•The M(H) magnetization curves of NiO nanoparticles (NPs) measured in pulsed fields of up to 250 kOe have been studied.•A model of NiO NP obtained from analysis of M(H) data have been ...proposed.•Surface and size effects as well as the origin and the magnitude of uncompensated magnetic moment have been revealed.
-The analysis of the M(H) magnetization curves of antiferromagnetic nanoparticles yields information about magnetic subsystems formed in these objects, which are characterized by a large fraction of surface atoms. However, in the conventionally investigated experimental magnetic field range of up to 60–90 kOe, this analysis often faces the ambiguity of distinguishing the Langevin function-simulated contribution of uncompensated magnetic moments μun of particles against the background of a linear-in-field dependence (the antiferromagnetic susceptibility and other contributions). Here, this problem has been solved using a pulsed technique, which makes it possible to significantly broaden the range of external fields in which the μun contribution approaches the saturation. Nanoparticles of a typical NiO antiferromagnet with an average size of ~ 4.5 nm have been investigated. Based on the thorough examination of the M(H) magnetization curves measured in pulsed fields of up to 250 kOe, a model of the magnetic state of NiO nanoparticles of such a small size has been proposed. The average moment is ~130 μB (μB is the Bohr magneton) per particle, which corresponds to 60–70 decompensated spins of nickel atoms localized, according to the Néel hypothesis (μun~ 3/2), both on the surface and in the bulk of a particle. A part of the surface spins unrelated to the antiferromagnetic core form another subsystem, which behaves as free paramagnetic atoms. Along with the antiferromagnetic core, an additional linear-in-field contribution has been detected, which is apparently related to superantiferromagnetism, i.e., the size effect inherent to small antiferromagnetic particles.
Ferrihydrite is a low-crystalline nanoscale matter. The uncompensated magnetic moment of the ferrihydrite caused by the antiferromagnetic ordering of the magnetic moments of iron atoms and leads to ...the magnetic properties very similar to those of ferro- and ferrimagnetic nanoparticles. In this study, we investigated the biogenic ferrihydrite nanoparticles with the narrow size distribution and an average diameter of ≈2 nm obtained by the bacteria life cycle. The features caused by the surface effects and the inhomogeneous structure of ferrihydrite have been examined in the temperature range of 4–300 K using Mössbauer spectroscopy and magnetometry. Based on the Mössbauer data, we identified the superparamagnetic blocking temperature at the temperature of 30 K for the largest ferryhidrite particles. We established that the exceptional magnetic anisotropy of ferrihydrite (KV=1.2⋅105 erg/cm3 and KS=0.1 erg/cm2) is reached because of the highly developed ferrihydrite nanoparticles’ surface. According to the Mössbauer data, we propose a core-shell structural model of the biogenic ferrihydrite particles. We found that the size of the dense core depends on the particle size. The well-crystallized core is formed only for nanoparticles larger than ≈2 nm, whereas smaller particles consist entirely of a matter with a lower density of iron atoms.
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•We discuss the core-shell structure of the biogenic ferrihydrite nanoparticles using Mössbauer spectroscopy and magnetometry in the temperature range of 4–300 K.•We found that the size of the dense core depends on the nanoparticle size. The particles less than ≈2 nm have no dense core.•It was shown that the surface magnetic anisotropy originates from the inhomogeneous structure of the ferrihydrite particles
•A sample of NiO nanoparticles with an average size of 8 nm was synthesized.•The sample exhibits the superparamagnetic behavior with a blocking temperature of 185 K.•The magnetization curves in ...fields up to 250 kOe at temperatures of 80–300 K was measured.•The FM, AFM contributions and paramagnetism of the surface spins was extract.
