Multipolar ordering and magnetization reversal in two-dimensional nanomagnet arrays

The low-temperature stable states and the magnetization reversal of realistic two-dimensional nanoarrays with higher-order magnetostatic interactions are studied theoretically. For a general calculus of the multipole-multipole interaction energy we introduce a Hamiltonian in spherical coordinates into the Monte Carlo scheme. We demonstrate that higher-order interactions considerably change the dipolar ground states of in-plane magnetized arrays favoring collinear configurations. The multipolar interactions lead to enhancement or decrease of the coercivity in arrays with in-plane or out-of-plane magnetization.     

Collective effects accompanying magnetization of two-dimensional lattices of nanosize magnetic particles

Collective effects arising in a two-dimensional lattice of nanosize magnetic particles as a result of the dipole interparticle interaction are investigated by Hall magnetometry. The experimental system consists of 10⁵ permalloy particles having a diameter of ∼40 nm and a height of ∼40 nm and forming a lattice with a rectangular unit cell (90 nm×180 nm). We attribute the characteristic features observed in the magnetization curves to quasi-one-dimensionality of the experimental lattice of particles and to the formation of solitons in chains of dipoles. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 475–479 (25 September 1998)     

Effect of the number of nanodisks in two-dimensional arrays on magnetization reversal processes

The magnetic properties of nanodisks packed into square arrays with various numbers of elements in the face have been studied. It has been shown that the vortex nucleation field oscillates with increasing number of nanodisks; the oscillation behavior depends on the nanodisk thickness. The synchronism of the vortex state formation varies with increasing number of nanodisks. The effect of the magnetostatic interaction of nanodisks on the critical fields of magnetization reversal has been estimated for the cases of vortex and single-domain states.     

二维无限深方势阱中粒子运动的路径积分解法

用路径积分的方法计算了二维无限深方势阱中粒子的传播子,并由传播函数推导出二维无限深方势阱中粒子的波函数和能量,进一步体现了路径积分与其他经典量子化方法的等价性,反映了路径积分应用于难以处理的量子力学问题的价值.     

Magnetic field induced phase branches of the superconducting transition in two-dimensional square π-loop arrays

Based on the results of explicit forms of free energy density for each possible arrangement of magnetization fluxes in large-scale two-dimensional （2D） square π-loop arrays given by Li et al [2007 Chin. Phys. 16 1450], the field-cooled superconducting phase transition is further investigated by analysing the free energy of the arrays with a simplified symmetrical model. Our analytical result is exactly the same as that obtained in Li＇s paper by means of numerical calculations. It is shown that the phase transition splits into two branches with either ferromagnetic or anti-ferromagnetic flux ordering, which depends periodically on the strength of external magnetic flux φe through each loop and monotonically on the screen parameter β of the loops in the arrays. In principle, the diagram of the phase branches is similar to that of its one-dimensional counterpart. The influence of thermal fluctuation on the flux ordering during the transition from normal to superconducting states of the π-loop arrays is also discussed.     

Collective dynamics in optomechanical arrays

Optomechanical systems couple light stored inside an optical cavity to the motion of a mechanical mode. Recent experiments have demonstrated setups, such as photonic crystal structures, that in principle allow one to confine several optical and vibrational modes on a single chip. Here we start to investigate the collective nonlinear dynamics in arrays of coupled optomechanical cells. We show that such "optomechanical arrays" can display synchronization, and that they can be described by an effective Kuramoto-type model.     

Collective resonances in gold nanoparticle arrays

We present experimental evidence of sharp spectral features in the optical response of 2D arrays of gold nanorods. A simple coupled dipole model is used to describe the main features of the observed spectral line shape. The resonance involves an interplay between the excitation of plasmons localized on the particles and diffraction resulting from the scattering by the periodic arrangement of these particles. We investigate this interplay by varying the particle size, aspect ratio, and interparticle spacing, and observe the effect on the position, width, and intensity of the sharp spectral feature.     

Oscillatory magnetization of two-dimensional electron gas

Diamagnetic moment of the two-dimensional electron gas is calculated for finite temperature using a simple energy band model and assuming no collisional broadening of Landau levels. Numerical results are presented for GaAs band parameters. The diamagnetic moment oscillates around zero value as a function of magnetic field strength and tends to zero at low fields. It is concluded that both the diamagnetic and the paramagnetic susceptibilities of the 2D electron gas vanish in the low field limit.     

