Relaxation of radiation-induced defects in CuO

The relaxation of radiation-induced defects in the CuO polycrystal and nanoceramic with the particle size d = 15 nm irradiated by an electron dose of F = 5 × 10¹⁸ cm^?2 has been studied. In the irradiated samples, a strong susceptibility increase with decreasing temperature T < 150 K is observed. This increase is due to the formation of ferromagnetic polarons in the antiferromagnetic matrix near the defects. The structural symmetry distortion makes the samples unstable. The time variations of the magnetic properties in the CuO samples prepared by different methods are compared.     

Superconducting Properties of Indium Nanostructured in Pores of Thin Films of SiO<Subscript>2</Subscript> Microspheres

Samples of a superconducting indium nanocomposite based on a thin-film porous dielectric matrix prepared by the Langmuir–Blodgett method are obtained for the first time, and their low-temperature electrophysical and magnetic properties are studied. Films with thickness b ≤ 5 μm were made from silicon dioxide spheres with diameter D = 200 and 250 nm; indium was introduced into the pores of the films from the melt at a pressure of P ≤ 5 kbar. Thus, a three-dimensional weakly ordered structure of indium nanogranules was created in the pores, forming a continuous current-conducting grid. Measurements of the temperature and magnetic field dependences of the resistance and magnetic moment of the samples showed an increase in the critical parameters of the superconductivity state of nanostructured indium (critical temperature T_c ≤ 3.62 K and critical magnetic field H_c at T = 0 K H_c(0) ≤ 1700 Oe) with respect to the massive material (T_c = 3.41 K, H_c(0) = 280 Oe). In the dependence of the resistance on temperature and the magnetic field, a step transition to the superconductivity state associated with the nanocomposite structure was observed. A pronounced hysteresis M(H) is observed in the dependence of the magnetic moment M of the nanocomposite on the magnetic field at T < T_c, caused by the multiply connected structure of the current-conducting indium grid. The results obtained are interpreted taking into account the dimensional dependence of the superconducting characteristics of the nanocomposite.     

Crystal field and magnetization of canted antiferromagnet CoCO<Subscript>3</Subscript>

The magnetization of the canted antiferromagnet CoCO₃ (T _N = 18.1 K) is calculated in the Weiss molecular field approximation taking into account the microscopic state of the Co²⁺ ion in the entire range of temperatures and magnetic fields. The values of T _N, magnetic susceptibility in the basal plane, and ferromagnetic moment were used as parameters. It is shown that the anisotropy of the g factor and of the exchange interaction at low temperatures (T < 30 K) including the magnetic ordering temperature is correctly described in the Abragam-Pryce approximation. At high temperatures, the g factor increases and becomes isotropic, but it cannot be described using the Abragam-Pryce approximation. The reasons for g factor variation and the magnitude of the magnetic moment are discussed.     

Role of the Cluster Structure of Amorphous Fe<Subscript>67</Subscript>Cr<Subscript>18</Subscript>B<Subscript>15</Subscript> Alloy in Magnetism and in the Changing of Electron Scattering Mechanisms under the Influence of Ion (Ar<Superscript>+</Superscript>) Irradiation

The contribution of clusters of different sizes to magnetism and the switching of electron scattering mechanisms in amorphous Fe₆₇Cr₁₈B₁₅ alloy during ion Ar⁺ irradiation is studied. The cluster magnetism is found to be related to the presence of clusters of the following two types: large α-(Fe, Cr) clusters of size D = 150–250 Å and small (D = 40–80 Å) clusters in a random intercluster medium. The generation of small ferromagnetic and antiferromagnetic clusters during ion irradiation leads to the formation of cluster glass, which affects the electrical properties of the alloy and causes a magnetic frustration. The temperature dependence of the barrier height is shown to characterize the magnetic state of the alloy in low fields. On the whole, the temperature dependence of the order parameter is a universal characteristic of the system. The temperature dependence of resistivity of initial alloys in the temperature range 98–300 K (ρ(T) ∝ T²) is determined by electron scattering by quantum defects, and the transition into a ferromagnetic state is revealed when the derivative ?ρ/?T ∝ T is analyzed. The increase in resistivity and the relation ρ ∝ T^1/2 in strongly inhomogeneous samples after irradiation at a dose Φ = 1.5 × 10¹⁸ ions/cm² are caused by weak localization effects, and the transition to a ferromagnetic state becomes obvious when the derivative ?ρ/?T ∝ T^–1/2 is considered. Irradiation by fluence Φ = 3 × 10¹⁸ ions/cm2 induces a giant (twofold) increase in the alloy density, restores the ferromagnetism of large clusters, decreases the resistivity by 37%, and restores the relation ρ(T) ∝ T², which results from the overlapping of the irradiation-induced small clusters when their concentration increases and from an increase in the alloy density. The overlapping of clusters lowers the barrier height and decreases the sensitivity of the alloy to an applied field. The relation ρ(T) ∝ T² is valid for the entire temperature range T = 2–300 K because of the partial screening of the magnetic moments of large clusters by a medium having the properties of cluster glass.     

