Table of contents

The observation of magnetic domains by magneto-optical microscopy, based on the Kerr and the Faraday effect, is one of the most prominent techniques for the visualization of distributions of magnetization within magnetic materials. The method has gained increased attention due to the possibility to visualize field and current induced phenomena in nanostructured magnetic materials on fast time-scales. Fundamental concepts and recent advances in methodology are discussed in order to provide guidance on the usage of wide-field magneto-optical microscopy in applied magnetism. Recent applications of magneto-optical microscopy in bulk and thin film materials are reviewed at the end.

The crystal structure and magnetic properties of AlFe_2−xMn_xB₂ (x = 0–0.5) compounds were investigated. With increasing Mn content, the magnetic properties of these compounds evolve gradually and the lattice parameters change continuously: Curie temperature (T_c) decreases from 312 K (x = 0) to 220 K (x = 0.5) and a new phase transition from the ferromagnetic state (FM) to spin-glass state (SPG) appears upon cooling. The lattice shrinks in the ab plane while it expands in the c direction. The formation of re-entrant SPG around 50 K was studied via temperature dependent ac and dc magnetization as well as the initial magnetization curves in 5 K. The reason for this glass state is the triangular configuration of magnetic atoms and the antiferromagnetic interactions introduced by Mn substitutions. The Curie transition leads to a conventional magnetocaloric effect (MCE). The Mn doping leads to a decrease in the MCE near room temperature, but there is a plane in entropy change (ΔS) for the x = 0.1 compound under low field which makes these compounds good candidates to produce composites used for magnetic refrigeration application in a wide temperature span.

The combination of cobalt with nanocarbons promises hybrid nanostructures that are ideal for the development of memory storage and spin-transfer electronics. Here, we report a dramatic effect of composition on the magnetic properties of the Co_xC₆₀ mixtures, whose nanostructure was organized upon simultaneous deposition and sequential exposure to air. We assert a critical change in the mixture's organization yielding either the composite nanostructure as array of the Co/CoO core–shell nanoparticles (NPs) in the C₆₀-based matrix at a high content of Co (a supersaturated mixture or SSM) or a coexistence of fcc-C₆₀ and Co_aC₆₀ fulleride when the Co content x is lower than some critical value ${{x}_{\text{c}}}$ (an ultradilute mixture or UDM). Magnetization of the SSM composite exhibits a superparamagnetic effect caused by the small Co/CoO NPs. Similar magnetization of the UDM with $x=0.7$ revealed a stable ferromagnetism and evidenced the formation of a magnetic Co₂C₆₀ fulleride. Phase composition in the UDM and SSM films was verified with the XRD and Raman spectra. The UDM and SSM films reveal great difference in content of the remaining oxygen which implies easy diffusion of O₂ molecules within the C₆₀-based phases and their splitting at the Co NP surface followed by formation of CoO shells. The results obtained indicate controlled access to a variety of promising Co–C₆₀ magnetic nanostructures.

Magnetization reversal of perpendicular anisotropy [Co/Au]$_{\text{N}}$ multilayers patterned into micrometer wide stripes with a coercivity gradient along the stripes was investigated with polar magnetooptical Kerr effect microscopy. 1 mm-long stripes were bombarded with He⁺ ions of 10 keV energy to induce the gradient. It was shown that short pulses of a magnetic field applied perpendicularly to the sample plane move domain walls between up- and down-magnetized areas in a direction of regions with higher coercivities.

Combining the features of graphene, spintronics and superlattices, this study reports huge tunnel magnetoresistance achieved using graphene-based magnetic tunnel junctions with superlattice barriers. Our calculation results show that the tunnel magnetoresistance of as much as 10⁷% is obtained using a magnetic tunnel junction with a six-cell superlattice barrier. This magnetoresistance increases as the number of cells in the superlattice barrier increases. This remarkable value can be linked to the artificially tailored band structures of the superlattice barriers. This generic nature provides new opportunities for other spintronics applications, using graphene superlattices.

