Magnetic Rare-earth Metals Research Articles

Growth of a quasicrystal-related structure and superstructure for ultrathin Ce-Ti-O films on Pt(111).

Herein, oxide quasicrystal-related (OQC-R) structure and Ce-Ti-O-(3 × 3) superstructure ultrathin films were prepared on Pt(111) and characterized using scanning tunneling microscopy (STM) and low-energy electron diffraction. The OQC-R structure with dodecagonal clusters consisting of triangles, squares, and rhombuses was observed in STM images. The first discovery of the OQC-R structure with a magnetic rare earth metal expands the possibility of discovering new oxide quasicrystals with novel magnetism or superconductivity. By depositing Ti on an OQC-R ultrathin film and post-annealing, a honeycomb lattice of the Ce-Ti-O-(3 × 3) superstructure was prepared. From X-ray photoelectron spectroscopy (XPS) and resonant-photoelectron spectroscopy, the chemical states of the Ce and Ti atoms in the OQC-R structure corresponded to the Ce3+ and Ti2+ states, while those for the Ce-Ti-O-(3 × 3) superstructure corresponded to the Ce3+, Ti3+, and Ti2+ states. The phase transformation from the OQC-R structure to the Ce-Ti-O-(3 × 3) honeycomb superstructure likely occurred when the amount of Ti increased and was more oxidized. The elemental atomic density was also calibrated using XPS and Rutherford backscattering spectroscopy. These results propose tentative structural models of the OQC-R structure as Ce18Ti14O41 and the Ce-Ti-O-(3 × 3) superstructure as CeTi6O9.

Rare-earth magnetic nitride perovskites

We propose perovskite nitrides with magnetic rare-earth metals as novel materials with a range of technological applications. These materials appear to be thermodynamically stable and, in spite of possessing different crystal structures and different atomic environments, they retain the magnetic moment of the corresponding elemental rare-earth metal. We find both magnetic metals and semiconductors, with a wide range of magnetic moments and some systems posses record high magnetic anisotropy energies. Further tuning of the electronic and magnetic properties can also be expected by doping with other rare-earths or by creating solid solutions. The synthesis of these exotic materials with unusual compositions would not only extend the accepted stability domain of perovskites, but also open the way for a series of applications enabled by their rich physics.

Combined effect of chemical pressure and valence electron concentration through the electron-deficient Li substitution on the RE4LiGe4 (RE=La, Ce, Pr, and Sm) system

Four members of the RE 4 LiGe 4 ( RE =La, Ce, Pr, and Sm) system have been prepared by high-temperature reaction method and characterized by X-ray diffractions. All compounds crystallize in the orthorhombic Gd 5 Si 4 -type structure (space group Pnma , Pearson code oP 16) with bonding interactions for interslab Ge 2 dimers. The Li substitution for rare-earth elements in the RE 4 LiGe 4 system leads to a combined effect of the increased chemical pressure and the decreased valance electron concentration (VEC), which eventually results in the structure transformation from the Sm 5 Ge 4 -type with all broken interslab Ge–Ge bond for the parental RE 5 Ge 4 to the Gd 5 Si 4 -type structure for the ternary RE 4 LiGe 4 ( RE =La, Ce, Pr, and Sm) system. Site-preference between rare-earth metals and Li is proven to generate energetically the most favorable atomic arrangements according to coloring-problem, and the rationale is provided using both the size-factor and the electronic-factor related, respectively, to site-volume and electronegativity as well as QVAL values. Tight-binding, linear-muffin-tin-orbital (TB-LMTO) calculations are performed to investigate electronic densities of states (DOS) and crystal orbital Hamilton population (COHP) curves. The influence of reduced VEC for chemical bonding including the formation of interslab Ge 2 dimers is also discussed. The magnetic property measurements prove that the non-magnetic Li substitution leads to the ferromagnetic (FM)-like ground state for Ce 4 LiGe 4 and the co-existence of antiferromagntic (AFM) and FM ground states for Sm 4 LiGe 4 . Reported is a combined effect of the chemical pressure and the reduced VEC caused by the smaller monovalent non-magnetic Li substitution for the larger trivalent magnetic rare-earth metals in the RE 4 LiGe 4 ( RE =La, Ce, Pr, and Sm) system. This results in the structure transformation from the Sm 5 Ge 4 -type to the Gd 5 Si 4 -type structure and the changes in magnetic properties. • Four Li-containing intermetallic compounds RE 4 LiGe 4 ( RE =La, Ce, Pr and Sm) were synthesized. • The combined effect of the chemical pressure and the reduced VEC caused by Li substitution for a rare-earth metal results in the structure transformation. • Electronic structures and magnetic properties of title compounds were thoroughly investigated.

