Two- and single-ion magnetic anisotropy in the rare-earth metals

The strong magnetic anisotropy of the rare-earth metals (basically due to the large oribtal contributions to the total ionic moments) has normally been attributed to the crystalline field acting on the ions. Within the last few years a number of experiments have revealed that anisotropic two-ion couplings are of comparable importance. A review of the experimental evidence for the presence of anisotropic coupling between the rare-earth ions is given. Such indications are found primarily in the excitation spectra of the metals, but are also implicit in the crystal-field parameters determined in dilute systems.     

Biaxial magnetic structures in rare-earth compounds: GdMg and HoP

The flopside spin structure, where the magnetic moments form two sublattices which at low temperatures are mutually perpendicular was first found in HoP and then in other rare-earth pnictides. There are large orbital contributions to the magnetic moments of these compounds and it had been thought that they play an important role in stabilizing the flopside spin structure. However, recently this spin structure has been found in GdMg. As Gd³⁺ is an S-state ion, there are negligible orbital effects. We have developed a model Hamiltonian which is able to explain both the occurence of initially a ferromagnetic phase and then at low temperature the flopside spin structure in two very dissimilar compounds GdMg and HoP. For GdMg we find that the competition between the near neighbor ferromagnetic and antiferromagnetic bilinear exchange interactions is such that while they produce a transition to a ferromagnetic phase at 110 K, an unusually small amount of biquadratic (quadrupolar) coupling is able to stabilize a flopside phase at low temperature which is able to resist collapse in a field as large as 150 kOe. For HoP we find that although anisotropic bilinear pair interactions - as for example pseudo-dipole - exist, they cannot be the primary origin of the flopside phase; quadrupole pair interactions are essential to explain the appearance of first the ferromagnetic and then the flopside phases found in HoP. On the basis of our model calculations we are able to explain the data extant on these compounds and we make some predictions which are open to experimental verification.     

The screening influence of the conduction electrons on the shallow donor levels in n-type GaAs

Using the approximative formula found by the author for the eigenvalues of Schrödinger equation for an electron in the screened coulombic field and the condition of the charge neutrality in partially compensated semiconductor, the ground-state and the first excited-state energy levels of shallow donors in n-type GaAs are computed for various donor concentrations and compensation ratios. The energy levels are found to be temperature dependent, which enables to explain the discrepancy between the ionization energies experimentally determined by optical methods at 4·2 K and those found by fitting the experimental temperature variation of the carrier concentration. The different values of the energies for the transition from the ground-state to the first excited-state at 4·2 K and at 15 K experimentally determined by other authors also confirm the temperature dependence of shallow donor levels which can be explained by the screening effect of conduction electrons.     

Scattering of conduction electrons by impurities in quantizing magnetic fields and calculation of electrical conductivity

The method of partial phases was used to obtain a formula for the electrical conductivity of conduction electrons scattered by impurities in quantizing magnetic fields for the case of an anisotropic carrier spectrum, when its isoenergetic surface is an ellipsoid of revolution, and the magnetic field is parallel to its major semiaxis. Equations for finding the phase shifts are given.Translated from Izvestiya Vysshikh Uchebynkh Zavedenii, Fizika, No. 2 pp. 98–102, February, 1978.In conclusion we thank Prof. A. Samoilovich and Docent S. L. Korolyuk for help in conducting this work.     

Many-body effects in core-level spectroscopy of rare-earth compounds

Many-body effects in core-level photoemission and core-level photoabsorption are discussed for rare-earth systems, especially for Ce and La compounds, both in metallic and insulating forms. Emphasis is put on effects of metallic mixed valency and insulating covalency of 4f electrons on these spectra. For the insulating compound CeO₂, detailed analyses of the 3d core photoemission (3d-XPS) and the 2p core photoabsorption (L₃-XAS) are presented by using the impurity Anderson model with a filled valence band. In order to give a consistent interpretation for 3d-XPS and L₃-XAS, it is shown to be essential to take account of the Coulomb interactions U _fd (between a 4f electron and a photoexcited 5d electron in the L₃-XAS) and -U _dc (between the 5d electron and a core hole), in addition to -U _fc (between the 4f electron and the core hole). Discussions are given on the physical information derived from the analysis, on similarities and differences in spectral features between insulating and metallic systems, and also on some related topics.     

Magnetoelastic and elastocaloric effects in rare-earth metals,their alloys and compounds in the region of magnetic phase transitions

This article discusses experimental data and their theoretical interpretation concerning the volume magnetostriction, spontaneous magnetostriction, variation of magnetization under the action of pressure, and elastocaloric effects in rare-earth metals, as well as their alloys and compounds. Particular attention is paid to the region of phase transitions. The volume magnetostriction ω of true magnetization was investigated near the Curie temperature Θ as a function of magnetization and determined from the change of magnetostriction under the action of pressure. From these data we obtained the dependence of the exchange integrals on the unit cell volume. Giant volume magnetostriction and magnetoelastic elastocaloric effects were discovered in the rare-earth metals and alloys in the region of their magnetic phase transitions. It was established that giant volume magnetostriction in RCo₂ compounds is caused by a critical increase of the magnetic moment of the 3d sublattice of cobalt in magnetic fields that exceeds the critical field at T > Θ. Giant volume magnetostriction in R₂Fe₁₇ compounds near the temperature Θ is shown to occur due to strong deformational dependences of exchange interaction and the value of the 3d electron bandwidth.     

