E. G. Gerasimov

Preparation of sintered Nd-Fe-B magnets by pressless process

Pressless process used for the preparation of sintered Nd-Fe-B magnets has been studied. The effect of the average particle size Dav, filling density ρf, and powder-texturing conditions for obtaining density ρ and desired magnetic hysteretic properties of sintered magnets has been investigated for both traditional technology (TT) and low-oxygen technology (LOT). The ρf magnitude ensuring the optimum relation between the density ρ of sintered magnets and their degree of texture was shown to be 2.5–3.0 and 2.2 g/cm3 for the TT and LOT, respectively. At lower ρf magnitudes, no required density of sintered magnets is reached, whereas at higher filling densities, a low level of texture and low remanence are realized. Optimum parameters of pulsed field inducing the high degree of texture in powders have been determined. The following properties were achieved for magnets prepared by pressless LOT: Br ≥ 14.2 kG, Hc ≥ 8 kOe, and (BH)max ≥ 47.9 MG Oe.

Magnetic-field-induced martensitic transformations in Ni47 − x Mn42 + x In11 alloys (with 0 ≤ x ≤ 2)

Magnetic properties and martensitic transformations in the Ni47 − xMn42 + xIn11 alloys (with 0 ≤ x ≤ 2) have been studied. The magnetic-field-induced martensitic transformation was found to be observed for all the alloys. The critical temperatures of magnetic and structural phase transformations, temperature dependences of spontaneous magnetization of austenite and martensite, and the critical field, at which the martensitic transformation occurs, have been determined based on magnetic measurements performed for the alloys under study. The spontaneous magnetization of the alloys in the martensitic state has been shown to be lower than that in the magnetic-field-induced austenitic state by a factor of six.

Martensitic transformations and magnetic properties of nonstoichiometric alloys of the Ni-Mn-In system

Martensitic transformations and magnetic properties of Ni89-xMnxIn11 (42 ≤ x ≤ 44) alloys have been investigated. Critical temperatures of magnetic and structural phase transitions in the studied alloy system have been determined. It has been shown that the martensitic transformation induced by the magnetic field is observed in all alloys. Temperature dependences of the spontaneous magnetization of austenite and martensite as well as the magnitude of the critical field, in which martensitic transformation occurs, have been determined.

Effect of Additions of Zinc Stearate on the Properties of Sintered Nd-Fe-B Magnets

Zinc stearate additions have been used to increase the remanence of sintered Nd-Fe-B magnets produced by the powder metallurgy without powder pressing. Zinc stearate acts as an internal lubricant, i.e., it decreases the friction forces between the particles and favors an increase in the degree of texture of the powders, which is induced by the magnetic field. It is shown that the density and the magnetic hysteresis characteristics of sintered magnets produced using additions of 0.15 wt % zinc stearate exceeds the corresponding values obtained for magnets produced without this addition at a filling density of powders in containers of more than 2.9 and 3.0 g/cm3 in dry and wet states, respectively. Using additions of zinc stearate in the amount of 0.15% with respect to the weight of the powder, magnets with a density of 7.55 g/cm3, Br = 14.02 kG, Hc = 7.91 kOe, and (BH)max = 46.1 MG Oe have been produced.

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.

Concentrational commensurate-incommensurate magnetic phase transition in Y1 − xTbxMn6Sn6

The magnetic state of Y1 − xTbxMn6Sn6 (with x ≤ 0.25) alloy is studied at different temperatures and magnetic fields by neutron diffraction. The alloy with xc = 0.22 exhibits an incommensurate-to-commensurate structure phase transition. The transition is accompanied by a decrease in the unit-cell volume. The weak dependence of the intensity of satellites on the x concentration and the monotonic displacement of their angular positions with increasing x allow us to conclude that the antiferromagnet → ferrimagnet transition occurs through the uniform magnetic state. The incommensurate phase in YMn6Sn6 exhibits four satellites, which are observed in the low-angle range of X-ray diffraction pattern and demonstrate different behavior with changing temperature. At 293 K, the antiferromagnet → ferromagnet transition in the Y1 −xTbxMn6Sn6 compound can be induced by applied magnetic field, in particular, the field μ0H = 0.3 T induces the transition in Tb0.15Y0.85Mn6Sn6.

Magnetic and Magnetocaloric Properties of (MnCo) 1- x Ge Compounds

The crystal structure, magnetic properties, and heat capacity of the (MnCo)1 − xGe compounds with x ≤ 0.05 have been studied. It was found that, as the deviation from the MnCoGe stoichiometric composition increases, the temperature of structural transition from the low-temperature phase with the orthorhombic TiNiSi-type structure to the high-temperature phase with the hexagonal Ni2In-type phase decreases rapidly, whereas the magnetic ordering temperature varies slightly. The temperature of structural transition for the composition with x = 0.02 approximately coincides with the Curie temperature of the hexagonal phase, and the transition is accompanied by a significant entropy change, namely, ΔS = 34 J/(kg K). The application of high magnetic field in the transition-temperature range causes an increase in the relative volume of the orthorhombic phase. An analysis of magnetocaloric properties of these compounds, which was performed with the formal application of the Maxwell’s relationship near the temperature of first-order structural phase transition, is shown to give overestimated values of the entropy change.

Effect of hydrogen intercalation on the critical parameters of YBa 2 Cu 3 O y

The effect of hydrogenation at T = 150 and 200°C on the electrophysical properties of highly textured YBa2Cu3Oy ceramics with different oxygen content has been investigated. Like hydration, hydrogenation results in the deterioration of these properties. However, in samples with high oxygen contents (y = 6.96) hydrogenated at T = 150°C after oxidation (400°C) or recovery annealing with subsequent oxidation, the critical current density and first critical field increase compared to the initial state. The improvement of the properties occurs mainly in a magnetic field applied perpendicularly to the c axis. As after hydration, this is connected with the formation of planar defects in the course of low-temperature annealing. In addition, in the process of the hydrogenation, the partial reduction of copper occurs with the formation of microinclusions of Cu2O and other products of chemical decomposition, which are extra pinning centers of magnetic vortices.

Effect of thermal cycling on structure and properties of Ni–Mn–In-based alloys

The results of investigations of the structure and properties of ternary alloys Ni47–xMn42 + xIn11 (0 ≤ x ≤ 2) after thermal cycling are presented. It has been shown that multiple cycles of heating and deep cooling result in a change in the shape of grain boundaries and an increase in microhardness. Thermal cycling does not cause any significant changes in the magnetic susceptibility of the investigated alloys.

Effect of structural water on the critical characteristics of highly textured YBa2Cu3O6.9

The effect of low-temperature treatment (200°C) in a humid argon atmosphere and subsequent annealing (930°C) on the critical parameters of a highly textured YBa2Cu3O6.9 has been studied. During annealing at T = 200°C, the absorbed water is incorporated into the structure of the compound, which is accompanied by the deterioration of its superconducting properties. However, after the recovery annealing at T = 930°C and subsequent oxidation, the superconducting characteristics (jc, B1c, and Fp) are improved. This is explained by the formation of 124-type planar defects, which are effective pinning centers, especially in high fields applied perpendicular to the c axis (⊥ c). The optimum conditions of double annealing substantially increasing the critical current density (jc ≥ 104 A/cm2) in an external magnetic field up to 10 T and also the first critical fields have been found. In fields up to ∼3 T, the critical current density jc is isotropic despite the conservation of high texture in the samples.