Magnetocaloric effect in R2Fe17 (R=Sm,Gd, Tb,Dy, Er)

A series of R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) have been synthesized. The magnetocaloric effect (MCE) of these compounds has been investigated by means of magnetic measurements in the vicinity of their Curie temperature. The Curie temperature of Er₂Fe₁₇ is 294 K. The maximum magnetic entropy change of Er₂Fe₁₇ under 5 T magnetic field is ∼3.68 J/kg K. In the R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) system, the maximum magnetic entropy change under 1.5 T magnetic field is 1.72, 0.89, 1.32, 1.59, 1.68 J/kg K corresponding to their Curie temperature (400, 472, 415, 364, 294 K), respectively.     

Magnetic studies on electrodeposited Cu1−xCox alloy films

Six Cu_1−xCo_x alloy films were prepared by an electrodeposition technique. The compositions of the films were determined to be , 0.13, 0.17, 0.19, 0.21, 0.26 by an atomic absorption spectrophotometer. The crystal structure is FCC-Cu for all films but a shift in the diffraction lines is observed with increasing Co content. The giant magnetoresistance effect was determined in some of the films below 200 K. Magnetisation curves showed no saturation at 10 kOe and the curves of the samples which have a large magnetoresistance value are inclined more than the curves of the other low magnetoresistance samples. This may be due to the degree of magnetic moment distribution in these samples.     

Magnetic properties of R2WO6 (where R=Nd, Sm, Eu, Gd, Dy and Ho)

The magnetic and electrical measurements carried out on the R₂WO₆ tungstates showed a paramagnetic behaviour for samples with R=Nd, Gd, Dy and Ho and more complex one for samples with R=Sm and Eu in the temperature range 4.2-280 K and an insulating state at room temperature. With increasing atomic number of the R element the Curie-Weiss temperature increases from −43.5 K for Nd₂WO₆ to −2.7 K for Ho₂WO₆, excluding Sm₂WO₆ and Eu₂WO₆ compounds for that the Curie-Weiss region is not observed and the imaginary part of susceptibility is close to zero. The effective magnetic moment is close to the theoretical one for the free R ion and the magnetic moment measured in magnetic field of 14 T and at temperature of 4.2 K, generally, does not reach the saturation state. The temperature independent residual susceptibility is negative for Nd₂WO₆ and positive for the remaining compounds suggesting different proportions of the Landau, Pauli and van Vleck contributions to the total susceptibility. An increase of the orbital magnetic contribution to the total magnetic moment is suggested from the fitting of the Landé factor in the compounds under study.     

Structural,microstructural and magnetic properties of nanocomposite isotropic Sm(CobalFe0.1MyZr0.04B0.04)7.5 ribbons with M=Ni,Cu and y=0.09 and 0.12

In boron-substituted melt-spun Sm(Co,Fe,Cu,Zr)_7.5-type alloys a nanocomposite microstructure and high coercivities in both as-spun and short-time annealed ribbons can be obtained. In the present study three different compositions, namely Sm(Co_0.73Fe_0.1Cu_0.09Zr_0.04B_0.04)_7.5, Sm(Co_0.70Fe_0.1Cu_0.12Zr_0.04B_0.04)_7.5 and Sm(Co_0.70Fe_0.1Ni_0.12Zr_0.04B_0.04)_7.5 have been examined in order to investigate the influence of composition on the magnetic properties and the microstructure. Melt-spun ribbons have been obtained and annealing has been followed under argon atmosphere for 30–75 min at 600–870 °C. For the as-spun ribbons the TbCu₇-type of structure and fcc-Co as a secondary phase have been identified in the X-ray diffraction patterns. For the annealed ribbons above 700 °C the 1:7 phase transforms into 2:17 and 1:5 phases. The TEM studies have shown a homogeneous nanocrystalline microstructure with average grain size of 30–80 nm. Coercivity values of 15–27 kOe have been obtained from hysteresis loops traced in non-saturating fields. The coercivity decreases with temperature, but it is sufficiently large to maintain values higher than 5 kOe at 380 °C.     

Magnetization plateau in the S=1/2 XXZ spin chain with period 3 magnetic field

We investigate the S=1/2 XXZ spin chain with period 3 magnetic field term. The magnetization plateau-nonplateau transition at m=±1/6 is expected to be of the Berezinski–Kosterlitz–Thouless type from the bosonization argument. By examining the level crossing of low-lying excitations numerically, we precisely determine the plateau phase diagram.     

