Giant kinks in the entropy change temperature dependence of the magnetocaloric effect in layered phase-separated metals.

In this work, the validity of standard magnetocaloric (MCE) scenarios is revisited for the Hubbard model for a square (two-dimensional) lattice to describe a layered metal. Different types of magnetic ordering (ferrimagnetic, ferromagnetic, Néel and canted antiferromagnetic states) with magnetic transitions between them are considered to minimize the total free energy. The phase-separated states formed by such first-order transitions are also considered consistently. We employ the mean-field approximation to focus attention on the vicinity of a tricritical point, where the order of the magnetic phase transition changes from first to second and phase separation bounds merge. Two types of first-order magnetic transition can be found: PM-Fi, Fi-AFM; with further temperature growth, the phase separation boundaries between them merge and a second order transition, PM-AFM, is observed. The temperature and electron filling dependencies of the entropy change in the phase separation regions are investigated in detail in a consistent way. The dependence of the phase separation bounds on the magnetic field results in the existence of two different characteristic temperature scales. These temperature scales are indicated by giant kinks in the temperature dependence of the entropy, which are an exceptional attribute of phase separation in metals.

Incommensurate magnetic order in rare earth and transition metal compounds with local moments.

Within the framework of thes-d(f) exchange model in the mean-field approximation for square, simple cubic, body-centered and face-centered cubic lattices, the formation of a ferromagnetic, spiral, and commensurate antiferromagnetic (AFM) order is investigated. The possibility of the formation of inhomogeneous states (magnetic phase separation), which necessarily arises during first-order phase transitions in the electron filling parameter, is taken into account. The saturation of the AFM and spiral states is studied depending on the parameters of the model. The results obtained include a rich variety of magnetic structures and phase transitions, allowing the interpretation of magnetic properties of semiconducting and metallic systems containing magnetic atoms.

Magnetic states, correlation effects and metal-insulator transition in FCC lattice.

The ground-state magnetic phase diagram (including collinear and spiral states) of the single-band Hubbard model for the face-centered cubic lattice and related metal-insulator transition (MIT) are investigated within the slave-boson approach by Kotliar and Ruckenstein. The correlation-induced electron spectrum narrowing and a comparison with a generalized Hartree-Fock approximation allow one to estimate the strength of correlation effects. This, as well as the MIT scenario, depends dramatically on the ratio of the next-nearest and nearest electron hopping integrals [Formula: see text]. In contrast with metallic state, possessing substantial band narrowing, insulator one is only weakly correlated. The magnetic (Slater) scenario of MIT is found to be superior over the Mott one. Unlike simple and body-centered cubic lattices, MIT is the first order transition (discontinuous) for most [Formula: see text]. The insulator state is type-II or type-III antiferromagnet, and the metallic state is spin-spiral, collinear antiferromagnet or paramagnet depending on [Formula: see text]. The picture of magnetic ordering is compared with that in the standard localized-electron (Heisenberg) model.

Spiral magnetism in the single-band Hubbard model: the Hartree-Fock and slave-boson approaches.

The ground-state magnetic phase diagram is investigated within the single-band Hubbard model for square and different cubic lattices. The results of employing the generalized non-correlated mean-field (Hartree-Fock) approximation and generalized slave-boson approach by Kotliar and Ruckenstein with correlation effects included are compared. We take into account commensurate ferromagnetic, antiferromagnetic, and incommensurate (spiral) magnetic phases, as well as phase separation into magnetic phases of different types, which was often lacking in previous investigations. It is found that the spiral states and especially ferromagnetism are generally strongly suppressed up to non-realistically large Hubbard U by the correlation effects if nesting is absent and van Hove singularities are well away from the paramagnetic phase Fermi level. The magnetic phase separation plays an important role in the formation of magnetic states, the corresponding phase regions being especially wide in the vicinity of half-filling. The details of non-collinear and collinear magnetic ordering for different cubic lattices are discussed.