•Pb2Fe2Ge2O9 single crystals were grown using a pseudo-flux technique.•Weak ferromagnetism arises due to single-ion anisotropy and Dzyaloshinskii–Moria mechanisms.•The principal magnetic anisotropy ...directions coincide with the orthorhombic axes.•Most prominent magnetodielectric response was observed in Pb2Fe2Ge2O9 at the spin-flop transition.
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 Neel 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 TN 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 TN, 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.
•Magnetization of ferrihydrite nanoparticles is studied by pulse field magnetometer in fields up to 250 kOe.•The contributions of magnetic subsystems are correctly deduced from M(H) data.•The ...quantitative model of magnetic state of antiferromagnetically ordered ferrihydrite nanoparticle is proposed.
Contributions of different magnetic subsystems formed in the systems of synthetic ferrihydrite nanoparticles (characterized previously) with an average size of < d>≈2.7 nm coated with polysaccharide arabinogalactan in different degrees have been separated by measuring the dependences of their magnetization M on magnetic field H of up to 250kOe on vibrating sample and pulsed magnetometers. The use of a wide measuring magnetic field range has been dictated by the ambiguity in identifying a linear M(H) portion for such antiferromagnetic nanoparticle systems within the conventional field range of 60–90 kOe. The thorough analysis of the magnetization curves in the temperature range of 100–250 K has allowed the verification of the contributions of (i) uncompensated magnetic moments µun in the superparamagnetic subsystem, (ii) the subsystem of surface spins with the paramagnetic behavior, and (iii) the antiferromagnetic susceptibility of the antiferromagnetically ordered ferrihydrite particle core. As a result, a model of the magnetic state of ferrihydrite nanoparticles has been proposed and the numbers of spins corresponding to magnetic subsystems (i)–(iii) have been estimated. An average magnetic moment μunof ∼ 145μB (μB is the Bohr magneton) per particle corresponds approximately to 30 decompensated spins of iron atoms in a particle (about 3 % of all iron atoms), which, according to the Néel’s hypothesis μun∼ 3/2, are localized both on the surface and in the bulk of an antiferromagnetically ordered particle. The fraction of free (paramagnetic) spins is minimal in the sample without arabinogalactan coating of the nanoparticle surface (7 %) and is attained 20 % of all iron atoms in the sample with the highest degree of spatial separation of particles. According to this estimation, paramagnetic spins are located mainly on the edges and protruding areas of particles. Most magnetic moments of iron atoms are ordered antiferromagnetically and the corresponding magnetic susceptibility of this subsystem behaves as in an antiferromagnet with the randomly distributed crystallographic axes, i.e., increases with temperature.
The relaxation of the remanent magnetization of antiferromagnetically ordered ferrihydrite nanoparticles at the exchange bias effect implemented in these systems has been investigated. The ...magnetization relaxation depends logarithmically on time, which is typical of the thermally activated hoppings of particle magnetic moments through the potential barriers caused by the magnetic anisotropy. The barrier energy obtained by processing of the remanent magnetization relaxation data under the field cooling conditions significantly exceeds the barrier energy under standard (zero field cooling) conditions. The observed difference points out the possibility of using the remanent magnetization relaxation to analyze the mechanisms responsible for the exchange bias effect in antiferromagnetic nanoparticles and measure the parameters of the exchange coupling of magnetic subsystems in such objects.
We report on the effect of interparticle magnetic interactions in an ensemble of superparamagnetic magnetite particles with an average size of ~8.4nm dispersed in the diamagnetic matrix on the ...blocking of this ensemble in external magnetic field. The two limit cases are investigated: the case of strongly interacting particles, when the value of magnetic dipole-dipole interaction between particles is comparable with the energy of other interactions in the ensemble (the interparticle distance is similar to the nanoparticle diameter) and the case of almost noninteracting particles distant from each other by about ten particle diameters. We demonstrate that the experimental dependence of the blocking temperature on external field is described well within the model 1, in which the density of particles in a nonmagnetic medium is taken into account and the correlation value depends on external magnetic field. The model for describing the magnetic properties of a disperse nanoparticle ensemble is proposed, which makes corrections related to the particle size and mean dipole-dipole interaction energy for the anisotropy constant. The surface magnetic anisotropy of Fe3O4 particles and parameters of the interparticle coupling are estimated.
