Conduction Electron Polarization Research Articles

Effect of conduction-electron polarization on the magnetism of hcp samarium metal

Peculiar magnetic properties of hcp Sm are adequately described by the calculations based on the model with ${\mathrm{Sm}}^{3+}$ ions in a metallic matrix, taking into account the lowest three multiplets, the spin-orbit interaction, the crystal fields, and the exchange interaction, and together with the conduction-electron spin polarization. It is found that the effect of the conduction-electron polarization is essential to them. By the analyses, it is concluded that the conduction-electron moment is larger than the localized $4f$ one and that the polarity of the spin part of the magnetic moment enhanced by the conduction-electron polarization is positive to the total moment. This polarity of the spin moment, which is opposite to the case for the free ${\mathrm{Sm}}^{3+}$ ion, is furthermore experimentally verified by the magnetic measurements on the hcp SmNd and SmGd alloys. It is also elucidated through the calculations that the difference between the temperature dependence of the spin moment and that of the orbital one due to the admixture of the $J$ multiplets, in general, results in a variety of temperature dependence of the ordered moment for each ${\mathrm{Sm}}^{3+}$ ion, just like the thermomagnetic behavior of ferrimagnets.

Crystal electric field effects on the magnetic form factor of the Kondo compound CeNi 2Al 5

Polarized neutron measurements were undertaken at T = 5.3 K on the orthorhombic Kondo compound CeNi 2Al 5 ( T N = 2.6 K). The magnetic density induced by a 7 T field is located on Ce sites only, with a shape slightly elongated along a . The anisotropy observed on the Ce magnetic form factor is well accounted for by the CEF parameters determined previously from magnetization and inelastic neutron spectroscopy results. A small positive polarization of conduction electrons is observed, as often in Ce compounds.

Magnetization density in the intermediate valence compound YbAl 3

Polarized neutron diffraction studies have been performed at various temperatures on a flux grown single crystal of YbAl 3. Only the Yb carries a field-induced magnetic moment, which is well described by a free ion Yb 3+ form factor. The thermal variation of the localized 4f Yb moment exhibits a second maximum at T ≈ 15 K, as recently observed in the bulk magnetization, in addition to the well-known maximum at T = 130 K. A small temperature-independent positive conduction electron polarization is suggested. These results are important to the understanding of the YbAl 3 ground state.

Local magnetization and hyperfine field systematics of s−p and noble impurities in Gd and Ni hosts

Magnetic hyperfine data of s−p and noble impurities diluted in ferromagnetic Gd and Ni hosts are described within a simple model which is an extension on that one of Daniel and Friedel. We also include in the present model the effect of next-neighbor perturbation, due to the translational invariance break introduced by the impurity. Performing a self-consistent calculation of the local magnetic moments at the impurity site, one obtains the conduction electron polarization (CEP) hyperfine field. It is found that the model can explain the different hyperfine field trends observed in Gd and Ni hosts, e.g., a remaining negative value along the s−p series in the Gd case and a change of sign behavior in the Ni case. Period effects observed in noble impurities are also discussed and the theoretical calculations are in good agreement with available experimental data.

Magnetic Polarization of Conduction Electrons at Au Layers in Fe/Au Multilayers by119Sn Mössbauer Spectroscopy

119 Sn Mössbauer measurements were performed for Fe/Au multilayers with 119 Sn probes to study magnetic properties of Au layers. 119 Sn Mössbauer probes were embedded in Au layers of Fe/Au multilayers which were made by ultra-high vacuum evaporation. 119 Sn Mössbauer spectra showed large magnetic hyperfine fields, indicating that Au layers were polarized by ferromagnetic Fe layers. Mössbauer spectra were measured with applied field at 5 K. By computer fitting it was found that the hyperfine fields due to the magnetic polarization have both parallel and antiparallel contributions to the applied field direction.

