Manifestation of geometric frustration on magnetic and thermodynamic properties of the pyrochlores Sm2X2O7 (X=Ti,Zr)

We present here magnetization, specific heat, and Raman studies on single-crystalline specimens of the first pyrochlore member Sm2Ti2O7 of the rare-earth titanate series. Its analogous compound Sm2Zr2O7 in the rare-earth zirconate series is also investigated in the polycrystalline form. The Sm spins in Sm2Ti2O7 remain unordered down to at least T=0.5 K. The absence of magnetic ordering is attributed to very small values of exchange (θcw∼−0.26 K) and dipolar interaction (μeff∼0.15 μB) between the Sm3+ spins in this pyrochlore. In contrast, the pyrochlore Sm2Zr2O7 is characterized by a relatively large value of Sm-Sm spin exchange (θcw∼−10 K); however, long-range ordering of the Sm3+ spins is not established at least down to T=0.67 K due to frustration of the Sm3+ spins on the pyrochlore lattice. The ground state of Sm3+ ions in both pyrochlores is a well-isolated Kramers doublet. The higher-lying crystal field excitations are observed in the low-frequency region of the Raman spectra of the two compounds recorded at T=10 K. At higher temperatures, the magnetic susceptibility of Sm2Ti2O7 shows a broad maximum at T=140 K, while that of Sm2Zr2O7 changes monotonically. Whereas Sm2Ti2O7 is a promising candidate for investigating spin fluctuations on a frustrated lattice, as indicated by our data, the properties of Sm2Zr2O7 seem to conform to a conventional scenario where geometrical frustration of the spin excludes their long-range ordering.

Electric and magnetic polarizabilities of hexagonal Ln(2)CuTiO(6)(Ln = Y, Dy, Ho, Er, and Yb)

We investigated the rare-earth transition-metal oxide series, Ln(2)CuTiO(6) (Ln = Y, Dy, Ho, Er, and Yb), crystallizing in the hexagonal structure with noncentrosymmetric P6(3)cm space group for possible occurrences of multiferroic properties. Our results show that while these compounds, except Ln = Y, exhibit a low-temperature antiferromagnetic transition due to the ordering of the rare-earth moments, the expected ferroelectric transition is frustrated by the large size difference between Cu and Ti at the B site. Interestingly, this leads these compounds to attain a rare and unique combination of desirable paraelectric properties with high dielectric constants, low losses, and weak temperature and frequency dependencies. First-principles calculations establish these exceptional properties result from a combination of two effects. A significant difference in the MO5 polyhedral sizes for M = Cu and M = Ti suppress the expected cooperative tilt pattern of these polyhedra, required for the ferroelectric transition, leading to relatively large values of the dielectric constant for every compound investigated in this series. Additionally, it is shown that the majority contribution to the dielectric constant arises from intermediate-frequency polar vibrational modes, making it relatively stable against any temperature variation. Changes in the temperature stability of the dielectric constant among different members of this series are shown to arise from changes in relative contributions from soft polar modes.

Transition metal oxide perovskites by photoelectron and x-ray absorption spectroscopy

X-ray and ultraviolet photoelectron spectroscopy as well as x-ray absorption spectroscopy have been employed to investigate transition metal oxide perovskites of the general formula ABOs (A=La or rare-earth ion, B=trivalent transition metalion). Systematics in the core levels and in the valence bands in the series of LaBOa compounds have been discussed. Lanthanum chemical shifts in the x-ray absorption spectra in this series show interesting trends. Photoelectron spectra of the solid solutions, LaNil_x Coxes, LaNix_x FexO8 and LaFel_x Coxes show that the rigid band model is applicable to these systems. It is shown that x-ray photoelectron spectroscopy can be employed to identify multiple oxidation states of transition metal ions in oxide perovskites.

Combustion synthesis and properties of Ln3Fe5O12 and yttrium aluminium garnets

The rare earth iron garnets Ln3Fe5O12 and Y3AlxFe5-xO12, where x=1.0-5.0, and Y1.5Gd1.5Al0.2Fe4.8O12 have been prepared by the combustion of redox mixtures containing corresponding metal nitrates and oxalyl dihydrazide, i.e. C2H6N4O2 at 350-degrees-C. The solid combustion products are amorphous, submicrometre-sized powders which, on heating at 750-degrees-C for 3 h, yield crystalline single-phase garnets. The particle size of the garnets is below 1 mum and the surface area ranges from 16 to 90 m2 g-1. Yttrium iron garnet could be sintered to a density of more than 95% at 1200-degrees-C for 3 h, giving an average grain size of 3-5 mum.

