N. V. Mushnikov

Kinetics of interaction of Mg-based mechanically activated alloys with hydrogen

Parameters of interaction of hydrogen with magnesium powders and structure of powder magnesium alloys alloyed with different metals and oxides (such as Fe, Ni, Al, Cu, Ti, Pd, NiPd, V2O5, and VH2) prepared by mechanical activation under either argon or hydrogen atmosphere in a vibration mill have been studied. The mechanically activated alloys absorb to 7 wt% hydrogen at 300°C for 20 min. For most of the additions used, the effect of the grain size and type of addition on the rate of hydrogen absorption was found to manifest itself only at the stage of the formation of the MgH2 phase upon mechanical activation in the hydrogen atmosphere; virtually no effect is observed upon subsequent hydrogenation. The temperature of the hydrogen desorption also depends only slightly on the addition kind. The increase in the hydrogenation rate of the Mg-based alloys resulting from the mechanical activation was shown to be due to the formation of a specific structural state of the particle surface, which exhibits a high catalytic activity for the hydrogen sorption. A study of the mechanically activated alloys by proton nuclear magnetic resonance showed a substantial increase in the rate of proton spin-lattice relaxation as compared to that observed for MgH2 produced by direct hydrogenation. This can be due to the interaction of protons with paramagnetic centers formed at the imperfect surface of mechanically activated Mg particles.

High-field magnetization and magnetic structure of Tb3Co

The measurements of the magnetization in high steady and pulsed fields together with neutron diffraction measurements on a powder sample and on a single crystal have been performed to study the magnetic state of the Tb3Co compound. It has been shown that the modulated antiferromagnetic structure which exists in Tb3Co below TN = 82 K transforms to the incommensurate magnetic structure with a strong ferromagnetic component along the c-axis with further cooling below the critical temperature Tt≈72 K. The phase transition from the high-temperature to the low-temperature magnetic state at Tt is of first order. The incommensurability of the low-temperature magnetic structure of Tb3Co is attributed to the non-Kramers character of the Tb3+ ion in combination with competition between the indirect exchange interaction and the low-symmetry crystal electric field.

Magnetic properties and structure of nonstoichiometric rare-earth transition-metal intermetallic compounds TbNi2Mnx (0 ≤ x ≤ 1.5)

Crystal structure, magnetization, coercive force, magnetic susceptibility, and anisotropic magnetostriction of nonstoichiometric rare-earth transition-metal intermetallic compounds TbNi2Mnx (0 ≤ x ≤ 1.5) have been studied. The samples with x ≤ 1 have an fcc structure, whereas TbNi2Mn1.25 has a rhombohedral structure of the PuNi3 type. It has been found that the magnetic ordering temperature increases sharply when manganese is added. As the Mn concentration grows, the magnetization and the magnetostriction decrease monotonically, while the coercive force increases. The experimental data obtained have been interpreted on the assumption that a partial substitution of manganese for terbium in TbNi2Mnx leads to local distortions of the crystal field acting on Tb ions, to the appearance of a local uniaxial random anisotropy, and to the formation of a noncollinear magnetic structure in the terbium sublattice.

Irreversible field-induced magnetic phase transitions and properties of Ho3Co

The results of magnetic susceptibility, magnetization, electrical resistivity and specific heat measurements performed on Ho3Co single crystals show that this compound exhibits two different antiferromagnetic structures: AFII at 8 K<T< 22 K and AFI below Tt≈8 K. Below the Neel temperature TN = 22 K the application of a magnetic field along the main crystallographic directions induces magnetic phase transitions which are accompanied by giant magnetoresistance. At T<Tt the field-induced phase transitions along the c- and b-axes are found to be irreversible, and a small ferromagnetic component is observed along the a-axis. These peculiarities are associated with the non-Kramers character of the Ho ion and with the presence of a complex incommensurate magnetic structure of Ho3Co below TN. The temperature coefficient of the electrical resistivity for Ho3Co above TN over a wide temperature range is found to differ from that observed for other R3Co compounds. Such a behaviour is attributed to the presence of an additional contribution to the conduction electron scattering by spin fluctuations induced by f–d exchange in the itinerant d-electron subsystem. The value of this extra contribution and its temperature range is suggested to depend on the spin value of the R ion. The excess of the effective magnetic moment per R ion, which is observed in Ho3Co and in other R3M type compounds, is also attributed to spin fluctuations induced by f–d exchange.

Magnetic state and properties of the Fe0.5TiSe2 intercalation compound

The Fe0.5TiSe2 compound with a monoclinic crystal structure has been prepared by intercalation of Fe atoms between Se-Ti-Se sandwiches in the layered structure of TiSe2. The crystal and magnetic structures, electrical resistivity, and magnetization of the Fe0.5TiSe2 compound have been investigated. According to the neutron diffraction data, the Fe0.5TiSe2 compound has a tilted antiferromagnetic structure at temperatures below the Néel temperature of 135 K, in which the magnetic moments of Fe atoms are antiferromagnetically ordered inside layers and located at an angle of approximately 74.4° with respect to the layer plane. The magnetic moment of Fe atoms is equal to (2.98 ± 0.05)μB. The antiferromagnetic ordering is accompanied by anisotropic spontaneous magnetostrictive distortions of the crystal lattice, which is associated with the spin-orbit interaction and the effect of the crystal field.