Competition between Kondo effect and magnetic interaction in Kondo systems

The phenomena of Kondo screening and moment reduction in almost localized Kondo systems are studied on the basis of a molecular-field approach. An interpretation of moment reduction as observed by elastic neutron scattering is given; it is shown how to describe the perturbative conduction electron polarization, and that the long-ranged Kondo screening cloud cannot be measured by neutron scattering. The interplay between Kondo reduction of magnetic moment and the effect of crystal fields is investigated. The model is applied to several cubic Ce systems, where the amount of Kondo reduction is found to be substantial.

THEORY OF THE MAGNETIC FORM FACTOR IN REDUCED-MOMENT KONDO SYSTEMS

The magnetic form factor in Kondo systems, as measured by polarized neutron elastic scattering, is uniformly reduced (with respect to the atomic form factor in the crystal field) due to the Kondo effect, and moreover it shows deviations at small scattering vector due to a spin polarization of conduction electrons: the sign of the deviations in Ce compounds is positive, which is opposite to what is observed in normal rare earths of the first half of the series, and expected from the s-f exchange Hamiltonian. In the present work the above two anomalies are interpreted within a theory for the magnetic form factor in Kondo systems. The uniform reduction of the form factor follows from a variational calculation for the one-impurity Anderson model in an effective magnetic field: Kondo reduction of the localized moment is distinguished here from the presence of a screening cloud, which cannot be observed by neutron scattering. The conduction electron polarization is first studied for the Coqblin-Schrieffer Hamiltonian, which we find to lead again to negative deviations of the form factor. Within the Anderson impurity model, the conduction electron contribution to the form factor is found to be the sum of two terms: a negative RKKY-like polarization, and a positive superexchange term, which is nonzero even for a filled (or empty) band. Dominance of the superexchange polarization could explain the observed sign of the deviations in Ce compounds. A variational basis which describes both Kondo reduction and conduction electron polarization effects is studied: the two effects are clearly separated, since they appear in different orders of the calculations. The conduction electron polarization is shown to be reduced due to the Kondo effect by the same spin-fluctuation factor which reduces the localized moment.

The field dependence of the hyperfine splitting of terbium in

The hyperfine splitting of the ground state of terbium in a single crystal of has been studied using spin-echo NMR at 1.3 K and in fields up to 8 T. The measurements are in perfect agreement with computations including corrections for the effect of J-mixing. From the field dependence of the quadrupolar splitting we have derived the electric field gradient at the terbium nucleus: and the ratio of antishielding factors: . The value obtained for is significantly different from that measured for the insulating compound . The contributions of the spin- and orbitally polarized conduction electrons to the total hyperfine field: T and T are obtained from the field dependence of the dipolar splitting.

Hybridization Effects on Conduction-Electron Polarization and Two-Ion Coupling in PrCu2Si2

The temperature dependence and the anisotropy of the magnetic susceptibility and the Knight shifts have been measured in PrCu 2 Si 2 and GdCu 2 Si 2 with magnetically aligned powder samples. For GdCu 2 Si 2 , the isotropic s-f exchange model provides a good description of its magnetic properties, but in PrCu 2 Si 2 an anisotropic two-ion coupling has been found. It has been revealed that the hyperfine coupling at the Si sites is enhanced in PrCu 2 Si 2 , thereby suggesting preferential hybridization of the f-electrons with the Si valence electrons. This effect is responsible for the unusually high Neel temperature of PrCu 2 Si 2 .

