Phase transition “polydomain state-single-domain state” with exchange springs in the antiferromagnetic interlayer of the spin-valve structure

The phase transition “polydomain state-single-domain state” with exchange springs in the antiferromagnetic interlayer of the spin-valve ferromagnet-antiferromagnet-ferromagnet structure, which occurs as the ratio of the interlayer thickness to the width of atomic steps on the interfaces between the layers increases, has been investigated using mathematical simulation. The critical value of this ratio, at which the phase transition occurs, and the deflection of exchange springs that appear at the interfaces between the layers have been determined in a wide range of values of the single-ion anisotropy constant.     

Mössbauer studies of the ultrafine antiferromagnetic cores of ferritin

The ultrafine iron cores found within the iron-storage protein ferritin are of interest to both the molecular biologist and physicist. The manner in which the protein shell (apoferritin) takes up, releases and sequesters iron is of great biological importance. Also, due to their nano-size (8 nm), the magnetically ordered cores are singly domained. Such particles possess interesting magnetic relaxation and dynamic properties for which Mössbauer spectroscopy is an ideal analytic tool. Horse ferritin molecules iron-loaded artificially contain ferrihydrite-like cores which behave as superparamagnets due to their nano-size. An anomalous decrease in thef-factor occurs above the blocking temperature of the superparamagnetic particles. This has been attributed to a magnetostriction effect resulting from superparamagnetic switching.     

Effect of the Aharonov-Bohm flux on the magnetic gap soliton in antiferromagnetic chain

Employing the double-sublattice, the coherent state ansatz, and the time-dependent variational principle, we have studied the effects of the Aharonov-Bohm flux on the soliton in one-dimensional antiferromagnetic chain. The results show that the Aharonov-Bohm flux have an effect on the peak, the width, the energy and the spatial configuration of the spin of the soliton.     

Single-site approximation for the s–f model of antiferromagnetic semiconductors

For the s–f model of an antiferromagnetic semiconductor, the effect of the antiferromagnetic ordering of the localized spins on the conduction-electron state is investigated over a wide range of exchange strengths by combining the effective-medium approach with the Green's function in the 2×2 sublattice Bloch function representation. The band splitting due to the reduced magnetic Brillouin zone occurs below the Néel temperature. There is a marked effect of the thermal fluctuation of the antiferromagnetically ordered localized spins on the conduction electron at the energies near the top (bottom) of the lower- (higher-) energy subband.     

Neutron-diffraction studies of the antiferromagnetic–ferromagnetic transition in anion-deficient cobaltites

Anion-deficient La_0.5Ba_0.5CoO_2.8 and Y_0.25Ca_0.25Sr_0.5CoO_2.62 cobaltites are studied under pressures up to 6.5 GPa in the temperature range of 5?300 K by neutron powder diffraction. Under ambient pressure both compounds are antiferromagnetic with T _N = 250 K. The applied pressure induces in the first compound a gradual transition from the antiferromagnetic to ferromagnetic state through a mixed magnetic state, whereas the second compound remains antiferromagnetic in the whole pressure range. We suggest that the magnetic ground state depends on the unit-cell volume and the magnetic transition is associated with the transition of cobalt ions from the mixed high-spin/low-spin state to the intermediate-spin/low-spin state.     

Destruction of the long-range antiferromagnetic order by defects of the “random local field” type

The behavior of diluted antiferromagnets in an external magnetic field has been considered. It has been shown that, because the “force” of induced defects of the “random local field” type depends on the magnetic induction, the destruction of the long-range antiferromagnetic order by these defects in space of dimension d ≥ 2 is impossible.     

Investigation of the character of the exchange interaction in the system Hg1−xMnxSe

A new method is proposed for determining the magnetic characteristics (magnitude and sign of the exchange interaction energy and the average size of clusters of magnetic ions) of dilute solid solutions of semimagnetic semiconductors at low temperatures based on oscillation measurements. The method makes it possible to find the magnetic characteristics of the indicated systems at temperatures between the point of the transition into the spin glass state and the temperature corresponding to the characteristic binding energy of magnetic atoms in clusters, for which standard methods based on the measurement of the magnetic susceptibility are not effective. The method is used to study the character of the exchange interaction in the system of solid solutions Hg_1–xMn_xSe as a function of their composition as well as under conditions of hydrostatic compression. To this end the oscillations of the magnetoresistance (Shubnikov-de Haas (SH) effect) in single-crystalline samples of Hg_1–xMn_xSe in the region of compositions 0.001 x 0.23 in magnetic fields H up to 65 kOe at temperatures T = (0.4–20) K and pressures up to 16 kbar were studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 29–39, November, 1989.     

