Anomalous thermoelectric power of rare earth metals and their compounds

We propose a new mechanism for an anomalous thermoelectric power (ATP) in the paramagnetic state of certain rare earth metals and their compounds, in which the ions possess a nonmagnetic ground state in a given crystal field. The ATP is found to be due to higher order inelastic scattering (second Born approximation) of the conduction electrons by the crystal field split rare earth ions. It has a peak at a temperatureΔ/3 ~Δ/2, whereΔ is the splitting energy between the ground state and the first excited state. Our main result is that the appearance of an ATP requires interactions between the conduction electrons and the ions of other than the simple isotropic exchange type. This implies that the ATP may serve as a valuable tool to detect more complicated types of thek-f interaction than the isotropic exchange.     

Exchange in rare earth metals and intermetallic compounds

Analytical expressions of the isotropic and the anisotropic exchange integrals in rare earth (RE) metals and intermetallic compounds are derived using the augmented-plane-wave (APW) formalism. Numerical results for DyZn indicate that the d electrons contribute predominantly to both isotropic and anisotropic exchange at most points on the Fermi surface.     

A simple theory of spin-wave relaxation in ferromagnetic metals

The anisotropic exchange interaction between localized spins and conduction electrons is described by an appropriate spin hamiltonian. This is used to calculate the lifetime of magnons for arbitrary values of Λ_eq, where Λ_e is the electron mean free path and q the magnon wavevector. At Λ_eq ? 1, this lifetime depends on the angle between q and the saturation magnetization. The antisymmetric part of anisotropic exchange (Dzialoshinsky-Moriya interaction) may dominate the relaxation of spin-waves of large q. The complicated band structure of transition metals gives rise to a magnon lifetime independent of Λ_e. The contribution of isotropic exchange is also considered.     

Phase diagram of the alternating-spin Heisenberg chain with extra isotropic three-body exchange interactions

For the time being isotropic three-body exchange interactions are scarcely explored and mostly used as a tool for constructing various exactly solvable one-dimensional models, although, generally speaking, such competing terms in generic Heisenberg spin systems can be expected to support specific quantum effects and phases. The Heisenberg chain constructed from alternating S = 1 and σ = 1/2 site spins defines a realistic prototype model admitting extra three-body exchange terms. Based on numerical density-matrix renormalization group (DMRG) and exact diagonalization (ED) calculations, we demonstrate that the additional isotropic three-body terms stabilize a variety of partially-polarized states as well as two specific non-magnetic states including a critical spin-liquid phase controlled by two Gaussinal conformal theories as well as a critical nematic-like phase characterized by dominant quadrupolar S-spin fluctuations. Most of the established effects are related to some specific features of the three-body interaction such as the promotion of local collinear spin configurations and the enhanced tendency towards nearest-neighbor clustering of the spins. It may be expected that most of the predicted effects of the isotropic three-body interaction persist in higher space dimensions.     

Influence of anisotropic fluctuations on the electrical resistivity in uniaxial ferromagnetic metals near Tc

The generalized Ornstein-Zernike correlation function appropriate to a uniaxial ferromagnet is applied to study the influence of anisotropic fluctuations on the electrical resistivity in uniaxial ferromagnetic metals near T_c. It is shown that both the anisotropic fluctuations and the Fermi surface anisotropy contribute on the same footing to an anisotropy of the spin-fluctuation resistivity. The results are consistent with those obtained for the isotropic system in the same approximation and may be useful for interpreting recent spin-fluctuation resistivity data obtained experimentally for uniaxial systems.     

Etude par RMN de la structure dynamique de l'alanate Na3AlH6

A low temperature wide-line NMR study has allowed a determination of the AI-H bond lengths in cryolite-type Na₃AlH₆. It was found from the thermal behavior of the proton lines that the |AIH₆|^3? octahedra reorient around a C₄ axis. Starting below 170 K, this rotation can be hindered by lattice defects. Above room temperature it becomes isotropic, and a quick protonic exchange appears.The thermal narrowing of the linewidth and the T₁ and T_1ρ relaxation times lead to activation energies of about 0.38 eV for axial rotation and 0.51 eV for protonic exchange. The minimum in T₁ is in good agreement with the exchange model. Absorption phenomena, as well as partial decomposition of Na₃AlH₆ during the heat treatments, explain the presence of small amounts of mobil hydrogen.     

The effective ion-ion interaction of heavy alkali metals

The effective ion-ion interaction for the heavy alkali metals is calculated by a full non-local model potential, including exchange and correlation in the screening according to Vashishta and Singwi and in the RPA. The inclusion of the s-d hybridization effects merely following Harrison's theory for transition metals turns out to be inadequate for these metals.     