NMR studies of magnetic multilayers

In connection with giant magnetoresistance and long-range exchange coupling between magnetic layers, both the spatial distribution of the local magnetic field in nonmagnetic layers and the interface magnetism in magnetic layers have been investigated using the NMR technique in [Ni/Cu/Ni], [Co/Cu/Co] and [Fe/V/Fe] multilayers. From the analysis of the Cu and V NMR spectra (distribution of the local magnetic field), two characteristic features have emerged: (a) the conduction electron polarization in the nonmagnetic spacer layers due to the magnetic layers has a long-range nature and manifests itself in an oscillatory distribution of the Cu NMR shift, and (b) the distribution of the shift, and hence the distribution of the magnetic-moment-induced spin polarization in the nonmagnetic layers, depends strongly on the orientation of the external magnetic field to the layer plane. The interface magnetism, which is strongly related to the interface atomic arrangements, has also been studied from the analysis of the distribution of the hyperfine field of ferromagnetic atoms observed by zero-field ferromagnetic nuclear resonance (FNR). In Cu/Co superlattices, there has been observed a correlation between the magnetoresistance ratio and the interface structure determined by the hyperfine field distribution.

Magnetic form factor Kondo systems: Kondo moment reduction versus conduction electron polarization

A theory for the magnetic form factor in Kondo systems is presented which allows us to distinguish Kondo reduction of the moments from conduction electron polarization effects. The sign of the deviations of the form factor at small scattering vector is ascribed to the dominance of a superexchange-like contribution to the conduction electron polarization, which is contained in the Anderson model in addition to the usual RKKY polarization.

197Au MÖssbauer Study of Au/Fe, Au/Co and Au/Ni Magnetic Multilayers

ABSTRACT197Au Mössbauer measurements have been performed for Au/Fe, Au/Co and Au/Ni magnetic multilayers. 197Au Mössbauer spectra observed from multilayers consist of at least 4 components having different magnitude of hyperfine fields and isomer shift values those depend on the local environments of the Au probe-atoms in multilayers. Rather large electron spin polarization at Au atoms has been observed in the interface with adjacent ferromagnetic layers, but nearly no magnetic hyperfine field has been observed to the interior of Au layer. It implies that the largest hyperfine field observed is not due to the conduction-electron polarization but induced by the hybridization in the interface with ferromagnetic layer.

Temperature-dependent electronic structure of gadolinium

We investigate the electronic structure of the ferromagnetic 4f-metal Gadolinium by use of a many-body evaluation of a generalized model of magnetism, the one-particle part of which is derived from an ASW-LSDA bandstructure calculation. A striking temperature-dependence of the conduction band states is traced back to a 4f-(5d, 6s) interband exchange. The conduction electron polarization (0.63 μ B atT=0) decreases forT→T c very similar to the 4f-magnetization. A red shift of the lower ↑-band edge of about 0.25 eV appears upon cooling fromT=T c toT=0. — The quasiparticle band-structure exibits a remarkable non uniform magnetic behaviour at different positions in the Brillouin zone, and in particular for different subbands. Weakly correlated (“s-like”) dispersions show a “Stoner-like”T-dependence of the exchange splitting. On the other hand, stronger correlated (“d-like”) dispersions split belowT c into four branches, two for each spin direction. TheirT-dependence mainly concerns the spectral weights of the quasiparticle peaks and not so much the energetic positions. An exchange caused splitting remains even forT<T c .

Magnetic Polarization of Au Layers in Co/Au Multilayers Observed by119Sn Mössbauer Effect

In order to investigate the magnetic properties of nonmagnetic metal layers in multilayers, we prepared Co/Au multilayers in which 119 Sn layers are inserted into Au layers, and applied Mossbauer spectroscopy. A fraction with large magnetic hyperfine fields was observed in the Mossbauer spectra, and thus magnetic polarization of conduction electrons in nonmagnetic layers is confirmed.

Magnetism and hyperfine fields in YFe10V2: a combined nuclear magnetic resonance and Mossbauer study

A combined nuclear magnetic resonance (NMR) and Mossbauer effect study of YFe10V2 has been carried out. Based on the 57Fe hyperfine fields (HFS) obtained from the Mossbauer results, the resonance lines of the NMR spectrum are assigned and the site preference of V atoms is determined. On the other hand, according to the site preference of vanadium atoms determined by NMR, together with neutron diffraction results, the 57Fe Mossbauer spectrum is fitted. It can be deduced from the NMR results that the existence of one nearest-neighbour Fe atom leads to an increase in the V HF of about 0.35, and the core electron polarization contribution Hcp, the 4s conduction electron polarization contribution Hs and the effect of the second-nearest-neighbour Fe atoms on the HF sites cannot be negligible.

