performance. For example, the limits imposed on the critical temperature by the Mermin-Wagner theorem can be escaped in three separate ways: magnetocrystalline anisotropy, long-range interactions, and shape anisotropy. Several recent experimental reports of atomically thin magnets with Curie temperatures above room temperature are highlighted

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

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.Near‐surface SANS is discussed for its potential as a probe of long‐wavelength magnetic modulations in specimens with reduced sample dimensions

Direct Measurement of the Intrinsic Sharpness of Magnetic Interfaces Formed by Chemical Disorder Using a He\u3csup\u3e+\u3c/sup\u3eBeam

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 FePt 3 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) FePt 3 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

Controlling the magnetic reversal mechanism of exchange biased MnxOy/Ni80Fe20 bilayers through O+ implantation

This work investigates the different magnetic reversal mechanisms of as-grown and oxygen-ion (O+) implanted MnxOy/Ni80Fe20 bilayers. A MnxOy/Ni80Fe20 bilayer was prepared by ion-beam sputter deposition in an Ar+ atmosphere using a partially oxidised Mn target. Oxygen implantations were then performed using 8 keV O+ ions at fluences of 1016, 1017 and 1018 ions/cm2 to modify the exchange bias strength at the MnxOy/Ni80Fe20 interface. The magnetic, crystallographic and chemical properties of the bilayers before and after O+ implantation were studied using transmission electron microscopy, X-ray reflectometry, magnetometry and polarised neutron reflectometry. The results show an overall improved exchange bias and coercivity for the O+ implanted bilayers. The 1017 ions/cm2 implanted sample shows the greatest improvement in exchange bias and was further studied for its detailed spin-reversal behaviour using polarised neutron reflectometry. Data analysis in the magnetically-trained state reveals a coexistence of coherent rotational and non-coherent magnetic spin reversal in the as-grown sample and a solely coherent spin rotation reversal mechanism for the O+ implanted MnxOy/Ni80Fe20 bilayer

The magnetic interfacial properties of an exchange biased nanocrystalline Ni80Fe20/α-Fe2O3 bilayer studied by polarized neutron reflectometry and Monte Carlo simulation

2019 The Japan Society of Applied Physics. The strength of exchange bias can be influenced by interface roughness and antiferromagnetic morphology. Here, we studied the interface profile of an exchange biased, nanocrystalline Ni80Fe20/α-Fe2O3 bilayer. Magnetometry determined the bilayer\u27s exchange bias is observed below a blocking temperature of 75 K. Polarized neutron reflectometry measurements revealed the Ni80Fe20 layer was fully saturated to yield a net-moment of 0.95 μ B/atom, while the majority of the Fe2O3 layer exhibited zero net-magnetization with the exception of the interfacial region with an uncompensated moment between 0.5 and 1.0 μ B/Fe2O3. Monte Carlo simulations of a ferromagnetic/antiferromagnetic bilayer incorporating a granular antiferromagnet indicate that an extrinsic uncompensated moment of ∼1.0 μ B/Fe2O3 can arise from grain boundary disorder. The size of the modeled moment is equivalent to the experimental value, and comparable with previous calculations. Furthermore, unlike intrinsic uncompensated spins, it is found that the disorder-induced moment in the granular antiferromagnet is not destroyed by interface roughness

Structural evolution of a Ni/NiOx based supercapacitor in cyclic charging-discharging: A polarized neutron and X-ray reflectometry study

Ni/Ni-oxide based supercapacitors with their excellent stability and long cycle lifetime are favorable due to their cost effectiveness and practicality. And yet, the full picture of their cyclic charging-discharging process is not well understood, and the influential factors on the cycle life of a supercapacitor are complicated. Using a combined polarized neutron and X-ray reflectometry approach, we have studied the structural evolution of a layered Ni/NiOxsupercapacitor electrode, operated in an alkaline electrolyte for cyclic charging-discharging. For the lower thousands of cycles, oxidation of Ni current-collecting backbone and dissolution of outer Ni oxide electroactive materials contribute to a total thickness consumption of 2 nm. Upon higher thousands of cycles, the two-dimensional NiOxsurface-layer evolves into a three-dimensional porous network as a result of the hole drilling depletion behavior of Ni oxide, i.e., dissolution of the more porous part and preservation of the more compact part in the NiOxlayer. The extra surface area of the Ni/NiOxsupercapacitor generated after cyclic charging-discharging gives rise to an increased capacitance. Evidenced by the increased derived density, the crystal defects of inner-layer NiOxare also eliminated with cycling, probably through Ni atom rearrangement, filling of oxygen vacancies within NiOx, or both

The magnetic interfacial properties of an exchange biased nanocrystalline Ni80Fe20/alpha-Fe2O3 bilayer studied by polarized neutron reflectometry and Monte Carlo simulation

The strength of exchange bias can be influenced by interface roughness and antiferromagnetic morphology. Here, we studied the interface profile of an exchange biased, nanocrystalline Ni80Fe20/α-Fe2O3 bilayer. Magnetometry determined the bilayer's exchange bias is observed below a blocking temperature of 75 K. Polarized neutron reflectometry measurements revealed the Ni80Fe20 layer was fully saturated to yield a net-moment of 0.95 μB/atom, while the majority of the Fe2O3 layer exhibited zero net-magnetization with the exception of the interfacial region with an uncompensated moment between 0.5 and 1.0 μB/Fe2O3. Monte Carlo simulations of a ferromagnetic/antiferromagnetic bilayer incorporating a granular antiferromagnet indicate that an extrinsic uncompensated moment of ∼1.0 μB/Fe2O3 can arise from grain boundary disorder. The size of the modeled moment is equivalent to the experimental value, and comparable with previous calculations. Furthermore, unlike intrinsic uncompensated spins, it is found that the disorder-induced moment in the granular antiferromagnet is not destroyed by interface roughness