Time-resolved scanning Kerr microscopy measurements have been performed upon arrays of square ferromagnetic nanoelements of different sizes and for a range of bias fields. The experimental results were compared to micromagnetic simulations of model arrays in order to understand the nonuniform precessional dynamics within the elements. In the experimental spectra acquired from an element of length of 236 nm and thickness of 13.6 nm, two branches of excited modes were observed to coexist above a particular bias field. Below this so-called crossover field, the higher frequency branch was observed to vanish. Micromagnetic simulations and Fourier imaging revealed that modes from the higher frequency branch had large amplitude at the center of the element where the effective field was parallel to the bias field and the static magnetization. Modes from the lower frequency branch had large amplitude near the edges of the element perpendicular to the bias field. The simulations revealed significant canting of the static magnetization and effective field away from the direction of the bias field in the edge regions. For the smallest element sizes and/or at low bias field values, the effective field was found to become antiparallel to the static magnetization. The simulations revealed that the majority of the modes were delocalized with finite amplitude throughout the element while the spatial character of a mode was found to be correlated with the spatial variation in the total effective field and the static magnetization state. The simulations also revealed that the frequencies of the edge modes are strongly affected by the spatial distribution of the static magnetization state both within an element and within its nearest neighbors. Furthermore, the simulations suggest that collective modes may be supported in arrays of interacting nanomagnets, which act as magnonic crystals.

We perform combined resistivity and compressibility studies of two-dimensional hole and electron systems which show the apparent metal-insulator transition -a crossover in the sign of ∂R/∂T with changing density. No thermodynamic anomalies have been detected in the crossover region. Instead, despite a ten-fold difference in rs, the compressibility of both electrons and holes is well described by the theory of nonlinear screening of the random potential. We show that the resistivity exhibits a scaling behavior near the percolation threshold found from analysis of the compressibility. Notably, the percolation transition occurs at a much lower density than the crossover. The apparent metal-insulator transition (MIT) in high-mobility two-dimensional systems remains a topic of fundamental interest [1] and continuing debate [2]. The anomaly of these systems is exemplified by the existence of a narrow range of carrier densities around n = n c where the slope of the temperature dependence of the resistance, ∂R/∂T , changes its sign. To unravel a complex interplay between interactions and disorder in this phenomenon, it is essential to combine transport measurements with other experimental probes, in particular measurements of the thermodynamic density of states (also referred to as the charge compressibility [3, 4]) χ = dn/dµ, where µ is the chemical potential. There have been only few measurements of χ near the apparent MIT [5,6,7], among which work [5] on a 2D hole gas with large values of the Coulomb interaction parameter r s ≡ 1/ πna 2 B ≈ 5 − 16 has attracted much attention. (Here a B = 18Å is the effective Bohr radius for the hole mass of 0.38 m 0 .) In their experiments done at T = 0.3 − 1.3 K the authors of Ref. [5] found that the inverse compressibility χ −1 (n) has a minimum which is positioned exactly at n c . This was interpreted as a thermodynamic signature of an interaction-driven phase transition discussed in theoretical works [8,9].An alternative explanation of the minimum of χ −1 (n) can be based on the nonlinear screening theory (NST) [10,11,12,13] that emphasizes the role of disorder. The basic premise of the NST is that a low-density metal is unable to screen fluctuations of potential, so that depletion regions with vanishingly small local density appear and grow as n decreases. The NST predicts that χ −1 (n) has a minimum at n = n m (determined by disorder), and a rapid upturn to positive values at n < n m .This theory also predicts a percolation threshold at n = n p [11], where n p ≈ n m /3 in typical GaAs systems [13]. There have been suggestions, based on the conductance scaling, that the percolation transition is closely related to the change in the sign of ∂R/∂T [12,14,15]. (The existence of the percolative MIT in 2D GaAs structures was proposed earlier in [11].) In this work we use combined compressibility and conductance measurements to shed light on the origin of the apparent MIT in 2D hole gases with large interactions between the carriers -a problem widely debated over the last few years...

Ferromagnetic cobalt nanowires with high‐crystalline quality are synthesized using a low‐voltage electrodeposition method. High‐resolution transmission electron microscopy (HRTEM) and X‐ray diffraction (XRD) results show that the nanowires are uniform in size, and consist of predominantly hexagonal close‐packed (hcp) structure with the magnetocrystalline easy axis (c‐axis) perpendicular to the wire axis. Superconducting quantum interference device (SQUID) measurements illustrate the dominance of shape anisotropy, manifested by the weak temperature dependence of the enhanced coercive field along the wire axis. Furthermore, the magnetic structures of individual, segmented, or intersected nanowires are studied using magnetic force microscopy. This reveals a strong dipole at the two ends of the wire, together with a spatial magnetization modulation along the wire. Based on theoretical modeling, such intrinsic modulation is attributed to magnetization frustration due to the competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the wire axis.

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We have studied the corrections to the Drude conductivity and Hall constant of a high-mobility twodimensional electron gas in a GaAs/AlGaAs heterostructure due to the electron-electron interaction in the presence of a mixed disorder. A parabolic, negative, temperature-dependent magnetoresistance (MR) and temperature-dependent Hall constant are observed. We show that these effects can be explained in terms of the electron interaction theory and obtain the values of the Fermi-liquid interaction parameter F σ o . In addition, a temperature independent, positive MR has been detected. This classical MR is also shown to be a consequence of the mixed disorder.