Search Results (4)

In recent years, the discovery of the two-dimensional (2D) intrinsically ferromagnetic monolayer CrI₃ has opened up promising avenues for the advancement of spintronic devices. Nevertheless, the relatively low Curie temperature poses a significant challenge for practical applications. Herein, we determine changes in the superexchange interaction of ferromagnetic coupling caused under pressure by using first-principles calculations and Monte Carlo simulations. Based on the superexchange interaction of ferromagnetic coupling, the effect of applying high pressure on the Curie temperature of monolayer CrI₃ is investigated. With a pressure coefficient of 2.0%, the Curie temperature is enhanced to 97.3 K, which is nearly double that of the monolayer CrI₃ without pressure. In addition, the direction of the easy magnetization axis changes from the out-of-plane to the in-plane one when the pressure coefficient is 1.2%. Meanwhile, the band gap of monolayer CrI₃ can be transformed from indirect to direct by applying high pressure. Our work enriches the process of modulating the magnetic and electronic properties of 2D monolayer materials. Full article

Multisubstituted barium ferrites with a magnetoplumbite structure were obtained by the method of solid-phase reactions with ferritization and pre-firing. Three-charged, mainly diamagnetic cations Al³⁺, Cr³⁺, Ga³⁺, and In³⁺ were chosen as substituents for the Fe³⁺ iron cations, the proportion of which in solid solutions did not exceed 50%. The values of the configurational mixing entropy were calculated for all the compositions. A slight deviation of the chemical composition of the obtained solid solutions from the nominal value was established by the energy-dispersive X-ray spectroscopy method. The phase purity and values of the unit cell parameters were refined from X-ray scattering data using full-profile analysis in the Rietveld method. A non-monotonic behavior of the unit cell parameters as a function of the B-sub-lattice average ionic radius of the magnetoplumbite structure was found. A minimum unit cell volume of ~667.15 ų was found for the composition BaFe_6.11Al_1.56Cr_2.17Ga_2.16O₁₉ with a B-sub-lattice average ionic radius of ~7.449 Å. The average crystallite size varied within 5.5–6.5 μm. The temperature and field dependencies of the magnetization have been measured. The values of the saturation magnetization, residual magnetization, hysteresis loop squareness, and coercivity at 50 K and 300 K were extracted from the experimental data. Using the Law of Approach to Saturation, the magnetic crystallographic anisotropy coefficient and anisotropy field were calculated. Multisubstitution leads to a significant decrease in such magnetic parameters as the magnetic ordering temperature and spontaneous magnetization at both temperatures. The maximum magnetic ordering temperature of ~297.7 K was found for the composition BaFe_5.84Ga_6.19O₁₉ with a B-sub-lattice average ionic radius of ~7.586 Å in a field of 500 Oe. A maximum saturation magnetization of ~24.7 emu/g was found for the composition BaFe_5.84Ga_6.19O₁₉ with a B-sub-lattice average ionic radius of ~7.586 Å at 50 K. A maximum hysteresis loop squareness of ~0.72 was found for the composition BaFe_6.11Al_1.56Cr_2.17Ga_2.16O₁₉ with an average ionic radius of ~7.449 Å at 50 K. A maximum magnetic crystallographic anisotropy coefficient of ~2.09 × 10⁵ Erg/g was found for the composition BaFe_6.19Al_1.25Cr_1.57Ga_1.74In_1.26O₁₉ with a B-sub-lattice average ionic radius of ~7.706 Å at 50 K. The frustrated magnetic state including the nano-sized clusters with an average diameter in the range of 50–200 nm was established from the results of measuring the ZFC and FC temperature magnetizations. The interpretation of the obtained experimental data is carried out taking into account the increased stability of high-entropy phases and regular changes in the intensity of the Fe³⁺(Al³⁺, Cr³⁺, Ga³⁺, In³⁺)–O²⁻–Fe³⁺(Al³⁺, Cr³⁺, Ga³⁺, In³⁺) indirect superexchange interactions as a result of magnetic dilution of the iron sub-lattice in the magnetoplumbite structure. Full article

Three high-entropy Sm(Eu,Gd)Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2O₃ perovskite solid solutions were synthesized using the usual ceramic technology. The XRD investigation at room temperature established a single-phase perovskite product. The Rietveld refinement with the FullProf computer program in the frame of the orthorhombic Pnma (No 62) space group was realized. Along with a decrease in the V unit cell volume from ~224.33 ų for the Sm-based sample down to ~221.52 ų for the Gd-based sample, an opposite tendency was observed for the unit cell parameters as the ordinal number of the rare-earth cation increased. The average grain size was in the range of 5–8 μm. Field magnetization was measured up to 30 kOe at 50 K and 300 K. The law of approach to saturation was used to determine the M_s spontaneous magnetization that nonlinearly increased from ~1.89 emu/g (Sm) up to ~17.49 emu/g (Gd) and from ~0.59 emu/g (Sm) up to ~3.16 emu/g (Gd) at 50 K and 300 K, respectively. The M_r residual magnetization and H_c coercive force were also determined, while the SQR loop squareness, k magnetic crystallographic anisotropy coefficient, and H_a anisotropy field were calculated. Temperature magnetization was measured in a field of 30 kOe. ZFC and FC magnetization curves were fixed in a field of 100 Oe. It was discovered that the T_mo magnetic ordering temperature downward-curve decreased from ~137.98 K (Sm) down to ~133.99 K (Gd). The spin glass state with ferromagnetic nanoinclusions for all the samples was observed. The <D> average and D_max maximum diameter of ferromagnetic nanoinclusions were calculated and they were in the range of 40–50 nm and 160–180 nm, respectively. The mechanism of magnetic state formation is discussed in terms of the effects of the A-site cation size and B-site poly-substitution on the indirect superexchange interactions. Full article

Monophasic Zn_1-xFe_xO nanoparticles with wurtzite structure were synthesized in the 0 ≤ x ≤ 0.05 concentration range using a freeze-drying process followed by heat treatment. The samples were characterized regarding their optical, structural, and magnetic properties. The analyses revealed that iron doping of the ZnO matrix induces morphological changes in the crystallites. Iron is substitutional for zinc, trivalent and distributed in the wurtzite lattice in two groups: isolated iron atoms and iron atoms with one or more neighboring iron atoms. It was also shown that the energy band gap decreases with a higher doping level. The samples are paramagnetic at room temperature, but they undergo a spin-glass transition when the temperature drops below 75 K. The magnetic frustration is attributed to the competition of magnetic interactions among the iron moments. There are a superexchange interaction and an indirect exchange interaction that is provided by the spin (and charge) itinerant carriers in a spin-polarized band situated in the vicinity of the Fermi level of the Fe-doped ZnO semiconductor. The former interaction actuates for an antiferromagnetic coupling among iron ions, whereas the latter constitutes a driving force for a ferromagnetic coupling that weakens, decreasing the temperature. Our results strongly contribute to the literature because they elucidate the controversies reported in the literature for the magnetic state of the Fe-doped ZnO system. Full article