Additivity of magnetic contributions to the x-ray magnetic circular dichroism spectrum

We have studied the behavior of the x-ray magnetic circular dichroism (XMCD) signals recorded at the Fe $K$ edge through the ${\text{Ho}}_{1\ensuremath{-}x}{\text{Lu}}_{x}{\text{Fe}}_{2}$ series in which the system is driven through a magnetic compensation by varying the concentration of the magnetic rare-earth metal, the temperature and the applied magnetic field. We report a direct experimental evidence of the additive character of the different magnetic species, Ho and Fe, to the XMCD signal at the conduction band. Both magnetic contributions are always present due to the hybridization of the conduction states, and only at ${T}_{Comp}$ the Ho contribution to the Fe $K$-edge XMCD disappears.

Temperature-Induced Reversal of Magnetic Interlayer Exchange Coupling

For epitaxial trilayers of the magnetic rare-earth metals Gd and Tb, exchange coupled through a nonmagnetic Y spacer layer, element-specific hysteresis loops were recorded by the x-ray magneto-optical Kerr effect at the rare-earth M5 thresholds. This allowed us to quantitatively determine the strength of interlayer exchange coupling (IEC). In addition to the expected oscillatory behavior as a function of spacer-layer thickness dY, a temperature-induced sign reversal of IEC was observed for constant dY, arising from magnetization-dependent electron reflectivities at the magnetic interfaces.

Influence of the short range order (SRO) on electrical resistivity of the magnetic rare earth metals in the paramagnetic region

The model of the magnetic rare earth metals which contains interacting itinerant electrons and localized spins was considered. The interaction between both kinds of electrons was assumed in the simple local exchange s–f form. The complicated many body problem was reduced to the one particle one with the help of coherent potential approximation (CPA). Itinerant electron self-energy was calculated using standard Green’s function technique with taking into account short range order (SRO) within the localized spin system. The magnetic part of the dc electrical conductivity σ in the paramagnetic region, expressed by Kubo–Greenwood formula, was calculated. The resistivity ρ = 1 / σ and its derivative over the temperature shows the structure near T C + 0 + . The quantities ρ , ∂ ρ / ∂ T were not constant versus temperatures as it is in the simple models often used. The influence of the important model parameters and single ion anisotropy on the resistivity was calculated and shortly discussed.

Rashba effect at magnetic metal surfaces

We give experimental and theoretical evidence of the Rashba effect at the magnetic rare-earth metal surface Gd(0001). The Rashba effect is substantially enhanced and the Rashba parameter changes its sign when a metal-oxide surface layer is formed. The experimental observations are quantitatively described by ab initio calculations that give a detailed account of the near-surface charge density gradients causing the Rashba effect. Since the sign of the Rashba splitting depends on the magnetization direction, the findings open up new opportunities for the study of surface and interface magnetism.

Evidence for Strong Electron-Magnon Coupling in Gadolinium

Tunneling spectroscopy measurements on normal state Gd–Gd oxide–Al junctions have been performed. Small deviations from the overall parabolic dependence of conductance on voltage were revealed and analyzed using a theoretical approach, taking into account interactions of tunneling electrons with elementary excitations in the oxide layer and metal electrodes. These processes have been studied by calculating the even and odd conductances that characterize the symmetrical (emission of oxide phonons) and asymmetrical (self-energy effects in electrodes) processes, respectively. It has been found that in gadolinium, one of the simplest magnetic metals known, an interaction of electrons with magnetic excitations is as important as the electron-phonon contribution. This result explains the difference between the calculated electron-phonon coupling parameters and the experimentally derived electron mass enhancement factors in magnetic rare-earth metals.

Josephson effect through a ferromagnetic layer

We report transport measurements on Superconductor/Ferromagnet/Superconductor (S/F/S) junctions: Nb/Al/Gd/Al/Nb where gadolinium (Gd) is a weakly polarized ferromagnet. A sizeable critical current Ic is observed in the I(V) characteristics. This current can be modulated by a weak magnetic field, as expected for a Josephson current. With these experiments, we establish that superconducting coherent transport survives across a small ferromagnetic layer. The penetration depth of Cooper pairs in Gd has been measured. An extensive study of the Josephson critical current in temperature for different thicknesses of magnetic compounds is presented. A comparison of transport measurements with S/N/S junction is given through measurements made on Nb/Al/Y/Al/Nb, where yttrium (Y) is used as non magnetic rare earth metal.

