Mössbauer studies of dilute Au:Er alloys: Obervation of hyperfine structure of ground and excited CEF levels

The hyperfine structure of dilute ¹⁶⁶Er impurities in Au has been investigated between 1.8 and 60 K by Mössbauer spectroscopy. The hyperfine spectrum of the Γ₇ electronic ground state is clearly observed below 4.2 K while at higher temperatures there is an indication of the contribution from the excited CEF-states Γ⁽¹⁾₈ and Γ₆. Using Hirst's relaxation theory for the Γ₇ ground state the magnetic hyperfine coupling constant A=(247±3) MHz and the exchange coupling constant J_sf=(0.10±0.02)eV were derived. A quadruple coupling constant B of about 1 MHz was estimated from the hyperfine pattern of the Γ⁽¹⁾₈ quartet.     

MnGa Acceptor Center in GaAs (Review)

Physics of the Solid State - The model of the MnGa acceptor in GaAs in which initial ground state of a coupled hole Γ8 is changed due to the antiferromagnetic exchange interaction with five 3d...     

Incommensurate phases in ferromagnetic spin-chains with weak antiferromagnetic interchain interaction

We study planar ferromagnetic spin-chain systems with weak antiferromagnetic inter-chain interaction and dipole-dipole interaction. The ground state depends sensitively on the relative strengths of antiferromagnetic exchange and dipole energies κ = J′a ² c/(g _L μ _B )². For increasing values of κ, the ground state changes from a ferromagnetic via a collinear antiferromagnetic and an incommensurate phase to a 120° structure for very large antiferromagnetic energy. Investigation of the magnetic phase diagram of the collinear phase, as realized in CsNiF₃, shows that the structure of the spin order depends sensitivly on the direction of the magnetic field in the hexagonal plane. For certain angular domains of the field incommensurate phases appear which are seperated by commensurate phases. When rotating the field, the wave vector characterizing the structure changes continously in the incommensurate phase, whereas in the commensurate phase the wave vector is locked to a fixed value describing a two-sublattice structure. This is a result of the competition between the exchange and the dipole-dipole interaction.     

Magnetization anisotropy in the AFM and SDW phases of CeB6

The angular dependences of the magnetization and Hall resistance have been investigated by the method of the sample rotation in the magnetic field in the high-quality single-crystal samples in the paramagnetic and magnetically ordered phases of CeB₆ in the magnetic field up to 60 kOe. It has been shown that, as CeB₆ undergoes the transition from the antiferromagnetic modulated phase to the so-called antiferroquadrupolar phase, the easy-magnetization axis in the [110] plane changes from 〈100〉 to 〈110〉. The magnetic field dependences of the anisotropic component of the magnetization differ radically in these magnetically ordered phases. The analysis provides evidence in favor of the formation of a state with the spin density wave (SDW phase) in the temperature range T _N ≈ 2.3 K < T < T _Q ≈ 3.3 K in CeB₆.     

Crystal field efects on the electron spin resonance of rare-earths in YPd3: Comparison with inelastic neutrons scattering experiments

We report low temperature Electron Spin Resonance experiments on diluted Er, Dy and Yb in YPd₃. The host cubic crystal field leaves a Γ₆ ground state for Er³⁺, and a Γ₇ excited state could be observed at 18 ± 4K for this system. A lower lying Γ₇ was measured for Yb³⁺ in YPd₃, and a broad undefined resonance for Dy³⁺ in this host. A comparison of our data with those obtained by Inelastic Neutrons Scattering is given.     

A tight-binding representation of electron-hole exchange interaction in semiconductors

The electron-hole exchange interaction in semiconductors is analyzed in the framework of the empirical tight-binding method. It is demonstrated that intra-atomic and interatomic contributions to the long-range exchange interaction enter in an inequivalent way. In particular, for the Γ₆×Γ₇ exciton in a spherical nanocrystal with a cubic lattice, the dipole-dipole contribution associated only with the intra-atomic (or intra-site) transitions does not lead to singlet-triplet splitting of the exciton level. The interatomic transitions, for example, anion-to-cation transitions between the nearest neighbors in binary semiconductor compounds, determine the so-called monopole-monopole contribution to the exchange splitting of the Γ₆×Γ₇ exciton, and this contribution does not vanish in a spherical nanocrystal.     

Anomalous specific heat of CeB6

Specific heat measurement of CeB₆ has been performed over the temperature region 1.6–77 K. Particularly the low-temperature anomaly, which consists of a sharp peak and a broad satellite, was examined in applied magnetic field up to 1.8 T. The observed field dependence and the entropy consideration reveal that both the two peaks are related to some intrinsic phase transitions which involve a complicated process of magnetic ordering. In the light of the reported data on magnetic properties, the anomalous features of this compound are discussed in terms of the unusual exchange interaction which differs remarkably in magnitude between the ground state and the excited state.     