Electron paramagnetic resonance and magnetic susceptibility of rare-earth hydrazone compounds

The magnetic properties of the rare earth molecular compounds with hydrazone ligands containing Nd³⁺, Gd³⁺, and Yb³⁺ have been investigated by electron paramagnetic resonance (EPR) and magnetization measurements. For the Gd-compound, partially resolved fine structure due to Gd³⁺ and exchange narrowing effects at low temperatures are observed in the EPR spectra, suggesting, consistent with the EPR and dc magnetic susceptibility, weak antiferromagnetic exchange interactions. Paramagnetic behavior sustained down to low temperatures is derived for Yb³⁺ ions, whereas substantial ferromagnetic exchange coupling is inferred for the lighter Nd³⁺ ions, indicating significant variations of the exchange integrals along the lanthanide series. Received 29 April 2002 Published online 31 July 2002     

Observation of independent cation and anion sublattice long- range magnetic orderings in complex ferricenium salts: Layered magnetic composites

The low temperature magnetism of some magnets based on ferricenium cations and tetrahedral tetraholaferrate anions is discussed. Some of these materials behave as layered magnetic composites with distinctly different ordering temperatures for their respective anion and cation sublattices based on iron-57 Mössbauer spectroscopy and low-field DC magnetic susceptibility studies. Exemplary results are presented for the specific case of [Cp₂Fe]₊[FeBr₄]^– and indicateT _critical (anion) 14 K, while T_critical (cation) 2.5 K where _M versusT suggests canted 3D-AF (likely Heisenberg) behavior for the former and 2D-Ising AF behavior for the latter.     

Binding energy between the magnetic impurity electron and the conduction electrons in the Anderson-Holstein model

A single-level Anderson-Holstein model is investigated using the Lang-Firsovtransformation followed by a zero-phonon averaging and the Kikuchi-Morita Clustervariation method as adopted by Bose and Tanaka in the case of Anderson model. The groundstate energy of the system at zero temperature and the binding energy between the magneticimpurity and the conduction electrons are calculated for the symmetric case ε _d = ?(U/2). Subsequently, the effect of the electron-phononinteraction on the ground state energy and the binding energy is investigated.     

Scattering of conduction electrons by vacancies in aluminium

The low-field Hall coefficientR _H ⁰ and transverse magnetoresistance /₀ were measured in high-purity polycrystalline aluminium samples which were quenched from temperatures between 350 and 500°C. The measurements were made at 4.2 K in magnetic fields up to 40 kG. It was found that the low-field Hall coefficientR _H ⁰ of aluminium containing vacancies lies between –1.0 and –2.5×10^–5 cm³ A^–1s^–1, which is in good agreement with the calculation of Pfändner, Böning and Brening.     

The effects of light and heavy rare-earth elements on magnetic ordering in Sm

The effect of 5% of Pr, Nd, Gd and Tb on the magnetic ordering temperatures of Sm has been examined by susceptibility and resistivity measurements. Neither transition correlates with the average de Gennes factor. The data for the as cast Gd and Tb alloys reveal the presence of the dhcp structure.     

Sample size effects on the spin resonance of conduction electrons in silver

The CESR linewidths of evaporated films of silver, deposited on quartz substrates, have been measured at 9.27 GHz. An increase in low temperature linewidth with decrease in thickness over the range 1.1 μm to 0.5 μm is observed. Below 13 K, ?, the probability of an electron losing its spin memory on colliding with the surface, is found to be about 4.10^-4.     

Determination of the 4f shell magnetic moment in cubic rare-earth intermetallic compounds

Different methods of determining the magnitude of the 4f shell electronic magnetic moment for cubic, ferromagnetic rare-earth intermetallics are critically reviewed. In particular, determination of the 4f ground state from magnetization measurements, polarized neutron, elastic and inelastic spectroscopy are sucessively reviewed. The interest in performing nuclear magnetic resonance quadrupolar hyperfine splitting analysis is then emphasized and limitations of the method underlined. A comparative set of 4f moments for ferromagnetic rare-earth-Al₂ and rare-earth-Zn obtained by the various methods discussed is presented.     

Cyclotron resonance study of conduction electrons in n-type indium antimonide under a strong magnetic field—II: Hot electrons

A system of hot electrons in the n-InSb under the application of a strong magnetic field has been studied by far IR cyclotron resonance. A three band model and an energy independent scattering time were assumed in analyzing the line shape variation with electric field applied either parallel or perpendicular to the magnetic field. Two kinds of electron temperature, inter- and intra-subband, were introduced to describe the electron distribution in energy space. The electron distribution function was found to deviate from an essentially Maxwellian form in the manner predicted by Yamada and Kurosawa. A remarkable difference exists between the two geometries: E∥H and E⊥U. A brief survey of cyclotron emission, and the reverse process of hot electron cyclotron absorption, is summarized at the end as an addendum.     

Predominance of d electrons in the conduction bands of rare earths and intermetallic compounds

With a view to study the nature of the conduction electrons in rare earth metals and intermetallic compounds self consistent augmented-plane-wave calculations have been performed for DyZn. The results indicate that 72 per cent of conduction electrons inside the APW sphere of Dy have d character. The dependence of the nature of the conduction electrons on the exchange potential has also been studied.