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The triangle-based magnetic subsystem of borates with the mineral norbergite structure M3BO6 (M = Fe, Cr, V) makes these compounds unique to investigate rare quantum ground states influenced by strong magnetic frustration. In this work, we investigated the thermal and magnetic properties of Cr3BO6 to find that despite very large negative Weiss temperature Θ = −160.7 K, it orders only at TN = 4.5 K and experiences a spin-flop transition at µ0H = 5 T. Density functional theory (DFT) calculations of exchange interaction parameters allow for suggesting the model of magnetic subsystem in chromium borate Cr3BO6. The results prove the decisive role of magnetic frustration on the formation of long-range order, providing therefore a basis for future study. Both experimental data and first-principles calculations point to the coexistence of chromium spin-singlets with long-range antiferromagnetic order.

Composite nanoparticles (NPs) based on nanocrystalline silicon (Si) with iron (Fe) content varied from 0.2 to 10 at.% are prepared by an arc-discharge plasma-ablative synthesis and investigated by means of the transmission and scanning electron microscopy, dynamic light scattering, Fourier-transform infrared spectroscopy, Mössbauer spectroscopy, electron paramagnetic resonance, magnetic susceptibility measurements, X-ray diffraction, and nuclear magnetic resonance relaxometry. It is shown that an increase in the Fe content results in an increase in the mean size of Si-Fe NPs from 30 to 400 nm. Iron-enriched agglomerates, which consist of α-FeSi2, α-Fe, and iron oxide, are detected in Si-Fe NPs with Fe content above 2.5 at.%. The magnetization relaxometry reveals a strong shortening of both the longitudinal and transverse proton relaxation times in aqueous suspensions of Si-Fe NPs. The enhanced proton relaxation is explained by the impact of both Fe ions and paramagnetic centers like Si dangling bonds, the contribution of which to the proton relaxation depends on the Fe content. In vitro experiments show that prepared Si-Fe NPs with concentration up to 50 μg/mL are characterized by low cytotoxicity, which slightly increases with increasing iron content. The obtained results demonstrate the possibility of using composite Si-Fe NPs as potential contrast agents for MRI-guided cancer theranostics.

Herein, we demonstrate the synthesis of sandwiched composite nanomagnets, which consist of hard magnetic Cr-substituted hexaferrite cores and magnetite outer layers. The hexaferrite plate-like nanoparticles, with average dimensions of 36.3 nm × 5.2 nm, were prepared via a glass crystallization method and were covered by spinel-type iron oxide via thermal decomposition of iron acetylacetonate in a hexadecane solution. The hexaferrite nanoplates act as seeds for the epitaxial growth of the magnetite, which results in uniform continuous outer layers on both sides. The thickness of the layers can be adjusted by controlling the concentration of metal ions. In this way, layers with an average thickness of 3.7 and 4.9 nm were obtained. Due to an atomically smooth interface, the magnetic composites demonstrate the exchange coupling effect, acting as single phases during remagnetization. The developed approach can be applied to any spinel-type material with matching lattice parameters and opens the way to expand the performance of hexaferrite nanomagnets due to a combination of various functional properties.

A magnetic structure of the sawtooth-chain antiferromagnet $$\\hbox {Fe}_2\\hbox {Se}_2\\hbox {O}_7$$ Fe 2 Se 2 O 7 was investigated by magnetization measurements, single crystalline and powder neutron diffraction experiments, and a further analysis on the Mössbauer spectra. These experiments revealed a nearly collinear antiferromagnetic structure with magnetic moments aligned along the b -axis, indicating dominant antiferromagnetic exchanges between Fe(1)–Fe(2) and Fe(2)–Fe(3) sites. The magnon dispersion relation derived from the linear spin wave approximation suggests the possible flat band nature of magnons.

Magnetically hard ferrites attract considerable interest due to their ability to maintain a high coercivity of nanosized particles and therefore show promising applications as nanomagnets ranging from magnetic recording to biomedicine. Herein, we report an approach to prepare nonsintered single-domain nanoparticles of chromium-substituted hexaferrite via crystallization of glass in the system SrO–Fe2O3–Cr2O3–B2O3. We have observed a formation of plate-like hexaferrite nanoparticles with diameters changing from 20 to 190 nm depending on the annealing temperature. We demonstrated that chromium substitution led to a significant improvement of the coercivity, which varied from 334 to 732 kA m−1 for the smallest and the largest particles, respectively. The results provide a new strategy for producing high-coercivity ferrite nanomagnets.

