Metastable states influence on the magnetic behavior of the triangular lattice: Application to the spin-chain compound Ca3Co2O6

It is known that the spin-chain compound Ca3Co2O6 exhibits very interesting plateaus in the magnetization as a function of the magnetic field at low temperatures. The origin of them is still controversial. In this paper we study the thermal behavior of this compound with a single-flip Monte Carlo simulation on a triangular lattice and demonstrate the decisive influence of metastable states in the splitting of the ferrimagnetic 1/3 plateau below 10 K. We consider the [Co2O6]n chains as giant magnetic moments described by large Ising spins on planar clusters with open boundary conditions. With this simple frozen-moment model we obtain stepped magnetization curves which agree quite well with the experimental results for different sweeping rates. We describe particularly the out-of-equilibrium states that split the low-temperature 1/3 plateau into three steps. They relax thermally to the 1/3 plateau, which has long-range order at the equilibrium. Such states are further analyzed with snapshots unveiling a domain-wall structure that is responsible for the observed behavior of the 1/3 plateau. A comparison is also given of the exact results in small triangular clusters with our Monte Carlo results, providing further support for our thermal description of this compound.Comment: 8 pages, 11 figures, submitted to PR

Dephasing Effects by Ferromagnetic Boundary on Resistivity in Disordered Metallic Layer

The resistivity of disordered metallic layer sandwiched by two ferromagnetic layers at low-temperature is investigated theoretically. It is shown that the magnetic field acting at the interface does not affect the classical Boltzmann resistivity but causes a dephasing among electrons in the presence of the spin-orbit interaction, suppressing the anti-localization due to the spin-orbit interaction. The dephasing turns out to be stronger in the case where the magnetization of the two layers is parallel, contributing to a positive magnetoresistance close to a switching field at low temperature.Comment: 11 pages, 3 figures. Title modified in journal versio

Giant magnetic enhancement in Fe/Pd films and its influence on the magnetic interlayer coupling

The magnetic properties of thin Pd fcc(001) films with embedded monolayers of Fe are investigated by means of first principles density functional theory. The induced spin polarization in Pd is calculated and analyzed in terms of quantum interference within the Fe/Pd/Fe bilayer system. An investigation of the magnetic enhancement effects on the spin polarization is carried out and its consequences for the magnetic interlayer coupling are discussed. In contrast to {\it e.g.} the Co/Cu fcc(001) system we find a large effect on the magnetic interlayer coupling due to magnetic enhancement in the spacer material. In the case of a single embedded Fe monolayer we find aninduced Pd magnetization decaying with distance $n$ from the magnetic layer as ~$n^{-\alpha}$ with $\alpha \approx 2.4$. For the bilayer system we find a giant magnetic enhancement (GME) that oscillates strongly due to interference effects. This results in a strongly modified magnetic interlayer coupling, both in phase and magnitude, which may not be described in the pure Ruderman-Kittel-Kasuya-Yoshida (RKKY) picture. No anti-ferromagnetic coupling was found and by comparison with magnetically constrained calculations we show that the overall ferromagnetic coupling can be understood from the strong polarization of the Pd spacer

Coexistence of glassy antiferromagnetism and giant magnetoresistance (GMR) in Fe/Cr multilayer structures

Using temperature-dependent magnetoresistance and magnetization measurements on Fe/Cr multilayers that exhibit pronounced giant magnetoresistance (GMR), we have found evidence for the presence of a glassy antiferromagnetic (GAF) phase. This phase reflects the influence of interlayer exchange coupling (IEC) at low temperature (T < 140K) and is characterized by a field-independent glassy transition temperature, Tg, together with irreversible behavior having logarithmic time dependence below a "de Almeida and Thouless" (AT) critical field line. At room temperature, where the GMR effect is still robust, IEC plays only a minor role, and it is the random potential variations acting on the magnetic domains that are responsible for the antiparallel interlayer domain alignment.Comment: 5 pages, 4 figure

Luttinger liquid superlattices

We calculate the correlation functions and the DC conductivity of Luttinger liquid superlattices, modeled by a repeated pattern of interacting and free Luttinger liquids. In a specific realization, where the interacting subsystem is a Hubbard chain, the system exhibits a rich phase diagram with four different phases: two metals and two compressible insulators. In general, we find that the effective low energy description amalgamates features of both types of liquids in proportion to their spatial extent, suggesting the interesting possibility of `engineered' Luttinger liquids.Comment: RevTeX, 5 pages, 3 figure

Phase Diagram of Multilayer Magnetic Structures

Multilayer "ferromagnet-layered antiferromagnet" (Fe/Cr) structures frustrated due to the roughness of layer interfaces are studied by numerical modeling methods. The "thickness-roughness" phase diagrams for the case of thin ferromagnetic film on the surface of bulk antiferromagnet and for two ferromagnetic layers separated by an antiferromagnetic interlayer are obtained and the order parameter distributions for all phases are found. The phase transitions nature in such systems is considered. The range of applicability for the "magnetic proximity model" proposed by Slonczewski is evaluated.Comment: 8 pages, 8 figure

Exchange Field Induced Magnetoresistance in Colossal Magnetoresistance Manganites

The effect of an exchange field on electrical transport in thin films of metallic ferromagnetic manganites has been investigated. The exchange field was induced both by direct exchange coupling in a ferromagnet/antiferromagnet multilayer and by indirect exchange interaction in a ferromagnet/paramagnet superlattice. The electrical resistance of the manganite layers was found to be determined by the absolute value of the vector sum of the effective exchange field and the external magnetic field.Comment: 5 pages, 4 figure

Self-similar magnetoresistance of Fibonacci ultrathin magnetic films

We study numerically the magnetic properties (magnetization and magnetoresistance) of ultra-thin magnetic films (Fe/Cr) grown following the Fibonacci sequence. We use a phenomenological model which includes Zeeman, cubic anisotropy, bilinear and biquadratic exchange energies. Our physical parameters are based on experimental data recently reported, which contain biquadratic exchange coupling with magnitude comparable to the bilinear exchange coupling. When biquadratic exchange coupling is sufficiently large a striking self-similar pattern emerges.Comment: 5 pages, 5 EPS figures, REVTeX, accepted for publication in Phys. Rev.

Enhancement of the Josephson current by an exchange field in superconductor-ferromagnet structures

We calculate the dc Josephson current for two superconductor-ferromagnet (S/F) bilayers separated by a thin insulating film. It is demonstrated that the critical Josephson current $I_{c}$ in the junction strongly depends on the relative orientation of the effective exchange field $h$ of the bilayers. We found that in the case of an antiparallel orientation, $I_{c}$ increases at low temperatures with increasing $h$ and at zero temperature has a singularity when $h$ equals the superconducting gap $\Delta$. This striking behavior contrasts suppression of the critical current by the magnetic moments aligned in parallel and is an interesting new effect of the interplay between superconductors and ferromagnets.Comment: to be published in PR

Conductance Quantization and Magnetoresistance in Magnetic Point Contacts

We theoretically study the electron transport through a magnetic point contact (PC) with special attention to the effect of an atomic scale domain wall (DW). The spin precession of a conduction electron is forbidden in such an atomic scale DW and the sequence of quantized conductances depends on the relative orientation of magnetizations between left and right electrodes. The magnetoresistance is strongly enhanced for the narrow PC and oscillates with the conductance.Comment: 4 pages, 4 figures, revised version with new figure