Critical bond lengths and their role in spontaneous magnetostriction of R2Fe17CX (R=Y,Nd, Gd,Tb, Er)

High-energy high-flux synchrotron X-rays have been used to study the spontaneous magnetostriction of R₂Fe₁₇ (R=Y, Nd, Gd, Tb, Er) and their carbides in the temperature range 10–1100 K. Addition of interstitial carbon greatly increases both the Curie temperatures (T_C) and the spontaneous magnetostrain of the compounds, while reduces the anisotropy of the magnetostrain by expanding the distances between rare-earth and neighboring Fe sites. The increase of T_C with carbon is due to the increased spatial separation of the Fe hexagon layers. On the basal plane, the Fe hexagons are squeezed and the contribution of Fe sublattice to spontaneous magnetostriction is attenuated, while that of rare-earth sublattice is enhanced. The average bond magnetostrain around Fe sites are in linear relation with their hyperfine field intensities.     

A crystal field investigation of the properties of RCuAs2 (R=Pr, Nd, Sm, Tb, Dy, Ho, Er and Yb)

A crystal field (CF) investigation of the magnetic properties and heat capacities of RCuAs₂ (R=Pr, Nd, Sm, Tb, Dy, Ho, Er and Yb) has been carried out using the observed average magnetic susceptibilities (1.8-300 K) of the title compounds. The CF parameters proposed for the systems show a systematic variation throughout the rare-earth series. Other physical properties dependent on the CF are also computed and compared with available experimental data. The experimental heat capacity data reported for a limited range of temperature agree well with computed heat capacity for all the compounds (except SmCuAs₂ and YbCuAs₂). CF J mixing was found to be appreciable for all the samples except YbCuAs₂.     

Structure and magnetostriction of RFex (1.6 ? x ? 2.0) and R(Fe1−yTiy)1.8 (y ? 0.2) alloys (RDy0.65Tb0.25Pr0.1)

Structure, Curie temperature and magnetostriction of RFe_x (1.6 ? x ? 2.0) and R(Fe_1−yTi_y)_1.8 (y ? 0.2) alloys (RDy_0.65Tb_0.25Pr_0.1) have been investigated using optical microscopy, X-ray diffraction, AC initial susceptibility and standard strain gauge techniques. The homogenized RFe_x alloys are found to be essentially single phase in the range of 1.8 ? x ? 1.85. The second phase is a rare-earth-rich phase when x ? 1.8, and (Dy, Tb, Pr)Fe₃ phase when x ? 1.85. X-ray diffraction indicates that the R(Fe_1−yTi_y)_1.8 alloys contain a small amount of Fe₂Ti phase when y ? 0.05, which increases with the increment of Ti content. The Curie temperature of R(Fe₁−_yTi_y)_1.8 alloys slightly enhances with increasing Ti concentration when y ? 0.05, then remains almost unchanged in the range of 0.05 ? y ? 0.20. The magnetostriction of RFe_x alloys is improved when x ? 1.80 and reduced by increasing Fe content when x ? 1.85. The magnetostriction of R(Fe_1−yTi_y)_1.8 alloys is lowered by increasing Ti content.     

Influence of Ho substitution for Nd on magnetic properties of the Nd1−xHoxMn2Ge2 germanides

The structure and magnetic properties of Nd_1−xHo_xMn₂Ge₂ (0.0≤x≤1.0) germanides were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. Substitution of Ho for Nd leads to a linear decrease in the lattice constants and the unit cell volume, and the magnetic interactions in the Mn sublattice cross over from a ferromagnetic character to an antiferromagnetic one. A typical SmMn₂Ge₂-like behavior is observed for x=0.6 and 0.8. The results are collected in a phase diagram.     

Magnetic properties of R3Co29Si4B10 (R=La, Ce, Pr, Nd, Sm, Gd and Dy) borides

The crystal structure and magnetic properties of quaternary rare-earth intermetallic borides R₃Co₂₉Si₄B₁₀ with R=La, Ce, Pr, Nd, Sm, Gd and Dy have been studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in a tetragonal crystal structure with the space group P4/nmm. Compounds with R=La, Ce, Pr, Nd and Sm are ferromagnets, while ferrimagnetic behavior is observed for R=Gd and Dy. The Curie temperatures vary between 149 K and 210 K. The Curie temperatures in R₃Co₂₉Si₄B₁₀ (R=Ce, Pr, Nd, Sm, Gd, Dy) compounds are roughly proportional to the de Gennes factors.     