It is well-known that the fraction of surface atoms and the number of defects in an antiferromagnetic particle increase with a decrease in the particle size to tens of nanometers, which qualitatively changes the properties of the particle. Specifically, in antiferromagnetic nanoparticles, spins in the ferromagnetically ordered planes can partially decompensate; as a result, an antiferromagnetic particle acquires a magnetic moment. As a rule, uncompensated chemical bonds of the surface atoms significantly weaken the exchange coupling with the antiferromagnetic particle core, which can lead to the formation of an additional magnetic subsystem paramagnetic at high temperatures and spin-glass-like in the low-temperature region. The existence of several magnetic subsystems makes it difficult to interpret the magnetic properties of antiferromagnetic nanoparticles. It is shown by the example of NiO nanoparticles with an average size of 8 nm that the correct determination of the contributions of the magnetic subsystems forming in antiferromagnetic nanoparticles requires magnetic measurements in much stronger external magnetic fields than those commonly used in standard experiments (up to 60–90 kOe). An analysis of the magnetization curves obtained in pulsed magnetic fields up to 250 kOe allows one to establish the contributions of the uncompensated particle magnetic moment μun, paramagnetic subsystem, and antiferromagnetic particle core. The μun value obtained for the investigated NiO particles is consistent with the Néel model, in which μun ∼ N1/2 (N is the number of magnetically active atoms in a particle), and thereby points out the existence of defects on the surface and in the bulk of a particle. It is demonstrated that the anomalous behavior of the high-field susceptibility dM/dH of antiferromagnetic NiO nanoparticles, which was observed by many authors, is caused by the existence of a paramagnetic subsystem, rather than by the superantiferromagnetism effect.
•Synthetic and biogenic ferrihydrite nanoparticles are synthesized.•Analysis of the Mössbauer spectra and magnetometry revealed the interparticle magnetic interactions in synthetic ...ferrihydrite.•Collective processes of freezing the magnetic moments of particles take place in synthetic ferrihydrite.
Samples of synthetic ferrihydrite with an average nanoparticle size of 2.7 nm have been examined by magnetometry and Mössbauer spectroscopy. Ferrihydrite is characterized by the antiferromagnetic interactions between the magnetic moments of iron atoms. In ferrihydrite nanoparticles, as in any other antiferromagnetic ones, structural defects induce the formation of an uncompensated magnetic moment, which determines the magnetic properties typical of single-domain ferro- and ferrimagnetic particles. The manifestation of the magnetic interactions between ferrihydrite nanoparticles in the magnetic properties of the material and in the temperature evolution of Mössbauer spectra has been in focus. The results obtained on synthetic ferrihydrite have been compared with the data for the biogenic ferrihydrite sample with a similar average size of particles surrounded by a polysaccharide shell, which weakens and screens the interparticle magnetic interactions. A clear manifestation of the effect of the interparticle magnetic interactions on the transition to the blocked state is the presence of a significant contribution of the relaxation component in the Mössbauer spectra at temperatures of the transition from the superparamagnetic to blocked state. The temperature dependence of the particle relaxation time obtained from the Mössbauer spectra points out the collective effect of freezing of the magnetic moments of particles due to the magnetic interactions between them.
•Fe3O4 nanoparticles modified with Ag were synthesized with thermal decomposition method.•Nanoparticles morphology and size do not depend on Ag concentration.•The Ag effect on nanoparticles magnetic ...properties was studied.•Strong influence of Ag on visible magnetic circular dichroism was revealed and discussed.
Effect of Ag inclusions on magnetic properties and magnetic circular dichroism (MCD) of Fe3O4 nanoparticles (NPs) in the mixed system of Fe3O4 and Ag NPs in dependence on the relative concentration of the components is presented. The samples were synthesized by the thermal decomposition of the mixture of constant concentration of Fe(NO3)3·9H2O and varied concentration of AgNO3. The synthesized powdered samples consisted of Fe3O4 and Ag NPs located very close with each other, and in the most cases the Fe3O4 NPs were bordered with the Ag nanocrystals. The Ag introducing in the samples does not effect, practically, in the Fe3O4 NPs morphology and size distribution. At the same time, Ag NPs in the powdered samples cause a decrease in the Fe3O4 NPs magnetization and shift of the blocking temperature to lower temperatures, both approximately proportional to the Ag concentration. Most significant changes are revealed in the MCD spectra in the energy region of 1.2–2.2 eV. We have discussed the influence of the Ag NPs on the MCD spectra features in terms of the charge-transfer electron transitions.