Magnetic properties and structure of nanocrystalline FINEMET alloys with various iron contents

The effect of the composition and annealing temperature on the structure and magnetic properties of soft magnetic nanocrystalline Fe-Cu-Nb-Mo-Si-B alloys has been studied. An increase in the iron content compared to that in the traditional FINEMET alloy is shown to allow one to increase the magnetic induction by 18% at a coercive force of no less than 6 A/m. It has been found that, along with the amorphous phase, rapidly quenched ribbons of alloys enriched in Fe contain crystalline α-Fe-based phase precipitates, the (100) crystallographic directions of which are perpendicular to the ribbon plane. Thermomagnetic analysis and differential scanning calorimetry were used to determine the temperatures of structural and magnetic phase transformations of the alloys with different iron contents. It was found that the separation of amorphous phase into areas of different compositions precedes the precipitation of nano-sized soft magnetic Fe-Si phase grains in the rapidly quenched iron-enriched ribbons.

High-strength magnetically hard Fe-Cr-Co-Based alloys with reduced content of chromium and cobalt

Structure and magnetic and mechanical properties of precipitation-hardening Fe-Cr-Co-W-Ga alloys with reduced Cr and Co contents have been studied. The compositions studied go beyond the limits of the miscibility gap in the phase diagram for conventional Fe-Cr-Co alloys. It has been established that after cold plastic deformation and low-temperature annealing the alloys are characterized by high values of mechanical and magnetic characteristics, which are significantly higher than those of known analogs. It is demonstrated that the treatment suggested leads to the decomposition of homogeneous solid solution based on α-Fe with the precipitation of disperse particles of a tungsten-rich phase, which promotes strengthening of alloys.

Magnetism of compounds with a layered crystal structure

This review presents the results of investigations of the crystal structure, magnetic ordering, magnetic anisotropy, and magnetic phase transformations in compounds of the RT2Z2 and RT6Z6 type (R is a rare-earth metal; T is a transition metal; and Z = Si, Ge, or Sn) and also in intercalated dichalcogenides of transition metals, such as MxTX2 and RxTX2. A specific feature of these compounds is a layered character of their crystal structures, in which the atoms that have a magnetic moment are located in separate crystallographic layers. Inside the layers of magnetic atoms and between the layers, there act different (in energy) exchange interactions of different type, which leads to a variety of magnetic structures and magnetic phase transitions in these compounds and makes them suitable objects for the investigation of physical phenomena inherent in quasi-two-dimensional magnetic systems.

Positive magnetoresistance and large magnetostriction at first-order antiferro–ferromagnetic phase transitions in RMn2Si2 compounds

The magnetostriction and magnetoresistance associated with the field-induced and spontaneous first-order antiferro–ferromagnetic (AF–F) phase transitions have been studied for quasi-single-crystalline samples of La0.25Sm0.75Mn2Si2, La0.25Y0.75Mn2Si2 and La0.27Y0.73Mn2Si2 compounds with natural layered ThCr2Si2-type structure. It was found that both the spontaneous and field-induced AF–F transitions are accompanied by a large volume magnetostriction ΔV/V≈2 × 10−3 and anisotropic linear changes of the lattice parameters Δa/a≈1.6 × 10−3, Δc/c≈−0.75 × 10−3. The field-induced AF–F magnetic phase transition has been observed in magnetic fields applied both along the c-axis and in the basal plane, and the magnetostriction value is virtually independent of the direction of applied field. It has been found also that the magnetoresistance is positive in these compounds (the value of the electrical resistance in the ferromagnetic state is higher than that in the antiferromagnetic state) for the fields applied both along the c-axis and in the basal plane. The value of the magnetoresistance observed along the c-axis is 30 times as high as that in the basal plane. The obtained results indicate that the electronic band structure changes are likely responsible for the AF–F magnetic phase transitions observed in the RMn2X2 compounds.

Magnetic anisotropy of La0.75Sm0.25Mn2Si2 compound

The magnetic anisotropy of the La0.75Sm0.25Mn2Si2 compound, which has a layered ThCr2Si2-type crystal structure, has been studied using a quasi-single-crystalline sample. It was found that the compound possesses a strong uniaxial magnetic anisotropy in both magnetically ordered and paramagnetic states. The magnetic anisotropy of the compound is mainly due to the Mn sublattice anisotropy. In the ferromagnetic state at 180 K, the magnetization process is characterized by the magnetic anisotropy constant K1 = 6 × 106 erg cm−3. In the paramagnetic state, a considerable anisotropy of paramagnetic Curie temperature Δθp ≈ 35 K and an appreciable difference between values of the effective magnetic moment measured along the c-axis and in the basal plane, Δμeff≈0.4 μB per formula unit, has been observed. The natural layered structure of the compound and partial unquenching of the Mn orbital momentum have been considered as possible reasons for the observed strong magnetic anisotropy.