Electronic structure and magnetic properties of Gd3Co11B4 compound

The electronic structure of hexagonal Gd₃Co₁₁B₄ compound has been studied by X-ray photoemission spectroscopy (XPS) and ab initio self-consistent tight binding linear muffin tin orbital (TB LMTO) method. We have found a good agreement between the experimental XPS valence band spectra and theoretical LMTO calculations. Results showed that the Gd₃Co₁₁B₄ compound is ferrimagnetic with the calculated total magnetic moment M=14.29 μ_B/f.u. The values of the magnetic moments on Co atoms strongly depend on the local environment. We have also compared the electronic structure and magnetic properties of Gd₃Co₁₁B₄ compound with those of Nd₃Co₁₁B₄ compound.     

NMR study of hyperfine interactions in R1−xLuxFe2 (R = Gd, Tb)

Spin-echo NMR measurements on ¹⁵⁵Gd, ¹⁵⁹Tb and ¹⁷⁵Lu have been done for the cubic Laves phase compounds Gd_1−xLu_xFe₂ and Tb_1−xLu_xFe_x. The observed hyperfine fields of Gd and Lu in those compounds increase with increasing Lu concentration. The concentration dependence of the Lu hyperfine field is qualitatively discussed.     

Magnetostriction in the rare-earth ferroborates RFe3(BO3)4, R=Pr and Tb

Recent experimental data for magnetostriction in the rare-earth (RE) ferroborates RFe₃(BO₃)₄ with R=Pr and Tb are discussed from a theoretical point of view. Multipole moments of RE ions are calculated in the framework of a crystal-field model for the RE ion and the molecular-field approximation. Quadrupole approximation is shown to be sufficient for interpretation of data for longitudinal magnetostriction at the magnetic field along the trigonal axis. Parameters of PrFe₃(BO₃)₄ are deduced when accounting for the experimental magnetization curves that manifest a spin-flop transition.     

Crystallographic and magnetic properties of (Nd,Dy)3Fe27.5(Ti,Mo)1.5 compounds

A systematic study of the formation, structure and magnetic properties of (Nd,Dy)₃Fe_27.5(Ti,Mo)_1.5 compounds has been performed. Rietveld analyses of the X-ray patterns of the samples indicate that the concentrations of Ti and Mo affect the formation and structural properties slightly, whereas different rare-earth (Nd and Dy) contents influence them significantly. It is found that high Dy contents make it difficult to form the 3:29-type structures. The Curie temperatures of Nd_2.1Dy_0.9Fe_27.5Ti_1.5−xMo_x decrease monotonically as more Ti was replaced by Mo but their saturation magnetizations remain almost unchanged; in contrast, for Nd_3−yDy_yFe_27.5TiMo_0.5, their saturation magnetizations decrease monotonically with increasing Dy contents while their Curie temperatures are constant.     

Magnetic properties of R2Fe17−xGax (R=Y and Gd) compounds

The magnetic properties of Y₂Fe_17−xGa_x for 3≤x≤7 and Gd₂Fe_17−xGa_x for 5≤x≤7 have been investigated using ⁵⁷Fe Mössbauer spectroscopy. These compounds have the rhombohedral Th₂Zn₁₇ structure. X-ray diffraction analysis of aligned powders shows that the easy direction of magnetization is parallel to the c-axis in Y₂Fe₁₀Ga₇ and Gd₂Fe₁₀Ga₇ and is perpendicular to the c-axis in Y₂Fe₁₄Ga₃, Y₂Fe₁₂Ga₅, Gd₂Fe₁₂Ga₅ and Gd₂Fe₁₁Ga₆. Mössbauer studies indicate that those samples are ordered ferromagnetically. The ⁵⁷Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for R₂Fe_17−xGa_x (R=Y and Gd) at room temperature is positive and the magnitude of δ increases with increasing Ga content.     

Magnetic,electronic and structural properties of ZnxFe3−xO4

A series of samples Zn_xFe_3−xO₄ have been prepared by the chemical coprecipitation technique and characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and X-ray photoelectron spectroscopy (XPS). XRD demonstrates all the samples of Zn_xFe_3−xO₄ have a spinel structure same as Fe₃O₄. The magnetic hysteresis loops of Zn_xFe_3−xO₄ obtained from VSM indicate that the saturation magnetization has a maximum when x is ∼1/3. The chemical states of Fe atoms and Zn atoms in zinc ferrites have been measured using XPS and Auger electron spectroscopy (AES). The Fe 2p core-level XPS spectra and Zn L₃M₄₅M₄₅ Auger peaks have been analyzed and the results have been discussed in correlation with the samples’ magnetic properties. These results suggest most of Zn atoms occupy the tetrahedral sites and a small amount of them occupy the octahedral sites.     