The magnetic structure of cubic ErAl3

The magnetic structure of cubic or β-ErAl₃ has been investigated by neutron diffraction from powder samples. ErAl₃ undergoes a transition at 5·1°K, to an antiferromagnetic state with an enlarged tetragonal unit cell. The moments are directed perpendicular to the tetragonal c-axis. Crystal field effects are large and dominant in this compound. The extrapolated saturation moment is 5·1/μ_B, which corresponds to the Γ₈ ground state.     

Elastic and magnetoelastic effects in CeB6

Elastic constants of CeB₆ were redetermined by ultrasonic and Brillouin scattering measurements. We found all elastic constants to be positive, especiallyc ₁₂. The temperature and magnetic field dependence of the elastic modesc ₄₄ and (c ₁₁-c ₁₂)/2 is explained on the basis of the newly proposed crystalline-electric-field level scheme for CeB₆ with the ₇ level 545 K above the ₈ ground state. The deduced quadrupolar coupling constants give an antiferroquadrupolar ordering temperature of 3.3 K in agreement with experiment.Supported by the DFG, SFB65, SFB125 and SFB126     

Neutron scattering investigation of the magnetic ground state of PrO2

We report on a series of neutron experiments on stoichiometric PrO₂. The material, which has the cubic CaF₂ structure, orders antiferromagnetically at 14 K with a type I magnetic structure as found in UO₂. The ordered moment is 0.6 ± 0.1 μ_B/Pr atom. No lattice distortion has been observed below T_N with a high-resolution neutron-diffraction experiment. Neutron inelastic scattering at the Intense Pulsed Neutron Source, Argonne National Laboratory, has been used to determine the crystal-field splitting of 130 ± 5 meV between the Γ₈ ground state and the Γ₇ excited state of the $\text{J =}\text{5}\text{2}$ multiplet. This represents the first direct observation of a crystal-field splitting in the rare-earth or actinide dioxides, and the largest such electronic splitting ever observed by neutron scattering. We propose that the ground state of PrO₂ is the Γ₈ quartet with the degeneracy lifted by a dynamic Jahn-Teller effect. The A₄〈r⁴〉 crystal field potential term for PrO₂ is - (57 ± 3) meV, and the significance of this determination for other oxides is discussed.     

Influence of two-particle excited states on the interatomic exchange interaction in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The effective spin Hamiltonian for undoped cuprates is constructed in the framework of the realistic multiband p-d model with the parameters calculated from first principles. The exchange interaction parameter is defined as the sum of the antiferromagnetic and ferromagnetic contributions, which are determined by the two-hole triplet terms. The ferromagnetic and antiferromagnetic contributions of the excited terms compensate each other to a large extent. It is shown that the antiferromagnetic contribution of the two-hole ground singlet ¹ A _1g to the exchange interaction is dominant. Original Russian Text. V.A. Gavrichkov, S.G. Ovchinnikov, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 6, pp. 1037–1041.     

Magnetic interactions and crystal field in RMg3 compounds (R = Ce,Pr, Nd)

We have carried out a study of magnetic properties, electrical resistivity and of crystal field excitations using inelastic neutron scattering, in cubic RMg₃ compounds with light rare earths R = Ce, Pr and Nd. The ground states are the Γ₇ and Γ₆ doublets for CeMg₃ and NdMg₃, which order at 3.5 and 6.5 K, respectively. PrMg₃ has the non-magnetic Γ₃ ground state and remains a Van Vleck paramagnet. The magnetic contribution to the resistivity has been separated. It allows us to evaluate the couplings between the 4f shell and the conduction band which is much larger for CeMg₃ than for the other compounds. The coupling constant Γ is negative in this case, as shown by an incipient Kondo effect in this compound.     

High-pressure effect on the crystal and magnetic structures of the frustrated antiferromagnet YMnO<Subscript>3</Subscript>

The high-pressure (to 5 GPa) effect on the crystal and magnetic structures of the hexagonal manganite YMnO₃ is studied by neutron diffraction in the temperature range 10–295 K. A spin-liquid state due to magnetic frustration on the triangular lattice formed by Mn ions is observed in this compound at normal pressure and T > T_N = 70 K, and an ordered triangular antiferromagnetic state with the symmetry of the irreducible representation Γ₁ arises at T < T_N. The high-pressure effect leads to a spin reorientation of Mn magnetic moments and a change in the symmetry of the antiferromagnetic structure, which can be described by a combination of the irreducible representations Γ₁ and Γ₂. In addition, it is observed that the ordered magnetic moment of Mn ions decreases from 3.27 μ_B (5 GPa) to 1.52 μ_B (5 GPa) at T = 10 K and diffuse scattering is enhanced at temperatures close to T_N. These effects can be explained within the model of the coexistence of the ordered antiferromagnetic phase and the spin-liquid state, whose volume fraction increases with pressure due to the enhancement of frustration effects.     

