I also investigate microstructure and magnetic properties. The chemical composition and layered information of the sensor are investigated using X-ray reflectivity and X-ray diffraction analyses and these show distinct face-centered-cubic fcc -Au phases, as dominated by the higher density of conduction electrons in Au as compared to Co. The sensor showed a maximum sensitivity of 5. New functionality can be achieved from the combined roles of generating surface plasmon oscillations in the artificially tailored MO structures when excited by a TM polarized p-polarized optical radiation that is further controlled by external magnetic, H fields [ 4 ].
The choice of Au in this configuration is due to its excellent plasmonic properties arising from the high electron density of about 5. All these features are essential in reducing attenuation and enhancing plasmon activity. Due to the high magnetic moment, it offers the possibility of having the strong magnetic modulation of permittivity at room temperature when excited by an optical radiation near infra-red.
By combining plasmonic Au and ferromagnetic Co into Ti buffer and a protective layer, P c and by optimizing the multilayer configuration, the influence of surface plasmons and magnetism on sensitivity and device performance can be exploited [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. As reported by us earlier [ 18 ], the references cited therein, as well as by many others [ 7 , 19 ], several modulation approaches have been explored to enhance the sensitivity of the SPR-based sensors.
Despite the higher sensitivity and improved performances shown by the MO-based SPR sensors, several technical challenges still prevail such as oxidation of sensor surface leading to degradation of performance, issues with reproducibility of the sensor surface due to the difficulty of removing adsorbed materials, and scratching of sensor surface during cleaning.
In addition to the composition and layer thickness, interface roughness between each layer of the sensor configuration also plays critical roles in defining MO effect and sensitivity of the sensor [ 7 ]. I analyze the MO effect using transfer matrix method similar to what has been described in my prior work [ 20 ]. Both variations in optical excitation wavelength and probed medium are also taken into consideration in the analysis.
Ultrathin Magnetic Structures II: Measurement Techniques and Novel Magnetic Properties (Pt. 2)
Furthermore, I have studied the effect of the protective layer, t Pc nanosized polycarbonate plastic of permittivity of 2. Three types of alcohol samples, namely, ethanol, propanol, and pentanol with increasing molecular weight and refractive index are used as probing samples.
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The study shows that the protective layer does not compromise the MO enhancement and sensitivity. The proposed sensor configuration is an excellent candidate for developing robust practical biosensors. High angle X-ray diffraction XRD or low angle X-ray reflection XRR analyses are widely used to characterize many unknown nanostructured materials in the field of geology, environmental science, material science, engineering, and biology, to name a few [ 21 ].
While XRD Rigaku SmartLab, Tokyo, Japan is primarily used to determine crystal structure, the lattice mismatch between the substrate and individual layers due to stress or strain, dislocation density, and quality of the nanostructure multilayer films, XRR Rigaku SmartLab, Tokyo, Japan is used to determine the layer or bilayer thickness, surface or interface roughness, and density of the film.
These studies help us to understand better light—matter interaction and relationship between the crystallographic plane and magneto-optics properties that are essential for biosensing. The profile shows a peak at an average interplanar distance of Co and Au. The high-intensity diffraction peaks indicate the presence of Au , Co , Au , and Au planes parallel to the multilayer interface. However thicker layers showed the signature of Au , Co , and Co planes in addition to the Au planes. The profile shows several peaks at an average interplanar distance of Co and Au and is controlled by the refractive index of Ti, Au, and Co layers.
The refractive index is related to the atomic density and scattering power of individual elements, in this case, again, Ti, Au, and Co. Note that the roughness caused by atomic level variation is termed as a continuous roughness and can vary continuously throughout the multilayer. It can arise from the lattice mismatch at the interface between two layers, dislocation, layer thickness variation, etc. Whereas, discrete roughness is associated with layer thickness consisting of an integer number of atomic layers and it usually results from nonuniform growth modes.
The detailed discussion on roughness is beyond the scope of this paper. Interested readers are referred to past papers [ 24 , 25 ]. This can also arise from the buffer layer and substrates. The beating seen in the measured XRR profiles in Figure 2 can arise either from the oxidation of the buffer layer or surface roughness of both buffer layer and substrate. Discrete roughness can also be a cause of the beating. After annealing, the dip in the reflection peaks is further reduced suggesting the improvement of the surface of the buffer layer. For additional information about roughness, I refer interested readers to my prior work [ 20 ].
The effect of the interaction of the magnetic field, H with optical radiation for paramagnetic or diamagnetic matter is very small at normal intensities and can be neglected. However, in the present work, in ferromagnetic multilayers consisting of Co, the interaction of light with the magnetic moment of Co has two effects depending on whether the incident optical radiation is TE-polarized s-polarized or TM-polarized p-polarized with respect to the orientation of magnetic moment.
For the TM-polarized light, a direct relationship exists between the interaction of optical radiation and magnetism M s —saturation magnetization and correspondingly induced dielectric tensor of Co, known as magneto-optic MO coefficient. The MO effect is directly related to the orientation of magnetic moment or the direction of the applied field in the sensor configuration and therefore, magnetic measurement is important to understand the physics of light—matter interaction in a multilayer configuration.
If a TM-polarized p-polarized light is perpendicularly incident on the sensor surface, the light is purely reflected this is further discussed in Section 5 meaning that the orientation of magnetic moment in Co has a strong effect on whether the light gets reflected or rotated. The inset in the right shows an enlarged view of the M-H curves, with a coercive force, H c of 40 Oe.