Influence of Si and Co substitutions on magnetoelastic properties of R2Fe17 (R=Y, Er and Tm) intermetallic compounds

The magnetostriction of the off-stoichiometric R₂Fe₁₇-type intermetallic compounds based on R₂Fe_14−xCo_xSi₂ (R=Y, Er, Tm and x=0, 4) was measured, using the strain gauge method in the temperature range 77-460 K under applied magnetic fields up to 1.5 T. All compounds show sign change and reduction in magnetostriction values compared to the R₂Fe₁₇ compounds by Si substitution. For Y₂Fe₁₄Si₂ and Er₂Fe₁₄Si₂, saturation behaviour is observed near magnetic ordering temperature (T_C), whereas for Tm₂Fe₁₄Si₂, saturation starts from T>143 K. Also, Co substitution has different effects on the magnetostriction of R₂Fe₁₄Si₂ compounds. In Er₂Fe₁₀Co₄Si₂ and Tm₂Fe₁₀Co₄Si₂, saturation occurs below the spin reorientation temperature (T_SR). In addition, in Er₂Fe₁₄Si₂, a sign change occurs in the anisotropic magnetostriction (Δλ) as well as the volume magnetostriction (ΔV/V) at their T_SR values. The volume magnetostrictions of the Tm-containing compounds show an anomaly around their T_SR. In R₂Fe₁₄Si₂ compounds, parastrictive behaviour is also observed in ΔV/V near their T_C values. In addition, the magnetostriction of the sublattices is investigated. Results show that in R₂Fe₁₄Si₂ compounds, the rare-earth sublattice contribution to magnetostriction is negative and comparable to the iron sublattice, whereas, in R₂Fe₁₀Co₄Si₂ compounds, the rare-earth sublattice contribution is positive and larger than Fe sublattice. These results are discussed based on the effect of Si and Co substitutions on the anisotropy field of these compounds. Influence of the spin reorientation transition on the magnetostriction of these compounds is discussed in terms of the anisotropic sublattice interactions.     

FORMATION AND MAGNETIC PROPERTIES OF NEW COMPOUNDS R3Fe29-xCrx(R=Y,Ce,Nd,Sm,Gd,Tb,and Dy)

A systematic investigation of structure and magnetic properties of the new R₃Fe_29-xCr_x compounds(R=Y,Ce,Nd,Sm,Gd, Tb,and Dy)has been performed. The Curie temperature of R₃Fe_29-xCr_x increased with increasing atomic number fromR=Ce to Gd and de creased from Gd to Dy. The saturation magnetization of R₃Fe_29-xCr_x at 4.2 K decreased gradually with increasing atomic number from R=Y to Dy,except for Ce. The spin reorientations of the easy magnetization d irection were observed at around 230 K for Nd₃Fe_24.5Cr_4.5 and 180 K for Tb₃Fe_28.0Cr_1.0,and the magnetohistory effects were obser ved for Nd₃Fe_24.5Cr_4.5 and Sm₃Fe_24.0Cr_5.0 in a low field of about 0.04 T. First order magnetization process occurs in magnetic field of around 2.3 T at room temperature for Tb₃Fe_28.0Cr_1.0. The saturation magnetization of Y₃Fe_27.2Cr_1.8 at 4.2 K is 52.2μ_B/f.u., which corresponds to an average magnetic moment of 1.92μ_B per each Fe atom.     

Oscillations in magnetocaloric effect and magnetic properties of RE2Al3Si2 (for RE=Dy,Ho and Er) and REAlSi (for RE=Ce and Pr)

We present the magnetic and thermal properties of a series of compounds RE₂Al₃Si₂ for RE=Dy, Ho, Er, and REAlSi for RE=Pr, Ce. The 2–3–2 family crystallizes with themonoclinic Y₂Al₃Si₂-type structure while the 1–1–1 family crystallizes in the body-centered tetragonal α-ThSi₂-type structure. The measurements were done on single crystals, grown using high-temperature flux technique and molten Al as a solvent . Susceptibility and heat capacity data were taken from 1.8 to 200 K, from the heat capacity data, the isothermal magnetic entropy change was obtained. Our results indicate signal oscillations in magnetocaloric properties for those compounds, in particular, Dy₂Al₃Si₂ that shows an anomaly that can be associated with a spin reorientation. Similar results are known for some Dy discilicides and dialluminades.