D.M. Edwards

Magnetic isotherms in the band model of ferromagnetism

Calculations are given for the dependence on temperature and magnetic field strength H of the magnetization M of ferromagnetic metals treated on the basis of the itinerant electron model. The spin wave contribution to the low temperature magnetization is considered in the limits of strong and very weak ferromagnetism. For the first limit, problems related to the well-known divergence of the zero field differential susceptibility are briefly discussed. The main part of the paper is concerned with the single particle contributions to the magnetization and associated differential susceptibility for very weak ferromagnetism. A simple equation for the magnetic isotherms is obtained in this limit and shown to be valid over a wide temperature range including 0 °K and the Curie temperature. This equation implies that plots of M2 against H/M at various temperatures in this range give a series of parallel straight lines. Recently measured isotherms for the material ZrZn2 are analysed on the basis of the theory, and several characteristic properties of this material are obtained from the analysis.

Ferromagnetism and electron-phonon coupling in the manganites

The physics of ferromagnetic doped manganites, such as La 1-x Ca x MnO 3 with x ≈ 0.2-0.4, is reviewed. The concept of double exchange is discussed within the general framework of itinerant electron magnetism. The new feature in this context is the coupling of electrons to local phonon modes. Emphasis is placed on the quantum nature of the phonons and the link with polaron physics. However it is stressed that the manganites fall into an intermediate coupling regime where standard small-polaron theory does not apply. The recently-developed many-body coherent potential approximation is able to deal with this situation and Greens recent application to the Holstein double-exchange model is described. Issues addressed include the nature of the basic electronic structure, the metal-insulator transition, a unification of colossal magnetoresistance, pressure effects and the isotope effect, pseudogaps in spectroscopy and the effect of electron-phonon coupling on spin waves.

High-temperature ferromagnetism of sp electrons in narrow impurity bands: Application to CaB6

Ferromagnetism with high Curie temperature Tc, well above room temperature, and very small saturation moment has been reported in various carbon and boron systems. It is argued that the magnetization must be very inhomogeneous with only a small fraction of the sample ferromagnetically ordered. It is shown that a possible source of high Tc within the ferromagnetic regions is itinerant electrons occupying a narrow impurity band. Correlation effects do not reduce the effective interaction which enters the Stoner criterion in the same way as in a bulk band. It is also shown how, in the impurity band case, spin wave excitations may not be effective in lowering Tc below its value given by Stoner theory. These ideas are applied to CaB6 and a thorough review of the experimental situation in this material is given. It is suggested that the intrinsic magnetism of the B2 and O2 dimers might be exploited in suitable structures containing these elements.

A resistor network theory of the giant magnetoresistance in magnetic superlattices

An explanation of the giant magnetoresistance effect observed in some magnetic superlattices is given in terms of an equivalent network of resistors. The model leads to a simple analytic formula that relates the magnetoresistance to the spin-dependent electron mean-free paths and thickness of the magnetic and nonmagnetic layers in the superlattice. The formula is used to study the giant magnetoresistance of Co/Cu and Fe/Cr superlattices, and is also used to predict the most favorable values of parameters for a large magnetoresistance effect. >

Oscillations in the exchange coupling of ferromagnetic layers separated by a nonmagnetic metallic layer

A general theory of oscillations in the exchange coupling J between two transition metal ferromagnets separated by a nonmagnetic transition metal spacer is developed. Detailed calculations of J as a function of the spacer layer thickness are made for a specific model based on a simple cubic tight-binding band structure. For a suitable choice of band filling a good account is given of the main features observed in Co/Ru, Co/Cr and Fe/Cr layered structures. An analysis of the numerical accuracy required in computing J from a total energy difference is given and this throws some light on difficulties experienced by previous workers using this approach. A general expression for J is obtained for an arbitrary band and finite temperature in the limit of large spacer thickness. A close analogy between oscillations in the exchange coupling and de Haas-van Alphen oscillations is established and the relation to RKKY theory is also discussed. It is shown that the period, asymptotic decay and temperature dependence of the oscillations in J are determined by properties of the Fermi surface of the spacer layer.

Superconductivity in the two-dimensional Hubbard model

Quasiparticle bands of the two-dimensional Hubbard model are calculated using the Roth two-pole approximation to the one-particle Greens function. Excellent agreement is obtained with recent Monte Carlo calculations, including an anomalous volume of the Fermi surface near half-filling, which can possibly be explained in terms of a breakdown of Fermi liquid theory. The calculated bands are very flat around the (\ensuremath{\pi},0) points of the Brillouin zone in agreement with photoemission measurements of cuprate superconductors. With doping there is a shift in spectral weight from the upper band to the lower band. The Roth method is extended to deal with superconductivity within a four-pole approximation allowing electron-hole mixing. It is shown that triplet p-wave pairing never occurs. A self-consistent solution with singlet

Oscillations in exchange coupling across a nonmagnetic metallic layer

{\mathit{d}}_{{\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2}}

Intrinsic and secondary mechanisms for biquadratic exchange coupling in magnetic trilayers

-wave pairing is found and optimal doping occurs when the van Hove singularity, corresponding to the flat band part, lies at the Fermi level. Nearest-neighbor antiferromagnetic correlations play an important role in flattening the bands near the Fermi level and in favoring superconductivity. However, the mechanism for superconductivity is a local one, in contrast to spin-fluctuation exchange models. For reasonable values of the hopping parameter the transition temperature

Self-consistent theory of current-induced switching of magnetization

{\mathit{T}}_{\mathit{c}}

Some current problems in itinerant electron magnetism

is in the range 10\char21{}100 K. The optimum doping