Inequivalent Sublattices Research Articles

Chirality-dependent magnon lifetime in a compensated half-metallic ferrimagnet

We report a first-principles investigation of magnetic excitations in a compensated half-metallic ferrimagnet using both the adiabatic Heisenberg model and the dynamic spin susceptibility of the electronic system. The combination of half-metallicity and spin compensation of inequivalent magnetic sublattices generates two acoustic magnon modes characterized by linear dispersions with equal spin wave velocities and an asymmetric Landau damping. The difference in the damping is the consequence of the half-metallicity leading to the gap in the energy of the up-to-down electronic spin-flip excitations, whereas down-to-up excitations are gapless. The engineering of the activation energy of the Stoner excitations opens an avenue for the chirality-selective manipulation of the lifetime of magnons.

Spin switching and magnetization reversal in single-crystal NdFeO3

We report an experimental and computational study of single-crystal NdFeO${}_{3}$, which features two inequivalent magnetic sublattices, namely, Fe and Nd sublattices that are coupled in an antiparallel fashion. This paper reveals that a strong interaction between 3$d$ and 4$f$ electrons of the two sublattices along with a spin-lattice coupling drives an extremely interesting magnetic state that is highly sensitive to the orientation and history of weak magnetic field. The following phenomena are particularly remarkable: (1) sharply contrasting magnetization $M$($T$) along the $a$ and $c$ axes; (2) a first-order spin switching along the $a$ axis below 29 K when the system is zero-field-cooled; and (3) a progressive magnetization reversal when the system is field-cooled. The intriguing magnetic behavior is captured in our first-principles density functional theory calculations.

Thermodynamic stability of calcium vanadium garnet ferrites upon formation of substitutional solid solutions

The thermodynamic stability of calcium vanadium garnet ferrites Ca3Fe3.5 − xTi2xV1.5 − xO12 upon isomorphic substitution of titanium ions for iron and vanadium ions was studied by the EMF method using ZrO2(Y2O3) ceramic solid electrolyte. Temperature-dependent ΔG0 was determined. Isomorphic substitutions of titanium ions for iron and vanadium ions in the inequivalent sublattices of the garnet structure in a temperature range of 1100–1483 K cause changes in ΔG0 due to the entropy and enthalpy components and has a minimal value when x = 0.15.

Supersolid polar molecules beyond pairwise interactions

We explore the phase diagram of ultracold bosonic polar molecules confined to a planar optical lattice of triangular geometry. External static electric and microwave fields can be employed to tune the effective interactions between the polar molecules into a regime of extended two- and three-body repulsions of comparable strength, leading to a rich quantum phase diagram. In addition to various solid phases, an extended supersolid phase is found to persist deep into the three-body dominated regime. While three-body interactions break particle-hole symmetry explicitly, a characteristic supersolid-supersolid quantum phase transition is observed, which indicates the restoration of particle-hole symmetry at half-filling. We revisit the spatial structure of the supersolid at this filling, regarding the existence of a further supersolid phase with three inequivalent sublattices, and provide evidence that this state is excluded also at finite temperatures.

Nonzero macroscopic magnetization in half-metallic antiferromagnets at finite temperatures

Combining density-functional theory calculations with many-body Green's-function technique, we reveal that the macroscopic magnetization in half-metallic antiferromagnets does not vanish at finite temperature as for the $T=0$ limit. This anomalous behavior stems from the inequivalent magnetic sublattices which lead to different intrasublattice exchange interactions. As a consequence, the spin fluctuations suppress the magnetic order of the sublattices in a different way leading to a ferrimagnetic state at finite temperatures. Computational results are presented for the half-metallic antiferromagnetic $\text{CrMn}Z$ $(Z=\text{P},\text{As},\text{Sb})$ semi-Heusler compounds.

Magnetic ordering and phase transitions in R 3Cu 4X 4

Ternary R 3Cu 4X 4 (R=Tb –Er; X=Si, Ge, Sn) compounds were investigated by means of neutron powder diffraction and magnetization measurements. The compounds crystallize in the orthorhombic Gd 3Cu 4Ge 4-type crystal structure (space group Immm). The rare-earth atoms occupy two inequivalent 2d and 4e sublattices. The magnetic moments are localized exclusively on the rare earths and order at low temperatures. On the basis of the collected data thermal dependence of the magnetic structure was determined.

Features of the nonlinear dynamics of domain walls in ferrimagnets

The nonlinear motion of a 180° domain wall with two inequivalent sublattices in the field of a sound wave propagating orthogonally to the plane of the domain wall is investigated. The dependence of the drift velocity of the domain wall on the polarization and the parameters of the sound wave is found. An analysis of the long-wavelength and short-wavelength approximations is carried out. The conditions for drift of the stripe domain structure in ferrites are determined.

