Tuning Ferromagnetism in a Single Layer of Fe above Room Temperature

The crystallographic and magnetic properties of an Fe monolayer (ML) grown on 2 ML Au/W(110) substrate are studied with spin-polarized low-energy electron microscopy, density functional theory, and relativistic screened Korringa–Kohn–Rostoker calculations. The single layer of iron atoms possesses hexagonal symmetry and reveals a ferromagnetic order at room temperature. We experimentally demonstrate the possibility of tuning the Curie temperature and the magnitude of magnetization of the Fe monolayer by capping with Au. Taking into account several structural models, the calculation results mostly show ferromagnetic states with enhanced magnetic moments of Fe atoms compared to their bulk value and a further increase in their value after covering with Au. The theoretically calculated Curie temperatures are in fair agreement with those obtained in the experiments. The calculations, furthermore, found evidence for the presence of frustrated isotropic Fe–Fe exchange interactions, and a discussion of the structural effects on the magnetic properties is provided herein.     

The Al Doping Effect on Epitaxial (In,Mn)As Dilute Magnetic Semiconductors Prepared by Ion Implantation and Pulsed Laser Melting

One of the most attractive characteristics of diluted ferromagnetic semiconductors is the possibility to modulate their electronic and ferromagnetic properties, coupled by itinerant holes through various means. A prominent example is the modification of Curie temperature and magnetic anisotropy by ion implantation and pulsed laser melting in III–V diluted magnetic semiconductors. In this study, to the best of our knowledge, we performed, for the first time, the co-doping of (In,Mn)As diluted magnetic semiconductors by Al by co-implantation subsequently combined with a pulsed laser annealing technique. Additionally, the structural and magnetic properties were systematically investigated by gradually raising the Al implantation fluence. Unexpectedly, under a well-preserved epitaxial structure, all samples presented weaken Curie temperature, magnetization, as well as uniaxial magnetic anisotropies when more aluminum was involved. Such a phenomenon is probably due to enhanced carrier localization introduced by Al or the suppression of substitutional Mn atoms.     

Ferromagnetism and Superconductivity in CaRuO3/YBa2Cu3O7-δ Heterostructures

The deposition of a ferromagnetic layer can affect the properties of high-temperature superconductors underneath. We investigated the influence of ferromagnetic CaRuO₃ on the properties of YBa₂Cu₃O_7-x (YBCO) superconducting thin films when the layers are either in direct contact or separated by a barrier layer of 5 nm SrTiO₃. Detailed measurements of the magnetic moment of the superconductor and ferromagnet as a function of temperature and magnetic field have been performed using SQUID magnetometry. Magnetometry and relaxation measurements show that the modification of the superconducting properties of YBCO strongly depends on the interaction with the ferromagnetic layer on top. The barrier layer has a significant impact on both the supercon-ducting properties of the YBCO film and the ferromagnetic ordering of CaRuO₃. The physical properties mentioned above were discussed in correlation with the materials’ structure determined by XRD analysis.     

Magnetocaloric and Giant Magnetoresistance Effects in La-Ba-Mn-Ti-O Epitaxial Thin Films: Influence of Phase Transition and Magnetic Anisotropy

Magnetic perovskite films have promising properties for use in energy-efficient spintronic devices and magnetic refrigeration. Here, an epitaxial ferromagnetic La_0.67Ba_0.33Mn_0.95Ti_0.05O₃ (LBMTO-5) thin film was grown on SrTiO₃(001) single crystal substrate by pulsed laser deposition. High-resolution X-ray diffraction proved the high crystallinity of the film with tetragonal symmetry. The magnetic, magnetocaloric and magnetoresistance properties at different directions of the applied magnetic field with respect to the ab plane of the film were investigated. An in-plane uni-axial magnetic anisotropy was evidenced. The LBMTO-5 epilayer exhibits a second-order ferromagnetic-paramagnetic phase transition around 234 K together with a metal–semiconductor transition close to this Curie temperature (T_C). The magnetic entropy variation under 5 T induction of a magnetic field applied parallel to the film surface reaches a maximum of 17.27 mJ/cm³ K. The relative cooling power is 1400 mJ/cm³ K (53% of the reference value reported for bulk Gd) for the same applied magnetic field. Giant magnetoresistance of about 82% under 5 T is obtained at a temperature close to T_C. Defined as the difference between specific resistivity obtained under 5 T with the current flowing along the magnetic easy axis and the magnetic field oriented transversally to the current, parallel and perpendicular to the sample plane, respectively, the in-plane magneto-resistance anisotropy in 5 T is about 9% near the T_C.     