Constant coupling approximation for antiferromagnets with mixed ferro- and antiferromagnetic interactions

The constant-coupling approximation is extended to an antiferromagnetic spin-$\text{1}\text{2}$ system with two distinct anisotropic exchange interactions. The thermodynamic properties such as the transition temperature, magnetization, susceptibility and specific heat are discussed for three special cases: (i) ferro- and antiferromagnetic Ising interactions, (ii) isotropic ferro- and antiferromagnetic Heisenberg interactions; and (iii) isotropic ferromagnetic Heisenberg interactions and antiferromagnetic Ising interactions, allowing in each case for two different nearest-neighbour coupling constants. Numerical calculations have been performed for a layer structure with z = 6 intraplanar and z' = 6 interplanar nearest neighbours and the results are compared with those obtained in other approximations. Applying the theory to FeCl₂, the exchange constants are evaluated. It is shown that the calculated magnitudes of the interactions strongly depend upon the exchange-interaction model assumed.     

EPR spectra of the double triangular polyoxovanadates with exchange and electron transfer effects: Symmetry breaking and effective motional averaging

The interplay of isotropic exchange, antisymmetric (AS) exchange, and electron delocalization has been theoretically investigated for a system comprising two triangles, one with three localized d¹ spins, which gives rise to spin frustration effects, and the other with one d¹ center delocalized on the three sites. In the localized limit the AS exchange is partially reduced by the low symmetry components of an isotropic exchange (inter-triangle interaction) and splits the spin triplets only in the second order, contrary to what is found for a single triangular system where splitting of the spin doublet levels occurs in the first order. Zero-field splitting parameters depend on both AS exchange parameters and low symmetry components of the isotropic exchange. The EPR spectrum consists of intratriplet (Heisenberg type) and intertriplet (non-Heisenberg type) lines. AS exchange also induces singlet-triplet transitions. The EPR spectra of powder samples are generated for a domain of key parameters. Two different situations occur depending on the symmetry of the electronic states when the electron delocalization is taken into account. The orbital doublets of the localized system indicate a complete averaging of the exchange interactions, which results in effective enhancement of AS exchange. Unlike triplets, the orbital singlets are unaffected by a transfer process, developing only second-order AS exchange. Coexistence of localized and delocalized states is expected to be an intrinsic feature of exchange systems with partial delocalization. The relevance of these effects to the magnetic properties of exchange clusters is briefly discussed.     

A new model for the ground-state spin determination in ferromagnetics: The influence of many-electron exchange effects on the ground-state total spin in titanium and nickel

A new approximate exchange interaction operator, derived from the exact atomic second quantization formalism, has been applied to the study of the magnetic properties of atoms with spin $\text{S =}\text{1}\text{2}$ and S = 1 which obey Hund's rule. The new operator involves the effects of one-pair and two-pair electron exchanges. The one-pair exchange effects are compared with the Heisenberg model. It is shown that our model comprises the symmetry properties of the atomic wavefunction and the interaction of all electrons in the unfilled shells of two atoms, which are not considered in the Heisenberg theory. The conditions under which the new one-pair exchange effect may be used to improve the old one are pointed out. The significance of the two-pair exchange effects is discussed. It is demonstrated that, owing to the simultaneous exchange of two electron pairs, the total ground-state spin of two atoms may have intermediate values in addition to those predicted by the Heisenberg model. The importance of the interrelation between the first- and second-order effects, with respect to both sign and magnitude, is pointed out. The existence of the intermediate values, even in the case where the first order exchange integral is negative, is taken as a conjecture by which one may explain the fact that the mean magnetic moment per atom in magnetic 3d metals is not equal to the spin magnetic moment of an individual atom and accounts for the existence of magnetic ordering when the first order exchange integral is negative.     

Spin-controlled negative magnetoresistance resulting from exchange interactions

We studied conductivity of AlGaAs–GaAs quantum well structures (where centers of the wells were doped by Be) at temperatures higher than 4 K in magnetic fields up 10 T. Throughout all the temperature region considered the conductivity demonstrated activated behavior. At moderate magnetic fields 0.1 T < H < 1 T, we observed negative isotropic magnetoresistance, which was linear in magnetic field while for magnetic field normal with respect to the plane of the wells the magnetoresistance was positive at H > 2T. To the best of our knowledge, it was the first observation of linear negative magnetoresistance, which would be isotropic with respect to the direction of magnetic field. While the isotropic character of magnetoresistance apparently evidences role of spins, the existing theoretical considerations concerning spin effects in conductance fail to explain our experimental results. We believe that such a behavior can be attributed to spin effects supported by exchange interactions between localized states.     

A reassessment of the role played by a 5d virtual bound state in the determination of crystal fields for heavy rare earth ions in gold and silver

Using the theory of Dixon and Wardlaw, it is shown that there is an important increase in the 5d virtual bound state contribution to the C₄ crystal field parameter for heavy rare earths in noble metals due to a significant decrease in the 4f-5d exchange interaction when screened.     

