Magnetic Spin Fluctuations Research Articles

Micro-probed raman scattering in La2-xSrxCuO4

Abstract The two-magnon scattering peak in La2-x Sr x CuO4 single crystals has been investigated by using a micro-probed Raman scattering technique. The peak intensity and the shape of two-magnon scattering are very sensitive to the Sr concentration. In La2CuO4, a micro analysis of the two-magnon scattering has been carried out as a function of position on the surface of the sample crystal. The peak frequency of two-magnon peak moves slightly, about 156 cm−1, without any change of spectrum shape as the observation position sweeps from 0 to 50 μm. On the other hand, the phonon peak does not show any anomalies. Furthermore, EPMA data shows that local compositions of Cu and La atoms distribute about 2% in the observed region of the sample. It suggests that the magnetic spin fluctuation is essentially affected by the local structure of the sample.

Heavy fermion behaviour in lanthanide and actinide materials

The electronic properties of Ce and U intermetallics are strongly affected by f-ligand hybridization leading to delocalization of the f-states, and by Coulomb interactions responsible for electron-electron correlations. Heavy-fermion behaviour is distinguished by a large electron specific heat coefficient γ pointing to a renormalization of the electron effective mass. In Ce compounds this appears in Kondo or Kondo lattice systems owing to a narrow resonance in the quasiparticle density of states at E F. The Ce charge state is very close to 4f 1 and the 4f 1 states are located well below E F. The U 5f electron wave functions tend to a more itinerant state, and the band picture is a more appropriate starting point for understanding U systems. The Ce and U materials exhibit different crossovers to magnetism. In Ce compounds, this is typically the transition from high γ Kondo systems to low γ local-moment antiferromagnets. The maximum γ values can be observed in U materials close to a magnetic instability or in the antiferromagnetic ordering of small 5f moments. The 5f states seem to be present at E F without any doubt, and the enhancement of γ is due probably to magnetic (spin) fluctuations. Itinerant 5f states can be concluded from a broad quasielastic response in inelastic neutron scattering and from electron spectroscopy experiments. The development towards the heavy-fermion state is compared for CeTX, CeT 2X 2, UTX and UT 2X 2 compounds.

Dynamical properties of critical and paramagnetic spin fluctuations in anisotropic Heisenberg magnets

Decay rates for long-wavelength spin fluctuations in the standard x-y model, and a second model described by the sum of the full Heisenberg exchange interaction and a single-site anisotropy, are calculated on the basis of a coupled mode theory in which the isothermal susceptibilities are consistent with corresponding spherical models (in consequence, quantum fluctuations are absent). Results for the decay rates are provided for two- and three-dimensional lattices and ferromagnetic and antiferromagnetic exchange interactions. Their dependence on the wave vector, q, and the inverse correlation lengths are derived from integral equations that admit homogeneous functions as solutions. The decay rates may thus be expressed as qd2/R1(q/ kappa 0) and q(d+2/2)R2(q/ kappa ,q/ kappa 0) where R1 and R2 are, respectively, scaling functions for the x-y and ferromagnetic single-site anisotropy models, d is the spatial dimension, and kappa (KO) is the out of plane (in plane) inverse correlation length. Analytical and numerical values of the scaling functions are presented for the limits of main physical interest.

Investigation of magnetic structure in R 2CuO 4 (R=La, Sr, Nd, Ce, Eu) by polarized neutrons

Experimental results on the magnetic structure, phase transitions and spin fluctuations in La 2CuO 4, La 1.92Sr 0.08CuO 4, Nd 2CuO 4, Nd 1.87Ce 0.19CuO 4 and Eu 2CuO 4 are presented.

Theory of paramagnetic and critical spin fluctuations in simple magnets

Recent theoretical work on the response functions observed in magnetic neutron scattering and muon spin relaxation experiments is briefly surveyed. Coupled-mode theory is found to provide a tolerable account of experiments performed on the paramagnetic and critical regions over a wide range of frequencies and wave vectors.

Magnetic properties of Pr in non-superconducting PrBa 2Cu 3O 7

The magnetic order and spin fluctuations of Pr in non-superconducting PrBa 2Cu 3O 7 have been studied by specific heat, susceptibility and neutron scattering measurements. The neutron data show that the basic ordering consists of a simple antiferromagnetic arrangement, with an ordering temperature of ∼17 K, while the specific heat data reveal a large value of the electronic specific heat coefficient γ, comparable to heavy fermion like materials. The observed magnetic inelastic scattering shows a broad quasi-elastic response as a function of energy, similar to mixed valent-like systems. These results suggest that there is substantial f-electron character at the Fermi level in this material.