Collective motion in superionic conductors

A theory of collective motion in superionic conductors is described by the use of a model of a crystalline cage immersed in a viscous liquid. The viscoelastic force and the interionic Coulomb force are considered as the cage—liquid interaction. The density-correlation functions and the frequency-dependent conductivity are calculated. The calculated conductivities for α-AgI are in good agreement with experiments. It is concluded that the structure in a.c. conductivity experimentally observed for α-AgI at frequencies below 10 cm^-1 can be ascribed to acoustic phonons.     

Study of magnetization process in ordered Fe nanowire arrays

Fe polycrystalline nanowires were electrodeposited in nanoporous anodized alumina membranes. Their magnetic properties were studied at 10 K. The behavior of the isothermal remanence _Mr$M_{\mathrm{r}}$ and the demagnetization remanence _Md$M_{\mathrm{d}}$ was determined. The corresponding ΔM$\mathrm{\Delta }M$ plot revealed the character demagnetizing of the dominant interactions. A simple analysis suggests that curling is the reversal magnetization mode in these samples. The forms of the _Mrev(_Mirr)Hi$M_{\mathrm{rev}}(M_{\mathrm{irr}}{)}_{H_{\mathrm{i}}}$ curves were different from those encountered until now, and it seems that these forms are associated with the curling reversal mode.     

Fabrication of two-dimensional colloidal arrays

We report the effective fabrication of two-dimensional (2D) arrays of submicron colloidal particles. These colloidal arrays are produced in thin layers of monodisperse colloid suspensions on flat surfaces of solids such as clean glass or cleaved mica. The process of colloid assembling includes two steps, nucleation and growth, similar to those found in crystallization in solution but each of the steps in detail progresses with different mechanisms. The nucleation process is initiated by a special kind of capillary force acting parallel to the surface. The growth is guided by a laminar flow of water to the crystals, which is driven by water evaporation from 2D arrays. What is distinguishable in the 2D assembling is its active nature governed by the forces and flows, making a contrast to the diffusive mechanism in ordinary crystallization. With this two-step mechanism, a domain of hexagonally packed colloidal array can grow with time. A large and uniform film of particle monolayer is, thus, formed in a short period, from several seconds up to several minutes depending on the conditions controlled.     

Collective multivortex states in periodic arrays of traps

We examine the vortex states in a 2D superconductor interacting with a square array of pinning sites. As a function of increasing pinning size or strength we find a series of novel phases including multivortex and composite superlattice states such as aligned dimer and trimer configurations at individual pinning sites. Interactions of the vortices give rise to an orientational ordering of the internal vortex structures in each pinning site. We also show that these vortex states can give rise to a multistage melting behavior.     

Discrete breathers in two-dimensional Josephson-junction arrays

We have proposed theoretically and studied numerically the existence of discrete breathers (intrinsic localized modes) in the dynamics of a two-dimensional Josephson-junction array biased by radio-frequency fields. The solutions are linearly stable in the framework of the Floquet theory and robust in the presence of thermal fluctuations. We have also discussed the conditions for realizing an experimental detection of these modes.     

Minor magnetization loops in two-dimensional dipolar Ising model

The two-dimensional dipolar Ising model is investigated for the relaxation and dynamics of minor magnetization loops. Monte Carlo simulations show that in a stripe phase an exponential decrease can be found for the magnetization maxima of the loops, M∼exp(−αN_l) where N_l is the number of loops. We discuss the limits of this behavior and its relation to the equilibrium phase diagram of the model.     

Collective magnetoplasma excitations in two-dimensional electron rings

The spectra of magnetoplasma excitations in two-dimensional electron disks and rings are studied by optical detection of resonance microwave absorption. For ring-shaped structures, two types of edge magnetoplasma modes localized along the inner and outer boundaries of the ring are observed. It is shown that the interaction between these modes leads to a strong modification of their magnetic-field dependences as compared to disks. In addition to the longitudinal edge magnetoplasma excitations, transverse plasma modes associated with the electron density oscillations along the ring radius are revealed. The spectra of magnetoplasma excitations are calculated in terms of the electrodynamic theory for both ring-shaped and disk-shaped structures. The classification of all modes of collective magnetoplasma excitations observed in the experiment is performed on the basis of the comparison between experimental and theoretical results.