Character Of Exchange Interactions Research Articles

Analysis of Structural Features of Periodic Fe/Pd/Gd/Pd Multilayered Systems

The structure of a material promising for magnetic memory elements based on Fe/Pd/Gd/Pd superlattices has been investigated by X-ray analysis and electron microscopy. Microstructural transformations in the superlattices and the influence of microstructure modification on the magnetic properties of the system are determined. It is shown that intense diffusion of Pd atoms to Gd layers occurs in the superlattices under study. The presence of a paramagnetic Pd layer changes the character of exchange interaction between neighboring ferromagnetic layers due to the induced magnetic moment on Pd atoms near the interface because of the interface imperfection. Nanocrystalline inclusions are found in the Pd/Gd/Pd layers, which are suggested to affect the specificity of magnetic ordering in these systems, as confirmed by magnetic measurements.

Structural and Magnetic Investigations of Yb Substituted Y1-xYbxBaCo4O7 (0 ≤ x ≤ 0.5) Compound

Structural and magnetic behavior of a series of new cobaltite Y1-xYbxBaCo4O7 (0 ≤ x ≤ 0.5), prepared using the conventional solid state reaction method have been investigated. Increasing Yb content leads to the increase in magnitude of magnetic moment and decreases the ferromagnetic transition (Tc) values. It is found that the predominant antiferromagnetic character of exchange interactions between cobalt ions, a ferromagnetic component, seemed to be strong enough to be detected still at room temperature. Our findings show that these materials exhibit ferromagnetic like transition at about Tc = 91 K, 80 K, 67 K, 55 K, 51 K and 45 K for composition x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5, respectively. We found that these materials exhibit spin-glass like state which dominates at low temperature.

Pressure-Controlled Migration of Paramagnetic Centers in a Heterospin Crystal.

A study of the single-crystal-to-single-crystal transformation induced by temperature variation for the chain polymer Cu(II) complex with nitronyl nitroxide showed that an increase in the hydrostatic pressure of up to ∼0.07 GPa completely changes the intracrystalline displacements of molecules relative to one another. This, in turn, significantly affects the interaction energy of the unpaired electrons of the paramagnetic centers and hence the form of the temperature dependence of the magnetic susceptibility χT. The cooling of crystals under normal conditions causes a rearrangement of the intrachain exchange clusters {>N-•O-Cu(II)-O•-N<} accompanied by a shortening of the distances between the paramagnetic centers. This changes the character of exchange interactions and generates multistage spin transitions. An increase in the hydrostatic pressure leads to a drastic change in the O···O distances between the nitroxyl fragments of adjacent chains, an increase in the antiferromagnetic exchange between them, and complete suppression of spin transitions.

Atom states and interatomic interactions in perovskite-like oxides: XXXV. Magnetic properties of solid solutions of lanthanum manganites doped with ytterbium and calcium in LaAlO3

The solid solutions La1–YYbyAlO3 (y = 0.01–0.20) and La1–0.397xYb0.067xCa0.33xMnxAl1–xO3 (x = 0.01–0.10) were studied by magnetic dilution method. The state of paramagnetic elements and the character of exchange interactions between them were determined on the basis of examination of magnetic properties. Clustering of manganese atoms in the La1–0.397xYb0.067xCa0.33xMnxAl1–xO3 solid solution, as distinct from the La1–0.67xCa0.33xMnxAl1–xO3 solid solutions studied before, is determined by the presence of ytterbium atoms in the perovskite structure of the complex oxides under study.

Peculiarities of magnetic field and spin effects in a three-spin system with regard to the long-distance character of exchange interaction

The biradical recombination probability and its field dependence in the presence of a paramagnetic particle was calculated in balance approximation taking into account the extended character of the exchange interaction in the biradical. Previously, to describe the effect of the third spin on the spin evolution in the biradical, we considered the constant spin exchange interaction both inside the biradical and between the added spin and one of the paramagnetic biradical centers. In the present work we considered the long-distance character of the exchange interaction of the biradical with the third spin. It is shown that the field dependence retains the main characteristic peculiarities of the three-spin system in the approximation of the constancy of the exchange interaction, i.e., the existence of several extrema, their shift along the field depending on the values of the exchange integrals, and the existence of a field-independent, nonzero recombination probability, known as spin catalysis.

Spintronics and spintronics materials

The review concerns the fundamentals of spintronics (spin-transport electronics). The material covers spin-spin interactions and spin relaxation in semiconductors as well as spin and spin injection related effects in the condensed matter. Examples of promising spintronic devices are given, requirements for spintronic materials are formulated, methods of synthesis of spintronic materials are described, and the physicochemical properties of some materials are characterized. Organic spintronic materials are briefly outlined and the state-of-the art in the field of research on inhomogeneous magnetic semiconducting materials possessing high-temperature ferromagnetism is described. The emphasis is placed on the chemical bonding and electronic structure of magnetic impurities in semiconductors, consideration of the nature of ferromagnetism, and on the character of exchange interactions between localized spins in novel spintronic materials.

