The interplay of the magnetically dead layer and structural defects in interacting ultrafine nickel ferrite (NiFe2O4) nanoparticles ( = 4 nm) have been investigated using transmission electron microscopy, X-ray diffraction, Fe-57 M & ouml;ssbauer spectrometry, and static (dc) magnetization and dynamic (ac) susceptibility measurements. According to the magnetic measurement data, there are three magnetic subsystems in NiFe2O4 nanoparticles. The first subsystem with the lowest blocking (spin freezing) temperature (T-S = 8 K) involves atomic magnetic moments of magnetically disordered particles with a size of d < 4 nm. The other two subsystems are formed by magnetic moments of the cores of nanoparticles more than 4 nm in size and by correlated surface spins in nanoparticle clusters. The magnetic moments of the ferrimagnetically ordered cores are blocked at a higher temperature (similar to 40 K). It has been shown that the most significant contribution to the energy dissipation is made upon blocking of the correlated nanoparticle surface spins from the magnetically dead layer on the nanoparticle surface. The magnetic measurements have shown that the thickness of this layer is d(md) approximate to 1 nm for a particle with a diameter of < d> = 4 nm. At the same time, the Fe-57 Mossbauer spectrometry study has revealed a structural disorder penetrating to a depth of up to d(cd) approximate to 0.6 nm in a particle with a diameter of < d> = 4 nm. This evidence for a faster violation of the magnetic order than in the case of the crystal order upon moving away from the center of a particle to its periphery.

The magnetic properties of rare-earth borates evoke keen interest, since the coexistence of two interconnected magnetic subsystems, 3d and 4 f ions, in the crystals of this family causes a diversity of magnetic structures and phase transitions between them. A comparative study of the effect of synthesis conditions on the structure and magnetic properties of the terbium chromoborate TbCr3(BO3)(4) single crystals grown from bismuth molybdate and lithium tungstate solvents has been carried out. In the single crystals with the monoclinic symmetry synthesized from the bismuth molybdate solvent, partial substitution of Bi(3+)ions for Tb3+ ions occurs, which entails a change in the magnetic anisotropy of the crystal and the formation of an angular magnetic structure, in which, unlike the crystals grown from another solvent, the Ising axis of Tb3+ ions deviates from the & Scy;(3) pseudo-axis direction by an angle of similar to 40 degrees. In the case of the lithium tungstate solvent, single crystals with the trigonal and monoclinic symmetry are formed and their temperature and field dependences of the magnetization coincide. At a N & eacute;el temperature of T-N1 = 9.2 K, the antiferromagnetic ordering with the magnetic moments lying in the basal plane is established in the Cr3+ ion subsystem. Due to the weakness of the exchange interaction between Cr3+ and Tb3+ ions, the antiferromagnetic order in the terbium subsystem of all the investigated TbCr3(BO3)(4) crystals is formed at a lower temperature: T-N2 = 5.5 K. The existence of two temperatures of the successive ordering of the magnetic subsystems of 3d and rare-earth ions has been found for the first time in the crystals of the huntite family and confirmed by the results of the magnetization and specific heat investigations. The lower ordering temperature of the terbium subsystem, which has a strong easy-axis magnetic anisotropy, also explains the nature of the easy plane -> easy axis orientational transition in the chromium subsystem discovered previously at a temperature of similar to 5 K.

The phase formation of terbium chromoborate TbCr3(BO3)4 in the bismuth trimolybdate and lithium tungstate melt-solutions has been studied. The absence of the terbium chromoborate trigonal phase in the bismuth trimolybdate-based system at all component ratios has been shown. The component ratio in the lithium tungstate-based system has been found at which the TbCr3(BO3)4 trigonal crystals are formed at temperatures above 1100 degrees C; below this temperature, the monoclinic phase dominates. The structural and magnetic properties of the grown crystals have been studied. It has been established that the trigonal and monoclinic TbCr3(BO3)4 crystals synthesized from the lithium tungstate-based solvent exhibit identical magnetic properties. At the same time, a significant difference of the magnetic properties of the single crystals synthesized from the bismuth molybdate melt-solution has been observed. This difference has been attributed to the effect of Bi3+ ions that partially replace Tb3+ ions.

