Phase diagram as a function of temperature and magnetic field for magnetic semiconductors

Using an extension of the Nagaev model of phase separation (E.L. Nagaev, and A.I. Podel'shchikov, Sov. Phys. JETP, 71 (1990) 1108), we calculate the phase diagram for degenerate antiferromagnetic semiconductors in the T-H plane for different current carrier densities. Both, wide-band semiconductors and 'double-exchange' materials, are investigated.Comment: 5 pages, 6 figures, RevTex, Accepted for publication in PR

Large nonzero-moment magnetic strings in antiferromagnetic crystals of the manganite type

The magnetic strings in antiferromagnetic crystals with the spin $S = 1 /2$ differ from the magnetic polarons (ferrons) by the absence of the additional magnetic moment. We show that in the $S > 1 /2$ double exchange crystals with the antiferromagnetic $s-d$ exchange, a new type of magnetic strings appears, which possesses a magnetic moment. It is concentrated at the center of the string, and the magnetized string is, in its essence, the state intermediate between the string and the ferron. In antiferromagnetic manganites, this moment is by an order of magnitude larger than that of a magnetic atom. Unlike the conventional ferrons, the magnetization of the strings exists at any parameters of the crystals under consideration. We argue that this new type of magnetic state can be relevant to some doped antiferromagnets including manganites.Comment: 7 pages, 1 eps figure, RevTeX, submitted to Phys. Rev.

Magnetic polarons in doped 1D antiferromagnetic chain

The structure of magnetic polarons (ferrons) is studied for an 1D antiferromagnetic chain doped by non-magnetic donor impurities. The conduction electrons are assumed to be bound by the impurities. Such a chain can be described as a set of ferrons at the antiferromagnetic background. We found that two types of ferrons can exist in the system. The ground state of the chain corresponds to the ferrons with the sizes of the order of the localization length of the electron near the impurity. The ferrons of the second type produce a more extended distortion of spins in the chain. They are stable within a finite domain of the system parameters and can be treated as excitations above the ground state. The ferrons in the excited states can appear in pairs only. The energy of the excited states decreases with the growth in density of impurities. This can be interpreted as a manifestation of an attractive interaction between ferrons.Comment: 6 pages, 5 figures, RevTex4, submitted to PR

Impurity Conduction and Magnetic Polarons in Antiferromagnetic Oxides

Low-temperature transport and magnetization measurements for the antiferromagnets SrMnO(3) and CaMnO(3) identify an impurity band of mobile states separated by energy E from electrons bound in Coulombic potentials. Very weak electric fields are sufficient to excite bound electrons to the impurity band, increasing the mobile carrier concentration by more than three orders of magnitude. The data argue against the formation of self-trapped magnetic polarons (MPs) predicted by theory, and rather imply that bound MPs become stable only for kT<<E.Comment: 4 pp., 4 fig

Exact stripe, checkerboard, and droplet ground states in two dimensions

Exact static nondegenerate stripe and checkerboard ground states are obtained in a two-dimensional generalized periodic Anderson model, for a broad concentration range below quarter filling. The random droplet states, also present in the degenerate ground state, are eliminated by extending the Hamiltonian with terms of different physical origin such as dimerization, periodic charge displacements, density waves, or distorsion lines.Comment: 12 pages, 8 figure

Phase diagram for Coulomb-frustrated phase separation in systems with negative short-range compressibility

Using numerical techniques and asymptotic expansions we obtain the phase diagram of a paradigmatic model of Coulomb frustrated phase separation in systems with negative short-range compressibility. The transition from the homogeneous phase to the inhomogeneous phase is generically first order in isotropic three-dimensional systems except for a critical point. Close to the critical point, inhomogeneities are predicted to form a BCC lattice with subsequent transitions to a triangular lattice of rods and a layered structure. Inclusion of a strong anisotropy allows for second- and first-order transition lines joined by a tricritical point.Comment: 4 pages, 3 figures. Improved figures and presentatio

Double-Exchange Model on Triangle Chain

We study ground state properties of the double-exchange model on triangle chain in the classical limit on $t_{2g}$ spins. The ground state is determined by a competition among the kinetic energy of the $e_g$ electron, the antiferromagnetic exchange energy between the $t_{2g}$ spins, and frustration due to a geometric structure of the lattice. The phase diagrams are obtained numerically for two kinds of the models which differ only in the transfer integral being real or complex. The properties of the states are understood from the viewpoint of the spin-induced Peierls instability. The results suggest the existence of a chiral glass phase which is characterized by a local spin chirality and a continuous degeneracy.Comment: 6 pages, 4 figure

Current fluctuations in a spin filter with paramagnetic impurities

We analyze the frequency dependence of shot noise in a spin filter consisting of a normal grain and ferromagnetic electrodes separated by tunnel barriers. The source of frequency-dependent noise is random spin-flip electron scattering that results from spin-orbit interaction and magnetic impurities. Though the latter mechanism does not contribute to the average current, it contributes to the noise and leads to its dispersion at frequencies of the order of the Korringa relaxation rate. Under nonequilibrium conditions, this rate is proportional to the applied bias $V$, but parametrically smaller than $eV/\hbar$.Comment: 6 pages, 2 figure

The phase-separated states in antiferromagnetic semiconductors with polarizable lattice

The possibility of the slab or stripe phase separation (alternating ferromagnetic highly- conductive and insulating antiferromagnetic layers) is proved for isotropic degenerate antiferromagnetic semiconductors. This type of phase separation competes with the droplet phase separation (ferromagnetic droplets in the antiferromagnetic host or vice versa). The interaction of electrons with optical phonons alone cannot cause phase-separated state with alternating highly-conductive and insulating regions but it stabilizes the magnetic phase separation. The magnetostriction deformation of the lattice in the phase-separated state is investigated.Comment: 17 Pages, 1 EPS Figur

Ferromagnetic spin-polaron on complex lattices

We present a simpler derivation of the exact solution of a spin-polaron in a ferromagnet and generalize it to complex lattices and/or longer range exchange interactions. As a specific example, we analyze a two-dimensional MnO$_2$-like lattice (as in the ferromagnetic layers in LaMnO$_3$) and discuss the properties of the resulting spin-polaron in various regimes. At strong couplings the solution is reminiscent of the Zhang-Rice singlet, however the electronic wavefunction involved in the singlet is dependent on the momentum of the singlet, and multiple bands may appear.Comment: 12 pages, 7 figure