Tuning of Magnetic Damping in Y3Fe5O12/Metal Bilayers for Spin-Wave Conduit Termination

In this work, we investigate the structural and dynamic magnetic properties of yttrium iron garnet (YIG) films grown onto gadolinium gallium garnet (GGG) substrates with thin platinum, iridium, and gold spacer layers. Separation of the YIG film from the GGG substrate by a metal film strongly affects the crystalline structure of YIG and its magnetic damping. Despite the presence of structural defects, however, the YIG films exhibit a clear ferromagnetic resonance response. The ability to tune the magnetic damping without substantial changes to magnetization offers attractive prospects for the design of complex spin-wave conduits. We show that the insertion of a 1-nm-thick metal layer between YIG and GGG already increases the effective damping parameter enough to efficiently absorb spin waves. This bilayer structure can therefore be utilized for magnonic waveguide termination. Investigating the dispersionless propagation of spin-wave packets, we demonstrate that a damping unit consisting of the YIG/metal bilayers can dissipate incident spin-wave signals with reflection coefficient R < 0.1 at a distance comparable to the spatial width of the wave packet.     

Magnetic Properties of GaAs/NiFe Coaxial Core-Shell Structures

Uniform nanogranular NiFe layers with Ni contents of 65%, 80%, and 100% have been electroplated in the potentiostatic deposition mode on both planar substrates and arrays of nanowires prepared by the anodization of GaAs substrates. The fabricated planar and coaxial core-shell ferromagnetic structures have been investigated by means of scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). To determine the perspectives for applications, a comparative analysis of magnetic properties, in terms of the saturation and remanence moment, the squareness ratio, and the coercivity, was performed for structures with different Ni contents.     

Strong Interfacial Perpendicular Magnetic Anisotropy in Exchange-Biased NiO/Co/Au and NiO/Co/NiO Layered Systems

The ability to induce and control the perpendicular magnetic anisotropy (PMA) of ferromagnetic layers has been widely investigated, especially those that offer additional functionalities (e.g., skyrmion stabilization, voltage-based magnetization switching, rapid propagation of domain walls). Out-of-plane magnetized ferromagnetic layers in direct contact with an oxide belong to this class. Nowadays, investigation of this type of system includes antiferromagnetic oxides (AFOs) because of their potential for new approaches to applied spintronics that exploit the exchange bias (EB) coupling between the ferromagnetic and the AFO layer. Here, we investigate PMA and EB effect in NiO/Co/Au and NiO/Co/NiO layered systems. We show that the coercive and EB fields increase significantly when the Co layer is coupled with two NiO layers, instead of one. Surrounding the Co layer only with NiO layers induces a strong PMA resulting in an out-of-plane magnetized system can be obtained without a heavy metal/ferromagnetic interface. The PMA arises from a significant surface contribution (0.74 mJ/m²) that can be enhanced up to 0.99 mJ/m² by annealing at moderate temperatures (~450 K). Using field cooling processes for both systems, we demonstrate a wide-ranging control of the exchange bias field without perturbing other magnetic properties of importance.     

DFT Insight into Conductive and Magnetic Properties of Heterostructures with BaTiO3 Overlayer

The ab initio calculations of a heterostructure based on the ferroelectric phase of barium titanate and dielectrics lanthanum manganese (LaMnO₃) or silicon (Si) are presented. We analyze structures of BaTiO₃/LaMnO₃ and BaTiO₃/Si interfaces, investigate magnetic properties and the impact of ferroelectric polarization. The use of ferroelectrics in the heterostructure plays a crucial role; in particular, ferroelectric polarization leads to the appearance of the conducting state at the interface and in the layers close to it. We show that defects (here, oxygen vacancies) incorporated into the system may change the electronic and magnetic properties of a system. Experimental results of magnetic susceptibility measurements for the Ba_0.8Sr_0.2TiO₃/LaMnO₃ heterostructure are also presented. It is shown that a correlation between the behavior of the ferromagnetic ordering and the resistance takes place. In addition, the ferromagnetic ordering at the interface of the heterostructure can be associated with the exchange interaction through current carriers that appear in high carrier concentration regions.     