Frustrated antiferromagnetism in the Sr<Subscript>2</Subscript>YRuO<Subscript>6</Subscript> double perovskite

The spins of Ru⁵⁺ ions in Sr₂YRuO₆ form a face-centered cubic lattice with antiferromagnetic nearest neighbor interaction J≈25 meV. The antiferromagnetic structure of the first type experimentally observed below the Néel temperature T _N =26 K corresponds to four frustrated spins of 12 nearest neighbors. In the Heisenberg model in the spin-wave approximation, the frustrations already cause instability of the antiferromagnetic state at T=0 K. This state is stabilized by weak anisotropy D or exchange interaction I with the next-nearest neighbors. Low D/J～I/J～10^?3 values correspond to the experimental T _N and sublattice magnetic moment values.     

High-pressure magnetic properties and <Emphasis Type="Italic">P-T</Emphasis> phase diagram of iron borate

The high-pressure magnetic states of iron borate ⁵⁷FeBO₃ single-crystal and powder samples have been investigated in diamond anvil cells by nuclear forward scattering (NFS) of synchrotron radiation at different temperatures. In the low-pressure (0 < P < 46 GPa) antiferromagnetic phase, an increase of the Neél temperature from 350 to 595 K induced by pressure was found. At pressures 46–49 GPa, a transition from the antiferromagnetic to a new magnetic state with a weak magnetic moment (magnetic collapse) was discovered. It is attributed to the electronic transition in Fe³⁺ ions from the high-spin 3d⁵ (S = 5/2, ⁶A_1g) to the low-spin (S = 1/2, ²T_2g) state (spin crossover) due to the insulator-semiconductor-type transition with extensive suppression of strong d-d electron correlations. At low temperatures, NFS spectra of the high-pressure phase indicate magnetic correlations in the low-spin system with a magnetic ordering temperature of about 50 K. A tentative magnetic P-T phase diagram of FeBO₃ is proposed. An important feature of this diagram is the presence of two triple points where magnetic and paramagnetic phases of the high-spin and low-spin states coexist.     

Transport properties and ferromagnetism of Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S sulfides

We have studied the resistivity and thermoelectromotive force (thermo emf) in a temperature range of T = 80–1000 K, the magnetic susceptibility and magnetization in a temperature range of T = 4.2–300 K at an external magnetic field of up to 70 kOe, and the structural characteristics of Co _x Mn_1?x S sulfides (0 ≤ x ≤ 0.4). Anomalies in the transport properties of these compounds have been found in the temperature intervals ΔT ₁ = 200–270 K and ΔT ₂ = 530–670 K and at T ₃ ～ T _N. The temperature dependences of the magnetic susceptibility, magnetization, and resistivity, as well as the current-voltage characteristics, exhibit hysteresis. In the domain of magnetic ordering at temperatures below the Néel temperature (T _N), the antiferromagnetic Co _x Mn_1?x S sulfides possess a spontaneous magnetic moment that is explained using a model of the orbital ordering of electrons in the t _2g bands. The influence of the cobalt-ion-induced charge ordering on the transport and magnetic properties of sulfides has been studied. The calculated values of the temperatures corresponding to the maxima of charge susceptibility, which are related to a competition between the on-site Coulomb interaction of holes in various subbands and their weak hybridization, agree well with the experimental data.     

Effect of structural defects on the magnetic properties of the EuBaCo<Subscript>1.90</Subscript>O<Subscript>5.36</Subscript> single crystal

The effect of structural defects in cobalt and oxygen sublattices with the constant average oxidation level 3+ of all cobalt ions on the magnetic properties of the EuBaCo_1.90O_5.36 single crystal has been studied. The magnetic properties of the single crystal and the polycrystalline sample of the corresponding composition are compared in the range T = 200–650 K. The results show that the cobalt-deficient EuBaCo_2–xO_5.5–δ samples demonstrate a three-dimensional XY ferromagnetic ordering of magnetic sublattices. The values of the effective magnetic moment at T > 480 K indicate the existence of the IS and HS states of Co³⁺ ions. The large difference of values of μ_eff of the EuBaCo_1.90O_5.36 single crystal and polycrystal can be due to that the magnetic ion spins lie in plane ab. The magnetic field directed along plane ab substantially influences the magnetic ordering at T < 300 K.     