A magnetic structure of the sawtooth-chain antiferromagnet Fe 2 Se 2 O 7 was investigated by magnetization measurements, single crystalline and powder neutron diffraction experiments, and a further analysis on the Mössbauer spectra. These experiments revealed a nearly collinear antiferromagnetic structure with magnetic moments aligned along the b -axis, indicating dominant antiferromagnetic exchanges between Fe(1)–Fe(2) and Fe(2)–Fe(3) sites. The magnon dispersion relation derived from the linear spin wave approximation suggests the possible flat band nature of magnons.

Mixed-valent Ba Mn Mn (SeO ) crystallizes in a monoclinic structure and has honeycomb layers of Mn ions alternating with triangular layers of Mn ions. We established the key parameters governing its magnetic structure by magnetization and specific heat measurements. The title compound exhibits a close succession of a short-range correlation order at = 10.1 ± 0.1 K and a long-range Néel order at = 5.7 ± 0.1 K, and exhibits a metamagnetic phase transition at with hysteresis most pronounced at low temperatures. The causes for these observations were found using the spin exchange parameters evaluated by density functional theory calculations. The title compound represents a unique case in which uniform chains of integer spin Mn ( = 2) ions interact with those of half-integer spin Mn ( = 5/2) ions.

Mixed-valent Ba[sub.2]Mn[sup.2+]Mn[sub.2] [sup.3+](SeO[sub.3])[sub.6] crystallizes in a monoclinic P2[sub.1]/c structure and has honeycomb layers of Mn[sup.3+] ions alternating with triangular layers of Mn[sup.2+] ions. We established the key parameters governing its magnetic structure by magnetization M and specific heat C[sub.p] measurements. The title compound exhibits a close succession of a short-range correlation order at Tcorr = 10.1 ± 0.1 K and a long-range Néel order at TN = 5.7 ± 0.1 K, and exhibits a metamagnetic phase transition at T < TN with hysteresis most pronounced at low temperatures. The causes for these observations were found using the spin exchange parameters evaluated by density functional theory calculations. The title compound represents a unique case in which uniform chains of integer spin Mn[sup.3+] (S = 2) ions interact with those of half-integer spin Mn[sup.2+] (S = 5/2) ions.

The triangle-based magnetic subsystem of borates with the mineral norbergite structure M BO (M = Fe, Cr, V) makes these compounds unique to investigate rare quantum ground states influenced by strong magnetic frustration. In this work, we investigated the thermal and magnetic properties of Cr BO to find that despite very large negative Weiss temperature Θ = -160.7 K, it orders only at = 4.5 K and experiences a spin-flop transition at µ = 5 T. Density functional theory (DFT) calculations of exchange interaction parameters allow for suggesting the model of magnetic subsystem in chromium borate Cr BO . The results prove the decisive role of magnetic frustration on the formation of long-range order, providing therefore a basis for future study. Both experimental data and first-principles calculations point to the coexistence of chromium spin-singlets with long-range antiferromagnetic order.

The triangle-based magnetic subsystem of borates with the mineral norbergite structure M[sub.3]BO[sub.6] (M = Fe, Cr, V) makes these compounds unique to investigate rare quantum ground states influenced by strong magnetic frustration. In this work, we investigated the thermal and magnetic properties of Cr[sub.3]BO[sub.6] to find that despite very large negative Weiss temperature Θ = −160.7 K, it orders only at T[sub.N] = 4.5 K and experiences a spin-flop transition at µ[sub.0]H = 5 T. Density functional theory (DFT) calculations of exchange interaction parameters allow for suggesting the model of magnetic subsystem in chromium borate Cr[sub.3]BO[sub.6]. The results prove the decisive role of magnetic frustration on the formation of long-range order, providing therefore a basis for future study. Both experimental data and first-principles calculations point to the coexistence of chromium spin-singlets with long-range antiferromagnetic order.