Physics of Metals and Metallography

One of the fundamental characteristics of a magnetic field detector is sensitivity to the external magnetic field. Of all the known magnetic effects, the giant magnetoimpedance (MI) has the highest sensitivity with respect to the external magnetic field. Here, we describe our experience in designing, fabrication, experimental and theoretical characterization of [FeNi(50 nm)/Ti(6 nm)]₆/Cu(500 nm)/[Ti(50 nm)/FeNi(6 nm)]₆ multilayered structures for biometric detector. The designed device operates at room temperature, with a maximum sensitivity of the order of 0.4 Ohm/Oe for the total impedance and its real part and 0.1 Ohm/Oe for the imaginary part of the total impedance. An automatic system based on a ZVA-67 (Rohde & Schwarz) vector network analyzer was built for one-scan microwave absorption studies of both magnetoimpedance and ferromagnetic resonance of multilayered sensitive elements. Measurements were made with the coplanar line type holder in the increasing and decreasing fields. The obtained experimental and theoretical results for MI range were in a satisfactory agreement with each other. They could be useful for optimization of the MI multilayered elements for practical applications, including applications in different types of magnetic biosensors.

Nanoscale [Tb–Co/Ti]_n and [Tb–Co/Si]_n multilayers were designed and prepared by the rf-sputtering technique. Their structure and magnetic properties have been experimentally studied. Low-angle X‑ray diffraction studies confirmed that a well-defined layered structure was formed in all investigated cases. The change in the magnetic properties of multilayers with a decrease in the thickness of the Tb–Co layers was due to both the size effect and the influence of the material of the non-magnetic spacers, namely the reduction of the Co atom moment because of the electron charge transfer from atoms of the Ti or Si spacers to atoms of the cobalt.

Magnetic properties and structure of the 1.4540 (S15500) stainless steel products manufactured by additive technology using the selective laser melting method (SLM) were studied. The structure of the SLM alloy differs from the conventional 1.4540 stainless steel. The volume fraction of austenite of 34% is found in the SLM sample. After solution treatment, the austenite is not observed. The martensitic thin plates and precipitations are observed in both as-build and solutionized samples. The solution treatment at standard temperature of 1038°C can be successfully used for the SLM 1.4540 steel products. Magnetic properties of the SLM 1.4540 stainless steel allow considering this material as applicable for soft-magnetic applications requiring high strength and high corrosion resistance.

Densities of states for simple (sc) and base-centered (bcc) cubic lattices with account of nearest and next-nearest neighbour hopping integrals t and t' are investigated in detail. It is shown that at values of τ ≡ t'/t = τ*, corresponding to the change in isoenergetic surface topology, the formation of Van Hove k lines takes place. At small deviations from these special values, the weakly dispersive spectrum in the vicinity of Van Hove lines is replaced by a weak k-dependence in the vicinity of a few van Hove points which possess huge masses proportional to |τ – τ*|^–1. The singular contributions to the density of states that originate from Van Hove points and lines are considered, as well as the change in the topology of isoenergetic surfaces in the k-space with the variation of τ. The closed analytical expressions for the density of states as a function of energy and τ in terms of elliptic integrals and power-law asymptotics at τ = τ* are obtained. Apart from the case of sc lattice with small τ (maximum of density of state corresponds to the energy level of X k-point), maximal value of the density of states is always achieved at energies corresponding to innerk-points of the Brillouin zone positioned in high-symmetry directions, and rather than at zone faces.

Using three-dimensional micromagnetic simulation, we studied dynamic processes in a domain wall moving under the action of the dc magnetic field in a magnetically soft film with an in-plane uniaxial anisotropy. The features of the dynamics are investigated depending on the film thickness. Visualization methods based on the calculation of topological invariants were used.

The nanocomposite artificial crystals with embedded magnetic nanoparticles are obtained from opal matrices composed of the submicron SiO₂ spheres. The introduced particles are sized from 5 to 60 nm. These artificial crystals contain particles of several rare earth titanates with pyrochlore structure. Low temperature magnetic properties of these nanocomposite materials have been investigated. The frequency and magnetic field dependences of AC magnetic susceptibility of nanocomposites with Gd₂Ti₂O₇, Dy₂Ti₂O₇, Sm₂Ti₂O₇, Nd₂Ti₂O₇ and Er₂Ti₂O₇ particles have been measured in temperature range from 2 to 10 K at frequencies from 1 Hz to 10 kHz. It has been established that magnetic field dependences of these rare earth nanocomposite titanates obey Cole-Cole-like formula written for magnetic field dependency of AC magnetic susceptibility.

An eight-port magnonic network was fabricated from 1 μm thick epitaxial yttrium iron garnet (YIG) film by the photolithography and ion-etching techniques. The network had a form of the 2 × 2 lattice of the 10 μm wide and 100 μm long YIG waveguides with inductive micro-antennas located at the ends and spaced by 90 μm. Effects of the spin waves (SW) propagation and interference in the network were studied experimentally and by micromagnetic simulation. It was shown that one can realize the constructive and destructive interference of the SW at the output transducers by changing the input signals phase. Possibilities to build logic gates and magnetic sensors based on the SW interference was demonstrated.