Magnetic and magnetoelastic properties of the TbMnSi and Tb0.5La0.5MnSi compounds

X-ray diffraction studies showed substitution of nonmagnetic lanthanum for terbium in the TbMnSi polycrystalline compound to initiate a structural transition from a TiNiSi-type orthorhombic structure (for TbMnSi) to a CeFeSi-type tetragonal phase (for Tb0.5La0.5MnSi). Magnetic measurements (of the magnetization, magnetostriction, thermal expansion) performed on Tb0.5La0.5MnSi revealed a change in the character of magnetic ordering, the appearance of a ferromagnetic component in the Mn magnetic moment, a strong increase in magnetization as compared to TbMnSi, and the appearance of a spontaneous magnetic moment. Insertion of the lanthanum ion onto the rare-earth sublattice of TbMnSi brings about a change in the unit cell size and, hence, in the Mn-Mn, Mn-Si, and R-Mn interatomic distances, which causes, in turn, a change in the character of exchange interactions in Tb0.5La0.5MnSi and the formation of a complex magnetic structure.

State of Nickel in Solid Solutions of Complex Bismuth Niobates with the Pyrochlore-type Structure

Magnetic behavior of nickel atoms in diamagnetic complex bismuth niobates having the pyrochlore-type crystal structure and magnesium and zinc in niobium sites was studied. It was found that the nature of the bivalent metal atoms essentially affects the nickel state and the character of exchange interactions.

Effect of lattice distortions on the competition between the double and superexchange mechanisms inLaMnO3

Double exchange (DE), superexchange (SE), and higher-order contributions to the exchange interaction energy between Mn layers in ${\mathrm{LaMnO}}_{3}$ are studied using the linear-muffin-tin-orbital method as generalized to treat noncollinear magnetic configurations. The degree of the internal lattice distortion $(f)$ and the angle $(\ensuremath{\theta})$ between ferromagnetically ordered Mn layers are considered as simulation parameters. We find that both global and internal lattice distortions dramatically influence the character of the exchange interactions: global distortions associated with variations of the apical Mn-O bond length promotes the DE contribution; the bending of the Mn-O bonds in the $(a\ensuremath{-}b)$ plane suppresses the DE and promotes the antiferromagnetic SE contribution to the interlayer exchange energy. Overall, the character of exchange interactions is determined by SE contributions: in hypothetical ferromagnetic phases DE interactions mediated by itinerant electrons is present but still rather small compared with SE; in antiferromagnetic phases this DE is negligible and non-Heisenberg terms are small compared with SE. We find further that a metal-to-semiconductor transition (in the sense of a band-gap opening) occurs in the range of parameters of magnetic and lattice distortion variations given by $\ensuremath{\theta}g~120\ifmmode^\circ\else\textdegree\fi{}$ and $fg~$ 0.7.

Comparison of magnetic phase diagrams for (Mn1-xFex)Pt3 alloys in the ordered and disordered states

Magnetic phase transitions as a function of concentration and temperature for ordered and disordered (Mn1-xFex)Pt3 alloys are investigated by neutron diffraction and magnetic measurements. On the basis of these results the magnetic phase diagrams are drawn. The diagram for the ordered alloys is characterized by a tricritical point in the region of the ferromagnetic-to-antiferromagnetic transition at Xc=2/3. A canted noncollinear magnetic structure and a region of two-phase mixtures are shown. The diagram for the disordered alloys is characterized by two regions of heterogeneous ferromagnets, both showing re-entrant spin-glass states at low temperatures, but differing in the character of exchange interaction between magnetic atoms. The diagrams are discussed within the framework of the phenomenological theory of the phase transitions with related order parameters.

Crystal chemistry of coordination compounds with anionic ligands based on stable nitroxide radicals

Due to a recent expansion of research into metal complexes with stable NR connected with prospects for the production of a radically new class of magnetic materials [50, 51], a great amount of data has been accumulated, in particular, on CC with anionic NR ligands. But until recently there were no reviews analyzing the structural features of these compounds. The reason for this is, evidently, dissimilarity of both the NR ligands and the CC based on them. The systematic studies on the synthesis, crystal structures, and magnetic properties of CC with 3-imidazoline NR permitted analysis of the crystallochemical structure of compounds. As the present paper is virtually the first publication that overviews structural studies of CC with NR, we did not undertake to give a complete analysis of the “structure-property” relationship. Nevertheless, the structural data obtained for CC with 3-imidazoline NR allow one to make some correlations between the character of exchange interactions and the electronic and spatial structure of CC [6, 7, 31]. It is known that to choose an exchange cluster model, one needs information on the relative arrangement of PMC. But the quantum-chemical calculations of the electronic states of a system require exact data on the stereochemistry of a coordination unit and mutual arrangement of PMC. Besides, it should be noted that the relationship between crystal symmetry (centrosymmetric or acentric space group) and magnetic properties will not become adequate unitl crystal structure and symmetry studies have been carried out in a magnetic field at appropriate temperatures, i.e., under conditions of measurement of magnetic parameters for compounds of a given class. In the near future, this research will certainly be continued, both in the field of synthesis of CC with 3-imidazoline NR and preparation of their single crystals, and in the field of studies on their structure, magnetic and other physical properties, as well as of quantum-chemical calculations.

Peculiarities of exciton-magnon light absorption by quasi-one-dimensional antiferromagnet CsMnCl 3·2H 2O

The optic absorption spectrum of antiferromagnetic CsMnCl 3·2H 2O is studied in the 6A 1g( 6S)→ 4T 2( 4D) transition region. The revealed peculiarities of the exciton-magnon absorption are accounted for by the quasi-one-dimensional character of exchange interactions in the crystal. They are absent in the well-known 3-dimensional AFM spectra.