Ising pyrochlore magnets: Low-temperature properties, "ice rules," and beyond

Pyrochlore magnets are candidates for what Harris et al. [Phys. Rev. Lett. 79, 2554 (1997)] call "spin-ice" behavior. We present theoretical simulations of relevance for the pyrochlore family R2Ti2O7 (R = rare earth) supported by magnetothermal measurements on selected systems. Ey considering long-ranged dipole-dipole as well as short-ranged superexchange interactions, we get three distinct behaviors: (i) an ordered doubly degenerate state, (ii) a highly disordered state with a broad transition to paramagnetism, and (iii) a partially ordered state with a sharp transition to paramagnetism. Closely corresponding behavior is seen in the real compounds.

The effect of mischmetal addition on the structure and mechanical properties of a cast Al-7Si-0.3Mg alloy containing excess iron (up to 0.6 Pct)

The effect of Fe content (0.2 to 0.6 pct) on the microstructure and mechanical properties of a cast Al-7Si-0.3Mg (LM 25/356) alloy has been investigated. Further, 1 pct mischmetal (MM) additions (a mixture of rare-earth (RE) elements) were made to these alloys, and their mechanical properties at room and at elevated temperatures (up to 200 degreesC) were evaluated. A structure-property correlation on this alloy was attempted using optical microstructure analysis, fractographs, X-ray diffraction, energy-dispersive analysis of X-rays (EDX), and quantitative metallography by image analysis. An increase in Fe content increased the volume percentage of Fe-bearing intermetallic compounds (beta and pi phases), contributing to the lower yield strength (YS), ultimate tensile strength (UTS), percentage elongation, and higher hardness. An addition of 1 pct MM to the alloys containing 0.2 and 0.6 pct Fe was found to refine the microstructure; modify the eutectic silicon and La, Ce, and Nd present in the MM; form different intermetallic compounds with Al, Si, Fe, and Mg; and improve the mechanical properties of the alloys both at room and elevated temperatures.

Thermodynamic properties of oxygen in RE-O (RE = Gd, Tb, Dy, Er) solid solutions

The oxygen potentials of four rare-earth metal – oxygen (RE–O: RE=Gd, Dy, Tb, Er) solid solutions have been measured by equilibration with yttrium – oxygen (Y–O) and titanium – oxygen (Ti–O) solid solutions. Rare-earth metal, yttrium and titanium samples were immersed in calcium-saturated CaCl2 melt at temperatures between 1093 and 1233 K. Homogeneous oxygen potential was established in the metallic samples through the fused salt, which contains some dissolved CaO. The metallic samples were analyzed for oxygen after quenching. The oxygen potentials of RE–O solid solutions were determined using either Y–O or Ti–O solid solution as the reference. This method enabled reliable measurement of extremely low oxygen potentials at high temperature (circa pO2=10−48 atm at 1173 K). It was found that the oxygen affinity of the metals decreases in the order: Y>Er>Dy>Tb>Gd>Ti. Values for the standard Gibbs energy of solution of oxygen in RE metals obtained in this study, permit assessment of the extent of deoxidation that can be achieved with various purification techniques. It may be possible to achieve an oxygen level of 10 mass ppm using an electrochemical deoxidation method.

Electrochemical deoxidation of RE–O (RE=Gd, Tb, Dy, Er) solid solutions

The removal of oxygen from rare-earth metals (RE, RE=Gd, Tb, Dy, Er) by an electrochemical deoxidation method was investigated. A titanium basket containing the rare-earth metal sample, submerged in molten CaCl2 electrolyte, formed the cathode of an electrolysis cell. A high-purity graphite anode was used. The calcium metal produced at the cathode effectively deoxidized the rare-earth metal. Carbon monoxide and dioxide were generated at the graphite anode. Rare-earth metals containing more than 2000 mass ppm oxygen were deoxidized to 10–50 mass ppm level by electrolysis at 1189 K for 36 ks (10 h). Cyclic voltammetry was used to characterize the molten salt at different stages of the process. The effectiveness of the process is discussed with the aid of a chemical potential diagram for RE–O solid solutions. The new electrochemical technique is compared with the conventional deoxidation methods reported in the literature. The possibility of nitrogen removal from the rare-earth metals by the electrochemical method is outlined.