Orthorhombic PbMnBO4 is a rare example of an insulating ferromagnet in which the magnetic properties, in particular the strong magnetic anisotropy, are determined by the Mn3+ Jahn-Teller ion. Here, we report on the detailed investigation of magnetic excitations in the subterahertz and terahertz frequency ranges, as well as lattice excitations in the terahertz range. Using polarized azimuthally resolved Raman spectroscopy, four branches of magnetic excitations were observed and assigned to one acoustic and three optical one-magnon modes. The coupling between ferromagnetic ordering at T-c = 31 K and the Jahn-Teller distortions of [MnO6] octahedra due to the spin-phonon interaction was directly observed for some specific phonon modes involved in these distortions. A spin-wave Hamiltonian was developed that allowed us to determine a unique set of exchange and effective anisotropy parameters. The intensity ratio of the anti-Stokes to Stokes acoustic magnon peaks is found to be anomalous and directly indicates the involvement of the magneto-optical parameters in the magnetic Raman scattering process.

Ferrihydrite is characterized by the antiferromagnetic ordering and, in ferrihydrite nanoparticles, as in nanoparticles of any antiferromagnetic material, an uncompensated magnetic moment is formed. We report on the investigations of ferrihydrite powder systems with an average particle size of ∼ 2.5 nm obtained (i) as a product of the vital activity of bacteria (sample FH-bact) and (ii) by a chemical method (sample FH-chem). In the first approximation, these samples can be considered to be identical. However, in sample FH-chem, particles contact directly, while in sample FH-bact, they have organic shells; therefore, the interparticle magnetic interactions in these samples have different degrees. The main goal of this work has been to establish the effects of the interparticle magnetic interactions and individual characteristics of ferrihydrite nanoparticles on ferromagnetic resonance (FMR) spectra. The FMR spectra have been measured at different (9.4–75 GHz) frequencies in a wide temperature range. It has been found that, at low temperatures, the field-frequency dependence ν(HR) of the investigated systems has a gap ν/γ = HR + HA, where HR is the resonance field and HA is the induced anisotropy, which decreases with increasing temperature. To estimate a degree of the effect of interparticle interactions on the results obtained and to correctly determine the temperature range of the superparamagnetic (or blocked) state, the static magnetic measurement and Mössbauer spectroscopy data have been obtained and analyzed. It has been shown that the most striking feature of the FMR spectra - a gap in the field-frequency dependences - is a manifestation of individual characteristics of ferrihydrite nanoparticles. The induced anisotropy is caused by freezing of a subsystem of surface spins and its coupling with the particle core, which is observed in both samples at a temperature of ∼80 K. The temperature range (below 80 K) in which the gap exists corresponds to the blocked state in the FMR technique. In sample FH-bact, the ratio between the FMR parameters HA and linewidth ΔH obeys the standard expression HA ∼ (ΔH)3. In sample FH-chem, however, the interparticle magnetic interactions dramatically affect the behavior of parameters of the FMR spectra, which change nonmonotonically upon temperature variation. This fact is attributed to the collective freezing of the magnetic moments of particles under the conditions of sufficiently strong interactions, which follows from the temperature dependence of the particle magnetic moment relaxation time determined from the Mössbauer spectroscopy and static magnetometry data obtained in weak magnetic fields.

In this work, the magnetodielectric response have been studied in a series of single crystals of Fe2-xMnxGe2Pb2O9 with substitution x from 0 to 0.43. The research results showed that an increase in x leads to a change in the orientation of the antiferromagnetism vector in a zero magnetic field. The application of an external magnetic field leads to spin-flop and spin-reorientation magnetic transitions, which are accompanied by anomalies in the behavior of the permittivity.When Fe3+ ions are replaced by Mn3+ ions, the response of the magnetic subsystem to an external magnetic field changes, which is naturally reflected in the change in the magnetodielectric response. The permittivity changes stepwise when the magnetic subsystem passes into a state with a nonzero vector of the weakly ferromagnetic moment, or when its magnitude and orientation change due to a spin-flop transition. The latter indicates that a sharp change in polarizability is a consequence of a change in the contribution of the exchange energy, as well as the contribution of the Dzyaloshinskii-Moriya term, due to a change in the skew angle between two interacting magnetic moments.

Single crystals of the Pb2Fe2_xMnxGe2O9 (x = 0.16) antiferromagnet have been grown. Using the specific heat measurements, a Ne acute accent el temperature of T-N = (42.0 +/- 0.5) K for the synthesized crystals has been found. It has been shown using the magnetic measurements that, due to the competition between the magnetoanisotropic contributions of the iron and manganese subsystems in the crystals, near a temperature of T-c = 22 K, a spontaneous spin-reorientation transition occurs, the tem-perature of which in an applied magnetic field changes with the field value and orientation relative to the rhombic axes of the crystal. Based on the analysis of the temperature and field dependences of the magnetization obtained at different orientations of the magnetic field, it has been established that, below T-c, an inclined magnetic structure is formed in the crystal. The antiferromagnetic vector of the inclined structure rotates smoothly in the rhombic bc plane with increasing tem-perature from a direction close to the b axis at T = 4.2 K and tends to the rhombic c axis at T = T-c. The rotation of the antiferromagnetic vector occurs also at fixed temperatures T < T-c with increasing magnetic field. In the temperature range of T-c < T < T-N, the antiferromagnetic vector is oriented along the rhombic c axis. Magnetic phase diagrams of states have been built for different magnetic field orientations relative to the rhombic axes of the crystal. The richest phase diagram is shown to correspond to the orientation H||c and contains, along with the above-listed states, one more inclined phase, in which the antiferromagnetic vector rotates toward the rhombic a axis direction with a change in temperature or magnetic field.

