Theory Of Metallic Magnetism Research Articles

Spontaneous distortion via the appearance of ferromagnetism in Pd ultrathin films: Observation of an inverse mechanism for the Stoner criterion

We study the crystal structure of Pd(100) ultrathin films, which show ferromagnetism induced by the quantum confinement effect, using in-situ X-ray crystal truncation rod measurement and density functional calculation. The energy gain due to the appearance of ferromagnetism in Pd results in flatter and uniform film growth of ferromagnetic Pd films compared with paramagnetic Pd. In addition, ferromagnetic Pd films expand the lattice constant in order to suppress the increase in kinetic energy of electrons accompanied by the occurrence of exchange splitting. Although the traditional theory of magnetism in metals indicates that the increase in density of states that induces ferromagnetism (Stoner criterion), our present finding reveals a mechanism of modulation in the density of states via the appearance of ferromagnetism, i.e., the inverse mechanism of Stoner's theory.

Local Moments in the Dynamic Spin-Fluctuation Theory of Metallic Magnetism

In the dynamic spin-fluctuation theory extended by taking into account higher-order terms of the free energy and uniform fluctuations along with the local ones, simple computational formulae for local magnetic moment are obtained. The temperature dependence of local magnetic moment in the Fe0.65Ni0.35 Invar is calculated. Good agreement with the experimentally estimated value of the magnetic volume change is achieved.

Developments of the theory of spin fluctuations and spin fluctuation-induced superconductivity.

Theory of spin fluctuations as developed in the past 30 years have played important roles in the theory of magnetism in metals, particularly in elucidating the properties around the magnetic instability or quantum critical points. Recently the theory has been extended to deal with the spin fluctuaion-mediated superconductivity with anisotropic order parameters in strongly correlated electron systems. These theoretical developments are briefly reviewed and the high temperature superconductivity of cuprates and organic and heavy electron superconductors are discussed in the light of these theories.

Theory of metallic magnetism at finite temperatures in bulk materials and thin films

A review of ‘first-principles’ theoretical work that describes the properties of magnetic metallic materials at finite temperatures is given. The key assumption is that a time-scale separation can be identified. There are the thermally induced spin fluctuations which are long-lived compared to the time electrons take to move from one lattice site to another. The dependence on the spin-polarised electronic structures of the systems is emphasised including the role of ‘local exchange splitting’ inferred even in the paramagnetic states. The disordered local moment picture which provides the basis of a mean field theory is discussed and its results for bulk transition metals and alloys recalled. Finally, new results for this picture in metallic thin films are reported. In particular comparison with calculated magnetic ordering temperatures of iron films on copper substrates is made with those deduced from experiment and an interpretation in terms of the electronic structure is given.

Metallic magnetism from crystals to amorphous structures in Fe, Co, and Ni

A theory of metallic magnetism that interpolates between crystals and amorphous structure has been developed on the basis of the functional integral technique and the distribution function method to investigate nonunique magnetism in amorphous Fe, Co, and Ni with different degree of structural disorder. Numerical results of various magnetic moments, susceptibilities, and magnetic phase diagram are presented as a function of the fluctuation of interatomic distance $[(\ensuremath{\delta}{R)}^{2}{]}_{s}^{1/2}/[R{]}_{s},$ the average coordination number ${z}^{*},$ and temperature. (Here $[{]}_{s}$ denotes the structural average.) It is demonstrated that there is a phase transition from the ferromagnetism to the spin glass in Fe with increasing structural disorder. The experimental data of amorphous Fe showing the ferromagnetism (the spin glass) are explained by the parameters ${z}^{*}\ensuremath{\approx}10.5$ ${(z}^{*}\ensuremath{\approx}11.5)$ and $[(\ensuremath{\delta}{R)}^{2}{]}_{s}^{1/2}/[R{]}_{s}\ensuremath{\approx}0.07.$ The Curie temperature ${(T}_{C})$ in Co is shown to increase monotonically with increasing the degree of structural disorder, but to have no maximum between the crystals and amorphous structure, while in Ni, ${T}_{C}$ decreases with increasing degree of structural disorder. The latter explains various ${T}_{C}$ as found by an extrapolation of the experimental data in Ni-rich amorphous alloys.

Theory of metallic magnetism from crystals to amorphous structure

A theory of metallic magnetism which interpolates between crystals and the amorphous structure is proposed. We show that the bcc Fe (fcc Ni) exhibits a transition from ferromagnetism to spin glass (paramagnetism) with increasing structural disorder. This results explains a variety of magnetism in Fe(Ni)-rich amorphous alloys.

Spin and charge fluctuations in itinerant-electron magnetism: A variational functional-integral approximation.

The single-site spin-fluctuation theory of metallic magnetism developed by Hubbard and Hasegawa is modified and extended to investigate the effects of spin and charge fluctuations on the magnetic properties at finite temperature. The key point of our scheme is to adopt a continuous decomposition of the interacting part of the Hubbard Hamiltonian in the Stratonovich-Hubbard transformation, resulting in the separation of the charge fluctuations from the spin fluctuations. This separation makes a better description for the contrary nature between the two types of fluctuations. In order to remove the ambiguity, a variational principle is introduced to determine the appropriate form of the decomposition of the Hubbard model

STRONG FERROMAGNETISM IN AMORPHOUS FE-NI ALLOYS

We elucidate the magnetic properties of amorphous Fe-Ni alloys by combining the geometrical-mean model for the ground-state electronic structure calculations of amorphous alloys with the finite-temperature theory of metallic magnetism. The numerical results show the strong ferromagnetism in the region 0.1≤c≤0.8 and the shift of the Curie-temperature maximum to the higher Fe concentration in agreement with the experimental data. We have clarified that these behaviors are not simply due to the volume change, but due to the change of electronic structure which causes the magnetic energy gain. The agreement between the calculated concentration dependence of effective Bohr magneton number and the experimental data for liquid alloys also supports our theoretical results.

Band theory of magnetism in metals and alloys

Band theory of magnetism in metals and alloys

Polarized neutron measurement of ’’forbidden magnon scattering’’ in nickel

We have performed inelastic neutron scattering on a Ni60 single crystal using neutrons polarized along and against the scattering vector and magnetic field direction. For temperatures above 0.9 TC we observe magnon stokes (antistokes) scattering for neutrons polarized along (opposite) the magnetization direction. The occurrence of such scattering, which is forbidden at zero temperature, is in disagreement with the conventional Stoner theory of magnetism in metals based on the RPA, but is in qualitative agreement with theories put forward by Korenman, Murray and Prange, Sokoloff, and others.

Band theory of magnetism in metals in context of exactly soluble model

The role of spatial forces, and of Hund's rule (exchange) forces, and the validity of the Hartree-Fock approximation in the band theory of magnetism of metals is reviewed. Some reasonable conjectures which have been made elsewhere by the author and others are examined in the light of an exactly soluble model of interacting electrons in one dimension. It is shown that within the framework of validity of this model, spatial i.e., direct (Coulomb) forces do not influence the spin susceptibility, and conversely, Hund's rule exchange forces do not affect the dielectric constant or plasmon spectrum. It is recalled that the time-independent Hartree-Fock approximation yields an incorrect spectrum of elementary excitations, and it is also shown explicitly that the magnetic susceptibility calculated in this approximation is incorrect. A paradox is noted, concerning whether ``correlations'' can in fact correct the errors in the H-F approximation. Finally, it is shown that when there are only space forces, such spin density waves as might be introduced in to the H-F ground state of the model are in fact spurious, because they are not representative of the correlations which exist in the true ground state.