AC susceptibility and 51^{51}V NMR study of MnV2_2O4_4

We report $^{51}$V zero-field NMR of manganese vanadate spinel of MnV$_2$O$_4$, together with both ac and dc magnetization measurements. The field and temperature dependence of ac susceptibilities show a reentrant-spin-glass-like behavior below the ferrimagnetic(FEM) ordering temperature. The zero-field NMR spectrum consists of multiple lines ranging from 240 MHz to 320 MHz. Its temperature dependence reveals that the ground state is given by the simultaneous formation of a long-range FEM order and a short-range order component. We attribute the spin-glass-like anomalies to freezing and fluctuations of the short-range ordered state caused by the competition between spin and orbital ordering of the V site

Competing interactions in two dimensional Coulomb systems: Surface charge heterogeneities in co-assembled cationic-anionic incompatible mixtures

A binary mixture of oppositely charged components confined to a plane such as cationic and anionic lipid bilayers may exhibit local segregation. The relative strength of the net short range interactions, which favors macroscopic segregation, and the long range electrostatic interactions, which favors mixing, determines the length scale of the finite size or microphase segregation. The free energy of the system can be examined analytically in two separate regimes, when considering small density fluctuations at high temperatures, and when considering the periodic ordering of the system at low temperatures (F. J. Solis and M. Olvera de la Cruz, J. Chem. Phys. 122, 054905 (2000)). A simple Molecular Dynamics simulation of oppositely charged monomers, interacting with a short range Lennard Jones potential and confined to a two dimensional plane, is examined at different strengths of short and long range interactions. The system exhibits well-defined domains that can be characterized by their periodic length-scale as well as the orientational ordering of their interfaces. By adding salt, the ordering of the domains disappears and the mixture macroscopically phase segregates in agreement with analytical predictions.Comment: 8 pages, 5 figures, accepted for publication in J. Chem. Phys, Figure 1 include

Short-range ordering in face-centered-cubic Ni3Al

Films of fcc Ni3Al with suppressed short-range order (SRO) were prepared by physical vapor deposition of Ni3Al onto room-temperature substrates. Extended electron energy-loss fine-structure spectra were obtained from both Al K and Ni L23 edges. After the samples were annealed for various times at 150 °C, a moderate growth of SRO was observed in the first-nearest-neighbor environments of both the Al and Ni atoms. As prepared, these fcc Ni3Al materials, and presumably others having similar heat evolutions as measured by differential scanning calorimetry, have a high degree of chemical disorder

A 2-D suspension of active agents: the role of fluid mediated interactions

Taking into account simultaneously the Vicsek short range ordering and also the far-field hydrodynamic interactions mediated by the ambient fluid, we investigate the role of long range interactions in the ordering phenomena in a quasi 2-dimensional active suspension. By studying the number fluctuations, the velocity correlation functions and cluster size distribution function, we observe that depending on the number density of swimmers and the strength of noise, the hydrodynamic interactions affect the ordering in a suspension. For a fixed value of noise, at large density of particles, long range interactions enhance the clustering in the system.Comment: 7 pages, 5 figure

The spin-1 two-dimensional J1-J2 Heisenberg antiferromagnet on a triangular lattice

The spin-1 Heisenberg antiferromagnet on a triangular lattice with the nearest- and next-nearest-neighbor couplings, $J_1=(1-p)J$ and $J_2=pJ$, $J>0$, is studied in the entire range of the parameter $p$. Mori's projection operator technique is used as a method which retains the rotation symmetry of spin components and does not anticipate any magnetic ordering. For zero temperature four second-order phase transitions are observed. At $p\approx 0.038$ the ground state is transformed from the long-range ordered $120^\circ$ spin structure into a state with short-range ordering, which in its turn is changed to a long-range ordered state with the ordering vector ${\bf Q^\prime}=\left(0,-\frac{2\pi}{\sqrt{3}}\right)$ at $p\approx 0.2$. For $p\approx 0.5$ a new transition to a state with a short-range order occurs. This state has a large correlation length which continuously grows with $p$ until the establishment of a long-range order happens at $p \approx 0.65$. In the range $0.5<p<0.96$, the ordering vector is incommensurate. With growing $p$ it moves along the line ${\bf Q'-Q}_1$ to the point ${\bf Q}_1=\left(0,-\frac{4\pi}{3\sqrt{3}}\right)$ which is reached at $p\approx 0.96$. The obtained state with a long-range order can be conceived as three interpenetrating sublattices with the $120^\circ$ spin structure on each of them.Comment: 13 pages, 5 figures, accepted for publication in Physics Letters