Both frequency swept and field swept ferromagnetic resonance measurements have been carried out for a number of different samples with negligible, moderate and significant extrinsic frequency independent linewidth contribution to analyze the correlation between the experimentally measured frequency and field linewidths. Contrary to the belief commonly held by many researchers, it is found that the frequency and field linewidth conversion relation does not hold for all cases. Instead it holds only for samples with negligible frequency independent linewidth contributions. For samples with non-negligible frequency independent linewidth contribution, the field linewidth values converted from the measured frequency linewidth are larger than the experimentally measured field linewidth. A close examination of the literature reveals that previously reported results support our findings, with successful conversions related to samples with negligible frequency independent linewidth contributions and unsuccessful conversions related to samples with significant frequency independent linewidth. The findings are important in providing guidance in ferromagnetic resonance linewidth conversions.

A pulsed magnetic field magnetic force microscope (PMF-MFM) is developed for evaluation of the magnetic properties of nano-scale materials and devices, as well as the characteristics of MFM tips. We present the setup of the PMF-MFM system, and focus on the evaluation of a FeCo soft magnetic tip by PMF-MFM. We find a new theoretical method to calculate tip magnetization curves (M-H curves) using MFM phase signals. We measure the MFM phase and amplitude signals for the FeCo tip during the presence of the pulsed magnetic fields oriented parallel and antiparallel to the initial tip magnetization direction, and acquire the tip coercivity H_c ~ 1.1 kOe. The tip M-H curves are also calculated using the MFM phase signals data. We obtain the basic features of the tip magnetic properties from the tip M-H curves.

Bandwidth extension and thickness reduction are now the two key issues of cloaks. In this paper, we propose to achieve broadband, thin uni-directional electromagnetic (EM) cloaks using metasurfaces. To this end, a wideband flat focusing lens is firstly devised based on high-efficiency transmissive metasurfaces. Due to the nearly dispersionless parabolic phase profile along the metasurface in the operating band, incident plane waves can be focused efficiently after passing through the metasurface. Broadband unidirectional EM cloaks were then designed by combining two identical flat lenses. Upon illumination, the incident plane waves are firstly focused by one lens and then are restored by the other lens, avoiding the cloaked region. Both simulation and experiment results verify the broadband unidirectional cloak. The broad bandwidth and small thickness of such cloaks have potential applications in achieving invisibility for electrically large objects.

We report that a band-tail emission at 3.08 eV, lower than near-band-edge energy, is observed in photoluminescence measurements of bulk Na-doped CuAlO₂. The band-tail emission is attributed to Na-related defects. Electronic structure calculations based on the first-principles method demonstrate that the donor-acceptor compensated complex of Na_Al-2Na_i in Na-doped CuAlO₂ plays a key role in leading to the band-tail emission and bandgap narrowing. Furthermore, Hall effect measurements indicates that the hole concentration in CuAlO₂ is independent on Na doping, which is well understood by the donor-acceptor compensation effect of Na_Al-2Na_i complex.

Accurate knowledge of the alignment of conduction and valence bands of layers at the heterojunction and warrant knowledge of the band offsets at the interface is essential for Zn_1−xSb_xO/ZnO based quantum well device designing and modeling. Under this scenario, valence band offsets of Zn_1−xSb_xO/ZnO heterostructures grown by the pulsed laser deposition technique was measured by photoelectron spectroscopy and consequently, the conduction band offset was calculated by UV-visible spectroscopy. The change in band alignment has been observed with the dopant (Sb) concentration. Ratios of conduction band offset to valence band offset were estimated to be 1.67 and 0.04 for x = 0.03 and 0.06, respectively, for Sb doped films. A Type-II band alignment was observed at the Zn_0.97Sb_0.03O/ZnO interface, whereas the Type-I band alignment took place at the Zn_0.94Sb_0.06O/ZnO interface.

We report on the performance enhancement of organic field-effect transistors prepared using cross-linked poly(vinyl alcohol) as gate dielectric and copper phthalocyanine as channel semiconductor through gate dielectric surface treatment. The gate dielectric surface was treated using either a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB), or an anionic surfactant, sodium dodecyl sulfate (SDS). We determined the charge-carrier field-effect mobility ( μ_FET) in these transistors as a function of the effective channel thickness in the channel bottleneck, near to the transistor source. When compared to the untreated devices, in the devices treated with CTAB or SDS, the channel formation occurs at lower gate voltage and the carrier mobility in the thinnest channel region, corresponding to the immediate vicinity of the insulator/semiconductor interface, is significantly higher. The surfactant treatment leads to a tenfold increase in μ_FET and significant enhancement in capacitance, on/off current ratio and transconductance of the transistor.