•We have synthesized high-quality single crystals of the FeTiF6 ‧ 6 H2O.•Magnetic properties investigation of the FeTiF6 ‧ 6 H2O single crystal has shown that this compound is a two-dimensional ...antiferromagnet with a N é еl temperature of TN = 8 K.•We observed giant easy-axis magnetic anisotropy with the value of 7000%.
Study of the magnetic properties of the FeTiF6 ‧ 6 H2O single crystal has shown that this compound is a two-dimensional antiferromagnet with a N é еl temperature of TN = 8 K and its magnetic moment anisotropy attains 7000% at a temperature of T = 4.2 K. The Mössbauer spectroscopy data unambiguously indicate the paramagnetic state of iron cations in the temperature range of 4.2–300 K. However, a sharp change in the difference between the quadrupole doublet linewidths at 10 K has been observed, which is consistent with the temperature of magnetic ordering. It has been suggested that the long-range magnetic order is established in the crystal through the formation of the exchange coupling revealed by the electron spin resonance measurements on the oriented single crystals.
An original magnetometer based on a dc superconducting quantum interferometer is described. The design features of the device are discussed and functional diagrams of its new units are presented. The ...magnetic moment measurement methods that optimize the measurement process are proposed. Examples of the investigations carried out on the samples of different materials are given. The factors affecting the reliability of the magnetic measurements are considered.
•Nickel oxide nanoparticles with an average size of ~8 nm were synthesized by thermal decomposition of nickel oxalate.•New magnetic subsystems are formed in these nanoscale antiferromagnetic ...particles.•Magnetic hysteresis in NiO nanoparticles was studied using both standard quasi-static (VSM) measurements and strong pulsed magnetic fields of up to 130 kOe.•Interaction between magnetic subsystems causes complex character of magnetization switching.
We report on the investigations of a system of 8-nm NiO particles representing antiferromagnetic (AFM) materials, which are weak magnetic in the form of submicron particles, but can be considered to be magnetoactive in the form of nanoparticles due to the formation of the uncompensated magnetic moment in them. The regularities of the behavior of magnetization switching in AFM nanoparticles are established by studying the magnetic hysteresis loops under standard quasi-static conditions and in a quasi-sinusoidal pulsed field of up to 130 kOe with pulse lengths of 4–16 ms. The magnetic hysteresis loops are characterized by the strong fields of the irreversible magnetization behavior, which is especially pronounced upon pulsed field-induced magnetization switching. Under the pulsed field-induced magnetization switching conditions, which are analogous to the dynamic magnetic hysteresis, the coercivity increases with an increase in the maximum applied field H0 and a decrease in the pulse length. This behavior is explained by considering the flipping of magnetic moments of particles in an external ac magnetic field; however, in contrast to the case of single-domain ferro- and ferrimagnetic particles, the external field variation rate dH/dt is not a universal parameter uniquely determining the coercivity. At the dynamic magnetization switching in AFM nanoparticles, the H0 value plays a much more important role. The results obtained are indicative of the complex dynamics of the interaction between magnetic subsystems formed in AFM nanoparticles.
Core-shell FexOy@C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static ...magnetic measurements, and optical magnetic circular dichroism (MCD). FexOy@C NPs synthesized by the pyrolysis process of the mixture of Fe(NO3)3 · 9H2O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO3 in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe3O4 and γ-Fe2O3. Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid FexOy@C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe3O4@C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the FexOy@C-Ag NPs is discussed.
•New oxyborate Co5/3Nb1/3BO4 was grown using the flux technique.•Crystal structure has been resolved in details.•The compound undergoes two magnetic transitions at TN1 = 27 K and TN2 = 14 K.
...Needle-shape single crystals of Co5/3Nb1/3BO4 warwickite were grown using the flux technique. X-ray diffraction measurements have revealed an orthorhombic structure (Sp. Gr. Pbnm) where the octahedral M1 site is occupied by a mixture of Co2+/Nb5+ ions and the M2 site is exclusively filled by Co2+ ions. Using dc magnetization measurements it was established that the new material undergoes two magnetic transitions: an antiferromagnetic transition at TN1 = 27 K and a ferrimagnetic one at TN2 = 14 K, below which a hysteresis cycle opens. Both magnetic transitions are marked by anomalies in the specific heat. High magnetic anisotropy with c-axis as a hard magnetization direction was detected.
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