Tuning the Magnetism in Boron-Doped Strontium Titanate

The magnetic and electronic properties of boron-doped SrTiO₃ have been studied by first-principles calculations. We found that the magnetic ground states of B-doped SrTiO₃ strongly depended on the dopant-dopant separation distance. As the dopant–dopant distance varied, the magnetic ground states of B-doped SrTiO₃ can have nonmagnetic, ferromagnetic or antiferromagnetic alignment. The structure with the smallest dopant-dopant separation exhibited the lowest total energy among all configurations considered and was characterized by dimer pairs due to strong attraction. Ferromagnetic coupling was observed to be stronger when the two adjacent B atoms aligned linearly along the B-Ti-B axis, which could be associated with their local bonding structures. Therefore, the symmetry of the local structure made an important contribution to the generation of a magnetic moment. Our study also demonstrated that the O-Ti-O unit was easier than the Ti-B-Ti unit to deform. The electronic properties of boron-doped SrTiO₃ tended to show semiconducting or insulating features when the dopant–dopant distance was less than 5 Å, which changed to metallic properties when the dopant–dopant distance was beyond 5 Å. Our calculated results indicated that it is possible to manipulate the magnetism and band gap via different dopant–dopant separations.     

Electrically Switchable Film Structure of Conjugated Polymer Composites

Domains rich in different blend components phase-separate during deposition, creating a film morphology that determines the performance of active layers in organic electronics. However, morphological control either relies on additional fabrication steps or is limited to a small region where an external interaction is applied. Here, we show that different semiconductor-insulator polymer composites can be rapidly dip-coated with the film structure electrically switched between distinct morphologies during deposition guided by the meniscus formed between the stationary barrier and horizontally drawn solid substrate. Reversible and repeatable changes between the morphologies used in devices, e.g., lateral morphologies and stratified layers of semiconductors and insulators, or between phase-inverted droplet-like structures are manifested only for one polarity of the voltage applied across the meniscus as a rectangular pulse. This phenomenon points to a novel mechanism, related to voltage-induced doping and the doping-dependent solubility of the conjugated polymer, equivalent to an increased semiconductor content that controls the composite morphologies. This is effective only for the positively polarized substrate rather than the barrier, as the former entrains the nearby lower part of the coating solution that forms the final composite film. The mechanism, applied to the pristine semiconductor solution, results in an increased semiconductor deposition and 40-times higher film conductance.     

Auxetic Metamaterials for Biomedical Devices: Current Situation,Main Challenges,and Research Trends

Auxetic metamaterials are characterized by a negative Poisson ratio (NPR) and display an unexpected property of lateral expansion when stretched and densification when compressed. Auxetic properties can be achieved by designing special microstructures, hence their classification as metamaterials, and can be manufactured with varied raw materials and methods. Since work in this field began, auxetics have been considered for different biomedical applications, as some biological tissues have auxetic-like behaviour due to their lightweight structure and morphing properties, which makes auxetics ideal for interacting with the human body. This research study is developed with the aim of presenting an updated overview of auxetic metamaterials for biomedical devices. It stands out for providing a comprehensive view of medical applications for auxetics, including a focus on prosthetics, orthotics, ergonomic appliances, performance enhancement devices, in vitro medical devices for interacting with cells, and advanced medicinal clinical products, especially tissue engineering scaffolds with living cells. Innovative design and simulation approaches for the engineering of auxetic-based products are covered, and the relevant manufacturing technologies for prototyping and producing auxetics are analysed, taking into consideration those capable of processing biomaterials and enabling multi-scale and multi-material auxetics. An engineering design rational for auxetics-based medical devices is presented with integrative purposes. Finally, key research, development and expected technological breakthroughs are discussed.     

Magnetic and Magnetostrictive Properties of Ni50Mn20Ga27Cu3 Rapidly Quenched Ribbons

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K—which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples.     