Spin fluctuations in the heavy-fermion paramagnetic U(In0.5Sn0.5)3

Abstract 119 Sn Mossbauer spectra of the heavy-fermion intermetallic system U(In 0.5 Sn 0.5 ) 3 have been measured in the temperature range 4.2–300 K. The room temperature spectrum is characterized by a quadrupole doublet with a splitting value of 1.45 mm/s. With decreasing temperature the spectral line width increases and a hyperfine magnetic structure is developed at helium temperatures. This hyperfine field may be interpreted as being transferred via the conduction-electron polarization from the U to the Sn nucleus. A detailed analysis of the spectral evolution with temperature proves the presence of spin fluctuations in U(In 0.5 Sn 0.5 ) 3 near the transition from local-moment magnetism to itinerant f-electron behavior. The temperature dependence of the spin relaxation rate has been determined.

155Gd Mössbauer spectroscopy study of the Gd 2Fe 17H x system

155Gd Mössbauer spectroscopy experiments have been undertaken on the Gd 2Fe 17H x ( x = 0, 3, 5) series and the results are analysed in the light of the structural and magnetic characteristics of these compounds. In contrast to carbon or nitrogen insertion, hydrogen is found to lower the positive electric field gradient ( V zz ) at the Gd nuclei. A decrease of the crystal field induced anisotropy is anticipated in the R 2Fe 17H x compounds. The reduction of the hyperfine field upon hydrogenation is attributed to a decrease of the (positive) field due to the conduction electron polarization. The larger isomer shift observed in the hydrides is assigned to a charge transfer from the Gd 6s orbital onto the H atoms.

Effect of spin injection on the conduction-electron spin resonance at a ferromagnet-semiconductor contact

A theory of the electric dipole spin-resonance (EDSR) absorption for conduction electrons at a ferromagnet-semiconductor interface is given on the basis of quasiclassical transport equations for moments of the spin density matrix. It is supposed that a uniform DC magnetic field is applied perpendicular to the interfacial contact plane. An electric current passing through the interface is attended by a spin density flow which causes polarization of conduction electrons in a semiconductor. It is shown that spin injection leads to a considerable increase (exceeding one order of magnitude) of the EDSR signal intensity in semiconductors having the zinc blende or the wurtzite structure.

Magnetization density in heavy-fermion UPd2Al3

The induced magnetization density has been measured at T = 36 K (above the antiferromagnetic ordering temperature of 14 K) in a single crystal of the heavy-fermion material UPd2Al3. A magnetic field of 5 T was applied along the easy axis of magnetization. The magnetization density as measured by the polarized-neutron technique is associated totally with the uranium atom; no spin transfer is found on the Pd atoms. The induced moment, as measured by neutrons, is some 12% below that measured by magnetization measurements on the same crystal. This indicates the presence of a significant positive (with respect to the uranium moment) conduction electron polarization. The ratio of the orbital to the spin moment of uranium found experimentally is -2.01(25), compared with the free-ion values of -2.56 (for U3+) and -3.29 (for U4+), and a recent theoretical prediction of-1.35.

On the microscopic model of Fe and Ni: the possible breakdown of the ferromagnetic fermi-liquid picture

A new model of strong itinerant ferromagnetism, taking into account the presence of a narrow density-of-states peak at EF, which is due to merging of van Hove singularities, is proposed. The states of the peak (eg-type states for Fe) are treated within the framework of the Hubbard model with strong correlations. The role of the non-quasi-particle states, which lie near EF and are not described by the theory of the ferromagnetic Fermi liquid, is investigated. In particular, experimental data on the spin polarization of conduction electrons and T-dependence of resistivity are discussed.