Controlling Exchange Coupling Strength in Ni x Cu1−x Thin Films

The thickness (d _F) and concentration (x) dependence of the Curie temperature of Ni _x Cu_1?x (d _F) ferromagnetic alloy layers (x=0.55, 0.65, d _F=[4.5 nm÷12 nm]) in contact with a vanadium layer was studied. The Curie temperature of the ferromagnetic layers depends on the thickness when it is comparable with the transition layer between the F and the vanadium layers, which is attributed to the proximity coupling of the interface region with the rest of the F layer. The present study provides valuable information for fabrication of samples with controlled exchange coupling strength for studies of superconductor/ferromagnet proximity effects.     

Pulsed laser reshaping and fragmentation of upconversion nanoparticles — from hexagonal prisms to 1D nanorods through “Medusa”-like structures

One dimensional (1D) nanostructures attract considerable attention, enabling a broad application owing to their unique properties. However, the precise mechanism of 1D morphology attainment remains a matter of debate. In this study, ultrafast picosecond (ps) laser-induced treatment on upconversion nanoparticles (UCNPs) is offered as a tool for 1D-nanostructures formation. Fragmentation, reshaping through recrystallization process and bioadaptation of initially hydrophobic (β-Na_1.5Y_1.5F₆: Yb³⁺, Tm³⁺/β-Na_1.5Y_1.5F₆) core/shell nanoparticles by means of one-step laser treatment in water are demonstrated. “True” 1D nanostructures through “Medusa”-like structures can be obtained, maintaining anti-Stokes luminescence functionalities. A matter of the one-dimensional UCNPs based on direction of energy migration processes is debated. The proposed laser treatment approach is suitable for fast UCNP surface modification and nano-to-nano transformation, that open unique opportunities to expand UCNP applications in industry and biomedicine.

     

High-temperature ferromagnetism of Si1 ? x Mn x films fabricated by laser deposition using the droplet velocity separation technique

The transport and magnetic properties of Si_{1 ? x} Mn _x films of thickness 55?C70 nm with various Mn content (x = 0.44?C0.6) are studied in the temperature range of 5?C400 K and in magnetic fields up to 2 T. The films are grown by pulsed laser deposition on Al₂O₃ (0001) substrates at a temperature of 340°C using velocity separation of deposited particles. The films exhibit metal conductivity and the resistivity ?? = (2?8) × 10^?4 ?? cm, typical of highly degenerate semiconductors. It is found that the anomalous component of the Hall effect dominates over the normal component at T = 300 K for the Si_{1 ? x} Mn _x alloy with x ?? 0.5, and that the Curie temperature significantly exceeds room temperature and is estimated as ??500 K from magnetization measurements (for MnSi silicide the Curie temperature is T _C = 30 K). It is shown that the anomalous component of the Hall conductivity at low temperatures is controlled by ??side-jump?? and (or) ??intrinsic?? mechanisms independent on the carrier scattering time. The results are explained by features of the formation of defects with localized magnetic moments in the case of Si_{1 ? x} Mn _x films with x ?? 0.5 and by the significant role of matrix spin fluctuations in the exchange between these defects.     

Magnetic Proximity Effects in V/Fe Superconductor/Ferromagnet Single Bilayer Revealed by Waveguide-Enhanced Polarized Neutron Reflectometry

Polarized neutron reflectometry is used to study the magnetic proximity effect in a superconductor/ferromagnet (SC/FM) system of composition Cu(32 nm)/ V(40 nm)/Fe(1 nm)/MgO. In contrast to previous studies, here a single SC/FM bilayer, is studied and multilayer artefacts are excluded. The necessary signal enhancement is achieved by waveguide resonance, i.e., preparing the V(40 nm)/Fe(1 nm) SC/FM bilayer sandwiched by the highly reflective MgO substrate and Cu top layer, respectively. A new magnetic state of the system was observed at temperatures below 0.7T _C manifested in a systematic change in the height and width of the waveguide resonance peak. Upon increasing the temperature from 0.7T _C to T _C, a gradual decay of this state is observed, accompanied by a 5% growth of the diffuse scattering. This behavior can be explained in a natural way by the polarization of the superconducting electrons upon the SC transition, i.e., an appearance of additional induced magnetization within the SC, due to the proximity of the FM layer.     

Epitaxial growth and properties of Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>O films produced by pulsed laser deposition

The Mg _x Zn_1-x O thin films with a Mg content corresponding to x = 0–0.45 are grown by pulsed laser deposition on ablation of ceramic targets. The conditions for epitaxial growth of the films on the single-crystal Al₂O₃ (00.1) substrates are established. The record limit of solubility of Mg in hexagonal ZnO, x = 35 is attained. In this case, the lattice mismatch for the parameter a of the ZnO and Mg_0.35Zn_0.65O films does not exceed 1%, whereas the band gaps of the films differ by 0.78 eV. The surface roughness of the films corresponds to 0.8–1.5 nm in the range of x = 0–0.27.     

Ion energy spectrum control in modified cross-beam pulsed laser deposition method

The time-of-flight curves of the ion current to a probe in a plasma beam formed by crossing plumes from two laser-ablated silicon targets have been measured using the Langmuir probe technique. It is established for the first time that the ion energy spectrum of the plasma beam formed by the two laser erosion plumes and oriented in the direction of the bisector of the angle between the initial plumes can be modified by changing the angle between them (i.e., between the erosion plume). A modified cross-beam pulsed laser deposition method is proposed, which allows the energies of deposited particles to be controlled in a broad range.