Brillouin-scattering studies of ferroelectric[NH3(CH3)]5Bi2Cl11

High-resolution Brillouin spectroscopy was used to investigate the elastic properties of $[{\mathrm{NH}}_{3}{(\mathrm{C}\mathrm{H}}_{3}{)]}_{5}{\mathrm{Bi}}_{2}{\mathrm{Cl}}_{11}.$ The temperature dependences of nine phonon modes propagating in the directions [100], [010], [001], [110], [101], and [011] were obtained in the temperature range from 120 to 330 K. Strong correlations were found with the electric polarization mode which exhibits anomalous behavior at 307 and 170 K. A theoretical explanation of the experimental observations was given in terms of Landau-type free energy expansion for two inequivalent sublattices. Discussion was provided regarding the possible types of couplings between strain tensor components and spontaneous polarization.

Simultaneous Antiferromagnetic Order and Spin-Glass-like Behavior in MnAsO(4).

A low-temperature time-of-flight neutron powder diffraction study of a simple new solid, MnAsO(4), in a sample also containing 20% Mn(2)As(2)O(7) has been performed. MnAsO(4) orders magnetically at 14.5(5) K, and the unusual antiferromagnetic structure below this temperature has been determined. Only half of the Mn(3+) spins are ordered, and the remaining "idle" spins show some spin-glass behavior evidenced by susceptibility measurements. The ordered moment is reduced to a value of 2.6 &mgr;(B) by frustration. It is not possible to determine which of the two crystallographically inequivalent Mn sublattices is magnetically ordered and which is idle. The antiferromagnetic structure of the minority phase Mn(2)As(2)O(7) which orders at 10.5(5) K has also been determined.

THEORY OF CORRELATION EFFECTS FOR DIFFUSION IN A TWO SUBLATTICE STRUCTURE

The tracer diffusion correlation factor has been studied for a simple cubic lattice gas with two energetically inequivalent sublattices and sites restricted to accommodate a maximum of one atom. The model serves as a paradigm for solids with site inequivalence but no algebraic formulae have previously been derived and compared with available Monte Carlo simulation data. An expression based on a kinetic equation theory which neglects the highest order in concentration fluctuations is shown to lead to a good description of the simulation results. The time dependence of the associated time correlation function appears to be unusually complex for diffusion in a two sublattice system. © 1997 Elsevier Science Ltd. All rights reserved

Mn2+ EPR study of the phase transition in an ammonium sulfate single crystal: Existence of two inequivalent sublattices.

Angular variations of X-band EPR spectra, for the magnetic field orientations in three mutually perpendicular planes, of a single crystal of ${\mathrm{Mn}}^{2+}$-doped (${\mathrm{NH}}_{4}$${)}_{2}$${\mathrm{SO}}_{4}$, have been recorded from 113 to 398 K. Two ${\mathrm{Mn}}^{2+}$ centers, the principal axes of whose zero-field splitting tensors (${b}_{2}^{m}$) are found to be oriented very close to each other, exhibit ferroelectric phase transitions of entirely different natures, although occurring at the same transition temperature (${T}_{c}$=223 K). At ${T}_{c}$, the EPR lines corresponding to one center exhibit a jump (first-order transition), while those for the other a continuity (second-order transition). For both the centers each EPR hyperfine line splits into two below ${T}_{c}$. The ${\mathrm{Mn}}^{2+}$ spin-Hamiltonian parameters in (${\mathrm{NH}}_{4}$${)}_{2}$${\mathrm{SO}}_{4}$ are evaluated at room temperature, using a rigorous least-squares-fitting procedure, combined with numerical diagonalization of the spin-Hamiltonian matrix. The present EPR data confirm the existence of two inequivalent sublattices in the (${\mathrm{NH}}_{4}$${)}_{2}$${\mathrm{SO}}_{4}$ crystal. The unusual crossing of the EPR line positions, below ${T}_{c}$, for the ${\mathrm{Mn}}^{2+}$ center which undergoes a first-order phase transition at ${T}_{c}$, has been related to the reversal of the spontaneous polarization. The dynamic behavior of the two ${\mathrm{Mn}}^{2+}$ centers, in this ferroelastic crystal, has here been interpreted to be due to the deformation of the ${\mathrm{SO}}_{4}^{2\mathrm{\ensuremath{-}}}$ groups and the spontaneous strain produced by the acoustic mode. The critical exponent \ensuremath{\beta} has been determined, for both the centers, to be 0.49\ifmmode\pm\else\textpm\fi{}0.03, from the line splitting below ${T}_{c}$. It has been verified that the scaling law and Rushbrooke inequality are well satisfied in (${\mathrm{NH}}_{4}$${)}_{2}$${\mathrm{SO}}_{4}$. .AE

Extended Hubbard model and copper-oxygen valence fluctuations

The extended Hubbard model for two inequivalent sublattices is used to calculate the coupling constant W in an exchange mechanism of holes between Cu and O ions and to draw some general conclusions.

