Narrow Results

The phase diagram of the single-orbit double exchange model for manganites with ferromagnetic Hund coupling between mobile e_g electrons and spins of localized t_2g electrons as well as antiferromagnetic superexchange coupling between t_2g electrons is investigated with a large scale Monte Carlo simulation in one dimension. The phase boundary is determined based on the internal energy, the electron density and the structure factor. In particular, low-temperature properties at quarter filling are studied in detail.

Double perovskites (DPs) of the form A₂BB′ O₆ usually involve a transition metal ion, B, with a large magnetic moment, and a nonmagnetic ion B′. While many DPs are ferromagnetic, studies on the underlying model reveal the possibility of antiferromagnetic (AF) phases as well driven by electron delocalization. In this paper we present a comprehensive study of the magnetic ground state and T_c scales of the minimal DP model in three dimensions using a combination of spin-fermion Monte Carlo and variational calculations. The effective magnetic lattice in three dimensions is face centered cubic (FCC) and so geometrically frustrated. This promotes noncollinear spiral states and "flux" like phases in addition to collinear AF order. We map out the possible magnetic phases for varying electron density, "level separation" ϵ_B - ϵ_B', and the crucial B′ B′ (next neighbour) hopping t′.

In order to promote suitable material to be used in spintronics devices, this study purposes to evaluate the magnetic properties of the titanium and vanadium-doped zinc-blende ZnO from first-principles. The calculations of these properties are based on the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA), using the local density approximation (LDA). We have calculated and discussed the density of states (DOSs) in the energy phase diagrams for different concentration values, of the dopants. We have also investigated the magnetic and half-metallic properties of this doped compound. Additionally, we showed the mechanism of the exchange coupling interaction. Finally, we estimated and studied the Curie temperature for different concentrations.

The ab initio study of electronic, magnetic properties and Curie temperature of AlAs doped by single and double impurities is investigated using LDA–KKR–CPA method. It is shown that the Al substituted by V and/or Ti induces a half-metallic character and ferromagnetism in the system for different concentrations except co-doping by x=y=0.035. The stability of magnetism between the ferromagnetic and DLM states as well as the mechanism of exchange interaction are discussed. The density of states are plotted in the energy diagram for different concentrations of dopants. The co-doping with (V, Ti) increases Curie temperature in AlAs with respect to single substituant. The finding of this work confirms that the new compounds have a great potential for spintronic devices.

Electron spin resonance spectra of copper phthalocyanine doped with alkaline metals (A_xCuPc) have been investigated. The temperature dependence of ESR spectra indicates ferromagnetic behavior. The Curie-Weiss temperature varies from 30 to 115 K depending on the stoichiometry x of the samples. Some particles of polycrystalline samples were attracted to a weak magnet at temperatures slightly higher than 77 K. The observed magnetism is caused by unpaired π-electrons of phthalocyanine anions on the eg doubly degenerated molecular orbital. The observed ferromagnetism can be understood within the framework of the McConnell-2 model proposed for organic ferromagnetic charge-transfer complexes. The high-temperature magnetism in A_xCuPc is considered to be a result of the Zener mechanism of double exchange between phthalocyanine molecular anions of different valence.