Nanofluids hold promise as a more efficient coolant for thermoelectric devices. Despite the capability of tailoring the thermo physical properties of nanofluids, by tuning the particle parameters such as shape, size and concentration, the toxicity of chemicals used for the preparation of nanoparticles is a serious concern. Green synthesis of nanoparticles is emerging as an alternative to the conventional chemical and physical methods for the preparation of nanoparticles. In this work, the results of the preparation of gold nanoparticles using plant extracts as reducing agents are presented. The green synthesis route employed for the present study provides particles of similar size, but the shape of the particles is found to vary depending upon the source of the natural reducing agents. The thermal diffusivity values of the gold nanofluid measured using laser based dual beam thermal lens technique elucidate the role of shape and concentration of the green synthesized nanoparticles on the effective thermal diffusivity values of the nanofluids.

Spatial resolution of laser micromachining of polymers can be improved with the use of femtosecond laser pulses. Due to the short interaction time, thermal effects are significantly reduced. Additionally, the non-linear character of the interaction of ultrashort laser pulses with transparent materials allows the modification inside their bulk also. However, this creates the challenge to accurately focus the laser beam in the surface when only surface ablation is required. Thus, this work presents a study of the laser ablation of a transparent polymer at different pulse energies and focusing positions controlled through z-scan transmittance measurements. Experiments were performed using an Yb:KYW laser with 450 fs pulses and 1027 nm wavelength. Morphological analysis of the polymer surface after irradiation was performed using scanning electron microscopy. Similar ablation craters were found for a range of sample positions around the beam waist. However, focused ion beam cross-sections of the craters unveil significant inner modifications under most of the focusing conditions leading to surface ablation. Hence, surface ablation without damaging the bulk material only occurs at critical positions where the beam waist is located slightly outside the sample. In situ monitoring of the sample position can be made through transmittance measurements.

With the progression towards higher aspect ratios and finer topographical dimensions in many micro- and nano-systems, it is of technological importance to be able to conformally deposit thin films onto such structures. Sputtering techniques have been developed to provide such conformal coverage through a combination of coating re-sputtering and ionised physical vapour deposition (IPVD), the latter by use of a secondary plasma source or a pulsed high target power (HiPIMS). This paper reports on the use of an alternate remote plasma sputtering technique in which a high density (>10¹³ cm⁻³) magnetised plasma is used for sputter deposition, and additionally is shown to provide IPVD and a re-sputtering capability. From the substrate I–V characteristics and optical emission spectroscopy (OES) data, it is shown that remote plasma sputtering is an inherently continuous IPVD process (without the need of a secondary discharge). Through the reactive deposition of Al₂O₃ onto complex structures, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) results demonstrate that applying a negative substrate bias during film growth can result in re-sputtering of deposited material and film growth on surfaces obscured from the initial sputter flux. Using 5 : 1 (height : width) aspect ratio trenches, the substrate bias was set to 0,−245 and  −334 V. At 0 V substrate bias, the alumina coating is predominantly deposited on the horizontal surfaces; at  −344 V, it is predominantly deposited onto the side walls and at  −245 V a more uniform layer thickness is obtained over the trench. The process was optimised further by alternating the substrate bias between  −222 and  −267 V, with a 50% residence time at each voltage, yielding a more uniform conformal coverage of the 5 : 1 aspect ratio structures over large areas.

We have performed first-principles density functional theory calculations, incorporated with van der Waals interactions, to study CO₂ adsorption and diffusion in nanoporous solid—OMS-2 (Octahedral Molecular Sieve). We found the charge, type, and mobility of a cation, accommodated in a porous OMS-2 material for structural stability, can affect not only the OMS-2 structural features but also CO₂ sorption performance. This paper targets K⁺, Na⁺, and Ba²⁺ cations. First-principles energetics and electronic structure calculations indicate that Ba²⁺ has the strongest interaction with the OMS-2 porous surface due to valence electrons donation to the OMS-2 and molecular orbital hybridization. However, the Ba-doped OMS-2 has the worst CO₂ uptake capacity. We also found evidence of sorption hysteresis in the K- and Na-doped OMS-2 materials.

