Local Band Theory Research Articles

A comparative study of the magnetoelastic properties of the YFe 10V 2 and NdFe 10V 2 compounds

The magnetoelastic properties of iron-rich REFe 10V 2 (RE=Nd, Y) compounds were studied via magnetostriction and thermal expansion measurements in the 5–300 K range of temperature in up to 6 T external fields. Results of thermal expansion analysis show that the spontaneous magnetostriction of the compounds mostly originates from itinerant magnetization. Besides, the small volume striction appearing in the thermal expansion of the Nd compound close to 50 K suggests the existence of a basal to conical spin re-orientation transition. The volume magnetostriction isotherms of both compounds take minimum values for external field corresponding to the anisotropy field. In addition, the anisotropic and the volume magnetostriction traces of the NdFe 10V 2 take marked maxima under low field, with a relatively large initial magnetostrictivity, again more pronounced at the conical–axial spin re-orientation transition ( T SR=130 K). Analysis of the anisotropic magnetostriction of the Nd compound leads to the conclusion that the contribution of Nd–Fe interactions is negligible. The temperature dependence of volume magnetostriction is in good agreement with prediction of a phenomenological model based upon a fluctuating local band theory. This analysis shows that the difference between the forced volume strictions of Y and Nd compounds below and above T SR originates from the Nd sublattice magnetization.

Short-range order in spin density wave antiferromagnets with chemical dimerization

The spin density wave model in a quasi-one-dimensional itinerant antiferromagnet with staggered potential at finite temperature is studied. Only short-range ordering exists in this system above the Neel temperature. The local-band theory of spin fluctuations is developed to calculate the spin density wave amplitude and the effective exchange integral. The one-electron spectrum and magnon spectrum are obtained in the short-range ordering regime.

Temperature dependence of the spin- and angle-resolved valence band photoemission spectra of Ni(110)

Abstract We have measured spin- and angle-resolved valence band photoemission (SARPES) spectra of ferromagnetic Ni(110) near X- and K-points in the Brillouin zone at various temperatures between T Tc =0.5 and 1.05. The spectral features were analyzed based on the assumption that they could be reduced into two components which show collapsing and noncollapsing behaviors. The results show that the observed wavevector dependence of the SARPES spectra is consistent with the one expected by the local band theory.

Temperature dependence of the spin- and angle-resolved valence band photoemission spectra of ferromagnetic Ni(110)

Abstract We have measured spin- and angle-resolved valence band photoemission spectra of Ni(110) at temperatures T / T C = 0.5−1.05. The temperature dependence of the spectral profiles can be reduced to two components, one that follows the local exchange field, and one that follows the magnetic field averaged over many local exchange fields, which is consistent with the local band theory.

Local-Band-Theory Analysis of Spin-Polarized, Angle-Resolved Photoemission Spectroscopy

We present the local-band theory of the temperature dependence of up- and down-spin Green's functions in itinerant-electron ferromagnets. The results, calculated for a temperature-independent, local, exchange field strength, and under the assumption of the existence of short-range magnetic order, agree with recent measurements of angle-, energy-, and spin-resolved photoemission in nickel. States which contribute to the magnetization, not seen in the measurements, remain split in energy even at ${T}_{C}$. Split states at ${T}_{C}$ are also expected in iron.

Spin-polarized photoemission study on the temperature dependence of the exchange splitting of Ni

Using spin-polarized photoemission with high energy- and angle resolution (ΔE=100 meV, ΔΘ=±3°) we have investigated the temperature dependence of the exchange splitting of Ni in the temperature range 0.5≦T/Tc≦0.94. At room temperature we findΔex=0.18 eV for the exchange splitting of theS4 band at theX point of the Brillouin zone. With increasing temperature the total (spin-averaged) energy distribution shows a narrowing and merges into one peak. The spin-resolved energy distribution curves approach each other and are strongly broadened. A discussion of the data within current theories of itinerant electron magnetism is given. The spectra indicate that neither the pure Stoner model nor the prediction of local band theory, assuming a temperature independent exchange splitting are justified for Ni. We conclude that the exchange splitting decreases with increasing temperature and that transverse as well as longitudinal spin fluctuations are responsible for the broadening of the spin-resoived energy distribution curves.

Temperature Dependence of the Exchange Splitting in Ni Studied by Spin-Polarized Photoemission

Angle- and spin-resolved photoemission spectra from Ni(110) have been measured in the temperature range $0.5<~\frac{T}{{T}_{C}}<~0.94$. The data cannot be reconciled either with the predictions of local band theory assuming a temperature-independent exchange splitting or with a pure Stoner model. It is concluded that the exchange splitting decreases with increasing temperature and that spin fluctuations strongly influence the photoemission line shapes.

Magnetovolume effects in the local-band theory of ferromagnetism: Pressure dependence of T c in iron and nickel

In the local-band theory the spontaneous volume magnetostriction and the pressure dependence of T c are related to one another through their dependence on the volume coefficient of A, the Bloch wall stiffness constant. The available data in iron and nickel are consistent with this relation, and bear a proper relation to the measured energy associated with the phase transition. We also show, in the context of a Heisenberg model, that it is short-range order which is mainly responsible for the good agreement.