Phase diagrams of La1?xCaxMnO3 in double exchange model with added antiferromagnetic and Jahn-Teller interaction

Within the effective mass approximation we theoretically studied the electronic properties of CdSe/ZnS and ZnS/CdSe core-shell quantum dots surrounded by wide-gap dielectric materials. The finite element method is used to obtain the lowest impurity levels and the carrier spatial distribution within the dot. We found that in these zero-dimensional semiconductor structures the electron energy is sensitively dependent on the dielectric constants of the embedding and on the heterostructure geometry. The influence of polarization charges on the binding energy of hydrogenic impurities off-center located is also investigated. The results suggest that in dielectrically modulated nanodots the donor energy can be tuned to a large extent by the structure design, the impurity position and a proper choice of the dielectric media.     

Similarity of the Fermi surface in the hidden order state and in the antiferromagnetic state of URu₂Si₂

Shubnikov-de Haas measurements of high quality URu2Si2 single crystals reveal two previously unobserved Fermi surface branches in the so-called hidden order phase. Therefore, about 55% of the enhanced mass is now detected. Under pressure in the antiferromagnetic state, the Shubnikov-de Haas frequencies for magnetic fields applied along the crystalline c axis show little change compared with the zero pressure data. This implies a similar Fermi surface in both the hidden order and antiferromagnetic states, which strongly suggests that the lattice doubling in the antiferromagnetic phase due to the ordering vector Q(AF)=(001) already occurs in the hidden order. These measurements provide a good test for existing or future theories of the hidden order parameter.     

Practical failures from the inclusion of exact exchange: how much exact exchange is appropriate?

The influence of exact exchange incorporated into exchange—correlation functionals on the predictions of relative energies, structures, electronic states, and vibrational spectra is examined numerically. Failures of widely used hybrid exchange—correlation functionals due to either the physical unacceptability of including exact exchange or an unbalanced mixing of exact exchange are considered. One set of examples involves tetraatomic chalcogen clusters and charge transfer complexes between diatomic chalcogens and diatomic oxygen. Poor energetic predictions from Hartree-Fock rule against the inclusion of exact exchange into the exchange—correlation functionals for these systems with significant left—right electron correlation effects. The energies of the conformers of [10]annulene are considered from an unusual viewpoint, namely, the empirical adjustment of the admixture of exact exchange to match the predictions of very high level theoretical methods. For this annulene with insignificant left-right electron correlation effects, a greater (50%) percentage of exact exchange should be included. The relationship of symmetry breaking to the inclusion of exact exchange is examined for seven linear radicals, OXO (X = B, Al, Ga, In, TI). AIOS, and OAIS. exchange—correlation functionals generate symmetry adapted solutions at the expense of an unusual ordering of the Kohn-Sham orbitals, which can cause uncharacteristic electronic states, incorrect vibrational spectra, and poorer predictions of energetics. These effects are greater when exact exchange is included. In all the examples considered, the appropriate focus for a detailed discussion of molecular properties involves consideration of the effects of exact exchange.     

Investigation of the parametric excitation and relaxation of phonons in antiferromagnetic α-Fe2O3

Parametric excitation of magnetoelastic waves was investigated in the easy-plane antiferromagnet α-Fe₂O₃ by parallel and perpendicular microwave pumping over a wide range of frequencies, magnetic fields, and temperatures, and the parametric resonance thresholds were measured. The frequencies of the natural magnetoelastic vibrations of the sample were investigated as a function of the magnetic field and temperature. The results of the measurements were used to calculate the parameters of the magnetoelastic wave spectrum and the rate of relaxation of the excited quasi-phonons. Possible mechanisms for quasi-phonon damping were analyzed.     