Optimization of the Growth Process of Double Perovskite Pr2−δNi1−xMn1+xO6−y Epitaxial Thin Films by RF Sputtering

Epitaxial thin films of Pr_2−δNi_1−xMn_1+xO_6−y (PNMO) double perovskite were grown on (001)-oriented SrTiO₃ substrates by RF magnetron sputtering. The influence of the growth parameters (oxygen pressure, substrate temperature, and annealing treatments) on the structural, magnetic and transport properties, and stoichiometry of the films was thoroughly investigated. It is found that high-quality epitaxial, insulating, and ferromagnetic PNMO thin films can only be obtained in a narrow deposition parameter window. It is shown that a careful selection of the growth conditions allows for obtaining a high degree of Ni/Mn cation ordering, which is reflected in the values of the Curie temperature, T_C, and saturation magnetization, M_S, which are very close to those of bulk material.     

Magnetopneumography: a general review

Magnetopneumography is the study of the remanent magnetism of foreign intrathoracic ferromagnetic particles after magnetization by an external magnetic field. Given knowledge of the magnetic characteristics of the inhaled particles, this highly sensitive and non-invasive technique allows the measurement of lung dust loads. Many groups of workers have been examined in this way, e.g. welders, coalminers, asbestos, foundry and steel workers. Magnetopneumography also allows analysis of the distribution of aerocontaminants in the different anatomical structures and, when repeated, the study of clearance speeds and migration from site to site of such particles. Emphasis has been laid on the importance of study of the fading of the remanent magnetic signal as time elapses. This short-term phenomenon, called relaxation, seems highly significant for the study of the dynamic properties of the immediate environment of extra pulmonary particles and especially for the study of macrophage activity.     

Microscopic origins for stabilizing room-temperature ferromagnetism in ultrathin manganite layers

La_0.7Sr_0.3MnO₃ is a conducting ferromagnet at room temperature. Combined with thin SrTiO₃ layers, the resulting heterostructures could be used as highly spin-polarized magnetic-tunnel-junction memories. However, when shrunk to dimensions below an apparent critical thickness, the structures become insulating and ferromagnetic ordering is suppressed. Interface spin and charge modulations are thought to create an interfacial dead layer, thus fundamentally limiting the use of this material in atomic-scale devices. The thickness of this dead layer, and whether it is intrinsic, is still controversial. Here we use atomic-resolution electron spectroscopy to demonstrate that the degradation of the magnetic and transport properties of La_0.7Sr_0.3MnO₃/SrTiO₃ multilayers correlates with atomic intermixing at the interfaces, and the presence of extended two-dimensional cation defects in the La_0.7Sr_0.3MnO₃ layers (in contrast to three-dimensional precipitates in thick films). When these extrinsic defects are eliminated, metallic ferromagnetism at room temperature can be stabilized in five-unit-cell-thick manganite layers in superlattices, placing the upper limit for any intrinsic dead layer at two unit cells per interface.     

Parameter Identification and Validation of Shape-Memory Polymers within the Framework of Finite Strain Viscoelasticity

Shape-Memory Polymers (SMPs) can be stretched to large deformations and recover induced strains when exposed to an appropriate stimulus, such as heat. This emerging class of functional polymers has attracted much interest and found applications in medicine and engineering. Nevertheless, prior to any application, their physical and mechanical properties must be thoroughly studied and understood in order to make predictions or to design structures thereof. In this contribution, the viscoelastic behavior of a polyether-based polyurethane (Estane) and its rate- and temperature-dependent behavior have been studied experimentally and by the mean of simulations. The model-inherent material parameters are identified with the assumption of the thermo-rheological complexity. Here, the numerical results of uni-axial stress relaxations were compared with the associated experiments in conjucation with the Levenberg-Marquard optimization method to determine the parameters of the Prony equation. The ability of the model to simulate the thermo-mechanical properties of Estane was evaluated by data-rich experimental observations on tension and torsion in various temperature ranges. Heterogeneous tests are included into the experimental program to cover a broader spectrum of loading scenarios.     

