G. A. Politova

Magnetocaloric properties of distilled gadolinium: Effects of structural inhomogeneity and hydrogen impurity

High-purity Gd prepared by distillation is a structurally inhomogeneous system consisting of needle-shaped crystals of cross section 0.5–2.5 μm with near-c-axis orientation embedded in a matrix of nanosized (30–100 nm) grains. By measuring the magnetocaloric effect (MCE) directly, we find that the MCE values differ markedly for the plate-shaped samples cut out of a distillate along and perpendicular to the crystals. The effect of small controlled amounts of impurity (hydrogen) on the properties of distilled Gd is further studied. We observe opposite trends in the MCE response to hydrogen charging with respect to the crystals orientation within the samples and discuss mechanisms interrelating the unique structural morphology with the impurity behavior. As an overall assessment, the Curie temperatures of α-GdHx solid solutions increase from 291 K up to 294 K when increasing hydrogen concentration x from 0 to 0.15. Hydrogenation is found to broaden the ferromagnetic-to-paramagnetic phase transition. Hydroge...

Multifunctional Phenomena in Rare-Earth Intermetallic Compounds With a Laves Phase Structure: Giant Magnetostriction and Magnetocaloric Effect

We report on the magnetic and thermal properties [magnetization, specific heat, thermal expansion, magnetostriction, and magnetocaloric effect (MCE)] for the three multicomponent systems Tb_xDy_yGd_zCo₂, Tb_xDy_yHo_z(Co, Fe)₂, and Tb_xDy_yEr_z(Co, Fe)₂ (x + y + z = 1). We show that for Tb_xDy_yR_zCo₂, the Curie temperatures, TC (which ranged from 130 to 300 K), and order of the phase transition (first or second order) could be controlled by composition. The highest MCE values (the adiabatic temperature change ΔT_ad = 2.2-2.3 K at μ₀ΔH = 1.8 T) were observed for the compounds exhibiting the transitions of the first order. Giant volume magnetostriction of 1500-2000 and 500-600 ppm is demonstrated at a field of μ₀H = 10 and 1.2 T, respectively. Structural and magnetic entropy contributions to the total isothermal entropy change are estimated for Tb_xDy_yR_zCo₂. Regular recurrence and/or change of the physical properties across the varied composition of the compounds allows us to find the materials with desired magnetic characteristics, such as T_C, MCE, and magnetostriction, to use them in practice.

Magnetocaloric effect in (Tb,Dy,R)(Co,Fe)2 (R = Ho, Er) multicomponent compounds

The magnetic, magnetocaloric, and magnetoelastic properties of TbxDyyRz(Co,Fe)2 (R = Ho, Er; x + y + z = 1) single-phase alloys prepared by arc melting were investigated. The high-purity samples with various compositions were used for the study, and a comprehensive investigation of the properties was performed. The measurements of the magnetocaloric effect (MCE) were carried out by a direct method; and the magnetostriction was determined using the strain-gauge technique. In the vicinity of the magnetic phase transition, an abrupt change of the magnetization, a large MCE, and the maxima of volume magnetostriction were observed for the compounds studied. Obtained results of MCE in (Tb,Dy)Co2 agree well with available theoretical calculations.

Magnetic ordering temperature of nanocrystalline Gd: enhancement of magnetic interactions via hydrogenation-induced “negative” pressure

Gadolinium is a nearly ideal soft-magnetic material. However, one cannot take advantage of its properties at temperatures higher than the room temperature where Gd loses the ferromagnetic ordering. By using high-purity bulk samples with grains ~200 nm in size, we present proof-of-concept measurements of an increased Curie point (TC) and spontaneous magnetization in Gd due to hydrogenation. From first-principles we explain increase of TC in pure Gd due to the addition of hydrogen. We show that the interplay of the characteristic features in the electronic structure of the conduction band at the Fermi level in the high-temperature paramagnetic phase of Gd and “negative” pressure exerted by hydrogen are responsible for the observed effect.

Magnetostriction in (Tb0.45Dy0.55)1−xErxCo2 (x = 0.1, 0.2): high-field investigation

Intermetallic compounds (Tb0.45Dy0.55)1−xErxCo2 (x = 0.1; 0.2) were synthesized using high purity rare-earth metals. Multicomponent compositions with compensated magnetic anisotropy (MA) were chosen with respect to theoretical predictions based upon the single-ion MA model. Magnetostriction was measured in magnetic fields up to 10 T within the temperature range of 1.5–200 K by means of strain gauges. In the vicinity of magnetic phase transitions (160–170 K), giant volume magnetostriction (~2×10−3 in 10 T) provided by the band magnetism of a Co sublattice was observed in the compounds studied.