•The interparticle interactions affect superparamagnetic behavior of nanoparticles;•Effective magnetic anisotropy constant depends on interparticle interaction energy;•Modified random anisotropy model was used for 3-D dispersed nanoparticles ensemble.
The electrical properties of a system of nanogranular amorphous Fe–SiO films with a SiO concentration between 0 and 92 vol % have been investigated. The samples with a low SiO content are ...characterized by the metal-type conductivity. With an increase in the dielectric content
x
in the films, the concentration transition from the metal to tunneling conductivity occurs at
x
≈ 0.6. At the same concentration, the ferromagnet–superparamagnet transition is observed, which was previously investigated by the magnetic method. The temperature dependences of the electrical resistivity ρ(
T
) for the compositions corresponding to the dielectric region obey the law ρ(
T
) ~ exp(2(
C
/
kT
)
1/2
), which is typical of the tunneling conductivity. The estimation of the metal grain sizes from the tunneling activation energy
C
has shown good agreement with the sizes obtained previously by analyzing the magnetic properties. In the dielectric region of the compositions, the giant magnetoresistive effect attaining 25% at low temperatures has been obtained.
•Due to competition between magnetic anisotropies of Fe3+ and Mn3+ spontaneous orientational transition occurs at Tc = 22 K.•Below Tc, antiferromagnetic vector rotates in rhombic bc plane under ...increasing temperature tending to c axis at T = Tc.•Its rotation to the c axis occurs also at fixed temperature T < Tc with increasing magnetic field.•For H||c orientation one more inclined phase is formed with antiferromagnetic vector rotates toward the rhombic a axis.
Single crystals of the Pb2Fe2−xMnxGe2O9 (x = 0.16) antiferromagnet have been grown. Using the specific heat measurements, a Néel temperature of TN = (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 Tc = 22 K, a spontaneous spin-reorientation transition occurs, the temperature 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 Tc, 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 temperature from a direction close to the b axis at T = 4.2 K and tends to the rhombic c axis at T = Tc. The rotation of the antiferromagnetic vector occurs also at fixed temperatures T < Tc with increasing magnetic field. In the temperature range of Tc < T < TN, 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.
We present a brief review of investigations and analysis of magnetization curves
M
(
H
) for NiO and ferrihydrite antiferromagnetic nanoparticles in external fields up to 250 kOe. For correct ...interpretation of magnetic properties of systems of antiferromagnetic nanoparticles, it is important to take into account the segment of
M
(
H
) dependences, which corresponds to high fields (exceeding 100 kOe). We analyze the regularities in the formation of additional magnetic subsystems in antiferromagnetically ordered nanoparticles due to the influence of size effects. These additional subsystems (the ferromagnetic subsystem associated with uncompensated magnetic moment and the subsystem of surface free spins) are estimated quantitatively. It is shown that antiferromagnetic nanoparticles with a size of 5 nm acquire the properties of “nanomagnets,” which are not inferior to those for iron-oxide ferromagnetic nanoparticles of the same size.
The magnetic-field dependence of the superparamagnetic-blocking temperature
T
B
of systems of antiferromagnetically ordered ferrihydrite nanoparticles has been investigated and analyzed. We studied ...two powder systems of nanoparticles: particles of “biogenic” ferrihydrite (with an average size of 2.7 nm), released as a result of vital functions of bacteria and coated with a thin organic shell, and particles of biogenic ferrihydrite subjected to low-temperature annealing, which cause an increase in the average particle size (to 3.8 nm) and burning out of the organic shell. The character of the temperature dependences of magnetization, measured after cooling in a weak field, as well as the shape of the obtained dependences
T
B
(
H
), demonstrate peculiar features, indicating the influence of magnetic interparticle interactions. A detailed analysis of the dependences
T
B
(
H
) within the random magnetic anisotropy model made it possible to estimate quantitatively the intensity of magnetic particle–particle interactions and determine the magnetic anisotropy constants of individual ferrihydrite particles.
•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
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