Both the in-plane and perpendicular-to-plane of the tri-layer surface only in-plane measured M-H curves are shown here for both the as-deposited and annealed samples showed isotropic in-plane easy axis magnetization along the multilayer interface.
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It seems that the interface states between the Au and Co are primarily responsible for this decrease. This information will be very useful in developing a practical biosensor. As shown by the M-H curves, the multilayer configuration shows an easy axis along the multilayer surface. That is, the orientation of magnetic moments in the 8 nm thick Co layer are parallel to the interface between Co and Au layers, this configuration is further discussed in Section 5.
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The protective layer is an inert and transparent plastic polycarbonate, in this case with a permittivity of 2. Figure 4 b—d show normalized sensitivity as the t pc is increased from 5 to 15 nm in the increment of 5 nm. The incident angle at which the maximum sensitivity occurs shifts towards higher angles More importantly, the t pc did not have any adverse effect on the magnitude of sensitivity.
All the calculated parameters obtained from Figure 4 are listed in Table 1.
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SPR sensitivity is not shown here for simplicity. The deposition method is as follows. First, a thin buffer layer of 2 nm of Ti 0. The critical switching fields of both strategies are substantially lower than that of precessional reversal for realistic materials.
Books by Bretislav Heinrich (Author of Ultrathin Magnetic Structures)
Nanoclusters, aggregates of a few tens to millions of atoms or molecules, have been extensively studied over the past decades. Core—shell nanoclusters have received increasing attention because of their tunable physical and chemical properties through controlling chemical composition and relative sizes of core and shell. The magnetic core—shell nanoclusters are of particular interests because these heterogeneous nanostructures offer opportunities for developing devices and cluster-assembled materials with new functions for magnetic recording, bio, and medical applications.
The purpose of this review is to report latest progress in the experimental and theoretical studies of bimetallic magnetic core—shell nanoclusters e. Due to page limit, a concise survey of synthetic techniques and main experimental characterizations for magnetic properties is presented. A more detailed overview is given to previous theoretical work.
The fabrication, structure, and magnetism of a variety of designed nanostructures are reviewed, from self-assembled thin-film structures and magnetic surface alloys to core—shell nanoparticles and clusters embedded in bulk matrices. The integration of clusters and other nanoscale building blocks in complex two- and three-dimensional nanostructures leads to new physics and new applications. Some explicitly discussed examples are interactions of surface-supported or embedded impurities and clusters, the behavior of quantum states in free and embedded clusters, the preasymptotic coupling of transition-metal dots through substrates, inverted hysteresis loops proteresis in core—shell nanoparticles, and nanoscale entanglement of anisotropic magnetic nanodots for future quantum information processing.
Clathrates are materials containing closed polyhedral cages stacked to form crystalline frameworks. With Si, Ge, and Sn atoms populating these frameworks, a wide variety of electronic and vibrational properties can be produced in these materials, by substitution upon framework sites or through incorporation of ions in cage-center positions.
Commonly formed structures include the type I, type II, and chiral clathrate types, whose properties will be described here. The enhanced T c in this compound has been shown to arise predominantly from very sharp features in the electronic densities of states associated with the extended sp 3 -bonded framework. Atomic substitution can tailor these electronic properties; however, the associated disorder has been found to inevitably lower the T c due to the disrupted continuity of the framework.
Efforts to produce analogous Ge-based superconductors have not been successful, due to the appearance of spontaneous vacancies, which also serve to disrupt the frameworks. The formation of these vacancies is driven by the Zintl mechanism, which plays a much more significant role for the structural stability of the Ge clathrates. The sharp density of states features in these extended framework materials may also lead to enhanced magnetic features, due to conduction electron-mediated coupling of substituted magnetic ions.
This has led to magnetic ordering in Fe- and Mn-substituted clathrates. Neutron scattering is a comprehensive tool for condensed matter research. After a brief description of the interaction of neutrons with matter, the usefulness of neutrons to probe the physical properties of magnetic materials is illustrated using examples taken from different research areas. Then a description of the crystal structure investigation, including in situ and time-resolved studies is given.
The use of polarized or unpolarized neutrons to study magnetic structures or magnetic phase transition is also illustrated. The potential of techniques such as small-angle neutron scattering or neutron scattering on magnetic surfaces is presented showing that neutron scattering now offers a wide range of useful techniques to probe the structural and magnetic properties of magnetic materials whatever their state: polycrystalline, single crystal, amorphous, bulk, or thin films.
Examples are taken from a wide range of research fields: hard magnetic materials, nanocomposite soft magnets, multilayers, superlattices, geometrically frustrated magnetic materials, etc. The experimental aspects are not covered in detail but relevant references are given throughout the chapter. Extrinsic control mechanisms of the interface magnetization in exchange bias heterostructures are reviewed. Experimental progress in the realization of adjustable exchange bias is discussed with special emphasis on electrically tunable exchange bias fields in magnetic thin film heterostructures.
Current experimental attempts and concepts of electrically controlled exchange bias exploit magnetic bilayer structures where a ferromagnetic top electrode is in close proximity of magnetoelectric antiferromagnets, multiferroic pinning layers, or piezoelectric thin films. Various experimental approaches are introduced and the potential use of electrically controlled exchange bias in spintronic applications is briefly outlined.