Maciej Czapkiewicz

The use of antiferromagnetic (AFM) materials in spin-electronic devices offers faster operation and immunity to the external magnetic fields. Controlling the Néel vector of AFMs can be achieved through the spin-orbit effect, in which spin-polarized electrons from the neighboring material with significant spin-orbit coupling exert a torque on the AFM layer. Recently, the results of current-induced reorientation of spins were shown by Moriyama or Baldrati for insulating monocrystalline antiferromagnets (AFM) with an adjacent heavy metal layer, either with or without an additional ferromagnetic layer, deposited by means of molecular beam epitaxy (MBE). While the orientation of the crystallization direction of monocrystalline AFM layer against spin-polarized current is crucial for switching of the Néel vector, the slow growth rate of monocrystalline layer is not suitable for practical spintronic applications. Here, we present a detailed structural analysis of the polycrystalline Pt and NiO growth on the following substrates: monocrystalline MgO(001) and MgO(111), and amorphous Si/SiOx (as a reference). Different crystal symmetries of NiO and Pt were observed, depending on the MgO substrate orientation, while Si/SiOx substrate induces amorphous growth. Our study proves feasibility of a control of the AFM growth even for fast magnetron sputtering process, enabling development of AFM-based spin-orbit-torque devices.

In this paper, we discuss the structural and magnetic properties of Ta(d)/Co₄₀Fe₄₀B₂₀/MgO/Ta multilayers. The CoFeB wedge layer was deposited on three buffers differing in Ta layer thickness: d = 5, 10, and 15 nm. A structural analysis showed that the Ta seed-buffer of 5 nm was amorphous, whereas thicker Ta grew in a β-tetragonal disordered structure. X-ray reflectivity measurements revealed that the Ta/CoFeB interface roughness for annealed samples ranged from 0.55 to 0.67 nm for a sample with a 0.85 nm CoFeB layer and decreased to approximately 0.47 nm for thicker CoFeB layers, while the average interface CoFeB/MgO thickness was about 0.2–0.3 nm. The morphological roughness of the amorphous single 5 nm Ta layer was the lowest, whereas crystalline grains in thicker Ta buffers induced higher roughness. The 5 nm thick MgO layer exhibited a strong (001)-oriented texture, which was the highest for the smoothest 5 nm Ta buffer. The magnetic dead layer thickness for the annealed sample with a 15 nm Ta buffer was 0.39 nm and increased with the decrease in the Ta buffer thickness. Temperature-dependent measurements offered further insight into the diffusion processes and the formation of the magnetic dead layer (MDL) at the Ta/CoFeB interface.

This study focuses on the development of new amino-functionalized magnetic Fe₂O₃/SiO₂ nanocomposites with varying silicate shell ratios (1:0.5, 1:1, and 1:2) for the efficient elimination of Hg²⁺ ions found in solutions. The Fe₂O₃/SiO₂–NH₂ adsorbents were characterized for their structural, surface, and magnetic properties using various techniques, including Fourier transform infrared spectrum (FT-IR), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Braunauer–Emmett–Teller (BET), thermogravimetric analysis (TGA), zeta-potential, and particle size measurement. We investigated the adsorption circumstances, such as pH, dosage of the adsorbent, and duration of adsorption. The pH value that yielded the best results was determined to be 5.0. The Fe₂O₃/SiO₂–NH₂ adsorbent with a silicate ratio of (1:2) exhibited the largest amount of adsorption capacity of 152.03 mg g⁻¹. This can be attributed to its significantly large specific surface area of 100.1 m² g⁻¹, which surpasses that of other adsorbents. The adsorbent with amino functionalization demonstrated a strong affinity for Hg²⁺ ions due to the chemical interactions between the metal ions and the amino groups on the surface. The analysis of adsorption kinetics demonstrated that the adsorption outcomes adhere to the pseudo-second-order kinetic model. The study of adsorption isotherms revealed that the adsorption followed the Langmuir model, indicating that the adsorption of Hg²⁺ ions with the adsorbent occurred as a monomolecular layer adsorption process. Furthermore, the thermodynamic analyses revealed that the adsorption of Hg²⁺ ions using the adsorbent was characterized by a spontaneous and endothermic process. Additionally, the adsorbent has the ability to selectively extract mercury ions from a complex mixture of ions. The Fe₂O₃/SiO₂–NH₂ nanocomposite, which is loaded with metal, can be easily recovered from a water solution due to its magnetic properties. Moreover, it can be regenerated effortlessly through acid treatment. This study highlights the potential use of amino-functionalized Fe₂O₃/SiO₂ magnetic nanoparticles as a highly efficient, reusable adsorbent for the removal of mercury ions from contaminated wastewater.

In this work, we comprehensively investigate and discuss the structural, magnetostatic, dynamic, and magnetoresistive properties of epitaxial Co/Mo superlattices. The magnetization of the Co sublayers is coupled antiferromagnetically with a strength that depends on the thickness of the nonmagnetic Mo spacer. The magnetization and magnetoresistance hysteresis loops clearly reflect interlayer exchange coupling and the occurrence of uniaxial magnetic anisotropy induced by the strained Co sublayers. Upon accounting for a deviation of the sublayer thicknesses from the nominal value, theoretical modeling, including both micromagnetic and macrospin approaches, precisely reproduces experimental magnetic hysteresis loops, magnetoresistance curves, and ferromagnetic resonance dispersion relations. The interlayer magnetic coupling as a function of the Mo spacer thickness is determined as a fitting parameter by modeling the experimental results.

We present a detailed study of Ta/Ru-based buffers and their influence on features crucial from the point of view of applications of Magnetic Tunnel Junctions (MTJs) such as critical switching current and thermal stability. We study buffer/FeCoB/MgO/Ta/Ru and buffer/MgO/FeCoB/Ta/Ru layers, investigating the crystallographic texture, the roughness of the buffers, the magnetic domain pattern, the magnetic dead layer thickness, and the perpendicular magnetic anisotropy fields for each sample. Additionally, we examine the effect of the current induced magnetization switching for complete nanopillar MTJs with lateral dimensions of 270 × 180 nm. Buffer Ta 5/Ru 10/Ta 3 (thicknesses in nm), which has the thickest dead layer, exhibits a much larger thermal stability factor (63 compared to 32.5) while featuring a slightly lower critical current density value (1.25 MA/cm2 compared to 1.5 MA/cm2) than the buffer with the thinnest dead layer Ta 5/Ru 20/Ta 5. We can account for these results by considering the difference in damping which compensates for the difference in the switching barrier heights.

In this paper the dynamics of the magnetization reversal process in perpendicularly biased [20 Å Pt/5 Å Co]₃/t Å Pt/100 Å IrMn/20 Å Pt multilayers with different Pt insertion layer thickness (0 Å ≤ t ≤ 12 Å) is studied. The insertion of 1 Å thick Pt enhances the exchange bias field (H_ex ) and for t > 3 Å H_ex decreases exponentially with increasing Pt layer thickness. We show by magnetization relaxation measurements and direct observation of magnetic domains that magnetization reversal takes place by the nucleation of isolated cylindrical domains with a different nucleation site density in the forward and backward branches of the hysteresis loop. All the results were quantitatively analyzed using the Fatuzzo model for the dynamics of domain reversal processes. The activation energies for magnetization reversal by domain nucleation and domain propagation were determined.