We review the dynamical mean-field theory of strongly correlated electron systems which is based on a mapping of lattice models onto quantum impurity models subject to a self-consistency condition. This mapping is exact for models of correlated electrons in the limit of large lattice coordination (or infinite spatial dimensions). It extends the standard mean-field construction from classical statistical mechanics to quantum problems. We discuss the physical ideas underlying this theory and its mathematical derivation. Various analytic and numerical techniques that have been developed recently in order to analyze and solve the dynamical mean-field equations are reviewed and compared to each other. The method can be used for the determination of phase diagrams (by comparing the stability of various types of long-range order), and the calculation of thermodynamic properties, one-particle Green's functions, and response functions. We review in detail the recent progress in understanding the Hubbard model and the Mott metal-insulator transition within this approach, including some comparison to experiments on three-dimensional transition-metal oxides. We present an overview of the rapidly developing field of applications of this method to other systems. The present limitations of the approach, and possible extensions of the formalism are finally discussed. Computer programs for the numerical implementation of this method are also provided with this article.

Dynamical mean-field theory of strongly correlated fermion systems and the limit of infinite dimensions

A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77 kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are discussed.

http://science.sciencemag.org/content/sci/264/5157/413.full.pdf

Thousandfold Change in Resistivity in Magnetoresistive La-Ca-Mn-O Films

Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

“Spintronics,” in which both the spin and charge of electrons are used for logic and memory operations, promises an alternate route to traditional semiconductor electronics. A complete logic architecture can be constructed, which uses planar magnetic wires that are less than a micrometer in width. Logical NOT, logical AND, signal fan-out, and signal cross-over elements each have a simple geometric design, and they can be integrated together into one circuit. An additional element for data input allows information to be written to domain-wall logic circuits.

http://science.sciencemag.org/content/sci/309/5741/1688.full.pdf

Magnetic Domain-Wall Logic

At room temperature a large magnetoresistance, \ensuremath{\Delta}R/R(H=0), of 60% has been observed in thin magnetic films of perovskitelike La-Ba-Mn-O. The films were grown epitaxially on ${\mathrm{SrTiO}}_{3}$ substrates by off-axis laser deposition. In the as-deposited state, the Curie temperature and the saturation magnetization were considerably lower compared to bulk samples, but were increased by a subsequent heat treatment. The samples show a drop in the resistivity at the magnetic transition, and the existence of magnetic polarons seems to dominate the electric transport in this region.

Giant negative magnetoresistance in perovskitelike La2/3Ba1/3MnOx ferromagnetic films.

Using mechanical alloying and a subsequent two‐step heat treatment we produced magnetically isotropic microcrystalline Sm2Fe17Nx samples with room‐temperature coercivities up to 24 kA/cm (30 kOe). The remanence and the energy product are equivalent to similarly prepared Nd‐Fe‐B samples, but the properties at elevated temperatures are superior because of the high Curie temperature of 470 °C and the large anisotropy field of 14 T at room temperature. From differential scanning calorimetry it is concluded that the 2:17 nitride is metastable. It decomposes into Sm nitride and α‐Fe above 600 °C.

High coercivity in Sm2Fe17Nx magnets

Sm–Fe alloys have been produced by rapid quenching and the resulting phases have been investigated in the as‐quenched state and after nitriding. Besides the well‐known equilibrium phases of the binary Sm–Fe system (Fe, Sm2Fe17, SmFe3, SmFe2 and Sm), a hexagonal TbCu7‐type phase shows up in melt spun ribbons (a=4.88 A, c=4.23 A). Its stoichiometry is about Sm1Fe9 and it is formed only at wheel velocities above 15 m/s. The Curie temperature and the saturation polarization of this new phase is 210 °C and 1.25 T, respectively. At higher Sm concentrations or lower quenching rates the structure changes to the Th2Zn17‐type. The Th2Zn17‐type ribbons are magnetically soft whereas the TbCu7‐type samples show moderate coercivities of up to 1.7 kA/cm. Nitrogenation leads to an expansion of the lattice and to an overall improvement of the hard magnetic properties for both phases. Their Curie temperatures are increased to 470 °C and the saturation polarizations are raised to 1.40 and 1.51 T for the TbCu7‐ and the Th2Zn17‐type phases, respectively. The best hard magnetic properties for isotropic TbCu7‐type material are obtained for quenched, annealed, and nitrided Sm10.6Fe89.4 which shows a coercivity, Hci, of 4.9 kA/cm, a remanence, Jr, of 0.86 T and an energy product, (BH)max, of 69.6 kJ/m3. For similarly treated Sm12Fe88, which crystallizes in the Th2Zn17 structure, a coercivity of 16.7 kA/cm, a remanence of 0.73 T and an energy product of 65.6 kJ/m3 are achieved.

Structural and hard magnetic properties of rapidly solidified Sm-Fe-N

Cu1−xCox alloys with x=0.1 and x=0.2 have been prepared by conventional melt spinning. The rapid solidification process results in an extended solubility of Co in Cu although some Co precipitates already during quenching. In the as‐quenched ribbons, the magnetoresistance (MR) is only of the order of 1.5%. It increases dramatically with the controlled nucleation and growth of Co precipitates from the supersaturated Cu matrix. The highest MR of 11% at 300 K occurs for Cu90Co10 after an aging at about 440 °C when the Co clusters are superparamagnetic. Saturation is possible only after a higher annealing or at lower measuring temperatures. For the optimally annealed samples the MR increases to 36% at 30 K.

Joachim Dr. Wecker

Papers

Giant negative magnetoresistance in perovskitelike La2/3Ba1/3MnOx ferromagnetic films.

High coercivity in Sm2Fe17Nx magnets

Structural and hard magnetic properties of rapidly solidified Sm-Fe-N

Giant magnetoresistance in melt spun Cu‐Co alloys

Magnetocrystalline anisotropy of Sm2Fe17N2