Structural Quality and Magnetotransport Properties of Epitaxial Layers of the (Ga,Mn)(Bi,As) Dilute Magnetic Semiconductor

Structural analysis of epitaxial layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor (DMS), together with investigations of their magnetotransport properties, has been thoroughly performed. The obtained results are compared with those for the reference (Ga,Mn)As layers, grown under similar conditions, with the aim to reveal an impact of Bi incorporation on the properties of this DMS material. Incorporation of Bi into GaAs strongly enhances the spin-orbit coupling strength in this semiconductor, and the same has been expected for the (Ga,Mn)(Bi,As) alloy. In turn, importantly for specific spintronic applications, strong spin-orbit coupling in ferromagnetic systems opens a possibility of directly controlling the direction of magnetization by the electric current. Our investigations, performed with high-resolution X-ray diffractometry and transmission electron microscopy, demonstrate that the (Ga,Mn)(Bi,As) layers of high structural quality and smooth interfaces can be grown by means of the low-temperature molecular-beam epitaxy method, despite a large difference between the sizes of Bi and As atoms. Depending on the applied buffer layer, the DMS layers can be grown under either compressive or tensile misfit strain, which influences their magnetic properties. It is shown that even small 1% Bi content in the layers strongly affects their magnetoelectric properties, such as the coercive field and anisotropic magnetoresistance.     

EPR Spectra of Sintered Cd1−xCrxTe Powdered Crystals with Various Cr Content

In this paper, we show a simple method of producing ferromagnetic materials with a Curie temperature above room temperature. The electron paramagnetic resonance (EPR) spectra of Cd_1−xCr_xTe (0.002 < x < 0.08) were measured with a dependence on temperature (82 K < T < 381 K). Obtained EPR lines were fitted to a Lorentz-shaped curve. The temperature dependencies of the parameters of the EPR lines, such as the peak-to-peak linewidth (H_pp), the intensity (A), as well as the resonance field (H_r), were studied. Ferromagnetism was noticed in samples at high temperatures (near room temperature). For a sample with a nominal concentration of chrome of x = 0.05, a very strong intrinsic magnetic field is observed. The value of the effective gyromagnetic factor for this sample is g_e = 30 at T = 240 K. An increase of chrome concentration above x = 0.05 reduces the ferromagnetic properties considerably. Analysis of the temperature dependencies of the integral intensity of EPR spectra was carried out using the Curie–Weiss law and the paramagnetic Curie temperature was obtained.     

Magnetic Control of Optical Reflectance from Metallic Thin Film Using Surface Plasmon Resonance and Faraday Rotation

We demonstrate magnetic control of optical reflectance with no ferromagnetic material via combining the Faraday rotation and the surface plasmon resonance (SPR) in a Kretschman configuration under magnetic fields < 0.5 T. The SPR produces the polarization sensitive reflectance from the Au or Ag thin film coated on a N-BK7 prism in which the Faraday rotation occurs. The gold (Au) or silver (Ag) metal film as a plasmonic film somewhat acts as an incident angle-dependent reflection polarizer that can sensitively sense the polarization change induced by the Faraday rotation that occurs in a prism. We find that combination of Faraday rotation and the surface plasmon can induce a significant magnetic modulation of reflectance normalized with respect to that obtained with no magnetic fields at a specific incident angle of light. The magnetic control of optical reflectance presented may find an application in polarizer-free photonic devices with no ferromagnetic material for magneto-optical modulation.     

Effects of Substrate Bias Voltage on Structure of Diamond-Like Carbon Films on AISI 316L Stainless Steel: A Molecular Dynamics Simulation Study

With the recent significant advances in micro- and nanoscale fabrication techniques, deposition of diamond-like carbon films on stainless steel substrates has been experimentally achieved. However, the underlying mechanism for the formation of film microstructures has remained elusive. In this study, the growth processes of diamond-like carbon films on AISI 316L substrate are studied via the molecular dynamics method. Effects of substrate bias voltage on the structure properties and sp³ hybridization ratio are investigated. A diamond-like carbon film with a compact structure and smooth surface is obtained at 120 V bias voltage. Looser structures with high surface roughness are observed in films deposited under bias voltages of 0 V or 300 V. In addition, sp³ fraction increases with increasing substrate bias voltage from 0 V to 120 V, while an opposite trend is obtained when the bias voltage is further increased from 120 V to 300 V. The highest magnitude of sp³ fraction was about 48.5% at 120 V bias voltage. The dependence of sp³ fraction in carbon films on the substrate bias voltage achieves a high consistency within the experiment results. The mechanism for the dependence of diamond-like carbon structures on the substrate bias voltage is discussed as well.