Reflexivity and Correlation of Magnetic States of Nanostructures in the Nb(70 nm)/Ni<Subscript>0.65</Subscript>Cu<Subscript>0.35</Subscript>(6.5 nm)/Si Ferromagnet–Superconductor Heterostructure

We have studied the Nb(70 nm)/Ni_0.65Cu_0.35(6.5 nm)/Si layered structure in the temperature range T = 1.5–10 K using polarized neutron reflectometry. The correlation of the states of magnetic structures is observed at temperature T = 9 K, which is slightly higher than the superconducting transition temperature T_c = 8.5 K of the structure. At temperature T = 4 K, which is lower than T_c, the effect of reflexivity of magnetic states existing at T = 9 K was observed.     

Intercluster conduction in lightly doped La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La _{1 ? x} Ca _x MnO₃ polycrystalline samples (x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T* ≈ 300 K, which is about 2–3 times higher than their Curie temperature T _C. The temperature dependence of the electrical resistivity measured from T* down to nearly T ≈ T _C is fitted by the relation logρ ～ T ^?1/2, which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T*, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.     

Colossal room-temperature magnetoresistance in thin La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ag<Subscript>y</Subscript>MnO<Subscript>3</Subscript> epitaxial films

The first thin La_1?xAg_yMnO₃ epitaxial films (_y ≤ x) were grown on SrTiO₃ (110) substrates with silver present in the ionized state (Ag⁺) only. The Curie temperatures T_C of the compositions with x = y = 0.05, x = y = 0.1, and x = 0.3 and y = 0.27 crystallizing in the hexagonal structure \(R\bar 3c\) above or close to room temperature. The temperature dependences of electrical resistivity ρ and of magnetoresistance ¦Δρ/ρ/¦ = ¦(ρ_H ? ρ _H = 0)/ρ_H=0¦ pass through maxima near T_C, with the magnetoresistance being negative and reaching colossal values of ～7–20% in a magnetic field H = 8.2 kOe not only at T_C but also at room temperature. The magnetic moment per formula unit as derived from the saturation magnetization at T = 5 K is substantially smaller than expected for complete ferromagnetic ordering. The magnetization in fields of up to 6 kOe depends on the actual sample cooling conditions, and the hysteresis loop of a field-cooled sample is displaced along the H axis by ΔH. The above properties can be accounted for by the fact that the films are in a two-phase magnetic (ferromagnetic-antiferromagnetic) state induced by strong s-d exchange. The maximum value of Δ H was used to calculate the energy of exchange coupling between the ferromagnetic and antiferromagnetic parts of a sample.     

Electrical and Magnetic Properties of Pb and In Nanofilaments in Asbestos near the Superconducting Transition

Bulk nanocomposites based on superconducting metals Pb and In embedded into matrices of natural chrysotile asbestos with the nanotube internal diameter d ~ 6 nm have been fabricated and studied. The low-temperature electrical and magnetic properties of the nanocomposites demonstrate the superconducting transition with the transition critical temperature T_c ≈ (7.18 ± 0.02) K for the Pb–asbestos nanocomposite (this temperature is close to T_{c bulk} = 7.196 K for bulk Pb). The electrical measurements show that In nanofilaments in asbestos have T_c ~ 3.5–3.6 K that is higher than T_{c bulk} = 3.41 K for bulk In. It is shown that the temperature smearing of the superconducting transition in the temperature dependences of the resistance R(T) ΔT ≈ 0.06 K for the Pb–asbestos and ΔT ≈ 1.8 K for the In–asbestos are adequately described by the fluctuation Aslamazov–Larkin and Langer–Ambegaokar theories. The resistive measurements show that the critical magnetic fields of the nanofilaments extrapolated to T = 0 K are H_c(0) ~ 47 kOe for Pb in asbestos and H_c(0) ~ 1.5 kOe for In in asbestos; these values are significantly higher than the values for the bulk materials (H\(H_{\rm{c}}^{\rm{bulk}}\) = 803 Oe for Pb and \(H_{\rm{c}}^{\rm{bulk}}\) = 285 Oe for In). The results of the electrical measurements for Pb?asbestos and In–asbestos agree with the data for the magnetic-field dependences of the magnetic moment in these nanocomposites.     