The present work deals with the injection and packing of a flexible polymeric rod of length L into a simply connected rectangular domain of area XY. As the injection proceeds, the rod bends over itself and it stores elastic energy in closed loops. In a typical experiment N of these loops can be identified inside the cavity in the jammed state. We have performed an extensive experimental analysis of the total length L(N, X, Y) in the tight packing limit, and have obtained robust power laws relating these variables. Additionally, we have examined a version of this packing problem when the simply connected domain is partially occupied with free discs of fixed size. The experimental results were obtained with 27 types of cavities and obey a single equation of state valid for the tight packing of rods in domains of different topologies. Besides its intrinsic theoretical interest and generality, the problem examined here could be of interest in a number of studies including packing models of DNA and polymers in several complex environments.

The ac response in the dielectric regime of thin films consisting of Pd nanoparticles embedded in a ZrO₂ insulating matrix, fabricated by co-sputtering, was obtained from impedance spectroscopy measurements (11 Hz–2 MHz) in the temperature range 30–290 K. The response was fitted to an equivalent circuit model whose parameters were evaluated assuming that, as a consequence of the bimodal size distribution of the Pd particles, two mechanisms appear. At low frequencies, a first element similar to a parallel RC circuit dominates the response, due to two competing paths. One of them is associated with thermally-activated tunneling conductance among most of the smallest Pd particles (size ~ 2 nm), which make up the dc tunneling backbone of the sample. The other one is related to the conductance associated with the capacitive paths among larger Pd particles (size  >  5 nm). At low temperature and intermediate frequencies (~1 kHz), a shortcut process between the larger particles connects regions initially isolated from the backbone at low frequencies. These regions, populated by some additional smaller particles located around two bigger particles, were isolated because the bigger particles separation is too large for the tunneling current. Once connected to the backbone, current may also flow through them by means of the so-called thermally-activated assisted tunneling resistive paths, yielding the second element of the equivalent circuit (a parallel RLC element). At high temperature, the thermal energy shifts the onset of the shortcut process high frequencies and, thus, only the first element is observed. Considering these results, controlling the particle size distribution could be helpful to tune up the frequency at which tunneling conductance dominates the ac response of these granular metals.

Ethanol is detected by graphene sheet, and it is found that the resistance change is proportional to the ethanol concentration. The dynamic response of the evaporation is traced, showing longer recovery time for higher ethanol concentrations. The influence of the residual adsorbed molecules on the graphene sheet was also studied, and it was found that the resistance change is negatively proportional to the initial resistance of the graphene sheet corresponding to the effective surface area. The influence of the structure defects on the response resistance is also studied, and it was found that the structure defects enhance the resistance change. These investigations suggest a better way to extract a reliable signal for physical adsorption based material detection.

Epitaxial NbO₂ (1 1 0) films, 20 nm thick, were grown by pulsed laser deposition on Al₂O₃ (0 0 0 1) substrates. The Ar/O₂ total pressure during growth was varied to demonstrate the gradual transformation between NbO₂ and Nb₂O₅ phases, which was verified using x-ray diffraction, x-ray photoelectron spectroscopy, and optical absorption measurements. Electric resistance threshold switching characteristics were studied in a lateral geometry using interdigitated Pt top electrodes in order to preserve the epitaxial crystalline quality of the films. Volatile and reversible transitions between high and low resistance states were observed in epitaxial NbO₂ films, while irreversible transitions were found in the case of Nb₂O₅ phase. Electric field pulsed current measurements confirmed thermally-induced threshold switching.

The achievement of high sensitivity and highly integrated transducers is one of the main challenges in the development of high-throughput biosensors. The aim of this study is to improve the final sensitivity of an opto-mechanical device to be used as a reliable biosensor. We report the analysis of the mechanical and optical properties of optical waveguide microcantilever transducers, and their dependency on device design and dimensions. The selected layout (geometry) based on two butt-coupled misaligned waveguides displays better sensitivities than an aligned one. With this configuration, we find that an optimal microcantilever thickness range between 150 nm and 400 nm would increase both microcantilever bending during the biorecognition process and increase optical sensitivity to 4.8   ×   10⁻² nm⁻¹, an order of magnitude higher than other similar opto-mechanical devices. Moreover, the analysis shows that a single mode behaviour of the propagating radiation is required to avoid modal interference that could misinterpret the readout signal.