System Gd-Rh-O: Thermodynamics and phase relations

Thermodynamic properties of GdRhO3 are investigated in the temperature range from 900 to 1300 K by employing a solid-state electrochemical cell, incorporating calcia-stabilized zirconia as the electrolyte. The standard Gibbs free energy of formation of GdRhO3 from component binary oxide Gd2O3 with C-rare earth structure and Rh2O3 with orthorhombic structure can be expressed as; Delta G(f(ox))(o)(+/- 60)/J mol(-1) = -56603 + 3.78(T/K) Based on the thermodynamic information on GdRhO3 from experiment and auxiliary data for binary oxides from the literature and estimated properties of Gd-Rh alloys, phase relations are computed for the system Gd-Rh-O at 1273 K. Gibbs free energies for intermetallic phases in the binary Gd-Rh are evaluated using calorimetric data available in the literature for two compositions and Miedema's model, consistent with the binary phase diagram. Isothermal section of the ternary phase diagram, oxygen potential-composition diagram and a 3-D chemical potential diagram for the system Gd-Rh-O at 1273 K are developed. Phase relations in the ternary Gd-Rh-O are also computed as a function of temperature at constant oxygen partial pressures. The ternary oxide, GdRhO3 decomposes to Gd2O3 with B-rare earth structure, metallic Rh and O-2 at 1759(+/- 2) K in pure O-2 and 1649(+/- 2) K in air at a total pressure P-0 -0.1 MPa. (c) 2012 Elsevier B.V. All rights reserved.

Estudo do efeito magnetocalórico nos compostos da série Gd1-xDyxAl2

Nesta dissertação, foram investigadas as propriedades magnéticas e magnetocalóricas nos compostos intermetálicos de terras-raras Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) usando abordagens teórica e experimental. Do ponto de vista teórico, a série Gd1-xDyxAl2 foi descrita através de um modelo para o hamiltoniano magnético, incluindo o efeito Zeeman, interação de troca e a anisotropia de campo elétrico cristalino. As entropias da rede e eletrônica foram consideradas nas aproximações de Debye e de gás de elétrons livres, respectivamente. A parte experimental inclui a preparação do material, sua caracterização e medidas das quantidades magnéticas e magnetocalóricas. Os resultados experimentais e os cálculos teóricos da variação adiabática da temperatura (ΔTad) e da variação isotérmica da entropia (ΔS T), sob variações de campo magnético ao longo da direção de fácil magnetização, estão de bom acordo. O efeito da aplicação do campo magnético ao longo de uma direção de difícil magnetização foi estudado e as componentes da magnetização em função da temperatura foram investigadas. Também foi observado que a temperatura de reorientação de spin, TR, diminui quando a intensidade do campo magnético aumenta. Além disso, as concentrações molares ótimas de um material híbrido formado pelos compostos Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) foram simuladas usando um método numérico de matriz proposto por Smaili e Chahine. O compósito apresenta um bom intervalo de temperatura para um refrigerador magnético de 60 até 170 K.

Investigação do efeito magnetocalórico convencional e anisotrópico no sistema Er(1-y)Ho(y)N

O efeito magnetocalórico, base da refrigeração magnética, é caracterizado por duas quantidades: a variação isotérmica da entropia (ΔST) e a variação adiabática da temperatura (ΔTad) as quais podem ser obtidas sob variações na intensidade de um campo magnético aplicado. Em sistemas que apresentam anisotropia magnética, pode‐se definir o efeito magnetocalórico anisotrópico, o qual, por definição, é calculado através da variação na direção de aplicação de um campo magnético cuja intensidade se mantém fixa. Nos materiais de nosso interesse, o efeito magnetocalórico é estudado teoricamente partindo de um hamiltoniano modelo que leva em conta a rede magnética (que pode ser composta por diversas sub-redes magnéticas acopladas), rede cristalina e a dinâmica dos elétrons de condução. No hamiltoniano magnético são consideradas as interações de troca, Zeeman e campo cristalino (esta ultima responsável pela anisotropia magnética). Recentemente, estudamos o efeito magnetocalórico convencional e o efeito magnetocalórico anisotrópico nos compostos mononitretos com terras-raras, a saber: Ho(y)Er(1-y)N para as concentrações y= 0,1,0.5 e 0.75. Comparações entre nossos resultados teóricos e os dados experimentais para o EMC foram bastante satisfatórias [3,9]. Além disso, diversas predições teóricas como a existência de uma fase ferrimagnética no sistema Ho(y)Er(1-y)N (para a concentração y=0.5) e reorientações de spin nas sub-redes do Ho e Er foram feitas [25].

