I. S. Tereshina

Transformations of magnetic phase diagram as a result of insertion of hydrogen and nitrogen atoms in the crystalline lattice of R2Fe17 compounds

Abstract The effect of hydrogen and nitrogen insertions on the magnetic properties (Curie temperatures, saturation magnetization, magnetocrystalline anisotropy) of R 2 Fe 17 compounds have been studied. Structural and magnetic investigations have been performed by means of X-ray diffraction, torque and pendulum magnetometer techniques and high-field magnetization measurements on R 2 Fe 17 H x single crystals and on magnetically aligned powder samples of R 2 Fe 17 N y . The hydrides and nitrides still retain the structure of their parent compounds, but the unit cell volumes are expanded. The Curie temperatures are increased by both hydrogenation and nitrogenation. Magnetic phase diagrams of the investigated compounds have been obtained. The effects of the nitrogen and the hydrogen atoms on the rare-earth sublattice anisotropy have been found to be opposite and this is determined by the orientation of the quadrupole moment q of the asymmetric 4f shell with respect to the direction of the resulting magnetic moment of 4f electrons.

Structural and magnetic properties of Lu2Fe17Hx (x=0; 3) single crystals

Abstract The structural and magnetic characterization of Lu2Fe17Hx (x=0; 3) single crystals is presented. The host alloy crystallizes with a disordered variant of hexagonal (Th2Ni17-type P63/mmc) structure. It is observed that the host alloy symmetry is retained upon hydrogenation. Hydrogen is found to be accommodated in the rare earth-rich octahedral position. The Neel temperature of the hydride Lu2Fe17H3 is increased by 35 K per hydrogen atom. The magnetization and magnetic anisotropy constant remains practically unaffected in the hydride Lu2Fe17Hx at x=3.

Magnetic anisotropy and magnetostriction in a Lu2Fe17 intermetallic single crystal

The magnetic anisotropy and magnetostriction of a Lu2Fe17 single crystal are investigated. The temperature dependence of the magnetic anisotropy constant K1 is measured in the range 4.2–300 K. The results obtained are compared with the data calculated using the Callen theoretical formula. It is found that the temperature dependence of K1 for the Lu2Fe17 single crystal deviates from the temperature curve predicted by the localized single-ion model. The inference is drawn that a certain contribution to the magnetic anisotropy of the Lu2Fe17 compound is made by the magnetic anisotropy of band electrons. The longitudinal, transverse, and volume magnetostrictions of the Lu2Fe17 single crystal are studied, and the magnetostriction constants are calculated. It is demonstrated that the exchange integral of Lu2Fe17, as for the Y2Fe17 compound, substantially depends on the atomic volume. This dependence is responsible for the considerable difference between the Curie temperatures of the Lu2Fe17 and Y2Fe17 compounds. It is revealed that the magnetostriction of the Lu2Fe17 single crystal in the temperature range of the magnetic phase transition is determined by the two-ion exchange and single-ion contributions.

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...

Direct Measurement of Magnetocaloric Effect in Metamagnetic Ni 43 Mn 37.9 In 12.1 Co 7 Heusler Alloy

The magnetocaloric effect in the metamagnetic Ni43Mn37.9In12.1Co7 Heusler alloy is directly studied experimentally under the adiabatic and quasi-isothermal conditions in a magnetic field with induction of up to 14 T.

Magnetocrystalline anisotropy of R2Fe17Hx (x=0, 3) single crystals

Abstract The magnetocrystalline anisotropy of R 2 Fe 17 (R=Y, Gd, Tb, Ho and Er) and their hydrides are studied by analyzing the magnetization curves of single crystal samples in the temperature range 4.2–300 K in magnetic fields up to 140 kOe. There is no noticeable influence of hydrogenation on the magnetic anisotropy in the Y 2 Fe 17 and Gd 2 Fe 17 compounds. An easy-plane to easy-cone transition is detected for a Tb 2 Fe 17 H 3 single crystal after hydrogenation. A significant change of the magnetization process has been observed in the hydrides Ho 2 Fe 17 H 3 and Er 2 Fe 17 H 3 . Hydrogenation induces a FOMP-type transition in the Ho 2 Fe 17 H 3 compound and, on the contrary, leads to the disappearance of the FOMP type transition in the Er 2 Fe 17 H 3 compound. The obtained results are discussed on the basis of a model based on the interaction of the quadrupolar moment and the magnetic moment of the 4f electron shell of the rare earth ion with the interstitial hydrogen. It is established that the orientation of the quadrupolar moment of the asymmetric 4f shell with respect to the direction of the resulting magnetic moment of 4f electrons plays an important role.