Glassy magnetic phase driven by short range charge and magnetic ordering in nanocrystalline La1/3_{1/3}Sr2/3_{2/3}FeO3−δ_{3-\delta}: Magnetization, Mossbauer, and polarised neutron studies

The charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3-\delta}$ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, M\"{o}ssbauer, and polarised neutron studies. A complex scenario of short range charge and magnetic ordering is realized from the polarised neutron studies in nanocrystalline specimen. This short range ordering does not involve any change in spin state and modification in the charge disproportion between Fe$^{3+}$ and Fe$^{5+}$ compared to bulk counterpart as evident in the M\"{o}ssbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe$^{3+}$ and Fe$^{5+}$ are about 3.15$\mu_B$ and 1.57$\mu_B$ for bulk, and 2.7$\mu_B$ and 0.53$\mu_B$ for nanocrystalline specimen, respectively. The destabilization of charge ordering leads to magnetic phase separation, giving rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K attains a value as high as 4.4 kOe for average size $\sim$ 70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass like transition around $\sim$ 65 K, below which EB appears. Overall results propose that finite size effect directs the complex glassy magnetic behavior driven by unconventional short range charge and magnetic ordering, and magnetic phase separation appears in nanocrystalline LSFO.Comment: 10 pages, 9 figures. Fig. 1 available upon request or in http://www.ffn.ub.es/oscar/Articles.html. Accepted in Phys. Rev.

Properties of charge density waves in La2−x_{2-x}Bax_{x}CuO4_4

We report a comprehensive x-ray scattering study of charge density wave (stripe) ordering in $\rm La_{2-x}Ba_xCuO_4 (x \approx 1/8)$, for which the superconducting $T_c$ is greatly suppressed. Strong superlattice reflections corresponding to static ordering of charge stripes were observed in this sample. The structural modulation at the lowest temperature was deduced based on the intensity of over 70 unique superlattice positions surveyed. We found that the charge order in this sample is described with one-dimensional charge density waves, which have incommensurate wave-vectors (0.23, 0, 0.5) and (0, 0.23, 0.5) respectively on neighboring $\rm CuO_2$ planes. The structural modulation due to the charge density wave order is simply sinusoidal, and no higher harmonics were observed. Just below the structural transition temperature, short-range charge density wave correlation appears, which develops into a large scale charge ordering around 40 K, close to the spin density wave ordering temperature. However, this charge ordering fails to grow into a true long range order, and its correlation length saturates at $\sim 230\AA$, and slightly decreases below about 15 K, which may be due to the onset of two-dimensional superconductivity.Comment: 11 pages, 9 figure

An effect of antiphase boundaries on the kinetics of short-range ordering by a vacancy mechanism

In Monte Carlo simulations of chemical short-range ordering on a square lattice, the number of single-atom domains was found to depend on the presence of antiphase boundaries and on the mechanism by which ordering occurred. When antiphase boundaries were present and the ordering occurred by a vacancy mechanism, the number of single-atom domains was found to increase with decreasing temperature, in contrast to thermodynamic predictions. This is understood as a consequence of highly correlated vacancy motions in those regions of the lattice away from antiphase boundaries

Local orientational ordering in fluids of spherical molecules with dipolar-like anisotropic adhesion

We discuss some interesting physical features stemming from our previous analytical study of a simple model of a fluid with dipolar-like interactions of very short range in addition to the usual isotropic Baxter potential for adhesive spheres. While the isotropic part is found to rule the global structural and thermodynamical equilibrium properties of the fluid, the weaker anisotropic part gives rise to an interesting short-range local ordering of nearly spherical condensation clusters, containing short portions of chains having nose-to-tail parallel alignment which runs antiparallel to adjacent similar chains.Comment: 13 pages and 6 figure