Magnetic order and spin fluctuations in oxide superconductors

A brief review is given of both the rare earth ( R ) and Cu magnetism in the R Ba 2Cu 3O 6+ x , R Ba 2Cu 4O 8, (La-Sr) 2CuO 4 and (Nd-Ce) 2CuO 4 systems. The Cu magnetism is dominated by the strong in-plane exchange interactions, which give rise to two-dimensional magnetic behavior in both the antiferromagnetically ordered insulating phase as well as in the superconducting state. The rare earth ions, on the other hand, order at low temperatures irrespective of the presence or absence of superconductivity. In the 1-2-3 and 2-4-8 materials, 2-d behavior is observed, while in the (Nd-Ce) 2CuO 4 system there is a substantial magnetic interaction between the Nd and Cu sublattices.

Analytic dynamics of the one-dimensional tight-binding model. III. Numerical studies

For pt.II see J. Phys. C., vol.21, p.4967 (1989). Properties of electron dynamics in a magnetic field and spin fluctuations in a modulated magnet are studied by implementing formulae for response functions derived in two previous papers. Results for the electron band structure and density of states are reported for a wide range of rational magnetic fields and (tight-binding) band-structure parameters. The special features of spin fluctuations in longitudinally modulated magnets are reported in terms of the transverse dynamic susceptibility observed in inelastic neutron scattering experiments.

Critical and paramagnetic spin fluctuations in Heisenberg magnets.

The frequency- and wave-vector-dependent spin response function is calculated for a Heisenberg ferromagnet using the mode-coupling theory. Results are presented for the critical and paramagnetic regions. The work is motivated largely by recent, high-resolution neutron spectroscopy studies of EuO and EuS. In view of this, the Heisenberg model used incorporates nearest- and next-nearest-exchange interactions between spins on a face-centered-cubic lattice. The coupled-mode theory is shown to be successful in describing the measurements. Additional features are predicted as possible subjects of future experimental work. We present a review of various derivations of mode-coupling equations and demonstrate the equivalence of some prescriptions. The question of incorporating an external magnetic field in the theory is addressed, and we conclude that readily attainable fields have a minimal effect on the spin dynamics.

Spin fluctuations in the magnetic superconductor Y9Co7

The experimental data of magnetisation, Curie temperature and spin-lattice relaxation time for the magnetic superconductor Y9Co7 were analysed in the frame of the recently developed magnetic spin fluctuation models. The basic parameters of the models and the relaxation frequency function of a spontaneous spin fluctuation were calculated.

Effect of spin scattering and magnetic order on the electronic heat capacity of magnetic superconductors.

An expression for the difference in thermodynamic potentials ${\ensuremath{\Omega}}_{s}$-${\ensuremath{\Omega}}_{n}$ of the superconducting and normal states is derived by use of thermodynamic Green's functions. The change in the heat capacity ${C}_{s}$-${C}_{n}$ in the normal-to-superconducting phase transition in the presence of a periodic magnetic order and spin fluctuations is obtained. The jump \ensuremath{\delta}C in the specific heat at superconducting transition temperature is calculated. The variation of electronic heat capacity C with temperature for T\ensuremath{\lessgtr}${T}_{c}$ is studied. It is noted that the presence of magnetic order reduces the jump in specific heat at ${T}_{c}$.

Superconductivity in amorphous Cr films

Superconducting transition has been observed for the first time in amorphous Cr films prepared by co-sputtering of Cr and BN. Superconducting transition temperatures ( Tc) up to 1.14 K have been observed with an orbital contribution (i.e.(d H c2⊥ /d T) Tc ) as large as 49 kOe K -1. The electronic specific heat coefficient (γ) has been estimated to be about 1.6 times as large as that of amorphous Mo-BN (or Mo-metalloid) alloys. This finding, i.e. the low Tc with the large γ for amorphous Cr is inconsistent with data reported for amorphous alloys of 4 d and 5 d transition elements. This inconsistency implies a formation of local magnetic moments or spin fluctuation in the amorphous Cr film.

Magnetism and superconductivity in intermetallic uranium compounds under high pressures

A survey of pressure effects on magnetic order, spin fluctuations and superconductivity in the uranium intermetallics is presented. In the limit of localived 5f electrons, pressure effects on the spontaneous magnetic moment are found to be weak, whereas in the itinerant case magnetism is appreciably suppressed by increasing pressures. Several of the uranium intermetallics combine superconductivity with large values for the coefficient of the electronic term in the specific heat. In this class of “heavy fermion” superconductors, positive as well as negative pressure effects on the superconducting transition temperature have been found. In the analysis of the high-pressure data, special attention is paid to magnetic order in UNi 2 and superconductivity in UPt 3.