Steps on (001) silicon surfaces

We investigate a topological method of eliminating antiphase domains in zinc-blende semiconductors heteroepitaxially grown on (001) Si substrates. Antiphase domains cannot occur if only one of the two atomic species of the polar overlayer bonds directly to the substrate at the heteroepitaxial interface (As, in the case of GaAs on Si) and if atoms from only one of the two inequivalent sublattices of the substrate form the termination boundary [only biatomic (a0/2) steps present]. We show that thermal equilibrium is a necessary and sufficient condition to realize this condition for (001) Si, a result of a π-bonded step reconstruction that lowers the relative enthalpy of reconstructed [110] biatomic steps by 0.04 eV per step atom for one type of biatomic step, and of correlation, which freezes out the step configuration entropy thereby suppressing the formation of all other types of steps. Our model explains selection rules and dimer orientations observed in infrared absorption, low-energy electron diffraction, and reflection high-energy electron diffraction measurements of vicinal (001) Si surfaces tilted toward (110). For general vicinal surfaces, competition from the non-π-bonded sides of the atomically flat terraces weakens the thermodynamic driving force. For surfaces tilted toward (100) it vanishes completely, indicating that antiphase-free polar material cannot be grown on these surfaces, in agreement with experiment. If a minimum threshold energy is needed per terrace to form a primitive (001) surface, then the boundaries between antiphase-free and antiphase-dominated regions for spherically figured surfaces should be parabolic. The minimum energy estimated from the results of Akiyama et al. on spherically figured surfaces suggests that terrace–terrace correlations are also important factors in forming the primitive surface.

Mössbauer investigation of W-type hexaferrite of composition BaZn2−xCoxFe16O27

Abstract Polycrystauine samples of composition BaZn2−xCox 16O27 (O<x<2) have been examined by thermomagnetic analysis and Mossbauer spectroscopy. Cobalt ions enter preferentially the octahedral sites of the spinel block and give rise to strong perturbations of the magnetic order. The 12k sublattice splits into two magnetically inequivalent sublattices. A spin reorientation transition is observed at a temperature that increases with increasing cobalt content. Co2-W undergoes a transition from planar to conical order at 515±5 K.

The Landau model of spontaneous metamagnetism involving inequivalent sublattices

A Landau-type free energy expansion is postulated for a metamagnet possessing two inequivalent magnetic sublattices. Stability analysis of the free energy reveals a number of possible sequences of spontaneous phase transitions which involve various ordered and disordered phases. Moreover, the inter-sublattice exchange coupling is examined as to its role in the associated magnetization processes. It is concluded that relatively small changes in the sublattice magnetization are most likely to occur. These changes are usually accompanied by minor shifts in the critical temperatures. Induced first order transitions to disorder resulting from the inter-sublattice exchange are highly unlikely to take place.

Mössbauer effect in R2Fe14B compounds

We report room temperature Mössbauer spectra for R2Fe14B compounds with R=Y, Ce, Pr, Nd, Gd, Dy and Ho. The hyperfine fields are obtained for the six inequivalent iron sublattices in the R2Fe14B crystal structure. The internal magnetic fields scale with the Curie temperature, but the spectra are otherwise virtually independent of the rare-earth component. These results indicate that the electronic configurations of the iron atoms are nearly the same for all these compounds, and that the iron 3d shell dominates the conduction electron polarization contribution to the internal fields.

Proton residence times in the solid solution phase of the Y—H system studied by quasi-elastic neutron scattering

A quasi-elastic neutron scattering study was made of the diffusion of hydrogen in polycrystals of yttrium hydride in the solid solution phase. The data are compared with a model for jump diffusion between inequivalent sublattices, and an empirical relation between the respective site residence times is obtained. The diffusion coefficient at 600 °C was found to be 7 × 10 −6 cm 2 s −1.

Phase transitions of solid methane at pressures up to 8 kbar determined by NMR: CH2D2 and CD4

Abstract The phase diagrams of solid CH 2 D 2 and CD 4 have been determined up to 8 kbar. The transformation to the new phase IV, which has been shown previously to occur in solid CH 4 , also occurs in the deuterated species. At normal pressure, phase IV is stable for at least several days. The T 1 data in the phases III and IV suggest the existence of several inequivalent sublattices with low site symmetries. The triple points of the I–II and II–III phase-lines of CH 2 D 2 and CD 4 have been established.

Electrical conduction and 19F NMR of solid solutions La 1−xBa xF 3−x

The electrical conductivity and 19F NMR spin-spin relaxation of single crystals of the anion-deficient solid solutions La 1−xBa xF 3−x (0 <× <0.095) have been investigated. In the temperature dependence of the conductivity a knee is observed, which is ascribed to exchange of fluoride ions between inequivalent sublattices. This is confirmed by spin-spin relaxation in pure LaF 3 and the solid solutions. Self-diffusion coefficients derived from the conductivity are in agreement with those from the spin-lattice relaxation time T 1ϱ in the rotating frame, indicating an absence of associates (Ba La V F) x above room temperature.

Probability renormalization group study of selective site percolation

Selective site percolation on a square lattice with two inequivalent sublattices is studied by the renormalization group technique. The phase boundary is controlled by a single fixed point if accordance with the universality hypothesis. An extension of parameter space results in an improvement of the critical concentration of standard percolation.