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We propose a simple hydrothermal synthesis of nanoflakes composed of alternating sulfide and hydroxide quasi-monolayers and their aqueous colloids, as a prospective family of novel multifunctional 2D materials.

We report the results of comparative analysis of magnetic properties of the systems based on ε-Fe2O3, nanoparticles with different average sizes (from ∼3 to 9 nm) and dispersions. The experimental data for nanoparticles higher than 6–8 nm in size are consistent with the available data, specifically, the transition to the magnetically ordered state occurs at a temperature of ∼500 K and the anomalies of magnetic properties observed in the range of 80–150 K correspond to the magnetic transition. At the same time, Mőssbauer and ferromagnetic resonance spectroscopy data as well as the results of static magnetic measurements show that at room temperature all the investigated samples contain ε-Fe2O3 particles that exhibit the superparamagnetic behavior. It was established that the magnetic properties of nanoparticles significantly change with a decrease in their size to ∼6 nm. According to high-resolution electron microscopy and Mőssbauer spectroscopy data, the particle structure can be attributed to the ε–modification of trivalent iron oxide; meanwhile, the temperature of the magnetic order onset in these particles is increased, the well-known magnetic transition in the range of 80–150 K does not occur, the crystallographic magnetic anisotropy constant is significantly reduced, and the surface magnetic anisotropy plays a decisive role. This is apparently due to redistribution of cations over crystallographic positions with decreasing particle size, which was established using Mössbauer spectra. As the particle size is decreased and the fraction of surface atoms is increased, the contribution of an additional magnetic subsystem formed in a shell of particles smaller than ∼4 nm becomes significant, which manifests itself in the static magnetic measurements as paramagnetic contribution.

A Pb 3 Mn 7 O 15 single crystal has been grown by the flux method and studied using x-ray diffraction and magnetization measurements. The crystal is hexagonal (P6 3 /mcm space group, Z = 4) and exhibits a pronounced layered nature. Along the [001] direction (c axis), the structure consists of layers of edge-sharing MnO 6 octahedra. Pairs of Mn atoms occupy the octahedral sites located between layers forming 'bridges' along the c axis, which link neighboring Mn layers. The magnetic properties of the crystal have been investigated using ac and dc magnetization measurements in the temperature range 2-900 K at magnetic fields up to 90 kOe. The experimental data obtained suggest that in the temperature region under study several different magnetic phases can be distinguished. Down to ∼250 K, the crystal is in the paramagnetic state. Below this temperature, short-range antiferromagnetic ordering apparently starts forming within Mn layers, although a transition to long-range magnetic order occurs at 70 K. The magnetization data obtained leads us to conclude that this state is canted antiferromagnetic with moments lying in the basal plane of the crystal. In addition, below 20 K the crystal undergoes one more magnetic transition that corresponds to spin reorientation.

Valleriite is of interest as a mineral source of basic and precious metals and as an unusual material composed of two-dimensional (2D) Fe−Cu sulfide and magnesium hydroxide layers, whose characteristics are still very poorly understood. Here, the mineral samples of two types with about 50% of valleriites from Noril'sk ore provenance, Russia, were examined using Cu K-and Fe K-edge X-ray absorption fine structure (XAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), 57 Fe Mossbauer spectroscopy, and magnetic measurements. The Cu K X-ray absorption near-edge structures (XANES) spectra resemble those of chalcopyrite, however, with a higher electron density at Cu + centers and essentially differ from those of bornite Cu 5 FeS 4 ; the Fe K-edge was less informative because of accompanying oxidized Fecontaining phases. The post-edge XANES and extended XAFS (EXAFS) analysis reveal differences in the bond lengths, e.g., additional metal−metal distances in valleriites as compared with chalcopyrite. The XPS spectra confirmed the Cu + and Fe 3+ state in the sulfide sheets and suggest that they are in electron equilibrium with (Mg, Al) hydroxide layers. Mossbauer spectra measured at room temperature comprise central doublets of paramagnetic Fe 3+ , which decreased at 78 K and almost disappeared at 4.2 K, producing a series of hyperfine Zeeman sextets due to internal magnetic fields arising in valleriites. Magnetic measurements do not reveal antiferromagnetic transitions known for bornite. The specific structure and properties of valleriite are discussed in particular as a platform for composites of the 2D transition metal sulfide and hydroxide (mono)layers stacked by the electrical charges, promising for a variety of applications.

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Abstract-Powders of undoped ferrihydrite nanoparticles and ferrihydrite nanoparticles doped with cobalt in the ratio of 5 : 1 have been prepared by hydrolysis of 3d-metal salts. It has been shown using Mössbauer spectroscopy that cobalt is uniformly distributed over characteristic crystal-chemical positions of iron ions. The blocking temperatures of ferrihydrite nanoparticles have been determined. The nanoparticle sizes, magnetizations, surface anisotropy constants, and bulk anisotropy constants have been estimated. The doping of ferrihydrite nanoparticles with cobalt leads to a significant increase in the anisotropy constant of a nanoparticle and to the formation of surface rotational anisotropy with the surface anisotropy constant K u = 1.6 × 10 -3 erg/cm 2 .