Theory of magnetic superconductors in an external magnetic field

The theory of Abrikosov and Gor'kov has been extended to study superconducting systems containing a lattice of magnetic ions. Differential equations have been set up for the Green's-function matrices which describe a magnetic superconductor in the presence of local-spin-conduction-electron exchange and external magnetic field. The self-consistent gap equation for the system has been formulated in terms of normal-state Green's functions. The solutions for the normal-state Green's functions ${?}^{0}$ have been obtained including the effects of magnetic order and spin fluctuations. The gap equation has been solved to obtain an analytical expression for the upper critical field ${H}_{c2}(T)$ of a magnetic superconductor. With the use of the results for the normal-state Green's functions ${?}^{0}$, the upper critical field has been calculated for a ferromagnetic superconductor in paramagnetic phase and a few antiferromagnetic superconductors in both paramagnetic and antiferromagnetic phases. The anomalous temperature dependence of the upper critical field ${H}_{c2}(T)$ has been analyzed in terms of contributions from magnetic order, spin fluctuation, and an indirect interaction of the conduction electron with staggered magnetization ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{M}}}_{Q}$ via magnetic induction field $\stackrel{\ensuremath{\rightarrow}}{\mathrm{B}}$.

Superconductivity and the electronic structure of Zr- and Hf-based metallic glasses

The results of a comprehensive study of the superconducting transition temperatures of Zr- and Hf-based metallic glasses are reported. The microscopic origins of superconductivity in these glasses are discussed in terms of recent ultraviolet photoelectron spectroscopy (UPS) measurements and calculations based on the renormalized atom technique. These calculations accurately predict the UPS spectra and the results of low-temperature heat-capacity measurements. The complete description of the electronic structure afforded by these calculations allows, for the first time, a consistent picture of the variation of Tc with X in the glasses Zr1-yXy (X=3d or 4d transition metals). In addition, the dependence of Tc with composition (y) can be understood in terms of the X d subband positions relative to EF. The results reported here support our recent contention that the strong depression of Tc observed for X=Fe, Mn, Cr, and V glasses is related to the formation of localized magnetic moments and spin fluctuations. An alternate explanation for the low Tc of X=V and Cr glasses based on the idea of an atomic-structure change is also discussed.

Superconductivity in Zr-3d metallic glasses

We report the results of T c and H c2 (ï) measurements in Zr 70X 30 metallic glasses. The relation between D(E F) and X is discussed in terms of recent UPS measurements. A strong depression of T c is observed for X = Fe, Mn, and Cr which we attribute to the formation of localized magnetic moments and spin fluctuations.

Nuclear Magnetic Relaxation and Spin Fluctuations in α-Mn by N.M.R.

The temperature dependence of the nuclear spin-lattice ( T 1 ) and spin-spin ( T 2 ) relaxation times in α-Mn metal has been studied for the four different Mn sites between 95 K ( T N ) and room temperature by means of cw and pulsed NMR techniques. The 1/ T 1 for sites I and II estimated from cw NMR linewidth was found to be almost temperature independent as ∼1.9 ×10 5 (s -1 ). This high relaxation rate is interpreted in terms of the existence of localized spin fluctuations at sites I and II. The temperature dependence of 1/ T 1 for sites III and IV was found to be 1/ T 1 =0.50 ×10 3 +4.4 T (s -1 ) from 120 K to room temperature. The first term arises from the localized spin fluctuations at sites I and II via RKKY type interaction. The second T-linear term is Korringa type relaxation effect due to the band electron in α-Mn.

Local-band theory of itinerant ferromagnetism. V. Statistical mechanics of spin waves

The functional-integral method of the previous paper is generalized to allow a discussion of the dynamical spin fluctuations in itinerant-electron magnets. The magnitude fluctuations and nonstatic spin fluctuations are treated as a perturbation on the nonlinearly interacting static spin fluctuations. The result is a free energy which is a sum of a Stoner contribution together with correlation corrections, a classical Heisenberg free energy as found in the previous paper, and a quantum spin-wave free energy minus its zero-frequency part.

The theory of spin fluctuations in itinerant-electron magnets

The thermodynamic properties of itinerant-electron magnets may be regarded in general as being governed by spin density fluctuations resulting from the strongly correlated motion of thermally excited electron-hole pairs. Itinerant- electron magnets are classified herein in terms of the relative strength of the electron-electron correlations and the various kinds of spin fluctuation that result. Two important limiting cases are the local-moment limit and the weakly-magnetic limit. It is possible to develop a theory based on the idea of approaching the subject from the latter limit, and this allows one to make significant improvements over conventional theories based on the static and dynamic Hartree-Fock approximations, by taking account of the mode-mode coupling in a self-consistent manner. Recent developments in this type of theory are discussed, and it is further extended to include a description of the spin-wave-like modes above Tc as observed in Ni and Fe by neutron scattering. A further discussion is given from a more general point of view.

Recent developments in the theory of spin fluctuations in itinerant electron magnets

The static and dynamical Hartree-Fock approximations (HFA) as a conventional approach to the itinerant electron magnetism are insufficient in describing either the equilibrium state or the spin density fluctuations and related physical properties at finite temperatures. Recent theoretical developments have been focussed on improving over HFA by taking account of the mode-mode coupling or the renormalization effect. The self-consistent renormalization theory, as an approach from the weakly ferro- and antiferromagnetic limits, seems to work well in resolving classical difficulties associated with HFA as well as in predicting qualitatively new results on various physical properties of weakly ferro- and antiferromagnetic metals.