Band structure and ordering of magnetic superlattices

The unit cell of a superlattice which contains two multiplane layers corresponding to two different constituent atoms, one of which is a long-range magnetic rare-earth metal, is represented along the superlattice direction by square-well potentials of appropriate depths. The electronic band structure of this bilayer is obtained numerically. The variation is charge density within the bilayer is discussed. The information so obtained allows also the magnetic ordering of the superlattice system to be determined by established the variation in the RKKY exchange interaction with distance. Results for this simple model show encouraging agreement with experimental evidence that the type of ordering is affected by the thickness of the magnetically inert multiplane layer and that long-range magnetic coherence may exist in these structures.

Depression of the superconducting transition temperature of the Heusler alloy Pd2YSn with the addition of magnetic rare-earth metals.

The compound ${\mathrm{Pd}}_{2}$YSn, having the cubic Heusler L${2}_{1}$ structure, is superconducting with a transition temperature (${T}_{c}$) of 4.55 K. The effect on ${T}_{c}$ of replacing Y by other magnetic rare-earth metal has been investigated in the series ${\mathrm{Pd}}_{2}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{R}}_{\mathrm{x}}$Sn (R=Gd, Dy, Ho, Er, Tm, and Yb). The depression in ${T}_{c}$ is the largest with Gd substitution. The initial rate of depression of ${T}_{c}$ shows deviation from the de Gennes function. The results of ${T}_{c}$ measurements are analyzed in terms of Abrikosov-Gor'kov theory, taking into account the effects of crystalline electric fields acting on rare-earth ions. Values of the density of states times the exchange constant, [N(0)${J}_{\mathrm{sf}{}^{2}}$], are derived from these measurements.

Crystal fields in Sc, Y, and the heavy-rare-earth metals Tb, Dy, Ho, Er, Tm, and Lu

Experimental investigations of the magnetic properties of dilute alloys of the rare-earth solutes Tb, Dy, Ho, Er, and Tm in the nonmagnetic hosts Lu, Y, and Sc have been performed. These measurements, which include and supplement earlier published results, have been analyzed and crystal-field parameters for all these 15 alloy systems deduced. The consistency of the parameters was confirmed by a variety of magnetic measurements, including neutron spectroscopy. Crystal-field parameters have also been derived for the ions in pure magnetic rare-earth metals and their alloys using the results for the dilute alloys supplemented with paramagnetic measurements up to high temperatures on the concentrated systems. Mean values and standard deviations of the higher-order crystal-field parameters for all Y and Lu alloys are $\frac{{B}_{40}}{\ensuremath{\beta}}=6.8\ifmmode\pm\else\textpm\fi{}0.9$ K, $\frac{{B}_{60}}{\ensuremath{\gamma}}=13.6\ifmmode\pm\else\textpm\fi{}0.7$ K, and $\frac{{B}_{66}}{\ensuremath{\gamma}}=(9.7\ifmmode\pm\else\textpm\fi{}1.1)\frac{{\ensuremath{\beta}}_{60}}{\ensuremath{\gamma}}$. These values---with the inaccuracies somewhat increased---are expected to be representative also for the magnetic rare-earth metals. For rare-earth ions in the Sc host the values $\frac{{B}_{40}}{\ensuremath{\beta}}=9.9\ifmmode\pm\else\textpm\fi{}1.9$ K, $\frac{{B}_{60}}{\ensuremath{\gamma}}=19.8\ifmmode\pm\else\textpm\fi{}1.5$ K, and $\frac{{B}_{66}}{\ensuremath{\gamma}}=(9.4\ifmmode\pm\else\textpm\fi{}0.9)\frac{{\ensuremath{\beta}}_{60}}{\ensuremath{\gamma}}$ were deduced. $\frac{{B}_{20}}{\ensuremath{\alpha}}$ is a host-sensitive parameter which has the average values of -102.7, -53.4, and 29.5 K for rare-earth ions in Y, Lu, and Sc, respectively. There is also evidence that this parameter varies with the solute. $\frac{{B}_{20}}{\ensuremath{\alpha}}$ for ions in the pure magnetic rare-earth metals and their alloys shows a linear variation with $\frac{c}{a}$ ratio characteristic of each ion. The results indicate a contribution from anisotropic exchange to the high-temperature paramagnetic anisotropy of approximately 20% for Tb, Dy, Ho, and Er, and approximately 10% for Tm.

Scattering of conduction electrons by localised moments in metals

The scatiering of conduction electrons from localized ionic moments in metals was studied by observing the change in residual resistivity of samples of pure lutetiurn, into which were dissolved small amounts of magnetic rareearth metals. The results indicate that the exchange scattering of the conduction electrons is proportional to the factor (g-1)/sup 2/J(J + 1), where J is the localized ionic moment of the solute and g is the Lande factor. The magnitude of the exchange scattering is comparable with that caused by the change in the electrostatic lattice potential, due to the impurity ion. (H.D.R.)