Yu. G. Pogorelov

Peculiar physical properties and the colossal magnetoresistance of manganites (Review)

An attempt is made to analyze the most important physical properties of manganites of the La-Ca-Mn-O type, which exhibit the colossal magnetoresistance effect. The primary focus is on the peculiarities of these compounds which are reflected in their crystalline, electronic, and magnetic structures and which determine the possible mechanisms by which an external magnetic field can exert a substantial influence on the transport characteristics of the current carriers in manganites. The combined effect of these factors is to create the necessary conditions for a metal-insulator phase transition that is sensitive to an external magnetic field. Another major topic in this review is a discussion of the scientific problems confronting the physics of manganites.

Tunnel magnetoresistance and magnetic ordering in ion-beam sputtered Co80Fe20/Al2O3 discontinuous multilayers

Discontinuous multilayered Co80Fe20(t)/Al2O3(30 A) thin films have been prepared by ion-beam sputtering. We report on structural, magnetic, and transport (for current in plane geometry) results obtained in this system. With growing nominal thickness t of the metal layers, which effectively characterizes the granular structure, a transition from tunnel to metallic conductance is observed, indicating the onset of infinite conducting paths at t>18 A. At t 13 A was detected from the magnetization data which display here a transition from superparamagnetic to ferromagnetic behavior. The measurements of tunnel magnetoresistance (MR) show that a sharp maximum of MR sensitivity to field takes place at this thickness, reaching ∼24%/kOe at room temperature. At least, MR itself as a function of t has a break at the same value. All these features suggest that some specific kind of percolation with respect to magnetic order occurs in o...

Ferromagnetic resonance in granular thin films

A theoretical analysis has been done of the effects of granule size, shape, orientation, and concentration and the temperature effects on the ferromagnetic resonance (FMR) field Hr in granular thin films. The granular CoxAg1−x thin films with 0.2 fp. For the Co–Ag system, persistence of a considerable SPM fraction is revealed by the superconducting quantum interference device data up to the highest f, and the effect at f=fp consists in a discontinuous jump of the Hr(f) slope. Otherwise, the FMR data for granular Fe–SiO2 films reveal a discontinuous jump in H...

Origin of fourfold anisotropy in square lattices of circular ferromagnetic dots

We discuss the fourfold anisotropy of the in-plane ferromagnetic resonance field Hr, found in a square lattice of circular Permalloy dots when the interdot distance a becomes comparable to the dot diameter d. The minimum Hr along the lattice 11 axes and the maximum along the 10 axes differ by 50 Oe at a/d=1.1. This anisotropy, not expected in uniformly magnetized dots, is explained by a mechanism of nonuniform magnetization mr in a dot in response to dipolar forces in the patterned magnetic structure under strong enough applied field. It is well described by an iterative solution of a continuous variational procedure.

Ground-state instability in systems of strongly interacting fermions

The stability of a fermion system is analyzed for a model repulsive pair interaction potential. The possibility of different types of restructuring of the Fermi ground state (at sufficiently great coupling constant) is related to the analyticity properties of such potential. In particular, for the screened Coulomb law it is shown that the restructuring cannot be of the Fermi condensation type, known earlier for some exactly solvable models, but instead belongs to the class of topological transitions. A phase diagram constructed for this model in the variables “screening parameter-coupling constant” displays two kinds of topological transitions: a “5/2” kind, similar to the known Lifshitz transitions in metals, and a “2” kind, characteristic for a uniform strongly interacting system.

Exact solution of Ising model on a small-world network.

We present an exact solution of a one-dimensional Ising chain with both nearest-neighbor and random long-range interactions. Not surprisingly, the solution confirms the mean-field character of the transition. This solution also predicts the finite-size scaling that we observe in numerical simulations.

Anomalous low-field magnetization in La2/3Ca1/3MnO3 near the critical point: Stable clusters?

The magnetic behavior of bulk La2/3Ca1/3MnO3 (TC=267 K) at low fields in the paramagnetic phase was studied. Near TC(T−TC<30 K) we find a sequence of steplike features in the effective Curie constant C(T)=M(T−TC)/H at well defined temperatures and intermediate plateaus, accompanied by temperature hysteresis. On approaching TC, the C values at each plateau follow a geometrical progression, with the effective spin S doubled at each step. The first plateau gives S=5.5, corresponding to a cluster of three Mn3+ ions sharing one extra hole. This suggests that close to TC short range magnetic order at low fields develops hierarchically through a series of most stable cluster states.

Low-temperature orientational ordering and possible domain structures in C60 fullerite

Based on a simple model for the ordering of hexagons on a square planar lattice, an attempt is made to consider the possible structure of C60 fullerite in its low-temperature phase. It is shown that hexagons representing fullerenes oriented along the C3 axes of the sc lattice can be ordered into an ideal structure with four nonequivalent molecules in the unit cell. Then the energy degeneracy for the rotation of each hexagon by π/3 around its C3 axis leaves the translational and orientational order in this structure but leads to a random distribution of π/3 rotations and hence to an “averaged” unit cell with two molecules. However, the most relevant structural defects are not these intrinsic “misorientations” but some walls between domains with different sequences of the above-mentioned two (nonideal) sublattices. Numerical estimates are made for the anisotropic intermolecular potential, showing that the anisotropy is noticeably smaller for molecules in walls than in domains.

Influence of co-evaporation technique on the structural and magnetic properties of CoCu granular films

Abstract Structural, magnetic and transport properties of granular Co x Cu 1 − x ( x = 0.11–0.45) films prepared by e-beam co-evaporation, were studied. Scanning tunneling microscopy showed the surface crystallites having an elongated shape. Magnetization and FMR data indicated a large in-plane uniaxial anisotropy in all the samples. A correlation between the orientation of the surface crystallites and the anisotropy easy axis has been found. Magnetoresistance in the fields up to 0.5 T is strongly different for transverse and longitudinal geometries.

Magnetization and magneto resistance in Fe-ion-implanted Cu and Ag thin films

Abstract Diluted granular films of CuFe and AgFe (iron content ⩽ 2%) were produced using 57 Fe ion implantation on Cu(Ag) films previously grown by laser ablation. Conversion electron Mossbauer spectroscopy shows that the implanted Fe forms either very small clusters (up to a few atoms) or large iron α-phase particles. These structural characteristics directly reflect on the magnetization, which exhibits ferromagnetic-like behaviour at room temperature (due to large clusters) superimposed by a significant paramagnetic contribution at low temperatures due to the small clusters. We observe deviations from strict superparamagnetic behaviour due to non-negligible local anisotropy effects at low temperatures and low fields. The Kondo effect is particularly enhanced in the CuFe films which have higher concentration of isolated Fe atoms and small sizes clusters. The magnetoresistivity Δ ϱ / ϱ of our films is dominated (for 0 ⩽ μ 0 H ⩽ 15 T) by a linear term in H , attributed to GMR-like effect from spin-dependent scattering when an electron passes between adjacent large and small clusters. At low fields we observe instead Δ ϱ / ϱ ∝ H 2 , due to the usual GMR effect between large clusters, during the alignment of their easy axes. The relevant physical differences (structural, magnetic and magnetoresistive) observed in our ion-implanted diluted Fe films, with respect to the concentrated granular films, are critically analysed.