Spontaneous and external magnetic field-induced magnetostriction in RCo2-based multicomponent alloys

The spontaneous and external magnetic field-induced magnetostrictions have been studied in multicomponent Tb0.3Dy0.35Ho0.35Co2 and Tb0.8Dy0.1Gd0.1Co2 alloys whose structures are isotypical to the C15 Laves phases. The temperature dependences of the lattice parameters have been measured in the range of 110–280 K. It has been found that the crystal lattice undergoes rhombohedral distortions at temperatures lower than the temperatures of magnetic ordering in these alloys (148.5 and 243 K, respectively). The temperature (4.2–280 K) and field (0–8 T) dependences of the longitudinal and transverse magnetostrictions have been studied. The high magnetoelastic effects have been detected.

Multifunctional phenomena in sublimed dysprosium in high magnetic fields: The magnetocaloric effect and magnetostriction

With the use of sublimed dysprosium having a controllable impurity composition, two physical functional phenomena induced by a magnetic field are studied: magnetostriction and the magnetocaloric effect, which is estimated by independent direct and indirect methods. A giant magnetostriction value at the “order–order” magnetic phase transition and a large magnetocaloric effect in high magnetic fields at the “order–disorder” transition are shown. Both phenomena are discussed in relation to the complex field and temperature behavior of metallic-dysprosium magnetization, the measurement of which is carried out in wide intervals of temperature and magnetic fields (to 14 T).

The magnetocaloric effect in hydrogen-doped Nd2Fe14B and Er2Fe14B intermetallic compounds

The nature of the magnetocaloric effect (MCE) in compounds R2Fe14B (R = Nd, Er) and their hydrides in a wide temperature range is investigated. The investigation is carried out on initial samples of a special purity including single-crystalline ones. The highest value of the magnetocaloric effect is established in the Curie-temperature region. The hydrogenation of samples affects the value of the MCE. The model explaining the dependence of the value and sign of the magnetocaloric effect on the hydrogen contents in R2Fe14B compounds with participation of erbium and neodymium is proposed.

A study of nanostructure magnetosolid Nd–Ho–Fe–Co–B materials via atomic force microscopy and magnetic force microscopy

Nanostructure Nd–Ho–Fe–Co–B alloys have been probed via atomic force microscopy and magnetic force microscopy (AFM and MFM, respectively). The ribbon samples with a thickness of ~30 μm are prepared via the rapid solidification on a rotating copper barrel. A part of samples has been subjected to hydration, whereas another one has undergone severe plastic deformation. AFM was mainly used to study the contact and free surface of ribbon samples. This has enabled us to establish the topography, structure, defects of both sides, morphology of magnetic inclusions of the initial quenched samples and the materials subjected to the subsequent external effects. The AFM and MFM data allowed the magnetic hysteresis properties of the bulk samples with the identical composition to be interpreted.

Structural, magnetic, and magnetothermal properties of the Tb0.3Dy0.7Co2 compound

A complex investigation of the structural, magnetic, and magnetothermal properties of the Tb0.3Dy0.7Co2 compound synthesized with the use of high-purity rare-earth metals has been performed. The phase composition has been controlled using the X-ray structural analysis, and the topology of the alloy surface has been investigated using atomic-force microscopy. It has been established that the Tb0.3Dy0.7Co2 compound is single-phase, while the samples selected for measurements possess a clearly pronounced texture. The magnetization has been measured using a vibrating-sample magnetometer in the fields up to 100 kOe in a temperature range from 4.2 to 200 K. The Curie temperature of the compound is 170 K. The data on the temperature dependence of heat capacity of Tb0.3Dy0.7Co2 have been obtained. The magnetocaloric effect ΔT has been measured by a direct method in the fields up to 18 kOe applied both along and perpendicularly to the texture axis. The anisotropic behavior of the magnitude ΔT for this compound, which possesses the cubic structure, has been found. The maximum value of the magnetocaloric effect ΔT = 2.3 K (ΔH = 18 kOe) has been observed upon applying the magnetic field along the texture axis.