The influence of magnetic and structural heterogeneity on thermopower,magnetothermopower, electrical resistivity,and magnetoresistivity of Nd<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganite

The thermopower, S, magnetothermopower, ΔS/S, resistivity, ρ, and magnetoresistivity, Δρ/ρ, depending on the temperature T and magnetic field H, have been studied in an Nd_0.5Sr_0.5MnO₃ single crystal consisting of three types of clusters: an antiferromagnetic CE-type with charge-orbital ordering (below the Neel temperature T_NCE ~ 145 K) and an A-type with T_NA ~ 220 K; a ferromagnetic at 234 ≤ T ≤ 252 K, and a ferromagnetic metal phase below the Curie temperature T_C = 248 K. The thermopower was found to be negative, indicating the dominance of the electronic type of conductivity. In the S(T) curves, a sharp minimum is observed in the temperature range of 100 K ≤ T ≤ 133 K, close to T_NCE, where the absolute S value attains 53 μV/K. With a further increase in temperature, the absolute S value decreases rapidly; at 200 K it is equal to 7 μV/K. It then slightly increases, reaching its maximum value of 15 μV/K at a temperature of 254 K, which is close to T_C. The absolute thermopower decreased under the influence of the magnetic field; i.e., a negative magnetothermopower occurs. In {ΔS/S}(T) curves, a sharp minimum is observed at T = 130 K close to T_NCE, where the magnetothermopower reaches a huge value of ~45% at H = 13.23 kOe. A broad minimum in the {ΔS/S}(T) curves is observed near the Curie temperature and its value is also high, viz., ~15% in the maximum measuring magnetic field of 13.23 kOe. The extremely high magnetothermopower values mean that the charge-orbital ordered nanoclusters or ferron type make the main contribution to the thermopower of the entire sample. The behavior of the ρ(T) and {Δρ/ρ}(T) curves is similar to that of the S(T) and {ΔS/S}(T) dependencies, which is in agreement with this conclusion.     

Thermal conductivity of partially graphitized biocarbon obtained by carbonization of medium-density fiberboard in the presence of a Ni-based catalyst

The thermal conductivity k and resistivity ρ of biocarbon matrices, prepared by carbonizing medium-density fiberboard at T _carb = 850 and 1500°C in the presence of a Ni-based catalyst (samples MDF-C( Ni)) and without a catalyst (samples MDF-C), have been measured for the first time in the temperature range of 5–300 K. X-ray diffraction analysis has revealed that the bulk graphite phase arises only at T _carb = 1500°C. It has been shown that the temperature dependences of the thermal conductivity of samples MDFC- 850 and MDF-C-850(Ni) in the range of 80–300 K are to each other and follow the law of k(T) ～ T ^1.65, but the use of the Ni-catalyst leads to an increase in the thermal conductivity by a factor of approximately 1.5, due to the formation of a greater fraction of the nanocrystalline phase in the presence of the Ni-catalyst at T _carb = 850°C. In biocarbon MDF-C-1500 prepared without a catalyst, the dependence is k(T) ～ T ^1.65, and it is controlled by the nanocrystalline phase. In MDF-C-1500(Ni), the bulk graphite phase formed increases the thermal conductivity by a factor of 1.5–2 compared to the thermal conductivity of MDF-C-1500 in the entire temperature range of 5–300 K; k(T = 300 K) reaches the values of ～10 W m^–1 K^–1, characteristic of biocarbon obtained without a catalyst only at high temperatures of T _carb = 2400°C. It has been shown that MDF-C-1500(Ni) in the temperature range of 40?300 K is characterized by the dependence, k(T) ～ T ^1.3, which can be described in terms of the model of partially graphitized biocarbon as a composite of an amorphous matrix with spherical inclusions of the graphite phase.     

Magnetic and transport properties of colossal magnetoresistance compound EuB<Subscript>6</Subscript>

The galvanomagnetic and magnetic properties of EuB₆ single crystal have been measured over wide temperature (1.8–300 K) and magnetic-field (up to 70 kOe) ranges, and the parameters of charge carriers and the characteristics of the magnetic subsystem are estimated in the paramagnetic and ferromagnetic (T < T _C ≈ 13.9 K) phases of this compound with strong electron correlations. In the temperature range T < T* ≈ 80 K, a magnetoresistance hysteresis Δρ(H)/ρ(0) is detected; it reaches a maximum amplitude of about 5% at T ≈ 12 K. The anomalies of charge transport observed in the temperature range T _C < T < T* are shown to be related to the magnetic scattering of charge carriers (m _eff = (15–30)m ₀, where m ₀ is the free-electron mass) that results from a short-range magnetic order appearing upon the formation of ferromagnetic nanoregions (ferrons).     

