In 3D momentum space, a topological phase boundary separating the Chern insulating layers from normal insulating layers may exist, where the gap must be closed, resulting in a "Chern semimetal" state with topologically unavoidable band crossings at the Fermi level. This state is a condensed-matter realization of Weyl fermions in (3+1)D, and should exhibit remarkable features, such as magnetic monopoles and Fermi arcs. Here we predict, based on first principles calculations, that such a novel quantum state can be realized in a known ferromagnetic compound HgCr2Se4, with a single pair of Weyl fermions separated in momentum space. The quantum Hall effect without an external magnetic field can be achieved in its quantum-well structure.

Chern semimetal and the quantized anomalous Hall effect in HgCr2Se4.

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

Stripline (SL), vector network analyzer (VNA), and pulsed inductive microwave magnetometer (PIMM) techniques were used to measure the ferromagnetic resonance (FMR) linewidth for a series of Permalloy films with thicknesses of 50 and 100nm. The SL-FMR measurements were made for fixed frequencies from 1.5to5.5GHz. The VNA-FMR and PIMM measurements were made for fixed in-plane fields from 1.6to8kA∕m (20–100Oe). The results provide a confirmation, lacking until now, that the linewidths measured by these three methods are consistent and compatible. In the field format, the linewidths are a linear function of frequency, with a slope that corresponds to a nominal Landau-Lifshitz phenomenological damping parameter α value of 0.007 and zero frequency intercepts in the 160–320A∕m (2–4Oe) range. In the frequency format, the corresponding linewidth versus frequency response shows a weak upward curvature at the lowest measurement frequencies and a leveling off at high frequencies.

/pdf/ferromagnetic-resonance-linewidth-in-metallic-thin-films-4nmku8h46u.pdf

Ferromagnetic resonance linewidth in metallic thin films: Comparison of measurement methods | NIST

The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-007233.pdf

First-principles calculations of electronic structure and spectra of strongly correlated systems: the LDA+U method

Over a long period of exploration, the successful observation of quantized version of anomalous Hall effect (AHE) in thin film of magnetically doped topological insulator (TI) completed a quantum Hall trio—quantum Hall effect (QHE), quantum spin Hall effect (QSHE), and quantum anomalous Hall effect (QAHE). On the theoretical front, it was understood that the intrinsic AHE is related to Berry curvature and U(1) gauge field in momentum space. This understanding established connection between the QAHE and the topological properties of electronic structures characterized by the Chern number. With the time-reversal symmetry (TRS) broken by magnetization, a QAHE system carries dissipationless charge current at edges, similar to the QHE where an external magnetic field is necessary. The QAHE and corresponding Chern insulators are also closely related to other topological electronic states, such as TIs and topological semimetals, which have been extensively studied recently and have been known to exist in various...

Quantum anomalous Hall effect and related topological electronic states

We study the magnetization and magnetoresistance of superlattices with biquadratic exchange. The equilibrium states are analyzed on the base of the free energy expression that includes all terms up to the fourth order in components of magnetizations of magnetic sublattices. The correlation between the magnetization curve and the magnetoresistance for various orientation of magnetic field relative to the film plane is established. The experimental studies are made on the samples of molecular-beam epitaxy grown Fe/Cr superlattices. The positive magnetoresistance was found for the perpendicular-to-plane magnetic field. It is shown that this effect as well as the characteristic features of the magnetization curves are connected with the noncollinear magnetic alignment, which exists in our samples, and the fourth order magnetic anisotropy of unfamiliar type. \textcopyright{} 1996 The American Physical Society.

Magnetoresistance and magnetization of Fe/Cr(001) superlattices with noncollinear magnetic ordering

We report results of integrated study of resistivity, magnetoresistance, Hall effect, thermopower, and magnetothermopower in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ single crystals with $x=0.15,0.20,0.25.$ The focus is on the vicinity of the Curie temperature. It is shown that near ${T}_{C}$ where the colossal magnetoresistance is observed the crystals are in insulator state. It is established that the conductivity in the manganites with different level of doping differs in nature. Near ${T}_{C},$ temperature and magnetic-field dependences of resistivity arise due to change of activation energy, which is linear in squared magnetization. The metal-insulator transition in the $x=0.20$ and 0.25 manganites occurs not at a certain temperature but in a temperature interval about 80 K wide.

Electronic transport in ferromagnetic La 1 − x Sr x MnO 3 single-crystal manganites

Pure spin currents provide the possibility to control the magnetization state of conducting and insulating magnetic materials. They allow one to increase or reduce the density of magnons, and achieve coherent dynamic states of magnetization reminiscent of the Bose-Einstein condensation. However, until now there was no direct evidence that the state of the magnon gas subjected to spin current can be treated thermodynamically. Here, we show experimentally that the spin current generated by the spin-Hall effect drives the magnon gas into a quasi-equilibrium state that can be described by the Bose-Einstein statistics. The magnon population function is characterized either by an increased effective chemical potential or by a reduced effective temperature, depending on the spin current polarization. In the former case, the chemical potential can closely approach, at large driving currents, the lowest-energy magnon state, indicating the possibility of spin current-driven Bose-Einstein condensation.

/pdf/chemical-potential-of-quasi-equilibrium-magnon-gas-driven-by-3or7jnfm1h.pdf

V. V. Ustinov

Papers

Magnetoresistance and magnetization of Fe/Cr(001) superlattices with noncollinear magnetic ordering

Electronic transport in ferromagnetic La 1 − x Sr x MnO 3 single-crystal manganites

Chemical potential of quasi-equilibrium magnon gas driven by pure spin current.

High frequency properties of magnetic multilayers

Magnetic properties of La0.7−xPrxCa0.3MnO3 single crystals: When is Banerjee criterion applicable?