The spin dynamics, magnetic structures and magnetic anisotropy of single crystals PrxY1-xFe3(BO3)(4) have been studied using antiferromagnetic resonance (AFMR) in a wide range of frequencies, magnetic fields, and temperatures. The frequency-field dependences of AFMR for the crystals with x = 0.25 and 0.45 are characteristic of antiferromagnets with the easy plane (EP) anisotropy. The crystals with x = 0.75 and 1.0 exhibit frequency-field dependences that are typical for antiferromagnets with the easy axis (EA) anisotropy. In these crystals, a significant decrease in the effective anisotropy fields of praseodymium upon the transition to the spin-flop state has been found. It is shown that this is the main reason for the large lability intervals, within which the regions of coexistence of the collinear and spin-flop states overlap. In the crystal with x = 0.67, the magnetic field applied along the trigonal axis of the crystal leads to the spin reorientation transition from the EA to the EP state. A magnetic phase diagram of the states on the plane "magnetic field - temperature" is built. In this crystal, the effective anisotropy field of praseodymium also decreases upon the transition to the field-induced EP state. Diamagnetic dilution of the praseodymium subsystem leads to the contribution of this subsystem to the total anisotropy field depending almost linearly on the praseodymium concentration. (C) 2022 Elsevier B.V. All rights reserved.

The Pb2Fe2-xMnxGe2O9 (x = 0.43) orthorhombic antiferromagnet single crystals have been synthesized by a modified pseudo-flux technique and their magnetic and magnetodielectric properties have been investigated. It has been established that partial substitution of highly anisotropic Mn3+ ions for iron ones significantly affects the magnetic structure of the crystal. Under magnetization of the crystal along the rhombic b and c axes, magnetization jumps have been detected, which are indicative of the occurrence of orientational transitions identified as first-order ones. No weak ferromagnetism characteristic of the pure crystal in the rhombic a axis direction has been detected. The field dependences of the magnetization for the pure and Mn-doped crystals have been analyzed using the thermodynamic potential that takes into account the crystal symmetry. It has been shown that, in the Mn-substituted crystal, the antiferromagnetic vector in the ground state is parallel to the rhombic b axis; in this state, weak ferromagnetism has not been observed. Under magnetization along the b axis, a conventional spin-flop transition occurs. The orientational transition under magnetization along the c axis has been attributed to the reorientation of the antiferromagnetic vector relative to the a axis with the simultaneous occurrence of a weak ferromagnetic moment along the c axis. Magnetic phase diagrams of the Mn-doped crystal for the magnetic fields H||b and H||c have been built. In the Mn-doped crystal, at E||c and H||c, the orientational transition-induced magnetodielectric response jump has been detected, which is higher than the jumps observed for the undoped crystal by a factor of 3. The magnetodielectric properties of the pure and Mn-doped crystals have been analyzed using their magnetic phase diagrams.

Ferrihydrite nanoparticles (2–3 nm in size), which are products of the vital activity of microorganisms, are studied by the ferromagnetic resonance method. The "core" of ferrihydrite particles is ordered antiferromagnetically, and the presence of defects leads to the appearance of an uncompensated magnetic moment in nanoparticles and the characteristic superparamagnetic behavior. It is established from the ferromagnetic resonance data that the field dependence of the frequency is described by the expression 2πν/γ = HR + H(=0)A(1 − T/T*), where γ is the gyromagnetic ratio, HR is the resonance field, HA ≈ 7 kOe, and T* ≈ 50 K. The induced anisotropy HA is due to the spin-glass state of the near-surface regions.