Magnetization of La- and Ce-doped CaMnO<Subscript>3</Subscript> manganite single crystals in a strong magnetic field

Studies of the magnetization curves of electron-doped single-crystal manganites Ca_{1 ? x} Ln _x MnO₃ (Ln = La³⁺, Ce⁴⁺; x ≤ 0.12) in strong pulsed magnetic fields of up to 350 kOe have revealed a metamagnetic transition in Ca_0.9Ce_0.1MnO₃ in the temperature range 77–190 K. The critical transition fields increase to ～350 kOe with the temperature decreasing to 100 K. The spin polarization is ～50% of the theoretical value. These results are interpreted as due to “melting” of the orbital/charge ordering below the temperature T _OO/CO = 185 K = T _N (of the C type AFM phase); this entails a decrease in the volume of the ordered phase with localized carriers and an increase in the volume of the ferromagnetic phase with delocalized carriers. The temperature and field dependences of the magnetization are used to compare two manganite systems in the region of the two-phase magnetic state.     

Magnetic and structural phase transitions in YMn<Subscript>2</Subscript>O<Subscript>5</Subscript> ferromagnetoelectric crystals induced by a strong magnetic field

Magnetic, magnetoelectric, and magnetoelastic properties of YMn₂O₅ ferromagnetoelectric single-crystals are investigated in strong pulsed magnetic fields of up to 250 kOe and in static magnetic fields of up to 12 kOe. It is found that, in YMn₂O₅ at T < T_N=42 K, a transverse weakly ferromagnetic moment of σ ₀=0.8 G cm³/g exists that is oriented along axis a and is attributed to the magnetoelectric interaction. When a magnetic field is directed along axis b, which is likely to be the axis of antiferromagnetism, a spin-flop transition is observed that is accompanied by jumps in magnetostriction and electric polarization. When a magnetic field is directed along axis a, the temperature of ferroelectric transition shifts from 20 to 25 K at H≈200 kOe. A theoretical analysis of the experimental results is given within phenomenological theory with regard to the fact that a YMn₂O₅ compound belongs to noncollinear antiferromagnetic crystals even in the exchange approximation.     

High-resolution spectroscopy of HoFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal: a study of phase transitions

The transmission spectra of HoFe₃(BO₃) multiferroic single crystals are studied by optical Fourier-transform spectroscopy at temperatures of 1.7–423 K in polarized light in the spectral range 500–10 000 cm^–1 with a resolution up to 0.1 cm^–1. A new first-order structural phase transition close to the second-order transition is recorded at T_c = 360 K by the appearance of a new phonon mode at 976 cm^–1. The reasons for considerable differences in T_c for different samples of holmium ferroborate are discussed. By temperature variations in the spectra of the f–f transitions in the Ho³⁺ ion, we studied two magnetic phase transitions, namely, magnetic ordering into an easy-plane structure as a second-order phase transition at T_N = 39 K and spin reorientation from the ab plane to the c axis as a first-order phase transition at T_SR = 4.7 ± 0.2 K. It is shown that erbium impurity in a concentration of 1 at % decreases the spin-reorientation transition temperature to T_SR = 4.0 K.     

High-pressure effect on the crystal and magnetic structures of the frustrated antiferromagnet YMnO<Subscript>3</Subscript>

The high-pressure (to 5 GPa) effect on the crystal and magnetic structures of the hexagonal manganite YMnO₃ is studied by neutron diffraction in the temperature range 10–295 K. A spin-liquid state due to magnetic frustration on the triangular lattice formed by Mn ions is observed in this compound at normal pressure and T > T_N = 70 K, and an ordered triangular antiferromagnetic state with the symmetry of the irreducible representation Γ₁ arises at T < T_N. The high-pressure effect leads to a spin reorientation of Mn magnetic moments and a change in the symmetry of the antiferromagnetic structure, which can be described by a combination of the irreducible representations Γ₁ and Γ₂. In addition, it is observed that the ordered magnetic moment of Mn ions decreases from 3.27 μ_B (5 GPa) to 1.52 μ_B (5 GPa) at T = 10 K and diffuse scattering is enhanced at temperatures close to T_N. These effects can be explained within the model of the coexistence of the ordered antiferromagnetic phase and the spin-liquid state, whose volume fraction increases with pressure due to the enhancement of frustration effects.