Magnetic anisotropy and Mössbauer effect studies of YFe11Ti and YFe11TiH

The magnetic properties and 57Fe Mossbauer spectra of the compounds YFe11TiHx (x = 0, 1) were investigated. The magnetocrystalline anisotropy of YFe11Ti and its hydride was studied by analysing the hard and easy magnetization curves of single crystal samples in the temperature range 4.2–300 K. The spontaneous magnetization was determined in wide temperature range 4.2–650 K. It is established that at T = 4.2 K magnetic anisotropy constants K1 and K2 for YFe11TiH1 single crystal reach values of 25.8 K f.u.−1 and 0.24 K f.u.−1, respectively. The Mossbauer effect spectra of YFe11Ti and its hydride were analysed in terms of a model which takes into account the local environment of Fe atoms on three crystallographic sites (8f, 8j and 8i) and an influence of the random distribution of titanium on the 8i site. Upon hydrogenation both the hyperfine fields and the isomer shifts increase. These results are discussed in terms of the hydrogen-induced unit cell expansion and the electron charge transfer from the conduction band onto the H atoms.

Magnetocaloric and thermomagnetic properties of Ni2.18Mn0.82Ga Heusler alloy in high magnetic fields up to 140 kOe

Measurements of the adiabatic temperature change (ΔT) and the specific heat transfer (ΔQ) of Ni2.18Mn0.82Ga Heusler alloy were taken in order to quantify the direct giant magnetocaloric effect of the alloy when it is in the vicinity of magneto-structural phase transition (PT) from paramagnetic austenite to ferromagnetic martensite, and their results are presented. A new vacuum calorimeter was used to simultaneously measure ΔT and ΔQ of magnetocaloric materials with a Bitter coil magnet in fields of up to H = 140 kOe. Other thermomagnetic properties of this alloy were investigated using standard differential scanning calorimetry and PPMS equipment. The maximal values of magnetocaloric effect in H = 140 kOe were found to be ΔT = 8.4 K at initial temperature 340 K and ΔQ = 4900 J/kg at 343 K. Using this direct method, we show that the alloy indeed demonstrates the largest value of ΔQ as compared with previously published results for direct measurements of magnetocaloric materials, even though at 140 kOe the ...

Rare-earth and transition metal sublattice contributions to magnetization and magnetic anisotropy of R(TM,Ti)12 single crystals

Abstract The substitution effect of Fe by Co on magnetic properties (magnetization, anisotropy field, Curie temperature) of R(TM,Ti) 12 (R=Y, Sm; TM=Fe, Co) single crystals have been studied. The temperature and composition dependence of magnetization and magnetic anisotropy constants K 1 and K 2 in the range 77–1000 K have been determined using torque and vibrating specimen magnetometry techniques. 57 Fe Mossbauer spectra of powdered samples of SmFe 11− x Co x Ti have been studied at room temperature. The easy magnetization direction of RFe 11− x Co x Ti single crystals was found to be coinciding with the c axis for x ≤5. A characteristic maximum in concentration dependence of saturation magnetization is found for x ≈3. The concentration dependence of magnetic anisotropy constant K 1 for RFe 11− x Co x Ti single crystals exhibited a maximum at x ≈1. It is found that the Sm-sublattice contribution to magnetic anisotropy decreases rapidly with increasing Co concentration.

Magnetostriction and magnetic anisotropy in TbFe11TiHX (x=0, 1) single crystals

Single crystals of TbFe11Ti and its hydride with tetragonal ThMn12-type structure were obtained. Magnetostriction measurements have been made on TbFe11TiHX (x=0, 1) single crystals by the applied strain gauge method in the temperature range from 77 to 400 K and in magnetic fields up to 13 kOe. Magnetization measurement along the main crystallographic directions of the tetragonal structure have been performed on RFe11TiHx (x=0, 1) single crystals in applied high magnetic fields up to 120 kOe in the temperature range from 4.2 to 300 K. It was concluded that hydrogenation results in an increase in rare-earth anisotropy that leads to the disappearance of the spin-reorientation transition in the TbFe11TiH single crystal. Hydrogenation lead to a change in sign of the magnetic anisotropy constant K3 and the magnetostriction constant λ2α,2.