Near the Curie temperature T-C = 30.3 K, the temperature dependences of the magnetization and heat capacity of a single-crystal ferromagnet PbMnBO4 in the magnetic fields of 1, 3, 10 and 30 kOe are studied. In the strong magnetic fields, both the magnetic contribution to the specific heat and the nonlinearity of the field dependences of the magnetization are maintained up to the temperatures exceeding T-C more than twice. It is assumed that in PbMnBO(4 )the difference between T-C, the paramagnetic Curie temperature theta = 49 K and the broad temperature region above T-C where the magnetic contribution to the specific heat is significant is due to the quasi-one-dimensional character of the magnetic structure of this ferromagnet. Using both the estimation of TC from the Ginzburg-Landau field theory and the theta value, the total exchange interaction parameters 2J approximate to 40.4 K (intrachain) and z'J' approximate to 8.8 K (interchain) are determined, with z' = 4 being the number of neighboring chains. The estimation shows that the Ginzburg-Landau field theory describing the quasi-one-dimensional behavior of PbMnBO4 is well applicable in the temperature range from to T = S(2)J approximate to 80 K. Above this temperature, the mean field approximation with the exchange parameter lambda(theta) based on the paramagnetic Curie temperature theta describes well the experimental temperature dependences of the magnetization in the strong magnetic field and the specific heat is determined by the lattice contribution.

Orthorhombic Pb2Fe2Ge2O9 antiferromagnetic single crystals have been synthesized by a modified pseudo-flux technique and their magnetic, thermodynamic, and magnetodielectric properties have been investigated. It has been found that, below the Ned temperature (45.2 K), iron moments are arranged in a canted antiferromagnetic structure with a weak ferromagnetic moment parallel to the a axis. According to the specific heat measurement data, the T-N value remains invariable in applied magnetic fields of up to 50 kOe within the experimental accuracy. The magnetic entropy in the investigated crystals attains 2Rln(2S + 1) right above T-N, which is indicative of a purely magnetic nature of the transition. It has been shown that the weak ferromagnetic moment is induced by the interplay between the single-ion anisotropy and antisymmetric Dzyaloshinskii-Moriya exchange interaction, with the latter contribution being dominant. It has been established from the angular dependences of the magnetization in three orthorhombic planes that the symmetries of the magnetic and crystal structure are identical. The magnetodielectric properties of the Pb2Fe2Ge2O9 single crystals have been studied at different mutual orientations of the electric and magnetic fields. The most prominent anomalies have been observed in the vicinity of the spin-flop transition in a magnetic field applied along the c axis.

The PbMn1-xFexBO4 (x approximate to 0.1) orthoborate single crystals have been grown for the first time by spontaneous crystallization and their magnetic and resonance properties and specific heat have been examined. It has been established that partial substitution of iron ions for manganese ones leads to an increase in the Curie temperature to 34.2 K from its value of 30.3 K in the unsubstituted crystal, enhances the magnetic anisotropy, and reduces the saturation magnetization. The magnetization drop is explained in the framework of the model of a ferrimagnetic-like structure, in which the magnetic moments of iron and manganese ions form ferromagnetic subsystems coupled by the antiferromagnetic exchange. It has been found that under magnetization along the rhombic b axis the magnetic moments switch stepwise to the magnetic field direction in a certain critical field. The spin-reorientation transition is the first-order one. This feature of the crystal magnetization does not allow the experimental ferromagnetic resonance frequency-field dependence for the rhombic b axis to be described using the calculation for a simple rhombic ferromagnet. It has been established that the increase in the magnetic anisotropy of the crystal upon substitution leads to an increase in the energy gap in the ferromagnetic resonance spectrum to 121.5 GHz at T = 4.2 K.

The magnetic phase transitions and the phase diagrams of the CuB2O4 multiferroic are studied. Phase diagrams of copper metaborate in a magnetic field directed along the [100] and [001] axes are plotted using the results of measuring the magnetic moment. Evidences for the existence of polycritical points are obtained.

Magnetic resonance characteristics of mixed chalcospinel nanocrystals CuCr2SxSe4-x (x = 0 and 2) and CoxCu1-xCr2S4 (x = 0.1 and 0.2) have been investigated. It has been established based on TEM, SEM and resonance data that all the samples contain both blocks with sizes from 1 to 50 m of compacted nanosized crystallites and individual nanoparticles with sizes from 10 to 30 nm. The studies provide evidence of strong interparticle interaction in all the samples leading to high values of the blocking temperature. Magnetic dipolar field arise in the boundary regions of interacting adjacent nanocrystals below the blocking temperature. This results in inhomogeneous broadening of the magnetic resonance spectrum along with appearance of additional absorption lines. With increase in magnetic anisotropy at low temperatures, a shift of the resonance field along with line broadening are observed for all the studied compounds due to freezing of the moments in the nanoparticles, both in the individual and compacted ones. A gapped characteristic of the resonance spectrum is established below the freezing temperature T-fr, with the energy gap defined by the averaged magnetic anisotropy . Anionic substitution of sulfur by selenium results in a decrease in the magnetic anisotropy. In contrast, cationic substitution of copper by cobalt increases the magnetic anisotropy due to a strong contribution from the latter ion. (C) 2017 Elsevier B.V. All rights reserved.