An average-magnetization analysis of R 1ρ relaxation outside of the fast exchange limit

Approximate expressions for the NMR spin relaxation rate constant in the rotating frame of reference R _1ρ are derived for two-site chemical exchange by consideration of the evolution of the average density operator using the stochastic Liouville equation. R _1ρ is obtained as a linearized approximation to the largest (least negative) eigenvalue of the matrix describing the evolution of the average density operator in the long-term limit. The expressions obtained are more accurate than existing expressions when exchange is not fast and the populations of the exchanging sites are close to equal. The new expressions for R _1ρ facilitate the interpretation of chemical exchange phenomena in proteins and other biological macromolecules.     

Multicritical behaviors of the antiferromagnetic Blume–Emery–Griffiths model with the external magnetic field on the Bethe lattice

The antiferromagnetic Blume–Emery–Griffiths model in an external magnetic field is studied by using the exact recursion relations on the Bethe lattice for arbitrary values of biquadratic and for negative values of bilinear interactions. We have studied the thermal variations of two-sublattice magnetizations belonging to spin-1 BEG model to obtain the phase diagrams on the (H/|J|,kT/|J|)$(H/|J|,\mathit{kT}/|J|)$ plane. As a result, we have found that the system presents second- and first-order phase transitions, therefore, tricritical points for appropriate values of K/|J|$K/\left|J\right|$, D/|J|$D/\left|J\right|$ and q  . We have also found that the second-order phase transition lines exhibit reentrant phenomena in temperature, besides it also shows reentrant phenomena for the first-order phase lines in external magnetic field for q=4$q=4$ and 6.     

Blume–Capel model on the antiferromagnetic fcc lattice

In the presence of a uniform external magnetic field, the spin-1 fcc antiferromagnetic Ising model in the four sub-lattice with the nearest-neighbor bilinear and crystal field interactions is investigated by using mean-field theory. The lattice is considered in blocks and each block contains 32 spins. While the central four spins forming a basic tetrahedron structure are allowed to fluctuate, the surrounding 28 spins take the mean-field values. The ground state phase diagram of the system is examined since it provides valuable information to make a prediction of the different phase diagram topologies. The phase diagrams are obtained on the reduced external field versus temperature, (H/|J|−kT/|J|)$(H/\left|J\right|-kT/\left|J\right|)$, plane for selected crystal field interaction values from the ground state phase diagram and it is found that the system exhibits reentrant behavior and some special points such as tricritical, bicritical and triple points.     

Magnetic phase diagram of the intermetallic compounds Gd1−x LaxMn2Ge2 and the effect of a field on transitions of the Mn subsystem from the antiferromagnetic into the ferromagnetic state

The magnetic properties of the intermetallic compounds Gd_1?x La_xMn₂Ge₂ (layered tetragonal crystal structure of the type ThCr₂Si₂), investigated in the present work and in [1] A. Sokolov, et al., Sol. State Commun. 105, 289 (1998), are discussed. It is shown that the basic characteristics of the magnetic ordering of these compounds—magnetic phase transitions from the ferrimagnetic into the antiferromagnetic state and vice versa—observed with increasing temperature in compositions with x < 0.1, the effect of a magnetic field on these transitions, the temperature dependences of the magnetization of these intermetallic compounds, as well as their magnetic T-x diagram can be described quantitatively in the Yafet-Kittel model for ferrimagnets with a negative exchange interaction in one of the sublattices using parameters determined in [2] A. Yu. Sokolov et al., JETP 89, 723 (1999) from investigations of the system Gd_1?x Y_xMn₂Ge₂, taking into account the dependence of the interplanar exchange interaction Mn-Mn on the crystal lattice parameter a.     

The conversion width of the σ- and ξ-hyperons in nuclei and the one-meson exchange

The conversion width in nuclear matter for and is estimated in the frame of one-mesonexchange model (OMEM). There is a considerable reduction of the width for if additional mesons besides are taken into account. The expected width for is about 3 MeV.Presented at the symposium Mesons and Light Nuclei, Bechyn, Czechoslovakia, May 27–June 1, 1985.