Mossbauer spectroscopic study of R3Fe29-xCrx and R3Fe29-xCrxH,(y)(R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

Fe-57 Mossbauer spectra for the series of R3Fe29-xCrx (R = Y,Ce, Nd, Sm, Gd, Tb, and Dy) compounds and their hydrides have been measured at 4.2 K. The weighted average hyperfine field at the Fe sites was separated into a 3d-electron contribution, proportional to the average Fe moment, and a transferred contribution due to rare earth moments. The latter was found to increase with the rare earth effective spin (g(J) - 1) J. Hyperfine fields in the hydrides were only slightly larger than in the corresponding alloys.

Fe-57 Mossbauer spectroscopic and magnetic studies of R3Fe29-xVx (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

Mossbauer spectra for Fe atoms in the series of R3Fe29-xVx (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) compounds were collected at 4.2 K. The ratio of 14.5 T/mu(B) between the average hyperfine field B-hf and the average Fe magnetic moment mu(Fe)(MS), obtained from our data, in Y3Fe29-xVx is in agreement with that deduced from the RxTy alloys by Gubbens et al. The average Fe magnetic moments mu(Fe)(MS) in these compounds at 4.2 K, deduced from our Mossbauer spectroscopic studies, are in accord with the results of magnetization measurement. The average hyperfine field of the Fe sites for R3Fe29-xVx at 4.2 K increases with increasing values of the rare earth effective spin (g(J) - 1) J, which indicates that there exists a transferred spin polarization induced by the neighboring rare earth atom.

Structural and magnetic properties of hydrides R3Fe29-xVxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic investigation of crystallographic and intrinsic magnetic properties of the hydrides R3Fe29 - xVxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed in this work. The lattice constants a, b, and c and the unit cell volume of R3Fe29 - xVxHy decrease with increasing rare-earth atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Hydrogenation results in regular anisotropic expansions along the a-, b-, and c-axes in this series of hydrides. Abnormal crystallographic and magnetic properties of Ce3Fe27.5V1.5H6.5, like Ce3Fe27.5V1.5, suggest that the Ce ion is non-triply ionized. Hydrogenation leads to the increase in both Curie temperature for all the compounds and in the saturation magnetization at 4.2 K and RT for R3Fe29 - xVx with R = Y, Ce, Nd, Sm, Gd, and Dy, except for Tb. Hydrogenation also leads to a decrease in the anisotropy field at 4.2 K and RT for R3Fe29 - xVx with R = Y, Ce, Nd, Gd, Tb, and Dy, except for Sm. The Ce3Fe27.5V1.5 and Gd3Fe28.4V0.6 show the larger storage of hydrogen with y = 6.5 and 6.9 in these hydrides. (C) 1998 Elsevier Science B.V. All rights reserved.

Crystallographic and magnetic properties of R3Fe29-xVxN4 (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic investigation of crystallographic and magnetic properties of nitride R3Fe29-xVxN4 (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. Nitrogenation leads to a relative volume expansion of about 6%. The lattice constants and unit cell volume decrease with increasing rare-earth atomic number from Nd to Dy, reflecting the lanthanide contraction. On average, the Curie temperature increases due to the nitrogenation to about 200 K compared with its parent compound. Generally speaking, nitrogenation also results in a remarkable improvement of the saturation magnetization and anisotropy fields at 4.2 K and room temperature for R3Fe29-xVxN4 compared with their parent compounds. The transition temperature indicates the spin reorientations of R3Fe29-xVxN4 for R = Nd and Sm are at around 375 and 370 K which are higher than that of R3Fe29-xVx, for R = Nd and Sm 145 and 140 K, respectively. The magnetohistory effects of R3Fe29-xVxN4 (R = Ce, Nd, and Sm) are observed in low fields of 0.04 T. After nitrogenation the easy magnetization direction of Sm3Fe26.7V2.3 is changed from an easy-cone structure to the b-axis. As a preliminary result, a maximum remanence B-r of 0.94 T, an intrinsic coercivity mu(0)H(C) of 0.75 T, and a maximum energy product (B H)(max) of 108.5 kJ m(-3) for the nitride magnet Sm3Fe26.7V2.3N4 are achieved by ball-milling at 293 K.