Low-temperature spectrum and damping of elementary excitations in biquadratic exchange systems (S = 1)

The diagrammatic technique for the standard basis operators and the expansion in powers of 1/z (where z is the number of spins interacting with any given spin) are used to calculate the energies of magnons (in the ferromagnetic and quadrupolar phases) and quadrons (in the ferromagnetic phase) and the bipolar and quadrupolar order parameters (in both phases) in the (1/z)¹ approximation and at low temperatures. it is shown that magnons are damped with the exception of the (ferromagnetic and quadrupolar) case of the Schrödinger exchange model and quadrupolar phase; the quadrons are undamped only in a special case of the ferromagnetic phase, i.e. in the ferrogmatic case of the Schrödinger exchange model. In the limit K → 0, i.e. the biquadratic exchange interaction tends to zero, the spectrum of magnons and their damping are identical with the results by Balakhonov et al. The spin waves and quadrupolar order parameter are discussed in more detail in the following cases of the quadrupolar phase: ferromagnetic-quadrupolar limit, isotropic purely quadrupolar interactions, and quadrupolar-antiferromagnetic limit.     

Magnetostriction of amorphous GdxAg1−x alloys

Magnetostriction measurements above 4.2 K and up to 2.2 T have been performed in the amorphous alloys Gd_xAg_1−x (0.30 ⩽ x ⩽ 0.40). Magnetostriction is purely of volume character. It shows with composition a maximum at x = 0.35, and this behaviour can be explained if forced volume magnetostriction derives form the strain dependence of the two-ion longitudinal spin correlation. The isotropic spin exchange correlation seems irrelevant in these series, as also put forward by thermal expansion measurements.     

Transition order and dynamics of a model with competing exchange and dipolar interactions

We performed Monte Carlo simulation of phase transitions from isotropic stripe phase with short-range order to long-range stripe phase in a model with competing ferromagnetic exchange and antiferromagnetic dipolar interactions on triangular lattice. We calculated phase diagram for different values of exchange and dipolar interaction constants ratio, η. We also determined the order of the transitions to stripe phases AFh of different stripe widths h: first-order phase transition was found to transitions into AF1 and AF2 phases, while transitions to AF3 and AF4 phases were of the second order. In the phase diagram the tricritical point was determined at the AF2 and AF3 phase boundary. We observed the peak of nematic phase at the transition region to the AF1 phase, but found it metastable at low values of η. We have also found that in AF1 phase spin relaxation corresponds to the Ising model dynamics. In phases AF3 and AF4 the dynamics slows down, and stripe domain growth with time is proportional to logt.     

Spatial dependence of the RKKY interaction in a semi-infinite d-dimensional medium

The expression proposed by Larsen for the form of the indirect exchange interaction in d-dimensional metals is proven to be correct for all d ⩾ 1. Corrections due to the presence of a specular surface are obtained and exhibited explicitly for d = 1, 2, 3.     

Detection of deep centers in semiconductors by strain modulated electron spin resonance: Pt+ in si

Strain-modulated ESR of p⁺-type silicon, doped with Pt (1.4 × 10¹⁷ Pt cm^-3) reveals a spectrum, characteristic of a triplet state. It is tentatively attributed to Pt⁺?Pt⁺ pairs. The g-factor (g = 1.94±0.01) is isotropic and extremely sensitive to strain: if we write δg = Fε we find ∥F∥? 2×10⁴ as an order of magnitude estimate. The exchange coupling is ferromagnetic and predominantly isotropic.     

Inelastic neutron scattering study of spin waves in MnO

The dispersion relation E(q) for spin waves in MnO has been measured at 4.2°K by neutron inelastic scattering. The isotropic exchange integrals as well as the anisotropy constants have been determined by fitting the Hamiltonian to the data. It is found that the exchange striction plays main role in the anomaly in the magnetic interactions in MnO and the biquadratic exchange interaction j₁(S₁·S₂)² is almost absent (j₁/J₁ ? 0.002).     

An EPR study of solid solutions of ferric oxide in γ-Al2O3

The EPR spectra of solid solutions of ferric oxide in γ-alumina have been studied. The dilute samples show two different spectra which are attributed to isolated ferric ions in the A and B sites of the spinel lattice. From the intensity dependence of the spectra on the iron concentration an approximate value of the range of the exchange forces is deduced. The concentrated samples show an isotropic spectrum at g = 2·033 which is attributed to clusters of ferric ions coupled by exchange. The temperature dependence of the intensity of the associated ions spectrum is attributed to the increase of the average spin value of the clusters with T.     

Large single-ion anisotropy and ferromagnetic intrachain interaction in four Ni-compounds of formula NiX2L2 with X = Cl,Br; L = N2C3H4, NC5H5

The specific heat has been measured of powdered samples of NiX₂L₂ (X = Cl, Br; L = pyrazole, pyridine), compounds which can be considered as weakly antiferromagnetically coupled ferromagnetic chains with large uniaxial single-ion anisotropy. The intrachain exchange constants and the zero-field splitting parameters for the four compounds have been deduced from the specific heat data, using accurate numerical calculations for the isotropic Heisenberg spin S = 1 linear chain model with uniaxial single-ion anisotropy.