Properties of the absorption spectrum due to the Gamma;1+ → Γ4− transition

We present results for the absorption spectrum due to a localized Γ₁⁺ → Γ₄^? transition that take into account the properties of the linear electron- phonon (e-p) interactions transforming as Γ₁⁺, Γ₃⁺ and Γ₅⁺ in the excited electronic state, in the cases of strong and weak e-p interaction coupling. We show that in the strong e-p interaction coupling limit the asymmetric shape of the structured band is due to the commutation relations of the e-p interaction matrices. Moreover, in the weak coupling limit we present an expression for the spectrum line shape obtained by taking into account the time ordering of the e-p interaction matrices and the phonon propagators at all times. It is shown in the latter case that the densities of phonon states corresponding to the electronic excited state are different from those corresponding to the ground state, and the e-p coupling constants are redefined due to the Jahn-Teller interactions.     

Anomalous magnetic field dependence of T AF in La 0.95 Sr 0.05 MnO 3

The magneto-elastic properties of single-crystalline La_0.95Sr_0.05MnO₃ have been studied ultrasonically. Our investigations focussed on the temperature interval where magnetic ordering starts to evolve and results in a spin canted antiferromagnetic ground state. In detail the experiments revealed that the magnetic order parameter in low-doped manganite is only weakly coupled to lattice strains. Furthermore, the anomalous temperature dependence of the order parameter as found resembles highly that in stoichiometric LaMnO₃. However, the main and most surprising finding is that external magnetic fields favor the spin canted phase in La_0.95Sr_0.05MnO₃. It is unclear at present how the exchange interaction can be tuned by magnetic fields in the way observed and we are not aware of existing theoretical concepts which might give a plausible explanation for the unexpected field dependent behavior of the critical temperature. We believe, however, that this behavior primarily results from the fact that the exchange interaction depends sensitively on the orbital configuration of the manganese d electrons. Received 27 March 2000     

Relaxation effects in the ESR of Gd in the metallic Van-Vleck paramagnet: PrSb

The ESR g shift and linewidth of Gd in the metallic Van-Vleck paramagnet, PrSb, exhibit appreciable change upon increasing the temperatures above T = 20 K. A model is suggested to explain the linewidth behavior. It is argued that while the g shift behavior is mainly associated with the admixture of the first excited crystalline field level, Γ₄, into the singlet ground state Γ₁, the relaxation is primarily via the Γ₅ excited state of Pr³⁺     

Influence of crystals fields on the magnetic form factor of samarium

It is calculated that crstal fields, with inclusion of mixing of higher multiplets into the ground $\text{J =}\text{5}\text{2}$ state, have a substantial effect on the magnetic form factor of Sm³⁺. From the neutron diffraction data obtained by Koehler and Moon for the cubic site in Sm metal it follows that the quartet Γ₈ is lowest, with the sixth-order crystalline field parameter A₆〈ritr⁶〉 positive. The conduction electron moment residing on Sm is estimated to be of the order of 0.4 μ_B.     

Neutron studies of crystal-field effects in PrB<Subscript>6</Subscript>

The roles of various physical mechanisms in the properties of the ground state of the Pr ion in PrB₆ are studied by measuring magnetic excitation spectra in the paramagnetic and magnetically ordered phases of polycrystalline PrB₆. The ground state of the Pr ion in the paramagnetic phase is experimentally found to be triplet Γ₅. During the transition into the magnetically ordered state, this triplet splits into three singlet levels. The results obtained do not exclude the appearance of an additional contribution to the splitting induced by a decrease in the local symmetry because of structural distortions.     

Magnetooptical evidence of exchange interactions in zero-gap Hg1−xFexTe mixed crystals

Interband magnetoabsorption is carried out on zero gap Hg_1-xFe_xTe alloys of x ～ 0.015.Γ₆ → Γ₈ magnetooptical spectra for σ⁺, σ^-, γ 6 H polarization are quantitatively interpreted within the “quasi Ge” model modified by the inclusion of exchange contributions. The field dependence of the magnetization provides evidence of antiferromagnetic interactions between localized spins.