Magnetovolume effects in Laves phase intermetallic compounds

First, general aspects of magnetovolume effects in various kinds of magnetic materials are summarized. Then the magnetovolume effects in rare earth-transition metal intermetallic compounds RM 2 are reviewed. RCo 2 show large magnetovolume effects which are attributable to an induced Co moment. We have recently observed a large spontaneous volume magnetostriction in the RMn 2 series. Among them YMn 2, which has been found to be an antiferromagnet, shows the largest volume magnetostriction of about 5%. On the other hand, it has been reported that some Np cubic Laves phase intermetallic compounds also show large pressure effects. These large magnetovolume effects can be interpreted generally in terms of itinerant electron ferromagnetism taking into account spin fluctuations, i.e. a local band theory of itinerant ferro- or antiferromagnetism.

Consequences of short range magnetic order above TC in iron and nickel

I review the Local-Band theory of itinerant electron ferromagnetism at finite temperatures, and some of its consequences in iron and nickel above T C. These include properties of spin waves in the paramagnetic region, energy considerations, including spontaneous volume magnetostriction and the pressure dependence of T C, and the temperature dependence of angle resolved photoemission. Recent experiments on diffuse neutron scattering, and on two electron pickup by grazing incidence deuterons, give further evidence of the reality of short range order above T C, in iron and nickel, respectively.

Magnetism in iron and nickel

The fluctuating local band theory of itinerant electron ferromagnetism in nickel and iron is investigated with the use of first-principles numerical calculations. In this theory the excitations predominantly responsible for the phase transition are fluctuations in the direction of local magnetization. The free energy in the presence of a fluctuation is evaluated numerically in the approximation that this direction changes in time and space slowly enough to justify the use of the static approximation and second-order perturbation theory. The energies and wave functions used to incorporate the band and wavevector dependence of the relevant interaction matrix elements were obtained by Slater-Koster fits to earlier ab initio self-consistent energy bands. Results for nickel and iron are obtained in terms of an effective classical Heisenberg exchange. This is compared with other theoretical calculations and available experimental data. From the numerical results, it is concluded that both quantum effects (the time dependence of the exchange field) and local-field effects are important to account for the transition temperature ${T}_{C}$.

Character of the Ferromagnetic Transition in Fe and Ni

The magnetism of Fe and Ni is investigated by a cluster theory with two novel features: (i) The coupling of magnetic moments formed by clusters (described by a local band theory) is determined, thus incorporating short-range order as well as collective modes. (ii) The clusters are coupled by source and sink terms describing transport processes. The results indicate that Fe is closer to a local moment system, whereas Ni seems to be a prototype for a local band theory.

Magneto-volume effects in the local-band theory of ferromagnetism

The volume change at the Curie temperature is analyzed according to the local-band theory of itinerant ferromagnetism. It is related to the volume coefficient of the Bloch wall stiffness. Plausible assumptions, in a simple one-band model, give the correct values for iron and nickel. A more speculative argument yields a good value for the Invar alloy ${\mathrm{Fe}}_{65}$${\mathrm{Ni}}_{35}$ as well.

Temperature Dependence of the Angle-Resolved Photoemission in Itinerant-Electron Ferromagnets

The prediction of the local band theory of itinerant-electron ferromagnetism for the temperature dependence of the single-hole Green's function is obtained. Qualitative agreement with observation is found, but the interpretation is novel. The underlying spectral function is more complex than the simple two-peaked structure heretofore assumed. Although the width of this complex shrinks with temperature, the shrinkage does not correspond to a diminution of the exchange splitting as that quantity is most conveniently defined.

Density-functional approximation for the quasiparticle dynamics of Rb

The quasiparticle mass variation over the Fermi surface of Rb has been evaluated using approximations for the mass operator based on the density-functional formalism. A nonlocal approximation for the mass operator is found to yield a Fermi surface more nearly in accord with experiment than the usual local band theory approximation. However, both local and nonlocal mass operator approximations predict a much more anisotropic quasiparticle mass than has been inferred from the recent experiments of Paul and Springford (see ibid., vol.8, p.1713 (1978)).

Local-band theory of itinerant ferromagnetism. II. Spin waves

The short-wavelength magnons which are observed by neutron scattering above as well as below the Curie temperature ${T}_{C}$ are studied by the methods of the local-band theory, described in the preceding paper. We find a temperature shift of the spin-wave constant $D$, of a type previously proposed, but which we estimate to be numerically small. A novel downward shift mechanism gives a lowering $\ensuremath{-}2D{|\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}|}_{\mathrm{av}}^{2}$, where ${|\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}|}_{\mathrm{av}}^{2}$ is proportional to the magnetic energy. ${|\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}|}_{\mathrm{av}}^{2}$ saturates shortly above ${T}_{C}$ at about 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}2}$ ${\mathrm{\AA{}}}^{\ensuremath{-}2}$ in both iron and nickel. The form of this term is strictly correct only for ${q}^{2}\ensuremath{\gg}{|\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}|}_{\mathrm{av}}^{2}$. However, it accounts for the greater part of the observed magnon energy shift, both in temperature dependence and shape of the dispersion curve. Finally, we find a new magnon-width mechanism ${\ensuremath{\Gamma}}_{q}$ proportional to ${|\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}|}_{\mathrm{av}}^{2}q$ which seems capable of accounting for the observed width.