Itinerant Electron Subsystem Research Articles

Metal-insulator transition and band magnetism in the spin-1∕2 Falicov-Kimball model on a triangular lattice with external magnetic field

Ground state properties of the spin$-1/2$ Falicov-Kimball model on a triangular lattice in the presence of uniform external magnetic field are explored. Both the orbital and the Zeeman field-induced effects are taken into account and in each unit cell only rational flux fractions are considered. Numerical results, obtained with the help of Monte Carlo simulation algorithm, reveal that the ground state properties strongly depend on the onsite Coulomb correlation between itinerant and localized electrons, orbital magnetic field as well as the Zeeman splitting. Strikingly, for the on-site Coulomb correlation $U/t \approx 1$, the Zeeman splitting produces a phase transition from paramagnetic metal/insulator to ferromagnetic insulator/metal transition in the itinerant electron subsystem accompanied by the phase segregation to the bounded/regular phase in the localized electrons subsystem. For the onsite Coulomb correlation $U/t \approx 5$, although no metal to insulator transition is observed but a magnetic phase transition from paramagnetic phase to ferromagnetic phase in the itinerant electron subsystem is observed with the Zeeman splitting. These results are applicable to the layered systems e.g. cobaltates, rare earth and transition metal dichalcogenides, $GdI_{2}$, $NaTiO_{2}$, $NaVO_{2}$ and $Be_{x}Zn_{1-x}O$ etc. It has been also proposed that the results can be realized in the optical lattices with mixtures of light atoms and heavy atoms using the cold atomic techniques.

D-magnetism instability in R-Co intermetallic compounds

Abstract Magnetic phenomena observed in R-Co intermetallic compounds with the d-magnetism instability are reviewed. The magnetic instability in these compounds is intimately related to the special position of the Fermi level in the hybridized 3d-5d (4d) band near to a local peak in N(e). In the presence of the f-d exchange interaction the magnetic state of the itinerant electron subsystem can essentially be modified giving rise to a number of field- and temperature-induced magnetic phase transitions. Following the band structure calculations these transitions as well as most of their fine details can be well understood theoretically. Magnetic, magnetoelastic and transport measurements of some R-Co compounds with d-magnetism instability and pseudobinary systems with R and Co substituted by either magnetic or nonmagnetic elements are presented and discussed.

Magnetocaloric effect in the Laves phase pseudobinary Er1−cYcCo2

In this article we study the magnetocaloric effect in the pseudobinary Er1−cYcCo2 using a theoretical model, in which the localized 4f spins are immersed in an effective subsystem of itinerant electrons. The interaction between the localized 4f spins is treated in the molecular field approximation and the disorder entering in the Er site is considered in the virtual crystal approximation. Our theoretical results for the adiabatic temperature change and for the isothermal magnetic entropy change with magnetic field are in good agreement with experimental data.

‘Lu-paradox’ as a consequence of the different magnetic ground states of the R(Co,Al)2 (R  heavy RE and Y) and Lu(Co,Al)2 compounds

The low temperature magnetic characteristics of the Y1−tLut(Co0.88Al0.12)2 system were determined. The drastic change from a weakly to strongly ferromagnetic ground state observed at t = 0.38 is discussed in the itinerant model by using the Landau expression for free energy expanded up to the eighth power of magnetization. The ‘Lu-paradox’, that the Curie temperature of Lu(Co1−xAlx)2 is higher than that of isostructural R(Co1−xAlx)2 with some heavy RE and Y (for x > 0.12), is interpreted considering the magnetic ground state of the itinerant electron subsystem to be strongly ferromagnetic in the Lu-based system and weakly ferromagnetic in the RE-based ones.

Thermodynamic properties of itinerant electrons in magnetic superlattices: Magnetic coupling and magnetoresistance

The itinerant electron subsystem of the superlattice of nonmagnetic mediators and magnetic slabs is considered. It is argued that the electrons are confined (in the transverse to the interface direction) in a set of quantum wells. These sets are different for the ferro- and antiferromagnetic ordering of the slabs. As a result, the difference of the energies (per unit volume) for these two types of orderings, which depend on the mediator thickness, change sign regularly. Such behaviour reproduces the oscillations of the antiferromagnetic exchange coupling strength. A qualitative description of the giant magnetoresistance is presented. The results of preliminary calculations are in reasonable correspondence with the experimental data.

A generalization of the Inoue-Shimizu model

Inoue and Shimizu [1] give the free-energy expansion for a system consisting of a set of identical local (rare-earth) magnetic moments interacting with an itinerant electron subsystem (Co2). In this paper the free-energy expansion is given for a generalized system consisting of an arbitrary number of interacting magnetic contributions. This generalization is necessary for a discussion of the type of magnetic transition in quasi-binary RE-Co2 compounds such as TbxHo1−xCo2.