Within the framework of the LDA + GTB multielectron approach to the electron structure of Mott–Hubbard insulators a scheme is developed for constructing the effective low-energy Hamiltonian that includes not only the ground state of the magnetic cations, but also the excited terms. The mathematical apparatus of the theory are Hubbard operators built on the many-electron states of the cation in the dⁿ configuration. The occupation of the excited term under optical pumping can change the sign of the exchange interaction of the excited cation with the neighboring cation in the ground state. Another variant of the occupation of the excited states is connected with a spin crossover when the excited and the ground terms change over, for example, at high pressure. Examples are given for such crystals as FeBO₃, Nd_0.5Gd_0.5Fe₃(BO₃)₄ and NiO.

The possibility of using the transverse relaxation time T2 of protons in aqueous media for quantitative measurement of the capture of magnetic nanoparticles by cells has been studied and demonstrated. The measurement of T2 was performed on a portable original NMR relaxometer with a measuring cell for a standard well of a biological plate. The novelty of the approach is that quantitative measurements of the capture kinetics were carried out using measurements of the proton relaxation time of the nutrient medium, which is determined by the remaining number of magnetic particles (not captured by the cells) in the medium. To study the kinetics of capture, two types of magnetic nanoparticles were synthesized: magnetite particles Fe₃O₄ and composite particles Fe@C with an iron-carbon shell structure. The surface of the particles was functionalized with amine-and carboxyl groups. The capture of aminated particles of Fe@C cells is established by microscopy and NMR-relaxometry by measuring the time T2. It is shown that the proposed method makes it possible to register very small concentrations of trapped magnetic nanoparticles equal to tens of pg/cell.

We present results of soft X-ray absorption spectroscopy studies of nonstoichiometric Tb_1 ‒ yBa_{1 +y}Co_{2 –x}O_{5.5 – δ} cobaltites and EuBaCo₂O_5.5 cobaltite powders subjected to mechanical impact (milling in a ball mill). It was established that in octahedra of Tb_1 – yBa_1 + yCo_2 – xO_5.5 – δ cobaltates Co³⁺ ions are in the high spin state. In EuBaCo₂O_5.5 powder after milling in a ball mill, a surface area (5–10 nm) of particles was found to be decomposed into Co₃O₄, BaCO₃, and EuCoO₃ phase components. A knowledge of the low-spin state of Co³⁺ ions in EuCoO₃ and Co₃O₄ and possibilities of surface-sensitive soft X-ray absorption spectroscopy have allowed to determine the composition of the EuBaCo₂O_5.5 sample (subjected to ball milling) near its surface, which is inaccessible to standard X-ray phase analysis.

The Ising model on a square lattice with arbitrary number of decorating spins, considering both the interactions between nodal and decorating spins is examined. A plethora of peculiarities such as heat capacity splitting, generation and suppression of multiple phase transitions and several kinds of partial ordering are thoroughly scrutinized. A rigorous analytical expression for the partition function closely resembling the one obtained by Onsager is presented.

In this study, some physical properties of a quaternary Cu-based alloy have been studied on the example of the alloy Cu–13Al–4.5Ni–1.5Ti (wt %) that was produced by using the arc-melting method. Four specimens were made from the alloy ingot, and three of them were subjected to heat treatment at 930°C for 30 min. Later, they were separately quenched into liquid nitrogen (–196°C), alcohol (0°C), and iced-brine (6°C) mediums. Differential Scanning Calorimetry (DSC) measurements of all samples were performed to determine the effect of quenching on the phase transformation temperatures. It was observed that the quaternary Cu–Al–Ni–Ti alloy is a high temperature shape memory alloy (HTSMA) and its transformation temperatures significantly were affected by heat treatments, e.g., the phase transformation temperatures increased via quenching in alcohol and iced brine by approximately 100 K. Moreover, optical microscopy (OM) and Scanning Electron Microscopy (SEM) images were taken to observe the changes in the microstructure after heat treatment. It was revealed that grain boundaries were more discernable in the samples that were quenched in alcohol and iced brine, also the volume fraction of martensite plates was increased. Crystal structures of samples at room temperature were determined by using X-ray diffractograms, whereby, the pattern showed the presence of β', γ₂, and X phases, which are of martensite origin.

The Al–Mg–Si composites employing aluminum (Al) as matrix, magnesium (Mg) and silicon (Si) as fillers with weight ratio of 60 : 20 : 20 have been prepared by the method of powder metallurgy. The mixed powders were pressed applying pressure of 700 MPa, followed by sintering at 500, 550, and 600°C for 3 h in inert atmosphere. The resulted samples have been studied using a scanning electron microscope (SEM), equipped with energy dispersive X-ray (EDX) and electron probe micro analyzer (EPMA). Experimental results showed that the distribution of Mg and Si were homogeneous in the Al matrix. According to the elemental mapping and diffusion equation under non-steady state condition, the atomic interdiffusion process at the interfaces was studied, resulting in the different diffusion coefficient (D) for each element as diffusant in the sample heated at 550°C. As a filler, the Mg diffusing into the Al matrix has the greater D (=0.708 × 10^–16 m² s^–1) than Si (D = 0.231 × 10^–16 m² s^–1), while oxygen diffusing into the Mg has the highest D (=8.333 × 10^–16 m² s^–1). Further probing by using the X-ray diffractometer (XRD), the phase formation covering metal oxides and intermetallic compounds was identified in the samples. Hardness test on the sample’s surface indicates the variation of Vickers hardness number (VHN) at several points on the sample’s surface.